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Effects of the host lattice and doping concentration on the colour of Tb3+
cation emission in Y2O2S:Tb3+ and Gd2O2S:Tb3+ nanometer sized
phosphor particles
Xiao Yan, George R. Fern*b, Robert Withnall (the late), Jack Silver*a
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Y2O2S and Gd2O2S phosphor lattices activated with a range of Tb3+ concentrations have been
successfully prepared as nanoparticles and their emission properties have been characterized using SEM,
XRPD, photoluminescence spectroscopy and cathodoluminescence. 5D3 – 5D4 cross relaxation processes
between Tb3+ cations were observed in both Y2O2S and Gd2O2S as a function of Tb3+ concentration. In
the Y2O2S host lattice, the predominant emission colour shifts from blue to green with increased Tb 3+
concentration. In contrast, green emission is always predominant in Gd 2O2S at Tb3+ concentrations from
0.1 mol% to 5 mol%. This finding is explained in accordance with previous reports on the bulk materials
that found the Gd2O2S lattice has a lower charge transfer state than the Y2O2S host lattice.
1. Introduction
Y2O2S and Gd2O2S crystals are well known wide band-gap (4.64.8eV) semiconductors that have been considered ideal host
matrixes for RE3+ cations 1. The crystal symmetry of the Ln2O2S
(Ln=Y, Gd) lattice is trigonal, and the space group is P-3m1.
Each Ln atom is bonded to four oxygen atoms and three sulphur
atoms and has a seven-coordinated geometry. Both Ln and
oxygen atoms have the same site symmetry of C3v and the S site
has symmetry of D3d2,3.
Tb3+ activated Y2O2S and Gd2O2S bulk phosphors have been
commonly used as scintillation materials for medical diagnostics
because of their inherent properties such as high X-ray
absorption, hard radiation stability, high conversion efficiency
from X-ray to visible light, short decay times and low afterglow4.
Gd2O2S:Tb3+ is the most frequently employed phosphor in X-ray
intensifying screens5. The high density of Gd2O2S (7.44 gcm-3)
makes it an effective trap of the incident X-ray photon, allied
with this are its further attributes of high intrinsic conversion
efficiency (20%) and high quantum yield of Tb3+. Together these
properties result in a high light output to meet the required
properties for such screens. Small particle phosphors with a
narrow size distribution offer the advantages of reduced structure
noise in the X-ray detection devices as well as facilitating high
screen resolution and high screen density6,7. Recently the
incorporation of Gd2O2S:Tb3+ into polyethylene microstructures
to fabricate flexible scintillators for next-generation flexible Xray image sensors has been reported. Pixel height and fill factor
are two factors that affect the sensitivities of the as-prepared
scintillators with different pitch sizes8.
In this paper, we report systematic studies on the effects of
varying the host cationic lattice and the Tb3+ doping
This journal is © The Royal Society of Chemistry [year]
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concentrations on the emission bands in nanometre-sized Y2O2S
and Gd2O2S crystals. This allows us to illustrate firstly the
versatility of a method of synthesising nanoparticles of yttrium
oxysulfide lattices and shows how easily the method facilitates
control of the doping of other cations (in this case Tb3+ ) into the
lattices. Secondly, the control of the Tb3+ concentration in turn
affects the luminescence. This means it is possible to easily
control the photoluminescent emission colour of the phosphor
nanoparticles, such that the Y2O2S:Tb3+ materials can give greens
through to blues whereas the Gd 2O2S:Tb3+ materials only give a
range of greens (thus we are able to colour tune the
nanoparticles). The reasons for these findings are discussed and a
unified explanation of the luminescence phenomena for both
Y2O2S:Tb3+ and Gd2O2S:Tb3+ phosphors is presented.
2. Experimental
Tb3+ activated Y2O2S and Gd2O2S nanometre sized phosphor
particles were synthesized by an analogous two-step method to
the previous procedure described in our earlier work 6, 9, 10, in
which the hydroxycarbonate precursors were prepared using the
urea homogeneous precipitation method.
