Cooperative Energy Transfer Up-Conversion and Quantum Cutting

Mar 31, 2009 - mechanical stability related to the silica-based glass matrix. The key to the achievement of efficient luminescence in these glass cera...
0 downloads 0 Views 285KB Size
Download PDF
6406

J. Phys. Chem. C 2009, 113, 6406–6410

Cooperative Energy Transfer Up-Conversion and Quantum Cutting Down-Conversion in Yb3+:TbF3 Nanocrystals Embedded Glass Ceramics Daqin Chen, Yunlong Yu, Yuansheng Wang,* Ping Huang, and Fangyi Weng State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ReceiVed: NoVember 13, 2008; ReVised Manuscript ReceiVed: February 03, 2009

Yb3+-doped oxyfluoride glass ceramics were fabricated by melt-quenching and subsequent heating. X-ray diffraction and high-resolution transmission electron microscopy analyses evidenced that orthorhombic TbF3 nanocrystals with a mean size of 22 nm were homogeneously precipitated among the aluminosilicate glass matrix. The incorporation of Yb3+ ions into TbF3 crystals was confirmed by energy-dispersive X-ray spectroscopy. Under 976 nm near-infrared laser excitation, the glass ceramics exhibited intense green Tb3+: 5 D4 f 7FJ (J ) 3, 4, 5, 6) up-conversion luminescence due to the cooperative energy transfer from two Yb3+ ions to one Tb3+ ion. In comparison, under 485 nm excitation, they yielded near-infrared quantum cutting down-conversion emission corresponding to Yb3+:2F5/2 f 2F7/2 transition, ascribing to the cooperative energy transfer from one Tb3+ ion to two Yb3+ ions. The energy transfer processes between Tb3+ and Yb3+ ions were discussed, and the energy transfer efficiency was evaluated.

Rare earth (RE) ion-doped nanostructured oxyfluoride glass ceramics, generally fabricated through controlled crystallization of the fluoride phases from the precursor glasses by heat treatment, have attracted much attention for their wide potential applications in optical communication, laser, solid-state threedimensional display, solar cells, and so on.1-6 These nanocomposites have not only comparative low phonon energies ascribed to the precipitated fluorides but also high chemical and mechanical stability related to the silica-based glass matrix. The key to the achievement of efficient luminescence in these glass ceramics lies in the partition of the optically active RE ions into the fluoride nanocrystals. Lanthanide trifluorides are very suitable for luminescent applications because of their low phonon energies and optically active properties.7-14The microstructure and optical spectroscopy for the RE ion-doped glass ceramics containing LaF3 or YF3 nanocrystals have been well-studied in recent years.15-24 Unlike La3+ or Y3+, Tb3+ can act as the optically active center or sensitizer for its partially full 4f electronic configuration. However, as far as we know, thus far, there has been no report on the fabrication and optical properties for RE ion-doped glass ceramics containing TbF3 nanocrystals. In the present work, the preparation, the selective partition of RE ions, and the cooperative energy transfer up-conversion and quantum cutting downconversion in the Yb3+-doped oxyfluoride glass ceramics containing orthorhombic TbF3 nanocrystals were investigated.

and then annealed at 550 °C for 2 h to relinquish the inner stress. To reduce the loss of the fluoride in the course of melting, a small amount of carbon powder was added to create a reductive atmosphere in the furnace. The as-made glass was then heattreated at 650 °C for 2 h to induce crystallization and form glass ceramic. DSC experiments of the precursor glass were performed in air at a heating rate of 10 K/min to follow its thermal behavior. To identify the crystallization phase and determine the mean size of the crystallites, X-ray diffraction (XRD) analysis was carried out with a powder diffractometer (DMAX2500 Rigaku) using Cu KR radiation (λ ) 0.154 nm). The microstructures of the samples were studied using a transmission electron microscope (TEM, JEM-2010) equipped with an energy-dispersive X-ray (EDX) spectroscope. The up-conversion emission spectra were detected using a Hamamatsu R943-02 photomultiplier tube (PMT) and a Spex 1000 M monochromator under 976 nm Ti sapphire laser excitation. The emission and excitation spectra in both the visible and near-infrared regions were recorded by an Edinburgh Instruments FLS920 spectrofluorometer equipped with a continuous (450 W) xenon lamp. Using the Hamamatsu PMT detector (R5509-72), near-infrared luminescence signals were detected. With the help of the Hamamatsu R928 PMT, the fluorescence decay signals of the Tb3+:5D 4 excited state were recorded under 485 nm excitation. To avoid the influence of the scattering light, the delay time was set to 0.1 ms. All the measurements were carried out at room temperature.

