White Emission by Frequency Up-Conversion in Yb3+-Ho3+-Tm3+

Oct 13, 2009 - White Emission by Frequency Up-Conversion in Yb3+-Ho3+-Tm3+ Triply ... People's Republic of China, and Key laboratory of low-Dimensiona...
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J. Phys. Chem. C 2009, 113, 18995–18999

18995

ARTICLES White Emission by Frequency Up-Conversion in Yb3+-Ho3+-Tm3+ Triply Doped Hexagonal NaYF4 Nanorods L. W. Yang,*,†,‡ H. L. Han,†,‡ Y. Y. Zhang,† and J. X. Zhong† Institute for Quantum Engineering and Micro-Nano Energy Technology and Faculty of Materials and Optoelectronic Physics, Xiangtan UniVersity, Hunan 411105, People’s Republic of China, and Key laboratory of low-Dimensional Materials and Application Technologies, Xiangtan UniVersity, Ministry of Education, Hunan 411105, People’s Republic of China ReceiVed: March 11, 2009; ReVised Manuscript ReceiVed: August 22, 2009

Hexagonal Yb3+-Ho3+-Tm3+ triply doped NaYF4 nanorods were synthesized via a hydrothermal method using oleic acid as a stabilizing agent. White up-conversion (UC) luminescence consisting of the blue UC radiations at 450 and 475 nm corresponding to the 1D2 f 3F4 and 1G4 f 3H6 transitions of Tm3+ ion, green at 545 nm to the 5S2 /5F4 f 5I8 transition of Ho3+ ions and red at 695 nm to the 3F3 f 3H6 of Tm3+ ion and at 650 nm including both the 5F5 f 5I8 transition of the Ho3+ ion and the 3F2 f 3H6 transition of Tm3+ ion, was observed in the as-prepared nanorods under the excitation of a 980 nm diode laser. The calculated color coordinates display that with the increase of pump power densities, the tendency of white color output turns toward the blue region, revealing that white UC light can be fine-tuned in a wide range of pump power densities. On the basis of spectral and pump power dependence analyses, the UC mechanisms for the fine-tuned white output were discussed in detail. I. Introduction Due to its potential applications in the fields of solid-state lasers, solar cells, multicolor three-dimensional displays, and especially biological fluorescent labels, white emission by frequency up-conversion (UC) in rare-earth (RE) ion doped nanocrystals deserves increasing attention.1-12 The realization of strong white emission requires the generation and an adequate combination of the three fundamental red, green, and blue (RGB) light colors, which greatly challenge people’s ability of material design including host composition and the suitable combination of sensitizers and activator ions. Previously, Downing et al. successfully generated blue, green, and red light exploiting the frequency UC process excited by two nearinfrared lasers with distinct wavelengths using fluoride glass samples codoped with Tm3+, Er3+, and Pr3+ ions.2 Unfortunately, no simultaneous generation of red, green, and blue emissions was demonstrated, particularly in the case with single infrared pump laser sources, which is mostly desirable. Recently, inspired by the report on white light simulation in triply doped fluoride glass host under the excitation of a 980 nm diode laser (LD), important progress on white UC emission was obtained in Y2O3 nanocrystals, cubic NaYF4 nanocrystals, and glass ceramics containing YF3 or Pb1-xCdxF2 nanocrystals.9-12 But the combination of sensitizer and activator ions is mainly limited in the Yb3+/Er3+/Tm3+ triply doped systems. Few investigations have focused on other activator ions, such as Yb3+/Ho3+/Tm3+ * To whom correspondence should be addressed. Phone: +86 732 8292 113. Fax: +86 732 8292 468. E-mail: [email protected] (L.W.Y.). † Institute for Quantum Engineering and Micro-Nano Energy Technology and Faculty of Materials and Optoelectronic Physics. ‡ Key laboratory of low-Dimensional Materials and Application Technologies.