For Tb3+ activated Y2O2S the Tb(NO3)3 stock solution was
obtained by dissolving Tb2O3 (Aldrich, 99.99%) in dilute nitric
acid. In a typical experiment, 25 ml Y(NO3)3 (0.5 M) and 12.6 ml
Tb(NO3)3 (0.01 M) were mixed and diluted to 500 ml with
deionised water. The solution was stirred and heated to 100 ˚C
before 30 g of urea was added. The solution became turbid and
was left to stand for 1h before filtering without cooling. The
phosphor precursor was obtained by drying the precipitates for 24
h at 100 ˚C. The as-prepared precursor was mixed with Na2CO3
and sulfur and then covered with a mixture of Y2O3, Na2CO3 and
sulfur (the mole ratio used was: precursor/Y2O3/Na2CO3/sulfur
[journal], [year], [vol], 00–00 | 1
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15
H2NCONH2 → NH4+ + OCN-
(101)
20000
Intensity (a.u.)
5
=1/1/1.5/2). After being fired at 900 ˚C for 1 h to yield the
required oxysulfide phosphors, the bottom layer was washed in
hot water (100 ˚C) for 20 min before filtration. The precipitates
were dried at 100 ˚C for 24 h giving white powders.
Tb3+ activated Gd2O2S nanometre sized phosphor particles were
prepared by the analogous method.
In addition, some of the obtained phosphors were re-fired at 1100
˚C. An alumina boat containing the required amount of phosphor
powders were put in the bottom of a quartz tube and fired at 1100
˚C for 1 h. The quartz tube was sealed with glass wool to ensure
an oxygen free atmosphere and purified nitrogen gas was passed
through the system at a rate of 200 cm3/min during both the firing
and cooling periods.
The chemical reactions are set out in the following equations:
(100)
10000
(102)
(110)
(200)
(111)
(003)
(202)
(201)
(113)
(001)
0
10
20
30
40
50
60
70
2 (degree)
Fig. 1 XPRD diffractogram of (red) Y2O2S:Tb3+ and (black)
60
Gd2O2S:Tb3+ nanoparticle phosphor samples.
OCN- + 2H+ + 2H2O → H2CO3 + NH4+
(a)
[REOH(H2O)n]2+ + H2CO3 → RE(OH)CO3·H2O + (n-1)H2O
(RE=Y, Gd, or Tb )
25
30
35
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2RE(OH)CO3·H2O + 3Na2CO3 + 4S + O2 → RE2O2S + 5CO2 +
2SO2 + 3Na2O + 2H2O + H2S
X-ray powder diffraction (XRPD) of the samples was performed
using a Bruker D8 Advance X-ray powder diffractometer fitted
with a copper source and LynxEyeTM silicon strip detector. The
diffractometer was previously calibrated using an aluminium
oxide line position standard from Bruker and the LaB6 NIST
SRM 660a line profile standard. The emission of the nickel
filtered Cu source and hence the instrumental line broadening
was determined by fitting the NIST standard using the software
Bruker TOPAS version 3. The samples were scanned from 5 to
100˚ (2θ) for 35 minutes in step scan mode. Furthermore the
crystal phase information including crystal size was analysed
using Bruker TOPAS software. The morphology and the particle
size of the samples were determined by SEM using a Zeiss Supra
35 VP field emission scanning electron microscope.
Photoluminescence (PL) measurements were carried out using a
Bentham Instruments dual monochromator system. Emission
spectra were recorded in the range of 300 nm to 800 nm. The CIE
coordinates were calculated from the spectra according to the
1931 CIE standard for colorimetry.
Cathodoluminescence (CL) properties were measured with an
electron gun in a high vacuum chamber. The samples were
excited by an electron beam with controlled accelerating voltage
of 5 kV. CL spectra were collected over the range of 300-800
nm.
(b)
Fig. 2 SEM images of (a) Y2O2S:Tb3+ and (b) Gd2O2S:Tb3+ nanoparticle
phosphor samples.
