Dy³⁺ doped thermally stable garnet-based phosphors: luminescence improvement by changing the host-lattice composition and co-doping Bi³⁺

Abstract

A series of (Gd_1−xY_x)_2.94−yAl₅O₁₂: 0.06Dy³⁺,yBi³⁺(x = 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, y = 0.000, 0.015, 0.030, 0.045, 0.060) phosphors has been prepared by a sol–gel combustion method. X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL), diffuse reflection, fluorescence decay curves, temperature-dependent photoluminescence and CIE color coordinates were used to characterize the prepared phosphors. The emission intensity of Dy³⁺ was greatly improved by co-doping the sensitizer of Bi³⁺ ions and changing the composition of the matrix via replacing gadolinium with Y³⁺ ions. The influence of different Y³⁺ and Bi³⁺ concentrations on the properties of the phosphor were discussed. The optimized composition was determined to be (Gd_0.2Y_0.8)_2.835Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺. Its intensity is almost double the intensity in YAG: Dy³⁺,Bi³⁺, four times stronger than that of GAG: Dy³⁺,Bi³⁺ and 15 times stronger than that of YGAG: Dy³⁺. The band gap and decay curves of the phosphors were investigated to explore the relationship between the intensity and the content of Y³⁺. In addition, good thermal stability of (Gd_0.2Y_0.8)_2.835Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺ was observed according to the temperature-dependent PL spectra. The enhanced luminescence intensity and good thermal stability of the phosphor are conducive to improved application in the field of white light-emitting diodes (W-LEDs).

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1. Introduction

White light-emitting diodes (W-LEDs) have been considered to be the fourth generation solid-state light source because of their superiorities, such as high efficiency, non-pollution and low-energy consumption.^1–3 The third category of W-LEDs is based on the combination of an ultraviolet (UV) LED chip with blue, green, and red phosphors. However, a low luminous efficiency owing to the reabsorption and different aging rate among the phosphors and complicated manufacturing process restrict their potential application.⁴ Thus, it is crucial to develop a single-phase white light-emitting phosphor excited with an UV LED. Taking account of the ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 emissions of Dy³⁺ in the blue and yellow regions, respectively, it is interesting to produce Dy³⁺-activated white phosphors due to the combination of blue and yellow emissions.^5–7 However, the Dy³⁺-doped phosphor suffers from a relatively low luminous intensity. Remarkably, energy transfer from sensitizers to activators is considered to be an efficient way to improve the emission intensity of activators. As sensitizer, Bi³⁺ plays a major role in the luminous field because of its external ⁶S₂ configuration. The Dy³⁺–Bi³⁺ co-doped single-composition white light emitting phosphors has been extensively investigated, such as Y₃Al₅O₁₂: Dy³⁺,Bi³⁺,⁸ Gd₃Al₅O₁₂: Dy³⁺,Bi³⁺,⁹ Sr₂SiO₄: Dy³⁺,Bi³⁺,¹⁰ YPO₄: Dy³⁺,Bi³⁺,¹¹ YVO₄: Dy³⁺,Bi³⁺,¹² CaSiO₃: Dy³⁺,Bi³⁺,¹³ YBO₃: Dy³⁺,Bi³⁺,¹⁴etc.

In addition, the composition of the matrix crystal has a great influence on the luminescent properties of the phosphor.¹⁵ Rare-earth aluminates garnets (Ln₃Al₅O₁₂, Ln: lanthanide and Y³⁺, hereafter referred to as LnAG) has exhibited superiorities in terms of high-optical transparency, high threshold for optical damage, and high chemical and radiation stability.^16,17 Therefore, LnAG might be the best-known compound and has been proved to be one of the best hosts for the incorporation of rare earth activators.¹⁶ Diverse luminescent properties of phosphors can be obtained by replacing the Ln ions with suitable trivalent rare earth ions and the solid solubility is still good.

