Luminescent Nanocrystals with A2B2O7 Composition Synthesized by

Jan 7, 2009 - Catalyst treatment could boost exhaust cleanup. The catalysts that clean up automotive emissions typically consist of particles of plati...
0 downloads 26 Views 477KB Size
Download PDF
1204

J. Phys. Chem. C 2009, 113, 1204–1208

Luminescent Nanocrystals with A2B2O7 Composition Synthesized by a Kinetically Modified Molten Salt Method Yuanbing Mao,† Xia Guo,‡ Jian Y. Huang,§ Kang L. Wang,‡ and Jane P. Chang*,† Department of Chemical and Biomolecular Engineering and Department of Electronic Engineering, UniVersity of California, Los Angeles, California 90095, and Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico 87185 ReceiVed: August 08, 2008; ReVised Manuscript ReceiVed: NoVember 14, 2008

This work focuses on the synthesis of nanocrystals with A2B2O7 composition (in this study, A ) La, Er, or their mixture, B ) Zr, Hf, or their mixture), their structural characterization, and luminescent property measurements. It was necessary to utilize a single-source complex precursor A(OH)3 · BO(OH)2 · nH2O to synthesize these A2B2O7 nanocrystals, by the synergistic interplay between the reduction of the transport distances of the reactive constituents to an atomic length scale and the enhanced diffusion of the reactants in the molten salt medium. This process is highly generalizable for the preparation of other complex oxide nanocrystals. Moreover, these nanocrystals possess outstanding luminescent properties, including strong photoluminescence in the infrared region, efficient pump-power dependence and excellent cathodoluminescence. These desirable properties make these nanocrystals promising for applications in display, bioanalysis, lighting, scintillating, and telecommunications. Introduction Materials with A2B2O7 composition (where A represents rareearth elements or their mixtures with oxidation state of +3 and B denotes fourth group transition metallic elements or their mixtures with oxidation state of +4)1,2 have broad application potentials because of their unique and attractive properties.3-5 For example, they are viable nuclear waste host materials because of their structural compatibility with radionuclide species3,6 and find applications as high-temperature heating elements, oxidation catalysts, and scintillator materials in X-ray computed tomographic detectors.4,7-10 Materials such as La2Zr2O7 are effective thermal barrier layers and have the potential as buffer layers for superconducting YBa2Cu3O7-xbased conductors on nickel tapes. Also, as a principle category of luminescent materials other than quantum dots, their rareearth-doped derivatives possess broad potential for light emitters, display devices, optical telecommunication components, active parts in lasers, and biolabels arising from the 4f electrons of rare-earth elements.5,8,11-14 Unlike the colloidal quantum dots, these A2B2O7 nanocrystals are near-infrared-active for bioanalysis due to their upconversion properties.12 Moreover, these A2B2O7 nanocrystals have great potentials in field emission displays due to their higher current density, higher efficiency, and better stability at low-voltage electron excitation as discussed later in this paper. Furthermore, despite the fact that a few groups have reported the preparation of luminescent A2B2O7 compounds, to the best of our knowledge, neither work on the synthesis of their erbium-related nanocrystals nor their photoluminescence in the infrared region or cathodoluminescence were reported.5,12,15 Trivalent erbium ion (Er3+) is one element that has attracted the most interest among luminescent * To whom correspondence should be addressed. † Department of Chemical and Biomolecular Engineering, University of California, Los Angeles. ‡ Department of Electronic Engineering, University of California, Los Angeles. § Sandia National Laboratories.

