Strain-induced structural and direct-to-indirect band gap transition in ZnO nanotubes

First-principles calculations have been employed to investigate the structural transformation and direct to indirect band gap transition of ZnO nanotubes under uniaxial strain. The results show that armchair and zigzag nanotubes can be transformed to each other via unusual fourfold-coordinated structures under the applied strain. Both the armchair and zigzag nanotubes exhibit direct band gap while the unusual fourfold-coordinated ones display indirect band gap. The origin of such a direct-to-indirect band gap transition is explained based on the analyses of atomic orbital contributions.