From the two aspects above, better luminescence of Dy³⁺ in the garnet matrix was expected through the synergistic effect via co-doping sensitizer while optimizing the composition of the host in this paper. Considering that in both Y₃Al₅O₁₂ and Gd₃Al₅O₁₂ host, the emission intensity of Dy³⁺ had been improved greatly by co-doping Bi³⁺, (Gd_1−xY_x)_2.94−yAl₅O₁₂: 0.06Dy³⁺,yBi³⁺(x = 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, y = 0.000, 0.015, 0.030, 0.045, 0.060) phosphors were synthesized by sol–gel low temperature combustion method in this work. Their photoluminescence properties were systematically studied. The phenomenon that luminescence of Dy³⁺ in (Y,Gd)AG is better than in YAG and GAG lattice was demonstrated in this paper. The thermal stability of the optimum composition was also demonstrated. The improved luminescent properties have laid a foundation for better application in the W-LED fields.

2. Experimental section

2.1 Materials and preparation

Ln₃Al₅O₁₂ (Ln = Y³⁺/Gd³⁺, hereafter referred to as YGAG) phosphors were synthesized via sol–gel combustion method, and RE₂O₃ [including Y₂O₃ (99.99%), Gd₂O₃ (99.99%), Dy₂O₃ (99.99%)] and Bi₂O₃ (AR) were used as the starting materials. Firstly, they were accurately weighed in stoichiometric amounts and dissolved in hot diluted nitric acid to obtain M(NO₃)₃ (M = Y³⁺, Gd³⁺, Dy³⁺, Bi³⁺). Then the required amounts of Al(NO₃)₃·9H₂O (AR) and citric acid monohydrate (citric acid–metal ion = 2 : 1) were added and dissolved in the above solution. The mixture was stirred for 10 min and the pH value was adjusted to about 3.0 with aqueous ammonia. Then the above solution was heated to 60 °C and continuously stirred using a magnetic agitator for about 2 h to obtain transparent sol after removing the excess water. Further being heated at 80 °C and stirred continuously, the sol gradually transformed into a yellowish gel. Secondly, the gel was combusted at 180 °C for 15 min. Then the obtained black products were annealed at 800 °C for 3 h in the muffle furnace to remove the residual carbon and later ground to fine powders. Finally, the obtained powders were calcined at 1300 °C for 3 h in air.

2.2 Characterization

The crystalline phase of samples was examined by the X-ray powder diffraction (XRD, Rigaku D/Max 2500 type, Japan) with CuKα radiation (λ = 1.5406 Å with 2θ step 0.02). The morphology and composition of samples were conducted on scanning electron microscope (SEM, Hitachi SU8010, Japan) equipped with an energy-dispersive X-ray spectrometer (EDS). The excitation and emission spectra of samples were tested by a fluorescence spectrophotometer (Lumina, America). The UV-vis diffuse reflectance spectra were measured by the UV-3100PC (Shimadzu, Japan). The fluorescent decay curves were tested by fluorescence spectrometer (FLsp920, British). The temperature-dependent luminescence properties were measured on the F4600 fluorescence spectrophotometer (HITACHI, Japan).