rare-earth ions in recent years, mainly because its emission band is around 1535 nm, a wavelength of great interest for telecommunications.16 So in this work we studied the photoluminescence in the infrared region and cathodoluminescence of the assynthesized nanocrystalline erbium derivatives of A2B2O7 compounds. Intensive research effort devoted to the development of nanocrystalline materials with controlled compositions, crystallographic phases, and morphologies has resulted in impressive progress in their synthesis, in order to access their unique chemical and physical properties over the past decade.17-21 One of the main challenges is to develop generalizable, scalable, and reliable synthetic methods to provide desirable nanocrystalline materials that can be readily reproduced by researchers whose scientific efforts are not on synthesis but on property investigation and device engineering. The molten salt synthesis, in which a molten salt is used as the reaction medium for reactant dissolution and product precipitation, is one such potential methodology that can be widely adapted by researchers in many fields. Previous studies have demonstrated that the characteristics of the products from molten salt method are generally affected by the synthesis conditions, such as the type of salt used, the annealing temperature, the temperature ramp rate, the precursor composition, and the solubility of the reactive constituents in the molten salt among others.21,22 In this work, we further demonstrate that it is necessary to employ a singlesource complex precursor, i.e., A(OH)3 · BO(OH)2 · nH2O, for the successful synthesis of nanocrystalline materials with the A2B2O7 composition. The conventional preparation of A2B2O7 involves many cycles of grinding of the component oxides and heating at high temperatures and usually yields inhomogeneous complex oxides. Therefore, researchers have moved to solution-processing chemical techniques since two decades ago in order to prepare pure and homogeneous A2B2O7 products, including fine particles or nanocrystals. These synthetic routes involve solution-based sol-gel process, gel combustion synthesis, co-precipitation, and

10.1021/jp807111h CCC: $40.75  2009 American Chemical Society Published on Web 01/07/2009

Luminescent A2B2O7 Nanocrystals 2 hydrothermal process, as just happened for other complex oxide systems.5,8,12,23-26 However, it is still a challenge to prepare these A2B2O7 nanocrystals with small particle sizes and correspondingly large surface areas. Sol-gel or co-precipitation routes require a calcination step for the formation of the final product, which results in some loss of the surface area.12,24,25 Hydrothermal crystallization of these oxides avoids this calcination step yet only particles with sizes larger than 50 nm were generated so far.23 To circumvent these difficulties, we applied the facile molten salt method as a means to prepare singlecrystalline A2B2O7 (A ) La, Er, or their mixture; B ) Zr, Hf, or their mixture) nanocrystals by using a single-source complex precursor A(OH)3 · BO(OH)2 · nH2O, which was prepared via coprecipitation of A(NO3)3 and BO(NO3)2/BOCl2 in a dilute ammonia solution at room temperature.27,28 The molten salt procedure involved an annealing of the mixture of the singlesource complex precursor and salt medium of NaNO3/KNO3 at 650 °C and generated A2B2O7 nanocrystals with an average size of 20 nm, the smallest A2B2O7 nanocrystal size reported so far. The formation of the single-source complex precursor rendered a homogeneous distribution and high intimacy of the reactive components at the atomic scale in the initial mixture of precursor and salt. Hence, the diffusion distance and rate of the reactive species in the molten salt medium are modified and efficient material transport is enabled to meet the minimal kinetic requirement for the reaction.19,29,30 In addition, the intrinsic simplicity, flexibility, and scalability of this strategy make it attractive for the preparation of a wider range of complex oxide nanocrystals. Experimental Section A. Sample Preparation. In a typical synthetic protocol, a single-source complex precursor, i.e., A(OH)3 · BO(OH)2 · nH2O, was prepared first by co-precipitation. Dilute ammonia aqueous solution (concentrated NH4OH (Alfa Aesar, 28.0-30.0%):H2O ) 1:9 (v/v)) was added dropwise into a mixed aqueous solution of A(NO3)3 (A ) La, Er, or their mixture in this study) and BO(NO3)2/BOCl2 (B ) Zr, Hf, or their mixture in this study) to co-precipitate the complex precursor A(OH)3 · BO(OH)2 · nH2O. For example, 5 mmol of La(NO3)3 · 6H2O (Alfa Aesar, 99%) and 5 mmol of ZrO(NO3)2 · xH2O (Alfa Aesar, 99.9%) were first added into 200 mL of deionized distilled water and then 200 mL of dilute ammonia aqueous solution was added dropwise into this mixed aqueous solution to form a complex precursor La(OH)3 · ZrO(OH)2 · nH2O. The precursor complex was then purified by filtration and washing with ∼300 mL of deionized distilled water. After being air-dried for at least 24 h, 0.35 g of single-source complex precursor A(OH)3 · BO(OH)2 · nH2O was mixed with 60 mmol of nitrate mixture (NaNO3: KNO3 ) 1:1 (molar ratio, and both were purchased from Alfa Aesar with purity of 99.0% minimum)) by hand-grinding with an agate mortar and pestle for at least 15 min. The mixture was transferred into a covered nickel crucible and heated to 650 °C at a rate of 10 °C/min with a box furnace in air and then isothermally annealed at 650 °C for 6 h. After being cooled to room temperature at a ramp-down rate of 10 °C/min, the resulting product slug in the crucible was immersed into deionized distilled water in a beaker to transfer the product into aqueous solution for subsequent separation and purification. The precipitate was centrifuged and purified with ∼200 mL of deionized distilled water several times. After drying in an oven at 120 °C overnight, A2B2O7 nanocrystals were obtained. B. Measurements. X-ray Diffraction. Synchrotron X-ray diffraction (XRD) was used to confirm the purity and obtain the crystallographic information on the as-synthesized A2B2O7

