Ga Substitution on Photoluminescence and

Article pubs.acs.org/JPCC

Effect of Al/Ga Substitution on Photoluminescence and Phosphorescence Properties of Garnet-Type Y3Sc2Ga3−xAlxO12:Ce3+ Phosphor Yi Luo† and Zhiguo Xia*,†,‡ †

School of Materials Sciences and Technology, China University of Geosciences, Beijing 100083, China School of Materials Sciences and Engineering, University of Science and Technology Beijing, Beijing 100083, China

‡

ABSTRACT: Garnet-type Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0−3) phosphors have been prepared by using the high temperature solidstate reaction. Al/Ga ratio dependent Y3Sc2(Ga,Al)3O12 phase structures, photoluminescence (PL) properties, and long-lasting phosphorescence (LLP) properties for the Ce3+-doped phosphors have been investigated in detail. The PL emission bands of Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ showed a red-shift tendency gradually from 503 to 520 nm with increasing Al content (x value), and the emission intensities increased first, maximized at x = 1, and then decreased. Y3Sc2Ga3O12:Ce3+ can show the green LLP emission, and afterglow can be obviously enhanced when (1) Al ions replaced Ga ions for a small amount and (2) the reaction atmosphere was varied from reducing to oxidation one. The afterglow emission, decay curves, and thermoluminescence (TL) of Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0−0.7) phosphors have been investigated, and related luminescence mechanisms are systematically discussed as well. can also be replaced by Al3+, or Si4+ or Ge4+, while maintaining the garnet crystal structure type. This wide solid solution compositional variation enables optimization of properties, and it possibly will produce some defects for the designed afterglow emission.9,10 To the best of our knowledge, there are few studies of the afterglow luminescence of Ce3+ in garnet compounds. Kanai et al. reported the afterglow properties of Gd3+δ(Al,Ga)5‑δO12:Ce3+ by X-ray excitation,11 and Jumpei Ueda has previously reported the afterglow luminescence in Y3Sc2Ga3O12:Ce3+.12 However, their structural evolution and the comparison of the photoluminescence (PL) properties and LLP properties still deserve to be studied for the understanding of the intrinsic physical chemistry properties of this host based on variation and control of the Al/Ga ratios. Herein, we have investigated the Al/Ga ratio dependent Y3Sc2(Ga,Al)3O12 phase structures, PL properties, LLP properties for the Ce 3+ -doped phosphors, and their related luminescence mechanisms. As for Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0−3) solid solutions, there is a red-shift tendency for the photoluminescence after Al ions replacing Ga ions under the excitation of 414 nm. A slight amount of replacement of Ga3+ by Al3+ (x < 1) enhanced the green afterglow luminescence obviously, and the phosphors prepared under oxidation (air) atmosphere show better afterglow performance when compared with those in reducing (H2/O2) atmosphere. The

1. INTRODUCTION Long-lasting phosphorescence (LLP) material is a kind of energy-storing material which can store the absorbed light energy and release it as luminescence after repealing of the excitation.1,2 As the environmentally friendly material, it has extensive practical and potential applications in various important fields, such as emergency signs, electronic displays, fiber optic thermometers, imaging or optical memory storage, and medical diagnostics.3−6 So far, there have been a number of reports on the afterglow luminescent material, and SrAl2O4:Eu2+-Dy3+, reported by Matsuzawa et al., which exhibited the brightest emission intensity and longest afterglow time among all the LLP materials ever reported.5 Recently, white light emitting diodes (LEDs) which consist of blue LEDs and yellow phosphors have started to be widely used as indoor illumination in place of fluorescent lamps for their high luminous efficiency, long lifetime, and energy saving ability.7 As is known to all, the Ce3+-doped garnet type yellow phosphors have lots of advantages, for example, strong absorption in the blue region, a broad emission band, and high quantum efficiency.8 Therefore, it will also be potentially useful if the Ce3+-doped garnet-type materials can be developed to the LLP phosphors, which will in turn make it a good candidate for some potential applications based on the LED devices. Garnet phosphors are unique in their tunability of the luminescence properties through variations in the {A}, [B], and (C) cation sublattice. The A site can be replaced by the rare earth ions, or divalent cations, such as Ca2+, Mn2+; and B site can be replaced by Al3+, Ga3+, Sc3+, and even Mg2+. The C site © 2014 American Chemical Society

