Optical Properties of Si Nanocrystals Enhanced by Ligands

Compared to bulk silicon, silicon nanocrystals (Si-NCs) show modified properties, such as tunable emission and enhanced radiative rate, as a result of the quantum confinement, surface chemistry and environment. While the effect of quantum confinement is well understood and experimentally confirmed on the hydrogen-capped Si-NCs, the surface effects in Si-NC with other types of ligands can be very complex and hard to predict. In our work, we argue that the surface chemistry, be it ligands and/or shell, can be designed to further improve the radiative rate of the Si-NCs, beyond what is achievable by the quantum confinement alone. Our experimental work shows a number of effects that indicate that in many instances, the core and surface capping cannot be separated, and optical properties cannot be clearly interpreted as “extrinsic” (related to the surface capping agent) or “intrinsic” (related to the core only). To this end, we performed also a detailed theoretical analysis of a number of surface ligands, to identify the role of chemistry and how that improves the optical properties of Si-NCs. Based on these investigations and findings, we realized two main things. Firstly, we argue that one cannot derive a simple rule to predict which type of element or molecule will improve or deteriorate the optical properties, because every individual element added (covalently) to the surface of Si-NC contributes to the electronic density via several mutually dependent effects, such as (i) orbital displacement, (ii) direct contribution of surface species into the density of states close to the bandgap, (iii) charge transfer due to the relative polarity of the surface capping element and Si, or (iv) ligand/matrix induced strain. Secondly, we realized that the k -space projections of the molecular orbitals, i.e., the band structure of the nanocrystal, are an essential and critical tool for investigations of the electronic and optical properties in materials with an originally indirect bandgap, since the surface chemistry in our simulations affects strongly the whole band structure.