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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 25 — Dec. 5, 2011
  • pp: 25471–25478
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Upconversion emissions from high-energy states of Eu3+ sensitized by Yb3+ and Ho3+ in β-NaYF4 microcrystals under 980 nm excitation

Lili Wang, Zhenyu Liu, Zhe Chen, Dan Zhao, Guanshi Qin, and Weiping Qin  »View Author Affiliations


Optics Express, Vol. 19, Issue 25, pp. 25471-25478 (2011)
http://dx.doi.org/10.1364/OE.19.025471


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Abstract

Under 980 nm excitation, multiple ultraviolet and visible upconversion luminescence from Ho3+ and Eu3+ ions were observed in Yb3+/Ho3+/Eu3+ tri-doped NaYF4 microcrystals (MCs). The high-energy states (5H3-7, 5L6, 5D3 and 5D2) of Eu3+ ions could be efficiently populated by two-step energy transfer (ET) processes of Yb Ho Eu. Four-, three-, two-photon UC processes of Eu3+ ions were confirmed by the dependence of 5H3-7, 5L6 and 5D0 levels emission intensities on the pumping power.

© 2011 OSA

1. Introduction

Rare earth ions have closely spaced energy levels and unique intra 4f transitions, which afford them the ability to absorb one or more low-energy near-infrared (NIR) photons and subsequently convert them to high-energy emissions [1

1. F. Auzel, “Upconversion and anti-Stokes processes with f and d ions in solids,” Chem. Rev. 104(1), 139–174 (2004). [CrossRef] [PubMed]

]. This anti-Stokes emission process is well known as upconversion (UC). In Yb3+ sensitized rare earth ions doped materials, efficient NIR-to-ultraviolet (UV) UC emissions have widely been investigated due to the need for developing short-wavelength solid-state lasers and photodynamic therapies in biomedicine [2

2. J.-C. Boyer, F. Vetrone, L. A. Cuccia, and J. A. Capobianco, “Synthesis of colloidal upconverting NaYF4 nanocrystals doped with Er3+, Yb3+ and Tm3+, Yb3+ via thermal decomposition of lanthanide trifluoroacetate precursors,” J. Am. Chem. Soc. 128(23), 7444–7445 (2006). [CrossRef] [PubMed]

4

4. W. P. Qin, D. S. Zhang, D. Zhao, L. L. Wang, and K. Z. Zheng, “Near-infrared photocatalysis based on YF3 : Yb3+,Tm3+/TiO2 core/shell nanoparticles,” Chem. Commun. (Camb.) 46(13), 2304–2306 (2010). [CrossRef] [PubMed]

]. Therefore studies of new approaches to obtain efficient UV UC luminescence are very necessary and valuable.

Recently, by using Yb3+ and Tm3+ (Er3+) as double sensitizers and 980 nm NIR diode laser as pump light, Eu3+ doped fluoride nanocrystals have been demonstrated to exhibit the unusual UC spectrum from visible to UV [5

5. K. Z. Zheng, L. L. Wang, D. S. Zhang, D. Zhao, and W. P. Qin, “Power switched multiphoton upconversion emissions of Er3+ in Yb3+/Er3+ codoped beta-NaYF4 microcrystals induced by 980 nm excitation,” Opt. Express 18(3), 2934–2939 (2010). [CrossRef] [PubMed]

, 6

6. L. L. Wang, X. J. Xue, F. Shi, D. Zhao, D. S. Zhang, K. Z. Zheng, G. F. Wang, C. F. He, R. Kim, and W. P. Qin, “Ultraviolet and violet upconversion fluorescence of europium (III) doped in YF(3) nanocrystals,” Opt. Lett. 34(18), 2781–2783 (2009). [CrossRef] [PubMed]

]. Analysis suggested upper levels of Eu3+ ions could be populated efficiently through internal ET between the optically active ions, while they could not be populated even in downconversion (DC) schemes of Eu3+ ions under vacuum UV excitations and at low temperatures. On the other hand, Eu3+ ions are excellent red emitters and famous structural probes under UV light excitation. Thus, it is attractive to explore the impact of ET routes on the spectra of Eu3+ ions. Particularly, Chen et al. have reported that Yb3+/Ho3+ doped NaYF4 is also an ideal model for studying NIR-laser-induced UV UC radiation of Ho3+ ions [7