3. Results and discussions

3.1 Crystalline phase and morphology

Fig. 1(a) shows the XRD patterns of (Gd_1−xY_x)_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺(x = 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9) phosphors. It was evident that the synthesized powders were garnet structure, and no other intermediate phases were detected. In addition, the diffraction peaks matched well with GAG phase (JCPDS 73-1371) when the value of x was 0.3, 0.4, and 0.5. Further increasing the concentration of Y³⁺, the diffraction patterns matched well with that of YAG phase (JCPDS 33-0040) which was mainly embodied in the changed intensity of diffraction peak (321) and (400). YAG and GAG have the similar garnet structure, but only the unit cell parameters are different from that of different concentrations of Y³⁺. Because the radius of Y³⁺ (0.1019 nm for CN = 8) is smaller than that of Gd³⁺ (0.1053 nm for CN = 8),¹⁸ the substitution of Y³⁺ for Gd³⁺ would reduce the lattice parameter of Gd₃Al₅O₁₂ structure. As can be seen from the Fig. 1(b), the diffraction peak (420) shifted towards higher angles with the increasing of Y³⁺ content. In order to verify the above results, rietveld refinement method was used to estimate the lattice parameters of (Gd_1−xY_x)_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺(x = 0.3, 0.7, 0.8, 0.9) phosphors using GSAS software as shown in Fig. 2. The fitting results are shown in Table 1. The R values (R_wp% and R_p%) of all samples were less than 10%. The obtained refined lattice parameter “a” for cubic system was presented in Table 1. It could clearly observed that the lattice parameter “a” decreases with the increase content of Y³⁺ ions.

Fig. 1 (a) X-ray diffraction patterns of (Gd_1−xY_x)_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺(x = 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9), (b) enlarged view of (420) peak.

Fig. 2 Rietveld refinement of the observed XRD patterns for (Gd_1−xY_x)_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺ phosphors (a) x = 0.3, (b) x = 0.7, (c) x = 0.8, (d) x = 0.9.

Table 1 The parameters of (Gd_1−xY_x)_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺(x = 0.3, 0.7, 0.8, 0.9) phosphors

Parameters	x = 0.3	x = 0.7	x = 0.8	x = 0.9
Space group	Ia3̄d(230)	Ia3̄d(230)	Ia3̄d(230)	Ia3̄d(230)
Phase	Cubic	Cubic	Cubic	Cubic
Lattice parameters a (Å)	12.0925	12.0495	12.0328	12.0191
Cell volume (Å^3)	1768.29	1749.47	1742.21	1736.28
R p (%)	3.03	2.74	2.76	3.05
R wp (%)	5.50	4.47	4.05	4.18

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The crystal structure of LnAG and ball-stick model of environmental coordination of Ln and Al atoms are shown in Fig. 3(a) and (b), respectively. The garnet structure can be viewed as a framework built up via corner sharing of the Al–O tetrahedral (24d sites) and Al–O octahedron (16a sites), with the Ln (24c sites) residing in dodecahedral interstices.¹⁸ The morphology and composition of YGAG: Dy³⁺,Bi³⁺ phosphor were investigated by SEM and EDS, as shown in Fig. 3(c) and (d), respectively. It can be seen that the shape of the particles look like quasi-sphere. Since the sample was limited and packaged by the network “cage” formed by citric acid cross-linked before the decomposition of gel, there was certain agglomeration between the particles.⁹ The EDS pattern provides the chemical position of the product, containing Y, Gd, Al, O, Dy and Bi. Combined with the XRD patterns, the samples are further proved to be YGAG.

Fig. 3 Crystal structure of (a) Ln₃Al₅O₁₂, (b) ball-stick model of environmental coordination of Ln and Al atoms, (c) the SEM image and (d) EDS pattern of YGAG: Dy³⁺,Bi³⁺ phosphor.

3.2 Photoluminescence properties

Fig. 4 presents the photoluminescence excitation (PLE) and emission (PL) spectra of the Dy³⁺–Bi³⁺ co-doped YGAG phosphors. The excitation spectra of YGAG: Dy³⁺,Bi³⁺ monitored at 482 nm exhibited a strong broad band at 280 nm corresponding to the ¹S₀ → ³P₁ transition of Bi³⁺. Excited at 280 nm, the emission spectrum consisted of two strong peaks located at 482 nm and 582 nm, which were assigned to the transition ⁴F_9/2 → ⁶H_15/2 (magnetic dipole transition) and ⁴F_9/2 → ⁶H_13/2 (electric dipole transition) levels of Dy³⁺, respectively.^19,20 Furthermore, the emission intensity of the blue emission was stronger than that of the yellow in YAG, GAG and YGAG phosphors, which suggested that Dy³⁺ is located at a high symmetry site in garnet substrate.^14,21 The emission intensity of Dy³⁺ with different Y³⁺ content is presented in Fig. 4(a). The emission intensity of Dy³⁺ at 482 nm gradually increased with increasing Y³⁺ content and reached a maximum value at x = 0.8. Keeping the molar ratio of Gd³⁺ to Y³⁺ from 2 to 8, the doping concentration of Bi³⁺ were also discussed as shown in the inset Fig. 4(b). It also presented a trend of increasing first and then decreasing with a maximum when y = 0.045.

