Abstract
his work focuses on the structure, stability, and mechanical properties of silicon nanowires (SiNWs) oriented along the < 111 > direction. A total energy calculation and a quenching-technique based on a non-orthogonal tight-binding molecular dynamics (MD) is used to study the equilibrium structure and relative stability of SiNWs of different diameters (d) ranging from 2 nm to 17 nm. Our study of the structural relaxation of wires of different diameters reveal the following two key findings: (i) A SiNW is composed of a crystalline core surrounded by a bond distortion-dominated surface region, and (ii) the width of the surface region is found to be similar to 1 nm and is independent of the diameter of the wire. These findings suggest that there exists a critical diameter (similar to 2 nm) below which a crystalline nanowire becomes unstable due to surface-surface interaction. We have also calculated the surface energy (E-s) for wires of different diameters (d), where the E-s versus d curve exhibits several local minima. A more careful study of the local minimum at d similar to 6 nm finds this minimum to be very sharp. The existence of local minima in the surface energy curve is indicative of the existence of magic diameters for SiNWs, while the sharpness of the minima is related to the existence of uniform-diameter wires. A study of the Young's modulus of the similar to 6 nm-SiNW suggests that surface effects are critically important in determining the mechanical properties of nanowires with the value of the Young's modulus reduced from its bulk value by about 40%.