Quantitative V-L-S Growth Model and Experiments of Fe Catalyzed Si Nanowire Formation
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Quantitative V-L-S Growth Model and Experiments of Fe Catalyzed Si Nanowire Formation Guangwen Zhou, Judith C. Yang, Fengting Xu, John A. Barnard Department of Materials Science and Engineering, University of Pittsburgh, PA 15261 Z. Zhang Beijing Laboratory of Electron Microscopy, Institute of Physics, Chinese Academy of Sciences, Beijing 10080, China
ABSTRACT We report the formation of Si nanowires (SiNWs) by vapor-liquid-solid (V-L-S) mechanism where Si atoms are pumped from Si wafer. This happens when a material has the large difference of activity in its pure (Si) and alloy state (Si-Fe). We developed a kinetic model to quantitatively describe the growth of the nanowires. The model predicts that the length of the nanowires increases linearly with the growth time. INTRODUCTION In the rapidly developing field of nanotechnology, formation of one-dimensional nanostructures has been intensively investigated because of their potential nano-device applications and the intrinsic interest in structures with reduced dimension. A variety of approaches, including thermal evaporation and laser ablation, have been successfully used to synthesize these one-dimensional structures, such as Si nanowires (SiNWs) and it was demonstrated that vapor-liquid-solid (V-L-S) reaction plays a key role in the formation of these structures [1-4]. The crystal growth by this mechanism involves vapor, liquid, and solid phases, and occurs in two steps. First, there is a condensation from the vapor to the liquid solution in a vapor-liquid system. The second step occurs in a liquid-solid system and is the precipitation from the supersaturated liquid solution at the liquid-solid interface. An important idea of the V-L-S growth model is that metal catalysts (Fe, Ni, Au, etc) act as liquid-forming agents, which react with the vapor phase, and form liquid solution droplets. Since the surface of the liquid droplets has a large accommodation coefficient and therefore the solution droplets are preferred sites for the condensation of vapor atoms which causes the liquid to become supersaturated. Crystal growth occurs by precipitation of the condensed materials from the supersaturated liquid at the solid-liquid interface and the unidirectional growth is the consequence of an anisotropy in solidliquid interfacial energy. The V-L-S model was proposed in the 1960s-1970s [5-7], however, what kind of specific role of the metal catalysts is acting during the V-L-S growth is still not very clear and quantitative growth model describing the growth process is still lacking except for the nanowires generally have alloy droplets on their tips. In order to be able to rationally control the size, structure, composition, and axial direction of the nanowires, a better understanding of the V-L-S model is necessary. Hence, in this work we chose SiNWs as a model system to study the V-L-S growth. The role of metal catalysts playing in V-L-S growth was discussed and a quantitative growth model was proposed to account for the nanowire growth.
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Figure 1. (a) TE
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