First-principles study of confinement effects on the Raman spectra of Si nanocrystals.

The Raman spectra of Si nanocrystals are studied as a function of nanocrystal diameter using pseudopotential density functional theory and the Placzek approximation. Our calculations reproduce the redshift and broadening of the optical Raman peak with decreasing nanocrystal size, and calculated peak frequencies show good agreement with experimental values. We also find that a surface induced softening of vibrational modes is largely responsible for the Raman redshift, with relaxation of momentum conservation playing only a minor role.