ANNEALING OF SILICON CLUSTERS

Cluster growth often occurs by sequential addition of atoms, a process which may not result in the lowest energy geometric structure. In this paper we describe experimental methods that have been developed to anneal the clusters, estimate activation energies for annealing, and estimate the relative stabilities of the isomers. The effects of annealing were investigated using the chemical reactivity of the clusters as a probe. Annealing does not appear to dramatically alter the structure of silicon cluster ions containing up to 50 atoms, but changes the relative abundances of the structural isomers that are present. For some clusters the more reactive isomer is the more thermodynamically stable and its relative abundance increases as these clusters are annealed. This indicates that the chemical reactivity of the silicon clusters is not related to their thermodynamic stability. For the larger clusters studied (n > 25) the estimated activation energies for interconversion between the different structural isomers are between 1.0 and 1.5 eV and do not vary systematically with cluster size. It was not possible to anneal most of the clusters to a single structure, suggesting that there are only small differences between the stabilities of the isomers. The stability differences, estimated from the experimental results, are typically approximately 0.2 eV.