JOURNAL OF RARE EARTHS, Vol. 29, No. 5, May 2011, P. 426

Foundation item: Project supported by National Basic Research Program of China (2007CB935502) and National Natural Science Foundation of China (20921002)

Corresponding author: LI Chengyu (E-mail: cyli@ciac.jl.cn; Tel.: +86-431-85262208)

DOI: 10.1016/S1002-0721(10)60473-5

Long-lasting phosphorescence study on Y3Al5O12 doped with different concentrations of Ce3+

ZHANG Su ( )1,2, LI Chengyu ( )1, PANG Ran ( )1, JIANG Lihong ( )1, SHI Lili ( )1,2, SU Qiang ( )1

(1. State Key Laboratory of Application of Rare Earth Resources, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China; 2. Graduate School of the Chinese Academy of Sciences, Beijing 100049, China)

Received 20 October 2010; revised 7 December 2010

Abstract: Long-lasting phosphorescence (LLP) was observed in Ce-doped Y3Al5O12 phosphors synthesized in reducing atmosphere. The characteristic emission of the 2D–2F5/2 and 2D–2F7/2 transition of Ce3+ in photoluminescence (PL) and LLP spectra was studied. It was inter-esting that the ratio between the peak areas of 2D–2F5/2 and 2D–2F 7/2 transitions in the PL spectrum was different from the ratio of that in LLP emission spectrum. And the ratios had different change regularities with increased Ce3+ concentration. The possible reason was attributed to the defect in the YAG host, which was affected by increasing the Ce3+ concentration. There were indications that the defect in the Ce3+-doped YAG samples was strongly associated with oxygen vacancy. And the defect levels were studied through thermoluminescence (TL) experi-ment. The results showed that the trap depth was between 0.6 and 0.65 eV, and the kinetic order of the LLP was 2.

Keywords: YAG; cerium; defects; long-lasting phosphorescence; rare earths

Recently much attention has been paid to YAG:Ce as a phosphor in lighting field accompanied with the blue emit-ting GaN LED[1–3]. Moreover, YAG:Ce crystal has been re-ported as a promising scintillator[4,5]. However, although sev-eral references reported the short time phosphorescence in garnet materials, the long-lasting phosphorescence (LLP) property in YAG was not noticed. In this paper, the LLP was observed in Ce3+-doped Y3Al5O12 phosphors synthesized in reducing atmosphere. The luminescence still can be seen by naked eyes in darkness even after 8 h after adequately ex-cited with 254 nm UV light. Therefore what attracts us is the LLP properties and how YAG:Ce3+ is endowed with such a property.

According to our previous researches, suitable trap is nec-essary to form LLP and the trap originates from the defects in the host lattice[6,7]. Up to now, comprehensive and sys-tematic studies have been given to the point defects in Ce doped YAG or other garnet-structure materials[8–15], because the YAl antisite defects and vacancy-type defects strongly affect the luminescence performance of these materi-als[12,16,17]. However, some certain defects in the host will be of great benefit to LLP. In this research, in order to study the defect in Ce3+ doped YAG and the relationship between de-fect and LLP, two sets of samples synthesized in air and re-ducing atmosphere were studied. The LLP was only ob-served in the samples synthesized in reducing atmosphere. Since the reducing atmosphere will generate plenty of oxy-gen vacancy (VO) in host, it is proposed that the LLP were

associated with the defect of VO. And the effects of the Ce3+ doping concentration on the defects were studied. In addition, Ref.[18] reported the relationship between PL emission color and Ce3+ concentration in garnet phosphors. Here, the char-acteristic emission of 2D–2F5/2 and

2D–2F7/2 transitions of Ce3+ in LLP spectra were studied. We found that the ratio between the two transitions in PL and LLP spectra had dif-ferent change patterns. The possible reason was attributed to the energy transfer between trap and Ce3+, which was af-fected by increasing the Ce3+ concentration. We considered that the study could deepen the defects analysis in YAG and the mechanism of LLP.

1 Experimental

A series of Y3Al5O12 samples doped with x mol.% Ce concentration were prepared (x=0.05, 0.1, 0.3, 0.6 and 0.9). The raw materials are Y2O3 (99.999%), Al2O3(99.999%) and CeO2(99.999%), which were ground thoroughly in an agate mortar. The mixtures were placed in alumina crucibles, and then sintered at 1550 ºC for 4 h in reducing atmosphere (CO) and in air, respectively.