Fig. 4 PL and PLE spectra for (Gd_0.2Y_0.8)_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺ phosphor. Inset shows the emission intensity of (a) (Gd_1−xY_x)_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺ and (b) (Gd_0.2Y_0.8)_2.94−yAG: 0.06Dy³⁺,yBi³⁺ phosphors.

In particular, a comparison of emission spectra of Dy³⁺ single-doped YGAG and Dy³⁺, Bi³⁺ co-doped YAG, GAG and YGAG is exhibited in Fig. 5. To be clearly, the obtained emission intensity of Dy³⁺ in Dy³⁺–Bi³⁺ co-doped YGAG was strongest, the second was in YAG, and in the GAG, the intensity was weakest. Meanwhile, the intensity of Dy³⁺ was greatly enhanced with Bi³⁺ co-doping in YGAG. Specifically, the intensity in YGAG: Dy³⁺,Bi³⁺ is almost double the intensity in YAG: Dy³⁺,Bi³⁺, four times stronger than GAG: Dy³⁺,Bi³⁺ and 15 times stronger than YGAG: Dy³⁺. To the naked eye, it can clearly see from the illustration that the sample of “d” as the YGAG: Dy³⁺,Bi³⁺ phosphor shows the brightest light under the UV lamp. Similarly as YAG and GAG, the incorporation of Bi³⁺ well improved the emission intensity of Dy³⁺ through effective energy transfer from Bi³⁺ to Dy³⁺.⁹ The energy transfer diagram is shown in Fig. 6. When excited by the UV light, the energy could be absorbed by Bi³⁺ ions via Bi³⁺: ¹S₀ → ³P₁ transition. Then the absorbed energy could be transferred to the ⁴F_9/2 level of Dy³⁺. Finally, the characteristic emissions from ⁴F_9/2 to ⁶H_J (J = 15/2, 13/2 and 11/2) occurred. For higher concentration of Bi³⁺ ions, Bi_n³⁺ aggregates might be formed which acted as trapping centers and dissipated the absorbed energy nonradioactive, instead of transferring it to the activator ions of Dy³⁺.²² Therefore, higher emission intensity of Dy³⁺ could be obtained by co-doping appropriate amount of Bi³⁺ in garnet substrate phosphors.

Fig. 5 Emission spectra of (a) YGAG: 0.06Dy³⁺ (b) GAG: 0.06Dy³⁺,0.045Bi³⁺, (c) YAG: 0.06Dy³⁺, (d) YGAG: 0.06Dy³⁺,0.045Bi³⁺ phosphors. The inset shows the photograph of corresponding phosphors of the above points under the UV lamp.

Fig. 6 The energy transfer scheme from Bi³⁺ to Dy³⁺.

