Surface effects on the mechanical behavior of silicon nanowires: Consequence on the brittle to ductile transition at low scale and low temperature

Understanding the origin of the brittle to ductile transition at low scale in Si requires the characterization of the elementary mechanisms governing crack formation or dislocation nucleation. By molecular dynamics simulations, we have investigated the role of three surface states of silicon nanowires (NWs), fresh cut, reconstructed by annealing at 300K and amorphized, for the activation of plastic mechanisms under tensile and compressive strains. We show that the onset of crack formation identified as wedge-shaped defect on the surface was only observed in fresh-cut NWs in tension. These NWs present high yield strain due to the high symmetry of the surface and the absence of surface defects favoring dislocation nucleation as for the other surface states. This result seems to confirm that the crack formation in nanostructures could be linked to dislocations interactions on intersecting glide planes as experimentally observed rather than direct crack opening.