The structures of the as-prepared samples were analyzed by Rigaku D/max-II X-ray diffractometer at 40 kV and 20 mA with Cu K 1 ( =0.15405 nm) radiation. The X-ray diffrac-tion measurements were taken at the interval of Bragg angle 2 (20º 2 90º) at scanning rate of 10 (º)/min. Photolumi-nescence excitation and emission spectra were measured

ZHANG Su et al., Long-lasting phosphorescence study on Y3Al5O12 doped with different concentrations of Ce3+ 427

with a Hitachi F-4500 fluorescence spectrofluorometer. The LLP spectra were measured using the same fluorescence spectrophotometer after the samples were irradiated by a UV lamp ( max=254 nm) with a power density of 2 mW/cm2. Thermoluminescence (TL) glow curves were carried out in the range of 20–500 °C on an FJ-427A1 TL meter (Beijing Nuclear Instrument Factory). All the experiments were per-formed at room temperature.

2 Results and discussion

2.1 XRD analysis

Fig. 1 shows the XRD patterns of the obtained YAG pow-der with the maximum content of doped Ce3+ ions we inves-tigated. All the XRD peaks were indexed in terms of the gar-net structure according to the standard PDF# 72-1315. The results indicate that the samples are single phase and the co-doping of Ce3+ does not cause any significant change of the host structure.

2.2 PL and long-lasting phosphorescence spectra

Fig. 2 shows the PL emission and excitation spectra of YAG:Ce3+ synthesized in air and CO atmosphere. For the sample synthesized in CO atmosphere, three excitation peaks

Fig. 1 XRD patterns of YAG doped with 0.9 mol.% Ce

Fig. 2 PL excitation spectra (left), and PL emission spectra (right)

of YAG:Ce3+ 0.6 mol.% synthesized in air (solid line) and in CO atmosphere (dashed line), measured at room temperature

located at about 270, 340 and 460 nm were observed. The 340 and 460 nm peaks are attributed to the splitting of the 5d levels of the Ce3+ [19], nevertheless the 270 nm peak is due to the defect absorption. Fig. 2 shows that the 270 nm peak disappears for the samples synthesized in air. Rotman et al.[13,20] reported the defect luminescent properties of YAG:Ce and the relationship between oxygen vacancy (VO) and preparing atmosphere. It is deduced that the excitation peak ranging from 300 to 200 nm is related with VO in the YAG host. Springis and Pujats et al.[21,22] reported that ca. 400 nm emis-sion and 240 nm excitation comes from F+ center. Therefore, we consider that the 270 nm peak is due to the VO defect.

The PL emission peaked at ca. 540 nm is ascribed to the allowed electric dipole transition 4f05d1 4f15d0 (2D 2F) of Ce3+ ions which substitute the dodecahedral sites of Y with D2 point group symmetry[23]. Fig. 2 shows that the influence of the reactive atmosphere on the PL emission spectra was revealed in the peak intensity, i.e. the emission of the sample synthesized in air is stronger than that of the sample synthe-sized in CO. The peak shape and position show no signifi-cant difference. This result means that the defect in YAG host will hamper the emission of Ce3+. But the interesting thing is that no LLP was observed in the samples synthe-sized in air. Therefore, it is reasonable to consider that the generating of LLP in YAG is related with VO.

2.3 Effect of Ce3+ concentration on PL and LLP emission

Figs. 3 and 4 show the PL emission and the LLP spectra of YAG doped with different concentrations of Ce3+. The emission can be deconvolved into two peaks, peak 1 and peak 2, located at about 520 and 580 nm which are due to transition from 4f05d1 to different 4f ground state, 2F5/2 and 2F7/2, respectively. The similar peak shape of LLP to that of PL emission spectrum means that the LLP emission origi-nates from the same transition, i.e. 2D–2F5/2 and 2D–2F7/2. The ratios of the areas of peak 2 to peak 1 in the PL and LLP spectra are shown in Fig. 5. It is interesting that the ratios for PL and LLP have different change regularities. Fig. 5 shows that for the PL emission, the ratio kept rising with increasing

Fig. 3 PL emission spectra of YAG doped with x mol% Ce (x= 0.05,

0.1, 0.3, 0.6 and 0.9) excited at 460 nm (Insert: Gauss fit emission of the YAG: 0.6 mol.% Ce3+ excited at 460 nm)

