Hardening mechanisms of silicon nanospheres: a molecular dynamics study.

Abstract

Much work has been done studying the compression of nanostructures of silicon as the measured properties can be related to structures present in MEMS and NEMS devices. In particular, spherical silicon nanoparticles are found to be much harder than bulk silicon during compression. Here, large scale molecular dynamics simulations are presented that investigate the yielding and hardening mechanisms of nanospheres. The resulting yield behavior is shown to vary with changes in temperature, sphere size, atomistic potential, and crystallographic orientation with respect to the loading direction. With the Tersoff potential, a strong temperature dependence is observed as hardness values near 0 K are much greater than 300 K values. beta-Sn forms during [100] crystallographic compressions which results in a slight hardening above 40 % strain. The Stillinger-Weber allowed for dislocation interactions to be studied in spheres comprised of up to one million atoms. Direct comparisons of the simulated results are made to experimental results indicating that the displacement excursions and low strain hardening behavior can be explained with dislocation activity. Further simulations investigated interactions affecting dislocations that might influence the properties of silicon nanostructures. The nature of dislocation-dislocation, dislocation-applied shear strain, and dislocation-free surface interactions are shown to be consistent with what is predicted by elementary dislocation theory. Presence of an oxide results in a more complex interaction as both the interface and the lattice strain associated with the oxide affect the dislocations. Depending on the geometry of the system, this oxide interaction may be repulsive resulting in dislocations becoming trapped in the system allowing for substantial hardening.