J. Phys. Chem. C 2008, 112, 11527–11531
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Vapor Phase Synthesis of Upconverting Y2O3 Nanocrystals Doped with Yb3+, Er3+, Ho3+, and Tm3+ to Generate Red, Green, Blue, and White Light Garry Glaspell,†,‡ John Anderson,‡ James R. Wilkins,† and M. Samy El-Shall*,† Department of Chemistry, Virginia Commonwealth UniVersity, Richmond, Virginia 23284, and United States Army Engineering Research and DeVelopment Center, Alexandria, Virginia 22315 ReceiVed: February 22, 2008; ReVised Manuscript ReceiVed: April 17, 2008
We report the vapor phase synthesis of upconverting Y2O3 nanocrystals doped with Yb3+, Er3+, Ho3+, and Tm3+ to generate red, green, blue, and white light. Incorporating Er3+ within the Yb3+ doped Y2O3 nanocrystals under 980 nm laser excitation produced orange and yellow upconversion luminescence tunable by varying the Yb3+ concentration. The Yb3+, Er3+, and Tm3+ codoped Y2O3 nanocrystals exhibited nearly equal intensities of the red, green, and blue emissions upon 980 nm laser excitation. White light can be produced by adjusting the concentrations of the Ln3+ ions within the Y2O3 nanocrystals. Introduction The conversion of near-infrared (NIR) photons to higher energy UV or visible light via multiple absorptions or energy transfer, known as upconversion (UC) phosphors, is an area of extensive research due to its fundamental interest and tremendous potential for photonic and biophotonic applications.1 The efficient generation of color-tunable and white light through the UC process is an important research goal in this area that offers a variety of applications such as displays, back light, sensors, biolabeling, and photodynamic therapy.1 Nanocrystals can provide several advantages including high efficiency due to minimal energy loss by scattering and the possibility of biological applications, which requires particle sizes 2900 K) necessary for the phase transformation. This is clearly an important advantage of the LVCC method in the synthesis of doped nanocrystals as compared to other methods.
Figure 2. (a) XRD data of (bottom) bulk Y2O3 target showing the cubic phase and (top) Y2O3 nanocrystals prepared using the LVCC method showing predominantly the monoclinic phase. Peaks marked with # are due to minor components of the cubic phase within the monoclinic phase. (b) XRD data of (bottom) bulk Y2O3 target containing 1% Er2O3 and 4% Yb2O3 powder showing the cubic phase and the presence of Yb2O3 as indicated by the peaks marked with * and (top) 1% Er2O3 and 4% Yb2O3 doped Y2O3 nanocrystals prepared using the LVCC method showing predominantly the monoclinic phase and the disappearance of the Yb2O3 peaks. Peaks marked with # are due to minor components of the Y2O3 cubic phase within the monoclinic phase.
Figure 3 compares the UC spectra of bulk Y2O3 powders doped with (8%) Yb3+/(6%) Er3+, (10%) Yb3+/(2%) Ho3+, and (6%) Yb3+/(2%) Tm3+ with the corresponding Y2O3 doped nanocrystals prepared by the LVCC method. While there is not much apparent difference in intensity of Er3+ Yb3+ codoped Y2O3 nanoparticles when compared to their bulk counterpart, as shown in Figure 3a, the Yb3+/Ho3+ and Yb3+/Tm3+ doped Y2O3 nanoparticles are significantly brighter than their bulk counterparts as shown in Figure 3b,c, respectively. UC emission spectra of Y2O3 nanocrystals doped with (16%) Yb3+/(6%) Er3+, (10%) Er3+, (8%) Yb3+/(6%) Er3+, (4%) Yb3+/ (6%) Er3+, (10%) Yb3+/(2%) Ho3+, and (6%) Yb3+/(2%) Tm3+ following the 980 nm laser diode excitations are shown in Figure 4. Control experiments with the bulk powder used to make the pellets as well as physical mixtures of the individual nanocrystals (Er2O3 + Y2O3, Yb2O3 + Er2O3 + Y2O3, Yb2O3 + Ho2O3 + Y2O3, or Yb2O3 + Tm2O3 + Y2O3) did not show any UC upon 980 nm excitation. This is demonstrated in Figure 4a through comparison of the UC emission spectra of 6% Er2O3 and 16% Yb2O3 in Y2O3 with the corresponding bulk powder (pellet) as
Y2O3 Nanocrystals Doped with Yb3+, Er3+, Ho3+, and Tm3+
Figure 3. UC emission spectra of bulk Y2O3 powder and Y2O3 nanocrystals doped with (a) (8%) Yb3+/(6%) Er3+, (b) (10%) Yb3+/ (2%) Ho3+, and (c) (6%) Yb3+/(2%) Tm3+.