7. G. Y. Chen, C. H. Yang, B. Aghahadi, H. J. Liang, Y. Liu, L. Li, and Z. G. Zhang, “Ultraviolet-blue upconversion emissions of Ho3+ ions,” J. Opt. Soc. Am. B 27(6), 1158–1164 (2010). [CrossRef]

]. It would be of great interest to extend the study and determine whether Ho3+ ions can sensitize other ions such as the Eu3+ ions and to uncover their fundamental optical UC mechanisms.

In this letter, we employed Ho3+ as the bridging ions between Yb3+ and Eu3+ ions to go on investigating the unusual radiative transitions of Eu3+ ions in NaYF4 MCs under NIR 980 nm excitation. From a fundamental point of view, more detailed information about the ET mechanism and interactions between the optically active ions can be obtained by changing the bridging ions from Tm3+, Er3+ to Ho3+, or changing the excitation light sources. Hexagonal NaYF4 was selected for embedding active ions, since it had been known to be an efficient host lattice for UC emissions [8

8. G. Wang, W. Qin, J. Zhang, L. Wang, G. Wei, P. Zhu, and R. Kim, “Controlled synthesis and luminescence properties from cubic to hexagonal NaYF4:Ln3+ (Ln=Eu and Yb/Tm) microcrystals,” J. Alloy. Comp. 475(1-2), 452–455 (2009). [CrossRef]

11

11. G. Y. Chen, H. C. Liu, G. Somesfalean, H. J. Liang, and Z. G. Zhang, “Upconversion emission tuning from green to red in Yb3+/Ho3+-codoped NaYF4 nanocrystals by tridoping with Ce3+ ions,” Nanotechnology 20(38), 385704 (2009). [CrossRef] [PubMed]

]. Hexagonal micropillars of Yb3+/Ho3+/Eu3+ tri-doped β-NaYF4 were synthesized via the ethylenediaminetetraacetic acid (EDTA)-assisted hydrothermal method. Mechanisms for UV-visible UC emissions of Eu3+ by two-step ET processes of Yb3+ → Ho3+ → Eu3+ have been proposed and demonstrated. The impact of energy populating routes on the spectra of Eu3+ in NaYF4:Yb3+/Ho3+/Eu3+ MCs was also investigated based on experimental data and analysis.

2. Experimental

Analytical grade Y(NO3)3 • 6H2O, Ho(NO3)3 • 6H2O, Yb(NO3)3 • 6H2O, Eu(NO3)3 • 6H2O, NaF, ethanol, and EDTA were obtained from Beijing Chemical Reagents, China. Deionized water was used throughout. All other chemical reagents were of analytical reagent grade.

In a typical synthesis, 1 mL of 0.5 M Ln(NO3)3 aqueous solution and 0.5 mmol of EDTA were dispensed into 20 mL of deionized water and magnetically stirred for 1 h, forming a chelated Ln-EDTA complex. Then 16 mL of 0.5 M NaF aqueous solution was added to the solution. After vigorous stirring for 1 h, the mixture was transferred into a 50-mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained in an oven at 160 °C for 18 h, and then cooled down slowly to room temperature. Subsequently, the suspension was centrifuged at 8000 rpm for 10 min. The resultant product was then washed thoroughly and dried in vacuum at 80 °C.

