PROPERTIES OF DEPOSITED SIZE-SELECTED CLUSTERS - REACTIVITY OF DEPOSITED SILICON CLUSTERS

The room temperature oxidation of deposited size-selected silicon clusters (Si(n), n=10, 13, and 40-50) has been examined using x-ray photoelectron spectroscopy (XPS). The size-selected clusters were deposited as positive ions on an amorphous carbon substrate at 5 eV. Silicon cluster sticking probabilities (determined from Rutherford backscattering measurements) were 95% +/- 5%. XPS spectra were recorded both before and after exposure to O2. The unoxidized clusters showed no significant (> 0.2 eV) core levels shifts relative to bulk silicon. Oxygen adsorption was monitored by following the evolution of the O1s and Si2p core levels. While the results are qualitatively similar to those obtained for bulk silicon surfaces, the O2 sticking coefficients are much smaller (approximately 0.001 compared to approximately 0.1). The O2 sticking coefficients are similar to those observed for the larger clusters in the ps phase, however, the spectacular variations in the reactivity of the smaller clusters in the gas phase are not reproduced. All the deposited clusters have similar O2 sticking coefficients. A number of observations suggest that there are substantial cluster-cluster interactions on the substrate (possibly leading to coalescence and sintering).