Thickness-dependent atomic structures of two-dimensional few-layer ZnO: A density functional theory study

The thickness-dependent atomic structures of two-dimensional (2D) few-layer (FL) ZnO are systematically investigated by the first-principles calculations. It is found that the structural transformation between thinner FL ZnO with graphitic structure (FL gZnO) and thicker FL ZnO with wurtzite structure (FL wZnO) takes place at the critical thickness of 9-12 Zn-O atomic layers. At the thickness of 9-12 layers, both graphitic and wurtzite structures can coexist at room temperature. In FL gZnO, the interlayer interaction is a long-range Coulomb interaction, and the charge population of Zn and O inside does not change during the structural transformation. Moreover, we demonstrate that the structural transformation of FL ZnO originates from the competition between the high energy of the O 2pz orbital in the graphitic structure and the polar-surface-induced dipole energy in the wurtzite structure. Our microscopic understanding guides a clear direction of regulating the atomic structure of FL ZnO, further optimizing its electronic properties, which benefits developing function-advanced 2D stacked devices.