Received: July 31, 2014 Revised: August 24, 2014 Published: September 11, 2014 23297

dx.doi.org/10.1021/jp507695n | J. Phys. Chem. C 2014, 118, 23297−23305

The Journal of Physical Chemistry C

Article

Figure 1. Rietveld analysis patterns for X-ray powder diffraction data of Y2.96Sc2Ga3O12:0.04Ce3+ sample. The red dots are calculated intensities, and the lines are the observed intensities; their differences are shown in a gray solid line. The inset is the schematic diagram of the crystal structure of Y3Sc2Ga3O12.

electric furnace. The TL spectra were recorded by using microcomputer TL dosimeters (FJ427-AL, Beijing Nuclear Instrument Factory, Beijing, China).

afterglow decay curves and thermoluminescence (TL) results are also discussed in detail, and a possible mechanism of afterglow properties generated in Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0−0.7) is also described.

3. RESULTS AND DISCUSSION The initial model compound Y3Sc2Ga3O12 belongs to a typical garnet structure possessing a cubic crystal system with Ia3d̅ symmetry. Y atoms occupy 24 (c) sites of 8-fold coordination (distorted cubic lattice), Sc atoms are at 16 (a) sites of 6-fold coordination, and Al atoms are at 24 (d) sites of 4-fold coordination. Each [ScO6] octahedron is connected to six [GaO4] tetrahedrons while each [GaO4] tetrahedron is connected to four [ScO6] octahedrons by sharing the corners and Y3+ locate at the space inside the [GaO4] and/or [ScO6] framework, as shown in the inset of Figure 1. In order to further study the crystal structure of the as-prepared phosphors, the Rietveld structural refinement for the selected composition of Y2.96Sc2Ga3O12:0.04Ce3+ is performed, and the result is shown in Figure 1. The black lines and red dots present the observed and calculated patterns, respectively, and it showed that they matched very well. The crystallographic data, details on the data collection, refinement parameters, and the fractional atomic coordinates and isotropic isotropic displacement parameters are listed in Tables 1 and 2, respectively. The cell parameters are a

2. EXPERIMENTAL PROCEDURE A series of crystalline samples of Y3Sc2Ga3−xAlxO12:Ce3+ (x = 0, 0.3, 0.5, 0.7, 1, 2, 3) phosphors were synthesized through a conventional solid-state reaction under controlled reaction atmosphere including the air atmosphere or 5% H2−95% N2 reducing atmosphere, respectively. Y2O3 (99.5%), Sc2O3 (99.5%), Ga2O3 (99.5%), Al2O3 (99.5%), BaF2 (99.5%), and CeO2 (99.995%) were used as raw materials. In a typical process, the starting materials were weighed in the given stoichiometric amounts with the 5% BaF2 as flux and thoroughly mixed in an agate mortar by grinding. Then, the mixture was fired at 1520 °C for 5 h. After that, the samples were furnace-cooled to room temperature and ground again into fine powders. Finally, the Y3Sc2Ga3−xAlxO12:Ce3+ phosphors were obtained for the following measurement. Powder X-ray diffraction (XRD) data were collected on an X-ray powder diffractometer (SHIMADZU, XRD-6000, Cu Kα radiation, λ = 0.154 06 nm, 40 kV, 40 mA), and the continuous scanning rate (2θ ranging from 10° to 70°) was 4° (2θ) min−1. The powder diffraction pattern for Rietveld analysis was collected with the same diffractometer. The step size of 2θ was 0.016°, and the counting time was 1 s per step. Rietveld refinement was performed by using TOPAS 4.2 software. Room temperature photoluminescence excitation (PLE) and emission (PL) spectra were characterized on F-4600 fluorescence spectrophotometer with a photomultiplier tube operating at 400 V, and a 150 W Xe lamp used as the excitation source. Before we can measure the afterglow decay curves and afterglow emission spectra, the samples were first irradiated for 15 min with a 365 nm UV lamp; after that, they are measured on the same F-4600 fluorescence spectrophotometer. The temperature dependent luminescence properties were measured on the same spectrophotometer, which was combined with a self-made heating attachment and a computer-controlled