2. Experimental Details

3. Results and Discussion

The precursor glass was prepared with the following composition (in mol %): (44 - x)SiO2-28Al2O3 -17NaF-11TbF3-xYbF3 (x ) 0, 0.25, 0.5, 1.0, 1.5, 3.0). The well-mixed stoichiometric chemicals were put into an alumina crucible and melted at 1400 °C for 30 min. The glass samples were fabricated by pouring the melt into a preheated brass mold

The DSC curve of the 1.5 mol % Yb3+-doped precursor glass is shown in Figure 1, where Tc (615 °C) stands for the crystallization temperature. XRD analysis evidenced that the crystallization peak was ascribed to the precipitation of TbF3 crystals. The XRD patterns of the precursor glass and glass ceramic, both doped with 1.5 mol % Yb3+, are presented in Figure 2. The precursor glass is amorphous with no sharp diffraction peaks. For the glass ceramic, some intense diffraction

1. Introduction

* Corresponding author. E-mail: [email protected].

10.1021/jp809995f CCC: $40.75  2009 American Chemical Society Published on Web 03/31/2009

Energy Transfer in Yb3+:TbF3 Nanocrystals

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6407

Figure 1. DSC trace of the 1.5 mol % Yb3+-doped precursor glass recorded at a heating rate of 10 K/min.

Figure 2. XRD patterns of the 1.5 mol % Yb3+-doped precursor glass (a), glass ceramic (b), and the standard orthorhombic TbF3 phase (c). Figure 4. EDX spectra from an individual TbF3 nanocrystal (a) and glass matrix (b) of the 1.5 mol % Yb3+-doped glass ceramic.

Figure 3. TEM micrograph of the 1.5 mol % Yb3+-doped glass ceramic; the inset shows the HRTEM image taken from the circle region of the TEM micrograph.

peaks appear, which are assigned to the orthorhombic TbF3 phase (JCPDS 84-0179). The mean size of the crystals was evaluated to be about 22 nm by the Scherrer formula. The TEM micrograph of the glass ceramic, shown in Figure 3, demonstrates that TbF3 nanocrystals sized 20-30 nm distribute homogeneously among the glass matrix. The high nanoparticle density shown in the TEM micrograph is actually a result of the projection of all the TbF3 crystallites in a region with a

certain thickness along the electron beam direction. The detailed lattice structure of an individual TbF3 nanocrystal is revealed by the high-resolution TEM (HRETEM) image shown in the inset of Figure 3. To detect the distribution of Yb3+ directly, the EDX spectra with nanosized probe taken from an individual TbF3 nanocrystal and the glass matrix were recorded separately. The spectrum from an individual crystallite, shown in Figure 4a, exhibits intense Tb, F, and Yb signals (the weak Al, Si, and O peaks are attributed to the glass matrix surrounding the nanosized crystal), whereas the spectrum from the glass matrix exhibits intense Si, Al, and O signals and weak Tb, F and Yb ones (Figure 4b). The Cu peaks are ascribed to the copper grid supporting the TEM sample. The content of Yb in the crystals is much higher (about 20 times) than that in the glass matrix, indicating that Yb3+ ions are mainly concentrated in the TbF3 nanocrystals after crystallization. The up-conversion emission spectra of the 1.5 mol % Yb3+doped precursor glass and glass ceramic under 976 nm laser excitation with pump power of 100 mW are shown in Figure 5. The spectra consist of seven emission bands centered at 380, 415, 437, 485, 543, 587, and 619 nm, ascribed to 5G6, 5D 3 f 7 F6; 5G6, 5D3 f 7F5; 5G6, 5D3 f 7F4; 5D4 f 7F6; 5D4 f 7F5; 5D4 f 7F4, and 5D4 f 7F3 transitions of Tb3+, respectively. Compared with those of the precursor glass, the emission bands of the glass ceramic are remarkably intensified. The influence of the Yb3+ content on the green (543 nm) up-conversion luminescence of Tb3+ in the glass ceramics is exhibited in the

6408 J. Phys. Chem. C, Vol. 113, No. 16, 2009

Figure 5. Up-conversion emission spectra of the precursor glass and glass ceramic doped with 1.5 mol % Yb3+ under 976 nm excitation. The inset shows the dependence of the 543 nm emission intensity on the Yb3+ content for the glass ceramic samples.

