Excitons in silicon nanocrystallites: The nature of luminescence

The absorption and emission spectra of silicon nanocrystals up to 1 nm diameter have been calculated within a first-principles framework. Our calculations include geometry optimization and the many-body effects induced by the creation of an electron-hole pair. Starting from hydrogenated silicon clusters of different sizes, different Si/O bondings at the cluster surface have been considered. We found that the presence of a Si-O-Si bridge bond causes significant excitonic luminescence features in the visible range that are in fair agreement with experiment.