VLS Synthesis and Characterization of SnO 2 Nanowires
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VLS Synthesis and Characterization of SnO2 Nanowires Dulce N. Castillo, Tomás D. Becerril, Enrique R. Andrés, Héctor J. Santiesteban and Godofredo G. Salgado. Posgrado en Dispositivos Semiconductores, Benemérita Universidad Autónoma de Puebla, Av. 14 Sur y San Claudio s/n, Ciudad Universitaria, C.P.72570, Puebla, Pue., México. ABSTRACT We have synthesized core-shell 1D nanostructures by the Vapor-Liquid-Solid (VLS) mechanism. Gold (Au) was used as a catalyst and tin oxide (SnO) powder as a precursor; the growth temperature was of 600 °C. These structures were characterized by XRD, SEM, TEM, EDS, and PL. The nanowires have an average diameter of 20 nm and their lengths are of tens of micrometers; the core is tin dioxide (SnO2) with the tetragonal rutile structure and it has an average diameter of 12 nm; the shell is amorphous Sn of 8 nm average thickness. Photoluminescence measurements show a broad band in the 400-800 nm range. On the same growth process, SnO2 nanoparticles and a mixture of SnO2 rods and wires were also obtained, at 400 °C and 800 °C, respectively. INTRODUCTION SnO2 is an n-type-semiconductor metal oxide whose bulk properties are: wide bandgap of 3.6 eV (at 300 K), low resistivity (10-4 to 10-6 ȍ-cm), high achievable carrier concentration (up to 6x1020 cm-3), high optical transparency in the visible range, and chemical and structural stability [1,2]. Nanomaterials have mechanical, chemical, thermal, electrical, and optical properties different from those of their bulk counterparts due to the enhanced surface-volume ratio and possible quantum confinement effects [3-5]. Therefore, SnO2 1D nanostructures are very interesting due to the various applications of SnO2 in optoelectronic and electronic devices and as transparent conductive electrodes, catalysts, anode materials for lithium-ion batteries, and gas sensors. It is used also in field effect transistors and it has been shown to act as a sub-wavelength waveguide [4, 6-10]. EXPERIMENTAL Core-shell nanowires were synthesized by the VLS mechanism [11]. The growth process was carried out inside a horizontal reactor (a quartz tube closed at one end) which was inserted into a quartz-tube furnace. Gold, deposited as an ~150 nm thick film over Si substrates in a vacuum evaporator, was used as a catalyst and powder SnO (99.9%) as a precursor. Three substrates and an alumina crucible containing the SnO powder precursor were placed inside the reactor in such a way that, when the reactor is inserted into the furnace quartz tube, each substrate lies in a different temperature zone of the furnace and so does the precursor powder
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crucible too. In this way, three samples were grown simultaneously, one at 400 °C, another one at 600 °C, and a third one at 800 °C; the precursor powder was at 1000 °C. A constant flow of nitrogen at a rate of 800 sccm (standard cubic centimeters per minute) was maintained all the time inside the reactor. Fig. 1 shows a schematic illustration of the growth system. After a growth process of one hour, the system was slowly cooled dow
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