Solvothermal synthesis and luminescence properties of BaCeF₅, and BaCeF₅:Tb³⁺ nanocrystals

Abstract

Cubic monodisperse BaCeF₅ and BaCeF₅:Tb³⁺ nanocrystals have been successfully synthesized by a citric acid assisted solvothermal method. The crystalline phase, size, morphology, and luminescence properties were characterized using powder X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), photoluminescence (PL), photoluminescent excitation spectra (PLE) as well as dynamics decay. The results reveal that the Tb³⁺-doped BaCeF₅ sample shows a strong green emission centered at 546 nm, corresponding to the ⁵D₄→⁷F₅ transition of Tb³⁺ due to an efficient energy transfer from Ce³⁺ to Tb³⁺. The decay lifetime of Ce³⁺ monotonically increases with increase of Tb³⁺ concentration. The critical energy transfer distance between Ce³⁺ and Tb³⁺ was also calculated by methods of concentration quenching and spectral overlapping. Experimental analysis and theoretical calculations reveal that the dipole–dipole interaction should be the dominant mechanism for the Ce³⁺–Tb³⁺ energy transfer.

---

1. Introduction

Over the past few decades, the study of nanometric luminescent materials, especially lanthanide ion-doped luminescent nanomaterials, has become one of the hottest topics in nanoscience because of their potential applications in high performance magnets, luminescent devices, catalysts, and other functional materials arising from 4f electrons.^1–5 As an important group of inorganic materials with unique optical and electronic properties, nano and submicroscale fluoride materials have drawn increasing attention. For example, the syntheses of metal fluoride nanomaterials, such as CaF₂ nanocubes,^6,7 SrF₂ nanospheres,⁸ and BaF₂ nanocubes⁹ and nanorods¹⁰ have been reported for their application in UV lithography, UV-transparent optical lenses, and surface conditioning of glass. Besides, binary lanthanide fluorides (LnF₃, Ln = lanthanide elements)^11–14 and ternary ALnF₄ (A = alkali metals, Ln = lanthanide elements)^15–22 fluorides were intensively researched with potential applications in display, laser, and biological labels in recent years. Compared with the fluorides mentioned above, alkaline-earth lanthanide ternary fluorides have obtained relatively little attention. Barium yttrium fluoride crystals such as BaY₂F₈ and BaYF₅ are prepared¹⁹ as excellent host matrixes that can be doped with divalent and trivalent lanthanide ions, exhibiting the strong broadband emission in the near UV spectra region (360–440 nm)²³ and highly efficient infrared-to-visible up-conversion light.²⁴ However, the lanthanide ions doped BaCeF₅ system has never reported up to now.

In general, alkaline-earth lanthanide ternary fluorides, for examples, bulk BaLn₂F₈/BaYF₅ crystals^25,26 and nonstoichiometric single crystals R_1−yM_yF_3−y (R = La–Er; M = Ca, Sr, Ba, Cd)²⁷ were prepared by traditional solid-state reaction method. Due to insufficient mixing and low reactivity of raw materials, several impurity phases easily co-exist in the product. Therefore, in recent years, several wet chemical techniques such as co-precipitation method,²⁸ hydrothermal method,^29–31 liquid-solid-solution procedure^32,33 and solvothermal method³⁴ were used to prepare the fluorides phosphor. Phosphor materials synthesized by these wet chemical methods have many advantages, i.e., high purity, homogenous composition and fine grains in the nanometer range. To the best of our knowledge, there has been no literature available on the wet chemical synthesis of cubic-phase BaCeF₅, so we have undertaken this work. In this paper, we realize the solvothermal synthesis of BaCeF₅, and BaCeF₅:Tb³⁺ nanocrystals and research the luminescence properties of Ce³⁺ and Tb³⁺ in cubic BaCeF₅. Moreover, the energy transfer efficiency from Ce³⁺ to Tb³⁺, the energy transfer critical distance (R_c) between Ce³⁺ and Tb³⁺, and the energy transfer mechanism of Ce³⁺–Tb³⁺ in BaCe_1−xTb_xF₅ nanocrystals have been discussed in detail.