Figure 6. The log-log plot of the up-conversion emission intensity versus the pump power of 976 nm excitation laser for the 1.5 mol % Yb3+-doped glass ceramic.

inset of Figure 5. With an increase in the Yb3+ doping content, the electron population of the Tb3+ 5D4 level is largely increased owing to the enhancement of energy transfer from Yb3+ to Tb3+. In this case, the up-conversion emission intensity increases gradually with an increase in the Yb3+ content from 0.25 to 1.0 mol %. However, the intensity obviously decreases when the Yb3+ content exceeds 1.0 mol % due to the concentration quenching of Tb3+. The dependence of up-conversion emission intensity (Iup) on the excitation power (Iin) follows the relationship of Iup ∞ Iinn, where n is the number of the pumping photons required to excite RE ions from the ground state to the emitting excited state. The power dependence of the aforementioned three typical emission transitions of 5D4 f 7FJ (J ) 6, 5, 4) is presented in Figure 6 by a log-log plot. The slopes of the linear fittings are 1.89 for 5 D4 f 7F6 (485 nm), 1.98 for 5D4 f 7F5 (543 nm), and 1.87 for 5 D4 f 7F4 (587 nm), indicating that two pumping photons are required to populate the 5D4 emitting level. On the basis of the energy-matching relationship of Tb3+ and Yb3+ levels, the up-conversion luminescence mechanism in the precursor glass and glass ceramic is proposed, as illustrated in Figure 7. Under 976 nm excitation, Yb3+ ions were excited from the ground state 2F7/2 to the excited state 2F5/2. Through the cooperative energy transfer (CET) from two Yb3+ ions to one Tb3+ ion (i.e., Yb3+:2F5/2 + Yb3+:2F5/2 f Tb3+:5D4), the electron population of the Tb3+: 5D4 level was achieved.25-29 Then, the

Chen et al.

Figure 7. Schematic energy level diagram of Tb3+ and Yb3+ ions and up-conversion luminescence mechanisms in the precursor glass and glass ceramic under 976 nm laser excitation.

electrons in the populated 5D4 level could radiatively relax to the lower 7FJ (J ) 6, 5, 4, 3) levels to produce four emissions centered at 485, 543, 587, and 619 nm, respectively. The other three emissions result from the following mechanism. The electrons in the 5D4 level could also be excited to the higher 5 D1 level by a one-photon absorption process; that is, excitedstate absorption (ESA) or energy transfer (ET) (or both) from Yb3+ ions. Then the electrons could nonradiatively relax to the 5 G6 and 5D3 emitting levels and further radiatively relax to the lower levels 7F6, 7F5, and 7F4, which generate three emissions at 380, 415, and 437 nm, respectively. As has been demonstrated in the EDX spectrum of the glass ceramic, the doping Yb3+ ions partition mainly into the precipitated TbF3 nanocrystals. Therefore, the Yb3+ and Tb3+ are located closer to each other than those uniformly dispersed in the precursor glass. The short distances between Yb3+ and Tb3+ favor the interionic interactions, resulting in an efficient cooperative energy transfer process; i.e., Yb3+:2F5/2 + Yb3+: 2 F5/2 f Tb3+:5D4 and, thus, the intense up-conversion emissions. Compared to the green one, the ultraviolet and blue upconversion emission intensities are very weak. This could be ascribed to the existence of cross relaxation (CR) between Tb3+ ions (i.e., 5G6, 5D3 + 7F6 f 5D4 + 7F0) since the Tb3+ content is very high in the investigated samples. As mentioned above, the green up-conversion luminescence of Tb3+ is achieved by the second-order cooperative energy transfer from two Yb3+ ions to one Tb3+ ion. Therefore, on the basis of the energy match relationship, the quantum cutting down-conversion luminescence of Yb3+ realized by cooperative energy transfer from one Tb3+ to two Yb3+ ions could occur, as well.30-33 Figure 8 shows the excitation and emission spectra of the 1.5 mol % Yb3+-doped glass and glass ceramic. From the excitation spectrum of the up-doped glass ceramic presented in the inset of Figure 8, we can observe that by monitoring Tb3+ 543 nm emission, the excitation band is located at 485 nm, corresponding to a Tb3+:7F6 f 5D4 transition. In the Yb3+doped glass ceramic, the excitation spectrum of Yb3+ nearinfrared emission at 976 nm is in good agreement with the Tb3+: 7 F6 f 5D4 absorption transition, which indicates the existence of energy transfer from Tb3+ to Yb3+. The Yb3+ near-infrared emission spectra of the precursor glass and glass ceramic are recorded under 485 nm excitation. The Yb3+ luminescence in the glass ceramic is much stronger than that in the precursor glass because of the favorable interionic interactions between Tb3+ and Yb3+ ions; i.e., the efficient cooperative energy transfer process Tb3+:5D4 f Yb3+:2F5/2 + Yb3+:2F5/2.