In addition, the UV-vis diffuse reflectance spectra of (Gd_1−xY_x)_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺(x = 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9) were measured as shown in Fig. 7. In the diffuse reflectance spectra, a broad band around 280 nm attributed to the transition from ¹S₀ to ³P₁ of Bi³⁺ was observed for all samples. But it's noteworthy that the lowest absorption band showed a visibly blue shift when the content of Y³⁺ increased to 60%. Meanwhile, the relative diffuse reflectivity presented a trend of increasing first and then decreasing with a maximum when x = 0.8. However, as seen from Fig. 4, the luminous intensity increased with the increasing of Y³⁺ content which means that the phosphor has the strongest excitation intensity whereas the absorption is lowest. It is consistent with the previously reported Ce³⁺ doped (Y_1−xGd_x)₃Al₅O₁₂ phosphors. In that system, the absorption intensity increased whereas the luminous intensity decreased with the increased content of Gd³⁺. The intensity of PL reflects the comprehensive effects of energy absorption and energy transfer in the phosphor. At higher concentration of Gd³⁺, the phosphor has a strong ability to absorb incident photons, but only a fraction of them convert to visible light.¹⁷ That is, the increase of (Gd_1−xY_x)_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺(x = 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9) luminescence upon Y³⁺ doping is not caused by the increased absorption.

Fig. 7 UV-vis diffuses reflectance spectra of (Gd_1−xY_x)_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺(x = 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9) samples. The inset shows the enlargement of spectra from 200 nm to 300 nm.

In order to show the absorption of the sample more directly, the relationship between the wavelength and the reflectivity is converted into the relationship between the absorbance and the wavelength as shown in Fig. 8 In addition, to further clarify the relationship between the intensity and the content of Y³⁺, different contents of Y³⁺(x = 0.3, 0.5, 0.7, 0.8, 0.9) were taken to be investigated to estimate the optical band gap energy (E_g). Firstly, F(R) was obtained by the Kubelka–Munk equation:²³

where R is the diffuse reflectance of the sample, α is the linear absorption coefficient and S is the scattering coefficient.

Fig. 8 The relationship figure between absorbance and wavelength of (Gd_1−xY_x)_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺(x = 0.3, 0.5, 0.7, 0.8, 0.9) phosphors. The inset shows the plot of [F(R)hν]²versus hν of (Gd_1−xY_x)_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺ phosphors.

Then the Tauc theory was used to calculate the optical band using the DR spectra. The relationship between the optical band gap with the absorbance and photon energy can be given by the following function:²³

[F(R)hν]² = C₂(hν − E_g)ⁿ (2)

where h is the Planck constant, ν is the photon energy, C₂ is a proportionality constant, and n is a constant associated to the different types of electronic transitions (n = 1, 2, 3, 4, 6 is for direct allowed transitions, nonmetallic materials, indirect allowed transitions and indirect forbidden transitions, respectively). The n = 4 is adopted and from the plot of [F(R)hν]²versus hν, the value of E_g is obtained by extrapolating the linear fitted regions to [F(R)h]² = 0.^23,24

As shown in the inset of Fig. 8, the obtained E_g values were slightly varies with the increased content of Y³⁺ ions. The values of E_g increased to the maximum at x = 0.8. Previous report had discussed the effect of different Gd³⁺ content on YAG: Ce³⁺ phosphor.¹⁷ It was generally consistent with this work. That is, with more and more Gd³⁺ doped into YAG, the band gap narrows. Meanwhile, the phenomenon that the luminous intensity decreased with the increasing content of Gd³⁺ was also observed in that paper. The author attributed the reason to the energy loss caused by the auto-ionization of electrons from excited state orbit to conduction band and the thermal delocalization of electrons from crystal defects to conduction band. In other words, the absorption enhanced but the energy loss also increased at the same time. Therefore, there might be two reasons for the results of this work. On the one hand, with the increasing content of Y³⁺, the band gap becomes wider and the crystal field strength increases which led to the increase of energy barrier between excited states to conduction band. On the other hand, it would naturally give rise to a substantial number of vacant sites in oxygen ion when the position of Gd³⁺ was occupied by Y³⁺ with smaller radius, and then increase the depth of the crystal defects with the increasing concentration of Y³⁺.^17,24 It may reduce the probability of the self ionization electron and thermal ionization of electrons.¹⁷ Finally, the luminous energy loss decreases and the luminous intensity increases gradually. When the content of Y³⁺ is 80.0%, the band gap is widest and the loss of energy is the least, which may lead to the maximum emission intensity.