2. Experimental

2.1. Synthesis of the samples

Materials. The samples were synthesized through a citric acid assisted solvothermal method. The raw materials BaCO₃ (99.0%), Tb(NO₃)₃·6H₂O (99.99%), Ce(NO₃)₃·6H₂O (99.99%), NH₄F (99.99%) and C₆H₈O₇·H₂O (99.5+%) were all purchased from Sinopharm Chemical Reagent Co. Ltd. and were used directly, without further purification.

Synthesis. In a typical procedure of preparing BaCeF₅ nanocrystals, the first mixture: 2 mmol of Ce(NO₃)₃·6H₂O was first added into 20 mL of isopropyl alcohol with stirring to form a transparent homogeneous solution. Subsequently, the second mixture: 20 mL of isopropyl alcohol containing 48 mmol of NH₄F (1.776 g), 2 mmol of BaCO₃ (0.39468 g) and 2 mmol of C₆H₈O₇·H₂O (0.42028 g) were added into the above solution with stirring to form a transparent homogeneous solution. After the addition of the second mixture into the first mixture, the end mixture became white and turbid. After stirring for about 60 min, the resultant solution was transferred into a 60 mL Teflon autoclave. Finally, the autoclave was sealed and heated at 180 °C for 12 h followed by cooling to the room temperature naturally. The resulting precipitates were washed with deionized water and ethanol each two times. The final product was dried at 60 °C for 12 h in air. Then, in a typical procedure of preparing BaCeF₅: (2 mol%) Tb³⁺ nanocrystals, the first mixture: 1.96 mmol of Ce(NO₃)₃·6H₂O and 0.04 mmol of Tb(NO₃)₃·6H₂O were first added into 20 mL of isopropyl alcohol with stirring to form a transparent homogeneous solution. The next steps were the same as the preparation of BaCeF₅ nanocrystals. Finally, we can obtain BaCeF₅: (2 mol%) Tb³⁺ nanocrystals. Similarly, other BaCe_1−xTb_xF₅(x = 0.04,0.06,0.08,0.10,0.12) samples are synthesized by the same process.

2.2. Characterization

The structural characteristics of the final products were examined by the powder X-ray diffraction (XRD) pattern using Cu-Kα (λ = 0.15405 nm) radiation on a Rigaku-Dmax 2500 diffractometer. The morphology and the size of the obtained samples were observed with field emission-scanning electron microscopy (FE-SEM, JSM-6700F, JEOL) and transmission electron microscopy (TEM, JEM-2010 JEOL). The ultraviolet-visible photoluminescence excitation and emission spectra were recorded with a Hitachi F-7000 spectrophotometer equipped with Xe-lamp as an excitation source. All the measurements were performed at room temperature.

3. Results and discussion

3.1. Crystal structure and morphologies

Fig. 1 presents representative XRD patterns of BaCe_1−xTb_xF₅ samples with different concentrations. The powder XRD data of the as-synthesized product shows nine characteristic diffraction peaks (25.61°, 29.67°, 42.50°, 50.28°, 52.69°, 61.61°, 67.81°, 69.81°, 77.62°) in 2θ range of 10° to 80°. Compared with the cubic structure BaCeF₅ (JCPDS # 43-0394, space group Fm-3m[225], cell parameters a = b = c = 6.018 Å), the absence of some diffraction peaks implies that the crystal structure of the as-synthesized product may have a higher symmetry in the space group. No impurity lines are observed in the patterns of the Tb³⁺ doped nanocrystals shown in Fig. 1(b)–(h), meaning that the RE³⁺ doping does not cause any significant changes in the crystal phases. In addition, it is worth noting that the diffraction peaks are widened as a result of the small-size effect of the nanocrystals. The mean crystallite size of the product was estimated from the XRD pattern according to the Scherrer formula D = Kλ/βcosθ, where λ is the X-ray wavelength (0.15406 nm), β is the full-width at half-maximum, θ is the diffraction angle, and K is a constant (0.89). The estimated mean crystallite size is 38.05 nm.

Fig. 1 XRD patterns of samples BaCe_1−xTb_xF₅ and reference date JCPDS#43-0394.

