First-principles study of ring to cage structural crossover in small ZnO clusters.

Density functional, full-potential computations are performed to study the origin and consequences of the ring to cage structural crossover in (ZnO)(n) (n = 2-16) clusters. The origin of this structural crossover, which is found to occur at n = 10, is studied by investigating the behavior of the Zn-O-Zn bond angle, the Zn-O bond strength, and the number of bonds in the systems. It is argued that 12 is the lowest magic number of ZnO clusters in the ground state, while finite temperature vibrational excitations enhance the relative stability of the (ZnO)(9) cluster to make it a magic system at temperatures above about 170 K. The obtained electronic structure of the clusters before and after applying the many-body GW corrections evidence a size-induced redshift originating from the ring to cage structural crossover in the system. The behavior of the electron density bond points of the clusters along with the extrapolated cluster binding energy at very large sizes may indicate the existence of a metastable structure for large ZnO nanostructures, different from the bulk ZnO structure.