Preparation and luminescence properties of Y_3−yAl_5−xGa_xO₁₂:Ce³⁺_y phosphors

Abstract

Y_3−yAl_5−xGa_xO₁₂:Ce³⁺_y phosphors were prepared by high temperature solid state reaction method. The crystal structures, the influence of Ga³⁺ concentration on the photoluminescence (PL), cathodoluminescence, thermal stability and morphology of the phosphors were studied in detail. The results indicated that diffraction angle of the samples decreased gradually with the increase of Ga³⁺ ions content in XRD pattern. The emission peak of the spectra show a progressive blue-shift, the intensity increased first and then decreased and the optimal Ga³⁺ concentration in Y_2.94Al_5−xGa_xO₁₂:Ce³⁺_0.06 phosphors is x = 0.75. The critical concentration of Ce³⁺ in YAGG:Ce³⁺ phosphors is affected with the ratio of Ga³⁺ to Al³⁺ and the Y_2.9Al_4.25Ga_0.75O₁₂:Ce³⁺_0.1 phosphor showed the best performance on PL. However, the optimum concentration of Y_3−yAl_5−xGa_xO₁₂:Ce³⁺_y phosphors is x = 1.5 and y = 0.04 when they were excited by cathode ray.

1 Introduction

Yttrium aluminum garnet (YAG) phosphors are an important class of materials for luminescence [1,2,3]. YAG crystal belongs to cubic crystal system with Ia3d (230) space group, the lattice parameters are a = b = c = 12.008 Å, α = β = γ = 90°, Z = 8. It possesses excellent chemical, physical and thermal stability, and is an excellent matrix for inorganic phosphor materials for lighting [4,5,6]. Rare-earth-doped phosphors have been used widely in field emission displays and white light-emitting diodes because of advantages such as excellent luminescent characteristics, good stability and high luminescence efficiency [7, 8].

In recent years, Ga³⁺-substituted YAG:Ce³⁺ phosphors have attracted much attention due to their high light yield compared with YAG:Ce³⁺ phosphors [9]. However, there is rare research on the occupancy sites of Ga³⁺ and Al³⁺ in garnet structure and their effects on optical properties in large Ga³⁺ concentration range. The main reason is that it is difficulty for Ga³⁺ to occupy tetrahedral site when three or more Al³⁺ were displaced, which leads to the appearance of impurities [10]. Meanwhile, as we know, there is no research on the comparison between cathodoluminescence and photoluminescence (PL) on YAGG:Ce³⁺ phosphors.

In this paper, a series of Y_3−yAl_5−xGa_xO₁₂:Ce³⁺_y phosphors were successfully synthesized by high temperature solid state reaction method. We solve the problem that the YAGG garnet phase is difficult to form when the amount of Ga³⁺ content is large and the optimal concentration of Ga³⁺ and Ce³⁺ in PL and cathodoluminescence was discussed in detail. What’s more, the effects of Ga³⁺ concentration on the thermal quenching, fluorescence lifetime and morphology properties were discussed.

2 Experimental

Y₂O₃ (99.99%), CeO₂ (99.99%), Al₂O₃ (99.99%), Ga₂O₃ (99.99%) were used as starting materials, which were precisely weighed according to the formula Y_3−yAl_5−xGa_xO₁₂:Ce³⁺_y with stoichiometric ratio. The appropriate amount of AlF₃ was additionally added as a fluxing agent. The raw powders were mixed, grained thoroughly and then sintered at 1500 °C for 4 h in reduction atmosphere.

Phases identification was performed by X-ray powder diffraction on a Rigaku D/max-II B diffractometer, operating at 40 kV and 20 mA, using Cu Kα radiation, (λ = 1.54056 Å). PL spectra were measured on a Hitachi F-7000 luminescence spectrophotometer equipped with a 150 W xenon lamp as the excitation. The temperature-dependence PL spectra were collected on a QE Pro high performance spectrometer equipped with a heating stage (THMS600, Linkam Scientific Instruments Ltd, UK), with a xenon lamp as the excitation source. The morphology was observed by scanning electron microscopy (SEM, JSM-6701F, JEOL, Japan). Cathodoluminescence (PL) spectra were measured on a OpticZenith Elec-ray system, operating at 6 kV. The luminescence decay curves were measured using a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) with a tunable laser (pulse width = 4 ns; gate = 50 ns) as the excitation (Continuum Sunlite OPO).

