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 N-2 on a liquid He cooled substrate. Raman spectra from monodisperse samples of the smaller clusters studied, Si-4, Si-6 and Si-7, show sharp, well-resolved, vibrations which are in good agreement with predictions based on ab initio calculations. From these comparisons we confirm that Si-4 is a planar rhombus, and assign Si-6 as a distorted octahedron and Si-7 as a pentagonal bypyramid. Si-5 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 Si-6. The spectra of the aggregated small clusters were identical to those observed for directly deposited larger cluster ``bands,{''} such as Si25-35. 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. (C) 1999 American Institute of Physics. {[}S0021-9606(99)70521-0].