J. Phys. Chem. C, Vol. 113, No. 4, 2009 1205 nanocrystals. For synchrotron XRD study, data were collected at beam line 2-1 of the Stanford Synchrotron Radiation Laboratory using an X-ray energy of 14 keV from 0.8 to 7.8 Å-1. The powder samples were pressed into the shallow cavity of a quartz sample holder. The loaded samples were rocked throughout the experiment in the X-ray beam to average the diffraction patterns. Electron Microscopy. The size and morphology of solid powder samples of A2B2O7 nanocrystals were characterized using a field emission scanning electron microscope (FESEM, Hitachi S4700) at an accelerating voltage of 4 kV. Specifically, the as-prepared samples of A2B2O7 nanocrystals, after centrifugation, were sonicated for about 1 min and then air-dried upon deposition onto clean silicon wafers, which were then attached onto the surfaces of SEM brass stubs for imaging. Transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images, selected-area electron diffraction (SAED) patterns, and energy-dispersive X-ray spectroscopy (EDS) data were carried out on an FEI Tecnai F30 transmission electron microscope equipped with an EDAX EDS detector. The microscope was operated at an accelerating voltage of 300 kV with a field emission source with a point-to-point resolution of 0.2 nm. (HR)TEM images were recorded by means of a Gatan CCD camera (4k × 4k). The EDS spectra were recorded using an EDS Si/Li detector with an ultrathin window. Specimens for the TEM studies were prepared by depositing a drop of the aqueous suspension of A2B2O7 nanocrystals onto a 300 mesh Cu grid, coated with a lacey carbon film. Prior to the deposition, the solution containing A2B2O7 nanocrystals was sonicated for 2 min to ensure adequate dispersion in solution. In Situ Study of Growth Process. In situ XRD measurements were taken to follow the growth process of the A2B2O7 nanocrystals using a PANalytical X-ray diffractometer. The system was operated in the Bragg configuration using Cu KR radiation (λ ) 1.54 Å) from 20 to 60° at a scanning rate of 0.152°/s with a step size of 0.008°. The mixture of a singlesource complex precursor A(OH)3 · BO(OH)2 · nH2O and NaNO3/ KNO3 was mounted into a ceramic sample holder, which was then heated and controlled by an Aaton Parr temperature controller. The ramp rate was kept at 5 °C/min, and the scans were taken every 25 °C while holding at that temperature. Optical Properties. To characterize the photoluminescent (PL) and cathodoluminescent (CL) properties, powder samples of A2B2O7 nanocrystals were added into ethanol, and the mixtures were subsequently sonicated for about 1 min and then air-dried upon deposition onto silicon wafers. Room-temperature PL measurements were performed with a 488 nm argon ion laser operated with liquid nitrogen cooled InGaAs detectors. Also, PL measurements were taken as a function of the laser pump power at 0.03-1.5 W. The CL properties of these nanocrystals were analyzed using cathodoluminescent panchromatic imaging system and spectroscopy attached to a Hitachi S2250-N scanning electron microscope. Results and Discussion The as-prepared A2B2O7 nanocrystals are pure with no detectable impurities, as verified by the synchrotron XRD analyses (Figure 1a). By indexing the XRD patterns, the assynthesized La2Zr2O7, La2Hf2O7, and Er2Zr2O7 are shown to be of their respective cubic phases with calculated cell constants of a ) 10.854, 10.816, and 10.410 Å, respectively, which agree well with the literature reported values of the bulk materials. The decreases of the cell constants are the results of the decreasing radii of A3+ ions (i.e., 1.16 Å (La3+) vs 1.03 Å (Er3+)

1206 J. Phys. Chem. C, Vol. 113, No. 4, 2009

Figure 1. (a) Synchrotron XRD patterns of as-synthesized La2Zr2O7, La2Hf2O7, and Er2Zr2O7 nanocrystals, indexed to JCPDS Nos. 00-0170450, 01-073-0445, and 01-078-1299, respectively. SEM images of as-synthesized nanocrystals with A2B2O7 composition: (b) La2Zr2O7; (c) La2Hf2O7; and (d) Er2Zr2O7.

