Stability and Mechanical Properties of Silicon Nanowires

Abstract

his work focuses on the structure, stability, and mechanical properties of silicon nanowires (SiNWs) oriented along the 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 themore » 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%.« less