J. Am. Ceram. Soc., 95 [10] 3229–3234 (2012)
DOI: 10.1111/j.1551-2916.2012.05306.x
© 2012 The American Ceramic Society
Journal
Comparative Investigation of Green and Red Upconversion
Luminescence in Er3+ Doped and Yb3+/Er3+ Codoped LaOCl
Zhiguo Xia,† Jing Li, Yi Luo, and Libing Liao
School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China
Pure tetragonal LaOCl:Er3+ and LaOCl:Yb3+/Er3+ samples
possessing upconversion (UC) luminescence behavior were synthesized using a simple solid-state reaction method and the
structural properties were investigated using X-ray diffraction,
crystal structure analysis, and SEM, respectively. The UC
spectroscopic properties of Er3+ doped and Yb3+/Er3+ codoped LaOCl were studied in terms of UC emission spectra.
The intense green and red emissions around 528, 550, and
662 nm corresponding to the 2H11/2?4I15/2, 4S3/2?4I15/2, and
4
F9/2?4I15/2 transitions of Er3+ ions were observed for
LaOCl:Er3+ sample under excitation at 980 nm, and the
power studies indicated that two-photon processes were responsible for the green and red upconversion luminescence. The
bright green emission is visible to the naked eyes for LaOCl:
Er3+. A great enhancement of the visible upconversion emissions in Yb3+/Er3+-codoped sample was observed, furthermore, it is interesting to find the bright red emission for
LaOCl:Yb3+/Er3+ samples. The mechanisms for the green
and red UC luminescence, especially the enhancement of red
UC emission were discussed based on the law of UC intensity
versus pump power and their lifetimes.
I.
compounds, many preparation strategies have been developed, such as, high-temperature solid-state methods,9 hydrothermal syntheses,5 mechanochemical grinding,3 sol-gel
processes,10 thermolysis methods of trifluoroacetate precursors,4 and pyrohydrolyisis methods of ternary ammonium
chlorometalates,7 and so on. However, to the best of our
knowledge, only few reports are concentrated on the comparative investigations of luminescence behaviors of different
activators in LnOX structure, especially the upconversion
(UC) luminescence.11 Recently, the UC luminescence of Er3+
doped and Yb3+/Er3+ codoped phosphors in the same hosts,
such as GdOCl, YVO4, Y2O3, and SrTiO3, have been widely
investigated, and some different luminescence mechanisms
have been proposed.11–14 In this study, we have successfully
prepared Er3+ doped and Yb3+/Er3+ co-doped LaOCl
phosphors using the conventional solid-state method, and the
two kinds of phosphors emit intense green and red UC luminescence for LaOCl:Er3+ and LaOCl:Yb3+/Er3+ samples,
respectively. Ground, excited state absorption, energy transfer (ET) process, and exchange interaction are discussed as
the possible mechanisms for these emissions.
Introduction
II.
ARE earth oxyhalides (represented by LnOX, Ln = Y,
lanthanide ions; X = F, Cl, Br) are previously rated as
preeminent solid electrolytes, yielding the highest reported
ion conductivities thus far for halogen ions conduction.1 In
these years, intensive investigations have been focused to
develop the luminescence phosphors of oxyhalides of lanthanides.2–5 Among them, lanthanide oxychlorides, LaOCl, is
known for the low maximum phonon cutoff energy, high
chemical stability, and ability to promote efficacious phonon
energy transfer to dopant ions, which are originating from
the low phonon vibrational energy and high ionicity of the
rare earth to chlorine bond.6,7 Accordingly, it will lead to the
minimal quenching probability of the excited state of lanthanide ions in the LaOCl host. Except for this, La3+ ion has
the largest ionic radius among the lanthanide series of ions,
and it can be easily substituted by different Ln3+ ions in the
structure.7 Therefore, LaOCl is an excellent host doped with
various Ln3+ ions. Based on this, LaOCl:Ln3+ are wellknown from their excellent luminescent properties as UVexcited and upconversion phosphors, and they are promising
candidates for practical applications.6–8
Considerable attention has recently been devoted to the
synthetic approaches on LnOX nanocrystals for both fundamental research of synthetic chemistry and a wide range of
potential applications.3–5,7–10 To fabricate the LnOX-type
Experimental Procedure
3+
LaOCl:0.01Er and LaOCl:0.10Yb3+,0.01Er3+ samples were
both prepared using the traditional high temperature solidstate method. Stoichiometric amounts of starting materials
La2O3, Yb2O3, and Er2O3 [(99.99%; Shanghai Yuelong NonFerrous Metals Limited, Shanghai, China) and NH4Cl (analytical reagent, A. R., 99.5%; Beijing Fine Chemical Company,
Beijing, China), were thoroughly mixed in an agate mortar.