Figure 2. TEM characterization of La2Zr2O7 nanocrystals: (a) TEM image; (b) HRTEM image; (c) EDS spectrum (the Cu and C peaks originate from the TEM grid); (d) ED pattern.

for La2Zr2O7 vs Er2Zr2O7) and those of B4+ ions (i.e., 0.79 Å (Zr4+) vs 0.78 Å (Hf4+) for La2Zr2O7 vs La2Hf2O7), respectively.15,31 More importantly, on the basis of these high-resolution synchrotron-based XRD patterns, the fluorite type of A2B2O7 phase was formed due to the absence of (311) and (511) peaks, which are considered signatures of the pyrochlore structure distinguished from the fluorite structure.12,23 Even though pyrochlores gained attention as potential nuclear waste host materials because of their structural compatibility with large-radionuclide species at the eight-coordinate A-stite,32 Sickafus et al. recently demonstrated, both theoretically and experimentally, that fluorites are inherently more radiation resistant than pyrochlores.3,6 Both SEM images (Figure 1b-d) and low-magnification TEM image (Figure 2a) demonstrated that the A2B2O7 nanocrystals have relatively uniform sizes. Using La2Zr2O7 nanocrystals as an example, TEM analyses show regularly shaped near-cubic crystalline particles with an average size of 20 nm (Figure 2a,b). The faces of these nanocrystals are essentially flat though some of their corners and edges are slightly truncated. The shape of these nanocrystals is likely determined by the relative specific

Mao et al. surface energies of the facets. In other words, the formation of cubelike A2B2O7 nanocrystals is likely a kinetic and morphological manifestation of the initial nuclei of the cubic form.19,20 The lattice image of an individual nanocrystal shows that it is single-crystalline (Figure 2b). Elemental analysis by EDS (Figure 2c) confirms the stoichiometry of La, Zr, and O in these nanocrystals to be 1:1:3.5. The electron diffraction pattern (Figure 2d) corroborates the XRD analysis that La2Zr2O7 is of the cubic phase (Figure 1a). We would like to point out that for rare-earth-doped nanocrystals, the size uniformity is not as critical as for semiconductor quantum dots and metallic nanocrystals, since the outer and less energetic 5s and 5p shells shield the 4f electrons of rare-earth ions from the influence of external forces. In other words, for rare-earth-doped nanocrystals, the peak position and shape of luminescent emission from the rareearth ions are not or only very weakly dependent on the particle size, even though the intensity of the luminescence and lifetime of the excited states from rare-earth-doped nanocrystals have been shown to be inversely proportional to the size of the host particles.33,34 The importance of generating a single-source complex precursor A(OH)3 · BO(OH)2 · nH2O that contains homogeneously distributed constituting elements, i.e., A and B at an atomic level is best demonstrated by parallel experiments in that neither phase-pure A2B2O7 products nor A2B2O7 nanocrystals were produced using different precursors, such as A(NO3)3 and BO(NO3)2 or commercially available BO2 nanoparticles (Nanostructured and Amorphous Materials, Inc.). The fact that no phase-pure A2B2O7 nanocrystals were prepared without mixing the single-source complex precursor with the salt under the same reaction conditions suggests that the chemical transport and interdiffusion have been modified in the molten salt medium with or without using the single-source complex precursor. Specifically, the atomic-level mixing and homogeneous distribution of the reactive constituents in the amorphous single-source complex precursor A(OH)3 · BO(OH)2 · nH2O at the same composition as the final A2B2O7 products minimize the concentration gradients and reduce the distance across which the reactive constituents have to transport to a minimum, in this case, to atomic distances.29,30 The mobile molten salt medium further serves to enhance the diffusion of the reactive constituents during the reaction to form A2B2O7 nanocrystals. Meanwhile, the minimal kinetics requirement for this reaction was met by heating the mixture of A(OH)3 · BO(OH)2 · nH2O and NaNO3/KNO3 to a moderate temperature, i.e., 650 °C, as demonstrated by the in situ XRD studies shown in Figure 3. From these in situ XRD patterns, the diffraction peaks disappeared as the temperature reached 225 °C, suggesting that the NaNO3/KNO3 salt mixture completely melted at this temperature. This phenomenon is consistent with the eutectic temperature of nitrate mixture of NaNO3 and KNO3 at a molar ratio of 1:1.35 A2B2O7 nanocrystals started to form only after the temperature reached 650 °C and continued to grow. Thus, it is plausible to conclude that the successful synthesis of A2B2O7 nanocrystals in this study is a result of the synergistic interplay between the reduction of the transport distances of the reactive constituents to an atomic length scale and the enhanced diffusion of the reactants in the molten salt medium. As reported in the literature, the formation of nanoparticles in molten salt synthesis follows either a diffusion-controlled mechanism or an interfacial reaction-controlled mechanism.36,37 Based on the fact that the as-prepared A2B2O7 nanocrystals have smooth surfaces and a faceted shape, it is highly possible that the synthesis of A2B2O7 nanocrystals by a single-source complex precursor A(OH)3 ·

