AN ABSTRACT OF THE DISSERTATION OF
Joayoung Jeong for the degree of Doctor of Philosophy in Chemistry
presented on December 6, 2007.
Title: High Quantum-Yield Phosphors via Quantum Splitting and Upconversion
Abstract approved:___________________________________________
Douglas A. Keszler
The Gd3+ ion has been used to induce quantum splitting in luminescent
materials by using cross-relaxation energy transfer (CRET). In Nd:LiGdF4, quantum
splitting results from a two-step CRET between Gd3+ and Nd3+, first involving a
transition 6G→6I on Gd3+ and an excitation within the 4f3 configuration of Nd3+
followed by a second CRET that brings Gd3+ to 6P7/2. The excited Nd3+ ion rapidly
relaxes nonradiatively to the emitting 4F3/2. The excited Gd3+ ion then transfers its
energy back to Nd3+, which gives rise to the second photon. The result is a quantum
yield of 1.05 ± 0.35 with emission in the NIR following excitation at 175 nm.
GdF3:Pr3+, Eu3+ also exhibits quantum splitting, but only at very low concentration of
Pr3+ (0.3%) and Eu3+ (0.2%), resulting in a quantum yield of approximately 20% under
160-nm excitation. Host intrinsic emission via a self-trapped exciton (STE) was also
examined as a means to sensitize Gd3+ emission. The material ScPO4:Gd3+ exhibits a
high absolute quantum yield of 0.9 ± 0.2 under 170-nm excitation, demonstrating a
potentially new and efficient pathway for exciting quantum splitting phosphors.
Single crystals of the material GdZrF7 were grown, and its structure was
established via single-crystal X-ray diffraction methods. Doped samples of
GdZrF7:Yb3+, Er3+ exhibit bright up-conversion luminescence with light output that is
up to twice that of a commercial material based on the host Gd2O2S. When doped
with Eu3+, the fluoride also emits a nearly white color under vacuum ultraviolet
excitation with an absolute quantum yield near 0.9. The new compound Gd4.67(SiO4)3S
was synthesized and studied. The structure was established via single-crystal X-ray
methods, and the luminescence of Tb3+ samples was investigated.
Copyright by Joayoung Jeong
December 6, 2007
All Rights Reserved
High Quantum-Yield Phosphors via Quantum Splitting and Upconversion
by
Joayoung Jeong
A DISSERTATION
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Doctor of Philosophy
Presented December 6, 2007
Commencement June 2008
Doctor of Philosophy dissertation of Joayoung Jeong presented on December 6, 2007
APPROVED:
Major Professor, representing Chemistry
Chair of the Department of Chemistry
Dean of the Graduate School
I understand that my dissertation will become part of the permanent collection of
Oregon State University libraries. My signature below authorizes release of my
dissertation to any reader upon request.
Joayoung Jeong, Author
ACKNOWLEDGMENTS
First of all I want to say “thank you” to Douglas Keszler for his endless support,
his patience, his guidance, and his acceptance of me as a doctoral student working
under his guidance.
I want to thank my committee members Drs. Janet Tate, Philip Watson,
Michael M. Lerner, William W. Warren, Jr., and John E. Baham for their
encouragement in helping me to successfully complete my doctoral program. In
addition, I also want to express my gratefulness to those professors who provided
instruction in my course work: Drs. Arthur W. Sleight, Wei Kong, Joseph W. Nibler,
William H. Warnes, and Milo D. Koretsky.
For device testing, everyday interactions, and friendship, I want to extend my
appreciation to the present and former members of the Keszler, Tate, Wager, and
Chang’s groups : Dr. Sangmoon Park, Dr. Cheol-Hee Park, Ji-Eun Yi, Kai Jiang, Dr.
Mike Hruschka, Mike Shoemaker, Jeremy Anderson, Jason Stowers, Dr. Peter Hersh,
Heather Platt, Stephen Meyer, Bahar Özmen, Dr. Liping Guo, Robert Kykyneshi,
Benjamin C. Nielsen, Paul Newhouse, Dr. Hai Chiang, David Hong, Doo-Hyoung Lee,
Seung-Yeul Han.
I want to thank Drs. K.C. Mishira and M. Raukas at Osram Sylvania for
numerous helpful discussions. I am also grateful to Ted Hinke for his contributions in
equipment design and Joe Magner for maintenance of research tools.
This research work was funded by the U.S. National Science Foundation, Grant
Nos, 0305400 (RSM) and 0305449 (DAK).
Most of all, I deeply appreciate my wife, Mikyeoung, and my son, Jinha, who
always stand by me with so much love.
CONTRIBUTION OF AUTHORS
I am most thankful to my co-workers Dr. Richard S. Meltzer and his graduate
student, Yi Zhou, at University of Georgia for measurement of VUV luminescent
characteristics and the precious answers to my many questions. Their contributions are
significantly represented in Chapter 2-5. Dr. Lev Zakharov provided invaluable
assistance in completing the crystal-structure analyses described in chapter 7 and 9.
TABLE OF CONTENTS
Page
CHAPTER 1. INTRODUCTION ........................................................................... 1
1.1. INTRODUCTION.................................................................................... 1
1.2. GENERAL CONSIDERATION.............................................................. 6
1.2.1 Selection of Host Compound for Rare Earth.................................. 6
1.2.2 Energetic postion of the lowest 5d Level ....................................... 9
1.2.3 The Stokes shift .............................................................................. 10
1.2.4 Energy transfer ............................................................................... 12
1.3. QUANTUM SPLITTING ........................................................................ 14
1.3.1 PCE Dynamics................................................................................ 14
1.3.2 Quantum Splitting by Cross Relaxation Energy Transfer
(CRET) ........................................................................................... 15
1.4. DISSERTATION SUMMARY ............................................................... 20
REFERENCES ............................................................................................... 22
CHAPTER 2. QUANTUM SPLITTING AND ITS DYNAMICS
IN GdLiF4:Nd3+ ....................................................................................... 23
2.1. INTRODUCTION .................................................................................... 24
2.2. EXPERIMENT ......................................................................................... 26
2.2.1. Demonstaration of the Quantum Splitting ..................................... 27
2.2.2. Excitation spectrum and quantum yield......................................... 33
2.2.3. Dynamics of the quantum splitting ................................................ 36
2.3. DISCUSSION........................................................................................... 43
2.4. CONCLUSION ........................................................................................ 46
REFERENCES ................................................................................................ 47
CHAPTER 3. SENSITIZATION OF Gd3+ AND THE DYNAMICS OF
QUANTUM SPLITTING IN GdF3:Pr,Eu ............................................... 49
3.1. INTRODUCTION ................................................................................... 50
3.2. RESULTS AND DISCUSSION.............................................................. 51
TABLE OF CONTENTS (Continued)
Page
REFERENCES ................................................................................................ 58
CHAPTER 4. RELAXATION OF THE 4fn-15d1 ELECTRONIC STATES
OF RARE EARTH IONS IN YPO4 AND YBO3 ..................................... 59
4.1. INTRODUCTION .................................................................................... 60
4.2. RESULTS AND DISCUSSION............................................................... 61
4.3. CONCLUSION ........................................................................................ 69
REFERENCES ................................................................................................ 70
CHAPTER 5. HOST SENSITIZATION OF Gd3+ IONS ON YTTRIUM
AND SCANDIUM BORATES AND PHOSPHATES FOR
APPLICATIONS IN QUANTUM SPLITTING .......................................71
5.1. INTRODUCTION .....................................................................................72
5.2. EXPERIMENTAL.....................................................................................74
5.3. RESULTS AND DISCUSSION................................................................76
5.3.1. ScBO3 ..............................................................................................76
5.3.2. YBO3 ...............................................................................................80
5.3.3. ScPO4 ..............................................................................................85
5.3.4. YPO4 ...............................................................................................91
5.4. ENERGY TRANSFER RATES................................................................94
5.4. CONCLUSIONS .......................................................................................97
REFERENCES .................................................................................................98
CHAPTER 6. LUMINESCENCE OF LANTHANIDES DOPED GdZrF7 ............ 100
6.1. INTRODUCTION .................................................................................... 101
6.2. EXPERIMENT ......................................................................................... 101
6.3. RESULTS AND DISCUSSION............................................................... 103
6.4. CONCLUSION ........................................................................................ 116
REFERENCES ................................................................................................ 117
CHAPTER 7. CRYSTAL STRUCTURE AND Eu3+ LUMINESCENCE OF
TABLE OF CONTENTS (Continued)
Page
GdMF7 (M=Hf4+, Zr4+) ........................................................................... 119
7.1. INTRODUCTION .................................................................................... 120
7.2. EXPERIMENT ......................................................................................... 121
7.3. RESULTS AND DISCUSSION............................................................... 124
7.3.1. Crystal structure ............................................................................. 124
7.3.2. luminescence characteristics .......................................................... 134
7.4. CONCLUSION ........................................................................................ 136
REFERENCES ................................................................................................ 137
CHAPTER 8. THE NEW EFFICIENT UPCONVERSION GREEN PHOSPHOR
GdZrF7 :Yb3+,Er3+ ........................................................................................... 138
8.1. INTRODUCTION..................................................................................... 139
8.2. EXPERIMENT ......................................................................................... 140
8.3. STRUCTURAL CHARACTERISTICS.................................................. 141
8.4. LUMINESCENCE CHARACTERISTICS............................................. 147
8.4.1. Optimal Er3+ and Yb3+ concentration............................................. 149
8.4.2. Color purity change vs. Yb3+ concentrration ................................. 152
8.4.3. Effect of Yb3+ concentration on red emission output..................... 157
8.4.4. Luminescence dependency on the excitation intensity .................. 159
8.5. CONCLUSION ........................................................................................ 160
REFERENCES ................................................................................................ 161
CHAPTER 9. CRYSTAL STRUCTURE AND LUMINESCENT
PROPERTIES OF THE APATITE Gd4.67(SiO4)3S .................................. 162
9.1. INTRODUCTION ................................................................................... 163
9.2. EXPERIMENT ........................................................................................ 164
9.3. RESULTS AND DISCUSSION.............................................................. 166
9.3. CONCLUSION ....................................................................................... 179
REFERENCES ............................................................................................... 180
TABLE OF CONTENTS (Continued)
Page
CHAPTER 10. CONCLUSION ............................................................................. 181
BIBLIOGRAPHY ................................................................................................... 183
APPENDICES ........................................................................................................ 188
Appendix A. Luminescent measurement system ............................................ 189
Appendix B. A HIGH MOBILITY TRANSPARENT THIN-FILM
TRANSISTOR WITH AN AMORPHOUS ZINC TIN OXIDE
CHANNEL.............................................................................................191
Appendix C. CURRICULUM VITA................................................................202
LIST OF FIGURES
Figure
Page
1.1. Energy level diagram of Pr3+ ion. PCE process is indicated by two
successive transitions from the 1S0 level following excitation into the
4f5d band.(adopted from A. P. Vink, P. Dorenbos, C. W. E. Van
Eijk, Journal of Solid State Chemistry, 171, 308-312 (2003)). ....................... 3
1.2. (a) Emission spectrum of YF3:Pr3+ (b) Schematic of PCE; initial
photon emission from 1S0 and the second photon emission from 3P0.............. 4
1.3. Schematic diagram showing the barycenter shift and crystal-field
splitting energy of the 5d levels of an ion in a host compound.. ..................... 7
1.4. Configurational coordinate diagram of excitation and emission
process............................................................................................................... 10
1.5. Schematic representation of sensitized emission. The energy is
absorbed by the sensitizer (S) and then transferred to acceptor (A),
which emits. ...................................................................................................... 12
1.6. Emission spectra of SrAlF5: Pr3+ under x-ray excitation. The dotted
line is measured at 100K, the solid line is at 350K (A.P.Vink et. al.,
Journal of Physics; Condensed Matter, 14, 8889 (2002), with
permission from publisher ................................................................................ 15
1.7. Quantum splitting process by CRET in two lanthanide ions ............................ 16
1.8. Energy level structure of Gd3+ in LiYF4. (adopted from R.T.Wegh,
H. Donker, A. Meijerink, Physical Review B 56, 21, 13841-13848
(1997))............................................................................................................... 17
1.9. 4fn-15d levels of free gaseous Ln3+ ions. (■) represents spin
forbidden 4f-5d transition energy and (♦) for dipole allowed 4f-5d
transition energy. (adopted from P. Dorenbos, J. Lumin. 91, 155-176
(2000))............................................................................................................... 18
2.1. (Color online) Relative quantum yield of GdLiF4:Nd 2% exciting at
160 nm (black, solid curve) and at 351 nm (red, dashed curve). The
spectra are normalized on the Nd3+ 4D3/2 and 2P3/2 quantum yields................... 28
2.2. (Color online) Energy level diagrams of Nd3+ and Gd3+ in
GdLiF4:Nd with the relevant energy levels labeled. The open box
LIST OF FIGURES (Continued)
Figure
Page
represents the 4f25d band of Nd3+. The boxed areas with horizontal
lines represent energy regions with a high density of 4fn levels. ET1
and ET2 indicate resonant energy transfer processes. Labels A, B,
and C next to the red (dashed) lines denote three cross relaxation
energy transfer processes. Some of the intrinsic lifetimes are
indicated .......................................................................................................... 30
2.3. (a) Absorption spectrum of YLiF4:Nd2% and (b) emission spectrum
of YLiF4:Gd5% [9] showing significant spectral overlap. .............................31
2.4. (Color online) Excitation spectrum of GdLiF4 containing 1, 2 and
3% Nd3+ and detecting the Nd3+ 4F3/2 emission using a cutoff filter
that transmits for λ>780 nm. Features of the 6GJ, 6DJ and 6IJ levels
of Gd3+ and the 4f25d bands of Nd3+ are indicated. ........................................33
2.5. (Color online) Comparison of the excitation spectra of
GdLiF4:Nd2% detecting only the 4F3/2 emission with λdetect>780 nm
with that of the case of detection for λdetect<780.............................................35
2.6. (Color online) Time evolution of the 6I (281 nm) and 6P7/2 (313 nm)
emission intensities of Gd3+ and the 4D3/2 and 4F3/2 emission
intensities of Nd3+ in a GdLiF4:Nd2% sample under 157 nm pulsed
laser excitation ................................................................................................37
2.7. (Color online) Time evolution of the 6I (281 nm) and 6P7/2 (313 nm)
emission intensities of Gd3+ under 157 nm pulsed excitation in
GdLiF4:Nd for 1, 2, and 3% Nd concentrations. The dashed lines
show the fits using the 6I decay times shown in the figure. Those
same times are used as the rise times in the fits to the 6P7/2 emission
for the sample with the same Nd3+ concentration ...........................................39
2.8. Time evolution of the 4D3/2 and 2P3/2 emission of Nd3+ in a sample of
GdLiF4:Nd2% under 355 nm excitation and the 4P3/2 emission under
157 nm excitation. The decay of 2P3/2 is the rate limiting state in the
feeding of 4F3/2. Also plotted as dashed lines are fits to the data
using the rise and decay times indicated on the figure. ..................................40
LIST OF FIGURES (Continued)
Figure
Page
2.9. (Color online) Time evolution of the 2P3/2 and 4F3/2 emission in a
GdLiF4:Nd2% sample under 355 nm and 157 nm excitation. The
fits shown on the figure are obtained using the rise and decay times
indicated in the legend. They percentage indicates the fraction of
population buildup which is contributed by this rise time. The
remainder of the population buildup is taken to appear immediately
after excitation.................................................................................................42
3.1. Energy level diagrams for Pr3+, Gd3+, and Eu3+ showing the various
energy transfer pathways labeled a through j. Processes a through d
are shown displaced downward by 2500 cm-1 reflecting half the
value of the Stoke’s shift for LaF3 for the Pr3+ 4f5d emission. .......................52
3.2. Emission spectra for a sample of GdF3 containing 0.3% Pr and 0.2%
Eu excited at 275 nm (6I state of Gd3+) and 160 nm (4f5d state of
Pr3+).................................................................................................................53
3.3. Excitation spectra of two samples of GdF3:Pr,Eu. Excitation spectra
obtained by detecting all wavelengths > 320 nm are referenced to a
Na salicylate standard. Excitation spectra obtained with filters
selectively for λ>580 nm and λ<560 nm are normalized for the 6I
peak but are not to the scale of the figure .......................................................55
3.4. Time-resolved emission for 6I and 6P7/2 of Gd3+ after pulsed
excitation at 193 nm showing that the decay of 6I corresponds to the
buildup of 6P7/2 and that energy transfer from Pr3+ predominantly
feeds 6I. The circles are the measurement and the dashed curves are
fits using an exponential decay and buildup of 2.4 µs with an initial
20% 6P7/2 population........................................................................................57
4.1. Energy level diagrams for Pr3+, Tm3+ and Er3+ in YPO4 and YBO3.
For Tm3+ and Er3+ the 5d levels are split into a lower-energy high
spin (HS) and higher energy low spin (LS) states. For Er3+ the room
temperature lifetimes are shown next to the emitting states.
Processes labeled A and B for the Pr3+-Tm3+ pair indicate energy
conserving cross relaxation paths ................................................................... 62
4.2. Excitation (dashed) and emission spectra (solid) for Pr3+, Er3+ and
Tm3+ ions in YPO4 at room temperature. The excitation spectra are
LIST OF FIGURES (Continued)
Figure
Page
relative to that of sodium salicylate. For Er3+ the distinct vertical
bars identify the emitting level........................................................................ 66
4.3. Excitation spectra at room temperature demonstrating the absence of
Pr3+ to Tm3+ energy transfer in YPO4 and YBO3. None of the
features of the Pr3+ excitation spectra appear in doubly doped
samples when only the Tm3+ emission is detected. The excitation
spectra of the doubly-doped samples are not to scale..................................... 68
5.1. Emission spectra of ScBO3, excited at160 nm. The instrinsic STE
emission is shown amplified by a factor of 20 ............................................... 76
5.2. Emission spectra of the Gd3+-doped borates in the red showing the
weak Gd3+ 6G→6P emission ........................................................................... 78
5.3. Excitation spectra of undoped and Gd3+-doped ScBO3 detecting the
total emission and measured relative to that of sodium salicylate.................. 78
5.4. Time resolved intrinsic emission of undoped and Gd3+-doped
ScBO3. The emission was excited at 157 nm and detected at 250
nm. Fitted decay curves are shown by the dashed lines. The fitted
values have a 5 ns instrumental contribution .................................................. 79
5.5. Observed decay of the Gd3+ 6G→6P emission in the Gd3+-doped
borates. The fitted decay curves are shown by the dashed lines with
the decay values shown in the legend ............................................................. 80
5.6. Fluorescence spectra of YBO3 excited at160 nm. The intrinsic
emission is shown expanded by a factor of 100.............................................. 81
5.7. Time resolved emission excited at 157 nm and detected at 340 nm.
The decay is a double exponential. The short decay component in
the figure is lengthened by the 5.9 kΩ oscilloscope input impedence.
Its actual decay time is < 2 ns ......................................................................... 82
5.8. Time-resolved emission spectra excited at 157 nm. The t=0
spectrum is obtained from the initial intensity of the fast decay
component. The spectrum of the slow decay component was
obtained from the intensity at 400 ns after the fast component had
decayed. It is identical to the time-averaged emission spectrum................... 83
LIST OF FIGURES (Continued)
Figure
Page
5.9. Excitation spectra of undoped and and Gd3+-doped YBO3 detecting
the total emission and measured relative to that of sodium salicylate ............ 84
5.10. Emission spectra of ScPO4, and YPO4 excited at 160 nm .............................. 86
5.11. Excitation spectra of undoped and Gd3+-doped ScPO4.The doped
sample is referenced to sodium salicylate (dashed curve). The
excitation of the UV portion of the emission is measured relative to
sodium salicylate (thin solid curve) while the red portion of the
emission is referenced to Y2O3:5%Eu3+ (dotted curve). The
estimated absolute quantum yield is shown by the bold solid curve .............. 87
5.12. Time-resolved emission of undoped and Gd3+-doped ScPO4 excited
at 157 nm. The fits are shown by the dashed lines and include a 5ns
instrumental contribution ................................................................................ 89
5.13. Time-resolved emission of ScPO4:1%Gd excited at 157 nm and
detected at 206nm and 600 nm (Gd3+ 6G emission) and 315 nm
(Gd3+ 6P emission). The inset shows the fit of the 6G decay.......................... 91
5.14. Excitation spectra of undoped and Gd3+-doped YPO4 for detection in
different wavelength regions showing the dependence of the spectra
on detection wavelength ................................................................................. 92
5.15. Time-resolved emission of undoped (solid curves) and Gd3+-doped
(dotted curves) YPO4. The 240 nm emission shows a 55 ns buildup
and 380 ns decay while the emission at longer wavelengths (340nm
and 460 nm shown in the figure) exhibit a decay with two
components. The dashed curves show fits to the 240 nm and 460 nm
data for the undoped sample ........................................................................... 94
6.1. Emission spectra of GdZrF7 a) undoped, b) doped with 1% Eu3+, c)
doped with 1% Pr3+, d) double doped with 1% Pr3+ and 1% Eu3+
under 160nm excitation................................................................................... 104
6.2. XRD patterns of GdZrF7:1%Eu3+ (a), the reference XRD pattern (b)
calculated from single crystal structure data................................................... 106
6.3. Excitation spectrum of GdZrF7:1%Eu3+ and 3% Eu3+ samples....................... 107
LIST OF FIGURES (Continued)
Figure
Page
6.4. Excitation spectra of GdZrF7 a) undoped, b) doped with 1% Eu3+ , c)
doped with 1% Pr3+, d) double doped with 1%Pr3+ and 1%Eu3+ for
whole emission above 300nm ......................................................................... 109
6.5. XRD patterns of (a) GdZrF7:1%Pr3+ (b) GdZrF7:1%Pr3+,1%Eu3+ ................. 111
6.6. Emission spectrum of GdZrF7 doped lanthanides. a) Doped with
1%Eu3+, b) doped with 1%Ce3+, c) codoped with 1%Eu3+ and
1%Tm3+ ........................................................................................................... 113
6.7. Emission spectrum of GdZrF7:1%Eu3+,1%Tb3+ for the whole
emission compared to that of GdZrF7:1%Eu3+. .............................................. 114
6.8. Excitation spectrum of several GdZrF7 samples doped with
lanthanides, a) doped with 1%Eu3+, b) doped with 1%Ce3+, c) co
doped with 1%Eu3+ and 1%Tm3+. All excitation spectra were
measured for whole emission spectrum except b) which was
measured excluding the emission peak of 6P of Gd3+..................................... 115
6.9. XRD patterns of (a) GdZrF7:Ce3+ and (b) GdZrF7:Eu3+,Tm3+ ........................ 116
7.1. Unit cell drawing of GdHfF7 ........................................................................... 125
7.2. Two views of the eight coordinated polyhedron of Gd3+ ion in
GdHfF7. ........................................................................................................... 126
7.3. A [001] directional view of GdHfF7 compound showing Hf 4+ and
Gd3+ ions composing the squares respectively. The crystal
coordinate was shown on the picture. ............................................................. 127
7.4. A [010] directional view showing the top and bottom layers
composed of Gd squares and Hf squares. Those layers form slabs of
[Gd2Zr2F12] 2+ with other bottom and top layers of next unit cells
along c-direction. ............................................................................................ 128
7.5. A fragment of the crystal structure of the two types of zig-zag -GdF-Gd-F- chains structure showing the disorder at F5 position ....................... 129
7.6. a) Experimental XRD pattern of the powder sample of GdHfF7, b)
XRD pattern calculated based on the single crystal structure of
GdHfF7 ............................................................................................................ 133
LIST OF FIGURES (Continued)
Figure
Page
7.7. Emission spectrum under 160nm excitation. Emission spectra of a)
GdHfF7, b) GdHfF7:1%Eu3+, c) GdZrF7:1%Eu3+, d)
Gd(Hf0.5,Zr0.5)F7:1%Eu3+. Y-coordinate is relative emission intensity .......... 134
7.8. Excitation spectrum of GdHfF7 samples compare to other analogous
compound. a)undoped GdHfF7, b) GdHfF7:1%Eu3+, c)
GdZrF7:1%Eu3+, d). Gd(Zr,Hf)F7:1%Eu3+ ..................................................... 136
8.1. Powder XRD data of Gd0.98-xZrF7:YbxEr0.02 samples. The bottom
peaks is for x=0.18 and top one is for x=0.98. The x value is
increased from x=0.18(bottom one) to 0.22, 0.26, 0.30, 0.34, 0.50
and 0.98(top one). Inset is the magnified one for Fig. 1 in the 2 θ
range of 21- 24 degree .................................................................................... 142
8.2. Reference XRD pattern of GdZrF7 compound calculated from the
single crystal structure solution data............................................................... 142
8.3. Cell parameter change according to the increase of Yb3+
concentration ,(a) cell parameter, (b) cell volume and β angle. ..................... 143
8.4. Raman spectrum of polycrystalline GdZrF7. The excitation source is
He-Ne green laser............................................................................................ 145
8.5. SEM pictures of Gd0.74ZrF7:Yb0.22,Er0.04sample at several
magnifications, (a) ×100, (b) ×300 and (c) ×1250.......................................... 147
8.6. Upconversion mechanism for green emission under near infrared
light excitation showing the energy transfer in Yb3+-Er3+ system. The
dotted curve explains the energy transfer from Yb3+ to Er3+ via
consecutive two or three photon absorption by Er3+, the downward
zigzag line is non-radiative transition, the straight thick downward
lines show the radiative transitions.[9] ........................................................... 148
8.7. Relative emission output of the Gd1-x-yZrF7:YbxEry samples were
measured during 30min compared with the reference one. The Er3+
concentration was varied as 1%, 2%, 3% and to 4% at three different
concentration of Yb3+. The emission output data for 18% Yb3+ are on
(a), for 22% Yb3+ on (b), for 26%Yb3+ on (c). In all graphs the black
line marked with black diamond represent the emission output of
reference sample. The line with brown triangle marker is for 2% Er3+,
the green cross marker is for 3% Er3+, the violet square marker is for
LIST OF FIGURES (Continued)
Figure
Page
1% Er3+ and the black cross maker is for 4% Er3+ in the downward
sequence from the top one .............................................................................. 150
8.8. Emission output results of GdZrF7 samples at three concentration
levels of Yb3+ and four concentration levels of Er3+ collected from
the experiment above. The data dispersion on each sample is caused
by the emission output increase as time pass by as we mentioned
already. The first group of dots express the emission output of
reference sample, the next three groups of dots represent the
emission output of 1% Er samples, the next three for 2% Er
samples, the next three for the 3% Er samples and the last three for
the 4% Er samples. At each Er concentration, the first group of dots
represent 18% Yb3+, the second one 22% Yb3+, and the third one
26% Yb3+ condition. ....................................................................................... 151
8.9. Emission output of Gd0.98-xZrF7:Yb3+x Er3+ 0.02 samples at further
increased concentration of Yb3+ up to 98% are measured
intermittently. The emission output was measured during 50min
intermittently and is represented as dots. x-abscise is the Yb3+
concentration, y-abscise is the relative emission output to that of
reference one. .................................................................................................. 152
8.10. Emission spectrum at various Yb3+ concentrations. (a) [Yb] =18%,
(b) [Yb] =22%, (c) [Yb] =26%, (d) [Yb] =30%, (e) [Yb] =34%, (f)
[Yb] =50% and (g) [Yb] =98%. Every emission spectrum are
compared with reference one which is shown by the blue solid line.............. 154
8.11. G/R ratio at various Yb3+ concentrations........................................................ 155
8.12. XRD data of reference up conversion green phosphor with the x-ray
pattern of Gd2O2S from ICDS file. ................................................................. 156
8.13. Emission spectrum excited by 379nm and 490nm (a) of 22%Yb3+
sample, (b) 50% Yb3+ sample and (c) reference one. In all pictures
the violet solid lines represent the emission spectrum under 490nm
excitation and the blue solid lines are that under the 379nm
excitation......................................................................................................... 158
9.1. Unit-cell drawing of Gd4.67(SiO4)3S................................................................ 168
LIST OF FIGURES (Continued)
Figure
Page
9.2. (a) Environment of the free O atom in Ln4.67(SiO4)3O apatite (b)
Environment of S atom in Gd4.67(SiO4)3S apatite ........................................... 169
9.3. Tricapped distorted trigonal prismatic environment of Gd(2) and
seven-coordinate site of Gd(1)........................................................................ 170
9.4. Sulfur column along c axis ............................................................................. 171
9.5. XRD patterns for Gd4.67(SiO4)3S (a) synthesis in flowing H2S(g) (c)
prepared in sealed tube and (b) reference pattern calculated from
single-crystal structure data ............................................................................ 173
9.6. Emission spectra for selected concentrations of Tb3+ in
Gd4.67(SiO4)3S ................................................................................................. 174
9.7. Emission spectra after correction for (a) 7% and (b) 10% Tb3+ doped
Gd4.67(SiO4)3S ................................................................................................. 175
9.8. Excitation spectrum of 7% and 10% Tb3+-doped Gd4.67(SiO4)3S (λem
= 544nm). ........................................................................................................ 176
9.9. Excitation spectrum at liquid helium temperature of (a) 10% Tb3+
doped Gd4.67(SiO4)3S and (b) 7% Tb3+ doped Gd4.67(SiO4)3O........................ 177
9.10. Excitation spectra 4f-4f transitions of 10% Tb3+ doped
Gd4.67(SiO4)3S ................................................................................................. 178
9.11. Comparison of emission spectra of Gd4.67(SiO4)3S:10% Tb3+ under
two different excitation wavelengths of 313nm and 370nm. The
emission intensity was calibrated with the intensity ratio of those
two excitation wavelength using Rhodamin-B. .............................................. 179
LIST OF TABLES
Table
Page
1.1. 4fn-15d energy of several Ln3+ ions doped in LiYF4 compound ..................... 19
2.1. Experimental energy transfer rates. ................................................................ 38
5.1. Wavelengths and decay times of the emission of undoped and Gd3+
doped scandium and yttrium borates and phosphates..................................... 95
7.1. Crystal data and some of details of X-ray diffraction experiment and
refinement of the crystal structure of GdMF7 (M=Zr, Hf) .............................. 123
7.2. Atomic position (x104) and equivalent isotropic displacement
parameters (Å2x 103). U(eq)is defined as one third of the trace of
the orthogonalized Uij tensor a) GdHfF7 ........................................................ 130
7.3. Selected bond lengths [Å] in GdMF7.............................................................. 131
7.4. Selected Bond angles [°] in GdMF7 ................................................................ 132
8.1. G/R ratio measured from the emission spectrum of each Yb3+
concentration. Data at two different excitation wavelengths of
490nm and 980nm are shown for two samples of 22%Yb3+ and 50%
Yb3 .................................................................................................................. 160
9.1. Crystal data and details of X-ray diffraction experiment for
Gd4.67(SiO4)3S. ................................................................................................ 165
9.2. Atomic positions and equivalent isotropic displacement parameters
(Å2 x 103) for Gd4.67(SiO4)3S. ....................................................................... 167
9.3. Bond lengths [Å]............................................................................................. 171
9.4. Selected Bond angles [°]................................................................................. 172
9.5. The shortest distance between Gd ions in two different sites. ........................ 177
HIGH QUANTUM-YIELD PHOSPHORS VIA QUANTUM
SPLITTING AND UPCONVERSION
CHAPTER I
1.1 INTRODUCTION
Phosphors are materials that emit light following excitation with
electromagnetic energy or high energy particles, e.g., electrons. They are currently
used in fluorescent and light-emitting diode lamps as well as variety of displays such
as the cathode ray tube (CRT), field emission display (FED), vacuum fluorescent
display (VFD), electroluminescent (EL) device, and plasma display panel (PDP). In
CRT, FED, and VFD applications, the phosphor is excited by an electron beam at
either high or low accelerating voltages, while in PDP and fluorescent lamp
applications, the phosphor is excited with high-energy photons in the ultraviolet (UV)
or vacuum ultraviolet (VUV) portions of the spectrum. Recently, large-area PDPs
have proven to be commercially successful as HDTVs. They operate on the basis of a
Xe discharge, which produces the VUV light for excitation of the red, green, and blueemitting phosphors. Similar technology using a Xe discharge is envisioned for
producing a fluorescent lamp that is free of Hg. A serious drawback of existing PDP
and related VUV-excited phosphors, however, is the low quantum ratio (R ) associated
with the high photon energy of the Xe discharge relative to the photon energies of
visible light. The average energy of light from a red, green, blue phosphor set
corresponds approximately to 500 nm. As shown below, this lead to R ~ 0.3 for a Xe
discharge and R ~ 0.5 for a Hg discharge.
•
PDP phosphor excited at 147 nm: R = hνem / hνex = λex / λem =
147 nm / 500 nm = 0.3
•
Mercury-based fluorescent lamp phosphor excited at 254 nm:
R = 254 nm / 500 nm= 0.5
2
Because of the low quantum ratio and the lower efficiency of light production
for a Xe discharge relative to a Hg discharge, the energy efficiency of a mercury-free,
Xe-based lamp cannot compete with that of a common fluorescent lamp, even with
phosphors having unit quantum efficiencies. For the energy efficiency of a Xe lamp
to be comparable to that of a conventional Hg-based fluroescent lamp, the quantum
efficiency of the phosphors in the lamp must be near 1.5 or higher , i.e., they must be
significantly greater than unity. Much of the work presented in this thesis is directed
to the development and study of new materials and processes that provide means to
realize these high quantum efficiencies under VUV excitation. Inorganic luminescent
materials exhibiting quantum efficiencies > 1 are well documented in the literature,
but none of these phosphors exhibits the necessary combination of strong absorption at
Xe discharge wavelengths, quantum efficiency, color purity, and stability for
application in a Hg-free lamp. Much of the work in this thesis is directed to realizing
this combination of attributes in a single material.
Phosphor quantum efficiencies > 1 have been realized in several ways. One
particularly useful method is described as Photon Cascade Emission (PCE), which
involves transitions between energy levels of lanthanide ions; PCE is also commonly
referred to as quantum cutting, quantum splitting, and multiphoton emission. While
quantum-cutting materials are differentiated from normal phosphors by having
quantum yields > 1, the physics associated with excitation and emission in a
multiphoton material is identical to that of a conventional luminescent substance.
Pr3+ is a representative lanthanide ion exhibiting PCE; its PCE was first reported
in 1974 by two separate groups: Piper and co-workers at General Electric and
Sommerdijk and co-workers at Philips [1, 2]. As shown in Fig. 1.1, the energy-level
structure of the 4f2 configuration of Pr3+ affords the opportunity for a two-step
emission. The energy of the 1S0 state in many compounds is located near 46,500 to
46,900 cm-1, importantly resting below the 4f1d1 levels. An excited electron in the 1S0
can relax to the ground state through a multiple-step process with transition from 1S0
to 1I6 involving emission of a photon as the first step and transition from 3P0 to 3H4
also involving emission of photon the second step,
3
60
4f15d1
Energy (*103cm-1)
50
1
S0
40
30
1
20
10
1
D2
1
G4
3
3
0
I6
P0
3
FJ
HJ
Fig. 1.1 Energy level diagram of Pr3+ ion. PCE process is indicated by two successive
transitions from the 1S0 level following excitation into the 4f5d band.(adopted from A.
P. Vink, P. Dorenbos, C. W. E. Van Eijk, Journal of Solid State Chemistry, 171, 308312 (2003))
4
incorporating a nonradiative relaxation process between the 1I6 to 3P0 levels, cf., Figs.
1.1. As shown in Fig. 1.2 for YF3:Pr3+, emission from the 1S0 level produces a photon
in the deep blue portion of the spectrum, while emission from 3P0 produces photons
spread across the blue-green, red, and NIR portions of the spectrum. For the overall
process, a quantum efficiency near 140% is observed. Because the initial deep blue
photon is positioned at a wavelength of low eye sensitivity, quantum splitting with
Pr3+ has no direct utility in lamp and display applications.
Wavelength(nm)
Wavelength(nm)
3P
0-
3F
3P
3,4
0-
Intensity
3P
0-
3 H 3F
6, 2
3H
4
1S
0-
1I
6
(a)
400
(b)
1
S0
500
1st
photon
600
Nonradiative
decay
1
I6
3
P0
4f15d
700
2nd
photon
3
3
FJ
H4
ground state
Excitation ~195 nm
Fig. 1.2 (a) Emission spectrum of YF3:Pr3+ (b) Schematic of PCE; initial photon
emission from 1S0 and the second photon emission from 3P0.
5
A more recent example of quantum splitting has been demonstrated in the
system, LiGdF4:Eu3+[3]. Here, energy migration and transfer between the ions Gd3+
and Eu3+ leads to a measured quantum efficiency of 190%. While this system
provides an optimum color purity on the basis of the red Eu3+ emission, the weak cross
sections of the f→f transitions of the Gd3+ do not provide an efficient excitation
pathway. The process of quantum splitting in the system is discussed in more detail in
Section 1.3.2.
Another method for realizing dramatically increased quantum efficiencies is
through electron multiplication. With high-energy excitation - usually ≥ 2.5 times the
band-gap energy - excited electrons can relax to energies above the band gap by
creating electron-hole pairs via inelastic collisions. These generated electron-hole
pairs can lead to photon generation in addition to that of the initially excited electron,
leading to enhanced quantum yields. The excitation energy required to observe
electron multiplication has been described by P. A. Rodnyi and co-workers [5] by
using Eq. 1.1,
9Eg
Et =
7-(me/mh)
Eq. 1.1
Where Et is the threshold energy, which is the minimum energy required to ionize
atoms in the solid as measured from the bottom of the conduction band, and me and mh
are the effective masses of the electron and hole, respectively. If the energy of an
exciting photon hνex exceeds the value of Eg + Et , a secondary electron-hole pair can
be created. For me ~ mh, Et = (3/2) * Eg, and for me >> mh, Et = (9/7) * Eg. From
experimental results Et ~ 1.5 * Eg for semiconductors and Et ~ Eg for ionic compounds.
