Ring structures of small ZnO clusters

We report results of an ab initio study of the atomic structures of small zinc oxide clusters (ZnO)nn ⩽ 6 using ultrasoft pseudopotential method and the generalized gradient approximation for the exchange-correlation energy. We optimize several isomers for each size in order to obtain the lowest energy structures and to understand the growth behavior. In all cases ring type structures are found to be most favorable. For n = 5 and 6, the ring structures are not planar suggesting that the bonding nature in these cluster has some covalent character. Small clusters are found to have small highest-occupied–lowest unoccupied molecular orbital (HOMO–LUMO) gap in contrast to the typical behavior of large gaps for small clusters that tend to decrease towards the bulk value with an increase in size.