Boundary and Symmetry Determined Exciton Distribution in Two Dimensional Silicon Nanosheets

Abstract

The size-dependent optical properties of silicon nanosheets (SiNSs) are investigated using the time-dependent density functional based tight-binding method (TD-DFTB). Evidence for the formation of self-trapped exciton states in SiNSs is provided by examining the localization properties in terms of the characteristic size of the electronic excitations. We show that the frontier molecular orbitals in the first excited state are highly localized in the central stretched Si–Si bond of the NSs due to structural relaxation, which leads to a significant red-shift of the optical gaps. For SiNSs, the boundary length and local structure of the central section in SiNSs influence the spatial extent of self-trapped excitons in the first excited state. The first excited states are observed to be spatially less localized for SiNSs with a longer boundary length. The distribution of electronic density perturbation is diverse in the case of central Si6 or a single Si–Si bond, thereby leading to different spatial confinement of structural relaxation. In contrast with zero- and one-dimensional Si nanomaterials, the two-dimensional SiNSs show a highly localized exciton when the width is less than 2 nm, which suggests it as a candidate for exploring the characteristics of exciton self-trapping.