Significant “smaller is softer” in amorphous silicon via irradiation-mediated surface modification

“Smaller is softer” is a reverse size dependence of strength, defying the “smaller is stronger” tenet. It usually results from surface-mediated displacive or diffusive deformation and is mainly found in some ultra-small-scale (below tens of nanometers) metallic materials. Here, making use of the surface modification via ion beam irradiation, we bring the “smaller is softer” into being in a covalently-bonded, hard, and brittle material-amorphous Si (a-Si) at a much larger size regime (< ∼500 nm). It is manifested as the transition from the quasi-brittle failure to the homogeneous plastic deformation as well as the decreasing yield stress with sample volume reduction at the submicron-scale regime. An analytical model of hard core/superplastic shell has been proposed to explain the artificially-controllable size-dependent softening. This surface engineering pathway via ion irradiation is not only of particular interest to tailor the strength and deformation behaviors in small-sized a-Si or other covalently-bonded amorphous solids but also of practical relevance to the utility of a-Si in microelectronics and microelectromechanical systems.