Table 1. Main Parameters of Processing and Refinement of the Y2.96Sc2Ga3O12:0.04Ce3+ Sample (Y0.96Ce0.04)3(Sc0.911(7)Ga0.089(7))2Ga3O12 space group a, Å V, Å3 Z 2θ, deg Rwp, % Rp , % Rexp, % χ2 RB, % 23298

Ia3d̅ 12.4801(1) 1943.80(5) 8 5−140 7.77 5.77 3.89 2.00 1.22



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Table 2. Fractional Atomic Coordinates and Isotropic Displacement Parameters (Å2) of (Y0.96Ce0.04)3(Sc0.911(7)Ga0.089(7))2Ga3O12 Sc1 Ga1 Ga2 Y Ce O

x

y

z

Biso

occ

0 0 0.375 0.125 0.125 0.02777(18)

0 0 0 0 0 0.05876(17)

0 0 0.25 0.25 0.25 0.15508(18)

0.41(4) 0.41(4) 0.33(3) 0.47(3) 0.47(3) 0.50(6)

0.9113(66) 0.0887(66) 1 0.96 0.04 1

Figure 2. XRD patterns of Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0, 1, 2, 3) are shown in lines a−d, together with the standard data for Y3Sc2Ga3O12 and Y3Sc2Al3O12 as reference (on the left) and variation of unit cell parameters a, V of Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ dependent on x values (on the right).

Figure 3. (a) PLE and (b) PL spectra of Y3−xSc2Ga3O12:xCe3+ with varying Ce3+ concentrations.

= b = c = 12.4801(1) (Å), and the unit cell volume V = 1943.80(5) (Å3). The final results of refinement are converged to Rwp = 7.77%, Rp (%) = 5.77%, and χ2 = 2, respectively. It is also found from the refinement result that Ga not only exists in a 4-fold coordination site, but also exists in a 6-fold coordination site for a small amount. It is thought that such a mixed crystallographic position should be related with the formation of the antisite defects, which will benefit the LLP behavior. As we know, the antisite defects happen when one atom (such as 4-fold Ga3+) replaces another atom (such as 6fold Sc3+) in the Y3Sc2Ga3O12 host, which make a distortion in

local environment. Furthermore, Figure 2 depicts the XRD patterns of the Y3Sc2Ga3−xAlxO12:Ce3+ (x = 0, 1, 2, and 3) phosphors together with the standard cards of JCPDS 25-1246 (Y3Sc2Ga3O12) and JCPDS 79-1846 (Y3Sc2Al3O12). It can be seen that all the diffraction peaks of the Y3Sc2Ga3O12:Ce and Y3Sc2Al3O12:Ce are well-indexed, and the diffraction peaks of the as-prepared samples shifted to higher angles with increasing Al content (x value) owing to the radius difference between Ga3+ and Al3+. This result suggests that the Ga3+ ions (0.47 Å: 4-fold) are substituted by the smaller Al3+ ions (0.39 Å: 4-fold). The variation of unit cell parameters (a) and unit cell volume 23299



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Figure 4. (a) PLE and (b) PL spectra of Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0, 0.1, 0.3, 0.5, 0.7, 1, 2, 3) obtained under O2 atmosphere, and (c) CIE chromaticity diagram and the selected phosphor images. The inset shows the normalized emission spectra of Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0, 0.1, 0.3, 0.5, 0.7, 1, 2, 3).