As an example, the quantum yield of the phosphor Zn2SiO4:Mn2+ is approximately
unity for excitation energies slightly exceeding the band gap (Eg = 5.5 eV). For
excitation energies above 14 eV, the quantum efficiency gradually rises to a value of
1.8 at an excitation energy of 21 eV [5]. The value of 14 eV corresponds to Eg+ Et.
6
Host-sensitized energy transfer is a potentially efficient excitation method for
injecting energy into a system exhibiting a high quantum efficiency. One advantage of
this method is that a secondary sensitizer ion is not required to increase the absorption
of excitation light. Instead, the host is used to strongly absorb the excitation energy,
and the resulting host emission (derived from a self-trapped exciton (STE)) is then in
turn used to sensitize the activator ion. This excitation process has not yet been
adapted in quantum splitting. Sensitizing an activator of interest as a quantum splitter
by host emission, however, has been reported for the 3PJ and 1D2 emissions of Pr3+ in
SrAlF5. [12] This host sensitization is examined in this work for excitation of the
Gd3+ 6G level. To observe quantum splitting from Gd3+, the host emission energy must
be sufficiently energetic and resonant to excite 6GJ of Gd3+, and the energy transfer
rate from the host to Gd3+ must be faster than the host emission decay rate.
1.2 GENERAL CONSIDERATIONS
Several fundamental characteristics of luminescent centers and their
interactions in solids must be considered in selecting and synthesizing materials for
observation of quantum splitting. A few of these characteristics are considered in this
section.
1.2.1 Selection of Host for Lanthanide
In suitable hosts Pr3+ commonly exhibits quantum splitting via PCE. Pr3+ is
excited via a parity allowed 4f →5d transition rather than directly into the 1S0 level,
because of the forbidden nature of the 3H4 → 1S0 transition. As such, the 5d level must
be energetically positioned above the 1S0 to sensitize its occupation. The position of
the 5d level is determined by two factors, the barycenter and the crystal field splitting
parameter Δ. The barycenter shift (centroid shift) is the energy shift in the average
energy of the crystal-field-split 5d levels, while the crystal field splitting affects the
position of the lowest 5d energy level.
7
Free ion
5d
Lowest 5d
centroid
shift (εc)
Red shift,
Depression E
D(Q+,A)
∆
Lowest 5d
∆(4fm, lowest 5d)= lowest
5d of free ion –D(Q+,A)
4fm
Fig. 1.3 Schematic diagram showing the barycenter shift and crystal-field splitting
energy of the 5d levels of an ion in a host compound.
A host with a band gap > 7 eV is necessary to prevent overlap of the lowest
4f5d level of Pr3+ with the conduction band (CB) of the host. As such, fluorides,
selected oxides, and chlorides are suitable hosts. The energy gap between the 4f5d and
1
S0 levels is also important. The free ion has an energy gap ∆E = 11,000 cm-1, but this
value is smaller because of the ligand field and nephelauxetic effects around the ion in
the host. This gap should not be too large or too small. A large gap leads to direct
transition from the 5d level to the ground state rather than the desired nonradiative
relaxation from 5d to 1S0. For a small gap, excessive thermal population of the 5d
level results.
As seen from Fig. 1.3, a small centroid shift (εc) is required for a high energy
position of the lowest 5d level. This shift tends to increase in the order fluoride <
sulfate < carbonate < phosphate < borate < silicate < aluminate [6]. A small
nephelauxetic effect associated with low ligand polarizability will force the lowest 5d
level to a high energy state. The centroid shift is modeled on the basis of Eq. 1.2 [6],
8
εc (cm-1)= 1.44*1017(N αsp/R6eff)
Eq. 1.2
where N is the number of the nearest neighbor ligands; Reff (pm) is the Ln3+ - ligand
distance; and αsp (Å3) is the spectroscopic polarizability.
To produce a high-energy position for the 5d level, a weak crystal field is also
needed. This crystal field can be realized with long Pr3+-ligand distances and high
coordination numbers (CN). In the fluoride YF3:Pr3+, as a result of the small
nephelauxetic effect and crystal field, the 5d level (48,900 cm-1) is situated above the
1
S0 level, resulting in PCE. In contrast, the oxide Y3Al5O12:Pr3+ has a large
nephelauxetic effect and crystal field; as a result, the 5d level (33,300 cm-1) is situated
13,500 cm-1 lower than the 1S0 level. In LaMgB5O10 and LaB3O6 with CN=10 for Pr3+,
the 5d level is located above 1S0, 1S0 emission is observed, but emission from 3P0, cf.,
Fig. 1.1, does not occur because of the high nonradiative transition rate from 3P0 to 1D2,
which derives from the high phonon energy of the borate. Considering the phonon
energy of BO33- at 1450 cm-1 the non-radiative and the radiative rates can be calculated
as Wnr = 107 s-1 and Wr of 3P0 = 3*105 s-1 respectively [7].
Lower frequency phonons are required to observe 3P0 Pr3+ emission in oxides,
SrAl12O19:Pr3+, for example, has a low phonon energy of 700 cm-1, and both steps of
PCE are observed [8]. The nonradiative and radiative transition rates from 3P0 are
calculated as Wnr = 3*103 s-1 and Wr of 3P0 = 3*105 s-1, respectively. The nonradiative
transition rate is related to the maximum phonon energy and the emission energy (Eq.
1.3) [7].
Wnr = β e
–α(∆E-2hω)
Eq. 1.3
β and α are materials-dependent constants; ∆E is energy gap associated with emission;
and 2hω is the maximum phonon energy. For a fixed emission wavelength,
nonradiative relaxation rates on the basis of phonon energies increase in the order
chloride < fluoride < oxide.
9
1.2.2 Energetic position of the lowest 5d Level
Because the position of the 5d level in Pr3+ can be used to predict PCE, it is
important to be able to estimate the energy of this level in a given host. This can be
done by using extensive compilations of the 5d-level positions of Ce3+ in various hosts
[9]. The Ce3+ ion has a simple ground-state electron configuration, 4f1, resulting in
excitation only to the 5d level. Emission generally occurs from this level, so an
excitation spectrum can be used to assign the position of the 5d level. From
examination of many hosts, it has been demonstrated that the energy difference
between Pr3+ and Ce3+ 5d levels is constant and approximately 12,240 ± 750cm-1 [9].
This relationship can be extended to all of the lanthanides through Eq. 1.4.
Lowest 5d level of Ln3+ = 49,340 cm-1 – D (Ln3+, A) + ∆ECe3+, Ln3+
Eq. 1.4
The 5d level of the free Ce3+ ion is positioned at 49,340 cm-1; D (Ln3+, A) represents
the crystal-field depression energy in host A; and ∆ECe3+, Ln3+ is the energy difference
between Ce3+ and the selected Ln3+ . To realize a 5d position above 1S0, the excitation
4f2→4f15d1 of Pr3+ must occur at λex < 205 nm, corresponding to Ce3+ having λex <
270 nm in a given host.
.
10
1.2.3 The Stokes Shift
As shown in Fig. 1.4, the excited state of an activator ion will have a minimum
energy at a cation-ligand separation that differs from the distance in the ground state.
The excited state relaxes to the lowest vibrational energy level from which the
emission spontaneously occurs. The vertical transition to the ground state results in the
occupation of an excited vibrational level, which also subsequently relaxes to the
lowest energy state. The energy difference between the absorption and the emission
energy
derived from these processes is referred to as the Stokes shift (ΔS).
excited
state
ground
state
emission
absorption
V’=0
V=0
R0
Fig. 1.4 Configurational coordinate diagram for luminescence process
To realize the energetic position of 5d > 1S0, not only must the energy of the
long wavelength absorption edge (4f-5d) exceed the energy of 1S0 (~47000cm-1), but
∆S must also be small. A short bond distance is preferable for observing a small
11
Stokes shift, because the shift increases as the square of the average bond length (Eq.
1.5) [10].
∆S ~ R2 (R = average bond distance)
Eq. 1.5
The position of the minimum of the 5d excited state can be estimated from Eq. 1.6
[10] by considering the Stokes shift.
Emin(5d) ≈ E (4f → 5d) - 0.5*∆S
Eq. 1.6
The site symmetry can also affect the position of the 5d energy level. In particular,
asymmetry in the site can significantly increase the Stokes shift.
12
1.2.4 Energy Transfer
Sensitization via energy transfer provides a means to deliver energy to an
activator that inefficiently couples to the excitation source. The sensitizer absorbs the
excitation energy and transfers it to the activator through a nonradiative process (Fig.
1.5).
Emission
Excitation
S
A
Energy
transfer
Fig. 1.5. Schematic representation of sensitized emission. The energy is absorbed by
the sensitizer (S) and then transferred to acceptor (A), which emits
This nonradiative energy transfer is generally modeled by the Forster-Dexter
theory. In the Dexter model, energy transfer occurs by an exchange interaction, where
the electron exchange occurs between sensitizer and activator dopants. Because this
exchange involves wave-function overlap of the sensitizer and activator, it occurs only
over very short distances. The transfer rate is expressed by Eq. 1.7 [15],
WDA = CDA e-2R/L
Eq. 1.7
where CDA is the donor acceptor interaction parameter; and R/L is the donor-acceptor
distance expressed in the Bohr-radius unit.
When the donor and acceptor are separated by large distances corresponding to
insignificant orbital overlap, energy transfer can proceed by dipole-dipole interactions
(Forster model). The electric field generated by an excited sensitizer (donor) can
13
induce a dipole at an acceptor impurity (acceptor). The probability of energy transfer
depends inversely on the square of the energy overlap and sixth power of the distance
between the donor and acceptor, cf., Eq. 1.8,
PABDD=(1.4*1024 fA fB S)/(∆E2 R6)
Eq. 1.8
where fA and fB are the oscillator strengths of the donor and acceptor, respectively;
S is the spectral overlap of donor emission and acceptor absorption; ∆E is the
transition energy; and R is the distance between the donor and acceptor.
As noted, a distinguishing feature of these two mechanisms involves the
sensitizer (S)-activator (A) distance. The Dexter model operates only at very short
distances, where wave-function overlap is significant. The Forster mechanism is
applied to longer S–A distances. Here, the transfer rate is associated with a dipoledipole interaction and the oscillator strengths of the S*→S and A→A* transitions.
This contrasts to the Dexter model, where energy transfer is independent of the
transition rate.
To achieve success in many quantum-splitting schemes, it is essential that the
energy-transfer rate is faster than the radiative decay rate of the sensitizer. The
radiative decay rate is typically 103 to 106 s-1 for forbidden 4f-4f transitions and 106 to
108 s-1 for parity allowed 4fn-15d - 4fn transitions. The Forster dipole-dipole transfer
rate involving the 4f-4f transitions of the lanthanides is estimated as ~105 s-1, assuming
5% impurity concentrations for the sensitizer and activator, S = 0.1 cm-1, 4f-4f
oscillator strengths = 10-6, and a transition energy = 3 eV. This result indicates that
energy transfer involving the 4f-4f transitions of the lanthanides can lead to efficient
transfer.
For a 4fn-15d -4f transition on the sensitizer and a 4f-4f transition on the
activator, the transfer rate is calculated as 107-108s-1, assuming a 5% impurity
concentration, a 4fn-15d -4f oscillator strength = 10-2, and S = 10-3. Wegh and coworkers, for example, reported an energy transfer rate of 109 s-1 between Er3+ and Gd3+
[16]. If the transitions of both ions are 4f n-15d-4f, then the calculated transfer rate is
1012 s-1 for nearest neighbors and 108 s-1 at a 1% dopant concentration [17].
14
1.3 QUANTUM SPLITTING
1.3.1 PCE Dynamics
Two mechanisms have been reported for excitation and luminescence of the Pr3+
ion. One involves excitation (λ ~ 190 nm) of the 1S0 level via relaxation from the 5d
level. Luminescence under this excitation can show PCE. When excited at 160 nm or
higher energies, e.g., by X-rays, host absorption occurs, and the emission spectrum
exhibits a strong enhancement of the 3P0 and 1D2 luminescence transitions. This
suggests the existence of an alternate energy-transfer pathway to the 3P0 or 1D2 levels
involving an STE; such luminescence has been described for the doped hosts BaSO4
[6], SrAlF5 [12], and LaF3-LiF compounds [11].
In BaSO4: 1% Pr3+, the excitation spectrum for 1D2 emission contains only the
host-absorption band, meaning it is not populated via 1S0 but by an STE. In
SrAlF5 :Pr3+, the STE-mediated energy transfer was observed in the emission spectrum
under X-ray excitation. In this case, the emission occurs only from 3P0 and 1D2 [12],
cf., Fig. 1.6. The STE emission. which dominates the emission spectrum at 100 K,
becomes weaker with increasing temperature. At 350 K, most of the energy of the
STE is transferred to 3P0 or 1D2. Additional evidence that the 3P0 state of Pr3+ can be
populated by energy transfer from a STE was found in the LaF3-LiF system [11] The
STE and 3P0 emissions exhibit similar thermal quenching, which implies that the 3P0
state is populated via the energy transfer from the STE. Meanwhile, the 1S0 emission
intensity rises with temperature to 350 K, suggesting a competitive relationship with
STE emission.
15
Fig. 1.6. Emission spectra of SrAlF5: Pr3+ under x-ray excitation. (dotted line - 100 K;
solid line – 350 K) (A.P.Vink, P. Dorenbos, J T M De Hass, H Donker, P A Rodnyi, A
G Avanesov, C W E van Eijk, Journal of Physics; Condensed Matter, 14, 8889 (2002),
with permission from publisher)
1.3.2 Quantum Splitting by Cross Relaxation Energy Transfer (CRET)
Quantum splitting mechanisms can generally be divided into two categories.
One is PCE, where a single ion, e.g., Pr3+, decays via a multiple-step transition. The
other is represented by cross relaxation energy transfer (CRET) between two ions, as
observed in LiGdF4:Eu3+, where Eu3+ receives excitation energy transferred from Gd3+
[3]. The energy-level diagram in Fig. 1.7 illustrates this process. Because of a
resonance between the 6GJ-6PJ energy of the Gd3+ ion and the 5DJ-7FJ energy on the
Eu3+ ion, the excitation energy on the Gd3+ can be transferred to the Eu3+ via cross
relaxation (①). In the process, the Gd3+ relaxes to the 6P0 level, while the Eu3+ is
excited to the 5D0 level. The excited Eu3+ ion can then emit a photon. The remaining
energy on the 6P level of Gd3+ is then transferred to Eu3+ (②), which generates another
photon from its 5DJ level. This process of PCE in Gd3+ provides a useful method for
designing quantum cutting phosphors. The multiple-step emission of Gd3+ can be
16
expected from its 4f energy level structure (Fig. 1.8), and experimentally the emission
transitions from 6GJ and 6PJ have been observed in Gd3+-doped LiYF4 [13 ].
LiGdF4:Eu
70000
60000
50000
-1
Energy (cm )
6
40000
30000
①
GJ
6
DJ
6
IJ ②
6
PJ
5
DJ
20000
①
10000
7
0
8
3+
Gd
S7/2
FJ
3+
Eu
Fig. 1.7 Quantum splitting process by CRET in two lanthanide ions.
17
13/2
3/2
11/2, 9/2, 5/2
7/2
6
GJ
630-636nm
50
1/2, 7/2, 3/2, 5/2
6
DJ
9/2
11/2, 15/2, 13/2
9/2, 17/2
7/2
6
IJ
6
PJ
311nm
30
7/2
5/2
3/2
202.1nm
Energy (*103cm-1)
40
0
8
S7/2
Fig. 1.8. Energy level structure of Gd3+ in LiYF4. (adopted from R.T.Wegh, H.
Donker, A. Meijerink, Physical Review B 56, 21, 13841-13848 (1997))
In LiGdF4:Eu3+, the internal quantum yield of 190% assumes all of the
excitation energy is absorbed into the 6GJ level of the Gd3+ and converted into
emission. The actual external quantum yield is only 32% [4], as a significant portion
of the incident light is not absorbed because of the forbidden character of the
excitation 8S7/2 to 6GJ of Gd3+.
To increase absorption sensitizers operating of the strong 4f-4f5d parity allowed
transitions have been examined in this work. Among the lanthanides Pr3+, Nd3+,
Ho3+,Er3+, and Tm3+ were considered as candidates. These ions exhibit relatively highenergy 4fn - 4fn-15d transitions and relatively uncongested 4f levels at high energies,
limiting internal relaxation of the 5d energy through these levels. The transition
energies from ground 4f state to the 4fn-15d excited level are summarized in Fig. 1.9.
18
110000
100000
Lu
--
Gd
90000
Yb
Energy, cm-1
Eu
80000
Pm Sm
Dy
Nd
70000
Tm
Tm
Dy
Tb
Pr
60000
Ho
Ho Er
Er
allowed
forbidden
Tb
50000
Ce
40000
0
2
4
6
8
10
12
14
16
number of 4f electron
Fig. 1.9. 4fn-15d levels of free gaseous Ln3+ ions. (■) represents spin forbidden 4f-5d
transition energy and (♦) for dipole allowed 4f-5d transition energy. (adopted from P.
Dorenbos, J. Lumin. 91, 155-176 (2000))
Krupar and Queffelec have reported the 4f5d positions of Ce3+, Pr3+, Nd3+, Eu3+,
Tb3+, Dy3+, Ho3+, Er3+, Tm3+ in LiYF4 at room temperature, over the range of energies
from 5 to 15eV [14]; results are summarized in Table 1.1. The results reproduce the
trend observed in Fig. 1.9. The excitation spectra also clearly reveal crystal-field split
4fn-15d levels. The five split levels of the 5d1 configuration are consistent with the site
symmetry S4 (D2d) in LiYF4.
19
Table 1.1 4fn-15d energy of several Ln3+ ions doped in LiYF4 compound
Ln3+
Energy of 5d absorption peaks (eV)
*H.A.
Ce3+
4.19
5.12
5.90
6.53
Pr3+
5.82
6.75
7.5
8.27
3+
7.10
7.77
7.94
8.92
Eu3+
8.15
8.67
9.61
Tb3+
5.87
6.74
6.96
7.66
8.27
10.55
Dy3+
7.38
7.75
8.43
9.18
9.54
10.53
Ho3+
7.65
8.00
8.49
9.05
9.46
10.55
Er3+
8.00
8.57
8.80
9.46
9.90
10.55
Tm3+
7.9
8.7
9.3
9.71
10.41
10.55
Nd
* Host absorption band
6.63
10.50
10.55
10.55
20
1.4 DISSERTATION SUMMARY
To develop efficient excitation processes for quantum splitting systems,
several stoichiometric Gd3+ hosts were examined. These were doped with selected
lanthanides (Nd3+, Pr3+, Sm3+, Tm3+) that were expected to exhibit 4fn→4fn-15d
transitions at sufficiently high energies to populate the 6GJ level of Gd3+ via CRET.
The energy of the absorption bands were determined for Nd3+ in LiGdF4 (LGF) and
GdPO4 (GPO); Pr3+ in GdF3 and NaGdF4; Sm3+ in NaGdF4 and LGF; and Tm3+ in
GdPO4, and they were then examined as sensitizers under VUV excitation.
LGF:Nd3+ provided an unexpected and unprecedented quantum-splitting
process. The details of this process are presented in Chapter 2. Quantum splitting was
also observed in the system GdF3:Pr3+,Eu3+, where Pr3+ is the sensitizer and Eu3+ is the
activator. Even though quantum splitting is observed in this system, the overall
quantum yield is low. A detailed description of the energy transfer and quantum
splitting processes are summarized in Chapter 3.
In Chapter 4, results of the VUV luminescence of the 4fn-15d state of Pr3+, Tm3+,
Er3+, and the results of CRET between the Pr3+-Tm3+ pair in YPO4 and YBO3 are
described.
The host intrinsic emission designated as STE was investigated as a new
sensitizing method to excite Gd3+ into 6G level in oxide hosts. To identify the
appropriate host features for STE emission at sufficiently high energy to sensitize
Gd3+, the host cation was varied across the phosphate series YPO4, LuPO4, and ScPO4.
The anion also was varied from phosphate to borate and silicate, e.g., ScPO4, ScBO3,
and Sc2Si2O7. Quantum splitting emission was observed in ScPO4:Gd3+ with the
quantum yield approaching unity. The experimental results on these compounds are
detailed in Chapter 5.
Results described in Chapter 2-5 were generated through collaborative work
with Dr. Richard S. Meltzer at the University of Georgia (UGA) and Drs. Kailish C.
Mishira and Madis Raukas at Osram Sylvania. Materials for study were selected
following discussions among the three groups. All samples were synthesized and
characterized with respect to structure and UV luminescence at Oregon State
21
University. VUV and energy-transfer experiments were conducted at UGA with some
assistance from Osram Sylvania.
The material GdZrF7 was prepared as a nearly white phosphor under VUV
excitation with high absolute quantum yield by doping with Eu3+ (Chapter 6), and its
crystal structure is described together with that of GdHfF7 in Chapter 7. The host
GdZrF7 was also developed as an anti-Stokes (upconversion) phosphor by codoping
with Yb3+ and Er3+. Its luminescence, compositional optimization, and particle
morphology are described in Chapter 8. While examining new Gd silicate systems for
quantum splitting, a new apatite sulfide Gd4.67(SiO4)3S was synthesized. Results of
luminescence characterization of Tb3+ samples and a single-crystal structure
determination are described in Chapter 9.
During my tenure at OSU, I also contributed to the development of a variety of
electronic materials. The performance of the n-type amorphous oxide semiconductor
ZnOx(SnO2)1-x (0<x<1) in transparent thin-film transistors is summarized in Appendix
B.
REFERENCES
[1] W. W. Piper, J. A. Peluca, F. S. Ham, Journal of Luminescence, 8, 344-348 (1974)
[2] J. L. Sommerdijk, A. Bril, A. W. de JAGER, Journal of Luminescence, 8. 341-343
(1974)
[3] R. T. Wegh, Harry Donker, Koenraad D. Oskam, Andries Meijerink, Science 283,
29, 663-666 (1999)
[4] C.Feldmann, T. Justel, C. R. Rondo, D. U. Wiechert, Journal of Luminescence, 92,
245-254 (2001)
[5] P. A.Rodnyĭ, Optics and Spectroscopy, 89, 4, 556-562 (2000)
[6] E.van der Kolk, P. Dorenbos, A. P. Vink, R. C. Perego, C. W. E. Van Eijk,
Physical Review B 64, 195129 (2001)
[7] P. A. Rodnyĭ, Optics and Spectroscopy 89, 4, 556-562 (2000)
22
[8] A. M. Srivastava, W. W. Beers, Journal of Luminescence, 71, 285-290 (1997)
[9] P. Dorenbos, Journal of Luminescence, 91, 155-176 (2000)
[10] P. A. Rodnyĭ, A. N. Mishin, A. S. Potapov, Optics and Spectroscopy 93, 5, 714721 (2002)
[11] P. A.Rodnyĭ, A. S. Potapov, A. S. Voloshinovskii, Optics and Spectroscopy 96, 6,
862-868 (2004)
[12] A. P. Vink, P. Dorenbos, C. W. E. Van Eijk, Journal of Solid State Chemistry 171,
308-312 (2003)
[13] R. T.Wegh, H. Donker, A. Meijerink, Physical Review B 56, 21, 13841-13848
(1997)
[14] J. C. Krupa, M. Queffelec, Journal of Alloys and Compounds 250, 287-292
(1997)
[15] A. J. De Vries, M. F. Hazenkamp, G. Blasse, Journal of Luminescence, 42, 275282 (1988)
[16] R. T. Wegh, E. V. D. van Loef, A. Meijerink, Journal of Luminescence, 90, 111122, (2000)
[17] R. S. Meltzer, private communication.
23
CHAPTER 2
QUANTUM SPLITTING AND ITS DYNAMICS IN GdLiF4:Nd3+
W. Jia, Y. Zhou, S.P. Feofilov, R.S. Meltzer
Department of Physics and Astronomy University of Georgia Athens, GA 30602
J. Y. Jeong and D. Keszler
Department of Chemistry 153 Gilbert Hall Oregon State University
Corvallis, OR 97331-4003
Modified version: Physical Review B: Condensed Matter and Materials Physics
(2005), 72(7)
24
ABSTRACTS
Efficient quantum splitting and sensitization of Gd3+ is demonstrated for the
Gd3+-Nd3+ system in GdLiF4:Nd 2%. The quantum splitting results from a two step
cross relaxation energy transfer between Gd3+ and Nd3+ which first involves a
transition 6G→6I on Gd3+ and an excitation within the 4f3 configuration of Nd3+
followed by a second cross relaxation energy transfer which brings Gd3+ to 6P7/2. The
excited Nd3+ ion rapidly relaxes, non-radiatively, to the emitting 4F3/2 state. The
excited Gd3+ ion then transfer its energy back to Nd3+ which gives rise to the second
photon. The process is studied by emission and excitation spectroscopy. The result is
a quantum yield for the emission of IR photons which has its maximum of about 1 ±
0.5, at 175 nm. The dynamics of both the Gd3+ and Nd3+ excited states are studied in
detail providing information about the mechanisms and rates for the various energy
transfer processes. It appears that the second step in the quantum splitting is less
efficient than the first. It is found that energy migration among the Gd3+ ions plays an
important role in the quantum splitting and that there is strong evidence that the
exchange interaction is the dominant mechanism in the energy transfer. This system
provides excellent insights into the quantum splitting process, especially with regard
to an evaluation of the details of the dynamics.
2.1 INTRODUCTION
It has been suggested that improvements in fluorescent lamps could be realized
by replacing the mercury discharge by xenon, thereby removing the deleterious
environmental impact of mercury and at the same time improving the energy
efficiency. Such innovations require a phosphor that absorbs one vacuum ultraviolet
(VUV) photon and emits two or more visible photons, an effect known as quantum
splitting or down conversion.[1]
25
Quantum splitting can occur either through a process of sequential cascade
emission[2] as an excited ion returns to its ground state by first radiating to an
intermediate state or by some cross relaxation process which enables the initially
excited ion to share its excitation energy with two or more ions, each of which emits a
visible photon. Both of these processes have been demonstrated. Cascade emission
was first demonstrated in YF3:Pr with a 140% quantum efficiency[3]. Cross
relaxation induced quantum splitting has been described for GdLiF4:Eu with an
internal quantum efficiency of 190% [4]. Unfortunately, neither of these schemes has
so far yielded a useful phosphor. For the cascade emission, the first photon occurs at
406 nm too far in the deep blue where the sensitivity of the human eye is very low.
For the cross relaxation scheme in GdLiF4:Eu, the absorption of the VUV photon is
too weak to produce a phosphor with high brightness. [5]
We attempted to sensitize the absorption by adding Nd3+ to GdLiF4: Eu3+. We
found that Nd3+ does effectively sensitize the excitation of Gd3+. However, in addition,
Nd3+ undergoes its own very strong cross relaxation with the Gd3+ system producing
efficient quantum splitting. A similar effect [6] has recently been reported for
GdLiF4:Tm3+. In this paper we study, in detail, the quantum splitting process for the
singly-doped system, GdLiF4:Nd. The result of exciting Nd3+ into the 4f25d state in
the VUV is the appearance of two infrared photons. While this material will not be a
commercially viable quantum splitting phosphor since the photons are in the infrared
and because of the large energy loss even if two photons were produced per input
photon, it does provide important insights into the dynamics and mechanisms of the
quantum splitting process. In this paper we (1) demonstrate the existence of the
quantum splitting, (2) obtain the actual quantum efficiency of the system relative to
the number of input VUV photons, (3) measure and analyze the dynamics of the
processes using time-resolved emission, and (4) discuss the mechanisms for the
energy transfer.
26
2.2 EXPERIMENT
Samples of GdLiF4:Nd containing 1, 2 and 3 mol% Nd were prepared in
powder form. GdF3 was first synthesized by heating a mixture of 1 Gd2O3 (99.99%,
Alfa Aesar) and 8 NH4F (99.99%, Alfa Aesar) at 900°C for 1.5 h. The resulting
product was then mixed with 1.15 LiF (99.99%, Alfa Aesar), 0.01, 0.02 or 0.03 Nd2O3
(99.99%, Alfa Aesar), and 4 NH4F (99.99%, Alfa Aesar) and thoroughly ground. The
mixture was then fired at 750°C for 1.5 h in a Pt crucible; the Pt crucible was covered
and positioned inside an alumina crucible filled with activated carbon and NH4F to
limit the exposure of the sample to air.
All spectra were obtained at room temperature. Emission spectra were
obtained by exciting the sample, contained in vacuum, with a deuterium lamp
spectrally filtered with an Acton Model VM-502 VUV monochromator containing a
concave grating so that selective excitation could be performed. The visible and UV
emission was dispersed with an Acton Spectrapro-150 spectrometer and was detected
with a Santa Barbara Instrument Group Model ST-6I CCD camera at the exit focal
plane. Emission spectra in the VUV were obtained by exciting the sample with a
GAM Laser, Model EX5, pulsed molecular F2 laser whose output is at 157 nm. The
sample emission was focused onto the entrance slit of the VUV monochromator. The
emission was detected with a solar blind PMT with a MgF2 window located at a third
slit of the VUV monochromator which was scanned to obtain the spectrum. All
emission spectra were corrected for the wavelength dependent response of the
detection system. For cw excitation in the UV, a UV-enhanced Ar+ laser was used at
351 nm.
Excitation spectra were obtained by scanning the VUV monochromator,
illuminated by the deuterium lamp, while detecting the emission with a PMT after
passing the luminescence through appropriate colored glass or interference filters to
select the desired components of the emission. Two PMT detectors were used, both
having quartz windows yielding a response in the UV down to 200 nm. One
(Hamamatsu R943) had a GaAs photocathode so that emission up to 900 nm could be
measured. The other had a photocathode with an S-20 response. The excitation
27
spectra of each sample were compared to that of a reference sample of sodium
salicylate whose quantum efficiency is assumed to be about 58% and constant over the
excitation wavelength range from 140 to 320 nm [7]. The measured quantum yield is
relative to input photons rather than absorbed photons since we have not obtained any
reflectance measurements for either the samples or the reference. This assumes
similar reflectivities of the sample and the sodium salicylate reference.
For the time-resolved data, the sample was excited with the pulsed laser at 157
nm (10 ns pulse width), while the emission was detected with the same PMTs
described above for the excitation spectra. Temporal resolution was about 20 ns. The
emission was selected with a 0.25m monochromator and additional colored glass or
interference filters to block light at other wavelengths from entering the
monochromator. The bandwidth of the instrument was ~3 nm. The main limitations
of the time-resolved spectra were extraneous signals at early times coming either from
broadband red/NIR emission from atomic fluorine in the laser discharge or from fast
decay of defect centers that were excited by the VUV excitation. This red/NIR
emission was so strong that it was very difficult to do any time resolved spectroscopy
from about 620 to 750 nm. For direct excitation of the 4f3 states of Nd3+ the third
harmonic of a pulsed Nd:YAG laser at 355 nm (10 ns pulse width) was utilized.
2.2.1 Demonstration of the Quantum Splitting
In Fig. 1 the emission spectrum is presented for two different excitation
wavelengths, 351 and 160 nm. The emission from 200 nm to 950 nm is dominated by
the 4F3/2 → 4I9/2 transition. However emission from the 4D3/2 and 2P3/2 states of Nd3+ is
also observed. Weak emission from the 6P7/2 state of Gd3+ is observed at 313 nm.
While it is not evident in this time-averaged spectrum, emission occurs at 281 nm
from the 6I state of Gd3+. Emission from the 4f25d state of Nd3+ in the wavelength
range of 180 nm to 270 nm, which dominates the spectrum of YLiF4:Nd [8], is not
observed in GdLiF4:Nd suggesting efficient energy transfer from Nd3+ to Gd3+, i.e.
strong sensitization.
28
When the spectra excited at the two different wavelengths are compared, by
normalizing them to the 4D3/2 and 2P3/2 emission, it is seen that under160 nm
excitation, the relative intensity of the 4F3/2 emission is more than double that observed
for 351 nm excitation. This suggests a process which enhances the excitation of 4F3/2
in a manner which was used to identify quantum splitting for GdLiF4:Eu [4]. This is
just the cross relaxation process responsible for quantum splitting.
GdLiF4:Nd 2%
Emission Spectra
3+ 2
D3/2
Nd
Nd
3+ 4
F3/2
P3/2
P3/2
6
3+
Nd
Gd
3+ 4
Nd
3+ 2
P7/2
Relative Quantum Yield
351nm excitation
160nm excitation
4
to IJ
9/2 11/2 13/2 15/2
200
300
400
500
600
700
800
900
1000
Wavelength (nm)
Fig. 2.1 (Color online) Relative quantum yield of GdLiF4:Nd 2% exciting at 160 nm
(black, solid curve) and at 351 nm (red, dashed curve). The spectra are normalized on
the Nd3+ 4D3/2 and 2P3/2 quantum yields.
The processes are illustrated in Fig. 2. The diagram shows the relevant 4f3 and
4f7 energy levels of Nd3+ and Gd3+, respectively. Boxed regions with horizontal lines
indicate a high density of states of the two 4fn configurations for which rapid
multiphonon relaxation occurs. The open box represents the 4f25d band of Nd3+. The
29
4f65d band of Gd3+ is off the energy scale and is not relevant here. The long vertical
arrow represents the VUV excitation of Nd3+ into the 4f25d band. Rapid energy
transfer to a nearly resonant 4f7 state of Gd3+, labeled by ET 1, followed by rapid nonradiative relaxation, populates the 6GJ states of Gd3+.
Cross relaxation energy
transfer from the 6G7/2 state of Gd3+ can occur via two paths. One of these, indicated
by the red(dashed) arrows labeled A on the energy level diagrams of Gd3+ and Nd3+,
results in a transition 6G7/2 →6PJ on Gd3+, as has been previously observed in the GdEu couple, with a simultaneous 4I9/2 → 4G5/2 excitation on Nd3+. These two transitions
have considerable overlap as shown in the room temperature spectra of Fig. 3 where
the 6GJ → 6PJ emission of Gd3+ observed [9] in YLiF4:Gd is compared to the 4I9/2 →
4
G5/2 absorption of YLiF4:Nd. Subsequently, rapid multiphonon relaxation leads to
feeding of the 4F3/2 metastable state from which strong IR emission occurs.
The second pathway involves a transition 6G7/2 →6IJ on Gd3+ coupled with a
4
I9/2 → 4F5/2, 2H9/2 or 4F7/2 transition on Nd3+ as indicated by the red(dashed) arrows
labeled B in Fig. 2.
Although the spectra are not available for comparison, the
transition energies for Nd3+ in absorption [10] and Gd3+ predicted for emission [6] are
likely to have good resonances. In addition, Peijzel et al. [6] have shown that the
reduced matrix elements for this second pathway are about an order of magnitude
greater than for the first, making this process about two orders of magnitude faster
under the similar resonance conditions. Indeed, as will be shown from studies of the
dynamics, the pathway involving the 6IJ levels does dominate the cross relaxation from
6
G7/2. However, 6IJ can further relax to 6PJ via another cross relaxation process, shown
by the red(dashed) arrows labeled C in Fig. 2, that excites the 4I13/2 state of Nd3+.
Evidence for this also exists from the dynamical studies discussed below.
30
GdLiF4:Nd
2
4f 5d
157nm
ET 1
6
GJ
2
G9/2
2
F7/2
A B
6
DJ
I
C 6J
PJ
6
ET 2
4
D3/2 τ=1µs
P3/2 τ=20µs
2
4
G5/2 4
F5/2
4
F3/2 τ=400µs
A B
C
Nd
3+
weak direct
excitation of
3+
Gd
4
IJ
8
Gd
3+
S7/2
Fig. 2.2 (Color online) Energy level diagrams of Nd3+ and Gd3+ in GdLiF4:Nd with
the relevant energy levels labeled. The open box represents the 4f25d band of Nd3+.
The boxed areas with horizontal lines represent energy regions with a high density of
4fn levels. ET1 and ET2 indicate resonant energy transfer processes. Labels A, B, and
C next to the red (dashed) lines denote three cross relaxation energy transfer
processes. Some of the intrinsic lifetimes are indicated
Absorption (arb.units)
31
560
Y L iF 4 N d
3+
580
4
2 % : I 9 /2
4
G 5 /2 A b s o r p ti o n
600
620
640
W a v e le n g t h ( n m )
YLiF4:Gd 5% Emission
(b)
Fig. 2.3 (a) Absorption spectrum of YLiF4:Nd2% and (b) emission spectrum of
YLiF4:Gd5% [9] showing significant spectral overlap.
32
The 6PJ states of Gd3+ then transfer their energy to the nearly resonant 4f3 states
of Nd3+, as shown by the blue(solid) arrow labeled ET 2. Above the 4D3/2 state of
Nd3+ there is a very dense, almost continuous forest of energy levels from the 4f3
configuration among which the 2L17/2 at ∼32,000 cm-1 is in closest resonance with the
6
P7/2 states of Gd3+. [10] Once excited, these will relax almost immediately to the 4D3/2
level which lives long enough to produce observable emission. Its decay, whose
lifetime is about 1 µs, is dominated by non-radiative relaxation to the 2P3/2 level which
lives much longer with a lifetime of ~20 µs. These and subsequent multiphonon
relaxations ultimately feed the 4F3/2 level leading to the emission of a second IR
photon. On the other hand, when the 4D3/2 state is excited directly at 351 nm, the cross
relaxation step is eliminated so that the relative intensity of 4F3/2 emission is less than
half of that obtained under 157 nm excitation. As described by Wegh et al. [4] for
GdLiF4:Eu, this is strong evidence for quantum splitting. The dynamics of the system
described below will provide further supporting evidence.
