Absorption and Photoluminescence of Silicon Nanocrystals Investigated by Excited State DFT: Role of Embedding Dielectric and Defects

Absorption and photoluminescence (PL) properties of silicon (Si) nanocrystals (NCs) covered with silicon dioxide ((Formula presented.)) are fairly well unraveled; corresponding information for silicon nitride ((Formula presented.)) coverage is scarce. We elucidate important optical and electronic features depending on the embedding dielectric and interface defect (dangling bond, DB) properties. Using density functional theory (DFT) and time-dependent (TD-) DFT for ground state (GS) and excited state (ES) properties, respectively, we compute fully (Formula presented.) - and OH-covered NCs of 11–26 Å size, enabling comparisons with experimental data. Our non-radiative Shockley-Read-Hall (SRH) recombination model of DBs at NC/dielectric interfaces demonstrates that SRH recombination is substantially higher for (Formula presented.) -covered NCs. An ensemble TD-DFT calculation of the eight lowest fundamental transitions accurately describes the absorption edge. Exciton binding energies are significantly smaller in (Formula presented.) - versus (Formula presented.) -covered NCs due to the delocalizing versus self-localizing impact of the dielectric onto the exciton. We find higher optical absorption rates for (Formula presented.) -embedded NCs versus (Formula presented.) -embedded NCs. However, SRH interface recombination renders the PL of (Formula presented.) -embedded NCs inferior to their (Formula presented.) -embedded counterparts. Finally, we explain a discrepancy in PL gaps of free-standing oxidized versus (Formula presented.) -embedded NCs by considering adequate phononic boundary conditions.