3. Results and discussion

3.1 Structure and morphology of NaYF4:Yb3+/Ho3+/Eu3+ microcrystals

The crystal structure was analyzed by a Rigaku RU-200b X-ray powder diffractometer (XRD) using a nickel-filtered Cu-Ka radiation (λ = 1.4518Å). The size and morphology were investigated by scanning electron microscope (SEM, KYKY 1000B). A power-adjustable laser diode (980 nm, 0 to 2W) with a lens making the beam parallel was employed as the UC pump source and a 100 W xenon lamp equipped in the Hitachi F-4500 fluorescence spectrophotometer was used as the DC pump source. The UC and DC luminescence spectra were recorded with a Hitachi F-4500 fluorescence spectrophotometer (1.0 nm for spectral resolution (FWHM) of the spectrophotometer and 400 V for PMT voltage) at room temperature. A 10-ns Raman laser running at 953.6 nm was used as the pulsed excitation source for temporal investigations. Low temperature emission spectra were obtained at 10 K with the samples mounted in a helium exchange gas chamber of a closed cycle refrigeration system. Figure 1
Fig. 1 SEM and XRD patterns of sample NaYF4: 20%Yb3+, 1.5%Ho3+, 3%Eu3+ MCs.
shows the XRD pattern of the MCs. All the diffraction peaks can be indexed to the pure hexagonal NaYF4 (JCPDS 16-0334). No other impurity peaks were detected. The corresponding SEM image (Fig. 1) shows that the NaYF4:Yb3+/Ho3+/Eu3+ MCs are hexagonal pillars.

3.2 Upconversion emissions of Ho3+ and Eu3+ ions

3.3. UC mechanisms for Eu3+ and Ho3+ions

Figure 4
Fig. 4 Energy level diagrams of Eu3+, Yb3+ and Ho3+ ions and possible UC emission processes.
describes schematically energy level diagrams of Eu3+, Ho3+, and Yb3+ ions and possible UC processes under 980 nm excitation [12

12. L. G. DeShazer and G. H. Dieke, “Spectra and Energy Levels of Eu3+ in LaCl3,” J. Chem. Phys. 38(9), 2190–2199 (1963). [CrossRef]

]. Yb3+ ions continuously absorb 980 nm photons and then transfer the energy to populate the 5I6, 5F2/5F4 and 5F2/3F2/5G2 levels of Ho3+ in turn. Different from Ho3+ ions, the population of the levels of Eu3+ ions can only originate through the ETs from excited Ho3+ ions. This can be confirmed from our experimental result: we have not observed any emissions of Eu3+ by UC in NaYF4: 20%Yb3+, 3%Eu3+ MCs. Multiple ET processes may exist since the trivalent Ho3+ ion has abundant energy levels and their small energy mismatches between levels of Eu3+ and Ho3+ ions. For briefness, four possible and representative ET processes should be considered in our paper: ET1 5S2/5F45I8 (Ho3+): 7F05D0 (Eu3+); ET2 5G45I8 (Ho3+): 7F05D3 (Eu3+); ET3 5F2/3F2/5G25I8 (Ho3+): 7F05D4 (Eu3+); ET4 5G4/5D4/3G45I8 (Ho3+): 7F05IJ/3P0/5FJ (Eu3+). Simultaneously, the low energy levels 5D3,2,1,0 of Eu3+ ions could be populated through a series of nonradiative relaxations from the neighboring higher excited levels.

To understand the UC processes well, we investigated the excitation power dependence of UC luminescence intensities. For an unsaturated UC process, the integrated UC luminescence intensity If is proportional to Pn [13

13. M. Pollnau, D. R. Gamelin, S. R. Luthi, H. U. Gudel, and M. P. Hehlen, “Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems,” Phys. Rev. B 61(5), 3337–3346 (2000). [CrossRef]

], where P is the pumping laser power, and n is the number of laser photons required in populating the upper emitting state. Figure 5
Fig. 5 Excitation power dependence of UC luminescence in NaYF4: 20%Yb3+, 1.5%Ho3+, 3%Eu3+ MCs under 980 nm excitation.
shows the typical pump-power dependence of UC luminescence of NaYF4: 20%Yb3+, 1.5%Ho3+, 3%Eu3+. The values of photon number n are 1.81 for the 5D07F0 transition of Eu3+, 2.15 for the 5D27F0 transition of Eu3+, 2.75 for the 5L67F0 transition of Eu3+, 2.98 for the 5G45I8 transition of Ho3+, 3.38 for the 3H4/5G4/1D45I8 transition of Ho3+, and 3.67 for the 5H3-77F0 transition of Eu3+, indicating that these transitions are of two-, three- and four-photon UC processes, respectively. Power dependence analyses illustrate that these levels of Eu3+ have the same multi-photon UC characters with the corresponding levels of Ho3+ ions and confirm that they are populated by the ETs from the corresponding levels of Ho3+ ions.