and Yb3+/Nd3+/Tm3+ systems. In this investigation, we demonstrate the design of white UC luminescence using Yb3+-Ho3+Tm3+ triply doped nanorods under the excitation of a 980 nm LD. II. Samples and Experiments In our studies, hexagonal phase NaYF4 (β-NaYF4) was chosen as the host material. To the best of our knowledge, among the reported UC materials, β-NaYF4 is identified as the highest efficient host material for the UC emissions.13-15 Here β-NaYF4 nanorods with designed triple doping (Yb3+-Ho3+-Tm3+) were synthesized by a hydrothermal method, using oleic acid as a stabilizing agent under a basic condition, which was similar to the method reported by Zhang et al.15 In a typical preparation, 0.7 g (17.5 mmol) of NaOH, 7.1 g (22.6 mmol) of oleic acid (90 wt %), and 10.0 g of ethanol were mixed well to get a white viscous solution. Then 12 mL (7.2 mmol) of 0.58 M NaF solution was added with vigorous stirring until a translucent solution was obtained. Then 1.5 mL (1.2 mmol) of 0.80 M Y(NO3)3 with designed RE ion doping content was poured into the above solution with vigorous stirring. Before being transferred to a Teflon-lined autoclave with an internal volume of 25 mL, the solution mixture was aged for 20 min at room temperature. The hydrothermal syntheses were conducted in an electric oven at 190 °C for 20 h. After the reactions, the obtained white products were harvested by centrifugation and thorough washing with deionized water, and dried at 60 °C for 24 h. The crystal structures of the obtained samples were determined by X-ray diffraction (XRD) measurements, using the copper KR radiation. The morphologies and microstructures of the synthesized samples were characterized by using field

10.1021/jp9021689 CCC: $40.75  2009 American Chemical Society Published on Web 10/13/2009

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Figure 2. Calibrated UC emission spectra of NaYF4 nanorods under the excitation of a 980 nm LD (a) doped with 0.2 mol % Ho3+ and 25/mol % Yb3+, (b) doped with 0.2 mol % Tm3+ and 25/mol % Yb3+, and (c) doped with 0.2 mol % Ho3+, 0.2 mol % Tm3+, and 25 mol % Yb3+. Figure 1. (a) Typical TEM image of Yb3+-Ho3+-Tm3+ triply doped NaYF4 nanorods, (b) SAED pattern, and (c) EDXA of a single triply doped NaYF4 nanorod. Note that the strong signals for Cu in panel c come from the copper TEM grid. (d) XRD patterns of NaYF4 nanorods synthesized at different hydrothermal temperatures.

emission scanning electron microscopy (SEM) and highresolutiontransmissionelectronmicroscopy(HRTEM,JEOL2100), with selected area electron diffraction (SAED). The UC luminescence spectra were recorded with a spectrophotometer (R-500) under the excitation of a 980 nm LD after the as-prepared powder samples were compressed into smooth slices. The dependence of UC emission intensity on pumping powers for different samples was obtained by changing the excitation powers. The fluorescence spot of the parallel laser beam was focused on the samples with a diameter of about 0.4 cm. The luminescence decays were recorded by a digital realtime oscilloscope under the excitation of 980 nm pulsed lasers. All measurements were performed at room temperature. III. Results and Discussion Figure 1a shows the typical TEM image of the RE ion-doped nanorods synthesized at 190 °C. These nanorods display uniform morphology and high quality with diameters of about 110 nm and lengths of about 1 µm. Figure 1b shows the electron diffraction pattern of single nanorods, which demonstrates unambiguously the single-crystalline nature of the sample. It can be readily indexed as hexagonal phase NaYF4, consistent with the XRD results (see Figure 1d), in which the diffraction peak is well indexed to β-NaYF4 (JCPDS 16-0334). During the TEM measurements, the elemental components of the nanorods were detected by energy-dispersive X-ray analysis (EDXA). All of the EDXA results in different RE ion-doped nanorods are consistent with their real elemental components, respectively. The EDXA results confirm that the main elemental components are Na, Y, F, and Yb (see Figure 1c). During the experiments, we found that temperature and hydrothermal reaction time greatly influence the final products. A mixture of the cubic R-NaYF4 phase and the hexagonal β-NaYF4 phase was obtained when the hydrothermal temperature is lower than 190 °C at the same reaction time (see Figure 1d). Similar results were obtained after hydrothermal reaction at 190 °C with shorter reaction time. Figure 2a shows typical UC luminescence spectra in Yb3+/ Ho3+ (25:0.2 mol %) codoped samples under the excitation of a 980 nm LD. They exhibit emission bands centered around