Meanwhile, the decay curves of (Gd_1−xY_x)_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺(x = 0.3, 0.5, 0.7, 0.8, 0.9) phosphors under excitation at 280 nm and monitored at 482 nm were also investigated as shown in Fig. 9. The fitted lifetimes were also given in the figure and all the decay curves can be well fitted to a double-exponential function as follows:^25,26

I(τ) = A + B₁ exp(−t/τ₁) + B₂ exp(−t/τ₂) (3)

where I(τ) is the luminescence intensities at time τ, A, B₁ and B₂ are the constants, τ₁ and τ₂ are the lifetimes for the exponential components, respectively. The average lifetime can be calculated by the following function:²⁵the value of A, B₁, B₂, τ₁ and τ₂, τ_avg were listed in Table 2. The lifetime values were determined to be 720.2355, 731.1796, 731.8307, 775.7064 and 895.9022 μs for different Y³⁺ concentrations of x = 0.3, 0.5, 0.7, 0.8 and 0.9, respectively. To be clearly, the average lifetime of ⁴F_9/2 of Dy³⁺ increased with the increasing Y³⁺ concentration. It was also indirectly indicated the explanations mentioned in the previous paragraph that the crystal field strength is weakened with the increased concentration of Y³⁺ and the depth of the crystal defects increases which illustrated the trapped electrons being hardly released to the conduction.

Fig. 9 Decay times obtained for (Gd_1−xY_x)_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺(x = 0.3, 0.5, 0.7, 0.8, 0.9) detected at an excitation wavelength of 280 nm, while monitoring at 482 nm.

Table 2 Decay times of (Gd_1−xY_x)_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺(x = 0.3, 0.5, 0.7, 0.8, 0.9) phosphors detected at an excitation wavelength of 290 nm, while monitoring at 482 nm

Samples	A	τ 1 (μs)	B 1	τ 2 (μs)	B 2	τ avg (μs)
x = 0.3	0.237	188.3677	388.395	808.3646	546.120	720.2355
x = 0.5	0.256	228.8441	317.432	815.7814	528.726	731.1796
x = 0.7	0.284	233.0223	391.681	824.9655	592.538	731.8307
x = 0.8	0.032	338.0215	411.537	893.5707	578.101	775.7064
x = 0.9	0.232	628.7451	285.902	996.7856	477.570	895.9022

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Additionally, the thermal stability of phosphor is important for the potential application in W-LEDs. To investigate the influence of temperature on the luminescence properties, Fig. 10(a) shows the PL spectra of (Gd_0.2Y_0.8)_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺ phosphor excited at 280 nm under different temperatures in the range of 30–200 °C. The inset shows the enlarged images of emission spectra from 470–500 nm. No obvious shifts were observed and the luminescence decreases slowly with the increasing temperature. The variations in the normalized emission intensity of (Gd_0.2Y_0.8)_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺ phosphor is exhibited in Fig. 10(b). The intensity at 150 °C is about 81% of the initial value at 30 °C, which means the phosphor has good thermal stability. To better understand the thermal quenching phenomena, the activation energy (ΔE) was obtained according to the modified Arrhenius equation:^25–29

where I_T is the emission intensity at different temperatures, I₀ is the initial intensity at 30 °C, C is a rate constant for the thermally activated escape, ΔE is the activation energy and k is the Boltzmamn constant. As shown in Fig. 10(c), the plot of ln[(I₀/I_T) − 1] vs. 1/kT yields a straight line, and the activation energy ΔE is obtained as 0.209 eV from the slope of the plot. The good color stability and thermal stability also shows great potential application in W-LEDs.