18
16
14
Frequency
20
12
10
8
6
4
2
0
80
50
55
3. Results and discussions
XRPD patterns of Y2O2S:Tb3+ and Gd2O2S:Tb3+samples are
presented in Fig. 1 and are in good agreement with the hexagonal
phases of Y2O2S and Gd2O2S reported in the literature 11-13,. Fig.
2 shows typical SEM images of the Y2O2S:Tb3+ and the
Gd2O2S:Tb3+ samples. All the samples exhibit discrete particles
of roughly spherical shape with apparently smooth surfaces. In
the case of the Gd2O2S:Tb3+ sample some of the particles are
present as well formed crystals. The mean particle size calculated
from Lorentian fitting of SEM observations is 109 nm for
Y2O2S:Tb3+ and 117 nm for Gd2O2S:Tb3+ (with R2 values of
0.835 and 0.978 respectively) as seen in the histograms presented
in Fig. 3, this in turn is consistent with the calculated data from
2 | Journal Name, [year], [vol], 00–00
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160
16
14
12
Frequency
45
100
Particle Size (nm)
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10
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6
4
2
0
80
100
120
140
160
Particle Size (nm)
Fig. 3 Histograms of Y2O2S:Tb3+ (top) and Gd2O2S:Tb3+ (bottom)
nanoparticle phosphor samples showing Lorentzian curve fit line in red.
This journal is © The Royal Society of Chemistry [year]
Table 1. The crystal sizes and hexagonal lattice parameters of
Re2O2S:Tb3+ (Re = Y or Gd) nanometre sized phosphor particles from
XRPD data.
General
formula
Bulk Y2O2S11
Y2O2S:Tb3+
Bulk Gd2O2S12
Gd2O2S:Tb3+
(900˚C)
Gd2O2S:Tb3
(1100˚C)
a (Å)
c (Å)
Crystal Size (nm)
3.750
3.7856(1)
3.8514(5)
6.525
6.5880(1)
6.667(2)
128.2(12)
3.8539(1)
6.6668(1)
157.0(11)
3.8542(6)
6.6677(1)
118.48(87)
7
5
7
D 3 - F6
Intensity (a.u.)
5
7
D 3 - F3
5
7
D 4 - F5
5
7
D 3 - F4
5
7
D 4 - F3
5
7
D 4 - F4
3+
Y2O2S:Tb
3+
400
500
600
700
Wavelength (nm)
5
50
Fig. 4 PL emission spectra of 0.1 mol% Tb3+ activated Y2O2S and
Gd2O2S. The intensities were normalised to that of the 544 nm
emission band.
Intensity (a.u.)
Y2O2S:Tb
Gd2O2S:Tb
55
60
65
260
280
300
320
340
360
380
400
420
440
Wavelength (nm)
Fig. 5 Excitation spectra (solid line) Y2O2S:Tb3+ and (dotted line)
Gd2O2S:Tb3+ nanoparticle phosphor samples monitored at 544nm.
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20
D3
G
To 7Fj
To 5D4
To 7Fj
To 5D4
40
7
D 4 - F6
Gd2O2S:Tb
15
D3
5
Figure 6 The 5Dj to CTS feeding and quenching transitions. Gj is the
rate of excitation to the 5D3. The 5D3 level is much closer to the CTS
band in the case of the Gd2O2S:Tb
D 3 - F5
45
10
5
G
5
240
CTS
T
30
35
220
CTS
T
ex=254nm
5
Gd2O2S:Tb
Y2O2S:Tb
25
the XRPD data (Table 1) for the Gd2O2S:Tb3+ but is lower for the
Y2O2S:Tb3+. In both the SEM images in Fig. 2 the samples can
be seen to show evidence of aggregation.
The emission spectra of 0.1 mol% Tb3+ activated Y2O2S and
Gd2O2S are presented in Fig. 4. The characteristic emission bands
of the Tb3+ cations can be assigned to transitions between the 5D3
and 5D4 excited states and the 7FJ (J = 6, 5, 4, 3) ground states.
