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