Surface Effects on the Piezoelectricity of ZnO Nanowires

TL;DR

This study uses molecular dynamics to examine how surface treatments affect the piezoelectric properties of ZnO nanowires, revealing that proper surface treatment reduces piezoelectricity and miniaturization alone does not enhance it.

Contribution

It demonstrates that surface treatments significantly influence the piezoelectric properties of ZnO nanowires and that miniaturization does not necessarily improve piezoelectric performance.

Findings

Surface treatments reduce piezoelectric constants in ZnO nanowires.

Piezoelectric properties are diminished due to bond length reduction near surfaces.

Miniaturization alone does not enhance ZnO nanowire piezoelectricity.

Abstract

We utilize classical molecular dynamics to study surface effects on the piezoelectric properties of ZnO nanowires as calculated under uniaxial loading. An important point to our work is that we have utilized two types of surface treatments, those of charge compensation and surface passivation, to eliminate the polarization divergence that otherwise occurs due to the polar (0001) surfaces of ZnO. In doing so, we find that if appropriate surface treatments are utilized, the elastic modulus and the piezoelectric properties for ZnO nanowires having a variety of axial and surface orientations are all reduced as compared to the bulk value as a result of polarization-reduction in the polar [0001] direction. The reduction in effective piezoelectric constant is found to be independent of the expansion or contraction of the polar (0001) surface in response to surface stresses. Instead, the surface polarization and thus effective piezoelectric constant is substantially reduced due to a reduction in the bond length of the Zn-O dimer closest to the polar (0001) surface. Furthermore, depending on the nanowire axial orientation, we find in the absence of surface treatment that the piezoelectric properties of ZnO are either effectively lost due to unphysical transformations from the wurtzite to non-piezoelectric d-BCT phases, or also become smaller with decreasing nanowire size. The overall implication of this study is that if enhancement of the piezoelectric properties of ZnO is desired, then continued miniaturization of square or nearly square cross section ZnO wires to the nanometer scale is not likely to achieve this result.