Size- and Surface-dependent Photoresistance in SnO 2 Nanowires
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0901-Rb15-02.1
Size- and Surface-dependent Photoresistance in SnO2 Nanowires Sanjay Mathur*, Sven Barth, Jae-Chul Pyuna and Hao Shen Nanocrystalline Materials and Thin Film Systems Division, Leibniz-Institute of New Materials a Korea Institute of Science and Technology (KIST) Europe Forschungsgesellschaft mbH D-66041 Saarbruecken, Germany ABSTRACT Nanostructured one-dimensional materials, such as nanowires, tubes and rods, are gaining increasing attention due to interesting properties and confinement effects, however controlled synthesis of these structures is still limited to a few methods. We present here the synthesis of SnO2 nanowires (Ø, 50 – 1000 nm) at moderate temperatures (550 – 900 °C) using a molecular source [Sn(OBut)4] with pre-existent Sn-O bonds. The growth occurs via a catalyst driven vaporsolid-solid mechanism. Size-selective synthesis of NWs in high areal density was achieved by choosing Au particles of appropriate size. HR-TEM analysis reveals the single crystalline behaviour of wires with a preferred growth direction [100]. Use of SnO2 nanowires as potential optical switches for UV applications was demonstrated by the photo-response measurements. Determination of band gap values confirmed the blue-shift of the main photo-response peak with shrinking radial dimensions of the wires. Furthermore, deposition of vanadium oxide onto SnO2 led to a red-shift of the main conduction value of the nanowires. INTRODUCTION Nanostructured metal oxide based semiconductors exhibit unique electronic, optical and transport properties, which scales up with the degree of dimensional confinement [1]. Tin oxide (SnIVO2) is transparent in the visible region due to the large band gap (3.6 eV) and is therefore useful as conductive electrodes [2]. In addition, tin oxide doped with lower or higher-valent cations (e.g. In(III) or Sb(V)) allows tailored material properties, such as increased sensitivity of sensor materials for reducing gases [3]. The density of charge carriers in SnO2 is strongly influenced by the degree of surfaced bonded O- and O2- species [4]. Removing these electron trapping oxygen species by incoming molecules leads to increase in the intrinsic conductivity of the material. Similarly, illumination of metal oxide semiconductors with UV light creates electron and hole carriers, which allows to switch between high (UV on) and low (UV off) conductivity states [5]. The most promising materials for electron transport and gas sensing materials are single crystalline high axial ratio nanostructures, such as wires, tubes and rods, due to the high surface to volume ratio and cable-like morphologies [6]. Compared to the large body of data dealing with tin oxide films, only few reports are available on the synthesis of SnO2 nanowires (NWs). SnO2 nanowires have been prepared by vapor transport synthesis at elevated temperatures [7], laser ablation of metallic tin target in Ar/O2 atmosphere [5], ethylene glycol template process in solution [8], oxidation of pre-synthesized Sn wires [9], a simple oxidation of tin at 9
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