650 and 542 nm corresponding to the 5F5 f 5I8 and 4S2/5F4 f I8 transitions, respectively, and also blue and near-infrared emission around 485 and 750 nm, which were respectively assigned to the 5F3 f 5I8 and 5I4 f 5I8 transitions of Ho3+ ions. Figure 2b shows typical UC luminescence spectra of the Yb3+/ Tm3+ (25:0.2 mol %) codoped samples. The intense blue emissions at 451 and 478 nm correspond to 1D2 f 3F4 and 1G4 f 3H6 transitions of Tm3+ ions, respectively. The weak red emissions at 650 and 695 nm are usually assigned to the 3F3/ 3 F2 f 3H6 or 1G4 f 3F4/3H5 of Tm3+ ions; however, in our experiment we find that they intensely depend on pump power densities. Besides the blue and red emissions, the intense nearinfrared emissions at 800 nm corresponding to 3H4 f 3H6 transitions of Tm3+ were also recorded in this sample. Figure 1c is the UC luminescence spectra of the NaYF4 nanorods doped with 0.2 mol % Ho3+, 0.2 mol % Tm3+, and 25 mol % Yb3+ ions with white emission (see the inset of Figure 3d). As compared with parts a and b of Figure 2, it is easy to assign the origins of the emission bands observed in the triply doped NaYF4 nanorods. The blue emission around 475 and 450 nm and the red emission at 696 nm can be assigned to the intra-4f electronic transitions of Tm3+ ions, and the green emission around 540 nm to Ho3+ ions. For the red emission at 650 nm, it should include the contribution of both Tm3+ and Ho3+ ions according to the change of fine spectral shape in the red band. Figure 3a shows typical UC spectra in Yb3+/Ho3+/Tm3+ triply doped NaYF4 nanorods under the excitation of a 980 nm LD with different pump power densities. One can find that the intensity of blue emissions increases faster than that of other green and red ones with the increase of pump power densities. Figure 3b shows the CIE chromaticity diagram of 1931 together with the calculated color coordinates in the triply doped NaYF4 nanorods with various outputs. The calculated color coordinates range from (0.424, 0.4184) to (0.305, 0.306), which correspond to the pump power densties ranging from 1.61 to 200.98 W/cm2. The arrow shown in the inset indicates the changing trends of the color coordinate with the increase of pump power densties. These color coordinates fall exactly within the white region of the 1931 CIE diagram. Particularly, the color coordinate (0.318, 0.335) at the pump power density of 14.09 W/cm2 is very close to the standard color coordinates (0.33, 0.33), pointing out their potential use as a standard white light source. In addition, one can find that with the increase of pump power densities, the tendency of color coordinates would be toward the blue region. The results originate from the fact that the intensity of blue 5

White Emission by Frequency Up-Conversion

J. Phys. Chem. C, Vol. 113, No. 44, 2009 18997 UC emissions centered at 695, 650, and 540 nm, while three and four pump photons are necessary to produce the UC emissions at 475 and 450 nm, which is consistent with other previous reports in Yb3+-Tm3+ codoped ZrO2, Y2O3, and BaYF5 nanocrystals.10-12,16 Furthermore, one can see that the slope values always tend to decrease for blue and green UC bands with the increase of pump power densities. This saturation effect could be attributed to two factors.16-18 One is the saturation of the Yb3+ absorption at high pump power densities. The other may be related to the saturation of the excited states of Tm3+(3H4) and Ho3+(5I7) owing to the efficient energy transfer (ET) processes from Yb3+ to Tm3+ or Ho3+ ions, resulting in the following ET processes populating the upper excited states being so efficient that it exceeds the spontaneous decay rate to the ground states.16,17 But the slope change abnormally increases for the red emission. For the abnormal dependence of the red emission with pump power densities increasing, we consider that it would originate from that at the low pump power densities, coming mainly from the 3F3 f 3H6 transition, which is a two-phonon process, then with the increase of excitation densities, the 1G4 f 3H5 transition of Tm3+ also contributes partly to the red emission, which is a three-phonon process. The UC mechanism of the 4f electron level of RE ions in different matrixes has been extensively discussed by Auzel.1 Figure 5 shows the energy level diagrams of the Tm3+, Ho3+, and Yb3+ ions and the possible UC mechanisms to produce the white radiation. For the blue emissions at 475 and 450 nm, the following multiple phonon-assisted ET processes from Yb3+ ions to Tm3+ ions might be responsible for the population of upper excited 1G4 and 1D2 levels of Tm3+ ions16,19

Figure 3. (a) Typical UC spectra in Yb3+/Ho3+/Tm3+ triply doped NaYF4 nanorods under the excitation of a 980 nm LD with different pump power densities. (b) The CIE chromaticity diagram of 1931 together with the calculated color coordinates under the excitation of a 980 nm LD with various outputs. The arrows indicate the changing trends of the color coordinate with the increase of pump power densities. The inset is the digital image of the white luminescence of Yb3+/Ho3+/ Tm3+ triply doped nanorods in toluene at the pump power of 50 mW.