Finally, it should be noted that the assumption that the initial Nd3+ → Gd3+
energy transfer (ET1 in Fig. 2) occurs to Gd3+ states resonant with the 4f25d state of
Nd3+ may not be a good one. Many possible cross relaxation energy transfer processes
are equally possible. These could excite many of the lower-lying states of Gd3+ below
the energy of the 4f25d state of Nd3+ (∼56,000 cm-1), shown on the Gd3+ energy level
diagram as the boxed area with many horizontal lines in Fig. 2. For example, cross
relaxation processes could leave Nd3+ in the 4IJ levels J=11/2, 13/2, 15/3 and Gd3+ in
states above 6GJ that conserve the total energy. Note that rapid multiphonon relaxation
would still lead to a build up in the population of the 6GJ levels of Gd3+ as had been
assumed. Cross relaxation processes are also possible in which the energy transfer
would result in Gd3+ being excited to 6DJ, 6IJ, or 6PJ by leaving Nd3+ in its 4F9/2 (14,800
cm-1), 4G7/2 (19,000 cm-1), or 4G11/2 (21,400 cm-1) states, respectively. However, these
processes would also still lead to quantum splitting since multiphonon relaxation
would populate 4F3/2 and the excited Gd3+ ion would still be capable of transferring its
energy to Nd3+ for producing the second photon. These processes would supplement
the energy transfer processes labeled as A and B that were previously discussed.
33
2.2.2 Excitation spectrum and quantum yield
The excitation spectra, detecting the 4F3/2 → 4I9/2 emission of Nd3+ at 780-910
nm, is shown in Fig. 4 for the 1% and 2% and 3% Nd samples. It contains features
associated both with Gd3+ and Nd3+ as indicated on the figure. One clearly sees the
states of the 4f7 configuration of Gd3+, namely 6GJ, 6DJ and 6IJ, indicating that energy
transfer between Gd3+ and Nd3+ occurs, as expected. The 4f25d bands of Nd3+ are also
clearly observed.
2.0
Relative Quantum Yield
1.8
Nd
3+
GdLiF 4 :Nd Excitation Spectra
2
4f 5d
Detect Nd
3+
Emission (λ >780nm)
1.6
1% Nd
2% Nd
3% Nd
1.4
1.2
1.0
0.8
Gd
3+
8
absorption from S 7/2 to:
0.6
6
0.4
6
GJ
6
0.2
IJ
DJ
0.0
140
160
180
200
220
240
260
280
300
Wavelength (nm)
Fig. 2.4 (Color online) Excitation spectrum of GdLiF4 containing 1, 2 and 3% Nd3+
and detecting the Nd3+ 4F3/2 emission using a cutoff filter that transmits for λ>780 nm.
Features of the 6GJ, 6DJ and 6IJ levels of Gd3+ and the 4f25d bands of Nd3+ are
indicated.
The quantum yield relative to that of the reference, sodium salicylate, achieves
a maximum of 1.8 in the 2% Nd sample for excitation into the 4f→ 5d bands of Nd3+
at 175 nm. This value is obtained by applying a number of corrections to the raw data.
34
First, the raw data are corrected for the fact that the relative quantum efficiency of the
PMT for the 4F3/2 → 4I9/2 emission wavelength of Nd3+ between 860 and 910 nm is
much less than that at the 380-460 nm emission wavelength range of sodium
salicylate. A correction factor for the relative response of the PMT is obtained by
convoluting the corrected emission of the sample and sodium salicylate reference each
with the quantum efficiency of the PMT and calculating the ratio of these products
yielding a correction factor of 20 ± 6. A great deal of effort was made to accurately
obtain the relative quantum efficiency of the PMT which, because of the rapid
decrease in response in the region above 860 nm, leaves this considerable uncertainty
of about ±30%. Secondly, it is estimated that only 33% of the 4F3/2 emitted photons
occur on the 4F3/2 → 4I9/2 transition, based on reported [11] emission spectra of
YLiF4:Nd and calculations of the branching ratios determined by a Judd-Ofelt analysis
[12], implying a further correction of about 3. An actual measurement of the
branching ratios obtained from the IR emission spectrum was performed by R. L.
Cone at Montana State University using an Applied Detector Corp. 403L Ge detector
at the exit slit of a Spex 1000M spectrometer. All spectra were referenced against a
tungsten halogen lamp operating at 2800K. The measurement yielded a value of
31.1% for the fraction of the emission occurring to 4I9/2, very close to the value
calculated. This result produced a correction factor of 3.22±0.3. Finally, there is an
uncertainty concerning the relative reflectivities of the samples and sodium salicylate
reference. Although these may be somewhat different, they are probably both less
than 20% in the strongly absorbing regions of the spectrum of interest. Thus this
should add not more than a ±10% error. Using an estimate that the absolute quantum
yield of sodium salicylate as 0.58, implies an absolute quantum yield for the 4F3/2
emission of about 1.05 ± 0.35. The estimated uncertainty is based on the accumulated
errors discussed above. This value for the quantum yield is about three times the
value of 0.32 [5] obtained for GdLiF4:Eu. However, it is still well below the
theoretical maximum quantum yield of 2 based on the quantum splitting scheme
described above. This highlights the fact that even in a system which exhibits highly
efficient quantum splitting, other losses can limit the absolute quantum yield. Indeed,
measurements of the quantum efficiency of the GdLiF4:Eu quantum splitting
35
phosphorError! Bookmark not defined. show that a broad defect absorption reduces the
quantum efficiency considerably. A study of the dynamics will allow for an
examination of some of the reasons for the reduced quantum yield for GdLiF4:Nd.
The excitation spectra for detection above and below 780 nm are compared in
Fig. 5. The spectra are normalized to the Gd3+ 6I transition. The black (dotted) curve
is obtained detecting wavelengths λ > 780 nm so that only the Nd3+ IR emission from
4
F3/2 is monitored. The red (solid) curve is the excitation spectrum for λ < 780 nm and
is dominated by Nd3+ emission from 4D3/2 which is not enhanced by the quantum
splitting. Both the 6G excitation features of Gd3+ and the 4f25d bands of Nd3+ are
enhanced when detecting the 4F3/2 emission supporting the conclusion that quantum
splitting plays an important role in the emission. For detection with λ < 780 nm, there
Excitation Efficiency (arb. units)
is evidence for an impurity or defect absorption band near 200 nm.
140
Excitation Spectrum
GdLiF 4: Nd2%
Nd
3+
2
4f 5d
Scaled to Gd
3+ 6
I
Detection: λ > 780 nm
Detection: λ < 780 nm
Gd
Gd
160
180
3+
200
3+
6
6
220
240
260
280
300
W avelength (nm )
Fig. 2.5 (Color online) Comparison of the excitation spectra of GdLiF4:Nd2%
detecting only the 4F3/2 emission with λdetect>780 nm with that of the case of detection
for λdetect<780.
36
2.2.3 Dynamics of the Quantum Splitting
Despite the fact that a great deal of work has been done on quantum splitting
due to cross relaxation energy transfer (CRET), there have been, to our knowledge,
only two studies [13,14] of the dynamics of this process. The studies considered the
Gd3+-Eu3+ couple in GdNaF4:Eu3+ and in GdLiF4:Eu3+. Both the cross relaxation and
direct transfer were observed with rates about two orders of magnitude slower than for
the Gd3+-Nd3+ couple studied here. As pointed out in Wegh et al. [4], the process
achieves its efficiency because of energy migration among the Gd3+ ions which are
stoichiometric in all known successful cross relaxation energy transfer quantum
splitters. Dipole-dipole energy transfer or exchange is just too slow except for ions
that are near neighbors. The fact that energy migrates within the Gd3+ ions ensures
that the excitation in the 6GJ levels of Gd3+ gets to spend a portion of its time as a near
neighbor of Nd3+. Thus the dynamics within the Gd3+ system is expected to play an
important role in the process.
When a sample of GdLiF4 containing 2% Nd3+ is excited at 157 nm with a
molecular F2 laser, one sees a buildup of the 6P7/2 transition of Gd3+ at 313 nm as
shown in Fig. 6 by the black (dark solid) curve. This buildup has two components.
One is very fast, at a rate which exceeds the time resolution of these experiments
(<50ns, limited by some background scattered light from the laser discharge and
defect luminescence), which represents about 20% of the population feeding. The
second is a slower buildup over several microseconds, representing about 80% of the
feeding. The cause of these two components becomes clear from the dynamics of the
6
I emission of Gd3+ at 281 nm shown by the purple (dotted) curve in Fig. 6. Its decay
rate coincides with the 6P7/2 population buildup rate. Also shown in Fig. 6 by the red
(dot-dashed) curve is the emission at 866 nm from the 4F3/2 state of Nd3+ which also
builds up within the temporal resolution of the experiment. Thus we conclude, as
suggested based on an earlier discussion of the reduced matrix elements, that cross
relaxation process B from Fig. 2 is the dominant one in the quantum splitting.
However, the fact that the 6P7/2 population does have a very fast component indicates
that there may also be a contribution from the cross relaxation energy transfer process
37
labeled as A in Fig. 2. The relaxation of Gd3+ from 6I to 6P in a few microseconds is
unlikely to occur due to multiphonon relaxation because of the large energy gap
(∼3000 cm-1) and low phonon energies of the GdLiF4 host, but rather most likely
occurs through the cross relaxation energy transfer process labeled C in Fig. 2.
Consistent with this suggestion is the fact that the relaxation is dependent on Nd3+
concentration as discussed below. In this process a Nd3+ ion is excited from the 4I9/2
ground manifold to 4I13/2, for which there is a good resonance match with the 6I → 6P
transitions on Gd3+.
Emission Intensity (arb. units)
GdLiF 4:Nd2% , Excitation: λ =157 nm
Nd
Gd
3+ 6
F 3/2 (866 nm )
P 7/2 (313 nm )
Nd
Gd
2
3+ 4
4
3+ 6
3+ 4
D 3/2 (358 nm )
I (281 nm )
6
8
10
Tim e( µ s)
Fig. 2.6 (Color online) Time evolution of the 6I (281 nm) and 6P7/2 (313 nm) emission
intensities of Gd3+ and the 4D3/2 and 4F3/2 emission intensities of Nd3+ in a
GdLiF4:Nd2% sample under 157 nm pulsed laser excitation.
The behavior of the dynamics of process C and its concentration dependence
provides important information on the role of donor-donor energy transfer among the
Gd3+ ions. The dynamics of the 6I and 6P emissions are shown as a function of
concentration in Fig. 7. The relaxation process is nearly exponential as seen by the
38
dashed lines plotted over the 6I time-resolved emission which are fits to the data
assuming an exponential decay of 6I. The values for the fit are shown on the figure
and are summarized in Table 1. The relaxation rate scales nearly linearly with
concentration as expected. Also shown are the time-resolved intensity of the 6P7/2
emission along with fits to the data using the 6I decay time as the feeding term in the
6
P7/2 population. Indeed, the same times describe both the 6I and 6P7/2 emissions. The
decay of 6P7/2 is also nearly exponential with a rate that depends on Nd3+
concentration. These rates are also summarized in Table 1. The nearly exponential
relaxation processes for all three concentrations suggests that energy migration among
the Gd3+ ions is fast compared to these CRET relaxation rates. In that case the Gd3+
excitation samples all sites thereby spending a fraction of its time nearby a Nd3+ ion
with which it can undergo CRET. If, after energy transfer from the 4f25d state of Nd3+
to Gd3+, the energy remained localized on that Gd3+ ion, the CRET rates would be
highly non-exponential. In addition, without energy migration, CRET process C
would be hindered as all of the energy resonances that we have discussed assume that
the Nd3+ ions are in their ground state. However, processes A and B leave the Nd3+
ion in an excited state for a time roughly equal to the lifetime of the 4F3/2 state of about
400 µs. Also, in the absence of rapid Gd3+-Gd3+ energy transfer, some of the possible
processes providing the initial Nd3+ → Gd3+ energy transfer could also leave Nd3+ in
an excited state, as discussed earlier, compromising the CRET processes A and B
which also assume that the Nd3+ ions are in their ground state.
Table 2.1 Experimental energy transfer rates.
Process
CRET A
CRET B
CRET C
Nd3+ conc.
All
All
Gd3+
G→6P
6
G→6I
6
I →6 P
6
Nd3+
I9/2→4G5/2
4
I9/2→4F5/2,2H9/2
4
I9/2→4I13/2
4
1%
2%
3%
Gd3+→Nd3+
6
1%
2%
3%
P7/2→8S7/2
4
I9/2→2L17/2
Expt ET rate(s-1)
>2x107
>2x107
3.8x105
5.7x105
8.0x105
4.3x104
6.7x104
9.1x104
39
Emission Intensity (arb. units)
GdLiF4:Nd, λex = 157 nm
2% Nd
1% Nd
6
P7/2
3% Nd
1% Nd
6
τd( I)=2.6 µs
2% Nd
6
τd( I)=1.75 µs
3% Nd
6
τd( I)=1.25 µs
2
4
6
8
10
Time(µs)
Fig. 2.7 (Color online) Time evolution of the 6I (281 nm) and 6P7/2 (313 nm) emission
intensities of Gd3+ under 157 nm pulsed excitation in GdLiF4:Nd for 1, 2, and 3% Nd
concentrations. The dashed lines show the fits using the 6I decay times shown in the
figure. Those same times are used as the rise times in the fits to the 6P7/2 emission for
the sample with the same Nd3+ concentration.
The excited Gd3+ ions in the 6P7/2 state then undergo energy transfer to the
nearly resonant 4f3 states of Nd3+ at a rate described by the decay of the Gd3+ 6P7/2
emission. Proof of this second step is seen by monitoring the 4D3/2 emission under 157
nm excitation. It is observed that this emission closely follows the Gd3+ 6P7/2
population with a small delay and that it has zero population immediately after the
laser excitation (see Fig. 6). This occurs because the intrinsic 4D3/2 lifetime (∼1 µs due
to multiphonon relaxation to 2P3/2) is much shorter than the 6P7/2 lifetime, as seen from
its decay under direct 355 nm excitation into the 4f3 states just above 4D3/2, as shown
in Fig. 8. The fact that the 4D3/2 population closely follows the excited Gd3+ population
40
demonstrates that energy transfer from Gd3+ to Nd3+ does occur, a process which is
necessary for the second step of the quantum splitting process. The observation that
the 4D3/2 emission (spectrally integrated) is more than an order of magnitude greater
than the Gd3+ 6P7/2 emission (see Fig. 1) indicates that a significant fraction of the Gd3+
ions transfer their energy to Nd3+ since the two populations follow one another
because of the short inherent lifetime of 4D3/2. Its greater time integrated intensity
results from its faster radiative rate than that of 6P7/2 which is spin forbidden. Since we
do not know the relative radiative rates, it is not possible to estimate from these
relative intensities the efficiency of this Gd3+→Nd3+ energy transfer.
GdLiF4:Nd2%
Emission Intensity (arb. units)
2
P3/2 λ ex=157nm
τ r = 4 µs
τd = 22 µs
4
D 3/2 λ ex=355nm
τ r = 0 µs
τ d = 1 µs
2
P3/2 λ ex=355nm
τr = 1 µs
τd = 20 µs
0
20
40
60
80
100
120
140
Time (µs)
Fig. 2.8 Time evolution of the 4D3/2 and 2P3/2 emission of Nd3+ in a sample of
GdLiF4:Nd2% under 355 nm excitation and the 4P3/2 emission under 157 nm
excitation. The decay of 2P3/2 is the rate limiting state in the feeding of 4F3/2. Also
plotted as dashed lines are fits to the data using the rise and decay times indicated on
the figure
41
The 4D3/2 state decays non-radiatively to 2P3/2 whose population dynamics is
also shown in Fig. 8 for both 355 nm and 157 nm excitation. Under 355nm excitation,
it builds up at the 4D3/2 decay rate and decays in 20 µs, its intrinsic non-radiative
lifetime. Under 157 nm excitation it has a slower buildup resulting from the
population feeding from 4D3/2 whose population is controlled by energy transfer from
6
P7/2 of Gd3+. The 2P3/2 decay ultimately feeds 4F3/2 through multiphonon relaxation
down the ladder of states of Nd3+ from whose radiative decay provides the second
photon in the quantum splitting arises. Thus the feeding of 4F3/2 for the second step in
the quantum splitting continues for ∼100 µs.
The temporal behavior of the 4F3/2 emission further supports the presence of
quantum splitting. As shown in Fig. 9, when the 4f3 Nd3+states just above 4D3/2 are
excited directly at 355 nm, such that there is no quantum splitting, the 4F3/2 emission
builds up with a rise time that is close to the value of the decay time of the 2P3/2 Nd3+
emission (20 µs). The 4F3/2 emission under 157 nm excitation, also shown in Fig. 9,
shows a much more rapid buildup as expected due to the first step in the quantum
splitting, namely the cross relaxation step. However, note that the 4F3/2 emission does
not immediately begin an exponential decay. Rather its population remains high due
to feeding from the second step in the quantum splitting which maintains a feeding
term for about ∼100 µs as 2P3/2 decays.
Attempts to fit the dynamics presented in Fig. 9 (dashed curves) with an
exponential rise and decay indicate that under 355 nm excitation, the 4F3/2 emission
has both a fast (immediate with respect to the experimental time resolution) followed
by an exponential rise with a 12 µs rise time. The latter represents only 33% of the
total contribution to the feeding of the 4F3/2 population. The source of the fast
component is unknown but it suggests the existence of some other channel of
relaxation for 355 nm excitation. Under 157 nm excitation there is again a fast
component, resulting from the first CRET step due to processes A and B, followed by
an additional feeding through 2P3/2 for about 100 µs (see Fig. 8). Here the additional
feeding contributes only 9% to the 4F3/2 population. Under ideal conditions of
quantum splitting, this should represent 50% of the contribution to the 4F3/2 population
through the process labeled ET 2 in Fig. 2. Because of the observation that even under
42
355 nm excitation there exists an unexplained very fast component to the 4F3/2
population, it may be that a somewhat lower value than 50% should be expected.
However, the fact that it is only 9% seems to explain, in part, the less than ideal
quantum yield.
3+
Emission Intensity (arb.units)
GdLiF 4:Nd 2%
4
2
P 3/2
λ ex=355 nm
λ ex=157 nm
λ ex=355nm
Expt
0
F 3/2
Fits
100
4
F 3/2 (λ ex=355nm) τr=12 µs(33%), τd=360 µs
4
F 3/2 (λ ex=157nm) τr=20 µs(9%),
2
P 3/2 (λ ex=355nm) τr=0.9 µs(100%), τd=20 µs
200
300
τd=350 µs
400
Time(µ s)
Fig. 2.9 (Color online) Time evolution of the 2P3/2 and 4F3/2 emission in a
GdLiF4:Nd2% sample under 355 nm and 157 nm excitation. The fits shown on the
figure are obtained using the rise and decay times indicated in the legend. They
percentage indicates the fraction of population buildup which is contributed by this
rise time. The remainder of the population buildup is taken to appear immediately
after excitation.
There are a number of potential sources for this reduced contribution including
radiative transitions from 4D3/2 and 2P3/2 that are observed in Fig. 1, radiative
transitions from 6P7/2 of Gd3+ prior to energy transfer to Nd3+, transfer of energy from
6
P7/2 of Gd3+ to impurities or defects, and cross relaxation among Nd3+ ions. In
addition, non-radiative processes involving 4F3/2 are possible. Indeed, the observed
43
lifetimes of the 4F3/2 emission are below the low concentration limit of 535 µs in
GdLiF4:Nd [15] and, in agreement with the results of Zhang et al [15], the 2 and 3%
samples exhibit significant non-exponential behavior indicative of Nd3+-Nd3+ cross
relaxation (not shown). However, while this would contribute to the reduced quantum
yield, it would not explain the lower than expected contribution to the feeding of 4F3/2.
2.3 DISCUSSION
It is of interest to examine the mechanisms for the cross relaxation energy
transfer (CRET) responsible for the quantum splitting. For closely spaced ion pairs,
this may occur by dipole-dipole interactions or exchange interactions [16]. For more
distant pairs, the exchange will become unimportant because of its rapid decrease with
distance. According to Forster-Dexter dipole-dipole energy transfer theory, the
transfer rate, PABdd can written [17] as
PABdd = 1.4 x 1024 fA fB SAB ∆E-2 R-6.
(1)
Here fA and fB, are the oscillator strengths of the transitions on Nd3+ and Gd3+, ∆E is
the transition energy of each ion (in eV), R is the distance between the two ions (in
Angstroms), and, SAB is the spectral overlap (in cm-1) of the downward and upward
transitions. In Fig. 3 it was shown for CRET process A that there are many 4I9/2 →
4
G5/2 transitions of Nd3+ that are nearly resonant with the 6GJ → 6PJ transitions of Gd3+.
The oscillator strength of each of these crystal field transitions of Nd3+ in YLiF4 are
typically about ∼5 x 10-7 based on spectral analysis of some of the individual crystal
field transitions at 20K. However, one can also estimate the oscillator strengths from
experimental and calculated values integrated over all transitions in the manifolds by
dividing by the number of final states which yields about the same average oscillator
strength per crystal field transition [18]. A similar situation holds for process B which
involves the 6GJ → 6IJ transitions of Gd3+ and the 4I9/2 → 4F5/2, 2H9/2 or 4F7/2 transitions
of Nd3+. These Nd3+ transitions also have oscillator strengths of about 5 x 10-7.
44
The oscillator strengths of the transitions within the 6G7/2 → 6PJ or the 6G7/2 →
6
IJ manifolds of Gd3+ have not been measured but their reduced matrix elements have
been calculated [6]. The reduced matrix elements for the 6G7/2 → 6IJ transitions are
almost a factor of 10 greater than those of the 6G7/2 → 6PJ transitions, yielding the
expectation that under similar resonance conditions, the probability for process B
should be one to two orders of magnitude great than for process A. As described
earlier, a factor of 5 was observed. The difference may be due to the quality of the
energy resonance for the two processes. The Gd3+ oscillator strengths are calculated
based on the reduced matrix elements [6] for Gd3+ and Judd-Ofelt parameters for Gd3+
in YLiF4 [19]. The total oscillator strength to all transitions 6G7/2 → 6I is 2 x 10-6 and
for 6G7/2 → 6P7/2 it is 1.5 x 10-8. Since there are 39 final states in 6I, each crystal field
transition, on average, has an oscillator strength of ∼5 x 10-8.
It is now possible to estimate the CRET transfer rates for dipole-dipole
interactions in process B from Eq. (1). Using typical values of 3 x 10-7 for each
transition of Nd3+ and 5 x 10-8 for each transition of Gd3+ and assuming a single
perfect energy resonance with a linewidth at room temperature of 10 cm-1 (spectral
overlap integral = 0.1) one finds a rate of ∼3.3 x 105 s-1 for a nearest neighbor pair
separated by 3.73 A. This rate falls to ∼5 x 104 s-1 for a next nearest neighbor pair
separated by 5.15 A. To predict what should be observed one has to know whether the
donor-donor transfer among the Gd3+ ions is occurring and whether it is faster than the
donor-acceptor CRET rates. The results from the dynamics of process C involving a
CRET from 6I to 6P suggest, based on the nearly exponential decay of 6I and rise of
the 6P7/2 population, that the donor-donor transfer occurs much more rapidly than the
observed CRET rate of ∼6 x 105 s-1 in the 2% Nd sample. If one assumes that the
same is true for process A where the CRET rates are >2x107 s-1, then the predicted
rates should take into account the fact that, on average, the excited Gd3+ excitation
spends a fraction, 4x, (x is the fractional concentration of Nd3+) of its time as one of
the four nearest neighbors of Nd3+. Thus for 2% Nd the nearest neighbor rate should
be multiplied by a factor of 0.08 yielding a result of ∼2.7 x 104 s-1. This rate is
obtained for one resonance between the Gd3+ 6G7/2 → 6I and the 4I9/2 → 4F5/2, 2H9/2 or
4
F7/2 transitions of Nd3+. Even if one were to assume that all Nd3+ transitions (11)
45
were perfectly resonant with a transition on Gd3+, which would be an extreme
assumption, and if contributions from more distant pairs are added, the maximum
predicted rate still would be less than 106 s-1.
The assumption of rapid energy transfer among the Gd3+ donors is supported
by studies of Gd3+-Gd3+ interactions. Studies of band-to-band exciton transitions in
GdCl3, Gd(OH)3, and Tb(OH)3 have shown that exchange interactions among nearest
neighbor ions can yield resonant energy transfer rates among nearest neighbors that
are as large as 1010 to 1011 s-1 for resonant energy transfer among Gd3+ ions in their
6
P7/2 state or Tb3+ ions in their 5D4 state [20]. These rates correspond to the condition
of resonance with homogeneous linewidths at 1.5 K of about 0.1 cm-1. At room
temperature, where these linewidths are ∼10 cm-1, corresponding rates would be 108 to
109 s-1. Even though the exchange interaction will probably be considerably smaller in
fluorides, the expectation that donor-donor transfer rates for the 6G state of Gd3+
should exceed 2 x 107 s-1 in GdLiF4 seems quite reasonable.
In the limit of no energy transfer among the Gd3+ ions then the relaxation after
the initial energy transfer from Nd3+ → Gd3+ would occur by interactions between a
pair of nearest neighbors. This rate would have a maximum value of ~5 x 106 s-1 if all
transitions of the two ions were resonant. Even this extreme assumption falls well
short of explaining the observed rate of > 2 x 107 s-1 and the absence of fast donordonor transfer seems unlikely. Thus the above analysis of the experiments points
strongly to the dominant role of exchange interactions in facilitating the CRET
responsible for quantum splitting in GdLiF4:Nd.
It would be interesting to model the full dynamics, taking into account the
energy migration of the Gd3+ excitations in both the 6G7/2 and 6P7/2 states. Although
this problem is a very interesting one, it is not the subject of this paper.
46
2.4 CONCLUSION
Efficient quantum splitting has been demonstrated for the Gd3+-Nd3+ system in
GdLiF4:Nd 2%. A VUV photon is absorbed by the Nd3+ ions whereupon the energy is
rapidly transferred to the high-lying excited states of the 4f7 configuration of Gd3+ in a
time scale of nanoseconds. A rapid and effective cross relaxation energy transfer then
occurs in two steps. In the first, a Gd3+ ion in its metastable 6G state undergoes a
transition to 6I while a Nd3+ ions makes a transition 4I9/2 → 4F5/2, 2H9/2 or 4F7/2 at a rate
>2 x 107 s-1. Multiphonon relaxation effectively brings the Nd3+ ions down to the 4F3/2
state where they radiate the first photon. For the remaining excited Gd3+ ion, there
occurs a second cross relaxation energy transfer in which Gd3+ undergoes a transition
6
I → 6P and Nd3+ is excited from 4I9/2 → 4I13/2. The resulting 6P7/2 excitation on Gd3+
transfers its energy to nearly resonant states of the 4f3 configuration of Nd3+ in a time
scale of about 10-20 µs whereby subsequent relaxation brings the population down to
4
F3/2 of Nd3+ where the second photon is emitted. This second step appears to be less
efficient than the first. The result is a quantum yield for the emission of IR photons
which has its maximum of about 1 ± 0.5, under 175 nm excitation. This is
considerably below the theoretical value of 2. Nonetheless, this system exhibits the
highest quantum yield for quantum splitting based on cross relaxation energy transfer
and provides excellent insights into the quantum splitting process, especially with
regard to an evaluation of the details of the dynamics and the mechanisms of quantum
splitting. An analysis of the dynamics and the theoretical limits of the dipole-dipole
contributions, leads to the conclusions that (1) there is rapid donor-donor energy
migration among the Gd3+ ions and (2) that exchange plays the dominant role in the
cross relaxation energy transfer responsible for the quantum splitting.
Acknowledgements
We gratefully acknowledge the financial support of the U.S. National Science
Foundation, Grants 0305400 (RSM) and 0305449 (DAK). We also express our
appreciation to K. Mishra and M. Raukas for helpful discussions and to A. Meijerink
47
for a crystalline sample of YLiF4:Nd. We are especially grateful to R. L. Cone for
measuring the IR emission spectrum of one of our samples.
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48
[15] X. X. Zhang, A. B. Villaverde, M. Bass, B. H. T. Chai, H. Weidner, R. I.
Ramotar, R. E. Peale, Journal of Applied Physics, 74, 790 (1993).
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49
CHAPTER 3
SENSITIZATION OF Gd3+ AND THE DYNAMICS OF QUANTUM
SPLITTING IN GdF3:Pr,Eu
S. P. Feofilovb, Y. Zhoua, J. Y. Jeongc, D. A. Keszlerc and R. S. Meltzera,*
a
Department of Physics and Astronomy, University of Georgia, Athens, GA, USA
b
c
Ioffe Physico-Technical Institute, St. Petersburg, Russia
Department of Chemistry, Oregon State University, Corvallis, OR, USA
Modified version; Journal of Luminescence, (2007), 122-123, 503-505
50
ABSTRACT
Efforts are reported to sensitize the Gd3+ excitation with Pr3+ for application to
quantum cutting in GdF3 doped with Eu3+ and Pr3+. Excitation and emission spectra
are reported for several samples with different concentrations of Eu3+ and Pr3+ at room
temperature. In addition, time resolved measurements are performed to obtain the
dynamics of the various excited states of Gd3+. Strong enhancement of the 5D0 Eu3+
emission is observed for excitation of 6G of Gd3+ relative to that for other Gd3+ excited
states as expected for quantum cutting. While some enhancement of the J=0 Eu3+
emission is also observed for excitation into the Pr3+ 4f5d states the maximum
quantum yield is disappointing, falling far below the desired goal of 2. The timeresolved studies indicate that one reason for the ineffective quantum cutting is that the
Pr3+→Gd3+ energy transfer occurs predominantly to the 6I state of Gd3+. Results on the
6
I→6P relaxation and the Gd3+(6P7/2)→Eu3+ energy transfer are described.
3.1 INTRODUCTION
Attempts to develop useful lamp phosphors using quantum splitting have
involved two routes, cascade emission and a cross relaxation energy transfer (CRET)
in which the initially excited ion transfers part of its energy to another ion and each
excited ion then emits a photon. Although both processes have been shown to yield
internal quantum yields greater than 1, neither of these has lead to a useful phosphor.
All of the systems which have exhibited efficient CRET quantum splitting
have involved Gd3+ and either Eu3+[1] or Nd3+[2] in which the Gd3+ ion is prepared in
the 6GJ states at about 49,000 cm-1. A downward transition on Gd3+ coupled with an
excitation of Eu3+ or Nd3+ leave both ions in excited states from which each emits a
photon. While several attempts to sensitize the absorption of Gd3+ using Er3+[3] and
Tm3+[4] have produced quantum splitting, the materials have not proven to be useful
phosphors.
51
3.2 RESULTS AND DISCUSSION
Here we attempt to use the 4f5d absorption of Pr3+ ions to sensitize the Gd-Eu
couple in GdF3. For Gd3+ sensitization it is necessary that the sensitizer ion should
have a strong absorption to a state that is above the 6G states of Gd3+. For Pr3+ this
requires a host with a very small crystal field to minimize the crystal field depression
of the 4f5d configuration. GdF3 satisfies this requirement and is the subject of this
study.
The energy level diagram along with the relevant energy transfer processes are
shown in Fig. 1. When the Gd3+ is excited to its 6G state, a cross relaxation energy
transfer with Eu3+ occurs, shown by the dashed arrow labeled e, whereby the Gd3+ ion
undergoes a transition 6G→6P while a nearby Eu3+ ion undergoes a resonant an
upward transition 7F1→5D0. The excited Eu3+ ion emits the first photon. The Gd3+ ion
in the 6P7/2 excited state then transfers its energy to Eu3+ as shown by the arrow labeled
j. This is followed by non-radiative relaxation among the 5DJ levels, all of which emit
producing the second photon in the quantum splitting.
The emission spectrum of GdF3: 0.3Pr, 0.2% Eu is shown in Fig. 2 for
excitation at two wavelengths. For 275nm excitation with an Ar+ laser, Gd3+ is
excited directly to the 6I states from which quantum splitting cannot occur because it
lies below the 6G levels. Energy transfer to Eu3+ still occurs and one sees emission
from all the 5DJ levels. Upon excitation at 160nm with a D2 lamp one again sees
emission from all the 5DJ states of Eu3+. However, the 5D0 is greatly enhanced and
emission from 6P7/2 of Gd3+ is now observed.
52
70000
CT
60000
-1
Energy (cm )
4f5d
Half
Stokes
Shift
50000
6
G
g
e, e'
h
i
6
I
f, f' 6 j
6
40000
D
a b c d
30000
P
k
3
2 5D
1 J
0
a b c d
20000
1
D2
10000
0
e'
1
G4
F3,4
e
3
H4
f
f'
3
3+
Pr
7
FJ
8
3+
Gd
S
3+
Eu
Fig. 3.1 Energy level diagrams for Pr3+, Gd3+, and Eu3+ showing the various energy
transfer pathways labeled a through j. Processes a through d are shown displaced
downward by 2500 cm-1 reflecting half the value of the Stoke’s shift for LaF3 for the
Pr3+ 4f5d emission.
53
GdF 3 :0.3%Pr,0.2%Eu
Emission Intensity
Excitation Wavelength
+
275 nm Ar
160 nm D 2 lamp
5
5
Gd
3+ 6
300
P 7/2
D0
D2
5
D3
400
5
500
D1
600
700
Wavelength (nm)
Fig. 3.2 Emission spectra for a sample of GdF3 containing 0.3% Pr and 0.2% Eu
excited at 275 nm (6I state of Gd3+) and 160 nm (4f5d state of Pr3+).
Excitation spectra, detecting all emission λ > 320 nm, for two samples with
different dopant concentrations are shown in Fig. 3. The peaks at ~275 nm are the 6I
states of Gd3+ while those at 202 nm and 195 nm are the 6G levels of Gd3+. Below 190
nm, the 4f5d bands of Pr3+ are observed. The band between 150 and 160 nm is the
charge transfer (CT) band of Eu3+; therefore at 160 nm both Pr3+ and Eu3+ are excited.
These excitation spectra given by the two solid curves are obtained relative to that of
sodium salicylate whose excitation spectrum is nearly independent of wavelength and
whose absolute quantum yield is about 0.6. Based on this, the maximum quantum
yield for total emission occurs for the 0.3% Pr, 0.2% Eu sample and has a
disappointing value of only about 0.2 at about 160 nm. Thus it appears that GdF3:Eu
sensitized by Pr3+ is not a useful quantum splitting phosphor. While excitation of the
54
4f5d states of Pr3+ does sensitize excitation of Gd3+ it does not lead to high quantum
yields. Nonetheless, it will be useful to study the dynamics and examine the possible
causes for its limited performance.
Evidence for strong quantum splitting from the 6G levels of Gd3+ is also shown
in Fig. 3 for a sample of GdF3: 0.3 Pr, 0.03% Eu by comparing excitation spectra
obtained by selecting different emission wavelength regions. When only 5D0 emission
is detected, the 6G excitation peaks are enhanced by more than a factor of 5 relative to
the 6I peaks as compared to detection wavelengths between 400-560 nm where only
the 5DJ (J>0) emission is detected. Here the excitation spectra are not presented
relative to Na salicylate but are normalized to the Gd3+ 6I excitation peak. This
preferential generation of population in 5D0 results from the cross relaxation energy
transfer shown by the dashed arrow labeled e in Fig. 1. Under ideal conditions of
quantum splitting, the enhancement should be no more than a factor of 2. The
observation of a much larger enhancement results from (1) the fact that a large fraction
of the Eu3+ emission occurs from states other than 5D0 and the possibility that the Gd3+
(6P7/2) →Eu3+ energy transfer may be much less than 100% efficient.
55
Quantum Yield Relative to Na Sal
0.60
GdF 3:Pr,Eu excitation spectra
0.55
0.50
Eu
3+
- CT
0.3% Pr,0.2% Eu λdet>320 nm
0.3% Pr,0.03% Eu λdet>320 nm
0.45
0.40
0.35
5
D0
λdet>580 nm
5
DJ J>0
λdet<560 nm
6
normalized to I
but not to absolute scale
0.30
Gd
3+
6
- I
0.25
0.20
to scale
0.15
Gd
0.10
0.05
Pr
3+
3+
6
- G
- 4f5d
0.00
160
180
200
220
240
260
280
300
Excitation Wavelength (nm)
Fig. 3.3 Excitation spectra of two samples of GdF3:Pr,Eu. Excitation spectra obtained
by detecting all wavelengths > 320 nm are referenced to a Na salicylate standard.
Excitation spectra obtained with filters selectively for λ>580 nm and λ<560 nm are
normalized for the 6I peak but are not to the scale of the figure.
The question of sensitization of Gd3+ is now considered with the aid of Fig. 1.
There exist a number of possible CRET routes by which Pr3+ can transfer energy to
Gd3+. These are shown by the solid arrows labeled a through d. Forster-Dexter
energy transfer requires overlap of the emission of the donor and absorption of the
acceptor. A number of emissive transitions from Pr3+ can occur which are nearly
resonant with absorptions of Gd3+. The Pr3+ emissive transitions labeled a through d
on the Pr3+ energy level diagram in Fig. 1 have been shifted to lower energy relative to
the bottom of the 4f5d band by half the Stoke’s shift or about 2500 cm-1, reflecting the
effect of the large Stoke’s shift known for the 5d emission in the isostructural LaF3
56
[5]. One sees that in view of the broad band characteristics of 5d→5f emission
(∼2000 cm-1), for all processes a-d, near resonances occur with transitions on Gd3+
from it 8S7/2 ground state. Unfortunately, only one of these, CRET labeled a, will
generate the desired 6G population.
The relative contribution of the different Pr3+→Gd3+ energy transfer processes
can be obtained by studies of the dynamics of the Gd3+ emission shown in Fig. 4 for
emission from 6I (279 nm) and 6P7/2 (312 nm) under 193 nm pulsed excitation to the
4f5d state of Pr3+. One sees that most of the initial population of Gd3+ appears in 6I
with less than 20% occurring to 6P7/2. 6I then relaxes to 6P in 2.4 µs as demonstrated by
the fits (dashed curves) to the data in Fig. 4 which show that the decay of 6I describes
also the buildup of 6P7/2. The fast component of the 6P emission (<20% of maximum
population) results either from energy transfer process a followed by the CRET
process e or some other rapid feeding of 6P. The mechanism of the relaxation process
6
I→6P in Gd3+ is unclear as its rate does not depend systematically on either the Eu3+
or Pr3+ concentrations. However multiphonon relaxation in a low phonon frequency
materials such as GdF3 is unlikely to explain the ∼2-5 µs decay time for an energy gap
as large as 3000 cm-1. The decay rate of the 6P7/2 emission of Gd3+ increases with Eu3+
concentration indicating that it results from energy transfer to Eu3+.