Luminescent A2B2O7 Nanocrystals

Figure 3. In situ XRD study of the molten salt synthesis of (La0.95Er0.05)2Zr2O7 nanocrystals between 25 and 650 °C and held at 650 °C for up to 300 min. Due to the decomposition and evaporation of NaNO3 and KNO3, the diffraction peaks from the corundum (Al1.896Cr0.104O3) ceramic sample holder emerged after the temperature reached 650 °C, as marked with /.

J. Phys. Chem. C, Vol. 113, No. 4, 2009 1207

Figure 5. Pump power dependence of the integrated photoluminescent intensity from as-prepared (La0.95Er0.05)2Zr2O7 nanocrystals excited at 488 nm at room temperature in the wavelength range of 1400-1625 nm: (a) PL spectra measured under different laser pump powers from 30 mW to 1.5 W. (b) Corresponding plot of PL yield (∆ symbols) as a function of pump power. The solid line is simply a visual guide.

Figure 6. Cathodoluminescent characterization of the (La0.95Er0.05)2Zr2O7 nanocrystals: (a) CL voltage as a function of beam current with a fixed beam voltage of 700 V and (b) CL voltage as a function of applied voltage with a fixed beam current of 150 pA. The insets of a and b are CL image and SEM image recorded simultaneously under 7 kV accelerating voltage, respectively.

Figure 4. (a) Schematic energy level diagram of Er3+ ions in A2B2O7 host corresponding to the theoretically predicted spectroscopic transitions from the 4I13/2 f 4I15/2 energy manifolds.41 (b) PL spectra of asprepared La2Zr2O7, Er2Zr2O7, and (La0.95Er0.05)2Zr2O7 nanocrystals excited at 488 nm at room temperature in the wavelength range of 1400-1625 nm.

BO(OH)2 · nH2O follows an interfacial reaction-controlled mechanism, even though further investigation is necessary. As shown in Figure 4, room-temperature PL spectra with a few well-resolved Stark features which correspond well to the theoretically predicted spectroscopic transitions from the Er3+ 4 I13/2 f 4I15/2 energy manifolds and maximum intensities at a wavelength of ∼1530 nm are clearly observed for Er2Zr2O7 and (La0.95Er0.05)2Zr2O7 nanocrystals, while no noticeable luminescence is observed for the La2Zr2O7 nanocrystals. Due to the concentration quenching, the PL intensity of Er2Zr2O7 nano-

