Illuminating the photomechanical effect: Exploring the relationship between electron-hole excitation and deformation mechanisms in nanotwinned boron carbide

Abstract

The photomechanical effect, a phenomenon in which electron-hole (e-h) excitation from light illumination influences the plastic deformation of semiconductors, remains poorly understood at the microscopic level. In this study, we employ constrained density-functional theory simulations to investigate the interaction between e-h excitation and twin boundaries in nanotwinned boron carbide (${\mathrm{B}}_{4}\mathrm{C}$) and their impact on deformation mechanisms. Our findings reveal that excited e-h pairs reduce both the critical displacement (or critical shear strain) and shear strength of nanotwinned ${\mathrm{B}}_{4}\mathrm{C}$ under shear-induced failure. This reduction stems from the redistribution of electrons and holes near the twin boundary, weakening the B--C bond connecting adjacent icosahedra and leading to cage destabilization and failure. Notably, e-h excitation alters the deformation mechanism of asymmetric twins while exacerbating that of symmetric twins. These simulation results offer a physical explanation for the observed softening effects in nanotwinned materials under e-h excitation, thereby contributing to the understanding of the photomechanical effect.