57
GdF 3:0.1%Pr,0.03%Eu
Excitation 193nm
Emission Intensity
1E-3
312 nm (Gd
τrise=2.4 µs
3+
6
P 7/2)
1E-4
279 nm (Gd
τdecay=2.4 µs
3+
6
I)
1E-5
2
4
6
8
10
12
14
16
Time (µs)
Fig. 3.4 Time-resolved emission for 6I and 6P7/2 of Gd3+ after pulsed excitation at 193
nm showing that the decay of 6I corresponds to the buildup of 6P7/2 and that energy
transfer from Pr3+ predominantly feeds 6I. The circles are the measurement and the
dashed curves are fits using an exponential decay and buildup of 2.4 µs with an initial
20% 6P7/2 population.
We acknowledge the support of the U.S. National Science Foundation, Grants
0305400 (RSM) and 0305449 (DAK).
58
REFERENCES
[1] R. T. Wegh, H. Donker, K.D. Oskam, A. Meijerink, Journal of Luminescence, 82,
93 (1999).
[2] W. Jia, Y. Zhou, S.P. Feofilov, R.S. Meltzer, J. Y. Jeong and D. Keszler, Phys.
Rev. B, in press (2005).
[3] R. T. Wegh, E. V. D. van Loef, A. Meijerink, Journal of Luminescence, 90, 111
(2000).
[4] S. Peijzel, W.J.M. Schrama, A. Meijerink, Molecular Physics, 102, 1285 (2004).
[5] P. Dorenbos, Journal of Luminescence, 91, 155 (2000).
59
CHAPTER 4
RELAXATION OF THE 4fn-15d1 ELECTRONIC STATES OF
RARE EARTH IONS IN YPO4 AND YBO3
W. Jia*, Y. Zhou*, D. A. Keszler**, Joa-Young Jeong**, K.W. Jang*** and R.S.Meltzer*
*
Department of Physics and Astronomy, University of Georgia, Athens,GA 30602
USA
**
Department of Chemistry, Oregon State University, Corvallis, OR 97331 USA
***
Dept of Physics, Changwon National University, Chang Won 641773, South
Korea
Modified version; Physica Status Solidi C: Conferences and Critical Reviews (2005),
2(1), 48-52
60
ABSTRACT
Large bandgap materials doped with rare earth ions are currently of great
interest as new vacuum UV phosphors for lighting and displays. In this report, the
optical properties of YPO4 and YBO3 doped with Pr, Tm, Er, and Eu are described.
The emission resulting from the VUV excitation of the parity allowed 4fn-15d1 states
and their quantum efficiencies are reported. Relaxation between the 4fn-15d1 and
nearby 4fn excited states is observed for some of these ions and the dynamics of these
excited states is reported. In doubly-doped samples, the prospects for quantum cutting
using cross relaxation energy transfer in which some of the energy of the initially
excited 4fn-15d1 state is transferred to an acceptor ion so that both donor and acceptor
ions are left in an excited state from which they each can emit a photon are examined.
4.1 INTRODUCTION
The high-lying excited states of rare earth ions are of considerable importance
for a wide range of applications including scintillators, vacuum ultraviolet (VUV)
phosphors, and UV lasers. In this work the problem of designing efficient VUV
phosphors will be considered. The potential utility of a scheme for converting the
exciting VUV photon into two visible photons using cross relaxation energy transfer
involving a parity allowed 4fn-15d → 4fn transition on one ion and a transition within
the 4fn’ configuration of a second ion will be tested in the hosts YPO4 and YBO3.
These materials have large band gaps that fall well into the VUV.
So called “quantum cutting” or “multiphoton” phosphors offer a significant
challenge for their realization. While the idea has been demonstrated in real systems,
these systems exhibit serious drawbacks preventing their commercialization. Cascade
luminescence from the 1S0 state of Pr3+ in YF3 was shown to occur with a quantum
yield of about 1.4 [1]. Unfortunately, most of the energy in the first step in the
stepwise relaxation back to the ground state occurs at about 400nm which is at a
wavelength for which the human eye is insensitive. Cross relaxation in LiGdF4:Eu3+
61
after initial excitation of the 6GJ levels of Gd3+ was shown to produce two visible
photons from Eu3+ with 1.9 quantum yield [2]. Here the cross relaxation occurs
between Gd3+ and Eu3+ involving transitions within the 4f7 configurations and 4f6
configurations, respectively. However, it is not possible to excite the 6G state of Gd3+
efficiently. Replacing the Eu3+ dopant in LiGdF4 by Er3+ and Tb3+ provides for
efficient excitation and results in a quantum efficiency that is somewhat in excess of
100%. It utilizes a cross relaxation scheme involving a parity allowed 4f115d → 4f12
transition on Er3+ coupled with a transition within the 4f7 configuration of Gd3+ [3].
The demonstration of the latter scheme suggests that the use of parity allowed
transitions on one of the ions in the ion-pair should be considered further and this
provides motivation for the present work.
4.2 RESULTS AND DISCUSSION
The 4fn-15d states of Ln3+ series are rather short-lived. For the first half of the
series, these lifetimes are tens of nanoseconds. In the second half of the series, there is
a low-lying high spin (HS) state in the 4fn-15d configurations whose lifetimes are
typically microseconds [4]. For cross relaxation energy transfer to be successful at
down converting the energy, its rate must be competitive with these radiative rates.
Fortunately, the multipolar interaction responsible for the energy transfer scales with
these radiative rates, making possible a competitive cross relation energy transfer rate.
The cross relaxation energy transfer scheme is indicated on the energy level
diagram shown in Fig. 1 for the ion-pair of Pr3+ and Tm3+. After excitation of Pr3+
into its 4f5d configuration, a cross relaxation can occur as shown by the dashed
arrows, labeled A, on both the Pr3+ and Tm3+ energy level diagrams. Here the Pr3+ ion
undergoes a downward transition from the 4f5d configuration to its 1D2 excited state
in the f2 configuration while simultaneously a Tm3+ ion undergoes an upward
transition from it ground state to its 1D2 level, conserving energy and leaving both ions
in excited states from which each can emit a photon.
62
LS
LS
12
4f 5d
60000
11
4f 5d
HS
HS
2
4f5d
4
-1
Energy(cm )
40000
20000
excitation: 157nm
<20ns
3
700ns
D1/2 2.7µs
P0,1,2
A
1
B
3
1
D2
A
Pr
H4,5,6
3+
Tm
3+
4
F9/2
I9/2
4
3
0
P3/2 4.0µs
G4
B
3
2
D2
1
P0,1,2
F7/2 60ns
4
H5
3
F4
3
H6
I11/2
4
I13/2
4
Er
I15/2
3+
Figure 4.1 Energy level diagrams for Pr3+, Tm3+ and Er3+ in YPO4 and YBO3. For
Tm3+ and Er3+ the 5d levels are split into a lower-energy high spin (HS) and higher
energy low spin (LS) states. For Er3+ the room temperature lifetimes are shown next
to the emitting states. Processes labeled A and B for the Pr3+-Tm3+ pair indicate
energy conserving cross relaxation paths
63
The rate for such a cross relaxation can be estimated based on the ForsterDexter dipole-dipole energy transfer theory. In Eq. 1 the rate, PABdd is expressed in
terms of fA and fB, the oscillator strengths of the two transitions labeled by A in Fig. 1,
the transition energies, ∆E, of each ion (in eV), the distance R between the two ions
(in Angstroms), and the spectral overlap, S, (in cm-1) of the downward and upward
transitions [5].
PAB = 1.4 x 1024 fA fB S ∆E-2 R-6
(1)
If we assume oscillator strengths of 10-2 and 10-6 for the 5d → 4f and 4f → 4f
transitions, respectively, ∆E = 3 eV, and S = 10-3, reflecting the fact that the 5d → 4f
downward transition is broad (about 1000 cm-1), then we find for nearest neighbors at
a distance of 3.5 A, a rate of ∼109 s-1 which is ten times greater than the radiative rate
of ∼108 s-1. At more typical phosphor dopant concentrations of 2-5%, the energy
transfer rate would be expected to be 107-108 s-1, still competitive with the radiative
rate. Exchange mediated energy transfer can be even much faster, but it will only be
important for nearest neighbor distances.
Samples of YPO4, YBO3 doped with trivalent rare earth ions were prepared by
solid state reaction, following procedure modified relative to those reported previously
[6]. For Y1-xRExPO4 samples, appropriate stiochiometric mixtures of Y2O3 (Cerac,
99.99%), (NH4)2HPO4 (Aldrich, 99.99%), Pr6O11 (Alfa aesar, 99.99%), Tm2O3 (Alfa
aesar, 99.99%), Er2O3 (Alfa aesar, 99.99%) were ground and then fired at 1150°C for
3h. The fired cakes were then mixed with an additional 3.5 wt% (NH4)2HPO4, and
heated again at 1300°C for 3 or 4 hours. For Y1-xRExBO3 samples, similar procedures
were followed, except a 4.2wt% excess of H3BO3 (Alfa aesar, 99.99%) was added to
each mixture. These samples were also subjected to two heat treatments. Formation of
a single phase of the correct crystal structure was confirmed on the basis of powder Xray diffraction by using a Siemens D5000 diffractometer. Excitation spectra were
performed with a deuterium lamp source and VUV monochromator (Acton VM502)
to select and scan the excitation wavelength. The excitation spectra are measured
relative to that of sodium salicylate whose absolute quantum efficiency is estimated as
nearly constant at 55-60% over the wavelength range of interest. The emission was
detected with a photomultiplier (PMT) and either glass or interference filters.
64
Emission spectra were recorded with a CCD detector (Santa Barbara ST-6B) attached
at the focal plane of a spectrometer (Acton Spectra Pro 150). Emission was excited
with monochromatic light from the deuterium lamp or using a F2 gas discharge
excimer laser (GAM EX5) emitting at 157nm. Time resolved emission spectra were
obtained with this laser which had a temporal pulse width of 10ns. All emission and
absorption spectra (except as noted) were fully corrected for the wavelength dependent
response of the CCD or PMT.
The emission and excitation spectra for 1% Pr3+ doped YPO4 is shown at room
temperature in the upper trace of Fig. 2, in agreement with what has been previously
reported [7]. The quantum yield obtained from the excitation spectrum is quite high,
estimated at about 0.5 at the highest peak. The emission is dominated by the
4f5d→4f2 transitions where the 4f2 final states are labeled in the figure. The large gap
between the 4f5d state and the 3P levels of Pr3+ results in dominant 4f5d emission.
The quantum yield drops with increasing concentration of Pr3+ with maxima of
approximately 0.25 and 0.08 for 5% and 10% concentrations, respectively.
The situation for Tm3+ doped YPO4 is quite different as shown in the lower
trace of Fig. 1. There is no evidence for emission from the initially excited 4f125d
configuration despite the large gap between the lowest 4f125d state and the highest 4f13
levels. The excitation spectrum shows a maximum quantum yield of just under 50%
relative to that of sodium salicylate, implying an absolute yield of about 0.25. As
noted previously,[4] the sharp feature at 160nm is the 4f125d state and the broader
feature at about 172nm is the charge transfer (CT) band of Tm3+ which lies below the
4f125d state. It is perhaps the presence of this low-lying CT state that mediates the
efficient relaxation to the 3P0 emitting level. In contrast to the case of Pr3+ the
quantum yield is much less sensitive to concentration, dropping only by half, i.e. about
0.13 at 10% Tm3+ concentrations. The quantum yield for 0.2% Tm3+ is a bit higher,
about 30%, than for 1% Tm3+ and this may be an underestimate since the absorption
has probably dropped significantly at only 0.2% Tm3+ levels.
Er3+, as for Tm3+, shows no emission from the 5d configuration inYPO4 and all
emission arises from the 4f12 configuration as shown in the middle spectrum of Fig. 2.
This is to be expected for Er3+ since the 4f115d states lie just above the 4f12 levels so
65
that multiphonon relaxation results in rapid relaxation to the metastable 2F7/2 state (see
Fig. 1). Because of the relatively high density of levels with smaller energy gaps,
multiphonon relaxation also causes non-radiative relaxation to lower-lying levels,
some of which are sufficiently long-lived to radiate as well. The emission spectrum
thus shows three groups of lines from the three metastable levels indicated by the bold
lines in the energy level diagram of Fig. 1. The initial state of each transition in the
emission spectrum of Er3+ in Fig. 2 is identified by a different vertical bar symbol
above each peak. The room temperature decay times of each level are indicated next
to the level in Fig. 1. Time resolved spectra at 300K show that all three of the
emitting levels appear immediately (<20ns) after excitation of the 4f115d state. This is
contrary to the expected mulitphonon relaxation process and implies some other nonradiative channel leading directly and rapidly from the 4f115d state to all three of these
states of the 4f12 configuration. The quantum yield relative to sodium salicylate is
quite low at room temperature (0.11) as shown on the excitation spectrum on left part
of the Er3+ spectrum. The spectrum, expanded by a factor of 8, shows a weak
excitation at 170nm into the high spin (HS) 5d level which occurs for all rare earths
with a fn configuration which is more than half filled (n>7). The host absorption
feature reported below 150nm at T=10K [4] is not observed at room temperature. The
observed quantum yield relative to sodium salicylate falls at lower Er3+
concentrations, probably due to decreased 5d absorption, suggesting that there is little
concentration quenching for Er3+.
66
Figure 4.2 Excitation (dashed) and emission spectra (solid) for Pr3+, Er3+ and Tm3+
ions in YPO4 at room temperature. The excitation spectra are relative to that of
sodium salicylate. For Er3+ the distinct vertical bars identify the emitting level.
We now turn to the issue of quantum cutting using cross relaxation energy
transfer from the 4fn-15d configuration of the trivalent rare earth ions. As noted in Fig.
1 by the dashed arrows labeled A and B, energy conserving cross relaxation schemes
exist for the Pr3+-Tm3+ pair. Process A is very attractive since it leaves both ions in
visible emitting excited states. However, as seen for YPO4 in the upper trace of Fig. 1,
emission from 4f5d to 1D2 is much weaker than that to the 3HJ and 3FJ levels. Thus the
oscillator strength of this transition is probably only about 10-4 reducing expected
energy transfer rates below 106 s-1 which is insufficient to compete with the radiative
rate. While process B would not leave Pr3+ in a visible emitting state, it can still
67
provide a proof of concept demonstration. Tm3+ absorptions to the 3PJ levels occur in
the range of 268-289 nm. This overlaps with the relatively strong 4f5d emission to
3
FJ. However, the excitation spectrum of a doubly doped sample containing 1% Pr3+
and 5% Tm3+ (lower half of Fig. 3) shows no evidence of the Pr3+ absorption when
only Tm3+ emission is detected; only the Tm3+ excitation features of the CT and 4f125d
states are observed.
Double doped samples of 1% Pr3+ and 5% of Tm3+ in YBO3 yield a similar
disappointing result as shown in the upper trace of Fig. 3. A singly doped sample
containing 1% Pr3+ exhibits, like YPO4, strong emission from the 4f5d state but with
an even larger peak quantum yield relative to sodium salicylate (∼1.2), implying an
absolute quantum yield of about 0.6 for excitation between 230 to 250nm. Both the
emission and excitation features are shifted by about 20nm to the red relative to YPO4,
as expected, since the 4f5d levels in YBO3 are predicted to lie about 3100 cm-1 below
the corresponding levels in YPO4 [8]. An excitation spectrum of the doubly doped
sample, detecting only the Tm3+ emission, shows little if any features of the Pr3+
excitation spectrum indicating the absence of energy transfer. The overlap of the 4f5d
emission of Pr3+ with the absorption from 3H6 to 3PJ states of Tm3+ is even more
favorable than in YPO4 and yet virtually no emission from Tm3+ is observed when
exciting the 4f5d levels of Pr3+.
Cross relaxation energy transfer for the Pr3+-Er3+ couple has also been
investigated in YPO4. The strong 4f5d emission of Pr3+ between 230-270nm overlaps
a number of absorptions of Er3+. Unfortunately these Er3+ transitions are quite weak
with small oscillator strengths. Indeed, excitation of this samples containing 1% Pr3+
and 10% Er3+ at 188nm where only Pr3+ absorbs yields no observable Er3+ emission.
We have also examined the Pr3+-Eu3+ couple in both YPO4 and YBO3. For
Eu3+ in these hosts, the CT transition overlaps strongly with the 4f5fd absorption of
Pr3+ so that it is difficult to identify cross relaxation energy transfer. There certainly is
overlap between the 4f5d emission of Pr3+ and the large density of Eu3+ transitions
within the 4f6 configuration. In samples containing 1% Pr3+ and 5% Eu3+ the Pr3+
emission is strongly quenched in YPO4 and is totally absent in YBO3 suggesting that
rapid energy transfer from Pr3+ to Eu3+ occurs and competes with the radiative decay
68
of Pr3+. However this is most likely to leave the Pr3+ ion in one of its low-lying levels
from which visible emission will not occur. Indeed, very little visible emission from
Pr3+ is seen in either of these samples.
Quantum Yield Relative to Sodium Salicylate
Excitation Spectra (Pr to Tm energy transfer?)
1
1%Pr, 5%Tm
1% Pr
3+
YBO3
0.5
0
1
3+
1% Tm (x2)
1% Pr
3+
YPO4
0.5
1%Pr, 5%Tm
0
140
160
180
200
220
240
260
Wavelength(nm)
Figure 4.3 Excitation spectra at room temperature demonstrating the absence of Pr3+ to
Tm3+ energy transfer in YPO4 and YBO3. None of the features of the Pr3+ excitation
spectra appear in doubly doped samples when only the Tm3+ emission is detected.
The excitation spectra of the doubly-doped samples are not to scale.
69
4.3 CONCLUSION
The quantum splitting of the high energy 4fn-15d electronic states of trivalent
rare earth sensitizer ions into shared excitation of lower energy on this sensitizer and
another activator ion using cross relaxation energy transfer does not seem promising in
the hosts YPO4 or YBO3. Of the ions Pr3+, Tm3+, Er3+ and Eu3+, only Pr3+ shows
emission from its 5d state at room temperature. Attempts to observe cross relaxation
energy transfer from Pr3+ to Tm3+ and Er3+ at 5% dopant levels were unsuccessful,
despite the fact the energy conserving pathways exist. In order to obtain sufficient
energy transfer, it will probably be necessary to invoke exchange interactions and
energy migration which will occur only in stoichiometric samples as previously
demonstrated for LiGdF4:Eu3+ [2]. For both Er3+ and Tm3+ in YPO4, the 5d emission
is quenched and emission from the 4fn configuration appears immediately (<20ns),
implying a very fast relaxation from the lowest 4fn-15d state to the 4fn states, even
when the energy gap between the lowest 4fn-15d state and 4fn states are much larger
than can be bridged by multiphonon relaxation.
We thank the National Science Foundation for their support of this work with
Grants 0305400 (RSM) and 0305449 (DAK).
70
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82, 93 (1999).
[3] K. D. Oskam, R. T. Wegh, H. Donker, E.V.D. van Loef, A. Meijerink, Journal of
Alloys and Compounds, 300/301, 421 (2000).
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045114 (2002).
[5] T. Kushida, Journal of the Physical Society of Japan, 34, 1318 (1973).
[6] R. P. Rao, D. J. Devine, Journal of Lumininescence, 87-89, 1260 (2000)
[7] L. van Pieterson, M. F., Reid R. T. Wegh, S. Soverna, A. Meijerink, Phys. Rev. B
65, 045113 (2002), and T. Justel, P. Huppertz, W. Mayr, D.U. Wiechert, Journal of
Lumininescence, 106, 225 (2004).
[8] P. Dorenbos, Journal of Lumininescence, 91, 155 (2000).
71
CHAPTER 5
HOST SENSITIZATION OF Gd3+ IONS IN YTTRIUM AND
SCANDIUM BORATES AND PHOSPHATES FOR
APPLICATIONS IN QUANTUM SPLITTING
S.P. Feofilova, Y. Zhoub, H.J. Seoc, J.Y. Jeongd, D.A. Keszlerd and R.S. Meltzerb
a
b
Ioffe Physical-Technical Institute, St. Petersburg, Russia
Department of Physics and Astronomy, University of Georgia, Athens, GA 30602
c
Department of Physics, Pukyoung National University, Pusan 608-737,
Republic of Korea
d
Department of Chemistry, Oregon State University, Corvalis, OR 97331, USA
Modified version; Physical Review B: Condensed Matter and Materials Physics
(2006), 74(8)
72
ABSTRACT
Energy transfer from the host excitations (STE) excited at λ~160 nm to Gd3+
impurity ions was observed in yttrium and scandium borates and phosphates. The
fluorescence and excitation spectra as well as time-resolved fluorescence data were
obtained. For ScPO4:1%Gd3+ efficient energy transfer to the Gd3+ 6G state was
observed followed by the cascade emission of the visible and UV photons yielding a
material exhibiting quantum splitting.
For ScBO3:Gd and ScPO4:Gd, absolute
quantum yields approach unity making these potential VUV excited phosphors. A
comparison of estimated dipole-dipole energy transfer rates with observations support
the importance of energy migration of the intrinsic excitations.
5.1 INTRODUCTION
Several schemes have now been demonstrated for implementing quantum
cutting which provides a means to obtain two or more photons for each photon
absorbed.
It therefore serves as a down converting mechanism that offers the
prospect for developing materials with quantum efficiency greater than unity and it
offers the prospect of providing improved energy efficiency in lighting devices. For
example, a new class of fluorescent lamps could be developed by replacing the
mercury discharge with xenon provided that phosphors with quantum efficiencies in
excess of 150% under vacuum ultraviolet (VUV) excitation could be discovered.
Examples of materials that emit two visible photons per absorbed ultraviolet photon
were demonstrated in the early 1970s when it was shown that photon cascade
emission from the high energy 1S0 level of Pr3+ can yield two visible photons in a
sequential two-step radiative process [1,2]. Detailed experimental studies of the
quantum efficiency showed that the actual visible (380-750 nm) quantum yield was
127%. It has been shown that Gd3+ ions can also exhibit photon cascade emission in
YLiF4 [3] and GdBaB9O16 [4] provided Gd3+ is excited to its 6G state.
73
In a second method using a combination of two lanthanide ions, cross relaxation
resonant energy transfer (CRET) in which each ion shares a portion of the energy of
the initially absorbed photon, was shown to yield two visible photons. For the Gd3+Eu3+ couple, an internal quantum efficiency as high as 190% was demonstrated [5].
After initial excitation of the 6G state of Gd3+ at about 50,000 cm-1, a CRET occurs
whereby the Gd3+ ion undergoes a non-radiative transition to its 6P state while the
Eu3+ ion undergoes a transition from it thermally populated 7F1 state to its 5D0 state [6].
The excited Eu3+ is responsible for the first visible photon. Resonant energy migration
among excited Gd3+ ions occurs from within the 6P state until the energy resides
nearby another Eu3+ ion to which it can transfer its energy. The second excited Eu3+
ion produces the second visible photon, achieving the quantum splitting. The quantum
splitting therefore requires that the Gd3+ be excited into its 6G excited state at about
50,000 cm-1. However, because of the weak absorption by Gd3+ resulting from the
parity-forbidden and spin-forbidden character of transitions from the ground state to
excited states of the f7 configuration, the direct absorption of this state is weak. Thus a
successful phosphor using the Gd-Eu couple will require sensitization of the Gd3+
excitation.
Sensitization of the 6G state of Gd3+ using the 4fn-15d states of other rare earth
ions has been examined. Allowed f-d transitions of many of the rare earth ions occur
in the VUV [7]. Tm3+ and Nd3+ have both been shown to sensitize Gd3+ in GdLiF4 but
they each also provide an alternate cross relaxation pathway for energy transfer which
is more efficient than the Gd-Eu CRET [8,9]. As a result, after the CRET, the Tm3+
and Nd3+ ions are left in low-lying excited states which yield, for the first photon,
infrared emission. This of course defeats the goal of a visible quantum splitter. While
sensitization of the 6G state of Gd3+ by Pb2+ has also been examined, it was concluded
that sensitization of the 6G state will be difficult with this and other heavy ns2 ions
[10].
Sensitization of Gd3+ using the intrinsic excited states of the host would be most
desirable since it does not require additional doping of ions into the system and the
host provides very strong absorption of the VUV excitation light. It requires that the
host excited state transfer its energy effectively to 6G of Gd3+. This could occur either
74
if the host emission were to overlap the Gd3+ absorption to 6G at about 204 nm (or
higher-lying states of Gd3+ which relax predominantly to 6G) or via some efficient
intersystem crossing between the self-trapped exciton and the Gd3+ 6G potential
surface. Excitation of 6G has been demonstrated in a number of hosts, including
GdPO4, using X-ray excitation [11]. In some sense, this is an example of host
sensitization of the 6G state of Gd3+
Host sensitization of Gd3+ to its 6P levels has been demonstrated in a number of
systems [12]. In the present paper we study several materials in which the absorption
of light by the host may have the potential for efficient transfer of energy to Gd3+ ions
in the 6G state, as required for quantum splitting. The main feature that distinguishes
these kinds of materials is that they should exhibit intense fluorescence from the
intrinsic excitations in the undoped materials. This emission is usually ascribed to selftrapped excitons (STE) [13]. The spectral overlap of the host emission with the
absorption of Gd3+ (or other) ion is the necessary condition of the nonradiative energy
transfer according to the Forster-Dexter energy transfer processes [14]. It is also
important that the rate of energy transfer from the host to the acceptor ions be faster
than the host excitation decay rate. Yttrium and scandium borates and phosphates
(YBO3, ScBO3, YPO4, ScPO4) are interesting materials for sensitizing Gd3+ ions
because they have relatively large band gaps and they emit short-wavelength UV
fluorescence efficiently when excited in the VUV. Using emission and excitation
spectroscopy along with time-resolved emission, we demonstrate host sensitization of
the 6G level of Gd and obtain its efficiency, estimate the absolute quantum yields of
both the undoped and Gd-doped samples, and examine the dynamics of both the host
to Gd3+ energy transfer and the dynamics of the photon cascade emission of Gd3+.
5.2 EXPERIMENTAL
Samples of doped YBO3, ScBO3, YPO4, ScPO4 were prepared in powder form
by using Y2O3 (99.999%, Standard Material Corporation), B2O3 (99.99%, Alfa Aesar),
Sc2O3 (99.999%, Standard Material Corporation), (NH4)2HPO4 (99.99%, Sigma-
75
Aldrich), and Gd2O3 (99.999%, Standard Material Cooperation). The oxides were
mixed according to the desired stochiometric ratios of each sample, including a 15
mol % excess of B2O3 or a 10 mol % excess of (NH4)2HPO4. The mixtures were
thoroughly ground and fired in alumina crucibles at 1150°C for 3h. For the phosphate
samples, the resulting products were ground a second time with an additional 10 mol%
excess of the phosphate reagent. This mixture was then heated at 1350°C for 3h. For
the borate compounds, the resulting products were ground and mixed two additional
times with a 10 mol % excess of B2O3 and fires twice at 1330°C for 4h.
All spectra were obtained at room temperature. Emission spectra were obtained
by exciting the sample, contained in vacuum, with a deuterium lamp spectrally filtered
with an Acton VM-502 VUV monochromator. The visible and UV emission was
dispersed with an Acton Spectrapro-150 spectrometer and was detected with a Santa
Barbara Instrument Group ST-6I CCD camera. All emission spectra were corrected
for the wavelength-dependent response of the detection system. Excitation spectra
were obtained by scanning the VUV monochromator, illuminated by the deuterium
lamp, while detecting the emission with a PMT after passing the luminescence
through colored glass or interference filters. The excitation spectra of each sample
were compared to that of a reference sample of sodium salicylate whose quantum
efficiency is assumed to be about 58% and constant over the excitation wavelength
range from 140 to 320 nm [15].
For the time-resolved data, the samples were excited with a GAM Laser, model
EX5 pulsed laser operating with F2 or ArF whose output was at 157 or 193 nm (pulse
width 10 ns). The laser emission was passed through an Acton Research VUV
interference filters in order to eliminate other wavelengths from the emission of the
laser discharge. The emission was selected with a 0.25 m monochromator and
additional colored glass or interference filters. The bandwidth of the instrument was
~3 nm. The emission was detected with a PMT and was averaged and stored in a
digital oscilloscope. A temporal resolution of 1-2 ns was obtained.
76
5.3 RESULTS AND DISCUSSION
5.3.1 ScBO3
Undoped ScBO3, under excitation in the VUV at 160 nm emits a broad emission
centered at 238 nm which is shown by the bold solid curve in Fig. 1. This emission
has been assigned to emission from the STE or to molecular transitions within the
BO3-3 group [16]. When ScBO3 is doped with Gd3+, the broad UV emission decreases
with an increase in Gd3+ concentration as shown by the dotted and dot-dashed curves
in Fig. 1. Accompanying the decrease of the broad emission is a dramatic increase in
the 6P→8S emission of Gd3+ at 313 nm, suggesting efficient energy transfer from the
host states to Gd3+. The energy transfer efficiency is so great that even the undoped
samples shows a weak Gd3+ 6P emission due to residual Gd3+ impurities.
undoped
1% Gd
5% Gd
8
P to S
Emission Intensity
ScBO3
λex=157nm
Gd
3+ 6
STE
x20
200
250
300
350
400
Wavelength (nm)
Fig. 5.1 Emission spectra of ScBO3, excited at160 nm. The instrinsic STE
emission is shown amplified by a factor of 20.
77
As seen in Fig. 2, there is also evidence for weak 6G→6P emission around 600
nm which has been previously identified in other materials containing Gd3+ [3,4]. The
time-resolved emission described later add support to this assignment. This means
that there is some energy transfer from the host states to 6G of Gd3+.
The excitation spectra of undoped and 1% Gd and 5% Gd doped samples of
ScBO3 fluorescence (detected at wavelengths longer than 305 nm) are shown in Fig. 3.
All three samples show the same spectral excitation features. The quantum yields,
measured relative to sodium salicylate, indicate very high quantum efficiencies. Since
the absolute quantum yield of sodium salicylate is 0.58, one sees that the 6P emission
of the 5% Gd3+ sample has an absolute quantum yield of 0.8±0.15. Thus, if efficient
energy transfer occurs from Gd3+ in the 6P state to another ion that is a good visible
emitter, this material could be a highly efficient VUV-excited phosphor. While the
broad intrinsic UV emission of the undoped sample has a considerably lower quantum
yield, it is still about 0.3.
The intrinsic emission decay is shown in Fig. 4. In the undoped ScBO3 the
decay time is 195 ns but it becomes much shorter in the Gd3+-doped samples. The
data is fitted with exponential curves with the best fitted results shown on the figure.
The instrumental response (laser and PMT) yields a 5 ns decay based on studies of
ZnO which is known to have a sub-nanosecond lifetime. Thus the estimated decay
time of the 1% and 5% Gd3+ samples is 15 and 3 ns, respectively. These shortened
decay times are consistent with the fact that the time-averaged intrinsic emission
intensities decrease with Gd3+ concentration (see Fig. 1). The increased quantum yield
in the Gd3+-doped samples supports this assertion. The weak emission around 600 nm
has a lifetime of 450 and 320 µs in the 1% Gd and 5% Gd samples, respectively as
seen in Fig. 5. This is consistent with expectations for the lifetime of the 6G state. The
6
G decay is dominated by radiative emission to 6P and is spin-allowed, in contrast to
the spin-forbidden 6P→8S emission whose lifetime is typically milliseconds. The
large energy gap of 5000 cm-1 between 6G and the next lower manifold, 6D leads to
low multi-phonon relaxation rates and hence to radiative decay.
Emission Intensity (arb. units)
78
Gd
560
3+ 6
6
YBO3: 5% Gd
YBO3: 1% Gd
ScBO3:5% Gd
ScBO3:1% Gd
G -> P Emission
600
640
680
Wavelength (nm)
Fig. 5.2 Emission spectra of the Gd3+-doped borates in the red showing the weak Gd3+
G→6P emission
6
Quantum Yield Relative to NaSal
1.6
Excitation Spectra
1.4
1.2 ScBO :5% Gd
3
1.0
0.8
ScBO3:1% Gd
0.6
0.4
0.2
0.0
140
ScBO3:undoped
160
180
200
220
Wavelength (nm)
Fig. 5.3 Excitation spectra of undoped and Gd3+-doped ScBO3 detecting the total
emission and measured relative to that of sodium salicylate.
79
10
-1
Intensity (arb. units)
Time Resolved Emission at 250 nm
ScBO3-undoped
τ=195 ns
10
-2
ScBO3-1% Gd
τ=20 ns
3+
ScBO3-5% Gd
τ=8 ns
10
3+
-3
0
100
200
300
400
500
Time (ns)
Fig. 5.4 Time resolved intrinsic emission of undoped and Gd3+-doped ScBO3. The
emission was excited at 157 nm and detected at 250 nm. Fitted decay curves are
shown by the dashed lines. The fitted values have a 5 ns instrumental contribution
The relatively weak 6G emission shows that the Gd3+ ions are mostly excited to
the 6P state; the excitation to 6G state is much less efficient in ScBO3. This may be
explained by the insufficient energy of the host excitations. The excitation of the 8S
→ 6G transition is at λ=205 nm and corresponds only to the high-energy wing of the
host emission so that spectral overlap with the host emission is not ideal. The spectral
overlap between host emission and Gd3+ absorption favors excitation of 6D and 6I at
254 and 276 nm, respectively. These states undergo rapid multi-phonon relaxation to
6
P. Despite the strong 6P emission, it is not possible to study its population buildup
with nanosecond resolution as the long lifetime limits the photon emission rate; thus
the details of the initial state distributions resulting from the energy transfer from the
host states have not be determined.
80
Gd
3+ 6
6
Material
(τFit)
ScBO3:5% Gd (320µs)
ScBO3:1% Gd (450µs)
YBO3: 5% Gd (200µs)
YBO3: 1% Gd (260µs)
G to P
λem=600 nm
-3
Intensity
10
-4
1x10
-5
1x10
0.0000
0.0005
0.0010
0.0015
0.0020
Time (s)
Fig. 5.5 Observed decay of the Gd3+ 6G→6P emission in the Gd3+-doped borates.
The fitted decay curves are shown by the dashed lines with the decay values shown
in the legend.
5.3.2 YBO3
The case of YBO3 shows some similarities with that of ScBO3. The emission
spectra of the doped and Gd3+-doped samples excited at 157 nm are shown in Fig. 6.
One sees a broad emission in the undoped sample (bold solid line) peaking at about
325 nm which is considerably weaker than the emission in ScBO3. In the Gd3+-doped
samples a strong 6P emission appears. The broad emission is not quenched in the 1%
Gd3+-doped samples but in the 5% Gd3+ it is reduced significantly.
81
YBO3
undoped
1% Gd
5% Gd
Gd
3+ 6
Relative quantum yield
8
P- S
Excitation 160 nm
X100
STE
YBO3-undoped
YBO3:1% Gd
YBO3:5% Gd
Gd
200
300
400
500
3+ 6
6
G- P
600
700
Wavelength (nm)
Fig. 5.6 Fluorescence spectra of YBO3 excited at160 nm. The intrinsic emission is
shown expanded by a factor of 100
The time-resolved emission exhibits a fast and slow component for all samples,
as shown in Fig. 7. The decay time of the fast component is very short, less than 2 ns,
but this is not shown in this figure because of the larger input impedence used to
obtain this data which limits the time resolution to about 1 µs. The slow component
has a decay time of about 36 µs in the undoped sample. The addition of 1% Gd3+ has
very little affect on the dynamics of the slower decay component, but with 5% Gd3+ it
does decrease somewhat (31 µs). This is in sharp contrast to ScBO3 which exhibited a
single exponential decay which became much faster with the addition of Gd3+. The
spectra of the fast and slow components, shown in Fig. 8, reveal that there are two
independent emission features associated with the two decays peaking at 285 nm (fast)
and 325 nm (slow). We suggest that the fast component is the intrinsic emission
whereas the slow component, which dominates the intensity of the undoped sample,
82
arises from some defect center. The peak of the fast component occurs at a slightly
longer wavelength than that of the intrinsic emission of ScBO3 which peaks 240 nm.
YBO3
Excited at 157 nm
λ=340 nm
-3
Intensity
10
-4
1x10
Undoped
1% Gd
5% Gd
-5
1x10
-6
10
0
20
40
60
80
100
Time (µs)
Fig. 5.7 Time resolved emission excited at 157 nm and detected at 340 nm. The
decay is a double exponential. The short decay component in the figure is
lengthened by the 5.9 kΩ oscilloscope input impedence. Its actual decay time is
< 2 ns.
83
slow
325 nm
t=0
Intensity (arb. units)
200
YBO3
λex= 157 nm
time-avg.
undoped
1% Gd
5% Gd
fast
285 nm
250
300
350
400
450
500
550
Wavelength (nm)
Fig. 5.8 Time-resolved emission spectra excited at 157 nm. The t=0 spectrum is
obtained from the initial intensity of the fast decay component. The spectrum of
the slow decay component was obtained from the intensity at 400 ns after the fast
component had decayed. It is identical to the time-averaged emission spectrum.
The excitation spectra, shown in Fig. 9, indicate that the total emission intensity
grows with the introduction of Gd3+ and that the 5% sample shows an estimated
absolute quantum yield of about 0.5 for excitation at 172 nm, about 60% as great as
for ScBO3 containing 5% Gd3+. The spectral features in the excitation spectra are
similar for the doped and undoped samples. Reflectance measurements indicate that
the band gap of YBO3 is at 7.65 eV (163 nm) [17]. This is just the spectral region
where the quantum yield drops. It is likely that this is related to the onset of very
strong absorption resulting in excitation close to the phosphor particle surface where
non-radiative processes can cause a decrease in quantum yield. The peak at 170 nm is
perhaps associated with excitation of the molecular BO3-3 group.