F5/2(Yb3+) + 3H6(Tm3+) f 3H5(Tm3+) +

2

F7/2(Yb3+) + phonon energy (3100 cm-1)≈ 3F4(Tm3+) + multiphonon relaxation (about 2 phonons) (I)

2

F5/2(Yb3+) + 3F4(Tm3+) f 3F2 / 3F3(Tm3+) +

2

2

emissions increases faster than that of other green and red UC ones, indicating that the simulated white color output could be modulated via adjusting the pump power densties of LD. Recently, Wang et al.8 presented an approach to fine-tuning the UC emission colors from visible to NIR under 980 nm excitation in cubic NaYF4 nanoparticles by precisely controlling the combinations of RE ion doping kinds and concentration, which is very effective in improving current luminescent probes that are particularly useful in multiplexed labeling. Hence our results imply an alternate approach. To clarify the UC mechanism and the dependences of the calculated color coordinate on the pump power densities, the power dependent UC behavior of the blue, green, and red radiations has been investigated. Generally, for unsaturated UC processes, the UC luminescence intensity (Iup) is related to the pump infrared one (IIR) via the following formula, IUP ∝ IIRn, where n is the number of pump photons required to populate the upper emitting level and its value can be obtained from the slope of the line in the plot of log IUP versus log IIRn.11 Figure 4 shows the log-log plot of the UC emission intensity versus the pump power in triply doped NaYF4 nanorods. The slope values of the linear fits with the experimental data are 1.70, 1.69, 1.73, 2.26, and 3.34 for the five observed visible UC bands under low pump power densities (see Figure 4a). The results indicate that two pump photons are necessary to produce the

F7/2(Yb3+) + phonon energy (0 cm-1)≈ 3H4(Tm3+) + multiphonon relaxation (about 6 phonons) (II)

F5/2(Yb3+) + 3H4(Tm3+) f 1G4(Tm3+) + 2F7/2(Yb3+) +

2

phonon energy (1300 cm-1) (III) F5/2(Yb3+) + 1G4(Tm3+) f 1D2(Tm3+) + 2F7/2(Yb3+) +

2

phonon energy (3000 cm-1) (IV) These are typical three- and four-photon processes, which might be consistent with our experiments. Usually the phononassisted ET rate (γ) can be expressed as γ ) Pe-R∆E, where P is the probability of the ET process without energy mismatch, ∆E is the mismatch for the energy transfer, and R is the electron-lattice coupling coefficient.20 From this equation, one finds that the energy mismatches in UC processes will largely influence the effective phonon-assisted ET rate and the larger the energy mismatch is, the smaller the ET rate will be. Considering the mismatch of energy between the 2F7/2 r 2F5/2 Yb3+ and 1G4 f 1D2 Tm3+ transitions (IV, Figure 5) is greater than 3000 cm-1, this final transfer should occur with little probability. As a result, the intensity of this peak at 450 nm would be extremely weak,16 which is against our experimental results. Therefore there should exist another

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Figure 4. Double-logarithm plots of the UC emission intensity versus the pump power densities in triply doped NaYF4 nanorods under the excitation of 980 nm LD: (a) low pump power densities, (b) moderate pump power densities, and (c) high pump power densities. (d) Temporal behaviors of blue emissions at 450 and 475 nm, green at 541 nm and red at 647 nm, and near-infrared at 800 nm under the excitation of 980 nm pulsed lasers as well as the infrared emission at 1040 nm from the 2F5/2(Yb3+) f 2F7/2(Yb3+) transition. The inset shows the calculated temporal behaviors of the cooperative UC (CUC) emisson according to the equations of INIR ∝ ne ∝ exp(-t/te) and ICUC ∝ ne2 ∝ exp(-2t/te), where ne is the population density of the excited Yb3+ ions.

Figure 5. Energy level diagram of the Tm3+, Ho3+, and Yb3+ ions, and the proposed UC mechanism to produce the blue, green, and red emissions.