Furthermore, a clear distinction between the two materials can be
observed in their emission spectra. The blue emissions from the
5D →7F transitions are predominant in the Y O S:Tb3+ spectrum
3
J
2 2
This journal is © The Royal Society of Chemistry [year]
while green emissions from 5D4→7FJ transitions predominate in
the Gd2O2S:Tb3+ spectrum. This has been related to the different
position of the charge transfer states (CTSs) in the host crystals 4,
14, 15 (see Fig. 5 which provides direct evidence for the previous
explanation).
The excitation spectra for Y2O2S:Tb3+ and Gd2O2S:Tb3+ are
presented in Fig. 5. The bands around 260 nm in both spectra are
due to the host lattice, the bands to the right arise from the charge
transfer states. The latter band for Gd2O2S:Tb3+ clearly ends to
the right of that of Y2O2S:Tb3+ showing the lowest energy states
in the former are much lower than the latter (by around 1000 cm1). This finding agrees well with the discussion in ref. 15. The
CTS in Gd2O2S lies closer to the 5D3 excited state of Tb3+ and
therefore most of the electrons on the 5D3 excited state of Tb3+ in
Gd2O2S are activated to the CTS and eventually fed to 5D4 state at
room temperature, yielding a very noticeable reduction on the
intensities of the emission bands from 5D3 → 7FJ transitions (see
figure 6)15. In contrast, this energy transfer cannot take place in
the Y2O2S lattice because the CTS in Y2O2S lies in a higher
position than that in Gd2O2S 15 (as shown in Fig. 5). Therefore the
blue emissions from 5D3 → 7FJ transitions are predominant in the
emission spectrum of 0.1 mol% Tb3+ activated Y2O2S phosphors.
Although for the 1 mol% Tb3+ activated Y2O2S phosphor the
green emission band at 545 nm is the most intense one, the
intensity ratio between the 5D4 → 7FJ and 5D3 → 7FJ transitions is
dependent on the doping concentration of Tb 3+ cations. Thus the
emission colour and intensity of Tb3+ emission bands are also
strongly affected by the doping concentration 10,16-25. This is due
to the cross relaxation process that occurs between two adjacent
Tb3+ cation pairs as illustrated in the following equation1:
Tb3+ (5D3) + Tb3+ (7F6) → Tb3+ (5D4) + Tb3+ (7F0).
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Fig. 7 presents a plot of the intensity ratio of green/blue emission
(I545/I418) as a function of Tb3+ concentration. The intensity ratio
of green/blue emission grows exponentially with Tb3+
concentration, which is in good agreement with a previous report
made by observing the bulk phosphor 23. In addition we have
previously reported both the PL spectra of Y2-xTbxO2S (where x
has values from 0.001 to 0.050) NPs (normalized to UV 385 nm
Journal Name, [year], [vol], 00–00 | 3
Tb3+ concentrations
1. 0.1 %
2. 0.5 %
3. 1%
4. 2%
5. 3%
6. 5%
Fig. 7 The intensity ratio of green/blue PL emission in Tb3+ activated
Y2O2S (I545/I418) as a function of Tb3+ concentration.
5
10
emission band) in the range 475nm to 675nm, and also the PL
spectra in the range 360nm to 560nm (normalized to the 545 nm
emission band10. In Fig. 8 we present the PL spectra of
Y2-xTbxO2S (where x has values from 0.001 to 0.050) NPs
(normalized to the 418nm emission band).
The intensity of the emission from that of 5D4 → 7FJ (J=6, 5, 4, 3)
transitions grows with increasing Tb3+ doping concentration, as
presented in Fig. 8. The green emission from 5D4 → 7F5 is clearly
dominant with increasing Tb3+ doping concentration.
35
Fig. 9 CIE chromaticity diagram showing the x and y coordinates of
Y2O2S:Tb3+ samples at various Tb3+ concentrations excited by 254nm UV
light.
3+
Relative Intensity (a.u.)
Tb concentration
0.1%
0.5%
1%
2%
3%
5%
500
520
540
560
580
600
620
640
660
Tb3+ concentrations
1. 0.1 %
2. 0.5 %
3. 1%
4. 2%
5. 3%
6. 5%
680
Wavelength (nm)
15
20
25
30
Fig. 8 PL spectra of Y2-xTbxO2S (where x has values from 0.001 to 0.050)
NPs (normalized to the 418nm emission band).