Some excessive NH4Cl (20%) is necessary for loss of Cl source
at high temperature to obtain the pure phase formation. Then
the mixtures were fired at 600°C for 2 h in air to produce the
final samples. The XRD measurements were carried out on a
XRD-6000 model diffractometer (Shimadzu Corporation,
Kyoto, Japan) using CuKa radiation (k = 0.15405 nm), operating at 40 kV, 30 mA. The morphologies of the samples were
inspected using a SEM (Tescan Vega, XM 5136, Brno, Czech
Republic). UC spectra were recorded on a JOBIN YVON
FL3-21 spectrofluorometer equipped with a R928 photomultiplier tube as the detector and an external power-controllable
980 nm semiconductor laser (Beijing Viasho Technology Company, Beijing, China) as the excitation source. Fluorescence
decay curves were measured by exciting with 980 nm from an
optical parametric oscillator pumped using a pulsed Nd:YAG
laser with a pulse duration of 10 ns and a repetition frequency
of 10 Hz. The signal was recorded using a monochromator
and an oscillograph. All the measurements were performed at
room temperature.
R
J. Varela—contributing editor
III.
Results and Discussion
The crystallization behavior and morphology of the studied
samples were investigated using XRD and SEM measurements.
It is found that heat treatment at 600°C was sufficient for the
crystallization of LaOCl and Er3+ and (or) Yb3+ doped LaOCl
Manuscript No. 31286. Received April 03, 2012; approved April 30, 2012.
†
Author to whom correspondence should be addressed: e-mail: xiazg426@yahoo.
com.cn
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Journal of the American Ceramic Society—Xia et al.
Vol. 95, No. 10
Fig. 2. SEM image of the LaOCl:0.01Er3+ prepared at 600 °C.
Fig. 1. (a) XRD patterns of tetragonal LaOCl: as-prepared LaOCl
(1), LaOCl:0.01Er3+ (2), LaOCl:0.10Yb3+,0.01Er3+ (3), and the
standard ICSD diffraction lines of LaOCl as reference; (b)Crystal
structure of LaOCl emphasizing the alternating (LaO)nn+ cation and
Cl anion layers, and the inset shows the coordination of La3+ in
LaOCl structure.
materials. Figure 1(a) shows the XRD patterns of as-prepared
LaOCl, LaOCl:0.01Er3+, and LaOCl:0.10Yb3+,0.01Er3+
samples. It can be seen that all the diffraction peaks of the
three samples can be well assigned to the reported data of
LaOCl (ICSD 84330). Furthermore, no second phase can be
detected for the present doping ions and concentration, indicating that the Er3+ and Yb3+ are completely dissolved in
the LaOCl host lattice by substitution for the La3+ owing to
their similar ionic radii and properties. As shown in
Fig. 1(b), the as-prepared LaOCl crystallize in the matlockite
tetragonal PbFCl structure (space group: P4/nmm; Z = 2)
with alternating (LaO)nn+ cation and Cl anion layers along
the crystallographic c direction.6,7 The local coordination
environment of La is characterized by C4v symmetry, and the
La3+ ions occupy the distorted square antiprismatic sites on
both sides of the central oxide sheet so that they are coordinated to four oxygen ions and five chloride ions.7 Specially,
the fifth Cl is distinctive from the remaining four proximal
Cl in being located in the next-to-nearest anionic layer, as
shown in the inset of Fig. 1(b).
Figure 2 shows the SEM image of the typical LaOCl:0.01Er3+ sample prepared at 600°C. LaOCl even prepared
under the solid-state reaction shows relatively regular and
rigid rectangular micro-sheets particle morphology. Lee et al.
explored three different synthetic methods, the liquid phase
process in HCl solution, the solvothermal reaction, and the
surfactant-assisted solvothermal reaction, to selectively control the particle shape of LaOCl.6 Kort et al. also reported a
novel synthetic strategy for the preparation of well-defined
LaOCl nanostructures with regular shapes based on the
ligand exchange and condensation of rare earth chlorides
and rare earth alkoxides in the presence of a coordinating
solvent.7 The previous reported liquid-phase synthetic strategies are obviously different from the present NH4Cl-based
solid-state technique in the reaction mechanism; however, the
phase formation of LaOCl demonstrates the same distinctive
preference for 2D growth along the (110) directions indicating strong confinement along the crystallographic c axis due
to layer structure of (LaO)nn+ cation and Cl anion layers
along the crystallographic c direction.6,7 Anyway, we should
admit that the introduction of some useful additive in the
liquid phase synthesis play an important role in the modification in the microstructure. However, we will herein pay more
attention to the UC luminescence behavior in the LaOCl
host, not the synthesis method as intensively investigated in
these years.
