Nanodots and Nanowires of Silicon

Abstract

When the size of silicon is reduced towards the nanometer range, new properties emerge due to a dramatic change in bonding conditions, and due to electron and hole state quantization. Bulk silicon is characterized by diamond-type crystal structure, with sp ³-hybridization and 4-fold coordination. With decreasing size, silicon undergoes a phase change to a more close-packed atomic arrangement, which characterizes atomic and electronic structures of small Si clusters. In order to study size-dependent properties of silicon nanoparticles, we apply scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). For silicon clusters we determine the energy gap as a function of size. We show that pristine silicon particles show a major transition in their electronic properties at about 15 Å. We find that by vapor-condensation in UHV, silicon is also able to form quasi-one-dimensional structures. Nanowires with diameters from 3 nm to 7 nm, more than 100 nm long were produced. Considering the calculated free energies and band gaps for several possible wire structures we suggest that silicon nanowires tend to grow with a fullerene-type Si24-based core structure.