84
Quantum Yield Relative to NaSal
1.0
YBO3 Excitation Spectra
0.8
5% Gd
1% Gd
undoped
0.6
0.4
0.2
0.0
140
160
180
200
220
240
260
Wavelenth (nm)
Fig. 5.9 Excitation spectra of undoped and Gd3+-doped YBO3 detecting the total
emission and measured relative to that of sodium salicylate.
For excitation at 157 nm the total absolute quantum yield of the undoped sample
is about 0.03.
Since approximately 10% of this occurs in the fast emission
component, it can be assumed that the short lifetime results from energy transfer to
non-radiative killer sites. The radiative lifetime would then be about 300 times the < 2
ns measured lifetime. Thus the radiative rate of the intrinsic emission in YBO3 is less
than 600 ns. Because the quantum yield increases with Gd3+ concentration due to
energy transfer to Gd3+ (evidenced by the appearance of 6P emission from Gd3+), the
Gd3+ must compete effectively with the killer sites. Thus the energy transfer to Gd3+
appears to be faster than in ScBO3; i.e. at room temperature it is likely that more rapid
energy migration of the exciton occurs in YBO3 than in ScBO3. It is possible that the
85
killer sites are associated with the slow component of the emission but this cannot be
proven.
As seen in Fig. 2, there is some excitation of the 6G level of Gd3+ as evidenced
by the weak emission at 600 nm, as was the case for ScBO3:Gd3+. The decay of this
emission is also shown on Fig. 5 and is similar to that of ScBO3 with decay times of
260 and 200 µs in the 1% Gd and 5% Gd samples, respectively. This is consistent
with the poor overlap of the fast component of the emission with the 6G absorption
which occurs for λ<204 nm.
5.3.3 ScPO4
The ScPO4:Gd3+ samples are definitely the most interesting of all studied in the
present paper from the point of view of energy transfer to the high energy excited
states of Gd3+ and for enabling quantum cutting. In contrast to the borates, ScPO4
phosphate does sensitize the 6G state of Gd3+ with considerable efficiency.
The
emission spectra of ScPO4, undoped and doped with 1% Gd3+, are shown in Fig.10.
The host emission has been ascribed to STEs [18] or intramolecular transitions of the
phosphate group[19].
Intrinsic emission from the host excitations, seen in the
undoped sample, consists of two broad features whose maxima are at 215 and 320 nm.
Emission from undoped ScPO4 under 140 nm excitation at 10K has been reported at
211, 350 and 470 nm [20]. The 211 nm emission can only be excited with excitation
energies above the band gap whereas the other bands can be excited at longer
wavelengths, supporting their assignment to impurities or defects. Indeed, single
crystals of ScPO4 have been reported to exhibit emission from a variety of impurities
[17]. Energy transfer from the intrinsic excitations to Yb3+ has been observed [19].
In a sample doped with 1% Gd3+, strong quenching of the host emission with the
appearance of intense emission from Gd3+ suggests the efficient energy transfer from
the host to Gd3+ ions. Even in the undoped sample, weak 6P emission from Gd3+ is
observed due to some residual impurity level. In the 1% Gd3+ sample a very weak
intrinsic luminescence persists. Note that in addition to the 6P emission, strong 6G →
8
S emission is observed at 204 nm along with 6G → 6P and 6G → 6I emission near 600
86
and 770 nm, respectively, indicating considerable sensitization of 6G. The integral 6G
emission intensity is of the same order of magnitude as 6P→8S emission at 313 nm.
40000
6
8
G to S
30000
8
25000
YPO4 undoped
YPO4 Gd 1%
ScPO4 undoped
ScPO4 Gd 1%
P to S peak at 430000
35000
cw 160 nm
STE
20000
6
Relative quantum yield (arb. units)
pulsed
157 nm
6
6
G to P
15000
10000
6
6
G to I
5000
0
200
300
400
500
600
700
800
Wavelength (nm)
Fig. 5.10 Emission spectra of ScPO4, and YPO4 excited at 160 nm.
The excitation spectra of Gd3+-doped and undoped ScPO4 samples are shown in
Fig.11. Since the emission consists of contributions through the UV, visible and near
IR, the excitation spectra were obtained separately in the different spectral regions.
The UV and blue emission was isolated with filters and referenced to the sodium
salicylate emission and the red and near IR emission was compared to that of
Y2O3:Eu5% whose absolute quantum yield as a function of wavelength is well
established [15]. All excitation spectra show the same general spectral features, with a
sharp onset at about 180 nm, indicating that they all result from the same initial
excitation centers and that the intrinsic center transfers its energy to Gd3+. The
undoped sample (dashed curve in Fig. 11) produces a quantum yield whose maximum
value is 0.23 relative to sodium salicylate at 170 nm, or an absolute quantum yield of
87
0.14. For the 1% Gd3+-doped ScPO4, the UV emission from 6P and 6G (light solid
line) shows a maximum quantum yield of 1.2 relative to sodium salicylate, or an
absolute quantum yield of 0.7. The quantum yield of the red and near IR emission
relative to that of Y2O3:Eu5%, (dotted line) is about 0.4. The maximum absolute
quantum yield of 0.25, based on the known absolute quantum yield of Y2O3:Eu5% at
170 nm of 0.6, [20] also occurs at 170 nm. The total absolute quantum yield (bold
solid line) is then obtained as the sum of these two contributions. It reaches as
maximum value of 0.92±0.2. Thus this could be an excellent phosphor if the Gd3+ ions
Quantum Yield / Relative Quantum Yield
can transfer their energy to a visible emitting activator.
1.2
ScPO4:1% Gd
relative to NaSal
(200 nm<λem<350 nm)
1.0
0.8
Estimated Absolute
Quantum Yield
0.6
0.4
0.2
ScPO4:1% Gd
relative to Y2O3:5% Eu
(λem>550 nm
0.0
140
150
160
ScPO4-undoped
relative to NaSal
170
180
190
200
Wavelength (nm)
Fig. 5.11 Excitation spectra of undoped and Gd3+-doped ScPO4. The doped sample
is referenced to sodium salicylate (dashed curve). The excitation of the UV portion
of the emission is measured relative to sodium salicylate (thin solid curve) while
the red portion of the emission is referenced to Y2O3:5%Eu3+ (dotted curve). The
estimated absolute quantum yield is shown by the bold solid curve
88
The relative contributions of the 6G and 6P emission can be estimated from their
relative quantum yields. The result is that about 20% of the total emission occurs
from 6G in ScPO4 containing 1% Gd3+. In the ideal circumstance for photon cascade
emission, all Gd3+ ions would be excited to 6G and would radiate in two steps, first to
6
P and then from 6P to the 8S ground state, producing equal contributions in the two
spectral regions.
If one assumes no non-radiative losses, the observed relative
contributions indicate that only about 25% of the Gd3+ ions start from the 6G state; the
remainder are excited to 6P, 6I, or 6D, where the latter two states rapidly decay to 6P
through multi-phonon emission. Indeed, as noted above, the 215 nm broad intrinsic
emission band overlaps both the 6G and 6D Gd3+ absorptions. In addition, the 320 nm
feature is strongly resonant with the 6P and perhaps the 6I absorptions. Therefore it is
to be expected that energy transfer will populate all of these states.
The results of time-resolved fluorescence measurements in ScPO4:1%Gd3+
confirms the above conclusions. The decay of the intrinsic emission is shown in are
shown in Fig. 12 along with an exponential fit to the data. The decay of the intrinsic
emission at 215 nm in the undoped sample is 75 ns after subtracting the instrumental
contribution.
89
10
-1
ScPO4
Excited at 157 nm
λem=220 nm
Undoped
Gd 1%
τfit=80 ns
-2
Intensity
10
τfit=13 ns
10
-3
10
-4
100
200
300
400
500
600
Time (ns)
Fig. 5.12 Time-resolved emission of undoped and Gd3+-doped ScPO4 excited at
157 nm. The fits are shown by the dashed lines and include a 5 ns instrumental
contribution
This is much longer than the 9 ns decay time reported in nominally undoped
single crystals prepared by flux growth [17]. The decay at 220 nm has been remeasured in single crystals supplied by Lynn Boatner of Oak Ridge National
Laboratory and a decay time of 130 ns was observed. This is value is much closer but
slightly longer than the 75 ns value obtained in the phosphor powders. In the sample
containing 1% Gd3+ the fitted decay time is 13 ns, yielding an actual measurable decay
time of 8 ns. This is assumed to arise from energy transfer to Gd3+ but the buildup of
the Gd3+ emission cannot be obtained with sufficient resolution. The dynamics of the
6
G and 6P states of Gd3+ are shown in Fig. 13 where 6G is obtained from emission
detected at both 206 nm and 600 nm while the dynamics of 6P is determined from the
emission at 313 nm. The 78 µs decay of Gd3+ 6G state is due to radiative transitions to
the 6P state: the corresponding build-up of 6P population may be seen in the 6P→8S
90
fluorescence kinetics which can be described by the same rate as the 6G decay. This
build-up makes up about 25% of the total 6P→8S emission intensity; 75% of the
excited population goes to 6P much faster (ns time scale). This means that the ratio of
the numbers of ions excited by energy transfer to 6G and 6P states is about 1:3; most of
the ions are excited by energy transfer from the host excitations directly to 6P or to 6P
via 6D and 6I. This is in close agreement with the conclusions based on the excitation
spectra where the ratio was determined to be 1:4.
The experimental results show that ScPO4:1%Gd3+ exhibits quantum cutting due
to the Gd3+ cascade emission process 6G →6P→ 8S. It is favorable for quantum cutting
that the dominant transition from 6G occurs to the 6P state, yielding visible emission in
the red. However, it is unfortunate that most of Gd3+ ions are excited by energy
transfer from the host states directly to 6P (or perhaps 6D or 6I) preventing the
occurrence of a quantum efficiency exceeding unity. In order to obtain a more
efficient phosphor material it will be necessary to find a material with a better
branching ratio for the transfer of energy to 6G rather than 6P population. Since the
second photon in the cascade emission occurs in the UV from 6P, it will also be
necessary to incorporate a second ion that emits in the visible which can receive the 6P
energy via an energy transfer process.
91
ScPO4:1% Gd
Intensity (arb. units)
6
8
P--> S
315nm
206nm
600nm 10-3
6
τfit=78 µs
8
G--> S
-4
10
0.0000
6
6
G--> P
0.0000
0.0002
0.0001
0.0002
Time (s)
0.0004
0.0006
0.0003
0.0008
Time (s)
Fig. 5.13 Time-resolved emission of ScPO4:1%Gd excited at 157 nm and
detected at 206nm and 600 nm (Gd3+ 6G emission) and 315 nm (Gd3+ 6P emission).
The inset shows the fit of the 6G decay.
5.3.4 YPO4
YPO4 also emits intrinsic emission in the UV and deep UV as shown in Fig. 10.
As for ScPO4 there are two broad emission bands, one centered at 240 nm and the
other at 400 nm. The emission is considerably weaker than that of ScPO4. Doping
YPO4 with 1% Gd3+ completely quenches the 240 nm emission feature and strong 6P
emission from Gd3+ is evident. A very weak 6G emission is observed at 600 nm (to
6
P) and 204 nm (to 8S7/2) whose lifetime is 72 µs. The excitation spectra of the doped
and undoped samples, under various detection conditions are reported in Fig. 14. All
the excitation spectra show an onset beginning at 180 nm. The excitation spectra for
the undoped sample differ somewhat depending on the detection wavelength range.
92
There appear to be two bands whose relative strengths are such that the shorter
wavelength excitation feature dominates for detection at longer wavelengths and visa
versa for detection at shorter wavelengths. Thus there appear to be two independent
sources in the excitation spectrum. Two features are also observed in the Gd3+-doped
YPO4 fluorescence excitation spectrum. The excitation spectrum of Gd3+-doped YPO4
is in agreement with that reported by Nakazawa [21] who assigned the feature at 152
nm to the host lattice absorption. The 4f65d and charge transfer bands of Gd3+ are at
much higher energies.
Relative Quantum Yield (arb. units)
0.12
YPO4:1% Gd (λem > 280 nm)
YPO4 (λem > 190 nm)
0.10
YPO4 (λem=210-320 nm)
0.08
0.06
0.04
YPO4 (λem > 280 nm)
0.02
YPO4:1% Gd (λem > 345 nm)
0.00
140
150
160
170
180
190
Wavelength (nm)
Fig. 5.14 Excitation spectra of undoped and Gd3+-doped YPO4 for detection in
different wavelength regions showing the dependence of the spectra on detection
wavelength
For detection wavelengths greater than 280 nm in the Gd3+-doped sample,
which includes essentially all of its emission, the 152 nm band is stronger; for
detection of the weak emission at wavelengths greater than 345 nm, two features of
93
nearly equal relative intensity are seen. The two peaks in the excitation spectra
probably correspond with the two distinct host states observed in the fluorescence
spectra. The strong peak at 152 nm, observed in excitation spectra of the total (Gd3+ +
host) emission may suggest a more efficient energy transfer from the higher energy
host excitations to Gd3+. However, the total quantum yield relative to sodium
salicylate is below 0.1 for both the doped and undoped samples.
The dynamics of the host emission is shown in Fig. 15. The dynamics are quite
unusual. First consider the undoped sample (solid curves). The decay of the emission
detected at 240 nm is exponential with a decay time of 380 ns as shown by the fit to
the data. This is about five times great than the lifetime in ScPO4. However, it does
not appear immediately but rather has a rise time of 55 ns as shown by the fit to the
time dependence of the 240 nm emission (dashed curve).
The 400 nm longer
wavelength emission feature exhibits a double exponential decay and does not display
any detectable rise time. The faster decay component is described by a 55 ns decay
time, the same as the 240 nm emission rise time; this is shown by the fit (dashed
curve). The longer decay time is 600 ns. The nearly identical dynamics of the 240 nm
buildup and 400 nm fast decay component suggest that the center responsible for the
400 nm feature feeds the center corresponding to the 240 nm feature. This does not
violate energy conservation provided that the 400 nm emission results from a very
large Stokes shift. In this way, its excited state energy may still be above that of the
center producing the 240 nm emission. The dynamics of the Gd3+-doped sample is
nearly identical to that of the undoped sample, except that the 240 nm feature is totally
absent. This can occur if the center responsible for the 240 nm emission transfers its
energy very efficiently to the Gd3+ ions. The small affect of Gd3+ on the 400 nm
emission implies that energy transfer from this center to Gd3+ is inefficient.
94
0.1
YPO4
Excited 157 nm
240 nm
55 ns rise
380 ns decay
0.01
Intensity
undoped
1% Gd
fit(undoped)
1E-3
340 nm
460 nm
55 ns decay
600 ns decay
1E-4
1E-5
0.000000
0.000001
0.000002
Time (s)
Fig. 5.15 Time-resolved emission of undoped (solid curves) and Gd3+-doped
(dotted curves) YPO4. The 240 nm emission shows a 55 ns buildup and 380 ns
decay while the emission at longer wavelengths (340nm and 460 nm shown in the
figure) exhibit a decay with two components. The dashed curves show fits to the
240 nm and 460 nm data for the undoped sample.
5.4 ENERGY TRANSFER RATES
The decay rates of the intrinsic and Gd3+ emission for the undoped and doped
samples are summarized in Table. 1. It should be possible to compare the observed
energy transfer rates from the host to the Gd3+ ions to estimates based on dipole-dipole
mediated Forster-Dexter energy transfer theory. The Forster-Dexter energy transfer
rates can be represented by the expression [22,9].
PABdd = (1.4*1024 fhost fGd S) /(∆E2 R6).
Eq. 5.1
95
where ∆E is the transition energy in eV, fhost and fGd are the oscillator strengths of the
host emission and Gd3+ absorptions, respectively, S is the spectral overlap expressed
in units of cm-1 of the host emission with the Gd3+ absorption and R is the distance
between the host excitation and Gd3+ expressed in Angstroms.
72 µs
55 ns decay
900 ns decay
55 ns decay
600 ns decay
400 nm
55 ns rise
240 nm
YPO4
320 nm
130 ns
215 nm (crystal)
380 ns decay
absent
3.2 ms
78 µs
4.8 ms
75 ns
215 nm (powder)
ScPO4
8 ns
260 µs 200 µs
4.5 ms
<2 ns <2 ns
< 2 ns
285 nm
YBO3
15 ns
195 ns
238 nm
5%
1%
3 ns
4.0 ms
1%
450 µs 320 µs
5%
G
6
Gd doped
undoped
ScBO3
Host emission
wavelength
Host emission (STE)
decay time
6
P
Gd3+ decay time
Table 5.1 Wavelengths and decay times of the emission of undoped and Gd3+ doped
scandium and yttrium borates and phosphates
96
An upper limit of the radiative decay rate of the intrinsic emission is given by its
observed decay rate. However, since the quantum yield of the intrinsic emission is of
order 1 in all but YPO4, the observed decay rate must, in fact, be close to the radiative
rate, typically 106-107 s-1, based on the data in Table 1. Such a decay rate corresponds
to an oscillator strength fA~10-3 to 10-2. For Gd3+ the absorptive transitions are spinforbidden so that oscillator strengths of about 10-6 to 10-7 are to be expected. The
overlap can be estimated as 2x10-4 cm-1 based on the observation that the host broad
band emission has a bandwidth that is much greater than that of Gd3+ and is about
5000 cm-1. Dipole-dipole mediated energy transfer rates for typical nearest neighbor
cation distances of 3.7 A are then estimated from Eq. 5.1 to lie between 5 x 105 to 5 x
107 s-1. For a 1% Gd3+ concentration, the typical nearest neighbor distance between a
localized host excitation and Gd3+ ion is about 7 A yielding estimated energy transfer
rates of 104 - 106 s-1. Since there are a number of Gd3+ ions in the vicinity of a host
excitation these estimates should probably be increased to the range 105-107 s-1. The
observed rates in these borates and phosphates doped with 1% Gd3+ are at least 108 s-1,
a value that is one to three orders of magnitude greater than the rates estimated based
on dipole-dipole Forster-Dexter energy transfer.
This suggests that the host
excitations are mobile allowing them to sample the whole lattice such that they spend
a fraction of their time as a nearest neighbor of the Gd3+ where the dipole-dipole
interactions will be about a factor of 100 larger or where much larger exchange
interactions can provide an additional energy transfer mechanism.
97
5.5 CONCLUSIONS
Yttrium and scandium borates and phosphates all exhibit intrinsic emission in
the UV and the lifetimes of these intrinsic emissions were determined. Efficient
energy transfer from the host excitations to Gd3+ was observed in the Gd3+-doped
materials showing that host sensitization occurs in these materials. A comparison of
the observed and estimated theoretical rates suggest that the host excitations are
mobile at room temperature. For ScPO4:1%Gd3+ about 30% of the energy transfer
from the host excitations to Gd3+ occurs to the 6G state, demonstrating for the first
time host sensitization of the 6G state of Gd3+. This excitation is followed by cascade
emission of photons making possible quantum cutting in which one visible and one
UV photon are emitted. Absolute quantum yields were determined for all samples
with measured values of 0.92 and 0.8 in ScPO4:Gd and ScBO3:Gd, respectively. If the
Gd3+ 6P excitation can be transferred efficiently to another ion emitting visible
radiation, these Gd3+-doped materials could be competitive with existing VUV excited
phosphors.
Acknowledgements
We acknowledge the support of the U.S. National Science Foundation, Grants
0305400 (RSM) and 0305449 (DAK). We thank Dr. Lynn Boatner of Oak Ridge
National Laboratory of the single crystal samples of ScPO4. We appreciate helpful
discussion with Drs. Madis Raukas and Kailash Mishra of OSRAM SYLVANIA.
98
REFERENCES
[1] Piper, W.W., DeLuca, J.A. and Ham, F.S., Journal of Luminescence, 8, 344, 1974.
[2] Sommerdijk, J.L., Bril, A.and de Jager, A.W., Journal of Luminescence, 8, 341,
1974.
[3] Wegh, R.T., Donker, H. and Meijerink, A., Physical Review B 56 13841, 1997.
[4] Yang, Z., Lin, J.H., Su, M.Z., Tao, Y. and Wang, W., Journal of Alloys and
Compounds. 308, 94, 2000.
[5] Wegh, R.T., Donker, H., Oskam, K.and Meijerink, A., Science 283, 663, 1999.
[6] Wegh, R.T., Donker, H., Oskam, K. and Meijerink, A., Journal of Luminescence,
82, 93, 1999.
[7] Dorenbos, P., Journal of Luminescence, 91, 155, 2000.
[8] Peijzel, P.S., Schrama, W.J.M. and Meijerink, A., Molecular Physics 102, 1285,
2004.
[9] Jia, W., Zhou, Y., Feofilov, S.P., Meltzer, R.S., Jeong, J.Y. and Keszler, D.,
Physical Review B 72, 075114, 2005
[10] Babin, V., Oskam, K.D., Vergeer, P. and Meijerink, A., Radiation Measurements,
38, 767, 2004.
[11] L.H. Brixner and G. Blasse, Chemical Physics Letters, 157, 283 (1989).
[12] Lin J., Su, Q., Journal of Alloys and Compounds, 210, 159 (1994).
[13] W. Hayes and A.M. Stoneham, Defects and Defect Processes in Non-metallic
Solids, J. Wiley, New York 1985.
[14] Dexter, D. L., Journal of Chemical Physics, 21, 836, 1953.
[15] J.K. Berkowitz, and J.A. Olsen, Journal of Luminescence, 50, 111, 1991.
[16] I. N. Orgorodnikov, V. A. Pustovarov. A. V. Kruchalov, L. I. Isaenko, M. Kirm,
G. Zimmerer, Phys, Solid State 42, 464 (2000) A. Meijerink, G. Blasse, M.
Glassbeek, Journal of Physics: Condensed Matter, 2, 6303 (1990)
[17] A. Mayolet, J.C. Krupa, J. SID, 4, 179 (1996).
99
[18] A. Trukhin and L.A. Boatner, Materials Science Forum, 239-241, 573 (1997).
[19] E. Nakazawa and F. Shiga, Journal of Luminescence, 15, 255 (1977) and 1 X.
Wu, H. You, H Cui, X. Zeng, G. Hong, C-H. Kim, C-H. Pyun, B-Y. Yu and C-H.
Park, Materials Research Bulletin, 37, 1531 (2002).
[20] L.van Pieterson, M.Heeroma, E. de Heer and A. Meijerink, Journal of
Luminescence, 91, 177, 2000.
[21] E. Nakazawa, Journal of Luminescence, 100, 89 2002).
[22] T. Kushida, Journal of Physical Sociey of Japan, 34, 1334, 1973.
100
CHAPTER 6
LUMINESCENCE OF LANTHANIDES DOPED GdZrF7
Joayoung Jeong and Douglas A. Keszler
Oregon State University, Department of Chemistry
Corvallis, OR 97331-4003
101
ABSTRACT
Single phase polycrystalline GdZrF7 compounds doped with several lanthanides
of Eu3+,Pr3+,Ce3+,Tb3+,Tm3+have been synthesized. The VUV luminescence characters
have been investigated using optical spectroscopy. GdZrF7 doped with Eu3+ and or
Pr3+ shows white emission with quantum yield of almost unity under 180nm excitation.
6.1 INTRODUCTION
There is considerable interest in phosphors that can be excited in the vacuum
UV (VUV) for applications in mercury-free fluorescent lamps and plasma displays.
We report here on a new highly efficient nearly white phosphor.
Kolk et al reported a VUV excitation study of Pr3+ doped LaZrF7.[1] They
demonstrated photon cascade emission (PCE) of Pr3+ when Pr3+ is excited to its charge
transfer band or to its 4f5d states. However, energy transfer from the STE to
Pr3+occurred predominantly to 3P0 and not 1S0 as required for PCE. Here we study the
related material, GdZrF7 doped with various lanthanides. We show that Eu3+ doped
GdZrF7 produces a nearly white emission with high quantum efficiency in the VUV.
While it does not exhibit quantum cutting, which could yield quantum efficiencies of 2,
it does perform with quantum efficiency near 1. In this paper we study GdZrF7 doped
with Eu3+ and with several other lanthanides for their potential as VUV phosphors.
The VUV emission and excitation spectra are investigated and quantum yields are
determined.
6.2 EXPERIMENT
6.2.1 SAMPLE PREPARATION
All samples were synthesized by solid state reaction. GdF3 (Alfa Aesar,99.99%),
ZrF4 (Aldrich,99.99%), NH4F (Aldrich,99.99%) and lanthanides of Eu2O3 (Stanford
102
Materials Corporation,99.99%), Pr6O11 (Alfa Aesar,99.99%), TbF3 (Alfa Aesar,99.9%),
Tm2O3 (Stanford Materials Corporation,99.99%), Ce(NO3)3·6H2O (Aldrich,99.9%)
depending on the kind of dopant were mixed well in stoichiometric ratio and was
charged into carbon crucible capped with another carbon crucible to provide the raw
mixture less oxygen atmosphere during heating. The ZrF4 was added 12 mol% in
excess than the stoichiometric ratio. The carbon crucibles were put into a bigger
aluminium crucible covered with lid and the space between the carbon crucibles and
alumina crucible was filled with carbon powder. The heating was carried at 730750°C during 1.5-2hs.
6.2.2 MEASUREMENT
All polycrystalline samples were measured in their structural character by x-ray
powder diffraction method. The diffraction data was collected at Siemens D5000
Diffractormeter using an in-house program and the λ=1.572 of Cu Kα radiation. The
reference x-ray pattern of GdZrF7 was calculated from the single crystal structure
solution data of our other research work.
All spectra were measured at room temperature. Emission spectra were obtained
by exciting the sample, contained in vacuum, with a deuterium lamp spectrally filtered
with an Acton VM-502 VUV monochromator. The visible and UV emission was
dispersed with an Acton Spectrapro-150 spectrometer and was detected with a SBIG
ST-6I CCD camera. All emission spectra were corrected for the wavelength-dependent
response of the detection system.
Excitation spectra were obtained by scanning the VUV monochromator, illuminated
by the deuterium lamp, while detecting the emission with a PMT after passing the
luminescence through colored glass or interference filters. The excitation spectra of
each sample were compared to that of a reference sample of sodium salicylate or
Y2O3:5% Eu3+. The blue and UV parts of the emission spectra were referenced to
sodium salicylate and the red regions of the spectra were referenced to Y2O3:5% Eu3+.
The absolute quantum yields were estimated based on absolute quantum yields of 0.6
103
for both of these reference standards over the UV and VUV for sodium salicylate and
at 160 nm for Y2O3:5% Eu3+.
6.3 RESULTS AND DISCUSSION
6.3.1 GdZrF7: undoped
Undoped GdZrF7 emits a strong broad emission band covering much of the
visible and the near UV as shown in Fig. 1. We assign this emission to that of the selftrapped exciton (STE). A STE emission was reported for LaZrF7 by van der Kolk and
coworkers at somewhat shorter wavelengths [1] but their LaZrF7 has a different crystal
structure as discussed below. The undoped GdZrF7 also exhibits a strong Gd3+ 6P
emission at 313 nm. Since Gd3+ has only parity and spin-forbidden transitions in the
VUV, its appearance indicates energy transfer from the STE to Gd3+. The overlap of
the high energy tail of the STE emission with Gd3+ 6P absorption provides a
mechanism for the energy transfer and the very poor overlap may explain the
incomplete transfer from the STE despite the fact that Gd is stoichiometric in the
sample.
6.3.2 GdZrF7:1%Eu3+
Following energy transfer from the STE to the 6P level of Gd ion a second
energy transfer occurs to the Eu3+ ions. Fig1 is the emission spectrum of GdZrF7
doped with Pr3+ and/or Eu3+.
104
120000
GdZrF7 doped and undoped
Excited λ= 160 nm, T=300 K
100000
a)undoped
b)1% Eu
c)1% Pr
d)1% Eu, 1% Pr
Intensity
80000
5
7
D0
60000
40000
J=1 2
5
D1 7F
J
5
FJ
3
4
J=1 2 3
7
D2 FJ J=1 2 3
D3 7FJ J=1 2 3
3+
Pr
20000
5
0
200
300
400
500
600
700
800
Wavelength (nm)
Fig. 6.1. Emission spectra of GdZrF7 a) undoped, b) doped with 1% Eu3+, c) doped
with 1% Pr3+, d) double doped with 1% Pr3+ and 1% Eu3+ under 160nm excitation.
The emission spectrum of the Eu3+-doped sample is composed of the STE broad
emission band peaking in the blue region and several emission peaks from the 5DJ
levels of Eu3+. The peaks from Eu3+ were assigned using the energy level structure of
trivalent lanthanides in LaF3 by W.T.Carnel [2]. The strong emission peak at 619nm
from the 5D0-7F2 transition is a sign of the hypersensitivity of Eu3+ emission at the non
symmetric site. Michel Poulain at 1972 [3] solved the crystal structure of SmZrF7 and
explained that SmZrF7 will be isostructural with lanthanide fluorozirconates LnZrF7
compounds (Ln=lanthanides, Y) in the space group of P21. We recently grew the
single crystal and solved the crystal structure of GdZrF7. Its basic frame of crystal
structure is almost same with SmZrF7 except one type of F- ion is disordered into two
positions resulting in P21/m space group [4]. According to our solution the Zr4+ ion is
six fold coordinated in a ZrF62- cluster with Oh site symmetry and the Gd3+ ion is eight
105
fold coordinated by F1- ions in GdZrF7. However each of the eight Gd-F bond lengths
is slightly different such that the Gd3+ site has a small deviation from inversion
symmetry which may be the origin of hypersensitivity of Eu3+ emission.
Many oxide compounds of borate (e.g. YBO3:Eu3+ emitting in red region), silicate or
phosphate doped with Eu3+ do not show a high level of emission from 5DJ (J>1) in the
blue, or green regions. On the other hand GdZrF7:1% Eu3+ shows several clear
emission peaks in blue and green region together with the red emission. This may be
related to the low phonon energy (ħω) of the host compound. Fluorides have a small
value for the maximum phonon energy, (typically <500cm-1 [5]) and this retards the
non radiative relaxation between two energy levels. The non radiative relaxation rate
can be expressed by Eq. 6.1 [6].
WNR = βel exp (-α(∆E-2ħωmax))
Eq. 6.1
where WNR is the non-radiative multiphonon transition probability, βel and α are
constants, ∆E=1750cm-1 is the energy difference between 5D1 to 5D0 level and ħωmax
is the maximum phonon energy. Schuurumans and van Dijk investigated βel and α in a
wide series of crystals and βel and α values of LaF3 is used here. The calculated non
radiative multiphonon transition rates between the 5D1 and 5D0 levels of Eu3+ using the
maximal phonon energy of 350cm-1 and the βel =1.9*107s-1 and α= 5.3*10-3cm-1 is
7.2*104s-1 which is much smaller than the value of the borate compound (8.4*109s-1)
The non-radiative transition rate from 5D2 to 5D1 and from 5D3 to 5D2 are calculated to
be decreased exponentially into 1.7*103 s-1 and 190s-1 respectively. Another factor to
be considered is the low Eu3+ concentration. The excited Eu3+ can also relax by cross
relaxation between two Eu3+ ions when the Eu3+ concentration is high. For 3% Eu3+ in
Y2O3, the emission spectrum is dominated by the 5D0-7FJ transition [7]. Even the
maximal phonon energy of Y2O3 is small (550cm-1 )[6] the high Eu3+ concentration
may remove the transition from high energy levels (J>1). The broad emission in blue
region is assigned STE emission as is in undoped GdZrF7.
The XRD results showed the samples are almost pure single phase of GdZrF7
with very small impurity peaks at 2theta = 23.3° and 28° coming from GdZr3F15 (Fig
2).
Intensity
-- -
106
2000
1800
1600
1400
1200
1000
800
600
400
200
0
(a)
10
15
GdZrF7:Eu1%
20
25
30
35
40
45
50
55
60
40
45
50
55
60
2θ
Intensity
-
12000
(b)
10000
8000
6000
4000
2000
0
10
15
20
25
30
35
2θ
Fig. 6.2 XRD patterns of GdZrF7:1%Eu3+ (a), the reference XRD pattern (b)
calculated from single crystal structure data.
The quantum yield of GdZrF7:Eu3+ samples are shown in Fig 3 and 1% Eu3+
doped one is estimated to have absolute quantum yield of almost unity under 180nm
excitation [14].
107
1.0
Estimated Absolute Quantum Yield
Excitation Spectra (Estimated Absolute Yield)
0.8
GdZrF7:Eu
Eu 1%
Eu 3%
0.6
0.4
0.2
0.0
140
160
180
200
220
240
Wavelength (nm)
Fig. 6.3 Excitation spectrum of GdZrF7:1%Eu3+ and 3% Eu3+ samples
6.3.3 GdZrF7:Pr3+, (Eu3+)
The emission spectrum was shown in Fig 1. Curve c) is the extended emission
spectrum of 1% Pr3+ doped sample and d) is the extended emission spectrum of double
doped sample with 1% Pr3+ and 1% Eu3+. The emission spectrum of Pr3+ doped
sample is also composed of a broad band and several sharp peaks. The broad emission
band peaking in the blue region is similar to that observed in both Eu3+ doped sample
and in the undoped sample while the sharp emission peaks represent the 4f-4f
transitions of Pr3+ energy states. In LaZrF7:Pr3+ the emission spectrum showed the
transition not only from 1S0 level but also from 3P0 level of Pr3+ resulting in PCE [1].
In Fig 1, the peaks at 480nm, 602nm and 640nm on curve c) are assigned to transitions
from 3P0-to 3H4, 3H6 and 3F2 of Pr3+ respectively while the emission peaks from 1S0
108
level are completely absent. As in LaZrF7,:Pr3+, the 1D2 emission appears to be
quenched. Usually the absence of emission from 1S0 of Pr3+ is explained by the
position of the lowest 5d level of Pr3+. For the occurrence of 1S0 emission, the lowest
5d level of Pr3+ should be located above the 1S0 energy level and under this
circumstance one can obtain PCE. Dorenbos [8] has surveyed the crystal depression
energy (red shift) for a number of compounds and has provided a very useful equation
to evaluate the lowest 5d level of lanthanides in those compounds. However his paper
does not include this information for GdZrF7. Instead we tried to estimate its 5d
position from the excitation spectrum data. From the excitation spectra obtained by
detecting all the emission above 300nm, one can see clearly in both samples doped
with Pr3+ that an additional shoulder appears near 207 nm which can be regarded as
the absorption peak of the lowest 5d level of Pr3+. This can be compared with the
excitation spectrum of LaZrF7: Pr3+ where the lowest absorption peak occurred at
205nm for the lowest 5d level of Pr3+ in the excitation spectrum for detection of either
the 1S0 or 3P0 emissions [1]. The crystal field splitting of a lanthanide ion depends
predominantly on the coordination number and on the type of nearest neighbor anion.
The higher is the coordination number the smaller is the crystal field splitting. In
addition, there is a dependence on the type of nearest neighbor cation such that the
higher is the charge to size ratio of the nearest neighbor cation the smaller is the
crystal field splitting. If the crystal structure of LaZrF7 is assumed isostructural with
GdZrF7 the main difference between GdZrF7 and LaZrF7 is that the cation size of Gd3+
(1.19Å) is smaller than La3+ (1.3Å). The distance between Gd3+ and F1- can be
predicted to be smaller in GdZrF7. Hence the Pr3+ (1.27Å), located at the Gd3+ site in
GdZrF7, should feel a stronger interaction with its F1- neighbors than in LaZrF7. As a
result, in GdZrF7 the Pr3+ ion will experience a higher crystal field splitting such that
the lowest 5d level of Pr3+ in GdZrF7 will be located at a lower energy than in LaZrF7.
However actual data show just a small difference of 2 nm in their lowest 5d levels and
the lowest 5d level of Pr3+ in GdZrF7 is still appears to be 1560cm-1 higher than 1S0
level. The x-ray data of LaZrF7 in Kolk’s paper is different with that of GdZrF7 even
the crystal structure was expected to be isostructural and this may explain the reason
why the estimated result about the lowest 5d level above was not repeated quite well
109
in the experimental data in both compounds. Therefore in GdZrF7 compound the
embedment of the lowest 5d level of Pr3+ and 1S0 level into the strong absorption band
arising from 225nm (Fig 4) will be the reason for the absence of 1S0 emission.
The other distinguishing feature of the Pr3+ doped sample is the strong emission peak
from the 6P level of the Gd ion. The strong 6P emission suggests that the energy
transfer from Gd3+ ion to Pr3+ ion is very inefficient. This is in sharp contrast with
Eu3+ doped GdZrF7 where the low intensity of the 6P emission suggests the efficient
energy transfer from Gd3+ to Eu3+. Only lanthanide ions such as Eu3+ [9], Tb3+ [10,11]
and Dy3+ [12] are known as acceptor ions for energy transfer from Gd3+ via the 6P or 6I
levels.
1.0
Excitation Spectra: GdZrF7
Relative excitation efficiency
(sodium saliclyate reference)
Detection: λ > 280 nm, T=300 K
0.8
0.6
0.4
a)undoped (arb. scale)
b)1% Eu
c)1% Pr
d)1% Eu, 1% Pr
0.2
0.0
140
160
180
200
220
240
Wavelength (nm)
Fig. 6.4 Excitation spectra of GdZrF7 a) undoped, b) doped with 1% Eu3+ , c) doped
with 1% Pr3+, d) double doped with 1%Pr3+ and 1%Eu3+ for whole emission above
300nm
110
To our knowledge there are no studies of energy transfer from Gd3+ to Pr3+ and no
energy gaps between 6G level and 6P levels in Gd3+ has the resonance with any of the
excitation energy into 3PJ(J=0,1,2),1I6 and 3D0 levels of Pr3+ from ground state.
