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 $(\mathrm{FL} g\mathrm{ZnO})$ and thicker FL ZnO with wurtzite structure $(\mathrm{FL} w\mathrm{ZnO})$ 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 $g\mathrm{ZnO}$, 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 $2{p}_{z}$ 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.