  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.
