Size-dependent Phase Transformation and Fracture of ZnO Nanowires

The structural and mechanical properties of ZnO nanowires (NWs) have been systematically investigated by using molecular dynamic simulations based on the empirical Buckingham potential. Under tensile loading in <0001> direction, ZnO NWs undergo four-stage deformation: elastic stretching of initial Wurtzite structure, Wurtzite to body-centered tetragonal (BCT) phase transformation, stretching of the resulting BCT structure and eventually brittle fracture. The entire deformation process is significantly size dependent. As the NW size decreases, the Young's modulus dramatically increases. The critical stress for both phase transformation and fracture decreases while the critical strain increases with increasing the NW size; both converge to constant values when the size is sufficiently large. The strain energy density for the initiation of phase transformation appears independent of the size, which implies that the size-dependent phase transformation is dominated by the size effect of the Young's modulus.