A representative panoramic FE-SEM image shown in Fig. 2(a) demonstrates that the product is composed of well dispersed particles with small sizes. The high-magnified TEM image (Fig. 2(b)) further shows the nanocrystals more distinctly. Most of the nanocrystals are pseudospherical particles with a mean size of about 40 nm, which is in consistent with the size estimated by Scherrer formula from the XRD pattern. The selected-area electron diffraction (SAED) in Fig. 2(c) exhibits dots, which are due to the diffraction of ensemble nanocrystals, indicating that the as-obtained nanocrystals are essentially single crystalline in nature. The interplanar distance obtained from the dot is 0.3475 nm, which can be assigned to (111) lattice planes of cubic structure BaCeF₅, respectively. In Fig. 2(d) the interplanar distance is 0.3475 nm, which is also matched with the (111) plane.

Fig. 2 (a) FE-SEM image of BaCe_0.90Tb_0.10F₅ sample. (b) TEM image of the sample. (c) SAED pattern of the particle. (d) HRTEM image of the sample.

3.2. Luminescence properties

Fig. 3(a) shows the photoluminescence excitation (λ_em = 344 nm) and photoluminescence emission (λ_ex = 292 nm) spectra of the BaCeF₅ nanocrystals. The excitation spectrum of the BaCeF₅ nanocrystals gives a broadband centered at 292 nm with a shoulder centered at 270 nm, which were attributed to the electric dipole-allowed transitions of the Ce³⁺ ions from the 4f shell to the 5d orbital. Owing to the influences of crystal field splitting and spin–orbit coupling, the 4f→5d transition of the Ce³⁺ ions will exhibit a subtle structure.³⁵ Under excitation at 292 nm, the emission spectrum of BaCeF₅ nanocrystals exhibits an intense ultraviolet emission band centered at 344 nm, which is assigned to the 5d–4f electronic transition of the Ce³⁺ ions. Fig. 3(b) shows the photoluminescence excitation (λ_em = 546 nm) spectrum of the BaCe_0.98Tb_0.02F₅ nanocrystals and photoluminescence emission (λ_ex = 292 nm) spectrum of the BaCeF₅ nanocrystals. The photoluminescence excitation (λ_em = 546 nm) spectrum of BaCe_0.98Tb_0.02F₅ nanocrystals in the Fig. 3(b) are similar with the photoluminescence excitation (λ_em = 344 nm) spectrum of the BaCeF₅ nanocrystals in the Fig. 3(a). On the basis of the above photoluminescence excitation spectrum of the Tb³⁺ doped samples and photoluminescence spectrum of no Tb³⁺ doped samples, we can find that the emission band of Ce³⁺ overlaps well with the excitation band of Tb³⁺. Therefore, it is expected that a resonance-type energy transfer from Ce³⁺ to Tb³⁺ in the Tb³⁺ doped BaCeF₅ nanocrystals may occur.

Fig. 3 (a) Excitation spectrum (λ_em = 344 nm) and emission spectrum (λ_ex = 292 nm) of BaCeF₅. (b) Excitation spectrum (λ_em = 546 nm) of BaCe_0.98Tb_0.02F₅ and emission spectrum (λ_ex = 292 nm) of BaCeF₅.

Fig. 4 gives the photoluminescence excitation (λ_em = 546 nm) and photoluminescence emission (λ_ex = 292 nm) spectra of the BaCe_0.86Tb_0.14F₅ nanocrystals. By monitoring the 546 nm emission of Tb³⁺, the BaCe_0.86Tb_0.14F₅ nanocrystal shows a broad excitation band peaking at 292 nm. With 292 nm excitation, the photoluminescence emission spectrum of BaCe_0.86Tb_0.14F₅ has characteristic transitions of forbidden 4f–4f transitions within the Tb³⁺ configuration in the wavelength range of 490–640 nm. The characteristic emissions of Tb³⁺ at 495, 546, 585, and 625 nm can be attributed to the transitions ⁵D₄→⁷F₆, ⁵D₄→⁷F₅, ⁵D₄→⁷F₄, and ⁵D₄→⁷F₃, respectively. The green emission (⁵D₄→⁷F₅) at 546 nm,³⁶ which is a magnetic dipole transition with ΔJ = 1, is more intense than the other transitions.³⁷

Fig. 4 Excitation (λ_em = 546 nm) and emission (λ_ex = 292 nm) spectra of BaCe_0.86Tb_0.14F₅ .