Figure 4. UC emission spectra of Y2O3 nanocrystals doped with (a) 16% Yb3+ + 6% Er3+, (b) 10% Er3+ and 8% Yb3+ + 6% Er3+, (c) 8% Yb3+ + 6% Er3+ and 4% Yb3+ + 6% Er3+, (d) 10% Yb3+ + 2% Ho3+, and (e) 6% Yb3+ + 2% Tm3+.
Figure 5. Energy level diagrams for UC emissions from Er3+, Tm3+, and Ho3+ under 980 nm laser excitation of Yb3+.
well as a physical mixture of 6% Er2O3, 16% Yb2O3, and 78% Y2O3 nanoparticles. It is clear that both the mixtures of bulk powders and the corresponding nanoparticles did not exhibit any UC properties as shown by the flat spectra in Figure 4a. A similar result also was found from the comparsion of the UC spectra of 2% Ho2O3 and 10% Yb2O3 doped Y2O3 nanocrystals with the corresponding bulk and nanoparticle mixtures as shown
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Figure 6. Power dependence of UC intensity of Y2O3 nanocrystals doped with (a) 8% Yb3+ + 6% Er3+, (b) 10% Yb3+ + 2% Ho3+, (c) 6% Yb3+ + 2% Tm3+, and (d) 6% Yb3+ + 2% Tm3+ measured at 671, 543, 475, and 486 nm, respectively.
Figure 7. UC emission spectra of Y2O3 nanocrystals doped with Yb3+/ Er3+/Ho3+ and Yb3+/Er3+/Tm3+.
in Figure 4d. These results confirm that the Er3+ or Ho3+ ions replace the Yb3+ and Y3+ ions throughout the Yb2O3 and Y2O3 crystal lattices since the doped nanoparticles display UC, while both the pellets and the mixtures of undoped nanoparticles do not. While it is possible to induce UC in the Er3+ doped Y2O3 nanocrystals directly with 980 nm excitation, this process is greatly enhanced by codoping with Yb3+, resulting in bright red emissions near 660-670 nm and green emissions near 530 and 550 nm as shown in Figure 4b. Both the (8%) Yb3+/(6%) Er3+ and the (4%) Yb3+/(6%) Er3+ codoped Y2O3 nanocrystals exhibited similar red and green emissions as shown in Figure 4c. The Yb3+ ions possess a large absorption cross-section at 980 nm, and energy transfer occurs as a result of the large spectral overlap between the Yb3+ emission 2F5/2 f 2F7/2 and the Er3+ absorption 4I11/2 r 4I15/2 bands as illustrated in the energy diagram shown in Figure 5. A second Yb3+ ion transfers its energy, resulting in the population of the 4F7/2 state of Er3+. Relaxation from the 4F7/2 state populates the 2H11/2, 4S3/2, and 4F 9/2 states, which results in the observed emission spectra. Specifically, as shown in Figure 4c, the red emissions observed at ∼670 nm correspond to the 4F9/2 f 4I15/2 transitions of Er3+, and the green emissions near 530 and 550 nm can be assigned to the 4H11/2 f 4I15/2 and 4S3/2 f 4I15/2 transitions, respectively (see Figure 5).11–13
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Figure 8. Photographs of emitted light showing red, green, blue, and white light produced upon 980 nm laser excitation of (16%) Yb3+/(6%) Er3+, (10%) Yb3+/(2%) Ho3+, (6%) Yb3+/(2%) Tm3+, and (10%) Yb3+/(1%) Er3+/(1%) Tm3+ doped Y2O3 nanocrystals, respectively.
