First-principles study of the dependence of ground-state structural properties on the dimensionality and size of ZnO nanostructures

The dimension- and size-dependent ground-state properties of ZnO systems are investigated through first-principles density-functional theory calculations. It is found that the effective elastic constants C33 of polar surface terminated ZnO nanofilms and [0001] oriented ZnO nanowires increase with structural size. For nanofilms, the effective C33 quickly approaches the bulk value and becomes almost stable when the film contains over six Zn-O double layers. For nanowires, the effective C33 is as small as around 16% of the corresponding bulk value when the diameter is very small, and it increases almost linearly to 88% of the bulk value when the diameter reaches about 2.4 nm. For ZnO nanoclusters with hexagonal prism structure, the original Zn-O double layers merge into single layers after relaxation. A shape-driven phase transition from the four-coordinate wurtzite to the six-coordinate rocksalt structure is found in a ZnO cluster with 48 atoms. Finally, a systematic energy analysis of all the above structures shows that the cohesive energies of the ZnO structures increase with both dimension and size.