Magic numbers of silicon clusters.

A structural model for intermediate sized silicon clusters is proposed that is able to generate unique structures without any dangling bonds. This structural model consists of bulk-like core of five atoms surrounded by fullerene-like surface. Reconstruction of the ideal fullerene geometry results in the formation of crown atoms surrounded by π-bonded dimer pairs. This model yields unique structures for Si33, Si39, and Si45 clusters without any dangling bonds and hence explains why these clusters are least reactive towards chemisorption of ammonia, methanol, ethylene, and water. This model is also consistent with the experimental finding that silicon clusters undergo a transition from prolate to spherical shapes at Si27. Finally, reagent specific chemisorption reactivities observed experimentally is explained based on the electronic structures of the reagents. PACS numbers: 36.40+d, 61.43.Bn, 61.46.+w, 68.35.Bs, 82.65.My Typeset using REVTEX ∗The Department of Physics, New York University, New York, NY 10003-6621.