crystals is weaker than that of (La0.95Er0.05)2Zr2O7 nanocrystals. These results indicate that the Er3+ ions occupy well-defined locations in the La2Zr2O7 lattice, namely, those of La3+ ions.38 They also indicate that the erbium-containing A2B2O7 nanocrystals are promising luminescent materials for optoelectronics and bioanalyses.39,40 In addition, the pump-power dependence of the PL was studied by taking the integrated photoluminescent intensity of the as-synthesized La2Zr2O7 nanocrystals with a relatively low erbium doping concentration, i.e., (La0.95Er0.05)2Zr2O7 nanocrystals, from 1420 to 1630 nm as a function of the laser pump power from 30 min to 1.5 W (Figure 5). Figure 5a indicates that the emission peaks do not shift as the excitation power changes. As the pump-power intensity increases, the PL intensity increases continuously with the pump power up to about 1.25 W, while, above 1.25 W, the photoluminescent intensity starts to saturate slightly in the studied range (∆ symbols in Figure 5b). This suggests an almost complete excitation at high pump powers. Another significant point is that the (La0.95Er0.05)2Zr2O7 sample exhibits a measurable PL intensity even at a pump power as low as 30 mW. Similar to PL, the (La0.95Er0.05)2Zr2O7 nanocrystals demonstrate excellent CL, as shown in the insets of Figure 6a,b by comparing the panchromatic CL and FESEM images. At a fixed acceleration voltage of 700 V, the CL voltage gradually increases with an increasing beam current of 150 pA (Figure 6a); at a fixed beam current, the CL voltage increases with an increasing acceleration voltage (Figure 6b). These properties

1208 J. Phys. Chem. C, Vol. 113, No. 4, 2009 suggest that the erbium-containing A2B2O7 nanocrystals are promising luminescent materials for field emission display applications.13,33 Conclusions In summary, single-crystalline nanocrystals with A2B2O7 composition and an average size of 20 nm have been synthesized by a simple, scalable, and efficient molten salt synthetic process using a single-source complex precursor A(OH)3 · BO(OH)2 · nH2O to modify the reaction kinetics. It is believed that their successful synthesis is a result of the synergistic interplay between the reduction of the transport distances of the reactive constituents to an atomic length scale and the enhanced diffusion of the reactants in the molten salt medium. This strategy is attractive for the preparation of a wide range of other complex oxide nanocrystals. Moreover, the outstanding luminescent properties, including strong photoluminescence in the infrared region, efficient pump-power dependence, and excellent cathodoluminescence, of the erbium-containing derivatives indicate that these nanocrystals are promising candidates for many applications such as displays, bioanalyses, and optoelectronics. Acknowledgment. The authors acknowledge the financial and program support from National Science Foundation (Grant CTS0522534), the Office of Naval Research (a Young Investigator Award), and the semiconductor Research Corporation (SRC) and its Focus Center Research Program (FCRP). Portions of this research were carried out at the Center for Integrated Nanotechnologies of Sandia National Laboratories and Standard Synchrotron Radiation Laboratory. The CINT, a U.S. Department of Energy, Office of Basic Energy Sciences, user facility, is supported by Laboratory Directed Research and Development (LDRD), Sandia National Laboratories. Sandia is a multiprogram laboratory operated by Sandia Corp., a Lockheed Martin Co., for the U.S. Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000. We also thank Dr. Michael Toney for his assistance with the synchrotron XRD experiments. References and Notes (1) Pruneda, J. M.; Artacho, E. Phys. ReV. B 2005, 72, 085107. (2) Klee, W. E.; Weitz, G. J. Inorg. Nucl. Chem. 1969, 31, 2367. (3) Sickafus, K. E.; Minervini, L.; Grimes, R. W.; Valdez, J. A.; Ishimaru, M.; Li, F.; McClellan, K. J.; Hartmann, T. Science 2000, 289, 748. (4) Cao, X.; Vassen, R.; Fischer, W.; Tietz, F.; Jungen, W.; Stover, D. AdV. Mater. 2003, 15, 1438. (5) Fujihara, S.; Tokumo, K. Chem. Mater. 2005, 17, 5587.