3 Results and discussion

Figure 1 shows XRD patterns of the Y_2.94Al_5−xGa_xO₁₂:Ce³⁺_0.06 (x = 0, 0.25, 0.5, 0.75, 1, 1.5, 2, 3, 3.5, 4, 4.5, 5) phosphors, the PDF#33-0040 corresponding to the standard card of YAG (Y₃Al₅O₁₂) and the PDF#43-0512 is the standard card of YGG (Y₃Ga₅O₁₂). The results indicate that the yttrium aluminium garnet phases were successfully obtained under the conditions of different Ga³⁺ doping concentration. From the figure, we can find that the diffraction angle decreased gradually with the increase of Ga³⁺ ions content. Which can be explained by the Bragg equation 2dsinθ = λ, the interplanar spacing d value becomes larger with the lattice expansion when Al³⁺ ions were replaced by Ga³⁺ ions (the lattice parameters of YAG are a = b = c = 12.008 Å, α = β = γ = 90°, Z = 8, and the lattice parameters of YGG are a = b = c = 12.27 Å, α = β = γ = 90°, Z = 8), the diffraction angle will decrease because of the ionic radius of Ga³⁺ is more than that of Al³⁺, exhibits a slight shift toward to lower 2θ angles. When Al³⁺ ions were completely replaced by Ga³⁺ ions, the sample becomes YGG. Besides, the diffraction intensity of (211), (321), (521), (532) crystal plane decreased gradually and the diffraction intensity of (400), (422), (642) increased simultaneously.

Fig. 1

XRD patterns of the Y_2.94Al_5−xGa_xO₁₂:Ce³⁺_0.06 (x = 0, 0.25, 0.5, 0.75, 1, 1.5, 2, 3, 3.5, 4, 4.5, 5) phosphors

Figure 2 displays the SEM images of the Y_2.94Al_5−xGa_xO₁₂:Ce³⁺_0.06 (x = x = 0, 0.75, 2, 3, 4, 5) phosphors. From the images, we can find that the particles of the sample replaced with a small number of the Ga³⁺ ions performs irregular shape, which consist of several small particles as shown in Fig. 2a–c. With the improvement of Ga³⁺ content, the size of individual particles increased gradually and the dispersion of the particles is getting better as shown in Fig. 2d–f. The doping of Ga³⁺ ions can help reduce the sintering temperature of YAG. Thus, the samples which content more Ga³⁺ ions are beneficial to the development of lattice.

Fig. 2

SEM images of the of the Y_2.94Al_5−xGa_xO₁₂:Ce³⁺_0.06 (x = 0, 0.75, 2, 3, 4, 5) phosphors. (a) x = 0.0, (b) x = 0.75, (c) x = 2, (d) x = 3, (e) x = 4, (f) x = 5

Figure 3 shows emission spectra and excitation spectra of the Y_2.94Al_5−xGa_xO₁₂:Ce³⁺_0.06 (x = 0, 0.25, 0.5, 0.75, 1, 1.5, 2, 3, 3.5, 4, 4.5, 5) phosphors. The emission spectra exhibit an emission band ranging from 480 to 650 nm under the 460 nm excitation, which corresponds to the 5d → 4f transition of the Ce³⁺ ions. The results indicate that with the increasing of Ga³⁺ ions concentration, the positions of the emission peak of Ce³⁺ ions show a greatly blue-shift. Meanwhile, the luminous intensity increased gradually first and then decreased and the luminous intensity becomes the strongest when x = 0.75. When Al³⁺ ions were completely replaced by Ga³⁺ ions, the sample shows no luminescence.

Fig. 3

Emission spectra (a) and excitation spectra (b) of the Y_2.94Al_5−xGa_xO₁₂:Ce³⁺_0.06 (x = 0, 0.25, 0.5, 0.75, 1, 1.5, 2, 3, 3.5, 4, 4.5, 5) phosphors

To explain the phenomenon, we draw the configuration coordinate diagram of YAG:Ce³⁺, YAGG:Ce³⁺, and YGG:Ce³⁺, which is showed in Fig. 4. From Fig. 4, we can find that the 5d energy level curve of Ce³⁺ will shift to the right with the increasing of Ga³⁺ ions concentration. Thus, the energy level difference between 5d and 4f in the garnet structure is YGG > YAGG > YAG. Then the luminous intensity of the samples increased with the increase of Ga³⁺ ions concentration. According to the equation of ΔE = h/λ (where h is Planck constant and λ is the wavelength), the λ will decrease when ΔE increased [11, 12]. But when the dopant volume of Ga³⁺ is too large, the 5D levels of Ce³⁺ are buried inside the conductance band that results in absence of Ce³⁺ luminescence in Y₃Ga₅O₁₂:Ce³⁺ at room temperature [13].

Fig. 4

Configuration coordinate diagram of YAG:Ce³⁺, YAGG:Ce³⁺, and YGG:Ce³⁺

As we all know, the optimal concentration of Ce³⁺ in YAGG:Ce³⁺ phosphors is affected with the ratio of Ga³⁺ to Al³⁺ [10, 12]. Figure 5 shows the emission spectra (a) and excitation spectra (b) of the Y_3−yAl_4.25Ga_0.75O₁₂:Ce³⁺_y (y = 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16) phosphors. The inset shows the trend of wavelength and luminous intensity with Ce³⁺ concentration. The inset indicates that with the increasing of Ce³⁺ ions concentration, the positions of the emission peak of Ce³⁺ ions show a red-shift, while the intensity increased first and then decreased, and the optimal concentration of Ce³⁺ in Y_3−yAl_4.25Ga_0.75O₁₂:Ce³⁺_y phosphors is y = 0.1.