UC process. Here we consider that the most likely one is the nearly resonant cross relaxation ET between Tm3+ ions: 3F2 + 3 H4 f 3H6 + 1D2 (CRET), which is also a four-photon UC process to populate the excited 1D2 level.12 For the green emission at 540 nm, the population on the upper excited level is accomplished through two successive phonon-assisted ET processes from Yb3+ ions, in which the exceeding energy is transferred to the host matrix (∼1600 cm-1). In the first transfer

(V, Figure 5), a Ho3+ ion is excited from its ground-state 5I8 level to the 5I6 level. Then a second transfer (VI, Figure 5) from a nearby Yb3+ ion excites the same Ho3+ ion from the excited state 5I6 to the 4S2/5F4 thermalized emitting levels. For the red emission at 650 nm, it included two UC channels. One is associated with Tm3+ ions, the other with Ho3+ ions. For the former, the population of the excited 3F2/3F3 level is accomplished through two successive ET processes from Yb3+ ions to Tm3+ ions at low excitation densities. For the latter, the population of the excited 5F5 level is realized by absorbing the energy (VII, Figure 5) of one excited Yb3+ ion after relaxing nonradiatively to the 5I7 level from the excited 5I6 level. Now we would like to discuss the time evolution behaviors of the blue, green, red, and near-infrared UC spectra in triply doped NaYF4 nanorods as well the infrared emission at 1040 nm from the transition of 2F5/2(Yb3+) f 2F7/2(Yb3+) under the excitation of a 980 nm pulsed laser. Figure 4d displays the room temperature luminescence decay profiles of the blue emissions at 475 and 450 nm, green emission at 540 nm, red emission at 650 nm, and near-infrared emission at 800 nm in triply doped NaYF4 nanorods as well the infrared emission at 1040 nm under the excitation of a 980 nm pulsed laser. Rise and decay components are observed, similar to other reports in Er3+ doped KPb2Cl5 crystals and Yb3+-Er3+ codoped NaYF4 nanoparticles.21,22 In analysis of the curves, due to the fact that when a rise and a decay component coexist in luminescence kinetics, decay is always the longer one, even if the population is slower than the depopulation, one has to realize that luminescence decay does not always represent the depopulation of the excited state, and luminescence rise does not always represent the population

White Emission by Frequency Up-Conversion of the excited state as well.22 In our experiment, the intermediate 3 H4 states have much longer lifetimes relative to the final UC emission states, such as 1G4 and 1D2 states. As a result, the rise components in our measurements should be determined by the lifetimes of the latter states, whereas the decay components of the UC luminescence kinetics are determined by the intermediate 3 H4 states. Furthermore, from the results of luminescence decays, we confirm qualitatively that the decrease of the slope values in the plot of log IUP versus log IIRn for the blue emissions at 450 and 475 nm mainly originates from the saturation effect of the excited Tm3+(3H4) states instead of the cooperative UC (CUC) mechanisms from the excited Yb3+ ions pairs to Tm3+ ions reported in ref 23. It is well-known that for CUC if INIR ∝ ne ∝ exp(-t/te), then ICUC ∝ ne2 ∝ exp(-2t/te), where ne is the population density of the excited Yb3+ ions.24,25 Accordingly, the decay of the blue upconverted signal should be faster than that of the near-infrared signal from the transition of 2F5/2(Yb3+) f 2F7/2(Yb3+) [see the inset of Figure 4d] since the former decay time should be almost half of the latter,26,27 which is inconsistent with our results. IV. Conclusion In summary, we have synthesized hexagonal Yb3+-Ho3+Tm3+ triply doped NaYF4 nanorods via a hydrothermal method using oleic acid as a stabilizing agent and observed bright white UC luminescence consisting of the blue UC emissions at 450 and 475 nm, green emission at 545 nm, and red emissions at 650 and 695 nm in the as-prepared nanorods under the excitation of a 980 nm LD. The spectral and pump power dependence analyses indicate that the two-pump photons processs is responsible for the red and green emissions, while three- and four-pump photons processes are responsible for blue emissions at 475 and 450 nm at low pump power densities. With the increase of pump power densities, the UC processes for the blue and green were shown to be saturated leading to a decreasing deviation in the power dependence studies from the expected values ascertained from the UC mechanism, while three-phonon processes are proposed to contribute partly to the red emission. The calculated color coordinates show that with the increase of pump power densities, the tendency of white color output turns toward the blue region, revealing that white UC light can be fine-tuned in a wide range of pump power. Our results indicate that RE ions-doped NaYF4 nanostructures will have potential application in the field of flat-panel displays, lasers, photonics, and biomedicine.