As a direct result, the intensity of the blue emission decreases
while that of the green emission bands increases. Here we employ
the CIE chromaticity diagram to illustrate/analyze the x and y
coordinates that can be achieved by changing the Tb 3+
concentrations in the Y2O2S:Tb3+ and Gd2O2S:Tb3+ nanometre
sized phosphor particles when excited by 254nm radiation. In the
CIE chromaticity diagram presented in Fig. 9 of Y2O2S:Tb3+ it is
apparent that increasing the Tb3+ concentrations allows the colour
to vary from blue to a yellowish green. A linear increase in both
the x and the y coordinates as a function of Tb3+ concentration is
apparent.
A similar trend can be observed for the Tb3+ activated Gd2O2S
although the colour shifts are confined in a relatively narrow
range of greens (see Fig. 10). This is because the initial emission
colour of Gd2O2S:Tb3+ at 0.1mol% is green due to the lower CTS
position in Gd2O2S. Here the non radiative relaxation is from 5D3
to 5D4 excited states.
4 | Journal Name, [year], [vol], 00–00
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Fig. 10 CIE chromaticity diagram showing the x and y coordinates of
Gd2O2S:Tb3+ samples at various Tb3+ concentrations excited by 254nm
UV light.
Evidence for the blue transitions in Gd2-xTbxO2S loosing intensity
as a function of the Tb3+ concentration is presented in Fig. 11,
which displays normalised overlay spectra of Gd2-xTbxO2S
phosphors fired at 1100 ˚C. In Fig. 11 it is apparent that the blue
bands at 380 nm, 418 nm, 438 nm, 458nm and 475nm all
decrease in intensity as the Tb3+ concentration increases.
In Fig. 12 PL spectra of the Gd2-xTbxO2S phosphors (fired at 1100
˚C) are presented. The green bands at 480 nm, 544 nm, and the
bands at 587 nm and 623 nm all increase in intensity as the Tb3+
concentration increases. These changes in intensity of the bands
This journal is © The Royal Society of Chemistry [year]
3+
Y2O2S: Tb
360
380
400
420
3+
Intensity (a.u.)
Intensity (a.u.)
Tb concentration
0.1%
0.5%
1%
2%
3%
5%
440
460
480
500
520
Wavelength (nm)
350
10
15
in Figs. 11 and 12 explain the origins of the changes in the green
colours presented in the CIE chromaticity diagram in Fig. 10
(showing the x and y coordinates of Gd2O2S:Tb3+). The fact that
the blue bands still show a decrease in intensity with increasing
Tb3+ concentration indicates that enough of the electrons on the
5D excited state of Tb3+ in Gd O S are not activated to the CTS
3
2 2
and these states can then lose intensity by the same mechanism as
that found in Y2-xTbxO2S. Thus this is evidence that the cross
relaxation process that occurs between two adjacent Tb 3+ cation
pairs in Y2-xTbxO2S also explains the findings presented here for
the Gd2-xTbxO2S phosphors.
500
550
600
650
700
750
3+
Tb concentration
0.1%
0.5%
1%
2%
3%
5%
2
0.24
-1
-2
-1
Radiance(W sr m nm )
5
450
Luminance (cd/m )
Fig. 11 PL spectra of Gd2-xTbxO2S (where x has values from 0.001 to
0.05) NPs (normalized to the 545 nm emission band)
400
Wavelength (nm)
Fig. 14 CL emission spectra of 1 mol% Tb3+ doped Y2O2S.
25
0.16
0
1
2
3
4
5
3+
Tb concentration (%)
0.08
0.00
Intensity (a.u.)
3+
Tb concentration
0.1%
0.5%
1%
2%
3%
5%
450
35
intensity ratio of green/blue emission grows almost exponentially
with Tb3+ concentration, which is in good agreement with a
previous report made by observing the bulk phosphor 17. In fact
the inaccuracy of the points from the lowest concentrations may
be disguising a totally exponential behaviour.