Er3+ is the most important active ion applied to UC luminescence, which can demonstrate green and red visible emission when excited by 980 nm laser.15 However, single Er3+
can hardly show efficient UC emission in many cases.2
Among the UC rare-earth ions, it is well-known that Yb3+
ion is a good sensitizer, which can greatly enhance UC efficiency through energy transfer owing to the strong absorption
in the region around 980 nm. Therefore, the Yb3+–Er3+ couple
is also used to produce visible green or red UC emission.
11–14,16
The host materials also play an important role in
obtaining highly efficient UC luminescence. Figures 3(a) and
(b) show the comparison of the upconversion luminescence
of LaOCl:0.10Yb3+,0.01Er3+ and LaOCl:0.01Er3+ upon
980 nm laser excitation with the same laser power. It is very
interesting to find that, both single Er3+ and codoped Yb3+–
Er3+ couple can show efficient UC emission. More interestingly, there are different visible UC emission colors, green
for single Er3+- doped sample and red for Yb3+–Er3+
codoped sample, respectively, as shown in the inset on luminescence images of the as-prepared samples irradiated by a
980 nm diode. As also shown in the UC spectra, two distinct
bands in the range of 500–700 nm were observed. The bands
in the green region 515–540 nm and 540–570 nm are associated with the 2H11/2?4I15/2 and 4S3/2?4I15/2 transitions of
Er3+ ions, respectively, whereas the band in the red region
640–690 nm is associated with the 4F9/2?4I15/2 transition of
Er3+ ions.2,11–17 The relative intensities of green and red
emission vary obviously with the adding Yb3+ contents.
Figure 3(c) gives the Yb3+ content dependent UC spectra of
LaOCl:xYb3+,0.01Er3+ sample with the same laser power. It
is found that the red/green intensity ratio increases with
Er3+ Doped and Yb3+/Er3+ Codoped LaOCl
October 2012
(a)
3231
(c)
(b)
Fig. 3. Comparison of the upconversion luminescence of LaOCl:0.10Yb3+,0.01Er3+ (a) and LaOCl:0.01Er3+ (b), the corresponding inset shows
the red and green luminescence images irradiated by a 980 nm diode, and the Yb3+ content dependent UC spectra of LaOCl:xYb3+,0.01Er3+
sample (c).
increasing Yb3+ concentration. When the doping concentration of Yb3+ reaches 10%, LaOCl:0.10Yb3+,0.01Er3+ sample shows strong red UC emission compared with the single
doped one. Therefore, it is reasonable to assume that there is
an energy transfer from the Yb3+ to Er3+ ions.
To understand the UC mechanisms of the green and red
UC luminescence, especially the enhancement of red UC
emission, the UC luminescence intensities were measured as
a function of the pump power to determine the number of
the photons responsible for the visible emission. Figure 4(a)
shows the up-converted emission spectra of LaOCl:0.01Er3+
under 980 nm laser excitation with different pumping power.
For the unsaturated upconversion process, the number of
photons required to populate the upper emitting state can be
described by the following relation18:
Iup / INIRn
(a)
(1)
where Iup is the fluorescent intensity, INIR is the pump laser
power, and n is the number of pump photons required. A
plot of log Iup versus log INIR yields a straight line with slope
n. As shown in Fig. 4(b), it gives the pump power dependence of the green and red upconversion emission in
LaOCl:0.01Er3+. The slopes (n values) obtained were
1.59 ± 0.13 and 1.53 ± 0.04 for the green emission transition
(2H11/2?4I15/2 and 4S3/2?4I15/2) and red emission transition
(4F9/2?4I15/2), respectively. These results indicate that both
the green and red emissions are two-photon process, and the
green UC emission is dominated, so that we can observe
green visible light by the naked eye. Similarly, Fig. 5(a) demonstrates
the
up-converted
emission
spectra
of
LaOCl:0.10Yb3+,0.01Er3+ with different pumping power;
and the pump power dependence of the green and red upconversion emission in LaOCl:0.10Yb3+,0.01Er3+ is also given
in Fig. 5(b). The slopes for the UC luminescence of the red
emission transition (4F9/2?4I15/2) and the green emission
transition (2H11/2?4I15/2 and 4S3/2?4I15/2) are 1.96 ± 0.06
and 0.92 ± 0.11, respectively. It is clearly found that the
two-photon process contribute to the upconversion of red
emission. However, the number of pump photons involved in
the Er3+ green emission in LaOCl:0.10Yb3+,0.01Er3+ is
almost 1, which is far deviated from 2 for those specific twophoton process. The same phenomenon has been found in
some other system, and the crystal structure type plays an
important role in the underlying mechanism for the NIR
(980 nm) to visible light UC process.19–21 In such a case, it is
proposed that the exchange interactions between neighboring
activator ions and the large excited ground absorption
(ESA), UC rate can explain the single-photon process.21
Moreover, we measured the emission decay curve of
2
H11/2?4I15/2 (525 nm) and 4F9/2?4I15/2 (674 nm) transition
in LaOCl:0.01Er3+ and LaOCl:0.10Yb3+,0.01Er3+ sample.
