Coordination imperfection enhanced electron–phonon interaction and band-gap expansion in Si and Ge nanocrystals

Correlation between the size-enlarged Stokes shift and band-gap expansion of Si and Ge nanocrystals has been investigated in terms of the bond order–length–strength correlation. It is shown that the bond order deficiency of surface atoms dictates electron–phonon coupling and crystal binding, and enhances the size dependence of the observed blue shift in photoemission and photoabsorption of Si and Ge nanocrystals. Compared with Si, Ge nanocrystal exhibits stronger electron–phonon coupling due to larger phonon-induced deformation of the crystal lattice.