To explore the possibility of the energy transfer from the Ce³⁺ to Tb³⁺ ions, Tb³⁺ ions with different concentrations were doped into the BaCeF₅ nanocrystals. Fig. 5 displays the emission spectra of the BaCe_1−xTb_xF₅ nanocrystals with different Tb³⁺ concentrations, it contains both the weak emission of the Ce³⁺ ions and the strong green emission of the Tb³⁺ ions. The emission intensity of the Tb³⁺ ions gradually increases at the expense of that of the Ce³⁺ ions with the increase of Tb³⁺ doping concentration, indicating that the energy transfer from the Ce³⁺ to Tb³⁺ ions is highly efficient since the emission band of the Ce³⁺ ions matches well with the f–f absorptions of the Tb³⁺ ions. Until BaCe_0.90Tb_0.10F₅, the emission intensity of Tb³⁺ ions reaches the strongest and then the emission intensity of Tb³⁺ ions gradually decreases with the increase of Tb³⁺ doping concentration.

Fig. 5 Emission spectra of BaCe_1−xTb_xF₅ (λ_ex = 292 nm).

In order to investigate the luminescence dynamics of the samples, we measured the photoluminescence decay curves and then calculated the lifetime as well as energy transfer efficiencies. All the decay curves can be well fitted by a single exponential function as I(t) = I₀exp(−t/τ), where I₀ is the initial emission intensity at t = 0, τ is the 1/e lifetime of Tb³⁺. The lifetimes of Tb³⁺ in BaCe_1−xTb_xF₅ samples are 2.80 ± 0.03, 2.36 ± 0.02, 2.14 ± 0.02, 1.70 ± 0.02, 1.66 ± 0.03, 1.44 ± 0.01, 1.36 ± 0.01, 1.19 ± 0.03, 1.16 ± 0.02, and 1.15 ± 0.01 ms for x = 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18 and 0.20, respectively.

On the basis of the above results, Ce³⁺ acts as a sensitizer to yield sensitized luminescence from Tb³⁺ in BaCeF₅. A simple operational definition of energy transfer efficiency η_T in terms of lifetimes is given by:

where τ_S0 is the decay lifetime of Ce³⁺ in the absence of Tb³⁺ and τ_S is the lifetime of Ce³⁺ in the presence of Tb³⁺. An alternative expression for η_T from the fluorescence yield can be expressed by the following formula:^38,39where I_S0 and I_S are the intensities of Ce³⁺ in the absence and in the presence of Tb³⁺, respectively. The η_T from Ce³⁺ to Tb³⁺ in the BaCe_1−xTb_xF₅ nanoparticles are calculated using eqn (2) and are illustrated in Fig. 6 as function of Tb³⁺ concentration (x). η_T equals 0, 0.157, 0.272, 0.366, 0.449, 0.522, 0.546, 0.569, 0.595, 0.605, and 0.610 for x is 0, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, and 0.20, respectively. The η_T monotonically increases with the increase of Tb³⁺ concentration and is as high as 0.610 when the Tb³⁺ concentration is 0.20. It slowly changes after x = 0.10, indicating that the η_T has nearly reached the maximum in the sample BaCe_0.80Tb_0.20F₅.

Fig. 6 Energy transfer efficiency η_T in BaCe_1-xTb_xF₅.

According to the energy transfer theories of Dexter and Schulman, concentration quenching is due to the energy transfer from one activator to another in many cases until an energy sink in the lattice is reached.⁴⁰ We approximated the unit cell as a sphere. The volume of the sphere can be expressed as follows: , (where r is the radius of the sphere, N is the number of sites that a lanthanide ion can occupy per unit cell, V is the volume of the unit cell). According to the above experimental results and the crystal structure of the BaCeF₅ nanocrystal, we use V = 219.2 ų, N = 2, and estimate the average separation R_Ce–Ce = 2r = 5.94 Å. In Fig. 5, it has been shown that the critical concentration of C_Tb is 0.10. As suggested by Verstegen et al.,⁴¹ the critical distance of the Ce³⁺–Ce³⁺ is the critical distance of Ce³⁺–Tb³⁺, because the Tb³⁺ ions replace the position of the Ce³⁺ ions in the each unit cell. So in the host of BaCeF₅ nanocrystals, the critical concentration of C_Tb is 0.10 and the corresponding critical distance R_c for the Ce³⁺–Tb³⁺ energy transfer is about 5.94 Å.