Energy Transfer in Yb3+:TbF3 Nanocrystals

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6409

Figure 10. Decay curves of the Tb3+:5D4 f 7F5 transition in a semilog plot for the undoped precursor glass and glass ceramics doped with various content of Yb3+ under 485 nm excitation.

Figure 8. (a) Excitation (λem ) 976 nm) and (b) near-infrared emission (λex ) 485 nm) spectra of the 1.5 mol % Yb3+-doped glass and glass ceramic; the inset shows the Tb3+ excitation (λem ) 543 nm) and emission (λex ) 485 nm) spectra of the undoped glass ceramic.

Figure 11. The decay lifetime of the Tb3+:5D4 f 7F5 transition and the efficiency of the Tb3+ f Yb3+ energy transfer versus Yb3+ content.

3+

3+

Figure 9. Schematic energy level diagram of Tb and Yb in the glass ceramic showing the cooperative energy transfer mechanism of the near-infrared quantum cutting emission under 485 excitation.

To get more information about the mechanism of Yb3+ luminescence, the dependence of the Yb3+ emission intensity on the 485 nm excitation power was measured, which exhibits a sublinear relationship with a slope of 0.49 similar to the case in the Tb3+-doped KYb(WO4)2 crystal.34,35 This result verifies that the energy of one excited Tb3+ ion is transferred to two different Yb3+ ions simultaneously. The cooperative energy transfer from one Tb3+ ion to two Yb3+ ions in the Yb3+-doped glass ceramic is illustrated in a schematic energy level diagram shown in Figure 9. When excited at 485 nm, Tb3+ emissions corresponding to 5D4 f 7FJ (J ) 5, 4, 3) transitions occur. Meanwhile, two near-infrared photons originating from Yb3+: 2 F5/2 f 2F7/2 transition are also obtained from one absorbed blue photon. The decay curves of the Tb3+:5D4 f 7F5 transition for the undoped precursor glass and glass ceramics doped with various contents of Yb3+ are exhibited in Figure 10. For the undoped precursor glass, the decay curve shows a near single-exponential feature, whereas for the undoped glass ceramic, the curve exhibits two different decay behaviors: one with a fast decay and the other with a slow decay similar to the case of the undoped precursor glass. As is known, in the glass ceramics, TbF3 nanocrystals were precipitated from the glass matrix, and some residual Tb3+ ions still stayed in the glass matrix, as demonstrated by the EDX spectrum shown in Figure 4. Therefore, it is concluded that the slow decay originates from the residual Tb3+ ions in the glass matrix and the fast one from

the Tb3+ ions in the TbF3 nanocrystals. With an increase in the Yb3+ content in the glass ceramic, the fast decay section changes obviously (the decay time decreases monotonically), but the slow decay one is not markedly altered. This result indicates that the energy transfer from Tb3+ to Yb3+ occurs mainly in the TbF3 nanocrystals. Because of the nonexponential decay feature of the glass ceramic, the effective experimental lifetime is evaluated using τeff ) ∫[I(t)t dt]/∫[I(t) dt], where I(t) represents the luminescence intensity at the time t after the cutoff of the excitation light. As shown in Figure 11, the decay lifetime decreases with the increasing of Yb3+ content, which can be explained by the introducing of the extra decay pathway; i.e., the cooperative energy transfer from Tb3+:5D4 to Yb3+:2F5/2. The energy transfer efficiency (ETE), ηtr,x%Yb, defined as the ratio of the number of Tb3+ ions depopulated by the energy transfer to Yb3+ ions over the total number of Tb3+ ions excited, is determined from the luminescence decay curves. By dividing the integrated intensity of the decay curve of the Yb3+-doped glass ceramic to that of the undoped one, ETE can be expressed as a function of Yb3+ content,

