Electronic structure and luminescence of 1.1-and 1.4-nm silicon nanocrystals: Oxide shell versus hydrogen passivation

The difference between oxide and hydrogen passivation of small Si nanocrystals is explored by all-electron, hybrid functional DFT calculations with unrestricted geometry optimization. Oxide passivation lowers the band gap by about 2.4 eV for Si-35 cores and by about 1.5 eV for Si-65 cores. The oxide-passivated nanocrystals have optically forbidden, indirect-gap-type transitions whereas the hydrogen-passivated nanocrystals have optically allowed, direct-gap-type transitions. The HOMO and LUMO are delocalized in both species. This result explains the experimental observation that hydrogen-passivated Si nanocrystals luminesce in the blue whereas oxide-passivated Si nanocrystals luminesce in the yellow-red.