428 JOURNAL OF RARE EARTHS, Vol. 29, No. 5, May 2011

Fig. 4 LLP spectra of YAG:x mol.% Ce (x=0.05, 0.1, 0.3, 0.6 and

0.9) measured after irradiated by a UV lamp ( max=254 nm) at room temperature (Insert: Gauss fit emission of YAG:0.6 mol.% Ce3+)

Fig. 5 Ratios of Gaussian peak 2/peak 1 of PL (circular) and LLP

(square) emission of YAG:Ce as a function of Ce3+ contents (0.05, 0.1, 0.3, 0.6 and 0.9 mol.%)

the Ce3+ concentration. However, for the LLP emission, the ratio was first increased and then decreased with the increase of Ce3+ concentration and reached the maximum intensity at 0.6 mol.% (shown in Fig. 5). Similarly, the change regulari-ties of total emission intensity of Ce3+ in PL and LLP were also different. Fig. 6 shows that the intensity of PL emission increased continuously and the intensity of LLP emission, however, reached its maximum at 0.6 mol.% with increasing the Ce3+ concentration.

It is well known that the 5d–4f emission of Ce3+ depends strongly on the crystal field. The coordination environment of Ce3+ was changed with increasing the doping concentra-tion. However, the different change regularities between PL and LLP mean that the PL emission and the afterglow emis-sion are different processes. For the former, the general case is that in the excitation period the electrons are pumped from the ground level to the excitation level and subsequently re-turn to the ground level to result in the radiative transitions. However, for the LLP, it is a more trap-related process. The energy after excitation is stored in the traps in the form of capturing carriers and then the LLP is performed through energy transfer between traps and emission centers. There-fore, a possible reason for the concentration-dependent LLP

Ce3+ emission is that the changing of the Ce3+ concentration will affect the energy transfer process. When the carriers thermally released from the traps, they tend to transfer to the Ce3+ ions having one certain dominant 2D 2F transition re-sulting in the different change trends of ratio. Ref. [18] re-ported that there is perturbed “sites” for Ce3+ ions in garnet lattice besides the intrinsic dodecahedral site. And this per-turbed “sites” gives a possible reason for the redshift of the d–f transition in garnet. Here, the different ratios between the transition 2D–2F7/2 and 2D–2F5/2 give us an evidence that multiple Ce3+ sites in these garnets could exist. And we de-duce that these multiple Ce3+ sites are possibly due to the perturbation of intrinsic defects.

Moreover, having a maximum of LLP intensity (observed in Fig. 6) is because that the density and depth of the trap are affected by increasing the Ce3+ doping concentration since the energy is stored in these traps. High density and suitable depth will be of benefit to LLP. And the properties of traps can be obtained by thermoluminescence experimental re-sults.

2.4 Thermoluminescence (TL) properties

The TL glow curves (Fig. 7) of the YAG materials doped with 0.6 mol.% Ce3+ were measured. It is obvious that the TL glow curve exhibits three components located at ca. 65,

Fig. 6 Intensity of PL (circular) and LLP (square) emission of YAG: Ce

as a function of Ce3+ contents (0.05, 0.1, 0.3, 0.6 and 0.9 mol.%)

Fig. 7 Thermoluminescence glow curves of the YAG:0.6 mol.% Ce3+

synthesized in air and reducing atmosphere with different delay time after the 254 nm UV irradiation 1 min operating at a linear heating rate of 3 ºC/s

ZHANG Su et al., Long-lasting phosphorescence study on Y3Al5O12 doped with different concentrations of Ce3+ 429

125 and 280 ºC. However, the 65 ºC peak is absent when the TL glow curve of the sample was measured after it was ex-cited for 12 h (as shown in Fig. 7 curve (2)). Now that the LLP could not be seen by naked eyes in darkness, it is rea-sonable to consider that the LLP of YAG:Ce at room tem-perature mainly arises from the recombination of the carriers escaped from the trap at about 65 ºC. It is similar to our pre-vious research that the depth of traps related with TL peaks ranging from 0 to 100 ºC is suitable for LLP at room tem-perature[24,25].

