Excitonic transitions in silicon nanostructures probed by time-resolved photoluminescence spectroscopy

Time-resolved photoluminescence spectroscopy has been utilized to reveal excitonic transitions in silicon nanowires and silicon nanocrystals. While previous works on porous silicon and silicon nanocrystals have shown a two-level splitting, e.g., singlet-triplet states, our measurements reveal the fine structure of the excitons including three semi-bright states and a ground dark excitonic state. Surprisingly, for silicon nanowires we have found the slowest semi-dark exciton to fall above the faster semi-bright excitonic state as opposed to silicon nanocrystals. The results are analyzed in terms of spin and orbital selection rules showing that the interchange in the level's hierarchy corresponds to a swap between spin-forbidden and orbitally-allowed states. We assign this surprising phenomenon to the relationship between the exchange interaction and the direct Coulomb interaction, which is affected by the dimensionality of the exciton.