J. Phys. Chem. C, Vol. 113, No. 44, 2009 18999 Acknowledgment. This work was supported by Grants from National Natural Science Foundation of China (No. 10802071) and the Across Subject Program of Xiangtan University (Grant No. 06IND03). Partial support was also provided by the Chang Jiang Scholars Program, Ministry of Education, China. References and Notes (1) Auzel, F. Chem. ReV. 2004, 104, 139–173. (2) Downing, E.; Hesselink, L.; Ralston, J.; Macfarlance, R. Science 1996, 273, 1185. (3) Van de Rijke, F.; Zijlmans, H.; Li, S.; Vail, T.; Raap, A. K.; Niedbala, R. S.; Tanke, H. J. Nat. Biotechnol. 2001, 19, 273. (4) Rosa, E. D.; Salas, P.; Desirena, H.; Angeles, C.; Rodrı´guez, R. A. Appl. Phys. Lett. 2005, 87, 241912. (5) Gao, S. Y.; Zhang, H. J.; Deng, R. P.; Wang, X. M.; Sun, D. H.; Zheng, G. L. Appl. Phys. Lett. 2006, 89, 123125. (6) Boyer, J. C.; Vetrone, F.; Cuccia, L. A.; Capobianco, J. A. J. Am. Chem. Soc. 2006, 128, 7444. (7) Boyer, J. C.; Cuccia, L. A.; Capobianco, J. A. Nano Lett. 2007, 7, 847. (8) Wang, F.; Liu, X. J. Am. Chem. Soc. 2008, 130, 5642. (9) Da Silva, J. E. C.; De Sa´, G. F.; Santa-Cruz, P. A. J. Alloys Compd. 2002, 344, 260. (10) L´ahoz, F.; Martı´n, I. R.; Calvilla-Quintero, J. M. Appl. Phys. Lett 2005, 86, 051106. (11) Chen, G. Y.; Liu, Y.; Zhang, Y. G.; Somesfalean, G.; Zhang, Z. G.; Sun, Q.; Wang, F. P. Appl. Phys. Lett. 2007, 91, 133103. (12) Chen, D.; Wang, Y.; Zheng, K.; Guo, T.; Yu, Y.; Huang, P. Appl. Phys. Lett. 2007, 91, 251903. (13) Mai, H. X.; Zhang, Y. W.; Si, R.; Yan, Z. G.; Sun, L. D.; You, L. P.; Yan, C. H. J. Am. Chem. Soc. 2006, 128, 6426. (14) Wang, L.; Li, Y. D Nano Lett. 2006, 6, 1645. (15) Zhang, F.; Wan, Y.; Yu, T.; Zhang, F. Q.; Shi, Y. F.; Xie, S. H.; Li, Y. G.; Xu, L.; Tu, B.; Zhao, D. Y. Angew. Chem. 2007, 119, 1. (16) Vetrone, F.; Mahalingam, V.; Capobianco, J. A. Chem. Mater. 2009, 21, 1847. (17) Pollnau, M.; Gamelin, D. R.; Luthi, S. R.; Gu¨del, H. U.; Hehlen, M. P. Phys. ReV. B 2000, 61, 3337. (18) Jacinto, C.; Vermelho, M. V. D.; Gouveia, E. A.; de Araujo, M. T.; Udo, P. T.; Astrath, N. G. C.; Baesso, M. L. Appl. Phys. Lett. 2007, 91, 071102. (19) Suyver, J. F.; Grimm, J.; van Veen, M. K.; Biner, D.; Kramer, K. W.; Gu¨del, H. U. J. Lumin. 2006, 117, 1. (20) Auzel, F. Phys. ReV. B 1976, 13, 2809. (21) Tkachuk, A. M.; Ivanova, S. E.; Joubert, M. F.; Guyot, Y.; Isaenko, L. I.; Gapontsev, V. P. J. Lumin. 2007, 125, 271. (22) Wang, Y.; Tu, L.; Zhao, J; Sun, Y.; Kong, X.; Zhang, H. J. Phys. Chem. C 2009, 113, 7164. (23) Xiao, S.; Yang, X.; Ding, J. W.; Yan, X. H. J. Phys. Chem. C 2007, 111, 8161. (24) dos Santos, P. V.; Vermelho, M.V. D.; Gouveia, E. A.; de Arau´jo, M. T.; Gouveia-Neto, A. S. Ann. Optic-XXV ENFMC 2002, 14. (25) Diaz-Torres, L. A.; De la Rosa, E.; Salas, P.; Desirena, H. Opt. Mater. 2005, 27, 1305. (26) Nakazawa, E.; Shionoya, S. Phys. ReV. Lett. 1970, 25, 1710. (27) Rosa, E. D.; Solis, D.; Dı´az-Torres, L. A.; Salas, P.; AngelesChavez, C.; Meza, O. J. Appl. Phys. 2008, 104, 103508.

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