(V) of Y3Sc2Ga3−xAlxO12:0.04Ce3+ dependent on x values is also given in Figure 2, which clearly found that they are proportional to the x value. The linear evolution result indicated the incorporation of Al/Ga substitution and formation of isostructural garnet-type solid solutions of Y3Sc2Ga3−xAlxO12. The photoluminescence excitation (PLE) and PL spectra of Y3−xSc2Ga3O12:xCe3+ (x = 0.02, 0.04, 0.06, 0.08, 0.1, 0.2) phosphors are presented in Figure 3a,b. As is shown in Figure 3a, two broad PLE bands are observed at near 354 and 414 nm by monitoring at 503 nm. The PLE bands at 414 and 354 nm are attributed to the transitions from the ground energy level to excited 5d1 and 5d2 levels of Ce3+, which are the lowest and the second lowest 5d levels, respectively.13 Y3Sc2Ga3O12:xCe3+ exhibits green emissions with typical unsymmetrical doublet bands peaking at 503 nm that extend from 450 to 650 nm which are shown in Figure 3b. The asymmetric emission band under the excitation of 414 nm can be fitted to two Gaussian profiles centered at 489 nm (20 429 cm−1) and 530 nm (18 869 cm−1) which is also similar to the theoretical value of 2000 cm−1 for 2F7/2 and 2F5/2 ground states of Ce3+. It can be also found in Figure 3b that the emission intensity of Ce3+ first increased with the increase in its concentration, and reached the maximum at x = 0.04, and then the emission intensity decreased with further increasing concentration ascribed to the concentration quenching effect. Herein, the optimum doping concentration of Ce3+ is determined to be 0.04 in Y3Sc2Ga3O12 host. It is accepted that concentration quenching is mainly caused by energy transfer among Ce3+ ions, the probability of which increased as the concentration of Ce3+ increased. It is known to all that the concentration quenching phenomena occurred when the average distance between identical Ce3+ ions is too small. Thus, the critical distance of energy transfer Rc become a key parameter. It can be calculated by the following formula suggested by Blasse:14

⎡ 3V ⎤1/3 R c ≈ 2⎢ ⎥ ⎣ 4πxcN ⎦

(1)

Here, V stands for the volume of the unit cell, xc is the critical concentration of Ce3+ ions, and N is the number of cations in the unit cell. For the Y3Sc2Ga3O12 host, the crystallographic parameters are V = 1943.80 Å3, N = 8, and xc is 0.04. The critical distance Rc is determined to be 22.64 Å indicating that the energy transfer mechanism in this system is not by exchange interaction but by electrostatic interaction.15 Variations in the PL and PLE spectra of the Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0, 0.1, 0.3, 0.5, 0.7, 1, 2, and 3) systems depending on different Ga/Al ratio are shown in Figure 4a,b. The similar PL and PLE spectral profile as shown in Y3Sc2Ga3O12:Ce3+ sample can be found except for the corresponding intensities. The emission intensity is enhanced first when x is below 1, and then decreased when x = 2 and x = 3. As also shown in Figure 4c, the emission bands of all the phosphors with various x values have a red-shift tendency gradually from 503 to 520 nm as the Al concentration increases from x = 0 to x = 3. Such a red-shift behavior should be attributed to the variation of the crystal environment energy of f−d transition of Ce3+ modified by Ga/Al replacement in the crystal lattice. After Al ions substituting Ga ions, the crystal lattice produces slight shrinkage, which induces the change in crystal fields surrounding the Ce3+ and enhances the splitting of the 5d state. Therefore, red-shift of the emission band appears along with the introduction of Al for Ga sites (the normalized PL spectra in the inset of Figure 4c), as also found from the CIE values and phosphor images in Figure 4c. Except for the photoluminescence behaviors, as-prepared Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ samples show green afterglow emission when x = 0 and x = 1. Accordingly, Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0, 0.1, 0.3, 0.5, 0.7) are selected as the studied compositions of the green-emitting afterglow phosphors. In order to explore the effect of the synthesis conditions 23300



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Figure 5. Afterglow spectra of Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0, 0.1, 0.3, 0.5, 0.7) under air atmosphere (a) and H2/O2 atmosphere (b), and the compared afterglow spectra under air and H2/O2 atmosphere.