Fig. 10 The PL spectra of (a) (Gd_0.2Y_0.8)_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺ (λ_ex = 280 nm) phosphor under different temperatures in the range of 30–200 °C and the inset is the enlarged images from 470–500 nm, (b) the variations in the emission intensity of (Gd_0.2Y_0.8)_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺, (c) a ln[(I₀/I_T) − 1] vs. 1/kT activation energy graph for thermal quenching of (Gd_0.2Y_0.8)_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺ phosphor.

Finally, the Commission Internationale de L'Eclairage (CIE) color coordinates (x, y) of Dy³⁺ singly doped YGAG, and Dy³⁺–Bi³⁺ co-doped LnAG (Ln = Y, YGd, Gd) samples are calculated and displayed in Fig. 11. The chromaticity coordinate shifted from blue edge area to green edge area when the Ln ions change from Y to Y/Gd to Gd. The value of color coordinates were listed in the insert table which was (0.2645, 0.2976), (0.2952, 0.3288), (0.308, 0.3401) and (0.3069, 0.372) for a, b, c and d, respectively.

Fig. 11 The CIE diagram of (a) YAG: 0.06Dy³⁺,0.045Bi³⁺, (b) YGAG: 0.06Dy³⁺,0.045Bi³⁺, (c) YGAG: 0.06Dy³⁺ and (d) GAG: 0.06Dy³⁺,0.045Bi³⁺.

4. Conclusions

Better luminescence of Dy³⁺ in the garnet matrix were synthesized via sol–gel combustion method through the synergistic effect by co-doping the sensitizer of Bi³⁺ and changing the composition of the matrix via replacing gadolinium with different contents of Y³⁺. The energy transfer from Bi³⁺ to Dy³⁺ was occurred in YGAG phosphors. The optimized composition was determined to be (Gd_0.2Y_0.8)_2.835Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺ and its emission intensity was 2 times stronger than Y_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺, 4 times stronger than Gd_2.895Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺, and 15 times stronger than (Gd_0.8Y_0.2)_2.94Al₅O₁₂: 0.06Dy³⁺. The band gap becomes wider and lifetime of ⁴F_9/2 of Dy³⁺ increased with the increasing Y³⁺ concentration. When the content of Y³⁺ was 80.0% (x = 0.8), the band gap was widest and the loss of energy was the least, which may lead to the maximum emission intensity. The thermal stability of (Gd_0.2Y_0.8)_2.835Al₅O₁₂: 0.06Dy³⁺,0.045Bi³⁺ was investigated based on the temperature-dependent PL spectra, and activation energy was also calculated about 0.209 eV. The enhanced luminescence intensity and thermal stability are useful for the application in the field of white LEDs.

Acknowledgements

The authors acknowledge the generous financial support from Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and National Natural Science Foundation of China (51402133) and (51202111).

