Orientation and size dependence of the elastic properties of zinc oxide nanobelts

Molecular dynamics simulations are performed to characterize the response of zinc oxide (ZnO) nanobelts to tensile loading. The ultimate tensile strength (UTS) and Young's modulus are obtained as functions of size and growth orientation. Nanobelts in three growth orientations are generated by assembling the unit wurtzite cell along the [0001], [01 (1) over bar0], and [21 (1) over bar0] crystalline axes. Following the geometric construction, dynamic relaxation is carried out to yield free-standing nanobelts at 300 K. Two distinct configurations are observed in the [0001] and [01 (1) over bar0] orientations. When the lateral dimensions are above 10 angstrom, nanobelts with rectangular cross-sections are seen. Below this critical size, tubular structures involving two concentric shells similar to double-walled carbon nanotubes are obtained. Quasi-static deformations of belts with [2 (1) over bar(1) over bar0] and [01 (1) over bar0] orientations consist of three stages, including initial elastic stretching, wurtzite-ZnO to graphitic-ZnO structural transformation, and cleavage fracture. On the other hand, [0001] belts do not undergo any structural transformation and fail through cleavage along (0001) planes. Calculations show that the UTS and Young's modulus of the belts are size dependent and are higher than the corresponding values for bulk ZnO. Specifically, as the lateral dimensions increase from 10 to 40 angstrom, decreases between 38-76% and 24-63% are observed for the UTS and Young's modulus, respectively. This effect is attributed to the size-dependent compressive stress induced by tensile surface stress in the nanobelts. [01 (1) over bar0] and [21 (1) over bar0] nanobelts with multi-walled tubular structures are seen to have higher values of elastic modull (similar to 340 GPa) and UTS (similar to 36 GPa) compared to their wurtzite counterparts, echoing a similar trend in multi-walled carbon nanotubes.