Theoretical investigation of size and shape effects on the melting temperature of ZnO nanostructures

We report a theoretical investigation concerning the melting temperature, T-m, of ZnO and Zn nanoparticles ( NPs), nanowires ( NWs) and nanotubes ( NTs). The shapes considered here for the zinc oxide low dimensional structures include spherical NPs, NWs with circular, rectangular ( nanobelts) and hexagonal sections and NTs with circular and hexagonal sections. A comparison between ZnO and Zn nanostructures demonstrates a higher stability of ZnO for most size and shape ranges considered. Moreover, the size effect on the melting temperature for ZnO is found to be quite strong: for a spherical ZnO NP with a radius of 5 nm, the size effect on T-m corresponds to a decrease of similar to 36% relative to the bulk melting temperature, whereas the reduction for the case of a metallic Zn NP with the same dimension is similar to 13%. Based on T-m estimations as a function of size and shape, we predict that certain ZnO nanostructures, such as small (< 10 nm) NTs, may not be viable for nanoelectronics or nanophotonic devices, since T-m is too close to, or in some cases even below, room temperature. The influence of the surface tension uncertainties on the calculated melting temperatures is also discussed. Finally, based on the determination of T-m at the nanoscale, the maximal intrinsic residual stress in a hexagonal ZnO NW and in a cylindrical Zn NW is estimated to be similar to 45 MPa and similar to 1.9 GPa, respectively.