We observed that the ratio of red to green emissions from Er3+ is strongly dependent on the Yb3+ concentration in the Y2O3 nanocrystals prepared by the LVCC method. As shown in Figure 4c, this ratio decreases with decreasing the Yb3+ concentration from 8 to 4%. A similar result was reported for Yb3+/Er3+ codoped Y2O3 nanocrystals prepared by the decomposition of RE precursors at 900 °C.11,21 Thus, it is possible to tune the overall emission color from the Yb3+/Er3+ codoped Y2O3 nanocrystals by varying the Yb3+ concentration. Bright green emissions near 540 nm and weak red emissions near 660 nm were observed from the 10% Yb3+ and 2% Ho3+ codoped Y2O3 nanocrystals as shown in Figure 4d. These emissions are consistent with the 5S2, 5F4 f 4I8, and 5F5 f 5I8 transitions of Ho3+, respectively. Figure 4e shows that the 6% Yb3+ and 2% Tm3+ codoped Y2O3 nanocrystals exhibit blue emissions near 458 and 475 nm, which can be assigned to the 1D2f 3F4 and 1G f 3H transitions of Tm3+, respectively (see Figure 5). In 4 6 addition, a weak red emission around 650 nm and very intense NIR emissions between 770 and 840 nm (not shown) can be attributed to the Tm3+ 1G4 f 3F4 and 3H4 f 3H6 transitions, respectively. The emission near 480 nm is consistent with the 1G f 3H transition of Tm3+, which requires a three-step energy 4 6 transfer from the incorporated Yb3+ ions.8 Briefly, energy transfer from the first Yb3+ ion exciting Tm3+ to 3H5 relaxes rapidly to the 3F4 level by nonradiative multiphonon decay. A second Yb3+ transfers its energy, populating the 3F2 and 3F3 levels, ultimately relaxing to the 3H4 level. Finally, a third energy transfer from the Yb3+ ions populates the 1G4 level, resulting in emission at ∼480 nm. Other emissions near 350 nm corresponding to the 1D2 f 3H6 transition and at ∼650 nm corresponding to the 1G4 f 3F4 and 3H5 transition also can take place. It is known that the UC intensity (I) depends on the excitation power (P) according to the power law I ≈ Pn, where the number
of pumping photons (n) required to excite ions from the ground state to the emitting state can be determined from the slope of the photoluminescence (PL) intensity versus the laser excitation power in a log-log plot.22 These plots are shown in Figure 6 for the Y2O3 nanocrystals doped with (a) 8% Yb3+ + 6% Er3+, (b) 10% Yb3+ + 2% Ho3+ (c), and (d) 6% Yb3+ + 2% Tm3+. From the power dependence plots, the UC emissions of Yb3+/ Er3+, Yb3+/Ho3+, and Yb3+/Tm3+ codoped Y2O3 nanocrystals observed at 671, 543, and 475 and 486 nm require two-, two-, and three-photon excitation processes, respectively. To demonstrate the flexibility and control of our synthesis approach in tuning the color of the UC emissions, we prepared two Y2O3 nanocrystal samples doped with 10% Yb3+/2% Er3+/ 2% Ho3+ and 16% Yb3+/1% Er3+/1% Tm3+. The UC emission spectra of these samples are shown in Figure 7. The incorporation of Ho3+ within the Yb3+/Er3+ codoped Y2O3 nanocrystals resulted in the enhancement of green emissions as shown in Figure 7a in comparison to the UC emission spectrum of the Yb3+/Er3+ codoped Y2O3 nanocrystals shown in Figure 4b. The nearly equal intensity of the red and green emissions from the 10% Yb3+, 2% Er3+, and 2% Ho3+ codoped Y2O3 nanocrystals resulted in an overall greenishyellow color. However, UC emissions from the 10% Yb3+/1% Er3+/1% Tm3+ codoped Y2O3 nanocrystals, shown in Figure 7b, exhibited nearly equal intensities of blue, green, and red emissions, which demonstrate the development of a nearly white emitting upconverter. This UC luminescence can likely be improved by adjusting the concentrations of the Ln3+ ions within the Y2O3 nanocrystals. Finally, Figure 8 displays representative images of UC emissions from the Ln3+ doped Y2O3 nanocrystals taken with a digital camera in dim light. These colors can easily be seen with the naked eye under dimmed light.