Actually this process is irrelevant because the population of 6G level Gd3+ was not
observed. We think there should be other pathway to populate the 3P0 level of Pr3+
creating emissions at 480nm, 602nm and 640nm.
The STE emission band of GdZrF7 overlaps with the excitation absorptions to the 3PJ
(J=0, 1, 2) and 6I0 levels of Pr (20935~22690 cm-1, [2]) and energy transfer from the
STE could excite Pr3+ to those levels. The weakness of the Pr3 + emission can be
explained by the small amount of Pr3+ which requires energy transfer over relatively
large distances. The weak direct excitation from 5d absorption of Pr3+ could be
another explanation for this Pr3+ emission. The STE emission band also overlaps with
the absorption of the 6P level of Gd and excites Gd into 6P states. However Gd3+ is a
stoichiometric element so that rapid energy transfer competes effectively with the STE
radiative emission favoring a high excitation probability and strong emission intensity.
In case of double doped sample GdZrF7:Pr3+, Eu3+, the total quantum yield still
remained almost the same as that of GdZrF7:Eu3+ (Fig 4). Based on the relative
excitation efficiency in Fig 4 we can conclude the quantum yield of GdZrF7:1%Pr3+
and GdZrF7:Eu3+, 1%Pr3+ are all near unity by comparison with that of GdZrF7:Eu3+
which was shown to have absolute quantum yield of almost unity. However, in the
case of GdZrF7:Pr3+, much of the emission is in the UV (6P→8S of Gd3+) so that the
visible quantum yield is somewhat reduced. The excitation spectrum in Fig 4 showed
that there is no quantum splitting in Eu3+ emission in GdZrF7:Eu3+ compound.
GdZrF7:Eu3+ does not have the absorption peaks of 6G of Gd at around 200nm or may
have weak peaks which is buried by the strong broad absorption band while
LiGdF4:Eu3+ or LiGdF4:Nd3+, known as a nice quantum cutter has distinct absorption
peaks of the 6G level of Gd3+ [9,13]. The emission from Eu3+ in GdZrF7:Eu3+ can not
be explained by the energy transfer via the 6G level of Gd as was the case in LiGdF4
compound. The Eu3+ emission in GdZrF7:Eu3+ will need another excitation source like
STE emission mentioned already.
111
The x-ray diffraction results showed that Pr3+ doped sample and Pr3+ and Eu3+ codoped sample are almost single phase of GdZrF7 compound (Fig 5)
Intensity
---
2400
GdZrF7:Pr 1%
(a)
2100
1800
1500
1200
900
10
15
20
25
30
35
40
45
50
55
60
2θ
---
1800
GdZrF7:Pr1%,Eu1%
(b)
1500
Intensity
1200
900
600
300
0
10
15
20
25
30
35
40
45
50
55
60
2θ
Fig. 6.5 XRD patterns of (a) GdZrF7:1%Pr3+, (b) GdZrF7:1%Pr3+,1%Eu3+
6.3.4 GdZrF7:Ce3+ or Tb3+ or Tm3+
Studies of energy transfer between Ce3+ and Gd3+ have shown this to be
efficient. Ce3+ was used as a sensitizer ion to absorb the excitation light efficiently and
deliver it to an activator that can not be excited directly by the excitation light. The
emission spectrum of GdZrF7:Ce3+ was essentially identical to that of the undoped
GdZrF7 and was composed of a broad emission band peaking in the blue and a strong
112
emission peak from 6P of Gd3+ ion at 313nm. Doping with Ce3+ did not provide any
improved quantum yield nor was there any 5d emission of Ce3+ under 160nm
excitation (Fig 6). This can be understood by estimating the wavelength of absorption
of the lowest 5d level of Ce3+ in GdZrF7 using the previously determined position of
the lowest 5d state of Pr3+ at 275nm [8]. The STE emission would then have little
overlap with the lowest 5d absorption of Ce3+ so that energy transfer to Ce3+ would not
be possible, thereby explaining the absence of 5d emission of Ce3+. As a result, this
also explains its inability to sensitize Gd3+. In Fig 9 curve a) shows the XRD data of
GdZrF7:Ce3+
The addition of Tm3+ or Tb3+ to GdZrF7:Eu3+ produced very little change in the
emission spectrum. The Tb3+ was added with the intension of modifying the color
index of GdZrF7: Eu3+ by creating additional emission in the green region. Tb3+ is well
known to accept energy from Gd via 6I level (about 274nm) [10]. However co-doping
of Tb3+ added only two weak emission peaks at 383nm and 544nm assigned as the
transition from 5D3-7F6 and 5D4-7F5 (Fig 7). The co-doping of 1%Tm3+ to
GdZrF7:1%Eu3+ produced only a loss of quantum efficiency (Fig 8). It did not create
new emission peaks from Tm3+ and instead decreased the intensity of Eu3+ emission a
little bit (Fig 6). The expected blue emission from the 1D2 levels of Tm3+ was not
observed in the emission spectrum. The x-ray data in Fig 9 shows Tm3+ co-doped
sample is single phase of GdZrF7 compound.
113
60000
50000
GdZrF7 Emission Spectra
40000
Excitation λ = 160 nm, T=300 K
a)1% Eu
b)1% Ce
c)1% Eu 1% Tm
Intensity
20000
10000
0
200
300
400
500
600
700
Wavelength (nm)
Fig. 6.6 Emission spectrum of GdZrF7 doped lanthanides. a) Doped with 1%Eu3+, b)
doped with 1%Ce3+, c) codoped with 1%Eu3+ and 1%Tm3+
114
Relative quantum yield (arb. units)
80000
a) Eu 1%
b) Eu 1%, Tb 0.5%
70000
GdZrF
Excited at 160nm
CCD Slit: 400 µm
VUV Slit: 3mm
No filter
Exposure time: 20s
Corrected
60000
50000
40000
30000
20000
10000
0
200
300
400
500
600
700
W avelength (nm)
Fig. 6.7 Emission spectrum of GdZrF7:1%Eu3+,1%Tb3+ for the whole emission
compared to that of GdZrF7:1%Eu3+.
115
Relative excitation efficiency
0.5
GdZrF7
VUV Slit: 400µm
PMT=900V
Filter=WG320
Dark current deducted
uncorrected
0.4
0.3
a) Eu 1%
b) Ce 1%
c) Tm1%, Eu1%
0.2
0.1
0.0
140
160
180
200
220
240
260
280
300
Wavelength (nm)
Fig. 6.8 Excitation spectrum of several GdZrF7 samples doped with lanthanides, a)
doped with 1%Eu3+, b) doped with 1%Ce3+, c) co doped with 1%Eu3+ and 1%Tm3+.
All excitation spectra were measured for whole emission spectrum except b) which
was measured excluding the emission peak of 6P of Gd3+.
116
1800
(a)
GdZrF7:Ce3+
Intensity
---
1500
1200
900
600
300
10
15
20
25
30
35
40
45
50
55
60
2θ
2100
(b)
GZF:1%Tm,1%Eu
---
1800
Intensity
1500
1200
900
600
300
10
15
20
25
30
35
40
45
50
55
60
2θ
Fig. 6.9 XRD patterns of (a) GdZrF7:Ce3+ and (b) GdZrF7:Eu3+,Tm3+
6.4 CONCLUSION
GdZrF7 was synthesized using the solid state reaction and its VUV
luminescence characters were investigated for several lanthanide dopants. In GdZrF7
which is known to be isostructural with SmZrF7 Gd3+ ion locates at the site
coordinated with eight F1- ions. The small distance difference among each of Gd3+-F1bonding can explain the hypersensitivity of Eu3+ emission which shows the strong
emission peak of 5D0-7F2 transition in GdZrF7:Eu3+. The quantum yield of this
phosphor was nearly unity. The Pr3+ in this compound did not show the PCE because
the lowest 5d level of Pr3+ and 1S0 states are included in the strong broad absorption
117
band. The emission of undoped GdZrF7 includes a broad band in blue region which is
assigned as STE emission.
From the experiment of the several lanthanides of Pr3+, Tb3+, Tm3+ and Ce3+ as
dopant we concluded that the STE emission formed from band to band excitation
transfers the excitation energy to 6P level of Gd. Only in the case of Eu3+ was there
efficient transfer from Gd3+. The GdZrF7:1%Pr3+ and GdZrF7:1%Pr3+,1%Eu3+ also
showed high quantum yield of almost unity similar to GdZrF7:1%Eu3+ but they did not
significantly change the color coordinates of the material.
Acknowledgements
Acknowledge National Science Foundation.
REFERENCES
[1] E. Van der Kolk, P.Dorenbos, C.W.E. van Eijk, Optics Communications. 197, 317326. (2001)
[2] Energy level structure and transition probabilities of the trivalent lanthanides in
LaF3, W.T.Carnel, Hannah Crosswhite, H.M.Crosswhite.
[3] Michel Poulain, Marcel Poulain et Jacques Lucas, Material Research Bulletin, 7,
319-326, (1972)
[4] Joayoung Jeong, L.N. Zakharov, Y. Zhou, R.S. Meltzer, D.A. Keszler, in
praperation
[5] A.P. Vink, P.Dorenbos, C.W.E. van Eijk, Journal of Solid State Chemistry, 171,
308-312 (2003)
[6] M.F.H.Schuurmans, J.M.F. van Dijk, Physica B+C, 123, 131 (1984)
[7] G.Blasse, B.C.Grabmaier, Springer,Verlag Berlin Heidelberg, Luminescent
Materials, 1994
[8] P.Dorenbos, Journal of Luminescence, 91, 155-176 (2001)
118
[9] Rene T.Wegh, Harry Donker, Koenrrad D.Oskam, Andries Meijerink, Science,
282, 663-666 (1999)
[10] M.J.J.Lammers, G.Blasse, Physical Status Solidi. (b), 127, 663 (1985)
[11] A.J. De Vries, M.F.Hazenkamp, G.Blasse, Journal of Luminescence, 42, 275-282
(1988)
[12] Yuji Saito, Takashi Kumagai, Shinji Okamoto, Hajime Yamamoto, Takashi
Kunimote, Japanese Journal of Applied Physics, 43,6A, 3456-3460 (2004)
[13] Jia, W.; Zhou, Y.; Feofilov, S. P.; Meltzer, R. S.; Jeong, J. Y.; Keszler, D.,
Physical Review B: Condensed Matter and Materials Physics, 72(7), (2005)
[14] Y. Zhou, J.Y. Jeong, D.A. Keszler, S.P. Feofilov and R.S. Meltzer, 16th
International Conference on Dynamical Processes in Excited States of Solids,
DCP 07.
119
CHAPTER 7
CRYSTAL STRUCTURE AND Eu3+ LUMINESCENCE OF
GdMF7 (M=Hf4+, Zr4+)
Joayoung Jeong, L.N. Zakharov and Douglas A. Keszler
Oregon State University, Department of Chemistry
Corvallis, OR 97331-4003
120
ABSTRACT
The crystals of GdMF7 (M=Hf 4+, Zr44+) has been grown by solid state reaction.
The crystal structure of GdHfF7 was determined by single crystal X-ray diffraction
method. Four Gd3+ ions and four M4+ ions form interlaced square planars respectively
along the [001] direction. The Gd3+ ion at one corner of the Gd3+ square locate at the
center of the M4+ square and the M4+ ion at M4+ square locate at the center of that
Gd3+ square making those two squares interlaced each other. Another layer
composing of Gd3+ square and M4+ square at the same pattern locates below this top
layer along c-direction. However Gd3+ square locates below M4+ square of the top
layer and M4+ square locates below the Gd3+ square of the top layer. This bottom layer
forms a slab with the top layer of next lower unit cell. The VUV emission of GdMF7
doped with Eu3+ was investigated under 160nm excitation.
7.1 INTRODUCTION
The phase diagram research in mixed compounds of LnF3 and HfF4 was carried
out by Fedorov, P. P. and coworkers (Ln=La, Nd, Sm, Gd, Ho, Er, Yb) [1]. They
showed the phases of LnHfF7 (Ln=La-Lu,Y), LnHf2F11 (Ln=La-Nd) and LnHf3F15
(Ln=Sm-Lu,Y) were formed in this binary systems. It was found that the LnHf2F11
compounds when the Ln is light lanthanide are isostructural with the analogous
fluorozirconate compounds with orthorhombic space group Ibam. Koreneve Yu. M
and coworkers indexed the powder x-ray diffraction patterns of the LnHfF7 in a
monoclinic lattice and provided their lattice parameters (Ln=La-Lu,Y) [2]. Michel
Poulain and coworkers solved the crystal structure of SmZrF7 and provided the cell
parameter of GdZrF7 from the powder XRD result [3]. However structural
establishment of single crystal of GdHfF7 or GdZrF7 has not been provided yet. In
luminescence study, there has been no research in gadolinium heptafluorohafnate
based host compound.
In this work we grew the single crystals of GdHfF7 and GdZrF7 and their
crystal structures were determined. The polycrystalline forms of these compounds
121
were also prepared and the VUV luminescence characteristics of Eu3+ in GdHfF7 was
investigated compared to that of Eu3+ doped GdZrF7 from the polycrystalline samples
7.2 EXPERIMENT
7.2.1 Sample preparation
For the powder samples the raw chemicals of 1 mole GdF3 (Alfa Aesar, 99.99%),
1. 2 mole HfCl2O·8H2O (Alfa Aesar, 98+%) were mixed in small amount of alcohol
and dried at oven. The dried raw mixture was ground with NH4F (Aldrich, 99.99%)
and charged into carbon crucible with lid. This carbon crucible was again placed
inside a bigger alumina crucible covered with Alumina lid and the space between the
two crucibles was filled with activated carbon powders. The carbon powders were
used to help prevent the mixtures from exposed to oxidizing atmosphere. This crucible
set was fired at 600°C 2hs and the fired cake was ground. The ground powder was
heated again at 850°C 2h in the same atmosphere. For the GdZrF7 compound 1.12
mole ZrF4 (Alfa Aesar, 99.9%) was mixed instead and the uniformly ground mixture
was heated in the same atmosphere above at 750°C 1.5h. The final powder samples of
GdMF7 (M=Hf 4+, Zr44+) prepared by heat treatment were charged in φ=3mm stainless
(sus#316) tube. This tube was sealed when it was vacuumed. The sealed tube was
heated at 950°C 2h followed by very slow cooling of 6°C/hr till 750°C and cooled
along the cooling speed of furnace.
7.2.2 X-ray crystallography.
Two types of crystals were found; one showed the unit cell of GdF3, the other
type was close to that of SmZrF7 which crystal structure was determined previously
[3]. It can be expected that compounds of GdMF7 are isostructural with SmZrF7. The
full single crystal diffraction studies were carried out with the latter type crystals and it
confirmed that the investigated compounds are GdMF7. (M=Hf4+, Zr4+).
122
X-Ray diffraction data were collected on a Bruker Smart Apex diffractometer
at 173(2) K using Mo Kα radiation (λ=0.71070 Å). Absorption corrections were made
by SADABS [4]. The structure was solved using direct methods and refined with fullmatrix least-squares methods based on F2. Crystal data and some of details of X-ray
diffraction experiment and refinement of the crystal structure of GdMF7 are given in
the Table 1. All calculations were performed using the SHELXTL (v. 6.10) package
[5].
In contrast to the structure of SmZrF7 obtained in space groups P21 [3] and
P21/n [6], the crystal structure of GdMF7 were solved and refined in space group
P21/m. It was found that one of F atoms (a µ2-bridge connecting Gd atoms) is
disordered over two positions related by a mirror plane. Thus in the crystal there are
two zig-zag -Gd-F-Gd-F- chains with different positions of the bridging F atoms, but
with the same occupations. Refinement of the crystal structure of GdMF7 in space
group P21 with different occupations for these two zig-zag -Gd-F-Gd-F- chains show
that such variant of the structure is not so good as the structure in the space group
P21/m.
123
Table 7.1 Crystal data and some of details of X-ray diffraction experiment and
refinement of the crystal structure of GdMF7 (M=Zr, Hf)
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
Unit cell dimensions
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Theta range for data
collection
Index ranges
Reflections collected
Independent reflections
Absorption correction
Max. and min.
transmission
Refinement method
Data / restraints /
parameters
Goodness-of-fit on F2
Final R indices
[I > 2σ (I)]
R indices (all data)
GdZrF7
381.47 u
173(2) K
0.71073 Å
monoclinic
P21/m
a = 6.1012(7)Å
b = 5.7051(7)Å
c = 8.2424(10)Å
α= 90°
β= 103.136(2)°
γ= 90°
279.39(6) Å3
2
4.534 g/cm3
13.694 mm-1
334
2.54 to 27.50°
GdHfF7
468.74 u
173(2) K
0.71073 Å
monoclinic
P21/m
a = 6.097(2)Å
b = 5.6876(18)Å
c = 8.229(3)Å
α= 90°
β= 103.065(5)°
γ= 90°
277.97(15) Å3
2
5.6 g/cm3
30.551 mm-1
398
2.54 to 28.18°.
-7≤ h ≤7, -7≤ k ≤7,
-10≤ l ≤10
3186
700 [R int = 0.0152]
Semi-empirical from
equivalents
0.6841 and 0.4474
-8≤ h ≤8, -7≤ k ≤7,
-10≤ l ≤10
3213
721 [R int = 0.0138]
Semi-empirical from
equivalents
0.4609 and 0.3103
Full-matrix least-squares
on F2
700 / 0 / 52
Full-matrix least-squares
on F2
721 / 0 / 53
1.087
R1a = 0.0221,
wR2b = 0.0524
R1 = 0.0235,
wR2 = 0.0531
1.075
R1 = 0.0261,
wR2 = 0.0664
R1 = 0.0272,
wR2 = 0.0673
0.077(3)
1.597 and -2.034 e/Å3
Extinction coefficient
Largest diff. peak and
1.484 and -1.283 e/Å3
hole
a
R1= Σ (|F0|-|Fc|) / Σ |F0|
b
wR2 = [ Σ {w(F02-Fc2)}2 / Σ wF02 ]1/2
124
7.2.3 luminescence measurement
The VUV emission spectra were obtained by exciting the sample, contained in
vacuum, with a deuterium lamp spectrally filtered with an Acton VM-502 VUV
monochromator. The visible and UV emission was dispersed with an Acton
Spectrapro-150 spectrometer and was detected with a SBIG ST-6I CCD camera. The
emission spectra were not corrected for the wavelength-dependent response of the
detection system. VUV Excitation spectra were obtained by scanning the VUV
monochromator, illuminated by the deuterium lamp, while detecting the emission with
a PMT after passing the luminescence through colored glass or interference filters.
The excitation spectra of each sample were calibrated with a reference sample of
sodium salicylate. All spectra were measured at room temperature
7.3 RESULTS AND DISCUSSION
7.3.1 Crystal Structure
Both GdHfF7 and GdZrF7 compounds have the same crystal structure with tiny
variations in matching bond lengths and bond angles between two compounds. A view
of unit cell of GdHfF7 compound is shown in Fig 1. In both structures the Gd3+ ion is
eight-coordinated with F1- ions forming distorted square antiprismatic structure (Fig.
2) and Hf (Zr)4+ ion is six-coordinated with F1- ions forming an octahedron around the
Hf (Zr) 4+ atom. In the polyhedron of Gd3+ ion the Gd-F distances are being in the
ranges 2.205(8)-2.374(6) Å (Hf) and 2.206(6)-2.380(4) Å (Zr). The octahedron of
Hf (Zr)4+ ions have small variations of the Hf (Zr)-F distances and F- Hf (Zr)-F angles
(table 3 and 4). Both GdZrF7 and SmZrF7 compounds have different structure against
lanthanide fluorozirconate compounds with small size Ln like Er,Tm,Yb and Lu
forming cubic (ReO3 structure) when they are quenched [7].
125
Fig. 7.1 Unit cell drawing of GdHfF7.
126
Fig. 7.2 Two views of the eight coordinated polyhedron of Gd3+ ion in GdHfF7.
127
Fig. 7.3 A [001] directional view of GdHfF7 showing Hf 4+ and Gd3+ ions composing
the squares respectively.
In the view of [001] direction in Fig 3 the four Gd3+ ions are locating at each corner of
a square and four of Hafnium ions are also forming a square. The Gd3+ ion at each
corner of a square is locating at the center position of the Hf 4+ square as if Gd3+ ion is
face centered in the Hafnium square and the Hf4+ ion at each corner of a square locates
vice versa. However those two squares are not exactly on a plane as is shown in Fig 4.
Below this top layer of Gd3+ and Hf4+ square planar structure there is bottom layer
composed of squares of Gd3+ and Hf4+. The different point from the top layer is that
Hf 4+ square in the bottom layer locates beneath the Gd3+ square of the top layer and
Gd3+ square in the bottom layer locates beneath the Hf 4+ square. The [001] directional
view in Fig 3 shows that each of the Gd3+ polyhedron and Hf 4+ octahedron are
connected by corner sharing in the plane of those layers via the fluorine atoms with
forming those two squares of Gd3+ and Zr4+ to be interlaced. The bottom layer form
slabs of [Gd2Hf2F12] 2+ with other top layer of the next lower unit cell along c-direction
and the slabs are connected by [F2] 2- layers as shown in Fig 4. This structure can be
128
described as a succession of [Gd2Hf2F12] 2+ layers and [F2] 2- layers equiberating
charges
Fig. 7.4 A [010] directional view showing the top and bottom layers composed of Gd
squares and Hf squares. Those layers form slabs of [Gd2Hf2F12] 2+ with other bottom
and top layers of next unit cells along c-direction.
129
c
b
Fig. 7.5 A fragment of the crystal structure of the two types of zig-zag -Gd-F-Gd-Fchains structure showing the disorder at F5 position. .
As was found in [2,3] the GdHfF7 is isostructural with monoclinic SmZrF7. It could be
that GdHfF7 compound shall be crystallized in monoclinic system with space group of
P21. However it was found in this research that the position of one of the F1- ion, F5
atoms connecting two Gd3+ ions is disordered over two possible positions related by
mirror plane. This is different point from the analogous of F1- ion whose position is
not disordered in SmZrF7 determined in P21. It indicates that in the crystal structure of
GdHfF7 there are two types of zig-zag -Gd-F-Gd-F- chains with different orientations
of the bridging F atoms (Fig. 5) in ratio 1:1. Only one possible orientation of the
similar –Sm-F-Sm-F- chains was found in SmZrF7 (P21). In another crystal structure
of SmZrF7 determined in space group P21/n there are also two types of chains just as
GdHfF7 does. However there is regular order in the packing of the two zig-zag- Sm-FSm-F- chains while in the GdHf(Zr)F7 positions of the two zig-zag -Gd-F-Gd-Fchains are random. Both Gd-F-Gd and Sm-F-Sm angles are same as 146.2(4).
The ionic size of Hf4+ and Zr4+ (0.85 Å and 0.86 Å respectively) is so similar that
replacing Hf4+ with Zr4+ would not affect the crystal structure a lot.
130
Table 7.2 Atomic position and equivalent isotropic displacement parameters (Å2x
103). U(eq)is defined as one third of the trace of the orthogonalized Uij tensor.
a) GdHfF7
x
y
z
U(eq)
Occupancy
Hf(1)
0.8148(1)
1/4
0.7269(1)
6(1)
0.5
Gd(1)
0.6547(1)
1/4
0.1868(1)
5(1)
0.5
F(1)
0.7587(11)
1/4
0.4778(8)
17(2)
0.5
F(2)
0.8618(11)
1/4
0.9748(8)
12(1)
0.5
F(3)
0.10529(12)
0.0107(12)
0.7370(7)
41(2)
0.5
F(4)
0.5904(14)
-0.0024(17) 0.7158(7)
66(3)
1.0
F(5)
0.4335(14)
0.0859(16) -0.0331(10)
11(2)
0.5
_____________________________________________________________________
b) GdZrF7
x
Zr(1)
Gd(1)
F(1)
F(2)
F(3)
F(4)
F(5)
0.8150(1)
0.6543(1)
0.7582(8)
0.8628(7)
0.10553(8)
0.5897(9)
0.4354(9)
y
z
U(eq)
1/4
1/4
1/4
1/4
0.0125(8)
-0.0031(12)
0.0838(11)
0.7268(1)
0.1867(1)
0.4783(5)
0.9752(5)
0.7377(5)
0.7156(5)
-0.0330(7)
7(1)
7(1)
17(1)
11(1)
42(1)
66(2)
11(1)
Occupancy
0.5
0.5
0.5
0.5
0.5
1.0
0.5
_____________________________________________________________________
Some of the important bond distances between Gd3+ ion and F1- ion and between M4+
ion and F1- ion and bond angles including in both GdHfF7 and GdZrF7 compounds are
listed in table 3 and table 4.
131
Table 7.3 Selected bond lengths [Å] in GdMF7.
GdHfF7
GdZrF7
___________________________________________________________________
Hf(1)-F(1)
Hf(1)-F(2)
Hf(1)-F(3)
2.001(6)
1.995(6)
1.978(5)
Zr(1)-F(1)
Zr(1)-F(2)
Zr(1)-F(3)
1.998(4)
2.002(4)
1.984(4)
Hf(1)-F(4)
Gd(1)-F(1)
Gd(1)-F(2)
Gd(1)-F(3)
1.971(6)
2.334(6)
2.374(6)
2.294(6)
Zr(1)-F(4)
Gd(1)-F(1)
Gd(1)-F(2)#8
Gd(1)-F(3)#4
1.981(4)
2.342(4)
2.380(4)
2.295(4)
Gd(1)-F(4)
Gd(1)-F(5)
2.324(6)
2.205(8)
Gd(1)-F(4)#6
Gd(1)-F(5)#2
2.322(4)
2.284(6)
Gd(1)-F(5) )#1
2.287(9)
Gd(1)-F(5)#1
2.206(6)
___________________________________________________________________
#1 x,-y+1/2,z
#2 -x+1,-y,-z
#4 -x+2,y+1/2,-z+1
#6 -x+1,-y,-z+1
#8 x,y,z-1
132
Table 7.4 Selected Bond angles [°] in GdMF7
GdHfF7
GdZrF7
___________________________________________________________________
F(4)-Hf(1)-F(4)#1
F(4)-Hf(1)-F(3)#1
F(4)-Hf(1)-F(3)
93.5(7)
176.7(4)
89.8(4)
F(3)#1-Hf(1)-F(3)
F(4)-Hf(1)-F(2)
F(3)-Hf(1)-F(2)
F(4)-Hf(1)-F(1)
87.0(5)
89.1(2)
91.3(2)
89.8(2)
F(4)-Zr(1)-F(4)#1
F(4)-Zr(1)-F(3)#1
F(4)#1-Zr(1)-F(3)#1
93.6(4)
176.3(3)
90.1(3)
F(3)#1-Zr(1)-F(3)
F(4)#1-Zr(1)-F(2)
F(3)-Zr(1)-F(2)
F(4)-Zr(1)-F(1)
86.2(3)
89.18(14)
91.10(14)
89.75(15)
90.04(14)
178.43(18)
F(3)-Hf(1)-F(1)
F(2)-Hf(1)-F(1)
89.8(2)
178.4(3)
F(3)-Zr(1)-F(1)
F(1)-Zr(1)-F(2)
F(5)-Gd(1)-F(5)#3
F(5)-Gd(1)-F(3)#4
F(3)#4-Gd(1)-F(3)#5
82.87(13)
142.3(3)
80.5(4)
F(5)-Gd(1)-F(5)#3
83.04(9)
F(5)-Gd(1)-F(3)#4
142.12(18)
F(3)#4-Gd(1)-F(3)#5
81.5(3)
F(5)-Gd(1)-F(4)#6
F(3)#4-Gd(1)-F(4)#6
F(3)#5-Gd(1)-F(4)#6
F(5)-Gd(1)-F(4)#7
72.8(3)
144.89(17)
92.0(3)
102.6(3)
F(5)-Gd(1)-F(4)#6
F(3)#4-Gd(1)-F(4)#6
F(3)#5-Gd(1)-F(4)#6
F(5)-Gd(1)-F(4)#7
73.00(18)
144.88(13)
91.5(2)
103.3(2)
F(4)#6-Gd(1)-F(4)#7
F(5)-Gd(1)-F(1)
F(3)#4-Gd(1)-F(1)
F(4)#6-Gd(1)-F(1)
74.6(5)
144.7(3)
72.89(18)
72.12(19)
F(4)#6-Gd(1)-F(4)#7
F(5)-Gd(1)-F(1)
F(3)#4-Gd(1)-F(1)
F(4)#6-Gd(1)-F(1)
74.7(4)
144.59(17)
73.11(12)
71.92(13)
F(5)-Gd(1)-F(2)#8
F(3)#4-Gd(1)-F(2)#8
F(4)#6-Gd(1)-F(2)#8
F(1)-Gd(1)-F(2)#8
74.0(3)
72.01(18)
138.1(2)
133.4(2)
F(5)-Gd(1)-F(2)#8
F(3)#4-Gd(1)-F(2)#8
F(4)#6-Gd(1)-F(2)#8
F(1)-Gd(1)-F(2)#8
73.92(18)
71.98(12)
138.17(15)
133.36(15)
Hf(1)-F(1)-Gd(1)
Hf(1)-F(2)-Gd(1)#9
Hf(1)-F(3)-Gd(1)#5
174.3(3)
140.8(3)
166.9(3)
Zr(1)-F(1)-Gd(1)
Zr(1)-F(2)-Gd(1)#9
Zr(1)-F(3)-Gd(1)#5
174.4(2)
140.5(2)
167.2(2)
Hf(1)-F(4)-Gd(1)#6
156.8(3)
Zr(1)-F(4)-Gd(1)#6
156.7(2)
Gd(1)-F(5)-Gd(1)#2
146.2(4)
Gd(1)-F(5)-Gd(1)#2
146.9(3)
____________________________________________________________________
Symmetry transformations used to generate equivalent atoms:
#1 x,-y+1/2,z #2 -x+1,-y,-z #3 -x+1,y+1/2,-z
#4 -x+2,y+1/2,-z+1 #5 -x+2,-y,-z+1 #6 -x+1,-y,-z+1
#7 -x+1,y+1/2,-z+1 #8 x,y,z-1 #9 x,y,z+1
133
The powder X-ray diffraction method was used to check the polycrystalline samples
of this compound obtained at different conditions. The XRD pattern for the
polycrystalline sample of GdHfF (Fig 6 a) prepared according to the synthesis process
explained above is in excellent agreement with XRD pattern calculated based on the
single crystal structure solution (Fig 6 b). So the powder sample obtained at such
conditions is pure. The difference in peak intensity ratio is attributed to the preferred
orientation of particles into (002) plane.
3500
a)
Intensity
--
3000
2500
2000
1500
1000
500
0
10
20
30
40
50
60
40
50
60
2θ
12000
b)
--
10000
Intensity
8000
6000
4000
2000
0
10
20
30
2θ
Fig. 7.6 a) Experimental XRD pattern of the powder sample of GdHfF7, b) The XRD
pattern calculated based on the single crystal structure of GdHfF7
134
7.3.2 Luminescence Characteristics
We investigated the luminescence characters of GdMF7 :Eu3+. The 1% Eu3+
doped and undoped polycrystalline samples of gadolinium heptafluorohafnate,GdHfF7
were prepared. In Fig 7 a), and 7 b) the VUV emission spectra under 160nm excitation
are shown and compared to the emission spectra of polycrystalline samples of
gadolinium fluoroheptazirconate, GdZrF7. Inset of Fig 7 is the extended one.
70000
a)
Relative quantum yield (arb. units)
65000
60000
GdHfF 7 undoped
b)
GdHfF 7: Eu 1%
c)
GdZrF 7: Eu 1%
d)
Gd(Hf,Zr)F7: Eu 1%
55000
50000
45000
40000
35000
30000
25000
300
20000
400
500
600
700
15000
10000
5000
0
300
400
500
600
700
Wavelength (nm)
Fig. 7.7 Emission spectrum under 160nm excitation. Emission spectra of a) GdHfF7,
b) GdHfF7:1%Eu3+, c) GdZrF7:1%Eu3+, d) Gd(Hf0.5,Zr0.5)F7:1%Eu3+.
135
GdZrF7 doped with Eu3+ was shown to be a high quantum yield phosphor under
180nm excitation with white emission in our other work [8]. The STE created by the
host intrinsic absorption transfer the excitation energy to the 6P level of Gd3+ and the
second energy transfer from Gd3+ to Eu3+ is followed generating Eu3+ emission. The
emission spectrum of 1% Eu3+ doped GdZrF7 in Fig. 7 c) is composed of a broad band
emission assigned as STE, strong emission peak from 6P of Gd3+ and those sharp
peaks in visible range from 5DJ-7Fj emission of Eu3+ [8].
In case of 1% Eu3+ doped GdHfF7, the 5DJ-7FJ emission of Eu3+ is strong as much as
GdZrF7:Eu3+ while the STE emission is decreased a lot in its emission intensity and
blue shifted (Fig 7 b). This shift of STE is shown more clearly in the emission
spectrum of undoped GdHfF7 sample in Fig 7 a) where we can see one additional peak
at 278nm designated as an emission of 6I of Gd3+ together with the very strong
emission peak at 312nm of 6P of Gd3+. The 6I state of Gd3+ is probably populated by
energy transfer from the STE which is blue shifted and becomes overlapped with 6I
absorptions. The two emissions, especially the 6I emission from Gd3+ ion, are almost
disappeared when Eu3+ is doped meaning the energy transfer from both 6I and 6P of
Gd3+ to Eu3+ are very efficient in GdHfF7. The emission spectrum of the intermediate
compound Gd(Zr,Hf)F7 between heptafluorozirconate and heptafluorohafnate, shows
that the STE has the same characters of heptafluorozirconate with about half emission
intensity while it does not show the emission peak at 278nm observed in pure Hafnate
sample (Fig 7 d)
Fig 8 shows the excitation spectrum in VUV region measured for the whole
emission spectrum relative to sodium salicylate. We can compare the relative
excitation efficiency compared to that of GdZrF7:1%Eu (curve c) which was already
shown to be almost unity in quantum yield [8]. It is clear that the quantum yield is
decreased as the amount of Hf4+ is increased and the pure heptafluorohafnate (curve a)
shows the lowest quantum yield. However the excitation spectrum of the pure
heptafluorohafnate is shifted to high energy consistent with the idea that the STE of
this is blue shifted in emission spectrum.
136
Relative excitation efficiency
1.2
1.0
a)
GdHfF7 undoped
b)
GdHfF7: Eu 1%
c)
GdZrF7: Eu 1%
d)
Gd(Hf, Zr)F7: Eu 1%
0.8
0.6
0.4
0.2
0.0
140
160
180
200
220
240
Wavelength (nm)
Fig. 7.8 Excitation spectrum of GdHfF7 samples compare to other analogous
compound. a)undoped GdHfF7, b) GdHfF7:1%Eu3+, c) GdZrF7:1%Eu3+, d).
Gd(Zr,Hf)F7:1%Eu3+
7.4 CONCLUSION
The single crystal structure of gadolinium heptafluorohafnate, GdHfF7 was
determined by X-ray diffraction methods. The Gd3+ squares and Hf 4+ squares
interlace each other and form top and bottom layers in a unit cell along c-axis direction.
The bottom layer has Hf 4+ square underneath Gd3+ square of top layer and Gd3+
square underneath the Hf 4+ square of top layer. These layers form slabs of
[Gd2Zr2F12] 2+ with other layers of next neighboring unit cells along c-axis and the
slabs are connected by [F2] 2- layers. The luminescence characters of GdHfF7 are
investigated when it is doped with Eu3+. The 1%Eu3+ doped GdHfF7 shows a broad
weak emission band in blue region and 5DJ-7FJ emission of Eu3+ under 160nm
excitation. By comparing with the emission and excitation spectrum of undoped
GdHfF7, 1%Eu3+ doped GdZrF7 and 1%Eu3+ doped Gd(Zr,Hf)F7, we can conclude
137
that the STE emission of the GdHfF7 is blue shifted than the GdZrF7 and the energy
transfer from Gd3+ to Eu3+ is very efficient.
Acknowledgements
We appreciate the help of Hinke Ted, staff in chemistry department of OSU, for
preparation of the vacuum sealed stainless tubes.
REFERENCES
[1] Korenev, Yu. M.; Antipov, P. I.; Novoselova, A. V.; Fedorov, P. P.; Sobolev, B. P.,
Zhurnal Neorganicheskoi Khimii (2000), 45(2), 214-219.
[2] Fedorov, P. P.; Val'kovskii, M. D.; Bondareva, O. S.; Sobolev, B. P. Zhurnal
Neorganicheskoi Khimii, 38(10), (1993),1611-13
[3] Michel Poulain, Marcel Poulain et Jacques Lucas, Material Research Bulletin, 7,
(1972),319-326
[4] G. M. Sheldrick, Bruker/Siemens Area Detector Absorption Correction Program,
Bruker AXS, Madison, WI, 1998.
[5] SHELXTL-6.10 "Program for Structure Solution, Refinement and Presentation"
BRUKER AXS Inc., 5465 East Cheryl Parkway, Madison, WI 53711-5373 USA
[6] Graudejus, O.;Schroetter, F.;Mueller, B.G.;Hoppe, R., eitschrift fuer Anorganische
und Allgemeine Chemie, 620, (1994), 827-832
[7] Michel Poulain, Bruce C. Tofield, Journal of Solid State Chemistry, 39 (1981)
314-318.
[8] Y. Zhou, J.Y. Jeong, D.A. Keszler, S.P. Feofilov and R.S. Meltzer, 16th
International Conference on Dynamical Processes in Excited States of Solids, DCP
07.
138
CHAPTER 8
THE NEW EFFICIENT UPCONVERSION GREEN PHOSPHOR
GdZrF7 :Yb3+,Er3+
Joayoung Jeong and Douglas A. Keszler
Oregon State Univeisity, Department of Chemistry
Corvallis, OR 97331-4003
139
ABSTRACT
The studies of Yb3+ and Er3+ doped GdZrF7 shows that its emission output is
almost twice as high as that of Yb3+ and Er3+ doped Gd2O2S upconversion phosphor
when it was pumped with near infrared laser source (980nm). It also has better color
purity because of the high green to red emission ratio and the shift of the red emission
band to higher energy side.