(b)
Fig. 4. (a) Up-converted emission spectra of LaOCl:0.01Er3+ under
diode laser excitation at 980 nm with different pumping power; (b)
Pump power dependence of the green and red upconversion emission
in LaOCl:0.01Er3+.
Figure 6(a) shows the decay profiles of 2H11/2?4I15/2 transition and 4F9/2?4I15/2 transition in LaOCl:0.01Er3+ sample.
Both of the two decay curves of the transitions could be
well-fitted to a single exponential function22:
IðtÞ ¼ Aexpðt=sÞ
(2)
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Vol. 95, No. 10
Journal of the American Ceramic Society—Xia et al.
(a)
(a)
(b)
(b)
Fig. 5. (a) Up-converted emission spectra of LaOCl:0.10Yb3+,
0.01Er3+ under diode laser excitation at 980 nm with different
pumping power; (b) Pump power dependence of the green and red
upconversion emission in LaOCl:0.10Yb3+,0.01Er3+.
where I is the luminescence intensity at time t, A is a constant, t is the time, and τ is the decay times for the exponential components. The calculated lifetimes of the 2H11/2 and
4
F9/2 states of Er3+ ions for LaOCl:0.01Er3+ are listed in
Table I. Furthermore, Fig. 6(b) shows the decay profiles of
2
H11/2?4I15/2 transition and 4F9/2?4I15/2 transition in
LaOCl:0.10Yb3+,0.01Er3+ sample. The decay profiles of
2
H11/2?4I15/2 transition can be fitted with a second-order
exponential decay model by the following equation:
IðtÞ ¼ A1 expðt=t1 Þ þ A2 expðt=t2 Þ
(3)
where I is the luminescence intensity; A1 and A2 are constants; t is the time; t1 and t2 are the lifetimes for the exponential components. The values of t1 and t2 are listed in
Table I, which means that Er3+ at 2H11/2 (4S3/2) state experience a relatively fast and subsequently slow decay time corresponding to different variation mechanism. Furthermore, the
average lifetime (t*) can be calculated as the following
Eq. (4):
t ¼ ðA1 t1 2 þ A2 t2 2 Þ=ðA1 t1 þ A2 t2 Þ
(4)
On the basis of Eq. (4), the decay time was determined
to be 0.39 ms, as shown in Table I. The decay profile of
4
F9/2?4I15/2 transition in LaOCl:0.10Yb3+,0.01Er3+ sample
Fig. 6. Decay profiles of 2H11/2?4I15/2 and 4F9/2?4I15/2 transition
in LaOCl:0.01Er3+ (a) LaOCl:0.10Yb3+, 0.01Er3+ (b) under 980 nm
excitation.
Table I. Lifetimes of the 2H11/2 and 4F9/2 States of Er3+ Ions
for LaOCl:0.01Er3+ and LaOCl:0.10Yb3+,0.01Er3+ Samples
Samples
Energy Level
LaOCl:0.01Er3+
H11/2
F9/2
2
H11/2
LaOCl:0.10Yb3+,
0.01Er3+
2
4
4
F9/2
Lifetime (ms)
0.43
0.59
τ1 = 0.48, τ2 = 0.13,
A1 = 5.99, A2 = 4.50,
τ* = 0.39
0.67
is the same as that in LaOCl:0.01Er3+ sample, and it could
be fitted to a single exponential function with the decay time
of 0.67 ms. It means that there are similar luminescence
mechanism between them.
