Growth and Characterization of [001] ZnO Nanorod Array on ITO Substrate with Electric Field Assisted Nucleation
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Growth and Characterization of [001] ZnO Nanorod Array on ITO Substrate with Electric Field Assisted Nucleation Young Jung Kim1, Huamei Shang, and Guozhong Cao Materials Science and Engineering, University of Washington, Seattle, WA 98195, U.S.A. 1 Permanent address: Materials & Chemical Engineering, Sun Moon University, Asan, Korea. ABSTRACT This paper reports direct growth of [001] ZnO nanorod arrays on ITO substrate from aqueous solution with electric field assisted nucleation, followed with thermal annealing. Xray diffraction analyses revealed that nanorods have wurtzite crystal structure. The diameter of ZnO nanorods was 60 ~ 300 nm and the length was up to 2.5 µm depending on the growth condition. Photoluminescence spectra showed a broad emission band spreading from 500 to 870 nm, which suggests that ZnO nanorods have a high density of oxygen interstitials. Low and nonlinear electrical conductivity of ZnO nanorod array was observed, which was ascribed to non-ohmic contact between top electrode and ZnO nanorods and the low concentration of oxygen vacancies. INTRODUCTION ZnO is of great interest for various photonic and electrical applications due to its unique physical and chemical properties, such as a wide band gap (3.37eV), large exciton binding energy (60meV) at room temperature, piezoelectricity, and surface chemistry sensitive to environment. Applications of ZnO include light-emitting diodes 1, diode lasers2, photodiodes 3, photodetectors 4, optical modulator wave guides 5, photovoltaic cells6, phosphor 7, varistor 8, data storage 9, and biochemical sensors10. Nanostructured ZnO, nanorods or nanowires in particularly, has attracted intensive research, primarily for their large surface area for applications relying on heterogeneous reactions such as sensors and detectors 4 , their light confinement for nano-lasers 2 , and their enhanced freedom in lateral dimensions for more sensitive piezoelectric devices11. Various fabrication techniques have been established for the growth of ordered ZnO nanorods and nanowires. Vapor-liquid -solid growth2 , chemical vapor deposition12, thermal evaporation 13, carbothermal evaporation 14, aque ous solution growth15, flux growth 16, and template-based synthesis 17 have all reported to successfully grow ZnO nanorod arrays. Wellaligned arrays of ZnO nanorods were grown by vapor-phase process at high temperature on the single crystal substrates such as Si18, GaN 19, and sapphire2 , which have crystallographic similarity to ZnO. This method has limitation to scale up the process because of expensive single crystal substrate and high processing temperature. Aligned arrays of [001] ZnO nanorods on glass and silicon substrates have also been readily grown from aqueous solution with nanocrystal seeding20; the alignment of nanorods was achieved by evolution selection growth 21, i.e., the crystal orientation with the higher growth rate and perpendicular to the substrate surface will survive and continue to growth. The nature of the evolution selection growth dictates the in
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