Mao et al. (6) Lumpkin, G. R.; Whittle, K. R.; Rios, S.; Smith, K. L.; Zaluzec, N. J. J. Phys.: Condens. Matter 2004, 16, 8557. (7) Qu, Z.; Wan, C.; Pan, W. Chem. Mater. 2007, 19, 4913. (8) Uno, M.; Kosuga, A.; Okui, M.; Horisaka, K.; Muta, H.; Kurosaki, K.; Yamanaka, S. J. Alloys Compd. 2006, 420, 291. (9) Ji, Y.; Jiang, D.; Shi, J. J. Mater. Res. 2005, 20, 567. (10) Zhang, L.; Fu, H.; Zhang, C.; Zhu, Y. J. Phys. Chem. C 2008, 112, 3126. (11) McCauley, R. A.; Hummel, F. A. J. Lumin. 1973, 6, 105. (12) Zhang, A.; Lu, M.; Zhou, G.; Wang, S.; Zhou, Y. J. Phys. Chem. Solid 2006, 67, 2430. (13) Yacobi, B. G.; Holt, D. B., Cathodoluminescence Microscopy of Inorganic Solids. Plenum Press: New York, 1990. (14) Tissue, B. M. Chem. Mater. 1998, 10, 2837. (15) Zhang, A.; Lu, M.; Yang, Z.; Zhou, G.; Zhou, Y. Solid State Sci. 2008, 10, 74. (16) Kenyon, A. J. Prog. Quantum Electron. 2002, 26, 225. (17) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2446. (18) Rao, C. N. R.; Nath, M. Dalton Trans. 2003, (1), 1. (19) Cushing, B. L.; Kolesnichenko, V. L.; O’Connor, C. J. Chem. ReV. 2004, 104, 3893. (20) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (21) Mao, Y.; Park, T.-J.; Zhang, F.; Zhou, H.; Wong, S. S. Small 2007, 3, 1122. (22) Yoon, K. H.; S., C. Y.; Kang, D. H. J. Mater. Sci. 1998, 33, 2977. (23) Chen, D.; Xu, R. Mater. Res. Bull. 1998, 33, 409. (24) Rao, K. K.; Banu, T.; Vithal, M.; Swamy, G. Y. S. K.; Kumar, K. R. Mater. Lett. 2002, 54, 205. (25) Dhas, N. A.; Patil, K. C. J. Mater. Chem. 1993, 3, 1289. (26) Fisher, M. J.; Wang, W.; Dorhout, P. K.; Fisher, E. R. J. Phys. Chem. C 2008, 112, 1901. (27) Urban, J. J.; Ouyang, L.; Jo, M.-H.; Wang, D. S.; Park, H. Nano Lett. 2004, 4, 1547. (28) Xu, G.; Ren, Z.; Du, P.; Weng, W.; Shen, G.; Han, G. AdV. Mater. 2005, 17, 907. (29) Jansen, M. Angew. Chem., Int. Ed. 2002, 41, 3746. (30) Weng, X.; Boldrin, P.; Abrahams, I.; Skinner, S. J.; Darr, J. A. Chem. Mater. 2007, 19, 4382. (31) Lide, D. R., CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 2001; Vol. 88. (32) Begg, B. D.; Hess, N. J.; McCready, D. E.; Thevuthasan, S.; Weber, W. J. J. Nucl. Mater. 2001, 289, 188. (33) Vetrone, F.; Boyer, J.-C.; Capobianco, J. A. In Encyclopedia of Nanoscience and Nanotechnology; Nalwa, H. S., Ed. American Scientific: Stevenson Ranch, CA, 2004; Vol. 10, p 725. (34) Meltzer, R. S.; Feofilov, S. P.; Tissue, B.; Yuan, H. B. Phys. ReV. B 1999, 60, R14012. (35) Berg, R. W.; Kerridge, D. H. Dalton Trans. 2004, (15), 2224. (36) Park, J.-H.; Lee, D.-H.; Shin, H.-S.; Lee, B.-K. J. Am. Ceram. Soc. 1996, 79, 1130. (37) Cai, Z.; Xing, X.; Li, L.; Xu, Y. J. Alloys Compd. 2008, 454, 466. (38) Van, T. T.; Chang, J. P. Appl. Phys. Lett. 2005, 87, 011907. (39) Kenyon, A. J. Curr. Opin. Solid State Mater. Sci. 2003, 7, 143. (40) Yan, J.; Estevez, M. C.; Smith, J. E.; Wang, K.; He, X.; Wang, L.; Tan, W. Nanotoday 2007, 2 (3), 44. (41) Kisliuk, P.; Krupke, W. F.; Gruber, J. B. J. Chem. Phys. 1964, 40, 3606.

JP807111H