References

1. K. Li, X. Liu, Y. Zhang, X. Li, H. Lian and J. Lin, Inorg. Chem., 2015, 54, 323–333.
2. X. Min, M. H. Fang, Z. H. Huang, Y. G. Liu, C. Tang, X. W. Wu and B. Dunn, J. Am. Ceram. Soc., 2015, 98, 788–794.
3. Q. Liu, L. X. Wang, L. Zhang, H. Yang, M. X. Yu and Q. T. Zhang, J. Mater. Sci.: Mater. Electron., 2015, 26, 8083–8088.
4. J. Zhao, C. F. Guo, X. Y. Su, H. M. Noh, J. H. Jeong and A. Srivastava, J. Am. Ceram. Soc., 2014, 97, 1878–1882.
5. Y. Liu, G. X. Liu, J. Wang, X. Dong and W. Yu, Inorg. Chem., 2014, 53, 11457–11466.
6. G. S. R. Raju, H. C. Jung, J. Y. Park, C. M. Kanamadi, B. K. Moon, J. H. Jeong, S. M. Son and J. H. Kim, J. Alloys Compd., 2009, 481, 730–734.
7. W. C. Lü, H. Zhou and C. Y. Tu, J. Phys. Chem. C, 2009, 113, 3844–3849.
8. Z. F. Mu, Y. H. Hu, L. Chen and X. J. Wang, J. Lumin., 2011, 131, 1687–1691.
9. L. Tian, L. X. Wang, L. Zhang, Q. T. Zhang, W. H. Ding and M. X. Yu, J. Mater. Sci.: Mater. Electron., 2015, 26, 8507–8514.
10. L. Zhang, P. D. Han, K. Wang, Z. Lu, L. X. Wang, Y. F. Zhu and Q. T. Zhang, J. Alloys Compd., 2012, 541, 54–59.
11. Y. Y. Cao, Y. X. Liu, H. Feng and Y. Yang, Ceram. Int., 2014, 40, 15319–15323.
12. D. C. Victory, G. Phaomei, N. Yaiphaba and N. Rajmuhon Singh, J. Alloys Compd., 2014, 583, 259–266.
13. S. F. Lai, Z. W. Yang, J. Y. Liao, J. B. Qiu, Z. G. Song, Y. Yang and D. C. Zhou, Mater. Res. Bull., 2014, 60, 714–718.
14. W. Zhang, S. X. Liu, Z. F. Hu, Y. L. Liang, Z. Y. Feng and X. Sheng, Mater. Sci. Eng., B, 2014, 187, 108–112.
15. M. S. Rezende and C. W. A. Paschoal, Opt. Mater., 2015, 46, 530–535.
16. J. K. Li, J. G. Li, S. H. Liu, X. D. Li, X. D. Sun and Y. S. Sakka, J. Mater. Chem. C, 2013, 1, 7614–7622.
17. L. Chen, X. L. Chen, F. Liu, H. Chen, H. Wang, E. Zhao, Y. Jiang, T. S. Chan, C. H. Wang, W. Zhang, Y. Wang and S. Chen, Sci. Rep., 2015, 5, 11514.
18. J. G. Li and Y. S. Sakka, Sci. Technol. Adv. Mater., 2015, 16, 014902.
19. L. L. Li, Y. L. Liu, R. Q. Li, Z. H. Leng and S. C. Gan, RSC Adv., 2015, 5, 7049–7057.
20. Y. Zhang, W. T. Gong, J. J. Yu, Y. Lin and G. L. Ning, RSC Adv., 2015, 5, 96272–96280.
21. R. Yu, D. S. Shin, K. Jang, Y. Guo, H. M. Noh, B. K. Moon, B. C. Choi, J. H. Jeong and S. S. Yi, Spectrochim. Acta, Part A, 2014, 125, 458–462.
22. G. Y. Dong, C. C. Hou, Z. P. Yang, P. F. Liu, C. Wang, F. C. Lu and X. Li, Ceram. Int., 2014, 40, 14787–14792.
23. S. Som, P. Mitra, V. Kumar, V. Kumar, J. J. Terblans, H. C. Swart and S. K. Sharma, Dalton Trans., 2014, 43, 9860–9871.
24. C. J. Shilpa, N. Dhananjaya, H. Nagabhushana, S. C. Sharma, C. Shivakumara, K. H. Sudheerkumar, B. M. Nagabhushana and R. P. S. Chakradhar, Spectrochim. Acta, Part A, 2014, 128, 730–739.
25. C. Y. Liu, Z. G. Xia, M. S. Molokeev, Q. L. Liu and H. Guo, J. Am. Ceram. Soc., 2015, 98, 1870–1876.
26. D. Song, C. F. Guo and T. Li, Ceram. Int., 2015, 41, 6518–6524.
27. Y. L. Huang and H. J. Seo, Mater. Lett., 2015, 156, 86–89.
28. X. C. Wang and Y. H. Wang, J. Phys. Chem. C, 2015, 119, 16208–16214.
29. W. Zhou, X. X. Ma, M. L. Zhang, Y. Luo and Z. G. Xia, Ceram. Int., 2015, 41, 7140–7145.

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