The energy transfer from excited Ho3+ to Eu3+ ions can be further proved by the dynamical analysis on Ho3+ excited states. The decay curves for the representative emissions from the 5F2/3F2/5G2, 5G4, 5S2/5F4 levels of Ho3+ ions in NaYF4: 20%Yb3+, 1.5%Ho3+ and NaYF4: 20%Yb3+, 1.5%Ho3+, 3%Eu3+ MCs were recorded under 953.6 nm pulsed Raman laser, as shown in Fig. 6
Fig. 6 Time-dependent emission profile of 5F2/3F2/5G2, 5G4, 5S2/5F4 levels of Ho3+ in NaYF4: Yb/Ho/Eu (|) and NaYF4: Yb/Ho (■) samples under 953.6 nm pulsed Raman excitation. The decay parts can be fitted into a single-exponential function as I = I0exp(-t/τ) (I0 is the initial emission intensity, τ is the lifetime of the emission center).
. Each of the decay curves can be fitted well into a single-exponential function as I = I0exp(−t/τ) (I0 is the initial emission intensity, τ is the lifetime of the level). The best-fitted results were listed in Table 1

Table 1. Average Lifetimes τ of Emission Levels of Ho3+ in NaYF4: Yb/Ho/Eu and NaYF4: Yb/Ho Samples Calculated from Time-dependent Emission Profiles.

table-icon
View This Table
. As can be seen clearly from Table 1, with the addition of Eu3+ ions, all the lifetimes of the 5F2/3F2/5G2, 5G4, 5S2/5F4 levels decrease greatly which explicitly demonstrate the energy transfer from Ho3+ to Eu3+ ions.

3.4. The spectral difference and analysis of Eu3+ ions under 980 nm and 394 nm excitation

Comparing Fig. 8a and Fig. 8b, we can observed obviously that the luminescence integral intensity ratios of (5D17F3)/(5D07F2) and (5D27F3)/(5D07F2) under 980 nm excitation are much larger than those under 394 nm excitation at the same doping concentration of Eu3+. Such results confirm again that the ETs from Yb3+ to Ho3+ and then from Ho3+ to Eu3+ play more efficient roles in populating the high-energy states of Eu3+ ions under 980 nm excitation than pumping them directly under 394 nm excitation [6

6. L. L. Wang, X. J. Xue, F. Shi, D. Zhao, D. S. Zhang, K. Z. Zheng, G. F. Wang, C. F. He, R. Kim, and W. P. Qin, “Ultraviolet and violet upconversion fluorescence of europium (III) doped in YF(3) nanocrystals,” Opt. Lett. 34(18), 2781–2783 (2009). [CrossRef] [PubMed]

].

Besides, we can found clearly in Fig. 8 that the intensity ratios of the (5D17F3)/(5D07F2) and (5D27F3)/(5D07F2) decrease dramatically with increasing the doping concentration of Eu3+ ions under 980 nm excitation comparing with those under 394 nm excitation at 300 K. The fast decrease of the ratios under 980 nm excitation can be attributed to the thermal effect of 980 nm infrared light, which increase the local temperature of samples. Cross relaxation, concentration quenching, and multiphonon relaxation rates increased sharply at higher temperatures under 980 nm excitation [17

17. H. W. Moos, “Spectroscopic relaxation processes of rare earth ions in crystals,” J. Lumin. 1–2, 106–121 (1970). [CrossRef]

]. The 5D1 and 5D2 emissions were quenched more and more with increasing the doping concentration of Eu3+ ions. Accordingly, the intensity ratios of the (5D17F3)/(5D07F2) and (5D27F3)/(5D07F2) decreased dramatically under 980 nm excitation, which is demonstrated in Fig. 8b. This can be further confirmed by the experimental data in Fig. 8b that when the samples were cooled from room temperature to 10 K, the intensity ratios of the (5D17F3)/(5D07F2) and (5D27F3)/(5D07F2) decrease gently with increasing the doping concentration of Eu3+ ions under 980 nm excitation, which are analogous to those under 394 nm excitation at a room temperature.

