Electronic Structure Engineering Achieved via Organic Ligands in Silicon Nanocrystals

A class of important semiconductors, such as Si, Ge, or C, has an indirect band gap, which critically limits their optical properties. Lack of efficient emission is especially unfortunate for silicon, where Si light sources could enable realization of the long-awaited on-chip-integrated Si laser for an integrated optical computing CPU architecture. Hence, methods toward the improvement of optical properties of Si-based materials are in high demand. Unlike most of the applied light-emitting semiconductor nanocrystals (NCs) with a direct band gap, the radiative rate in covalent silicon NCs (SiNCs) is size-dependent but remains low even for the smallest SiNCs. Additionally, the radiative rate is also ligand-sensitive, and the covalent bond with ligands is very rigid and static and could be, in principle, used for straining via steric hindrance, further influencing the radiative rates. In this work, we use the self-consistent density functional theory (DFT) simulation together with a "fuzzy"band-structure concept to show the effect of covalently bonded ligands on the electronic structure of NCs and their k - -space projection. For instance, in 2 nm large SiNCs with C-linked organic ligands, we demonstrate that radiative rates can be manipulated by ligands to a considerable extent through an intricate interplay between charge transfer from the core to the ligand, orbital delocalization, and strain by steric hindrance. We propose that the tunability of electronic properties achieved via ligands in covalent systems offers a possible direction toward the design of an ideal Si light-emitting system.