Synthesis, Morphology, and Optical Characterization of Nanocrystalline Er 3+ :Y 2 O 3 Sreerenjini Chandra, † Francis Leonard Deepak, †,‡ John B. Gruber, † and Dhiraj K. Sardar* ,† Department of Physics and Astronomy, UniVersity of Texas at San Antonio, San Antonio, Texas 78249-0697, and International Iberian Nanotechnology Laboratory, AVda Mestre Jose Veiga, Braga 4715, Portugal ReceiVed: October 2, 2009; ReVised Manuscript ReceiVed: NoVember 4, 2009 We describe a methodology to synthesize trivalent-erbium (Er 3+ )-doped yttrium oxide (Y 2 O 3 ) nanoparticles. An in-depth morphology analysis indicates that the average diameter of the individual nanoparticles is about 25 nm. To optically characterize the nanocrystalline material, the room-temperature absorption spectrum has been obtained between 400 and 900 nm. The spectrum consists of six absorption bands, including 2 G(1) 9/2 , 4 F 3/2 + 4 F 5/2 , 4 F 7/2 , 2 H(2) 11/2 + 4 S 3/2 , 4 F 9/2 , and 4 I 9/2 . The room-temperature fluorescence spectra of the Er 3+ (4f 11 ) 2 H(2) 11/2 + 4 S 3/2 f 4 I 15/2 and 4 F 9/2 f 4 I 15/2 transitions were analyzed for the crystal-field splitting of the energy levels of these states of erbium. We have measured the lifetimes for the 2 H(2) 11/2 + 4 S 3/2 and 4 F 9/2 metastable states and have investigated the effects of Er 3+ concentrations and particle size on the emission intensity and decay times. The experimental energies (Stark levels) agree well within the experimental error with the theoretical values reported earlier for bulk single crystalline Er 3+ :Y 2 O 3 . Detailed structural and optical analyses suggest that the nanoparticles of Er 3+ :Y 2 O 3 have potential applications in diverse fields of photonics including laser systems and optical communication devices. Introduction Depending on their size and shape, rare-earth (RE)-ion-doped nanoparticles have attracted significant attention due to their potential applications in photonic and biophotonic technologies. 1-9 It has been reported recently that the RE nanophosphors emit energy in the visible range by a process of upconversion. 10-13 Among the entire group of trivalent rare-earth (RE 3+ ) ions, the spectra of the erbium ion (Er 3+ ) have been investigated extensively due to its intense green emission and efficiency as an optical activator in a large number of mid-infrared solid- state laser host materials. 14-17 The RE 3+ -doped nanocrystalline Y 2 O 3 host has attracted considerable interest as well, due to its high chemical durability, thermal stability, and success as phosphors in fluorescent lamps, projection TV systems, and field emission systems. 3,10,11,18 Er 3+ ions doped in Y 2 O 3 (Er 3+ :Y 2 O 3 ) possess interesting photonic applications due to their ability to fluoresce at either infrared or visible wavelengths. 19 In addition, Y 2 O 3 can be a more efficient material than the popular yttrium aluminum garnet (Y 3 Al 5 O 12 or YAG) as a laser host when its material and optical properties are critically evaluated for certain applications. 19-21 In the present study, we report the synthesis, morphology, and a detailed spectroscopic analysis of Er 3+ :Y 2 O 3 nanoparticles of ∼20-30 nm in diameter. The structural characterization and the elemental analyses of the synthesized nanoparticles have been performed using various microscopy and energy dispersive X-ray analysis techniques. The optical characteristics have been investigated by analyzing the room-temperature absorption and emission spectra corresponding to various manifold transitions of Er 3+ in the nanoparticles. We have also measured the fluorescent lifetimes of the 2 H(2) 11/2 + 4 S 3/2 and 4 F 9/2 metastable states and performed a comparative study of emission decay with varying concentrations of Er 3+ ions. Finally, a detailed crystal-field energy splitting analysis has been performed for the Stark transitions in the 2 H(2) 11/2 + 4 S 3/2 f 4 I 15/2 and 4 F 9/2 f 4 I 15/2 emission spectra and the results obtained were compared with the values measured earlier for the single crystalline Er 3+ : Y 2 O 3 . Experimental Section A. Synthesis. The Er 3+ :Y 2 O 3 nanoparticles were prepared by hydrothermal precipitation of a homogeneous solution of dissolved ErCl 3 and YCl 3 (1 mmol, totally) and NaOH (4.5 mmol) in deionized (DI) water. The chemicals are 99.99% pure and were purchased from Sigma-Aldrich. The OH - ions were generated through the solubility of NaOH in water. A solution of ErCl 3 , YCl 3 , and NaOH was dissolved in DI water taken in a Teflon container and sealed in an autoclave and heated to 175 °C for 4 h. The result was the formation of gelatinous colloidal RE hydroxides. The hydroxides were then filtered from the solution, washed thoroughly with DI water and ethanol, dried at 75 °C, and finally sintered at 1000 °C for 2 h to obtain the corresponding oxide nanoparticles. We opted the sintering temperature to be 1000 °C to ensure the removal of impurity materials. An overview of the synthesis scheme is given through the following reaction steps: RECl 3 + 3NaOH 9 8 175 °C RE(OH) 3 + 3NaCl 2RE(OH) 3 9 8 1000 °C RE 2 O 3 + 3H 2 O where RE ) Y, Er. For the comparative study of the physical and optical properties, we synthesized and characterized five different samples of Er 3+ :Y 2 O 3 nanoparticles having different Er 3+ concentrations in atomic weight percentage, i.e., 0.1, 0.3, 0.5, 1.0, and 3.0 at. %, respectively. X-ray powder diffraction * Corresponding author. E-mail: dsardar@utsa.edu. † University of Texas at San Antonio. ‡ International Iberian Nanotechnology Laboratory. J. Phys. Chem. C 2010, 114, 874–880 874 10.1021/jp909457g  2010 American Chemical Society Published on Web 12/07/2009