As a comparison to the sample prepared in reducing at-mosphere, the sample with 0.6 mol.% Ce was prepared in air atmosphere. The TL curve of the sample is also shown in Fig. 7, curve (3). It is obvious that no peak is located in the range from 20 to 100 ºC. Taking into account these findings and considerations, it can be concluded that the trap with a depth located around 65 ºC is associated with the defects of oxygen vacancy (VO), which determines the LLP in Ce- doped YAG.

Moreover, the TL glow curves (Fig. 8) of the YAG mate-rials doped with Ce3+ at various doping concentrations were measured. It is obvious that the TL glow curves of the 0.3, 0.6 and 0.9 mol.% doped materials exhibit three components located at ca. 65, 125 and 280 ºC. However, the samples with Ce3+ concentration lower than 0.3 mol.% show much simpler glow curves, with only two bands peaking around 65 and 280 °C. Therefore, the glow curves show concentration dependence: (1) the TL peaks located at ca. 65 and 280 °C were increased and then decreased with the increase of Ce3+ con-centration and reached the maximum intensity at 0.6 mol.% of Ce3+; (2) a TL peak located at ca. 125 ºC appeared with 0.3 mol.% of Ce3+. Accordingly, it is deduced that the Ce3+ ions affect the defect in YAG and do not just serve as the ac-tivator. And it is reasonable to deduce that the different change regularity of the ratios is possibly related to the change of the defects.

Utilizing the peak-shape method and the usual general- order kinetics expressions, the kinetic order and depths of the traps in YAG:Ce can be calculated from the TL curve by the following function[26]:

I(T)=snoexp(–E/kT)/o

( 1)

1 ( 1) exp( )T

T

b b

b E kT dT (1)

where I(T) is the TL intensity; s=s nob 1, b is the kinetic order;

no is the number of trapped electrons before readout; k is the Boltzmann constant, T represents the absolute temperature of heating, To is the initial absolute temperature; is the heating rate. Here, =2 ºC/s. As shown in Fig. 9, the calculated curves fit very well with the experimental curve. The calcu-lated trap depths and parameters of the TL curve of YAG:Ce (curve1 in Fig. 8) are shown in Table 1. According to the calculated results, the broad TL band can be fitted with seven sub-curves peaking at 320, 347, 385, 425, 487, 544 and 604 K, respectively, and the corresponding depth of traps are 0.6, 0.61, 0.62, 0.66, 0.754, 0.84 and 0.94 eV, re-spectively. Combining the results of Fig. 7, the low tem-perature TL peaks (320, 347 and 385 K) are thought to ac-

Fig. 8 Thermoluminescence glow curves of the YAG:Ce3+ synthe-

sized in reducing atmosphere as a function of the Ce3+ con-centrations

Fig. 9 Peak-shape method and the usual general-order kinetics ex-

pressions calculated thermoluminescence glow curve of the YAG:0.6 mol.% Ce3+ synthesized in reducing atmosphere

Table 1 Parameters of the TL glow curve for the YAG:0.6 mol.% Ce3+ sample

Peaks E/eV s/s–1 no/cm–1 Tm/K b

1 0.6 8.53×108 4.28×106 320 2

2 0.61 1.87×108 3.47×106 347 2

3 0.62 2.36×107 5.08×106 385 2

4 0.66 1.24×107 2.54×106 425 1.7

5 0.75 1.01×107 1.99×106 487 2

6 0.84 1.00×107 1.29×107 544 1.8

7 0.94 1.00×107 4.64×106 604 2

count for the LLP at room temperature in the present work. The E of the three peaks are around 0.6–0.65 eV. And the parameter b of peak 1, 2 and 3 were 2, namely the kinetic order of LLP is 2.

3 Conclusions

With the increasing of Ce3+ concentration, the change regularities of the ratios between the transition 2D 2F5/2 and 2D 2F7/2 in PL and LLP spectra were different. This differ-ent regularities were due to the change of the defects in the host and the energy transfer process between trap and emis-sion center. Furthermore, VO played a key role in forming the

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LLP in Ce3+ doped YAG phosphors. Therefore, it was rea-sonable to propose that defects related with VO were the traps responsible for the temporary trapping of charges. And the TL results showed that the trap depth was between 0.6 and 0.65 eV, and the kinetic order of the LLP decay was 2. This study would contribute to a deep understanding of the mechanism of LLP and the defects in YAG:Ce materials.

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