8.1 INTRODUCTION
There are growing interests in converting near infrared light into shorter visible
emission as the solid state laser technology is developed and the new application area
like bio imaging appears. The up converting concept was introduced at 1966 by Auzel
in Yb3+-Er3+ doped glasses for laser. Ovsyankin and Feofilov observed the
upconversion process via cooperative sensitization in Yb3+-Tm3+ system [1]. To date
several compounds such as LaF3 [2], YF3 [3], BaYF3, NaYF4 [4], KY3F10 [5], YOCl,
Y2O2S [6], La2O2S and Gd2O2S [7] are reported as efficient hosts for upconversion of
infrared light. Among them the hexagonal structure NaYF4 host is known to show the
highest luminescence efficiency. The oxysulfide host also has attraction for the bio
application purpose because of its lack of toxicity and stability.
The small phonon energy of the host compound is promising for high upconversion
efficiency by suppressing the non-radiative energy loss. In this aspect the GdZrF7
compound could be better than NaYF4 having the phonon energy of 300-400cm-1 [8].
The GdZrF7 compound has heavier elements of Gd and Zr compared to Y and Na ions
in NaYF4 that probably induce lower phonon energy. Fluoride compounds are difficult
to synthesize as a pure phase and usually some specialized tube furnace is required
with the toxic HF gas. All previous fluorides were synthesized through this difficult
process to get pure phase. In this paper GdZrF7 compound which is pure enough to act
as a nice upconversion host was prepared by more simple method explained in
experimental part.
140
8.2 EXPERIMENT
8.2.1 sample preparation
For the polycrystalline samples the raw chemicals of 1 mole GdF3 (Alfa Aesar,
99.99%), 1.12 mole ZrF4 (Alfa Aesar, 99.9%), YbF3 (Alfa Aesar, 99.99%), ErF3
(Aldrich, 99.99%) were mixed with 2 mole of NH4F (Aldrich, 99.99%) in mortar
uniformly. The mixture was charged and packed into a carbon crucible with a carbon
lid. This carbon crucible was capped by an alumina crucible and was positioned inside
a bigger alumina crucible covered with an alumina lid. The space between the two
crucibles was filled with carbon powders. The carbon powder was used to help
prevent the mixtures from becoming oxidized. This crucible set was fired at 760°C for
1.5h and ground with additional small amount of ZrF4 and 2mol of NH4F. The ground
powder was heated again at 860°C for 1.5h in same atmosphere. Our preparation
method for fluorides is similar to normal solid state reaction process except that we
use carbon crucible and carbon powders.
8.2.2 X-ray crystallography
All polycrystalline samples were structurally charactericterized by powder x-ray
diffraction. The reference x-ray pattern of GdZrF7 was calculated with Findit software
using the single crystal structure solution data of our previous research [9]. The
diffraction data were collected with a Siemens D5000 Diffractormeter using an inhouse program and λ=1.572 of Cu Kα radiation
8.2.3 luminescence measurement
The visible emission spectra were obtained by exciting the sample with a
100mW 980nm near infrared laser diode (Word Star tech, UH5-100G-980). The
visible emission from sample was dispersed with an Acton Spectrapro-150
spectrometer and was detected with a PMT tube (Hamamatsu, R636-10) and the
141
electric signal is delivered to computer. The emission spectra were not corrected for
the wavelength-dependent response of the detection system. All spectra were
measured at room temperature. The emission output is measured by the same PMT
tube without dispersing the emission output into monochromator under the fixed
intensity of near infrared laser.
8.3 STRUCTURAL CHARACTERISTICS
All polycrystalline samples were characterized by XRD confirming that the
crystal system of Yb3+ and Er3+ co-doped GdZrF7, even at very high Yb3+
concentration, is monoclinic with P2(1)/m space group [8]. There is one Gd3+ site
with point group symmetry C2v which can be occupied by the Yb3+ or Er3+ ions. The
powder x-ray results at various concentration of Yb3+ from low concentration to
almost pure YbZrF7 are shown in Fig. 1. As is clear from the inset picture of Fig. 1
showing 2θ =21˚to 2θ =24˚, as the concentration of Yb3+ is increased, the peaks are
shifted to high 2θ values which can be expected when the smaller size cation of Yb3+
replaces the bigger Gd3+ ion. The other note is that YbZrF7 sample shows the same
crystal structure with GdZrF7. The difference in the peak intensity ratio compared with
the reference pattern (Fig. 2), which is calculated from our single crystal structure
solution, results from the preferred orientation of our polycrystalline samples into 002
planes.
Fig. 3 shows the cell parameter variation with the change in Yb3+ concentration
from the XRD data. As the Yb3+ concentration is increased, the cell parameter and cell
volume decrease linearly as expected from the refraction peaks shift at XRD data. The
relatively faster cell shrinkage along the a and c axis are accompanied by the increase
of the β angle.
142
Yb18
Yb20
Yb22
Yb24
12000
Yb19
Yb21
Yb23
Xray results at vaious [Yb]
-
10000
intensity
8000
21
22
23
24
6000
4000
2000
0
10
15
20
25
30
35
40
45
50
55 [Yb] 60
---
Fig. 8.1 Powder XRD data of Gd0.98-xZrF7:YbxEr0.02 samples. The bottom peaks is for
x=0.18 and top one is for x=0.98. The x value is increased from x=0.18(bottom one) to
0.22, 0.26, 0.30, 0.34, 0.50 and 0.98(top one). Inset is the magnified one for Fig. 1 in
the 2 θ range of 21- 24 degree.
300
Intensity
250
GdZrF7-calculated
200
150
100
50
0
10
15
20
25
30
35
40
45
50
55
2θ
Fig. 8.2 Reference XRD pattern of GdZrF7 compound calculated from the single
crystal structure solution data.
60
143
Cell parameter change as [Yb]
8.5
Distance(Å)
---
y = -0.0014x + 8.2412
8
7.5
a
b
c
(a)
7
6.5
y = -0.0012x + 6.0977
6
y = -0.0008x + 5.7123
5.5
10
30
50
[Yb], mol%
70
90
110
Cell volume and beta agnle change
103.55
280
y = 0.0036x + 103.14
278
274
β
V
(b)
272
103.35
270
103.3
--
103.4
276
beta angle
--
103.45
Volume(Å)
103.5
268
103.25
266
103.2
y = -0.1418x + 279.47
103.15
0
20
40
60
80
[Yb], mol %
100
264
262
120
Fig 8.3 Cell parameter change according to the increase of Yb3+ concentration, (a)
cell parameter, (b) cell volume and β angle.
144
The phonon energy of GdZrF7, which will be important to realize high
upconversion efficiency, can be assumed smaller than that of NaYF4 considering the
bond distances and bond angles. As the bond distance increases the phonon energy of
the bond will be decreased. The bond distances between rare earth ion and F ion are
distributed in a wider range from 2.205 Å to 2.374 Å in the case of GdZrF7 than the
range from 2.226 Å to 2.318 Å in the case of NaYF4. The average distances of those
bonds in GdZrF7 and NaYF4 are 2.3045 Å and 2.2949 Å respectively, and the
difference is rather small. The rare earth ion in both compounds are eight fold
coordinated. However the bonding angle in those two compounds and the point
symmetry of the rare earth ions are quite different. None of the bonding angles of F-Gd3+-F- in GdZrF7 approach 180 degrees while all the bonding angles of F--Y3+-F- in
NaYF4 are near to 180 degrees which means the phonon energy of GdZrF7 compound
is mostly composed of bending mode whereas the higher ones in phonon energy of
NaYF4 depends mostly on the stretching vibration mode. The Gd3+ ion locating in a
distorted square anti-prismatic structure in GdZrF7 is on the non-centro symmetric site
while the Y3+ site in NaYF4 has inversion symmetry. It would be realistic to assume
that the inversion symmetry at the Y3+ site in NaYF4 can be attributed to higher
phonon energy through stretching vibration mode even the bond distance is similar to
that of GdZrF7.
Fig. 4 is the Raman spectrum of GdZrF7 measured at WITec alpha300R Raman
spectroscopy with He/Ar laser 514nm. The dominant peaks appear in 280 - 420 cm-1
which is similar to the known values, 300 - 420cm -1 in NaYF4 [9]. The heavierness
of atomic weight of Gd and Zr than that of Y and Na will be another factor
determining the lower phonon energy of GdZrF7.
145
Fig. 8.4 Raman spectrum of polycrystalline GdZrF7. The excitation source is He-Ne
green laser.
The particle morphology of GdZrF7 is shown in the SEM pictures of Fig. 5 at
several magnifications. On the left side of Fig. 5(c) is the agglomeration reaction
shown between the small particles resulting in a few tens um size particle which is
bigger than the reference sample having particle size of 4-5um. The commercial
product of green upconversion phosphor, PTIR545F obtained as a sample from
“Phosphor Technology” company was used as a reference sample in this paper. The
reference one is assumed as Gd2O2S:Yb3+, Er3+ from the XRD data shown later in this
paper
146
(a) × 100
(b) × 300
147
(continued)
(c) × 1250
Fig. 8.5 SEM pictures of Gd0.74ZrF7:Yb0.22, Er0.04 sample at several magnifications, (a)
×100, (b) ×300 and (c) ×1250.
8.4 LUMINESCENCE CHARACTERISTICS
The up conversion emission mechanism of Yb3+-Er3+ system has been
previously studied as a representative green emitting ion pair in many host materials.
Fig. 6 is the energy level diagram explaining the upconversion mechanism in Yb3+Er3+ system [9].
148
4G
11/2
Energy, 103 cm-1
25
2H
9/2
②
4F
7/2
20
4S
3/2
15
①
4F
9/2
4I
9/2
10
4I
11/2
2F
5/2
4I
13/2
5
0
4I
15/2
Er3+
2F
7/2
Yb3+
Fig. 8.6 Upconversion mechanism for green emission under near infrared light
excitation showing the energy transfer in Yb3+-Er3+ system. The dotted curve explains
the energy transfer from Yb3+ to Er3+ via consecutive two or three photon absorption
by Er3+, the downward dotted line is non-radiative transition, the straight thick
downward lines show the radiative transitions.(adopted from J. F. Suyber, J. Grimm,
M. K. Van Veen, D. Biner, K. W. Krämer, H. U. Güdel, Journal of Luminescence,
117, 1-12 (2006))
The green upconversion mechanism is explained by the two photon absorption process
into 4F7/2 energy state of Er3+ which is populated by excited state absorption at 4I11/2
level of the second photon. The fast non-radiative relaxation from the 4F7/2 state
populates the 2H11/2 or 4F9/2 energy states in a short time followed by subsequent
transition of green or red emission to the ground state of Er3+. We investigated the
visible emission output and emission spectrum at several different concentrations of
sensitizer, Yb3+ and activator, Er3+ to ultimately find the optimal composition at this
new host compound of GdZrF7.
149
8.4.1 Optimal Er3+ and Yb3+ concentration
Er3+ concentration was varied from 1% to 4% by increment of 1% at each three
concentration level of Yb3+ (18 %, 22% and 26%). Fig. 7 (a), (b), (c) show the
emission output change at various dopant concentrations mentioned above.
Emission output at 18% Yb
Relative intensity
--
140%
120%
100%
80%
commercial
[Er]1s
60%
[Er]2s
(a)
40%
[Er]3s
[Er]4s
20%
time(30m15s)
0%
0
2
4
6
8
10
12
14
16
Emission potput at 22%Yb
--
160%
140%
Relative Intensity
120%
100%
80%
commercial
[Er] 1%
60%
40%
[Er] 2%
[Er] 3%
(b)
[Er] 4%
20%
time(30m20s)
0%
1st
2nd
3rd
4th
5th
6th
7th
8th
9th
10th 11th
12th
13th
150
(continued)
Emission output at 26%Yb
Relative Intensity
--
250%
200%
150%
100%
commercial
[Er] 2%
[Er] 4%
50%
(c)
[Er] 1%
[Er] 3%
time(30m15s)
0%
1
2
3
4
5
6
7
8
9
10
11
12
Fig. 8.7 Relative emission output of the Gd1-x-yZrF7:YbxEry samples were measured
during 30min compared with the reference one. The Er3+ concentration was varied as
1%, 2%, 3% and to 4% at three different concentration of Yb3+. The emission output
data for 18% Yb3+ are on (a), for 22% Yb3+ on (b), for 26%Yb3+ on (c). In all graphs
the black line marked with black diamond represent the emission output of reference
sample. The line with brown triangle marker is for 2% Er3+, the green cross marker is
for 3% Er3+, the violet square marker is for 1% Er3+ and the black cross maker is for
4% Er3+ in the downward sequence from the top one.
The emission output was measured intermittently under 980nm infrared laser diode
excitation with the reference sample to compare. After one cycle of measurement was
finished for all samples, the same measurement process was repeated for all samples
during 30min. As it is clear the emission output of GdZrF7 at every Er3+ concentration
and at every Yb3+ concentration are maintained above their initial emission output
while the reference one shows a slight decrease as time passes. At all three
concentration of Yb3+, the 2% Er3+ condition shows the highest result which is
represented as brown triangle markered lines in Fig. 7 (a), (b) and (c). The highest
emission output is about two orders higher than that of reference one at the condition
of 2% Er3+ and 26% Yb3+ concentration.
151
Fig. 8 collects all the emission output data of Fig.7 (a), (b), and (c) into one picture. It
is more clear to figure out at 2% Er3+ concentration we can get the highest emission
output and at 2% Er3+ concentration the emission output of GdZrF7 materials keeps
increasing as Yb3+ concentration is increased in this experimental range. We increased
the Yb3+ concentration further to higher concentration to see the saturation point.
Emission output vs. [Yb] and [Er]
Relative light output
--
250%
200%
150%
100%
[Er]=1
50%
[Er]=2
[Er]=3
[Er]=4
[Yb], [Er], mol%
0%
0
2
4
6
8
10
12
14
Fig. 8.8 Emission output results of GdZrF7 samples at three concentration levels of
Yb3+ and four concentration levels of Er3+ collected from the experiment above. The
data dispersion on each sample is caused by the emission output increase as time pass
by as we mentioned already. The first group of dots express the emission output of
reference sample, the next three groups of dots represent the emission output of 1%
Er samples, the next three for 2% Er samples, the next three for the 3% Er samples
and the last three for the 4% Er samples. At each Er concentration, the first group of
dots represent 18% Yb3+, the second one 22% Yb3+, and the third one 26% Yb3+
condition.
The further increase in Yb3+ concentration was carried at 30%, 34%, 50% and 98%
Yb3+ concentration. The 98% Yb3+ sample means the pure YbZrF7 having 2% Er3+. In
Fig. 9 the emission output results at all Yb3+ concentration range including the former
data set are shown. It maybe realistic assuming that the saturation concentration
152
inYb3+ be around 34% even we do not have data between 34% and 50% Yb3+
concentration.
Emission output vs. [Yb]
Relative light output
--
250%
200%
150%
100%
50%
[Yb],mol%
0%
0
20
40
60
80
100
Fig. 8.9 Emission output of Gd0.98-xZrF7:Yb3+x Er3+ 0.02 samples at further increased
concentration of Yb3+ up to 98% are measured intermittently. The emission output was
measured during 50min intermittently and is represented as dots. x-abscise is the Yb3+
concentration, y-abscise is the relative emission output to that of reference one.
8.4.2 Color purity change vs. Yb3+ concentration
The color purity is another important characters as to be a quality phosphor and
this can be interpreted from the emission spectrum. Fig. 10 (a) to (g) shows the
emission spectrum for the same GdZrF7 sample set in Fig. 9. We calculated the green
to red emission ratio (G/R ratio, the ratio between the maximum peak intensity in
green emission band and red emission band) to figure out the color purity change by
the variation in Yb3+ concentration quantitatively. We assumed that the maximum
peak intensity ratio can represent the emission ratio. The calculated G/R ratio is
graphed into Fig. 11. The emission spectrum show increase in their emission output
until the Yb3+ concentration reaches up to 34%. At 50% Yb3+ the green emission
153
decreased drastically to the similar level of 18% Yb3+ sample which has the same
concentration difference from the optimal Yb3+ condition (34%) as 16%. However the
red emission at 50% Yb3+ is much stronger. This is related to the energy back transfer
from Er3+ to Yb3+ with populating the 4F3/2 level and resulting in red emission. This
will be explained later in detail.
emission spectra at various [Yb]
emission spectra at various [Yb]
3.50E-08
(a)
3.00E-08
[Yb] 18
(b)
[Yb] 22
commercial
-
commercial
-
3.00E-08
3.50E-08
Relative Intensity
Relative Intensity
2.50E-08
2.00E-08
1.50E-08
2.50E-08
2.00E-08
1.50E-08
1.00E-08
1.00E-08
5.00E-09
5.00E-09
0.00E+00
3900
4400
4900
5400
5900
0.00E+00
3900
6400 (Å)6900
5900
3.00E-08
[Yb] 26
(c)
6400 (Å) 6900
Relative Intensity
2.50E-08
2.00E-08
1.50E-08
2.00E-08
1.50E-08
1.00E-08
5.00E-09
5.00E-09
4400
4900
5400
5900
6400 (Å) 6900
commercial
2.50E-08
1.00E-08
0.00E+00
[Yb] 30
(d)
commercial
-
-
5400
3.50E-08
3.00E-08
Relative Intensity
4900
emission spectra at various [Yb]
emission spectra at various [Yb]
3.50E-08
3900
4400
0.00E+00
3900
4400
4900
5400
5900
6400 (Å) 6900
154
(continued)
emission spectra at various [Yb]
emission spectra at various [Yb]
3.50E-08
3.50E-08
3.00E-08
2.00E-08
commercial
2.50E-08
2.00E-08
1.50E-08
1.50E-08
1.00E-08
1.00E-08
5.00E-09
5.00E-09
0.00E+00
3900
[Yb] 50
(f)
Relative Intensity
--
commercial
2.50E-08
Relative Intensity
3.00E-08
[Yb] 34
(e)
0.00E+00
4400
4900
5400
5900
6400 (Å)6900
3900
4400
4900
5400
5900
6400 (Å) 6900
emission spectra at various [Yb]
3.50E-08
3.00E-08
[Yb] 98
(g)
Relative Intensity
-
commercial
2.50E-08
2.00E-08
1.50E-08
1.00E-08
5.00E-09
0.00E+00
3900
4400
4900
5400
5900
6400 (Å)6900
Fig. 8.10 Emission spectrum at various Yb3+ concentrations. (a) [Yb] =18%, (b) [Yb]
=22%, (c) [Yb] =26%, (d) [Yb] =30%, (e) [Yb] =34%, (f) [Yb] =50% and (g) [Yb]
=98%. Every emission spectrum are compared with reference one which is shown by
the blue solid line.
The G/R ratio is continuously decreased in the order of third power of Yb3+
concentration as shown by the fitting equation on the trend line in Fig. 11. For the
optimal condition in Yb3+ and Er3+ concentration the emission output data of Fig. 9
and the color purity data of Fig. 10 are considered together and the optimal condition
for GdZrF7:Yb3+, Er3+ is expected to be near 2% Er3+ and 34% Yb3+ concentration. At
155
34% Yb3+ even the G/R ratio is not the highest, it is still high enough to emit purer
green color compared to the reference (G/R ratio of reference sample is 1.34) as is
shown in the emission spectrum of Fig. 10 (e)
G/R ratio vs Yb concent.
G to R ratio
--
3.00
G/R
Poly. (G/R)
2.50
2.00
1.50
y = -5E-06x3 + 0.0014x2 - 0.1357x + 4.8656
R2 = 0.9876
1.00
0.50
[Yb]
0.00
10
30
50
70
90
Fig. 8.11 G/R ratio at various Yb3+ concentrations.
The best result in this paper shows more than two orders of emission output and
higher color purity than the reference one. The reference is assumed to be Gd2O2S
doped with Yb3+and Er3+ by the XRD data. Fig. 12 is the XRD data of reference
sample with the x-ray pattern of Gd2O2S in ICDS file.
Intensity
--
156
5000
Reference
4000
3000
2000
1000
0
10
15
20
25
30
35
40
45
50
55
60
Intenisty
--
2θ
3000
2500
2000
Gd2O2S-trigonal
1500
1000
500
0
10
15
20
25
30
35
40
45
50
55
60
2θ
Fig. 8.12 XRD data of (a) reference up conversion green phosphor and (b) of Gd2O2S
from ICDS file.
The G/R ratio is decreased as Yb3+ concentration is increased. In the range of 18%
Yb3+ to 34% Yb3+ not only the red emission output keeps increasing but also the
portion of red emission in the whole emission flux is increased. However at very high
Yb3+ concentration like 50% the red emission output is decreased even the portion of
red emission in the whole emission output is increased
There is another feature in the emission spectrum of GdZrF7 compound. The emission
spectrum is shifted to high energy side compared to the reference one which would
contribute to higher color purity. The shift of emission band position to lower energy
side in the reference sample shows the nephelauxetic effect of oxysulfide compound
relative to fluoride compound.
157
8.4.3 Effect of Yb3+ concentration on red emission output
We investigate the reason for the increase in the portion of red emission output
as the concentration of Yb3+ is increased. There are two possible ways to populate the
4
F9/2 energy level of Er3+ from which red emission can be generated. The first one is
two photon absorption process marked with (①) and the second one is the three
photon absorption process marked with (②) in Fig. 6. The first process can generate
both green and red emission by transition from 2H11/2 or 4S3/2 and 4F9/2 levels
respectively to the ground state after non-radiative relaxation from 4F7/2 level. In the
second process, the energy back transfer to Yb3+ of the transition energy from 2H9/2 to
4
F9/2 in Er3+ can populate the 4F9/2 energy state of Er3+.
We measured the emission spectrum of two chosen samples, one of which is 22%
Yb3+ and the other one is 50% Yb3+, under 379nm excitation (this can excite Er3+ into
4
G11/2 energy level) and 490nm excitation (this can excite Er3+ into 4F7/2 energy level)
and compared the green and red emission intensity in the emission spectrum . The
reference one is also investigated under the same excitation sources.
2.50E-08
(a)
2.09E-08
--
2.00E-08
1.58E-08
Intensity
1.50E-08
1.36E-08
1.00E-08
5.00E-09
2.55E-09
0.00E+00
5000
5500
6000
wavelength(A)
6500
7000
158
(continued)
2.50E-08
(b)
2.00E-08
Intensity
-
1.90092E-08
1.50E-08
1.0897E-08
1.00E-08
5.2935E-09
5.00E-09
1.46126E-09
0.00E+00
5000
5500
6000
6500
7000
wavelength(A)
4.00E-07
3.50E-07
3.67091E-07
(c)
-
3.00E-07
Intnesity
2.50E-07
2.00E-07
1.50E-07
1.00E-07
6.07103E-08
5.26735E-08
5.00E-08
0.00E+00
5000
5500
6000
2.82728E-08
6500
7000
wavelength(A)
Fig. 8.13 Emission spectrum excited by 379nm and 490nm (a) of 22%Yb3+ sample,
(b) 50% Yb3+ sample and (c) reference one. In all pictures the violet solid lines
represent the emission spectrum under 490nm excitation and the blue solid lines are
that under the 379nm excitation.
When both 22% and 50% Yb3+ samples are excited with 490nm, the G/R ratio in the
emission spectrum is similar (8.2 for 22% Yb3+ and 7.47 for 50% Yb3+ sample). This
means that second photon absorption process is not the main reason for the increase in
159
red emission at 50% Yb3+ sample. Meanwhile the G/R ratio of both samples under the
379nm excitation is quite different (0.86 for 22% Yb3+ and 0.28 for 50% Yb3+) and the
50% sample shows stronger red emission than 22% Yb3+ sample. This stronger red
emission at high concentration of Yb3+ under 379nm excitation could mean the three
photon absorption process is participating for that emission increase. As the
concentration of Yb3+ is increased the portion of three photon absorption process
relative to the two photon absorption process in the excitation process becomes more
important and at high Yb3+ concentration like 50% the three photon absorption process
becomes mainly responsible to the emission. The emission spectrum of reference one
(Fig. 13 (c)) shows that it relies more seriously on the three photon absorption process
for its emission. Its lower G/R ratio under 490nm excitation will be the results of the
high phonon energy and the high non-radiative relaxation rate between 4G11/2 and 2H9/2
levels of oxysulfide compound.
8.4.4 Luminescence dependency on the excitation intensity
In the previous research, the green upconversion emission of Yb3+-Er3+ system
was explained by the two photon excitation process from luminescence intensity
dependency result on excitation laser intensity. The relationship is expressed with the
equation of
n
Iemit α (Iexci.) , n ≤ 2
However in the GdZrF7 compound it probably is not correct to say the emission is
attributed to two photon excitation process. According to the emission spectrum data
of our experiment under the different excitation wavelength of 379nm and 490nm that
can differentiate the two photon absorption excitation and three photon absorption
excitation process exclusively, more than two photon absorptions are expected to
participate in the excitation process of GdZrF7 compound. If the green emission in
GdZrF7 compound under 980nm excitation is determined only by the two photon
absorption process in excitation then the G/R ratio in emission spectrum should be
similar to the value when it is excited by 490nm light. This value in GdZrF7
compound will be high because of the small phonon energy compared to the energy
160
gap between 4G11/2 and 2H9/2 levels and actual result in this experiment is high.
However the G/R ratio data under 980nm excitation in the whole experimental
concentration range of Yb3+ show lower value than the expected ones from 490nm
excitation.(Table 1)
Table 8.1 G/R ratio measured from the emission spectrum of each Yb3+ concentration.
Data at two different excitation wavelengths of 490nm and 980nm are shown for two
samples of 22%Yb3+ and 50% Yb3
excitation
[Yb]18%
[Yb]22%
[Yb]26%
[Yb]30%
[Yb]34%
[Yb]50%
[Yb]98%
490nm
-
8.2
-
-
-
7.4
-
980nm
2.85
2.46
2.38
1.79
1.73
1.01
0.366
As is clear here, the red emission is much stronger under 980nm excitation than the
expected red emission from the transition branching ratio of 4F7/2 energy level into
which Er3+ will be excited by the two photon absorption process (the same excited
state by 490nm excitation).
8.5 CONCLUSION
The optimal concentration of Yb3+ and Er3+ was investigated in GdZrF7
compound as a new upconversion green phosphor. The best emission output is more
than two orders higher than that of commercial Gd2-x-yO2S:Yb3+x, Er3+y sample. It also
has high G/R ratio imparting better color purity. The increase in red emission as Yb3+
concentration is increased can be interpreted with the increased three photon
absorption process in excitation process which results in population of the 4F9/2 level
responsible for red emission. The XRD data shows that unit cell shrinks as the Yb3+
concentration is increased at faster rate along a and c axes than along the b axis and a
continuous increase of β angle is observed.
161
Aknowledgement
I appreciate Phosphor Technology in England for the Gd2O2S based commercial
upconversion phosphor samples.
REFERENCES
[1] V. V. Ovsyankin, and P. P. Feofilov, Soviet Phys.-JETP Letters, 4, 317-318,
(1966)
[2] J. L. Sommerdijk, Journal of Luminescence, 4, 441-449, (1971)
[3] N. M. P. Low, A. L. Major, Journal of Luminescence, 4, 357-368, (1971)
[4] N. Menyuk, K. Dwight, and J. W. Pierce, Applied Physics Letters, 21(4), 159-161,
(1972)
[5] Alexandra Lapaport, Janet Milliez, Journal of Display Technology, 2(1), 68-78,
(2006)
[6] Xi-xian Luo, Wang-he Cao, Materials Letters, 61, 3696-3700, (2007)
[7] P. N. Yacom, J. P. Wittke, and I. Ladany, Metallugical Transations, 2, 763-676,
(1971)
[8] J. F. Suyber, J. Grimm, M. K. Van Veen, D. Biner, K. W. Krämer, H. U. Güdel,
Journal of Luminescence, 117, 1-12 (2006)
[9] Joayoung Jeong L. N. Zakharov Y. Zhou, R. S. Meltzer and D. A. Keszler,
3+
“Crystal structure and Eu luminescence of GdMF7(M=Hf,Zr)” in preparation
162
CHAPTER 9
CRYSTAL STRUCTURE AND LUMINESCENT PROPERTIES OF
THE APATITE Gd4.67(SiO4)3S
Joayoung Jeong, L.N. Zakharov and Douglas A. Keszler
Oregon State University, Department of Chemistry
Corvallis, OR 97331-4003
To be submitted to Solid State Sciences
163
ABSTRACT
The defect apatite material Gd4.67(SiO4)3S has been prepared by solid state
reaction, and its crystal determined by single crystal X-ray diffraction methods. The
compound crystallizes in space group of P63m. One of the Gd atoms, Gd(2), located at
the center of a tri-capped trigonal prism with nine oxygen atoms. The other Gd atom,
Gd(1), occupies a distorted seven coordinate environment of five of oxygen and two
sulfur atoms. The distorted tetrahedral SiO4 groups are connected to the Gd atoms by
sharing vertices. The sulfur is coordinated by six Gd(1) atoms in a site having S6
symmetry. The luminescent properties of the Tb3+-doped compound have been studied
and compared with those of the oxide analog Gd4.67(SiO4)3O:Tb3+.
9.1 INTRODUCTION
Apatite materials typified by compositions such as Ca3-(PO4)3Cl and Sr5(PO4)3F, have been widely adopted for luminescent and laser materials. As phosphor
materials, the alkaline earth halophosphates such as Ca5(PO4)3Cl doped with antimony
and manganese[1] have been used in fluorescent lamps. Apitites are also known as
chemically diverse materials; the Ca2+ in Ca5(PO4)3F can be replaced by Sr2+, Ba2+,
Y3+, and lanthanides, and PO43- can be replaced by SiO44- and VO43-. The halide anion
can also be replaced with OH1-, O2-, or S2-. The structure of apatite derivative
7Gd2O3·9SiO2 was reported in 1969[2], and the luminescence characteristics of Prand Ce- doped samples were described by A. J. de Vries and co-workers[3]. They
examined emission from the lowest 5d energy levels of Pr3+ and Ce3+. As solid-state
laser materials, A. M. Anderson and J. C. Wang reported the efficient Yb- and Ndbased lasers in the phosphate apatites X5(PO4)3Z (X = Ca,Sr,Ba; Z = F,Cl)[4]. In this
work we reported the crystal growth and structure of the apatite Gd4.67(SiO4)3S. The
powder form of this compound is also prepared for characterization Tb3+ luminescence.
164
9.2 EXPERIMENTAL
9.2.1 Sample Preparation
For crystal growth the chemicals Gd2O3 (Stanford Materials Corp., 99.995%),
SiO2 (CERAC, 99.5%) were mixed in the molar ratio 2.335:3 with 10 wt% LiF
(CERAC, 99.9%) and 100 wt% precipitated S (Fisher Scientific). The mixture was
ground and placed in a covered carbon crucible. This crucible was placed in a covered
alumina crucible with the space between the two crucibles filled with mixture of
powdered sulfur and carbon. This crucible set was fired at 1200°C for 3h and then
cooled to1000°C at 5°C/min and to 850°C at 10°C/min; the power to the furnace was
then turned off for rapid cooling to room temperature. Small crystals having needle
shapes were extracted and analyzed via X-ray diffraction in detail, vide infra.
Powder samples were synthesized according to the formula derived from the
structural analysis on the needle-shaped single crystal. A mixture of 2.33 Gd2O3, and 3
SiO2 with 5 wt% LiF was heated at 1150°C for 3h, the resulting mixture was then
heated under flowing H2S (g) at 1200°C for 2h. Powder samples were also synthesized
by grinding stoichiometric mixtures of Gd2O2S, Gd2O3, and S in vacuum, sealing said
mixtures in evacuated silica tubes, and heating at 1150°C for 2h. The Gd2O2S was presynthesized by heating a mixture of Gd2O3, Na2CO3, and S in an alumina crucible at
1250°C for 3h. The Gd2O2S was extracted by dissolving the flux with water.
9.2.2 X-ray diffraction analysis
X-Ray diffraction data were collected on a Bruker Smart Apex diffractometer at
173(2) K by using Mo Kα radiation. The structure was solved with direct methods and
refined with full-matrix least-squares methods based on F2 by using the SHELXTL (v.
6.10) package [6]. Crystal data and details of the experiment are given in the Table 1.
Absorption corrections were made by using the computer program SADABS [5].
165
Table 9.1 Crystal data and details of X-ray diffraction experiment for Gd4.67(SiO4)3S
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
Unit cell dimensions
Gd4.67(SiO4)3S
1042.67 u
173(2) K
0.71073 Å
Hexagonal
P63/m
a = 9.7379Å
b = 9.7379Å
c = 6.6470Å
545.87(9) Å3
2
3.170 g/cm3
14.275 mm-1
453
2.42 to 28.18°
-12≤ h ≤11, -8≤ k ≤12, -8≤ l ≤8
3436
482 [Rint = 0.0227]
98.6 %
Semi-empirical from equivalents
1.000 and 0.650
Full-matrix least-squares on F2
482 / 0 / 40
1.209
R1a = 0.0219, wR2b = 0.0560
R1 = 0.0221, wR2 = 0.0562
0.0055(4)
1.268 and -1.489 e/Å3
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to θ = 28.18°
Absorption correction
Max. and min. transmission
Refinement method
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
Extinction coefficient
Largest diff. peak and hole
a
R1= Σ (|F0|-|Fc|) / Σ |F0|
b
wR2 = { Σ [w(F02-Fc2)]2 / Σ wF02 }1/2
166
9.2.3 Luminescence measurements
To record the emission spectra, a Xenon lamp (Oriel 300W) was used in
conjunction with a Carry model-15 prism monochromator to select a suitable
excitation wavelength. The emission from the sample was passed through a filter to
eliminate second-order effects and then dispersed by a grating monochromator (Oriel
22500) before detection with a PMT tube (Hamamatsu, R636-10). The measured
emission data were corrected by using a calibrated tungsten lamp (Eppley Laboratories,
Inc.) Excitation spectra were recorded on the same system by scanning the excitation
monochromator at a fixed emission wavelength. Depending on the wavelength range,
the spectra were corrected with sodium salicylate or Rhodamin-B. Low temperature
excitation spectra were measured by using a flow cryostat and liquid helium. Powder
samples were spread directly on a copper plate in the cryostat.
9.3 RESULTS AND DISCUSSION
Final positional parameters for Gd4.67(SiO4)3S are listed in Table 2. The
compound is similar to the apatite derivatives Ln4.67(SiO4)3O (Ln = La [7] and Gd [2])
that crystallize in space group P63/m. The position of the S atom in Gd4.67(SiO4)3S,
however, differs from that of the O′ atom in Ln4.67(SiO4)3O′.
A view of the unit-cell contents of Gd4.67(SiO4)3S is given in Fig. 1. The Gd(1) and
Gd(2) atoms occupy positions on the mirror plane (Wykoff 6h position) and on the
three-fold axis (Wykoff 4f position), respectively. Refinement of the occupation
factors for these positions reveals that the position 6h is fully occupied, while the
position 4f is partially occupied at 83.2%.
167
Table 9.2 Atomic positions and equivalent isotropic displacement parameters (Å2 x
103) for Gd4.67(SiO4)3S.
_____________________________________________________________________
x
y
z
Ueq
Occupancy
_____________________________________________________________________
Gd(1) 0.7473(1)
0.9867(1)
1/4
6(1)
Gd(2)
1/3
2/3
-0.0044(1) 12(1)
0.832(7)
S
0
0
0
10(1)
Si
0.3821(3)
0.9702(3)
1/4
6(1)
O(1) 0.4747(7)
0.8687(7)
1/4
9(1)
O(2) 0.7365(5)
0.0939(6)
-0.0556(8) 17(1)
O(3) 0.5003(10)
0.8434(8)
3/4
22(2)
_____________________________________________________________________
The Si atom has a distorted tetrahedral coordination by O atoms. The Si atom
and two O atoms (O(1) and O(3)) in the tetrahedron are on mirror planes and the two
other O atoms are in general positions. Thus the structure of Gd4.67(SiO4)3S has a
framework similar to that in Ln4.67(SiO4)3O; the Gd(1) and Gd(2) atoms form parallel
to the c axis interconnected via SiO4 bridges (Figure 1). However positions of the
“free” O atom in Ln4.67(SiO4)3O and the S atom are different. The “free” O atom in
Ln4.67(SiO4)3O is at a symmetrical µ3-bridge between three Gd(1) atoms, occupying
the geometrical center of the Gd(1) triangle (Figure 2a). The distance Gd-O is 2.238 Å
[2]. The S atom in Gd4.67(SiO4)3S is not inside the above-mentioned Gd triangle, but
between them (Figure 2b). Thus six Gd(1) atoms form a distorted octahedron around
the S atom with the Gd-S distance of 2.9180(4) Å. The S atom locates outside of the
Gd triangle because of the larger radius of S2- (Shannon radii, 1.84 Å) in comparison
to that of O2- (Shannon radii, 1.40 Å). The calculated distance of Gd(1)-center of Gd
triangle in Gd4.67(SiO4)3S is about 2.4 Å which is somewhat longer than that of Gd(1)O in Ln4.67(SiO4)3O. However this value is much smaller than the sum ~2.98 Å of radii
for S and Gd. The middle value of between 6 coordinated and 8 coordinated atomic
radii was chosen as atomic radius of Gd3+.
168
Fig. 9.1 Unit-cell drawing of Gd4.67(SiO4)3S
169
(a)
(b)
Fig. 9.2 (a) Environment of the free O atom in Ln4.67(SiO4)3O apatite (b) Environment
of S atom in Gd4.67(SiO4)3S apatite
170
Fig. 9.3 Tricapped distorted trigonal prismatic environment of Gd(2) and sevencoordinate site of Gd(1).
