A Non-Traditional Vapor-Liquid-Solid Method for Bulk Synthesis of Semiconductor Nanowires
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A Non-Traditional Vapor-Liquid-Solid Method for Bulk Synthesis of Semiconductor Nanowires Shashank Sharma, and Mahendra K. Sunkara* Department of Chemical Engineering, University of Louisville, Louisville, KY 40292, USA Guoda Lian and Elizabeth C. Dickey Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY 40506, USA * Corresponding author: [email protected] ABSTRACT Multiple silicon nanowires were synthesized using large gallium pools and microwave plasma. Results showed that nanowires growing out of different sized large gallium drops show little variation in diameters, suggesting that our non-traditional technique can be used to synthesize bulk amounts of monodispersed nanowires out of thin films of molten gallium. INTRODUCTION Size constraints and surface effects induce novel characteristics in materials at nanoscale.1-6 In order to utilize nanometer scale structures in applications such as electronic, optoelectronic, and catalytic, bulk amounts of nanostructures will be required with tunable composition and structure. In this regard, bulk synthesis of semiconductor nanowires has been traditionally achieved using several variations of transition metal catalyzed techniques such as vapor-liquidsolid (VLS). 7-20 In such techniques, the nanowire diameter is limited by the catalyst droplet size. However, creation of nanometer size catalyst droplets is a non-trivial task. Furthermore, given the fact that transition metals form high solubility eutectics with various semiconductor elements, it would be very difficult to obtain abrupt compositionally modulated structures. Nanowires have also been synthesized by restrictive growth methods such as nanotube-confined reactions 21-23 and with atomic scale step edges as templates. 24 In all of these techniques, it will be difficult to control the nanowire diameter and diameter distribution. Recently our group demonstrated bulk synthesis of silicon nanowires using gallium as the molten metal solvent and microwave plasma to carry out the gas phase chemistry. 25 It was reported that multiple nuclei form and grow as nanowires out of a large gallium pool. Thus, this gallium and microwave plasma based technique eliminates the need for nanometer sized metal droplets and has a potential to work at much lower temperatures than the conventional techniques using transition metals. However, there are certain important aspects of this technique that remained unaddressed. In this paper, we discuss issues related to control over nanowire diameter and diameter distribution in our non-transition metal-based technique. We also present evidence for in-situ generation of silyl radicals thus confirming silicon source to be through gas phase.
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EXPERIMENTAL DETAILS The experimental setup for bulk synthesis of silicon nanowires has been previously described. 25 Chemical vapor transport experiments were also performed, in which galliumcovered p-BN and quartz substrates surrounded by silicon pieces were exposed to a diluted hydrogen plasma. An Ocean Op
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