Surface-induced polarity inversion in ZnO nanowires

In this work we use ab initio calculations to address the polar behavior of ZnO nanowires. Moving from a description based on Wannier functions, we employ a computational approach that allows one to express the polarization of a nanostructure in terms of local contributions. In particular, we discuss the changes in the nanostructure polarity in terms of two contributions, one related to changes in the equilibrium lattice parameters and the other related to surface effects. The former contribution is also interpreted on the basis of piezoelectric constants. Surprisingly, we find that for the smallest nanostructures, the average dipole is opposite to that of an infinite bulk structure.