The energy transfer from a sensitizer to an activator can take place via radiative energy transfer, exchange interaction, and multipole-multipole interaction.⁴² In general, the existence of radiative energy transfer from a sensitizer to an activator can be confirmed by the spectral dips in the emission spectrum of the sensitizer. The absence of the dips in the emission band of the Ce³⁺ ions corresponding to the f–f absorption lines of the Tb³⁺ ions means that the radiative energy transfer between the Ce³⁺ ions and the Tb³⁺ ions can be neglected. Moreover, the radiative energy transfer does not change the decay time of the sensitizer. The decrease of the decay time of the Ce³⁺ ions also does not support a radiative energy transfer process. Exchange interaction is strongly influenced by the distance between the sensitizer and activator and needs a large overlapping between sensitizer and activator orbitals. While both the Ce³⁺ and Tb³⁺ ions are reducing ions, such an exchange would require very high energy. Generally, the value of the critical distance is about 3–4 Å if the exchange is dominated.⁴³ In our case, the critical distance of Ce³⁺ and Tb³⁺ is estimated to be 5.94 Å, suggesting that energy transfer via exchange interaction can be excluded either. Thus we suspected that the energy transfer in BaCe_1−xTb_xF₅ nanocrystals takes place via electric multipole–multipole interaction. On the basis of Dexter's energy transfer formula of multi-polar interaction and Reisfeld's approximation,^41,44,45 the following relation can be given as:

where η₀ and η are the luminescence quantum efficiency of the Ce³⁺ ions in the absence and in the presence of the Tb³⁺ ions, respectively; C_Tb is the doping concentration of the Tb³⁺ ions; and n = 6, 8, 10 corresponding to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. The value η₀/η can be approximately calculated by the ratio of related luminescence intensities (I_S0/I_S) of Ce³⁺. The I_S0/I_S–C_Tb^n/3 plots are illustrated in Fig. 7 for BaCe_1−xTb_xF₅. As n = 6, the curve exhibits the best linear relation in the three plots implying that the energy transfer from Ce³⁺ to Tb³⁺ is considered to be a dipole–dipole mechanism in the BaCeF₅ host.

Fig. 7 Dependence of I_S0/I_S of Ce³⁺ on (a)C_Tb^6/3, (b)C_Tb^8/3, and (c)C_Tb^10/3.

According to Dexter's energy transfer theory,⁴⁵ the energy transfer process through multipolar interaction depends on the extent of overlap of the emission spectrum of the sensitizer with the absorption spectrum of the activator, the relative orientation of interacting dipoles and the distance between the sensitizer and the activator. For a dipole–dipole interaction, the energy transfer probability (P_SA) from a sensitizer to an activator is given by the following formula:

here f_d is the oscillator strength of the involved dipole absorption transition of the activator, τ_s is the radiative decay time of the sensitizer, and R is the sensitizer-activator average distance, f_S (E) represents the normalized emission shape function of the sensitizer, and F_A (E) is the normalized absorption shape function of the activator, and E is the energy involved in the transfer (eV).

The critical distance (R_c) of the energy transfer from the sensitizer to activator is defined as the distance for which the probability of transfer equals the probability of radiative emission of the sensitizer, i.e., the distance for which P_SA ×τ_s = 1. Therefore, R_c can be obtained from eqn (5):

The f_d of the Tb³⁺ transition is 0.3 × 10⁻⁶.⁴¹ Using this value and the calculated spectral overlap, the critical distance for a dipole–dipole interaction mechanism is estimated to be 5.89 Å, which little deviates from the value estimated from the critical concentration (5.94 Å), indicating that the electric dipole–dipole interaction as the main energy transfer mechanism. In addition, the dipole–dipole interaction can generally be expected to dominate in the energy transfer when both the sensitizer and the activator ions are characterized by electric dipole-allowed transitions, while the f–f transitions of Tb³⁺ are allowed by the selection rules of electric dipole transitions. Further, the dipole–dipole interaction mechanism in the energy transfer can be determined, too. According to the above results, we believe that the energy transfer mechanism from Ce³⁺ to Tb³⁺ in the BaCeF₅ host should be predominated by dipole–dipole interactions.⁴⁶