Figure 6. Afterglow emission spectra (a) and afterglow decay curve (b) of Y2.96Sc2Ga3O12:0.04Ce3+ sample after the removal of excitation source.

Al/Ga substitution, the reaction atmosphere also plays an important role in the LLP behaviors, which is related with the formation of the defects. Compared with the samples prepared in O2 atmosphere, the samples prepared in H2 atmosphere which are presented in Figure 5 b possess the same broad persistent emission band at around λ = 495 nm. But the LLP intensities are obviously lower than that obtained in O2 atmosphere, as given in Figure 5c. Furthermore, there are several kinds of traps in Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ phosphors, such as oxygen vacancies, antisite defects, and impurity ions. When the antisite defects are formed, these defect can be electron traps or hole traps where Ce3+ can work as electron or

on the LLP properties, the afterglow emission spectra of Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0, 0.1, 0.3, 0.5, 0.7) prepared in air (O2) atmosphere and 5% H2−95% N2 reducing (H2) atmosphere are presented in Figure 5a,b. As is shown in Figure 5a, there are obviously strong persistent emission bands peaking at about 495 nm which is in agreement with the PL spectra, corresponding to the 5d−4f allowed transitions of Ce3+, and the peak positions and the shape of the emission spectra are identical in different samples. This result also indicates that a slight replacement of Ga3+ by Al3+ is suitable to produce defects in Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ phosphors and show better afterglow luminescence. Except for the effect of 23301



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hole capture.11,12 According to the results above, we choose the phosphors prepared in O2 atmosphere to study the afterglow luminescence properties. Figure 6a shows the afterglow emission of Y2.96Sc2Ga3O12:0.04Ce3+ sample obtained under O2 atmosphere after the removal of excitation source with different delay times, and Figure 6b gives its afterglow decay and the corresponding fitting curve. The persistent time is defined as the time when the intensity of afterglow luminescence becomes 1/10000 of the saturated fluorescence intensity under excitation (τ1/100000). From Figure 6a, we can see that the afterglow emission intensity goes down quickly at the beginning and then slows down. In addition, the decay process consists of a fast decay process and a slow decay part which are well-fitted into biexponential function as follows: I(t ) = I0 + A1 exp( −t /τ1) + A 2 exp(−t /τ2)

in the host causes the traps level to change and makes a contribution to store the energy which extends the afterglow time. However, when the composition is at x = 1, the afterglow emission decreased sharply, which should be ascribed to the defect equilibrium for the Y2.96 Sc 2 Ga 2.7 Al 0.3O 12:0.04Ce 3+ phosphor. It is generally accepted that the most efficient technique used to study the variation of the LLP properties is thermoluminescence (TL) measurement. By using TL technique, it is easier to evaluate the density and depth of traps generated in materials under the irradiation of UV light, and the position of TL bands represents the trap depth while the lower/higher temperature corresponds to the shallow/deeper traps.16−18 It is normal that the energy stored in the excessive shallow traps will be released at a very fast ratio under room temperature thermal balance; the energy stored in a relatively deep trap cannot return to the excited state under room temperature for the reason that the electrons have been strongly immobilized in trap.19 Therefore, appropriate traps are important to create LLP emission. As is reported in the previous work, the ideal trap depth for excellent long persistent luminescence was reported at 0.6−0.7 eV.5 In order to get further information about the traps and detect the trapping levels, TL flow curves of these phosphors are recorded. TL measurements are performed for Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0, 0.3, 0.5, 0.7), and the collected TL curves are exhibited in Figure 8. It can be seen

(2)

Here, I(t) and I0 are the phosphorescence intensities at time t and 0, A1 and A2 are constants, t is time, and τ1 and τ2 are the decay times. For exploration of the afterglow decay behavior in more detail, the decay curves of Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0, 0.1, 0.3, 0.5, and 0.7) samples are recorded as shown in Figure 7 and are in accord with eq 2. The fitting parameters are

Figure 7. Afterglow decay curve of Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0, 0.3, 0.5, 0.7) after being irradiated for 15 min under 365 nm UV light.