40
50
Fig. 13 The intensity ratio of green/blue PL emission in Tb3+ activated
Gd2-xTbxO2S (where x has values from 0.001 to 0.05) (I545/I418) as a
function of Tb3+ concentration.
This journal is © The Royal Society of Chemistry [year]
650
Fig. 15 Overlay of the CL emission spectra of Y2O2S: Tb3+ at the Tb3+
concentrations given in the Fig. Inset: CL luminance variation with Tb3+
concentration in Y2O2S.
45
Fig. 13 presents a plot of the intensity ratio of green/blue
emission (I545/I418) as a function of Tb3+ concentration. The
600
30
Wavelength (nm)
20
550
Wavelength (nm)
480 500 520 540 560 580 600 620 640 660 680 700
Fig. 12 PL spectra of Gd2-xTbxO2S (where x has values from 0.001 to
0.05) NPs (normalized to the 380 nm emission band).
500
55
In both Y2O2S:Tb3+ and Gd2O2S:Tb3+ nanometre sized phosphor
particles it has been shown here that the synthesis allows total
control of the Tb3+ dopant concentration as is apparent from the
plot in figs. 7 and 13 and also in the CIE diagrams in figs. 9 and
10. Fig. 14 displays CL spectra of 1 mol% Tb3+ doped Y2O2S.
The emission bands in the CL spectra are slightly different from
those in the PL spectra. For example in the CL spectra the 5D3 →
7F transitions are stronger than in the PL spectra relative to the
5
545nm transitions for 1 mol% Tb3+ doped Y2O2S. This finding is
most likely a result of the two different excitation processes for
PL and CL.
For the PL of Tb3+ activated Y2O2S, the Tb3+ cations are excited
from the ground state energy level to a higher excited state
energy level by directly absorbing photon(s) of incident light.
While for CL excitation, these Tb3+ luminescence centres are
predominantly excited by recombination of pairs of electrons and
holes that are generated inside the crystal by the incident electron
beam 26-27.
Journal Name, [year], [vol], 00–00 | 5
10
15
2
Luminance(cd/cm )
20000
15000
2
Luminance (cd/cm )
5
It’s has also been suggested that the sulphur anions in Y2O2S
rather than the crystal structure itself are probably responsible for
the indirect CL excitation of the luminescence centres in Y2O2S:
Tb3+ phosphors.
In Fig. 15 an overlay of CL emission spectra of Y2O2S: Tb3+ in
the visible light region is presented. The intensity of emission
band of the 5D3 → 7FJ transitions can be seen to decrease with
increasing Tb3+ concentration due to the cross relaxation process
discussed above. The inset displays a plot of the luminance
intensity of Y2O2S: Tb3+ as a function of various Tb3+
concentrations. The highest radiances were obtained at 545 nm
emission from the 0.5 and 5 mol% doped Y2O2S:Tb3+ samples.
The drop in radiance between these values might result from the
cross relaxation effects. The regain of radiance intensity up to 5
mol% doped Y2O2S:Tb3+ makes Y2O2S:Tb3+ a promising
candidate for green phosphors in field emission display devices.
18000
12000
0
1
2
3
4
5
3+
10000
Tb Concentration (%)
3+
Tb concentration
0.1%
0.5%
1%
2%
3%
5%
5000
0
1000
2000
3000
4000
5000
Voltage (V)
40
Fig. 17 CL luminance of Gd2O2S:Tb3+ samples as a function of
accelerating voltages (using 8.6 μA emission current and a defocused ebeam. Inset: The luminance as a function of Tb 3+ concentration at 5 kV
accelerating voltage.
45
Intensity (a.u.)
4. Conclusions
50
400
500
600
700
Wavelength (nm)
55
Fig. 16 The CL spectra of 2 mol% Tb3+ doped Gd2O2S.
20
25
30
35
Fig. 16 presents the CL spectra of 2 mol% Tb3+ doped Gd2O2S.
The emission bands in the CL spectra are slightly different from
those in the PL spectra as found for the Tb3+ doped Y2O2S. In this
case for the Tb3+ doped Gd2O2S it is the 5D4 → 7F6 transitions
that are the second strongest under CL which differs from the PL
behaviour where this is not so apparent. Again this finding is
most likely a result of the two different excitation processes for
PL and CL as explained above.
