The Structures and Properties of Silicon Clusters in the Shape Transition Region

Koblar Jackson

Physics Department, Central Michigan University

The abrupt, prolate to spherical shape transition in silicon clusters over the intermediate size range is an outstanding example of the sensitive size dependence of properties in sub-nanometer-sized clusters. Using a new “big bang” optimization method and extensive first-principles density functional theory (DFT) calculations, we recently determined ground state structures for the clusters spanning this transition, confirming the structures through comparisons of calculated and measured ion mobilities and dissociation energies. Using these structures, we study the evolution of a variety of cluster properties over this range. First, we show that the prolate structures fall into two distinct families, moderately prolate and “stretched” structures, in precise agreement with the ion mobility data (Hudgins et al., J. Chem. Phys. 111, 7865 (1999)). We also show that calculated ionization energies for the lowest-energy neutral clusters agree with the measurements of Fuke et al. (J. Chem. Phys. 99, 7807 (1993)), including the precipitous drop in IP that occurs at n=23. Finally, we examine the electronic properties of the clusters, showing that the prolate structures have systematically larger band gaps, dipole moments and polarizabilities than the spherical clusters. Interestingly, cluster size trends in the polarizability indicate a metal-like response to external electric fields. Taken as a whole, these properties give significant insight into the physical and chemical trends of semiconductor clusters in the intermediate size range.