Infrared Spectroscopy of Impurities in ZnO Nanoparticles
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Infrared Spectroscopy of Impurities in ZnO Nanoparticles W. M. Hlaing Oo and M. D. McCluskey Department of Physics Washington State University Pullman, WA 99164-2814, U.S.A. ABSTRACT Semiconductor nanoparticles have a range of potential applications in electronic, optoelectronic and spintronic devices. Zinc oxide (ZnO), a wide-bandgap semiconductor, has emerged as an important material for such applications. In this work, impurities in ZnO nanoparticles were investigated with infrared (IR) spectroscopy, and the results show the presence of CO2 impurities in ZnO nanoparticles. Isotopic substitution was used to verify the frequency assignment and the results demonstrate conclusively that the impurities originate from the precursors. Isochronal annealing experiments were performed to study the formation and stability of the CO2 molecules. In addition to unintentional CO2 impurities, we intentionally introduced hydrogen into ZnO nanoparticles. Our results show that post-growth annealing in hydrogen dramatically changes IR transmission, reflection and electrical properties of the nanoparticles. INTRODUCTION Zinc oxide (ZnO) has received considerable attention because of its potential applications such as varistors, piezoelectric transducers, and transparent conducting thin films [1-2]. A variety of wet-chemical methods, using zinc salts such as zinc acetate or zinc nitrate as precursor materials, have been used to synthesize the ZnO nanoparticles. Although many approaches have been used to prepare ZnO nanoparticles, the presence of impurities remaining from the precursor materials and reaction products is still a challenging problem. In this paper, we report IR spectroscopy of unintentional CO2 impurities and intentionally introduced hydrogen in ZnO nanoparticles. SYNTHESIS OF ZnO NANOPARTICLES ZnO nanoparticles were synthesized by reaction of zinc acetate dehydrate, Zn(CH3COO)2⋅2H2O, and sodium hydrogen carbonate, NaHCO3 [3]. To prevent contamination from ambient the mixture was sealed in an Ar gas filled quartz ampoule. The reaction was performed at 200 °C for 2 hours. After the reaction, the by-product sodium acetate was washed away with distilled water several times. The powder was dried overnight at room temperature and then annealed at 350 °C for 2 hours to remove the remaining water. The TEM image and Xray diffraction pattern of the nanoparticles are shown in figure 1. The sizes of the particles are about ~15 nm and XRD pattern confirms the wurtzite structure. The powder was pressed into thin pellets (~0.25 mm) to perform the IR spectroscopy.
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Figure 1. TEM image and X-ray diffration pattern of ZnO nanoparticles. INFRARED SPECTRA OF ZnO NANOPARTICLES Infrared absorbance spectra of the ZnO nanoparticles were taken with a vacuum Bomem DA8 FTIR spectrometer. A strong absorption peak was observed at a frequency of 2342 cm-1 (figure 2). In addition a weak absorption was observed at 2277 cm-1. These peaks are assigned to be asymmetrical stretch frequencies (ν3) of 12CO2 and 13CO2, respectively.
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