ηtr,x%Yb )

∫ I0%Yb dt - ∫ Ix%Yb dt ∫ Ix%Yb dt )1∫ I0%Yb dt ∫ I0%Yb dt (1)

where I stands for the decay intensity, and x % Yb denotes the Yb3+ content. Worthy of noticing, the ETE increases monotonically with the increasing of Yb3+ content, as shown in Figure 11. For the 3.0 mol % Yb3+-doped glass ceramic, the ETE

6410 J. Phys. Chem. C, Vol. 113, No. 16, 2009 reaches a value of 41%, meaning that the depopulation of Tb3+: 5 D4 level proceeds 41 out of 100 times by exciting two Yb3+ ions simultaneously from the ground state 2F7/2 to the excited state 2F5/2. It is worth mentioning that the transparent glass ceramic with intense near-infrared quantum cutting down-conversion luminescence may find application in enhancing the efficiency of silicon solar cells by reducing the thermalization of the electron-hole pairs.5,30-33 The 485 nm absorption band of Tb3+ is located at the maximum intensity region of the solar spectrum, and the Yb3+ emission band via cooperative energy transfer from Tb3+ to Yb3+ is around 976 nm, where silicon solar cells exhibit their greatest spectral response. 4. Conclusions The oxyfluoride glasses and glass ceramics containing TbF3 nanocrystals were fabricated by melt quenching and subsequent heating. Doping of Yb3+ ions resulted in both the visible upconversion luminescence of Tb3+ and the near-infrared quantum cutting down-conversion emission of Yb3+, through cooperative energy transfer Yb3+:2F5/2 + Yb3+:2F5/2 T Tb3+:5D4. Compared with the precursor glass, the up- and down-conversion intensities in the glass ceramic were remarkably enhanced due to the partition of Yb3+ into the precipitated TbF3 nanocrystals and, thus, the favorable interionic interactions between Yb3+ and Tb3+ ions. Acknowledgment. This work was supported by NSFC (50672098), the Science & Technology Projects of Fujian (2007HZ0002-2, 2008F3114, 2006L2005), the projects of CAS (KJCX2-YW-M05)andFJIRSM(SZD07004,2006K02,2006KL002), and the Knowledge Innovation Program of CAS and SKLSC (20080039). References and Notes (1) Wang, Y. H.; Ohwaki, J. Appl. Phys. Lett. 1993, 63, 3268. (2) Dantelle, G.; Mortier, M.; Vivien, D.; Patriarche, G. Chem. Mater. 2005, 17, 2216. (3) Pisarski, W. A.; Goryczka, T.; Pisarska, J.; Ryba-Romanowski, W. J. Phys. Chem. B 2007, 111, 2427. (4) Lahoz, F.; Herna´ndez, S. E.; Capuj, N. E.; Navarro-Urrios, D. Appl. Phys. Lett. 2007, 90, 201117. (5) Ye, S.; Zhu, B.; Chen, J. X.; Luo, J.; Qiu, J. R. Appl. Phys. Lett. 2008, 92, 141112. (6) Bueno, L. A.; Gouveia-Neto, A. S.; da Costa, E. B.; Messaddeq, Y.; Ribeiro, S. J. L. J. Phys.: Condens. Matter 2008, 20, 145201. (7) Yan, R. X.; Li, Y. D. AdV. Funct. Mater. 2005, 15, 763.