4. Conclusion

In summary, a simple solvothermal method has been used to prepare BaCeF₅ and BaCeF₅:Tb³⁺ nanocrystals. The XRD, FE-SEM and TEM analysis indicated that the samples crystallizes in a cubic structure with spherical morphology and an average diameter of 40 nm. The photoluminescence spectra of BaCe_1−xTb_xF₅ nanocrystals demonstrate that the energy transfer from Ce³⁺ to Tb³⁺ is highly efficient. The photoluminescence spectra of the BaCe_1−xTb_xF₅ nanocrystals show this result, as the concentration quenching phenomenon occurs when the x = 0.10. The average separation between Ce³⁺ and Tb³⁺ is calculated and the critical distance R_c is 5.94 Å determined by the method of concentration quenching. The R_c calculated by spectral overlapping method proves this. By comparison of theoretical calculation results to those of experiments, we can infer that the energy transfer from Ce³⁺ to Tb³⁺ in the nanocrystals occurs predominantly via the dipole–dipole interaction.

Acknowledgements

This work was supported by the National Science Foundation of China (no. 11004081), partially supported by the Science and Technology Innovation Projects of Jilin Province for overseas students and by the Open Project of State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (RERU2011005).

References

1. W. Xu, Y. Wang, X. Bai, B. Dong, Q. Liu, J. S. Chen and H. W. Song, J. Phys. Chem. C, 2010, 114, 14018–14024.
2. C. X. Li and J. Lin, J. Mater. Chem., 2010, 20, 6831–6847.
3. T. Yu, J. Joo, Y. I. Park and T. Hyeon, Angew. Chem., Int. Ed., 2005, 44, 7411–7414.
4. H. X. Mai, Y. W. Zhang, R. Si, Z. G. Yan, L. D. Sun, L. P. You and C. H. Yan, J. Am. Chem. Soc., 2006, 128, 6426–6436.
5. M. Yu, H. Wang, C. K. Lin, G. Z. Li and J. Lin, Nanotechnology, 2006, 17, 3245–3252.
6. X. M. Sun and Y. D. Li, Chem. Commun., 2003, 1768–1769.
7. C. M. Zhang, C. X. Li, C. Peng, R. Chai, S. S. Huang, D. M. Yang, Z. Y. Cheng and J. Lin, Chem.–Eur. J., 2010, 16, 5672–5680.
8. Y. Jin, W. P. Qin and J. S. Zhang, J. Fluorine Chem., 2008, 129, 515–518.
9. P. Gao, Y. Xie and Z. Li, Eur. J. Inorg. Chem., 2006, 3261–3265.
10. G. H. De, W. P. Qin, J. S. Zhang, J. S. Zhang, Y. Wang, C. Y. Cao and Y. Cui, J. Solid State Chem., 2006, 179, 955–958.
11. H. Hu, Z. G. Chen, T. Y. Cao, Q. Zhang, M. X. Yu, F. Y. Li, T. Yi and C. H. Huang, Nanotechnology, 2008, 19, 375702–375712.
12. C. X. Li, J. Yang, P. P. Yang, H. Z. Lian and J. Lin, Chem. Mater., 2008, 20, 4317–4326.
13. X. L. Yang, S. G. Xiao, J. W. Ding and X. H. Yan, J. Appl. Phys., 2008, 103, 093101.
14. Z. L. Wang, Z. W. Quan, P. Y. Jia, C. K. Lin, Y. Lou, Y. Chen, J. Fang, W. Zhou, C. J. O'Connor and J. Lin, Chem. Mater., 2006, 18, 2030–2037.
15. J. L. Zhuang, J. Wang, X. F. Yang, I. D. Williams, W. Zhang, Q. Y. Zhang, Z. M. Feng, Z. M. Yang, C. L. Liang, M. M. Wu and Q. Su, Chem. Mater., 2009, 21, 160–168.
16. P. Ptacek, H. Schäfer, K. Kömpe and M. Haase, Adv. Funct. Mater., 2007, 17, 3843–3848.
