First-principles calculation of mechanical properties of Si <001> nanowires and comparison to nanomechanical theory

TL;DR

This study uses first-principles density functional theory to analyze the mechanical properties of hydrogen-passivated Si <001> nanowires, revealing size-dependent behaviors influenced by surface interactions and charge density variations.

Contribution

It provides a detailed first-principles analysis of Si nanowire mechanics, comparing results with continuum and classical models to understand size effects and surface interactions.

Findings

Young's modulus scales with surface area to volume ratio.

Surface hydrogen interactions significantly influence mechanical properties.

Discrepancies between models are explained by charge density variations.

Abstract

We report the results of first-principles density functional theory calculations of the Young's modulus and other mechanical properties of hydrogen-passivated Si <001> nanowires. The nanowires are taken to have predominantly {100} surfaces, with small {110} facets according to the Wulff shape. The Young's modulus, the equilibrium length and the constrained residual stress of a series of prismatic beams of differing sizes are found to have size dependences that scale like the surface area to volume ratio for all but the smallest beam. The results are compared with a continuum model and the results of classical atomistic calculations based on an empirical potential. We attribute the size dependence to specific physical structures and interactions. In particular, the hydrogen interactions on the surface and the charge density variations within the beam are quantified and used both to parameterize the continuum model and to account for the discrepancies between the two models and the first-principles results.