Low-frequency Raman scattering of Ge and Si nanocrystals in silica matrix

Ge and Si nanocrystals (nc-Ge and nc-Si) with average sizes in the range of 2–7nm, embedded in silica matrix, were fabricated for investigating their acoustic-phonon vibrational properties. The freely elastic sphere theory was found to be unsuitable for explaining the low-frequency phonon vibration character of both nc-Ge and nc-Si in our current experiments. We have assigned the observed low-frequency Raman peaks to “LA-like mode” and “TA-like mode” in terms of their polarization and depolarization behaviors. In addition, it is revealed that the lattice contraction phenomenon exists in nc-Ge and nc-Si with sizes smaller than 4nm, which leads to a contrary effect against matrix traction on the phonon vibrational frequencies.