Magic Numbers in a One-Dimensional Nanosystem: ZnS Single-Walled Nanotubes

The structure and stability of a range of zigzag (n,0) and armchair (n,n) ZnS SWNTs of increasing diameter are investigated theoretically using density functional theory. Zigzag and armchair SWNTs are found to be most energetically stable when considerably structurally reconstructed with respect to the corresponding metastable relatively smooth SWNTs. A strong relation between the average Zn-S(Zn)-Zn dihedral angles in the SWNTs and their relative energetic stability points to the out-of-plane tendency of the highly polarizable S anions as the driving force behind the observed distortions. The internal energetic strain in reconstructed armchair ZnS SWNTs is found to gradually monotonically decrease toward that in a reconstructed single 2D ZnS sheet with increasing diameter. This tendency follows that in previously reported calculations for both armchair and zigzag SWNTs of C, BN, MoS2, and TiO2 For reconstructed zigzag ZnS SWNTs, however, the general decrease in energetic strain with increasing diameter possesses clear oscillations, with (3a,0) SWNTs possessing the highest stability and largest band gap. We compare this discrete pattern of stability with that often encountered for particularly stable nanoclusters possessing magic numbers of atoms. As far as we are aware, this is the first report of magic numbers being predicted to occur in a 1D nanosystem.