17. X. Liang, X. Wang, J. Zhuang, Q. Peng and Y. D. Li, Adv. Funct. Mater., 2007, 17, 2757–2765.
18. Z. J. Wang, F. Tao, W. L. Cai, L. Z. Yao and X. G. Li, Solid State Commun., 2007, 144, 255–258.
19. Y. J. Huang, H. P. You, G. Jia, Y. H. Song, Y.H. Zheng, M. Yang, K. Liu and N. Guo, J. Phys. Chem. C, 2010, 114, 18051–18058.
20. C. C. Cao, H. K. Yang, J. W. Chung, B. K. Moon, B. C. Choi, J. H. Jeong and K. H. Kim, J. Mater. Chem., 2011, 21, 10342–10347.
21. D. M. Yang, C. X. Li, G. G. Li, M. M. Shang, X. J. Kang and J. Lin, J. Mater. Chem., 2011, 21, 5923–5927.
22. D. G. Yang, X. J. Kang, M. M. Shang, G. G. Li, C. Peng, C. X. Li and J. Lin, Nanoscale, 2011, 3, 2589–2595.
23. X. R. Liu, G. Xu and C. P. Richard, J. Solid State Chem., 1986, 62, 83–91.
24. L. F. Johnson, H. J. Guggenheim, T. C. Rich and F. W. Ostermayer, J. Appl. Phys., 1972, 43, 1125–1137.
25. S. Bigotta, D. Parisi, L. Bonelli, A. Toncelli and M. Tonelli, J. Appl. Phys., 2006, 100, 013109.
26. V. Toccafondo, S. A. Cerqueira and S. Faralli, J. Appl. Phys., 2007, 101, 023104.
27. N. I. Sorokin and B. P. Sobolev, Phys. Solid State, 2011, 50, 416–421.
28. G. S. Yi, H. C. Lu, S. Y. Zhao, G. Yue, W. J. Yang, D. P. Chen and L. H. Guo, Nano Lett., 2004, 4, 2191–2196.
29. J. Zhuang, L. Liang, H. H. Y. Sung, X. Yang, M. Wu, I. D. Williams, S. Feng and Q. Su, Inorg. Chem., 2007, 46, 5404–5410.
30. F. Zhang, J. Li, J. Shan, L. Xu and Dm. Zhao, Chem.–Eur. J., 2009, 15, 11010–11019.
31. X. Wang and Y. Li, Chem.–Eur. J., 2003, 9, 5627–5635.
32. X. Wang, J. Zhuang, Q. Peng and Y. Li, Inorg. Chem., 2006, 45, 6661–6665.
33. S. Li, T. Xie, Q. Peng and Y. Li, Chem.–Eur. J., 2009, 15, 2512–2517.
34. H. Mai, Y. Zhang, R. Si, Z. Yan, L. Sun, L. You and C. Yan, J. Am. Chem. Soc., 2006, 128, 6426–6436.
35. H. Lai, A. Bao, Y. M. Yang, Y. C. Tao, H. Yang, Y. Zhang and L. L. Han, J. Phys. Chem. C, 2008, 112, 282–286.
36. M. Yu, J. Lin, J. Fu, H. J. Zhang and Y. C. Han, J. Mater. Chem., 2003, 13, 1413–1419.
37. S. D. Cheng, C. H. Kam and S. Buddhudu, Mater. Res. Bull., 2001, 36, 1131–1137.
38. G. A. Kumar, P. R. Biju, G. Jose and N. V. Unnikrishnan, Mater. Chem. Phys., 1999, 60, 247–255.
39. P. I. Paulose, G. Jose, V. Thomas, N. V. Unnikrishnan and M. K. R. Warrier, J. Phys. Chem. Solids, 2003, 64, 841–846.
40. D. L. Dexter and J. A. J. Schulman, Chem. Phys., 1954, 22, 1063–1070.
41. J. M. P. J. Verstegen, J. L. Sommerdijk and J. G. Verriet, J. Lumin., 1973, 6, 425–431.
42. G. K. Liu and B. Jacquier, Spectroscopic Properties of Rare Earthsin Optical Materials, Springer, Beijing, 2005.
43. B. M. Antipeuko, I. M. Bataev, V. L. Ermolaev, E. I. Lyubimov and T. A. Privalova, Opt. Spectrosc., 1970, 29, 177.
44. U. aldiño. G, J. Phys.: Condens. Matter, 2003, 15, 3821–3830.
45. D. L. Dexter, J. Chem. Phys., 1953, 21, 836–850.
46. Z. Zhang, J. Wang, M. Zhang, Q. Zhang and Q. Su, Appl. Phys. B: Lasers Opt., 2008, 91, 529–537.

---