4. Conclusions

In summary, Yb3+/Ho3+/Eu3+ tri-doped NaYF4 MCs were fabricated through a simple hydrothermal process. It was found that not only unusual UV emissions of Ho3+ ions, but also unusual UV emissions of Eu3+ ions could be observed in this tri-doped system. Power-dependence analysis and dynamical analysis on Ho3+ excited states confirm that Ho3+ is an important dopant that served as a “bridging ion” in the efficient UC excitation processes of Eu3+. Besides, the spectra of NaYF4: Yb3+/Ho3+/Eu3+ MCs under 980 nm and 394 nm excitation were compared. The luminescence integral intensity ratios of (5D17F3)/(5D07F2) and (5D27F3)/(5D07F2) under 980 nm excitation are much larger than those under 394 nm excitation at the same doping concentration of Eu3+. These results indicate that a UC scheme is a better way to populate the high-energy levels of Eu3+ than a DC scheme.

Acknowledgment

This work was supported by the National High Technology Research and Development Program of China (863 program: 2009AA03Z309) and the National Natural Science Foundation of China (NNSFC) (grants 10874058, 51072065, and 60908031).

References and links

1.

F. Auzel, “Upconversion and anti-Stokes processes with f and d ions in solids,” Chem. Rev. 104(1), 139–174 (2004). [CrossRef] [PubMed]

2.

J.-C. Boyer, F. Vetrone, L. A. Cuccia, and J. A. Capobianco, “Synthesis of colloidal upconverting NaYF4 nanocrystals doped with Er3+, Yb3+ and Tm3+, Yb3+ via thermal decomposition of lanthanide trifluoroacetate precursors,” J. Am. Chem. Soc. 128(23), 7444–7445 (2006). [CrossRef] [PubMed]

3.

K. Z. Zheng, D. Zhao, D. S. Zhang, N. Liu, and W. P. Qin, “Ultraviolet upconversion fluorescence of Er3+ induced by 1560 nm laser excitation,” Opt. Lett. 35(14), 2442–2444 (2010). [CrossRef] [PubMed]

4.

W. P. Qin, D. S. Zhang, D. Zhao, L. L. Wang, and K. Z. Zheng, “Near-infrared photocatalysis based on YF3 : Yb3+,Tm3+/TiO2 core/shell nanoparticles,” Chem. Commun. (Camb.) 46(13), 2304–2306 (2010). [CrossRef] [PubMed]

5.

K. Z. Zheng, L. L. Wang, D. S. Zhang, D. Zhao, and W. P. Qin, “Power switched multiphoton upconversion emissions of Er3+ in Yb3+/Er3+ codoped beta-NaYF4 microcrystals induced by 980 nm excitation,” Opt. Express 18(3), 2934–2939 (2010). [CrossRef] [PubMed]

6.

L. L. Wang, X. J. Xue, F. Shi, D. Zhao, D. S. Zhang, K. Z. Zheng, G. F. Wang, C. F. He, R. Kim, and W. P. Qin, “Ultraviolet and violet upconversion fluorescence of europium (III) doped in YF(3) nanocrystals,” Opt. Lett. 34(18), 2781–2783 (2009). [CrossRef] [PubMed]

7.

G. Y. Chen, C. H. Yang, B. Aghahadi, H. J. Liang, Y. Liu, L. Li, and Z. G. Zhang, “Ultraviolet-blue upconversion emissions of Ho3+ ions,” J. Opt. Soc. Am. B 27(6), 1158–1164 (2010). [CrossRef]

8.