Figure 8. TL curves of Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0, 0.3, 0.5, and 0.7) phosphors prepared under air atmosphere.

Table 3. Decay Curve Fitting Results of Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0, 0.3, 0.5, 0.7) x value x x x x

= = = =

0 0.3 0.5 0.7

A1

A2

τ1

τ2

39.22 61.22 59.57 44.45

79.06 124.46 130.84 71.62

45.71 76.87 78.61 95.53

8.54 14.57 13.33 14.57

that there is merely a single band located at 310 K for Y3Sc2Ga3O12:0.04Ce3+. For Al3+-doped ones, another band appears at around 400 K, and the relative intensity of TL curves increases with the increasing Al3+ concentration. It is known that the intensity of TL curves represents the concentration of carriers which are captured by traps. For the sake of clarifying the traps, a method provided by Chen is used to obtain the TL parameters by fitting experimental data according to the general order kinetics formula.20 Two deconvoluted bands are denoted as bands 1 and 2, respectively, and the depth values of each individual band can be calculated by the following eq 3:21

listed in Table 3. Obviously, the initial relative intensities increase in the order Y2.96Sc2Ga3O12:0.04Ce3+ < Y2.96Sc2Ga2.3Al 0.7 O 12 :0.04Ce 3+ < Y 2.96 Sc 2 Ga 2.5 Al 0.5 O 12 :0.04Ce 3+ < Y2.96Sc2Ga2.7Al0.3O12:0.04Ce3+. This result implies that the Ga/Al ratio in the Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ matrix can influence the afterglow properties. A slight amount of Al doping

E = Tm/500

(3)

E is the activation energy which stands for the activation energy which means the trap depth; Tm is the temperature of 23302



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performance. Otherwise, the intensity could decrease as a result of the escape of electrons under higher temperature. Therefore, the intensity of both excitation and emission spectra decreasing with the increasing temperature can be observed. In addition, the Arrhenius fitting of the emission intensity of Y2.96Sc2Ga2.3Al0.7O12:0.04Ce3+ phosphor and the calculated activation energy (ΔE) for thermal quenching are also investigated and given in Figure 9c. The activation energy (ΔE) can be expressed by eq 4:24

corresponding TL peaks. The calculated values of trap depths (E) of samples Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0, 0.3, 0.5, 0.7) for the three TL bands are listed in Table 4. It is reported Table 4. Estimated Trap Depth (E) of Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0, 0.3, 0.5, 0.7) Samples band 1

band 2

sample

Tm (K)

E (eV)

Tm (K)

E (eV)

x=0 x = 0.3 x = 0.5 x = 0.7

310 308 310 314

0.620 0.616 0.620 0.628

382 394 396

0.764 0.788 0.792

IT =

I0

(

ΔE

1 + c exp − kT

)

(4)

Here I0 is the initial emission intensity of the phosphor at room temperature, IT is the emission intensity at different temperatures, c is a constant, ΔE is the activation energy for thermal quenching, and k is the Boltzmann constant (8.617 × 10−5 eV). As shown in Figure 9c, the plot of ln[(I0/IT) − 1] versus 1/kT yields a straight line, and the activation energy ΔE is obtained 0.337 eV. So far, there is still a lack of a convincing mechanism of the LLP. Among all the reports about the LLP mechanism, the generally accepted mechanism is thermostimulated recombination of holes and electrons.25,26 On the basis of the abovementioned results, a possible mechanism is proposed to explain the generation of green LLP in Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ phosphor which is shown in Figure 10. Part of the defects may serve as hole capture centers, for example, antisite defects, while the others may serve as electron capture centers, for example,

