Atomic-scale origin of piezoelectricity in wurtzite ZnO

Abstract

ZnO has been extensively studied by virtue of its remarkably high piezoelectric responses, especially in nanowire forms. Currently, the high piezoelectricity of wurtzite ZnO is understood in terms of the covalent-bonding interaction between Zn 3d and O 2p orbitals. However, the Zn 3d orbitals are not capable of forming hybridized orbitals with the O 2p_z orbitals since the Zn ion is characterized by fully filled non-interacting 3d orbitals. To resolve this puzzling problem, we have investigated the atomic-scale origin of piezoelectricity by exploiting density-functional theory calculations. On the basis of the computed orbital-resolved density of states and the band structure over the Γ–M first Brillouin zone, we propose an intriguing bonding mechanism that accounts for the observed high piezoelectricity – intra-atomic 3d_z²–4p_z orbital self-mixing of Zn, followed by asymmetric hybridization between the Zn 3d_z²–4p_z self-mixed orbital and the O 2p_z orbital along the polar c-axis of the wurtzite ZnO.