Role of surface charge distribution on band-structure and optical properties of silicon nanocrystals

Quantum confinement effects in silicon nanostructures have been studied over the past 3 decades [1], with aim to convert silicon into direct bandgap-like semiconductor for applications as efficient light emitters, even amplifiers and lasers. We have shown that slightly electronegative ligands on the surface of silicon quantum dot (Si-QD) and/or varying electrostatic field from the environment manipulates the electronic density inside the Si-QD’s core, ultimately resulting in an indirect-to-direct bandgap conversion [2,3]. To test the role of charge distribution induced by ligands and environment experimentally, we synthesize colloidal Si-QDs capped by butyl chains using oxygen-free wet chemical method [3] and introduce two terminations, amine (-NH2) and carboxylic acid (-COOH). Si-QDs are dispersed in aqueous solutions of varying pH. The push-pull effect on the electronic wave-functions translates into changes in band-gap (emission spectrum and absorption band-edge) and emission lifetime. Results are interpreted and discussed within the frame of our theoretical simulations by tight binding and DFT.