Structures and coalescence behavior of size-selected silicon nanoclusters studied by surface-plasmon-polariton enhanced Raman spectroscopy

The structures and coalescence behavior of size-selected, matrix-isolated silicon clusters have been studied using surface-plasmon-polariton (SPP) enhanced Raman spectroscopy. The cluster ions were produced in a laser vaporization source, mass selected then deposited into a co-condensed matrix of Ar, Kr or N₂ on a liquid He cooled substrate. Raman spectra from monodisperse samples of the smaller clusters studied, Si₄, Si₆ and Si₇, show sharp, well-resolved, vibrations which are in good agreement with predictions based on ab initio calculations. From these comparisons we confirm that Si₄ is a planar rhombus, and assign Si₆ as a distorted octahedron and Si₇ as a pentagonal bypyramid. Si₅ depositions down to 5 eV did not reveal a measurable Raman spectrum under our experimental conditions. Evidence for cluster-cluster aggregation (or fragmentation) was observed under some conditions, even for a "magic number" cluster such as Si₆. The spectra of the aggregated small clusters were identical to those observed for directly deposited larger cluster "bands," such as Si_25-35. The Raman spectra of the aggregated clusters bear some similarity to those of bulk amorphous silicon. Cluster-deposited thin films were prepared by sublimating the matrix material. Even under these "soft landing" conditions, changes in the Raman spectrum are observed with the thin films showing even greater similarity to amorphous silicon.