G. Wang, W. Qin, J. Zhang, L. Wang, G. Wei, P. Zhu, and R. Kim, “Controlled synthesis and luminescence properties from cubic to hexagonal NaYF4:Ln3+ (Ln=Eu and Yb/Tm) microcrystals,” J. Alloy. Comp. 475(1-2), 452–455 (2009). [CrossRef]

9.

V. Mahalingam, R. Naccache, F. Vetrone, and J. A. Capobianco, “Sensitized Ce(3+) and Gd(3+) ultraviolet emissions by Tm(3+) in colloidal LiYF(4) nanocrystals,” Chemistry 15(38), 9660–9663 (2009). [CrossRef] [PubMed]

10.

C. Liu, H. Wang, X. Li, and D. Chen, “Monodisperse, size-tunable and highly efficient beta-NaYF4:Yb,Er(Tm) up-conversion luminescent nanospheres: controllable synthesis and their surface modifications,” J. Mater. Chem. 19(21), 3546–3553 (2009). [CrossRef]

11.

G. Y. Chen, H. C. Liu, G. Somesfalean, H. J. Liang, and Z. G. Zhang, “Upconversion emission tuning from green to red in Yb3+/Ho3+-codoped NaYF4 nanocrystals by tridoping with Ce3+ ions,” Nanotechnology 20(38), 385704 (2009). [CrossRef] [PubMed]

12.

L. G. DeShazer and G. H. Dieke, “Spectra and Energy Levels of Eu3+ in LaCl3,” J. Chem. Phys. 38(9), 2190–2199 (1963). [CrossRef]

13.

M. Pollnau, D. R. Gamelin, S. R. Luthi, H. U. Gudel, and M. P. Hehlen, “Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems,” Phys. Rev. B 61(5), 3337–3346 (2000). [CrossRef]

14.

F. Auzel, “Multiphonon-assisted anti-Stokes and Stokes fluorescence of triply ionized rare-earth ions,” Phys. Rev. B 13(7), 2809–2817 (1976). [CrossRef]

15.

M. J. Weber, “Multiphonon Relaxation of Rare-Earth Ions in Yttrium Orthoaluminate,” Phys. Rev. B 8(1), 54–64 (1973). [CrossRef]

16.

M. J. Weber, “Radiative and Multiphonon Relaxation of Rare-Earth Ions in Y2O3,” Phys. Rev. 171(2), 283–291 (1968). [CrossRef]

17.

H. W. Moos, “Spectroscopic relaxation processes of rare earth ions in crystals,” J. Lumin. 1–2, 106–121 (1970). [CrossRef]

OCIS Codes
(190.4180) Nonlinear optics : Multiphoton processes
(300.6540) Spectroscopy : Spectroscopy, ultraviolet

ToC Category:
Nonlinear Optics

History
Original Manuscript: September 13, 2011
Revised Manuscript: November 9, 2011
Manuscript Accepted: November 14, 2011
Published: November 29, 2011

Citation
Lili Wang, Zhenyu Liu, Zhe Chen, Dan Zhao, Guanshi Qin, and Weiping Qin, "Upconversion emissions from high-energy states of Eu3+ sensitized by Yb3+ and Ho3+ in β-NaYF4 microcrystals under 980 nm excitation," Opt. Express 19, 25471-25478 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-25-25471