Fig. 3 shows the Gd(2) atom located in a tricapped distorted trigonal prism with
coordination by nine oxygen atoms. Three O(1) atoms form one trigonal face, and
three O(3) atoms form the other face. These faces are rotated one relative to the other
slightly. The three O(2) are positioned in capping sites beyond the three rectangular
faces of the prism. The distances Gd(2)-O(1) and Gd(2)-O(3) at 2.432(4) and 2.339(5),
respectively, are normal while the Gd(2)-O(2) distance is long, cf., Table 3. Relevant
bond angles are listed in Table 4..
Gd(1) has seven coordination with two types of oxygen atoms (O(1),four of
O(2)) and two sulfur atoms and files up along c direction as Gd(2) atom does.(Fig 1)
The distances between Gd(1) and each of the coordinating atoms are listed in Table 3.
The Gd(1)-O(2) distance is normal, and the Gd(1)-S distance is comparable to the sum
of crystal radii ( 2.8 Å). As mentioned earlier the sulfur atom is located between the
two trigonal Gd(1) planes in a site with S6 symmetry. The sulfur atoms are arranged in
columns extending along the c axis, cf., Fig. 4.
171
Fig. 9.4 Sulfur column along c axis.
Table 9.3 Bond lengths [Å].
_____________________________________________________
Gd(1)-O(1)
2.306(6)
Gd(2)-O(1)
2.432(4)
Gd(1)-O(2)#1
2.311(5)
Gd(2)-O(1)#6
2.432(4)
Gd(1)-O(2)
2.311(5)
Gd(2)-O(1)#5
2.432(4)
Gd(1)-O(2)#2
2.482(5)
Gd(2)-O(2)#7
2.765(5)
Gd(1)-O(2)#3
2.482(5)
Gd(2)-O(2)#8
2.765(5)
Gd(1)-S(1)#4
2.9180(4)
Gd(2)-O(2)#2
2.765(5)
Gd(1)-S(1)
2.9180(4)
Si(1)-O(3)#7
1.590(7)
Gd(2)-O(3)#6
2.339(5)
Si(1)-O(2)#7
1.635(5)
Gd(2)-O(3)
2.339(5)
Si(1)-O(2)#13
1.635(5)
Gd(2)-O(3)#5
2.339(5)
Si(1)-O(1)
1.637(6)
_____________________________________________________
Symmetry transformations used to generate equivalent atoms:
#1 x,y,-z+1/2
#2 x-y+1,x,-z
#3 x-y+1,x,z+1/2
#4 -x+2,-y+2,z+1/2 #5 -x+y,-x+1,z #6 -y+1,x-y+1,z
#7 -x+1,-y+2,-z
#8 y-1,-x+y,-z #13 -x+1,-y+2,z+1/2
172
Table 9.4 Selected Bond angles [°]
____________________________________________________________________
O(1)-Gd(1)-O(2)#1
85.65(14)
O(2)#2-Gd(1)-S(1)
70.77(12)
O(1)-Gd(1)-O(2)
85.65(14)
O(2)#3-Gd(1)-S(1)
105.28(13)
O(2)#1-Gd(1)-O(2)
123.1(3)
S(1)#4-Gd(1)-S(1)
69.429(12)
O(1)-Gd(1)-O(2)#2
O(2)#1-Gd(1)-O(2)#2
O(2)-Gd(1)-O(2)#2
O(1)-Gd(1)-O(2)#3
72.86(17)
144.32(19)
84.07(8)
72.86(17)
Gd(1)#2-S(1)-Gd(1)#9 180
Gd(1)#2-S(1)-Gd(1)
89.225(9)
Gd(1)#9-S(1)-Gd(1)
90.775(9)
Gd(1)#2-S(1)-Gd(1)#10 89.225(8)
O(2)#1-Gd(1)-O(2)#3
O(2)-Gd(1)-O(2)#3
O(2)#2-Gd(1)-O(2)#3
84.07(8)
144.32(19)
62.7(2)
Gd(1)#9-S(1)-Gd(1)#10
Gd(1)-S(1)-Gd(1)#10
Gd(1)#2-S(1)-Gd(1)#11
90.775(9)
90.775(8)
90.775(9)
O(1)-Gd(1)-S(1)#4
O(2)#1-Gd(1)-S(1)#4
O(2)-Gd(1)-S(1)#4
O(2)#2-Gd(1)-S(1)#4
139.23(7)
72.96(14)
135.11(12)
105.28(13)
Gd(1)#9-S(1)-Gd(1)#11
Gd(1)-S(1)-Gd(1)#11
Gd(1)#10-S(1)-Gd(1)#11
Gd(1)#2-S(1)-Gd(1)#12
89.225(9)
89.225(8)
180
90.775(9)
O(2)#3-Gd(1)-S(1)#4
O(1)-Gd(1)-S(1)
O(2)#1-Gd(1)-S(1)
O(2)-Gd(1)-S(1)
70.77(12)
139.23(7)
135.11(12)
72.96(14)
Gd(1)#9-S(1)-Gd(1)#12 89.225(9)
Gd(1)-S(1)-Gd(1)#12
180
Gd(1)#10-S(1)-Gd(1)#12 89.225(8)
Gd(1)#11-S(1)-Gd(1)#12 90.775(9)
Symmetry transformations used to generate equivalent atoms:
#1 x,y,-z+1/2
#2 x-y+1,x,-z
#3 x-y+1,x,z+1/2
#4 -x+2,-y+2,z+1/2 #5 -x+y,-x+1,z #6 -y+1,x-y+1,z
#7 -x+1,-y+2,-z
#8 y-1,-x+y,-z
#9 -x+y+1,-x+2,z
#10 -y+2,x-y+1,z
#11 y,-x+y+1,-z #12 -x+2,-y+2,-z
#13 -x+1,-y+2,z+1/2 #14 x,y,-z-1/2
X-ray powder diffraction analysis was used to monitor the powder form of this
compound obtained under selected synthesis conditions. The XRD pattern for the
sample (Fig. 5(a)) prepared by heating at 1200℃
℃ under H2S(g) agrees will the pattern
calculated on the basis of the single-crystal data (Fig. 5(b)). So the powder sample
obtained at such conditions is single phase. In contrast, the powder sample synthesized
by mixing Gd2O2S, Gd2O3, and S in a sealed tube at 1150℃
℃ is not pure. The XRD
pattern of the powder sample obtained using this way (Fig. 5(c)) has additional peaks
173
in comparison to the calculated pattern. Several weak peaks around 2θ = 30° are likely
to derive from unreacted starting materials.
1600
--
1200
intensity
1400
1000
Gd4.67(SiO4)3S-powder
(a)
800
600
400
200
0
10
20
30
40
50
60
2θ
12000
Gd4.67(SiO4)3S-reference
(b)
--
10000
Intensity
8000
6000
4000
2000
0
10
20
30
40
50
60
2θ
700
anealing in sealed tube
intensity
-
600
(c)
500
400
300
200
100
0
10
20
30
2θ
40
50
60
Fig. 9.5 XRD patterns for Gd4.67(SiO4)3S (a) synthesis in flowing H2S(g) (c) prepared
in sealed tube and (b) reference pattern calculated from single-crystal structure data.
174
We investigated the Gd4.67(SiO4)3S apatite as a luminescent host by doping with
Tb3+ at concentrations of 2, 4, 7, and 10 mol%. In Fig 6, emission spectra as a function
of Tb3+ concentration are summarized. The emission intensity continually increases as
the concentration of Tb3+ increases. No concentration quenching up to 10 mol% is
observed and probably more space is left for increasing the emission intensity by
adding more Tb3+. Corrected emission spectra of 7% and 10% Tb3+ doped samples are
shown at Fig. 7. The emission transitions are assigned to those from 5D4 → 7FJ as
usual for Tb3+ emission.
2.00E-08
[Tb]2%
[Tb]4%
[Tb]7%
[Tb]10%
--
1.80E-08
1.60E-08
relative intensity
1.40E-08
1.20E-08
1.00E-08
8.00E-09
6.00E-09
4.00E-09
2.00E-09
0.00E+00
3800
4300
4800
5300
5800
6300
6800
7300
wave length(Å)
Fig. 9.6 Emission spectra for selected concentrations of Tb3+ in Gd4.67(SiO4)3S.
175
1.00E+00
5438Å
5
relative intensity
--
9.00E-01
D3-7F5
a) [Tb]7%
b) [Tb]10%
8.00E-01
7.00E-01
6.00E-01
5.00E-01
4.00E-01
4895Å
5
3.00E-01
D3-7F6
4160Å
2.00E-01
1.00E-01
5
5866Å
5
D3-7F4
7
D3- F5 4381Å
5
D3-7F4
0.00E+00
3800
4300
4800
5300
5800
wavelength(Å)
6218Å
5
D3-7F3
6300
6800
7300
Fig. 9.7 Emission spectra after correction for (a) 7% and (b) 10% Tb3+ doped
Gd4.67(SiO4)3S.
M. J. J. Lammers and co-workers reported Gd3+ absorption peaks at 250, 280,
and 315nm in the UV excitation spectrum of Gd4.67(SiO4)3O:Tb3+ apatite[8]. Our data
exhibited a broad excitation band with no distinct Gd absorption peaks (Fig. 8). To
understand this difference in excitation spectra, the low temperature excitation spectra
for Gd4.67(SiO4)3S:10%Tb3+ and Gd4.67(SiO4)3O:7%Tb3+ were measured at liquid
helium temperature; the results are given in Fig. 9. As note, Gd4.67(SiO4)3O:Tb
exhibits the sharp absorption peaks attributed to 6DJ, 6IJ, and 6PJ states of Gd while
Gd4.67(SiO4)3S:Tb3+ has broad absorption bands near 320 nm without the sharp Gd
absorption. As a result, no or little energy migration between Gd ions is expected in
Gd4.67(SiO4)3S. For an effective energy transfer involving Gd, the distance limit
between donor and acceptor is less than ~ 4Å for an exchange process. The distances
between Gd ions of each Gd site in Gd4.67(SiO4)3O and Gd4.67(SiO4)3S compounds are
shown in Table 5. Considering the distance limit in energy transfer, we can assume
that Tb ion should replace the Gd(2) at 4f site to enable the energy transfer among Gds
or between Gd and Tb in Gd4.67(SiO4)3O, while it should replace Gd(1) at 6h site in
Gd4.67(SiO4)3S. The long distance over 4Å between Gd ions in 6h site at
Gd4.67(SiO4)3S should limit efficient energy transfer between Gd ions or Gd and Tb
176
ions and result in no Gd absorption peaks in the excitation spectrum. If Tb3+ ion
occupies the Gd(2) site as we thought intuitively from the partial occupancy of Gd(2)
site the excitation spectrum should have several Gd absorption peaks because Gd(2)
site in Gd4.67(SiO4)3S has the same coordination of O atoms with Gd4.67(SiO4)3O.
1
[Tb]7%
relative intensity
--
[Tb]10%
0.8
0.6
0.4
0.2
3520
to 5L9,5D2
0
2400
2600
2800
3000
3200
3400
3600
3800
wavelenth(Å)
Fig. 9.8 Excitation spectrum of 7% and 10% Tb3+-doped Gd4.67(SiO4)3S (λem = 544nm).
177
1.40E-09
GSiS:10%Tb
GSiO:7%Tb
--
1.20E-09
relative intensity
1.00E-09
8.00E-10
6.00E-10
4.00E-10
2.00E-10
0.00E+00
2400
2600
2800
3000
3200
3400
3600
wavelength(Å)
Fig. 9.9 Excitation spectrum at liquid helium temperature of (a) 10% Tb3+ doped
Gd4.67(SiO4)3S and (b) 7% Tb3+ doped Gd4.67(SiO4)3O.
Table 9.5 The shortest distance between Gd ions in two different sites.
6h
Between 6h-4f
4f
Gd(1)- Gd(1)
Gd(1)- Gd(2)
Gd(2)- Gd(2)
Gd4.67(SiO4)3S
4.099*
4.033
3.264(3.383)**
Gd4.67(SiO4)3O
4.099*
4.036
3.435
Gd site
* The inter-plane Gd(1)-Gd(1) distance is listed here in consideration of energy
transfer.
** Two different distances between Gd(2) exist in the sulfide.
The 4f2-4f5d absorptions of Tb3+ are calculated at 209 nm (spin allowed) and
244 nm (spin forbidden); they are beyond the range of the excitation measurements.
The absorption increase high energy area beyond 260 nm in excitation spectrum
shows the onset of the 5d band. These 4f5d energy levels were calculated by using the
equation suggested by P. Dorenbos to predict the lowest 5d level of lanthanides in
178
many hosts and the crystal depression data of Gd4.67(SiO4)3O[9]. The absorption band
arising from 320 nm in Gd4.67(SiO4)3S is assumed to be the charge-transfer (CT) state
of Tb-S.
The excitation spectrum in Fig. 10 shows absorption peaks associated with 4f-4f
transitions of Tb3+ in the range from 330 to 500 nm. The features are assigned to
excitations into the 5LJ levels from the ground state of Tb3+.
--
1.00E-09
relative intensity
1.20E-09
8.00E-10
7
7
F6-5L9
F6-5L7
7
F6-5L10
7
F6-5G6
6.00E-10
4.00E-10
7
2.00E-10
0.00E+00
3300
3800
4300
F6-5D4
4800
wavelength(Å)
Fig. 9.10 Excitation spectra 4f-4f transitions of 10% Tb3+ doped Gd4.67(SiO4)3S.
The emission spectra of 10% Tb3+ doped sample measured under two excitation
wavelengths at 313 and 370 nm are compared in Fig. 11. The emission intensity under
both excitations are calibrated to compensate for the intensity difference of excitation
light using the intensity ratio of Rhodamin-B between these two wavelengths. The
emission under 370 nm excitation created by 4f-4f transition from 5L10 level to ground
state is weak whereas the emission under 313 nm excitation showed much stronger
intensity consistent with the proposed CT nature of the absorption.
179
1.60E-09
370nmexcitation
313nmexcitation
--
1.40E-09
relative intensity
1.20E-09
1.00E-09
8.00E-10
6.00E-10
4.00E-10
2.00E-10
0.00E+00
4600
5100
5600
6100
6600
wavelength(Å)
Fig. 9.11 Comparison of emission spectra of Gd4.67(SiO4)3S:10% Tb3+ under two
different excitation wavelengths of 313nm and 370nm. The emission intensity was
calibrated with the intensity ratio of those two excitation wavelength using RhodaminB.
9.4 CONCLUSION
The framework of the apatite sulfide Gd4.67(SiO4)3S has been established to be
the same as that found in Gd4.67(SiO4)3O. The position of S atom, however has been
determined to be different from that of free O′ atom in Gd4.67(SiO4)3O′. The S resides
in a distorted octahedral environment of Gd atoms, while the O′ atoms rest in the
center of a Gd triangle.
The Gd(2) atom occupies a distorted tricapped prism. The Gd(1) has seven
coordination with five oxygen and two sulfur atoms. Both Gd(2) and Gd(1) files up
along lines parallel to the c axis. The SiO4 distorted tetrahedral units bridge the two
types of Gd atoms. The possibility of Gd4.67(SiO4)3S apatite as a luminescent host was
investigated by doping with Tb3+. The intensity of green luminescence increases with
Tb3+ concentration up to the maximum tested concentration of 10 mol%. Strongest
absorption is associated with S→Tb CT transitions at λ < 320.
180
Acknowledgements
Acknowledge National Science Foundation.
REFERENCES
[1] Shigeo Shinoya,William M. Yen, Phosphor handbook, CRC press, Boca Raton,
394.
[2] Smolin, Yu. I.; Shepelev, Yu. F. Izvestiya Akademii Nauk SSSR,
Neorganicheskie Materialy 5(10), 1823-1825 (1969).
[3] De Vries, A. J., Blasse, G. Materials Research Bulletin 21(6), 683-694 (1986).
[4] A. M. Anderson, J. C. Wang, Advanced Solid State Lasers 26, 664-673 (1999).
[5] G. M. Sheldrick, Bruker/Siemens Area Detector Absorption Correction Program,
Bruker AXS, Madison, WI, 1998.
[6] SHELXTL-6.10 "Program for Structure Solution, Refinement and Presentation"
BRUKER AXS Inc., 5465 East Cheryl Parkway, Madison, WI 53711-5373 USA
[7] Toropov, N. A., Kougiya, M. V., Doklady Akademii Nauk SSSR 182(3), 614-16
(1968).
[8] M. J. J. Lammers, G. Blasse, Journal of Electrochemcal Society, 134(8), 20682071 (1987).
[9] P. Dorenbos, Journal of Luminescence, 91, 155-176 (2000).
181
CHAPTER 10
CONCLUSION AND FUTURE WORK
Several systems have been examined in the development of new quantumsplitting phosphors. For the first time, efficient sensitization of the high-enery levels
of Gd3+ ion has been demonstrated by using Nd3+. The strong 4f3→4f25d1 highenergy absorption transition of Nd3+ has been used to deliver energy into the densely
packed 4f levels of Gd3+ above the 6G state in LiGdF4:Nd. This energy on Gd3+ is
back-transferred to the Nd3+ ion, resulting in a unique quantum-splitting process with
emission near 980 nm. Because of the rapid back transfer of energy from Gd3+ to
Nd3+, it is not possible to transfer the energy efficiently from the Gd3+ to another ion
that emits in the visible. The investigation on the energy transfer processes from
sensitizer to Gd3+ and from Gd3+ to Nd3+ provided good information on the
luminescence dynamics of quantum splitting process. In Pr3+,Eu3+:GdF3, Pr3+ acts as a
sensitizer to trigger the quantum splitting. Unfortunately, the effect is observed only
at very low concentrations of Pr3+ (0.3%) and Eu3+ (0.2%), and a correspondingly low
absolute quantum yield of 20% results.
Quantum splitting via PCE on a single ion was also examined with the ions
Tm3+ and Er3+. Their emission spectra are characterized by f-f transitions without
emission from the 5d state; no quantum splitting was observed. However, in the case
of Tm3+, the non-radiative transition from the 5d level to the next lower 4f level (3PJ or
1
I6) may be a possible first transition for CRET to a second ion. An experiment to
convert the emission color in near UV transition 1S0 → 1I6 of Pr3+ into visible range
was unsuccessful for Sm3+ co-doping, even though a resonance exists between the
energy levels of the two ions.
Host intrinsic emission via STEs was tested as a means to sensitize Gd3+ into the
6
G level. Gd3+-doped oxides were examined, and factors affecting the STE emission
were discussed. The fact that Gd3+ was excited beyond 6G level by the host intrinsic
emission in ScPO4 peaking at 215 nm provides a new possible excitation method for
the quantum splitting phosphor. The ScPO4:1%Gd3+ shows high absolute quantum
182
yield of 0.9 ± 0.2 following excitation at 170nm. The kind of cation in its atomic radii
and ionization energy is shown to have very strong relationship with host intrinsic
energy. Smaller cation radii or higher ionization energies lead to higher STE emission
energy in phosphate compounds.
Several new compounds are synthesized. Single crystals of the material GdZrF7
were grown for structure refinement by using the flux method. The up-conversion
process involving the ion pair Yb3+-Er3+ was investigated in this host. Compositionally
optimized samples in this system were demonstrated as potentially useful upconversion phosphors with almost 200% of the light output of the commercial material
Gd2O2S;Yb,Er; relative to the commercial sample, the fluoride sample also exhibits a
better green color purity. Continued development of this material with emphasis on
the production of nanoparticles could extend use into the bioimaging field as a probe
material. The luminescence of several lanthanides in this compound was also
investigated; among them the Eu3+ doped GdZrF7 was found to be a nearly whiteemitting phosphor with an absolute quantum yield near 0.9 under VUV excitation. The
crystal structure of the new apatite compound Gd4.67(SiO4)3S was also solved from
single-crystal X-ray diffraction data; luminescence from Tb3+ doped samples revealed
a bright-green emission.
183
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(1988)
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A. L. Harmer, A. Linz and D. R. Gabbe, Journal of Physics and Chemistry of solids,
30, 1483 (1969).
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188
APPENDIX
Carry model-15
Excitation
monochromator
Oriel 300W
Hamamatsu,
R636-10
PMT
Oriel 22500
Emission
monochromator
Water
filter
Xe
lamp
Reader
Data
processing
189
Appendix A
LUMINESCENT MEASUREMENT SYSTEM
1. UV/VIS Spectroscopy
190
2. VUV Luminescent Measurement spectroscopy
( Dr. Richard S. Meltzer’s Group at UGA)
191
Appendix B
A HIGH MOBILITY TRANSPARENT THIN-FILM TRANSISTOR
WITH AN AMORPHOUS ZINC TIN OXIDE CHANNEL
H. Chiang and R. L. Hoffman
Hewlett-Packard Company 1000 NE Circle Blvd. Corvallis, OR 97330-4239
J. F. Wager
School of Electrical Engineering and Computer Science Oregon State University
Corvallis, OR 97331-3211
J. Jeong and D. A. Keszler
Department of Chemistry Oregon State University Corvallis, OR 97331-4003
ABSTRACT
A new type of transparent thin-film transistor (TTFT) with an amorphous zinc tin
oxide channel layer formed via RF magnetron sputter deposition is demonstrated.
Typical field-effect mobilities of 5-15 and 20-50 cm2V-1s-1 are obtained for devices
post-deposition annealed at 300 and 600°C, respectively. TTFTs processed at 300 and
600°C operate in an enhancement-mode with extracted threshold voltages of 15-20 V
and 0-10 V, respectively. Under both processing conditions, a drain current on-to-off
ratio greater than 107 is obtained. Zinc tin oxide is one example of a new class of high
performance TTFT channel materials involving amorphous oxides composed of
heavy-metal cations with (n-1)d10ns0 (n≥4) electronic configurations.
192
1 INTRODUCTION
The recent development of transparent thin-film transistors (TTFTs) [1, 2, 3, 4,
5, 6] represents a major advance in the emerging field of transparent electronics. These
transistors have been fabricated on the basis of crystalline oxide channels that are
typically produced at relatively high processing temperatures. To appraise the
potential for reducing these temperatures, while retaining or improving transistor
performance, we have examined the use of amorphous channel materials. Here, we
describe the first example of a high-performance TTFT having an amorphous oxide
channel.
Amorphous oxides composed of heavy-metal cations with (n-1)d10ns0 (n≥4)
electronic configurations constitute an interesting class of transparent conductors,
since they possess relatively high electron mobilities despite their amorphous
character. [7, 8, 9] Examples of such materials include indium oxide doped with tin,
ITO, [10] and zinc tin oxide [11] for which amorphous-state mobilities as large as 40
and 30 cm2V-1s-1, respectively, have been reported. Such high mobilities may be
a consequence of a conduction band primarily derived from spherically symmetric,
heavy-metal cation ns orbitals. Such orbitals have large radii, leading to a high degree
of overlap between adjacent orbitals and considerable band dispersion. Moreover, the
spherical symmetry of an s orbital makes delocalised electronic transport less sensitive
to local and extended structural order as compared with band formation from
anisotropic p or d orbitals. Furthermore, multicomponent oxide semiconductors are
preferred to binary oxides for ensuring that the oxide remains amorphous under
a range of processing conditions.
2 RESULTS AND DISCUSSION
Zinc tin oxide is a wide band-gap, n-type semiconductor; its stoichiometry can
be most generally described as (ZnO)x(SnO2)1-x (0 < x < 1). Two crystalline forms
have been reported, [11, 12, 13, 14, 15, 16, 17, 18] trigonal ilmenite (ZnSnO3) [15]
193
and cubic spinel (Zn2SnO4) [18]. The ilmenite phase exhibits limited thermal stability
[12, 13, 15]. In bulk samples, decomposition of ZnSnO3 to Zn2SnO4 and SnO2 has
been noted at temperatures as low as 600°C, [15] but the rate of this process is rather
slow.
Zinc tin oxide thin films have been primarily investigated for transparent
conductor applications [11, 12, 13, 14, 15, 17, 18]. Thin films typically exhibit a
direct optical band gap of 3.3 to 3.9 eV [17, 18]. This broad range of reported band
gaps likely arises from a combination of a large Burstein-Moss shift and
compositional or structural variations of the zinc tin oxide thin films; the fundamental
band gap of Zn2SnO4 has been reported to be 3.35 eV [18]. Attractive attributes of
zinc tin oxide include its chemical stability with respect to oxidation and etching, [13,
17] its physical robustness and extreme resistance to scratching, [17] and its tendency
to possess an exceedingly smooth surface in thin films [17, 18].
Zinc tin oxide TFTs are fabricated by using two similar bottom-gate test
structures -the first employing a glass substrate for realization of a fully transparent
device and the second employing a heavily-doped Si wafer with a SiO2 gate-dielectric
layer formed via thermal oxidation. The description provided here deals explicitly
with the completely transparent structure; the oxidized Si structure differs only in the
nature of the gate-electrode / gate-dielectric portion of the device stack. The electricalperformance characteristics of the transparent and the Si-based structures are similar,
although the Si-based devices exhibit less device-to-device and substrate-to-substrate
variability.
TTFTs are prepared on Nippon Electric Company glass substrates (NEG OA2)
coated with a 200-nm sputtered ITO gate-electrode film and a 220-nm atomic layer
deposited superlattice of AlOx and TiOx (ATO) [20]. A zinc tin oxide channel layer
(typically 20-90 nm) and ITO (typically 250 nm) source and drain electrodes are
deposited via RF magnetron sputtering in Ar/O2 (90%/10%) and Ar (100%),
respectively, at a substrate temperature of 175°C. Zinc tin oxide films are deposited
with a target fabricated in the Oregon State University Department of Chemistry (a
mixture of ZnO and SnO2 powder with a ZnO:SnO2 molar ratio of 1:1 is sintered
overnight at 1100 °C and with several targets purchased from a commercial supplier
194
(ZnO:SnO2 molar ratios of 2:1 and 1:1). Devices are typically furnace annealed in air
at 600°C for 1 hour following zinc tin oxide deposition. The complete staggered,
bottom-gate device structure is shown in the inset of Fig. 1. Six devices with W/L =
7100 µm/1500 µm (~4.7/1) are fabricated on each 1in. × 1in. substrate. Channel and
source/drain layers are patterned through the use of shadow masks.
Figure 1. Optical transmittance as viewed through the source/drain (~84% in the
visible portion of the electromagnetic spectrum) of a zinc tin oxide channel TTFT.
(Inset) Prototypical TTFT device structure.
The zinc tin oxide thin films described here, as TTFT channel layers, are
essentially amorphous. X-ray diffraction (XRD) patterns are obtained from two
systems; the results from these systems vary slightly. The first system is a Philips
MRD instrument that employs Cu Kα radiation (0.5º incident angle); the XRD
patterns from this system exhibit a single, extremely broad peak at 2θ∼34 º,
characteristic of amorphous zinc tin oxide films previously reported in the literature
[11, 17, 18]. The second system is a Rigaku Rapid image-plate machine with Cu Kα
195
radiation (10 º incident angle) provided by a rotating anode and a 0.5-mm pinhole
collimator; an XRD pattern obtained from this system is shown in Fig. 2. Broad XRD
profiles are observed at 2θ values near 34, 59, and 90 º [17]. These bands encompass
the major diffraction peaks for both ilmenite ZnSnO3 and spinel Zn2SnO4, so it is not
possible to explicitly identify the phase(s) present in the film. Moreover, the breadth
of the diffraction peaks indicates that any crystallites within the film do not exceed a
diameter of approximately 5 nm. Thus, the exact description of the zinc tin oxide film
as multiphase nanocrystalline or amorphous is difficult to discern. In contrast, above
an annealing temperature near 650°C, a multiplicity of sharp XRD peaks appear,
indicating a dramatic increase in the crystalline nature of the zinc tin oxide films.
Figure 2. XRD pattern obtained from an ~200 nm zinc tin oxide thin film deposited
from a target of stoichiometry ZnSnO3 and subjected to a 600°C anneal: reference
stick patterns for ilmenite ZnSnO3 (black), and spinel Zn2SnO4 (gray) are
superimposed. This XRD pattern is representing of the amorphous nature of zinc tin
oxide films employed as TTFT channel layers with post-depostion annealing
treatments up to 600°C. (Inset) 100000x SEM cross-sectional image of a zinc tin oxide
thin film: the scale bar on the image is 200nm.
196
The scanning-electron-microscope (SEM) image shown in the inset of Fig. 2 is
a representative cross-sectional view of a thin-film sample. This image is obtained at a
magnification of 100,000x and provides qualitative information regarding the
morphology of these zinc tin oxide thin films. In agreement with XRD results, the
image does not indicate any grain formation in the sample. Similar to the XRD results,
there is little, if any, observable difference in the SEM cross-sectional view of
unannealed samples and those samples that are annealed up to 600°C.
The optical transmittance versus wavelength through the source/drain region of
a typical TTFT is shown in Fig. 1. The average transmittance in the visible portion of
the electromagnetic spectrum (400 -700 nm) is ~ 84% for this device. The data
represent raw transmission through the entire structure, including the substrate, i.e.,
the measured transmission is reduced by both absorption and reflection.
Representative log(ID)-VGS and log(IG)-VGS characteristics (for VDS = 40 V),
representative of ~50 devices with a 600°C channel layer anneal, where ID, VGS, IG,
and VDS are the drain current, gate-to-source voltage, gate current, and drain-to-source
voltage, respectively, are illustrated in Fig. 3. An ID -VDS characteristic (for VGS = 3-15
V in 3V steps) is shown in the inset of Fig. 3. Qualitatively, ideal TFT behavior is
observed, including drain current saturation.
The electrical parameters characterizing a thin-film transistor are typically
threshold voltage, drain current on-to-off ratio, and channel mobility. The threshold
voltage (extracted from an ID -VGS measurement with a small VDS [21] for device
operation in the linear region) is typically 0-10V for zinc tin oxide channel TTFTs.
However, precise identification of the threshold voltage for a given device is
somewhat ambiguous. A less ambiguous device parameter, although not explicitly
quoted as often in the literature, is the turn-on voltage (Von). Von is the gate voltage at
the onset of channel conduction, i.e., the gate voltage at the onset of the initial sharp
increase in current in a log(ID)-VGS characteristic [22]. Von is equal to -5V in Fig. 3,
and it is typically -5 to 5V for ~50 zinc tin oxide channel devices with a 600°C postdepostion anneal.
197
Figure 3. Representative Log(ID)-VGS and log(IG)-VGS characteristic (VDS = 40 V) for
zinc tin oxide (sputter deposited from a ZnSnO3 target) TTFTs annealed at600°C.
Inset shows ID-VDS characteristic (VGS is varied from 3 to 15 V in 3 V steps and ID
increases with increasing VGS).
The typical ID on-to-off ratio is ∼07 -108. As shown in Fig. 3, the transistor ”off”
current is established by gate leakage, which is typically ∼10-10 A for the ATO gate
dielectric employed here. Channel mobility is arguably the most important TFT
electrical parameter, as it quantifies the semiconductor channel layer performance,
specifically with respect to current drive capability and maximum switching
frequency. The channel mobility is assessed by differentiating the drain current
characteristic in the linear (or triode) region of device operation, with respect to VGS,
i.e., the field-effect mobility (µ FE) [21, 22]. µ FE is typically 20-50 cm2V-1s-1 for our
zinc tin oxide TTFTs. This compares to ~25 cm2V-1s-1 for the best polycrystalline
ZnO TTFT reported to date, [22] and ~80 cm2V-1s-1 for an engineered superlattice
single-crystal TTFT prepared by pulsed laser deposition and a high-temperature
anneal of 1400°C [5]. Finally, note for comparison that opaque a-Si:H and
polycrystalline-Si TFTs typically have channel mobilities on the order of 1.5-2.0 and
100-200 cm2V-1s-1, respectively [23, 24, 25].
198
The zinc tin oxide channel layer has been deposited from sputter targets with
two different stoichiometries, (ZnO)x(SnO2)1-x (x = 1/2 and x = 2/3), corresponding to
the compositions of the ilmenite and spinel structures, respectively. Device
performance, specifically in terms of channel mobility, exhibits little variation
between these two stoichiometries, indicating the possibility of a surprising degree of
insensitivity to stoichiometry (specifically the Zn:Sn ratio).
Finally, we should point out that relatively low processing temperature, i.e., a
300°C channel layer anneal (in place of the 600°C anneal as discussed above), have
been employed to yield devices with channel mobilities of 5-15 cm2V-1s-1, a modest
performance reduction in light of the substantial decrease in processing temperature.
Typical µ FE -VGS characteristics for devices annealed at 300 and 600°C are illustrated
in Figure 4.
Figure 4. Representative µ FE -VGS (VDS =1 V ) characteristics for zinc tin oxide TTFTs
fabricated on a Si substrate and annealed at 300°C and 600°C. The higher annealing
temperature yields a higher mobility and a lower turn-on volatage.
199
This mobility trend (i.e., increasing mobility with increased annealing
temperature) is tentatively attributed to modification of the semiconductor-insulator
interface with annealing or improved local atomic enhanced local atomic
rearrangement at higher temperature rather than to enhanced long-range crystallinity
since zinc tin oxide films annealed at 300 and 600°C have virtually identical XRD
patterns.
The results presented in this letter identify zinc tin oxide as a viable TTFT
channel layer material with respect to electrical performance and processing
requirements. Furthermore, other types of amorphous oxides composed of heavymetal cations with (n-1)d10ns0 (n≥4) electronic configurations may provide
opportunities for identification of TTFT channel materials with further improved
electron transport and transparency.
ACKNOWLEDGEMENTS
The authors would like to thank Arto Pakkala, Jarmo Maula, and Sey-Shing Sun for
supplying ITO /ATO coated substrates, Alexander Yokochi for XRD measurements,
Jeffrey P. Bender for electrical characterization of ATO thin films, and David Hong
for many useful discussions. This work was funded by the U.S. National Science
Foundation under Grant No. DMR-0071727 and by the Army Research Office under
Contract No. MURI E-18-667-G3.
200
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Appendix C
CURRICULUM VITA
EDUCATION:
PhD, Chemistry,
Oregon State University
Graduation
Dec, 2007
Dissertation Title
- High Quantum Yield phosphors via Quantum Splitting and Upconversion
Dissertation Advisor
- Douglas A. Keszler, Dept. of Chemistry, Oregon State University
BA, Agricultural Chemistry, Seoul National University
April, 1986
EXPERIENCE
09/2002~ present
Oregon State University
Corvallis, Oregon
Graduate Research Assistant in Department of Chemistry
- High quantum yield phosphor using Quantum Splitting luminescence.
- Worked on p-type semi-conductor materials.
- Synthesizing compounds by several solid state reaction and characterizing via X-ray
analysis.
03/1986~ 08/2002
Samsung SDI
Soowon. Korea
Principal Engineer
- Developed PDP phosphors and burning-free Projection TV phosphors
- Conducted feasibility study for a new type display device which combines phosphors
and LCD
- Developed pigmented ZnS:Ag,Cl phosphor and set up mass production condition in
line
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PUBLICATIONS:
- Crystal structure and luminescent properties of the apatite Gd4.67(SiO4)3S, J.Y.Jeong,
L.N.Zakharov and D.A. Keszler – in preparation
- Crystal structure and Eu3+ luminescence of GdMF7 (M=Hf4+,Zr4+), J.Y.Jeong,
L.N.Zakharov Y.Zhou, R.S.Meltzer and D.A.Keszler – in preparation
- Luminescence of Lanthanides doped GdZrF7, J.Y.Jeong, Y.Zhou, R.S.Meltzer,
Madis Raukas and D.A. Keszler – in preparation
- The new efficient green upconversion phosphor, GdZrF7:Yb3+,Er3+, J.Y.Jeong, and
D.A. Keszler – in preparation
- Energy transfer from the host excitations to Ce3+ ions in scandium borate. Journal of
Luminescence, 125, 1-2, 80-84 (2007), S.P. Feofilov, Y. Zhou, J.Y. Jeong, D.A.
Keszler and R.S. Meltzer
- Sensitization of Gd3+ and the dynamics of quantum splitting in GdF3:Pr,Eu. Journal
of Luminescence, 122-123, 503-505 (2007), S.P. Feofilov, Y. Zhou, J.Y. Jeong,
D.A. Keszler and R.S. Meltzer
- Host sensitization of Gd3+ ions in yttrium and scandium borates and phosphates:
Application to quantum cutting. Physical Review B: Condensed Matter and Materials
Physics, 74(8) (2006), Feofilov, S. P.; Zhou, Y.; Seo, H. J.; Jeong, J. Y.; Keszler, D.
A.; Meltzer, R. S.
- Quantum cutting in GdxY1-xLiF4:Nd3+ - dynamics and mechanisms. Journal of
Luminescence, 119-120, 264-270 (2006), Zhou, Y.; Feofilov, S. P.; Jeong, J. Y.;
Keszler, D. A.; Meltzer, R. S.
- Quantum splitting and its dynamics in GdLiF4:Nd3+. Physical Review B: Condensed
Matter and Materials Physics, 72(7) (2005), Jia, W.; Zhou, Y.; Feofilov, S. P.;
Meltzer, R. S.; Jeong, J. Y.; Keszler, D.
- Relaxation of the 4fn-15d1 electronic states of rare earth ions in YPO4 and YBO3.
Physica Status Solidi C: Conferences and Critical Reviews, 2(1), 48-52 (2005), Jia,
W.; Zhou, Y.; Keszler, D. A.; Jeong, Joa-Young; Jang, K. W.; Meltzer, R. S.
- High mobility transparent thin-film transistors with amorphous zinc tin oxide
channel layer. Applied Physics Letters, 86(1) (2005), Chiang, H. Q.; Wager, J. F.;
Hoffman, R. L.; Jeong, J.; Keszler, D. A.
- Yellow ZnS-based phosphor, process of preparing the same, and display device
using the phosphor. U.S. Pat. Appl. Publ., 7 pp (2004), Lee, Sanghyuk; Shin,
204
Sanghoon; You, Yongchan; Jeong, Joayoung
- Display device apparatus. Ger.Offen., 9 pp (1996), Do,Young-rag;You,Young-chul;
Jeong,Joa-young; You,Yong-chan
AWARDS AND HONORS:
- President Award for the year best patent at Company (88-11305)
Soowon, Korea