Synthesis and temperature-dependence of hydrogen-terminated silicon clusters

A novel laser photolysis source has been used to synthesize hydrogen-terminated silicon clusters SinHx+ where n less than or equal to 50. Using mass spectrometry, the hydrogen content of the SinHx+ clusters with n = 10-30 was measured as a function of temperature from 300 to 950 K. For n = 10-22 at the lower temperatures, there is a broad distribution of [H]/[Si] ratios which peak around compositions expected for clusters with bulklike silicon cores (x/n > 1). SinHx+ clusters with x/n congruent to 1 appear to be favored at intermediate temperatures. This stoichiometric composition suggests three-dimensional, SinHn+ cage-like structures built from Si-H units. The stability of the cage geometries is supported by density functional calculations. For cluster cations with n greater than or equal to 23 an abrupt transition occurs and there is a sharp drop in the hydrogen content at the lower temperatures. For these clusters, SinH(n-y)+ (y = 1-5) compositions are prominent at the higher temperatures. These compositions suggest cage-like geometries with one or more internal silicon atoms. By adjusting the source conditions it is possible to generate SinHx+ (n greater than or equal to 23) clusters with a narrow distribution of x (less than two hydrogens wide).