Y2O3 Nanocrystals Doped with Yb3+, Er3+, Ho3+, and Tm3+ Conclusion Ln3+
In conclusion, we synthesized ion doped Y2O3 nanocrystals via a LVCC method. Incorporating Er3+ within the Yb3+ doped Y2O3 nanocrystals under 980 nm laser excitation produced orange and yellow UC luminescence that was tunable by varying the Yb3+ concentration. The Yb3+, Er3+, and Tm3+ codoped Y2O3 nanocrystals exhibited nearly equal intensities of red, green, and blue emissions upon 980 nm laser excitation. White light can be produced by adjusting the concentrations of Ln3+ ions within the Y2O3 nanocrystals. A variety of doped nanocrystals with controlled dopant concentrations can be designed and tested using the LVCC method. Acknowledgment. We thank the National Science Foundation (CHE-0414613) for support of this work. References and Notes (1) Prasad, P. N. Nanophotonics; John Wiley and Sons: New York, 2004. (2) Silver, J.; Martinez-Rubio, M.; Ireland, T.; Fern, G.; Withnall, R. J. Phys. Chem. B 2001, 105, 948. (3) Matsuura, D. Appl. Phys. Lett. 2002, 81, 4526. (4) Capobianco, J.; Boyer, J.; Vetrone, F.; Speghini, A.; Bettinelli, M. Chem. Mater. 2002, 14, 2915. (5) Vetrone, F.; Boyer, C.; Capobianco, J.; Speghini, A.; Bettinelli, M. J. Phys. Chem. B 2003, 107, 1107.
J. Phys. Chem. C, Vol. 112, No. 30, 2008 11531 (6) Huignard, A.; Buissette, V.; Laurent, G.; Gacoin, T.; Boilot, J. P. Chem. Mater. 2002, 14, 2264. (7) Hirai, T.; Orikoshi, T.; Komasawa, I. Chem. Mater. 2002, 14, 3576. (8) Patra, A.; Saha, S.; Alencar, M. A. R.; Rakov, N.; Maciel, G. S. Chem. Phys. Lett. 2005, 407, 477. (9) Vetrone, F.; Boyer, J.; Capobianco, J.; Speghini, A.; Bettinelli, M. J. Mater. Res. 2004, 19, 3398. (10) Patra, A.; Ghosh, P.; Chowdhury, P. S.; Alencar, M. A. R.; Lozano, W.; Rakov, N.; Maciel, G. S. J. Phys. Chem. B 2005, 109, 10142. (11) Matsuura, D.; Hattori, H.; Takano, A. J. Electrochem. Soc. 2005, 152, 39. (12) Sivakumar, S.; van Veggel, F. C. J. M.; Raudsepp, M. J. Am. Chem. Soc. 2005, 127, 12464. (13) Boyer, J.; Vetrone, F.; Cuccia, L. A.; Capobianco, J. A. J. Am. Chem. Soc. 2006, 128, 7444. (14) Heer, S.; Lehmann, O.; Haase, M.; Gudel, H. U. Angew. Chem., Int. Ed. 2003, 42, 3179. (15) Sun, Y.; Liu, H.; Wang, X.; Kong, X.; Zhang, H. Chem. Mater. 2006, 18, 2726. (16) Qin, Z.; Yokomori, T.; Ju, Y. Appl. Phys. Lett. 2007, 90, 73104. (17) Li, S.; Silvers, S. J.; El-Shall, M. S. J. Phys. Chem. 1997, 101, 1794. (18) El-Shall, M. S.; Abdelsayed, V.; Pithawalla, Y.; Alsharach, E.; Deevi, S. J. Phys. Chem. B 2003, 107, 2882. (19) Glaspell, G.; Abdelsayed, V.; Saoud, K.; El-Shall, M. S. Pure Appl. Chem. 2006, 78, 1671. (20) Patra, A.; Friend, C. S.; Kapoor, R.; Prasad, P. N. Appl. Phys. Lett. 2003, 83, 284. (21) Heer, S.; Kompe, K.; Gudel, H.; Haase, M. AdV. Mater. 2004, 16, 2102. (22) Pollnau, M.; Gamelin, D. R.; Luthi, S. R.; Gudel, H. U. Phys. ReV. B. 2000, 61, 3337.
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