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References

  1. F. Auzel, “Upconversion and anti-Stokes processes with f and d ions in solids,” Chem. Rev.104(1), 139–174 (2004). [CrossRef] [PubMed]
  2. J.-C. Boyer, F. Vetrone, L. A. Cuccia, and J. A. Capobianco, “Synthesis of colloidal upconverting NaYF4 nanocrystals doped with Er3+, Yb3+ and Tm3+, Yb3+ via thermal decomposition of lanthanide trifluoroacetate precursors,” J. Am. Chem. Soc.128(23), 7444–7445 (2006). [CrossRef] [PubMed]
  3. K. Z. Zheng, D. Zhao, D. S. Zhang, N. Liu, and W. P. Qin, “Ultraviolet upconversion fluorescence of Er3+ induced by 1560 nm laser excitation,” Opt. Lett.35(14), 2442–2444 (2010). [CrossRef] [PubMed]
  4. W. P. Qin, D. S. Zhang, D. Zhao, L. L. Wang, and K. Z. Zheng, “Near-infrared photocatalysis based on YF3 : Yb3+,Tm3+/TiO2 core/shell nanoparticles,” Chem. Commun. (Camb.)46(13), 2304–2306 (2010). [CrossRef] [PubMed]
  5. K. Z. Zheng, L. L. Wang, D. S. Zhang, D. Zhao, and W. P. Qin, “Power switched multiphoton upconversion emissions of Er3+ in Yb3+/Er3+ codoped beta-NaYF4 microcrystals induced by 980 nm excitation,” Opt. Express18(3), 2934–2939 (2010). [CrossRef] [PubMed]
  6. L. L. Wang, X. J. Xue, F. Shi, D. Zhao, D. S. Zhang, K. Z. Zheng, G. F. Wang, C. F. He, R. Kim, and W. P. Qin, “Ultraviolet and violet upconversion fluorescence of europium (III) doped in YF(3) nanocrystals,” Opt. Lett.34(18), 2781–2783 (2009). [CrossRef] [PubMed]
  7. G. Y. Chen, C. H. Yang, B. Aghahadi, H. J. Liang, Y. Liu, L. Li, and Z. G. Zhang, “Ultraviolet-blue upconversion emissions of Ho3+ ions,” J. Opt. Soc. Am. B27(6), 1158–1164 (2010). [CrossRef]
  8. G. Wang, W. Qin, J. Zhang, L. Wang, G. Wei, P. Zhu, and R. Kim, “Controlled synthesis and luminescence properties from cubic to hexagonal NaYF4:Ln3+ (Ln=Eu and Yb/Tm) microcrystals,” J. Alloy. Comp.475(1-2), 452–455 (2009). [CrossRef]
  9. V. Mahalingam, R. Naccache, F. Vetrone, and J. A. Capobianco, “Sensitized Ce(3+) and Gd(3+) ultraviolet emissions by Tm(3+) in colloidal LiYF(4) nanocrystals,” Chemistry15(38), 9660–9663 (2009). [CrossRef] [PubMed]
  10. C. Liu, H. Wang, X. Li, and D. Chen, “Monodisperse, size-tunable and highly efficient beta-NaYF4:Yb,Er(Tm) up-conversion luminescent nanospheres: controllable synthesis and their surface modifications,” J. Mater. Chem.19(21), 3546–3553 (2009). [CrossRef]
  11. G. Y. Chen, H. C. Liu, G. Somesfalean, H. J. Liang, and Z. G. Zhang, “Upconversion emission tuning from green to red in Yb3+/Ho3+-codoped NaYF4 nanocrystals by tridoping with Ce3+ ions,” Nanotechnology20(38), 385704 (2009). [CrossRef] [PubMed]
  12. L. G. DeShazer and G. H. Dieke, “Spectra and Energy Levels of Eu3+ in LaCl3,” J. Chem. Phys.38(9), 2190–2199 (1963). [CrossRef]
  13. M. Pollnau, D. R. Gamelin, S. R. Luthi, H. U. Gudel, and M. P. Hehlen, “Power dependence of upconversion luminescence in lanthanide and transition-metal-ion systems,” Phys. Rev. B61(5), 3337–3346 (2000). [CrossRef]
  14. F. Auzel, “Multiphonon-assisted anti-Stokes and Stokes fluorescence of triply ionized rare-earth ions,” Phys. Rev. B13(7), 2809–2817 (1976). [CrossRef]
  15. M. J. Weber, “Multiphonon Relaxation of Rare-Earth Ions in Yttrium Orthoaluminate,” Phys. Rev. B8(1), 54–64 (1973). [CrossRef]
  16. M. J. Weber, “Radiative and Multiphonon Relaxation of Rare-Earth Ions in Y2O3,” Phys. Rev.171(2), 283–291 (1968). [CrossRef]
  17. H. W. Moos, “Spectroscopic relaxation processes of rare earth ions in crystals,” J. Lumin.1–2, 106–121 (1970). [CrossRef]

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