Interatomic force fields for silicon microclusters

We define an interatomic potential for silicon. As with previous work, this potential is based on bulk interactions that are adjusted to describe ‘‘covalent→metallic’’ phase transitions instead of small-amplitude atomic vibrations. It includes the transfer of bond strength from dangling bonds to back bonds. However, this potential has been slightly modified to reduce its range. With the modified potential we determine the energies and structural properties of ${\mathrm{Si}}_{\mathit{n}}$, where n≤30. For n≤10, we find this potential leads to a significant improvement over previous work for both the binding energies and the bond lengths of these clusters when compared with quantum-mechanical methods. For 10n≤20 we find as before, and in agreement with experiment, that ${\mathrm{Si}}_{\mathit{n}}$ clusters follow an icosahedral pentagonal growth sequence with n=13 and 19 being special structures. For 20n≤30 we find this growth sequence is weakened, but a general pentagonal sequence is retained. We examine the role of back-bond strengthening by varying the strength of the corresponding interaction. We find that with increasing back-bond strength a ‘‘first-order’’ phase transition occurs that mimics the bulk ‘‘covalent→metallic’’ transition. The ability to vary this interaction will allow us to examine intrinsic differences in the nucleation of covalent versus metallic clusters.