that the TL bands are situated somewhere between 350 and 420 K if the materials show excellent LLP performance.22,23 Therefore, another reason for improving LLP performance may be that the new bands appeared at around 400 K in Al3+-doped samples. Figure 9 gives the temperature dependent excitation and emission spectra of Y2.96Sc2Ga2.3Al0.7O12:0.04Ce3+ from 30 to 250 °C under excitation at 414 nm. As shown in Figure 9, the intensity of both excitation and emission spectra decreased rapidly with the increasing temperature. This phenomenon can be explained as follows. On the basis of the TL discussion above, there are two bands in Y2.96Sc2Ga2.3Al0.7O12:0.04Ce3+, and the band locating at 314 K plays a key role in afterglow luminescence property. When the temperature is close to thermoluminescence temperature, it makes a good afterglow

Figure 9. Temperature dependent excitation (a) and emission (b) spectra of Y2.96Sc2Ga2.3Al0.7O12:0.04Ce3+ from 30 to 250 °C under excitation at 414 nm, and the Arrhenius fitting of the emission intensity and the calculated activation energy for thermal quenching (c). 23303



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-10-8237-7955. Fax: +86-10-8237-7955. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was supported by the National Natural Science Foundations of China (Grants 51002146, 51272242), Natural Science Foundations of Beijing (2132050), the Program for New Century Excellent Talents in the University of the Ministry of Education of China (NCET-12-0950), Beijing Nova Program (Z131103000413047), Beijing Youth Excellent Talent Program (YETP0635), the Funds of the State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University (KF201306), and the excellent tutor section of the Fundamental Research Funds for the Central Universities (2-9-2014-043).

Figure 10. Schematic diagram of the possible mechanism on LLP for the studied compositions of Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0, 0.3, 0.5, and 0.7).

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oxygen vacancies. Ce3+ works as activator itself as well as the trap provider. Under the light irradiation, the valence band electrons are promoted to the conduction band and generated excited carriers in the Y3Sc2Ga3O12 matrix (1), and the luminescence center Ce3+ acquired most of the excitation energy ascribed to the excited carries from the host through the energy transfer (2), and eventually generate the characteristic emissions of Ce3+ as the luminescence (5). However, there are some excited carriers captured by different trapping centers during a relaxation process (3). When turning off the light, the trapped carriers are passed to Ce3+ ions through the valence band and conduction band (4), eventually producing the green light emitting LLP. As discussed above, with increasing Al content in Y2.96Sc2Ga3−xAlxO12:0.04Ce3+, the 5d1 level and the conduction band become farther because of the increase in the crystal field strength and band gap. It has a bad influence on generating trap level and produces LLP, and that is the reason why afterglow luminescence is observed only in a small range of x.

REFERENCES

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4. CONCLUSIONS In conclusion, a series of garnet-type Y3Sc2Ga3−xAlxO12:Ce3+ (x = 0−3) phosphors are successfully synthesized by conventional high temperature solid-state reaction. As for Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0−3) solid solutions phosphors, green photoluminescence peaks possess a red-shift tendency from 503 to 520 nm after Al ions replace Ga ions under the excitation of 414 nm. A slight replacement of Ga3+ by Al3+ (x < 1) also exhibits enhanced green afterglow luminescence, and phosphors prepared under O2 atmosphere show better afterglow performance when compared with those obtained under H2 atmosphere. The afterglow decay curves reveal that the optimum studied composition of phosphor is Y2.96Sc2Ga2.7Al0.3O12:0.04Ce3+, and the fitting results show the doubleexponential decay mode in the phosphor. Thermoluminescence results indicate that there are several suitable stable traps with different depths in this system, and Y 2.96 Sc 2 Ga 2.3 Al0.7O12:0.04Ce3+ possesses the best afterglow behaviors. A possible mechanism of afterglow properties generated in Y2.96Sc2Ga3−xAlxO12:0.04Ce3+ (x = 0−0.7) is also described. 23304



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