Finite size effects on the dynamics of amorphous and nanocrystalline materials

Abstract

The dynamics of amorphous and nanocrystalline materials exhibit substantial finite size effects in very small particles. Raman scattering measurements performed in ultrahigh vacuum on ultrathin island films composed of clusters exhibit appreciable changes in the phonon response functions. For amorphous Ge clusters, this is indicated by large changes in the phonon density of states and appreciable sensitivity to surface chemisorption. These effects are associated with a large fraction of cluster dangling bonds and orbital rehybridization. In Bi island films a size-dependent transformation is noted from a nanocrystalline semimetal to an amorphous system for an 8 Å thick film. The vibrational properties of the a-Bi state differ from that of metallic a-Bi thick films, suggesting semiconducting character. For nanocrystalline C thin films prepared by annealing a-C, the Raman spectra indicate new disorder induced scattering associated with finite size effects. The spectra exhibit distinctly different character of amorphous and nanocrystalline carbon of dimensions < 15 Å.