Short-Period Superlattice Structure of Sn-doped In 2 O 3 (ZnO) 4 and In 2 O 3 (ZnO) 5 Nanowires
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Short-Period Superlattice Structure of Sn-doped In2O3(ZnO)4 and In2O3(ZnO)5 Nanowires Chan Woong Na, Seung Yong Bae, and Jeunghee Park Department of Chemistry, Korea University, Jochiwon 339-700 Korea; ABSTRACT Two longitudinal superlattice structures of In2O3(ZnO)4 and In2O3(ZnO)5 nanowires were exclusively produced by thermal evaporation method. The diameter is periodically modulated in the range of 50-90 nm. They consist of one In-O layer and five (or six) layered Zn-O slabs stacked alternately perpendicular to the long axis, with a modulation period of 1.65 (or 1.9) nm. These superlattice nanowires were doped with 6-8 % Sn. X-ray diffraction pattern reveals the structural defects of wurtzite ZnO crystals due to the In/Sn incorporation. High-resolution X-ray photoelectron spectrum suggests that In/Sn withdraw the electrons from Zn, and enhance the number of dangling-bond O 2p states, resulting in the reduction of band gap. Photoluminescence exhibit the peak shift of near band edge emission to the lower energy as the In/Sn content increases. INTODUCTION The natural short-period superlattice structures have been found in the oxide compound system, such as In2O3-ZnO compounds that the In-O and Zn-O layer slabs stacks alternately. Since Kasper first observed the structure of In2O3(ZnO)m (m=2-5 and 7),1 Kimizuka group investigated extensively the superlattice structure of homologous compounds InMO3(ZnO)m (M = In, Fe, Ga, and Al) using high-resolution transmission electron microscopy and X-ray diffraction.2-4 Yan et al. reported the high-resolution Z-contrast image for a modulated structure In2O3-ZnO.5 As a benefit from unique crystal structure, high electrical/thermal conductivity, optical transparency, and excellent thermoelectric properties of In2O3(ZnO)5 have been observed.6,7 High-performance transparent field-effect transistors were fabricated using InGaO3(ZnO)5 film.8,9 Recently, as an effort to produce the 1-dimensional nanostructures, Jie et al. synthesize the superlattice nanowires having diverse composition which is approximately In2O3(ZnO)m (m=5, 7, 18).10 EXPERIMENTAL DETAILS Mixture of ZnO (99.98%, Aldrich), In (99.99%, Aldrich), and Sn (99.99%, Aldrich) powders was placed in a quartz boat located inside a quartz tube. Silicon substrate was coated with HAuCl4·3H2O (98%, Sigma) ethanol solution, and then positioned on the source boat. Highdensity nanowires were grown on the Au-deposited substrates at 900-1000 °C, by the evaporation of Zn/In/Sn source for 1 h under argon flow. As the growth temperature increases, the In/Sn content usually increases. We also synthesized pure ZnO nanowires using Zn/ZnO powder mixture at 800 °C. The morphology and composition of products were examined by scanning electron microscopy (SEM, Hitachi S-4300), transmission electron microscopy (TEM, Jeol JEM-2010, 200 kV), high-voltage transmission electron microscopy (HVEM, Jeol JEM ARM 1300 S, 1.25 MV), XRD patterns were measured using the 8C2 beam line of the Pohang Light Source (PLS) with monochromatic radiation (λ=1.54
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