The UC excitation processes can be populated by several
well-known mechanisms12–14: (1) ground state absorption
(GSA); (2) excited state absorption (ESA), (3) energy transfer
(ET), and (4) photon avalanche. Photon avalanche was
excluded in the present study as no inflection point was
observed in the power study.13 Figure 6 presents the energy
level diagram for the Er3+ and Yb3+ ions as well as the proposed UC mechanisms to produce green and red up-converted emissions. For the single Er3+ model in the
LaOCl:0.01Er3+ sample, the collaborative effect of GSA and
Er3+ Doped and Yb3+/Er3+ Codoped LaOCl
October 2012
3233
Fig. 7. Schematic representation of the energy level diagram for the Er3+ and Yb3+ ions as well as the proposed UC mechanisms to produce
green and red up-converted emissions.
ESA can pump the Er3+ ion from the ground state to 4I11/2
level, and then raise to 4F7/2 level by absorbing two 980 nm
laser photons, as shown in Fig. 7. The Er3+ ions at 4F7/2
level undergo multi-phonon relaxation to green luminescent
2
H11/2 level, further, there is a fast thermal equilibrium
between 2H11/2 and 4S3/2 levels. Accordingly, we can observe the
green emission transitions (2H11/2?4I15/2 and 4S3/2?4I15/2).
As the 2H11/2 (4S3/2) and 4F9/2 levels are separated by a relatively high energy gap, the possibility for the thermal relaxation between them will be low, hence we can only observe
relatively weak red UC emission. For the Yb3+–Er3+ coupled model in the LaOCl:0.10Yb3+,0.01Er3+ sample, a great
enhancement of UC emissions ascribed to the efficient energy
transfer between Yb3+ and Er3+ except for the self-absorption of Er3+. As shown in Fig. 7, the Er3+ ions can
be excited to the 4I11/2 level by GSA and energy transfer
from excited Yb3+ ions. This ET process can be
described as: 4I15/2 (Er) + 2F5/2 (Yb)?4I11/2 (Er) + 2F7/2 (Yb)
[ET1, Fig. 7), and the ET1 process is the dominant one
owing to the strong absorption of Yb3+ in the region around
980 nm. In such a case, ET2 process will only populates
the 4F9/2 level by the following mode, 4I13/2 (Er) + 2F5/2 (Yb)
? 4F9/2 (Er) + 2F7/2 (Yb)], as shown in Fig. 7. The intensity
of red emissions increases faster than that of green emissions
with Yb3+ ions codoping, so that we can also find the decay
time of 4F9/2 level is a little long compared with that of Er3+
single doped sample. Simultaneously, ET3 process and ESA
process will competitively pump the Er3+ from the 4I11/2
level to the 4F7/2 level. However, ESA process is the dominate one as there are larger probability for the exchange
interaction in the same Er3+ ions than that of the energy
transfer between Yb3+ and Er3+ ions. The decreased decay
times and the second-order exponential decay model can also
prove this proposed assumption. Therefore, it will lead to the
single-photon process for the large ESA UC rate.21
IV.
Conclusions
3+
In conclusion, LaOCl:Er
and LaOCl:Yb3+/Er3+ samples
were synthesized using the solid-state reaction method. Both
green and red emission bands can be found for the Er3+ singly doped and Yb3+/Er3+ codoped LaOCl samples, and the
bands in the green region, 516–537 nm and 537–570 nm are
assigned to the transitions 2H11/2?4I15/2 and 4S3/2?4I15/2 of
Er3+ ions, respectively, whereas the band in the red region
640–690 nm is associated with the 4F9/2?4I15/2 transition of
Er3+ ions. The relative intensities of green and red emission
vary obviously with the adding Yb3+ contents. Under excitation at 980 nm, bright green emission is visible to the naked
eyes for LaOCl:Er3+, whereas LaOCl:Yb3+/Er3+ sample
shows enhanced red emission at the same conditions. A great
enhancement of UC emission is ascribed to the efficient
energy transfer between Yb3+ and Er3+ except for the selfabsorption of Er3+, furthermore, the codoping of Yb3+
greatly enhanced the red (4F9/2–4I15/2) upconversion emission
for LaOCl:Yb3+/Er3+ samples. The mechanisms for the
green and red UC luminescence, especially the enhancement
of red UC emission were discussed.
Acknowledgments
This present study was supported by the National Natural Science Foundations of China (Grant No. 51002146), the Ph.D. Programs Foundation of
Ministry of Education of China (Grant No. 20090022120002), and the Fundamental Research Funds for the Central Universities (2010ZY35,
2011YYL131).
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