Synthesis of Silicon Nano-Dendrites
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Synthesis of Silicon Nano-Dendrites Saion Sinha Department of Physics, Southern Connecticut State University, 501 Crescent Street, New Haven, CT 06515, U.S.A. Bo Gao and Otto Zhou Department of Physics and Astronomy and Curriculum in Applied and Materials Sciences, University of North Carolina, Chapel Hill, NC 27599, U.S.A. Recent interests on the science at nanometer-scale, have led to successful synthesis of a growing number of nanostructures such as zero-dimension semiconductor quantum dots [1] and C60 fullerene [2] and one-dimensional carbon nanotubes [3], metallic nanowires [4] and inorganic nanorods [5]. More complex structures/junctions beyond simple dots and lines that are essential for eventual molecular electronic applications, however, have not been realized via nonlithographic methods. Here we report the synthesis of a new branched silicon nanostructure which we call Silicon Nano-Dendrite (SiND) by Laser ablation. The materials were synthesized by ablating a sintered Si/Fe target using 532nm beam of a pulsed Nd:YAG laser inside a furnace heated to 1175oC in an argon environment [6]. During the Laser ablation process, the Argon flow was switched on and off very fast and simultaneously decreasing the flow rate, to create a non-steady-state flow. Transmission electron microscopy (TEM) examination on the product formed in the front of the tube showed that a significant fraction of the raw materials have dendritic structures (Fig. 1). Normal SiNWs are formed at the back of the tube [7] on a uniform Argon flow . The SiNDs typically comprise of a long primary backbone with a diameter of 20-100nm, short and narrow secondary and, in some cases, tertiary branches that are orthogonal to each other. The length, diameter, and spacing of the arms are roughly uniform. The backbones and branches are made of sp3 Si with the Raman mode downshifted by 2-5cm-1 from the bulk crystalline Si. The outer surfaces are coated with silicon oxide. TEM and electron diffraction measurements indicated that all the arms are terminated by FeSi2 particles which are believed to be the nuclei for the nanostructures. We argue that nucleation and growth of the backbone structure occur via the same vaporliquid-solid (VLS) mechanism proposed for silicon whiskers [8] and later for silicon nanowires [5] (Step 1 in Figure 2). Such branching and kinking of the silicon whiskers have been observed [9] before but in a much larger scale. These features were believed to have formed on the sudden change of temperature gradient. Nucleation of the dendritic arms is attributed to the formation of protrusions on the molten FeSi2 surface due to thermal instability, and their subsequent propagation into a supercooled environment (Step 2 in Figure 2) [10]. This is similar to the formation of conventional dendrites (10-100µm in diameter) in metals during solidification [11]. In the latter case, it is known that a planar solid/liquid interface will form if the solid propagates in a superheated liquid . This mechanism keeps the nanostructures growing inside the fu
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