Optimisation of the Tb3+ activator concentration by CL
measurements indicated that a 2 mol% Tb3+ concentration gave
the highest luminance in the Gd2O2S:Tb3+ phosphor samples. The
luminance of the Gd2O2S:Tb3+ phosphor samples plotted against
accelerating voltage for an emission current of 8.6 μA are
presented in Fig. 17.
The luminance as a function of Tb3+ concentration at 5 kV
accelerating voltage is presented in the inset in Fig. 17. It can be
seen that the luminance intensity grows as Tb3+ concentration
increases from 0.5 mol% to 2 mol% and then decreases as it
increases from 2 mol% to 5 mol%.
6 | Journal Name, [year], [vol], 00–00
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To sum up, the conclusions are demonstrated as follows: In this
work we have shown that nanometre-sized Y2O2S and Gd2O2S
phosphors doped with various Tb3+ concentrations (ranging from
0.1 mol% to 5 mol%) were easily prepared and the resulting
phosphors luminescent properties were directly related to the
Tb3+ concentration. It has been demonstrated herein that in both
Y2O2S:Tb3+ and Gd2O2S:Tb3+ nanometre sized phosphor particles
the synthesis allows fine control of the Tb3+ dopant concentration.
The emission colour of the resulting phosphors was strongly
affected by both the different host lattices and the Tb3+ cation
doping concentration. The cross relaxation effect between two
adjacent Tb3+ cation pairs yields a colour shift towards green in
both Y2O2S:Tb3+ and Gd2O2S:Tb3+ as the concentration of the
dopant increases.
The initial emission colour of Tb3+ at 0.1 mol% is determined by
the relative CTS position of the host lattices. The cross relaxation
effect was observed in both the PL and CL spectra of Y2O2S:Tb3+
phosphors, indicating the emission colour of Y2O2S:Tb3+ could be
tuneable by simply varying Tb3+ doping concentration. The cross
relaxation effect might be responsible for the highest luminance
of Y2O2S:Tb3+ at 5 mol%.
The luminescent properties of nanometre sized Gd2O2S:Tb3+
phosphor samples have been investigated. Characteristic emission
spectra of Tb3+ have been observed. The intensity of emission
bands was shown to be dependent on both the activator
concentration and the firing temperature. Cross-relaxation of
Tb3+ cations could be observed in the spectra of Gd2O2S:Tb3+
phosphor which reduces the intensity of emission bands from 5D3
→ 7FJ (J= 6, 5, 4, 3) transitions and enhances that of emission
bands from 5D4 → 7FJ (J= 6, 5, 4, 3) transitions. The green
emission bands at 545 nm from 5D4 → 7F5 transition are the most
intense bands in every spectrum of Gd2O2S:Tb3+ phosphor in the
Tb3+ concentration range studied herein. This results from the fact
This journal is © The Royal Society of Chemistry [year]
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that the bottom of the CTS in Gd2O2S lies close to the 5D3 energy
level of Tb3+, and that the resulting thermal quenching between
these greatly reduces the electron population in the 5D3 level, but
does not take all of the population so that some blue bands can
still be seen. This residual blue band intensity is not sufficient to
dominate the spectra so that all are in the green region for the
Gd2O2S:Tb3+ phosphors but the trend with concentration is the
same as that in the Y2O2S: Tb3+ phosphors going more pure green
with increase in Tb3+ concentration as the blue intensity
disappears by cross relaxation.
55
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65
Acknowledgement
The authors thank the China Scholarship council (CSC) for PhD
support to X. Yan.
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Notes and references
a
20
Centre for Phosphor and Display Materials, Wolfson Centre for
Materials Processing, Brunel University, Uxbridge, Middlesex, UB8
3PH, UK. Fax: +44(0)1895 269737; Tel: +44(0)1895 266116; E-mail:
jack.silver@brunel.ac.uk b Tel: +44(0)1895 265628; E-mail:
george.fern@brunel.ac.uk
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