Fig. 5

Emission spectra (a) and excitation spectra (b) of the Y_3−yAl_4.25Ga_0.75O₁₂:Ce³⁺_y (y = 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16) phosphors. The inset shows the trend of wavelength and luminous intensity with Ce³⁺ concentration

According to the crystal field theory, the red-shift of the emission spectra of Ce³⁺ mainly depends on the splitting of the 5d levels. The crystal field splitting (Dq) can be expressed as below:

$$\text{D}_{\text{q}}=\frac{1}{6}\text{Ze}^{2}\frac{\text{r}^{4}}{\text{R}^{5}}$$

(1)

where Dq represents the extent of energy level separation, Z is the charge of anion, e is the electron charge, r is the radius of the d wavefunction, and R is the bond distance between the Ce³⁺ and O²⁻. Ga³⁺ ions with the bigger ion radius and stronger electronegativity comparing with Al³⁺ have shorter bond and the stronger interactions with the coordinate O²⁻, which will induce the ligands of Al³⁺ sites shrinking [14,15,16]. So, the Dq decreases, and the emission spectra shift to longer wavelength (red-shift).

YAGG:Ce³⁺ phosphors are not only can be excited by blue light but also can be excited by cathode ray. Figure 6 shows the emission spectra of the Y_2.94Al_5−xGa_xO₁₂:Ce³⁺_0.06 (x = 0, 0.5, 0.75, 1, 1.5, 2, 3, 4, 5) and Y_3−yAl_3.5Ga_1.5O₁₂:Ce³⁺_y (y = 0.004, 0.006, 0.008, 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14) phosphors excited by cathode ray (6 kV). The results shows that the location of the emission peak is similar to the PL spectra, but the optimum concentration of the samples is x = 1.5 and y = 0.04 when they were excited by cathode ray. What’s more, the cathodoluminescence intensity is much higher than that of the PL. It might be because of the energy of cathode ray is higher than blue light. All these results indicate that the YAGG:Ce³⁺ phosphors are excellent in cathodoluminescence.

Fig. 6

Emission spectra of the Y_2.94Al_5−xGa_xO₁₂:Ce³⁺_0.06 (x = 0, 0.5, 0.75, 1, 1.5, 2, 3, 4, 5) (a) and Y_3−yAl_3.5Ga_1.5O₁₂:Ce³⁺_y (y = 0.004, 0.006, 0.008, 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14) (b) phosphors excited by cathode ray

Furthermore, we measured fluorescence lifetimes of Ce³⁺ in Y_2.94Al_5−xGa_xO₁₂:Ce³⁺_0.06 (x = 0, 0.75, 2, 3, 4, 5) phosphors (with emission at 535 nm upon excitation at 330 nm). As displayed in Fig. 7, it is observed that all the decay curves can be well fitted with a single-exponential equation. The lifetime increased first and then decreased with the increasing of the incorporation content of Ga³⁺ ion, which is accord with the trend of the emission intensity discussed above.

Fig. 7

Decay curves and fluorescence lifetimes of Y_2.94Al_5−xGaO₁₂:Ce³⁺_0.06 (x = 0, 0.75, 2, 3, 4, 5) samples

As is known, the thermal quenching is one of the important parameters for phosphors. Figure 8 displays the temperature-dependent luminescence intensity of Y_2.94Al_5−xGaO₁₂:Ce³⁺_0.06 (x = 0, 0.75, 2, 3, 5) excited by 460 nm, Fig. 8f shows the trend of the normalized emission intensity. The results indicate that all intensities drop gradually with temperature increase and the decline is getting worse with the increase of the concentration of Ga³⁺. The thermal quenching may be resulted from the two main reasons: (1) The garnet structure of Y_2.94Al_5−xGaO₁₂:Ce³⁺_0.06 phosphors become more and more relaxed and rigidity gradually decreases with Ga³⁺ increasing; (2) The thermal ionization is more likely to arise as the Ga³⁺ concentration increases [17,18,19].

Fig. 8

Temperature-dependent luminescence intensity of Y_2.94Al_5−xGaO₁₂:Ce³⁺_0.06. (a) x = 0.0, (b) x = 0.75, (c) x = 2, (d) x = 3, (e) x = 5, (f) shows the trend of normalized intensity

4 Conclusions

A series of Y_3−yAl_5−xGa_xO₁₂:Ce³⁺_y phosphors were successfully synthesized by high temperature solid state reaction method. The diffraction angle decreased gradually with the increase of Ga³⁺ ions content because of the he lattice expansion. In PL, the critical concentration of Ga³⁺ ions in Y_2.94Al_5−xGa_xO₁₂:Ce³⁺_0.06 is x = 0.75 and the optimal concentration of Ce³⁺ in Y_3−yAl_4.25Ga_0.75O₁₂:Ce³⁺_y is y = 0.1. The fluorescence lifetimes of the Y_2.94Al_5−xGa_xO₁₂:Ce³⁺_0.06 phosphors showed a similar trend to the emission intensity. The temperature-dependent luminescence intensity drop gradually with temperature increase and the decline is getting worse with the increase of the concentration of Ga³⁺. In cathodoluminescence, the optimum concentration of the samples is x = 1.5 and y = 0.04. With the improvement of Ga³⁺ content, the particle size increased gradually and the dispersion of the particles is getting better.