The formation of new silicon cages: a semiempirical theoretical investigation

A systematic theoretical study on the Si-n (n = 26-36, 60) cages using semiempirical methods is herein presented. Equilibrium geometries, harmonic vibrational frequency analyses, enthalpies of formations, HOMO-LUMO energy gaps and other properties are calculated at the level of the AM1 theory. Present theoretical results show that all the Si-n (n = 26-36, 60) cage isomers exhibiting maximum symmetry undergo slight distortions into more stable structures of lower symmetry. No simple function for the clusters stability with respect to the number of Si atoms can be discerned in the whole Si-n (n = 26-36, 60) series. However, the stability in the Si-n (n = 33-36) series decreases as the number of Si atoms drops whereas the Si-n (n = 26-32) series shows the same trend with the exception of n 28 and 31. Several instances of four-fold coordinated, distorted sp(3) hybridized Si atoms carrying large net charges have been detected. Calculated HOMO-LUMO energy gaps of these Si-n cages lie in the range of 3.9-4.6 eV, being larger than those of both smaller Si-n clusters and bulk Si. Comparison with available theoretical and experimental data give firm support to the present application of semiempirical methods to large Sin clusters. (C) 2003 Elsevier Science B.V. All rights reserved.