Chen et al. (8) Zeng, J. H.; Su, J.; Li, Z. H.; Yan, R. X.; Li, Y. D. AdV. Mater. 2005, 17, 2119. (9) Li, C. X.; Yang, J.; Yang, P. P.; Lian, H. Z.; Lin, J. Chem. Mater. 2008, 20, 4317. (10) Li, C. X.; Liu, X. M.; Yang, P. P.; Zhang, C. M.; Lian, H. Z.; Lin, J. J. Phys. Chem. C 2008, 112, 2904. (11) Sivakumar, S.; van Veggel, F. C. J. M.; Raudsepp, M. J. Am. Chem. Soc. 2005, 127, 12464. (12) Bovero, E.; van Veggel, F. C. J. M. J. Phys. Chem. C 2007, 111, 4529. (13) Zhang, M. F.; Fan, H.; Xi, B. J.; Wang, X. Y.; Dong, C.; Qian, Y. T. J. Phys. Chem. C 2007, 111, 6652. (14) Dong, B.; Song, H. W.; Qin, R. F.; Bai, X.; Lu, S. Z.; Ren, X. G.; Pan, G. H.; Zhang, H.; Wang, F.; Fan, L. J. Nanosci. Nanotechnol. 2008, 8, 3921. (15) Dejneka, M. J. J. Non-Cryst. Solids 1998, 239, 149. (16) Tanabe, S.; Hayashi, H.; Hanada, T.; Onodera, N. Opt. Mater. 2002, 19, 343. (17) Goutaland, F.; Jander, P.; Brocklesby, W. S.; Dai, G. J. Opt. Mater. 2002, 22, 383. (18) Yanes, A. C.; Del-Castillo, J.; Me`ndez-Ramos, J.; Rodrı´guez, V. D.; Torres, M. E.; Arbiol, J. Opt. Mater. 2007, 29, 999. (19) Chen, D. Q.; Wang, Y. S.; Yu, Y. L.; Liu, F.; Huang, P. Opt. Lett. 2007, 32, 3068. (20) Chen, D. Q.; Wang, Y. S.; Zheng, K. L.; Guo, T. L.; Yu, Y. L.; Huang, P. Appl. Phys. Lett. 2007, 91, 251903. (21) Chen, D. Q.; Wang, Y. S.; Yu, Y. L.; Huang, P. Appl. Phys. Lett. 2007, 91, 051920. (22) Chen, D. Q.; Wang, Y. S.; Ma, E.; Yu, Y. L.; Liu, F.; Li, R. F. J. Appl. Phys. 2007, 102, 023504. (23) Chen, D. Q.; Wang, Y. S.; Bao, F.; Yu, Y. L. J. Appl. Phys. 2007, 101, 113511. (24) Chen, D. Q.; Wang, Y. S.; Yu, Y. L.; Huang, P.; Weng, F. Y. Opt. Lett. 2008, 33, 1884. (25) Martı´n, I. R.; Yanes, A. C.; Mendez-Ramos, J.; Torres, M. E.; Rodriguez, V. D. J. Appl. Phys. 2001, 89, 2520. (26) Vermelho, M. V. D.; dos Santos, P. V.; de Aroujo, M. T.; GouveriaNeto, A. S.; Cassanjes, F. C.; Ribeiro, S. J. L.; Messaddeq, Y. J. Lumin. 2003, 102-103, 762. (27) Salley, G. M.; Valiente, R.; Gu¨del, H. U. Phys. ReV. B 2003, 67, 134111. (28) Huang, L. H.; Yamashita, T.; Jose, R.; Arai, Y.; Suzuki, T.; Ohishi, Y. Appl. Phys. Lett. 2007, 90, 131116. (29) Huang, L. H.; Qin, G. S.; Arai, Y.; Jose, R.; Suzuki, T.; Ohishi, Y.; Yamashita, T.; Akimoto, Y. J. Appl. Phys. 2007, 102, 093506. (30) Vergeer, P.; Vlugt, T. J. H.; Kox, M. H. F.; Den Hertog, M. I.; van der Eerden, J. P. J. M.; Meijerink, A. Phys. ReV. B. 2005, 71, 014119. (31) Zhang, Q. Y.; Yang, C. H.; Pan, Y. X. Appl. Phys. Lett. 2007, 90, 021107. (32) Zhang, Q. Y.; Yang, C. H.; Jiang, Z. H.; Ji, X. H. Appl. Phys. Lett. 2007, 90, 061914. (33) Zhang, Q. Y.; Yang, G. F.; Jiang, Z. H. Appl. Phys. Lett. 2007, 91, 051903. (34) Strek, W.; Deren´, P.; Bednarkiewicz, A. J. Lumin. 2000, 87-89, 999. (35) Strek, W; Bednarkiewicz, A.; Deren´, P. J. J. Lumin. 2001, 92, 229.

JP809995F