Direct to indirect band gap transition in ultrathin ZnO nanowires under uniaxial compression

The direct to indirect band gap transition in ultrathin [0001] ZnO nanowires with the structural transformation from the regular wurtzite structure to a more close-packed hexagonal structure during uniaxial compression is studied by using the first-principles calculations. The results show that all ZnO nanowires exhibit direct band gap in wurtzite structure and indirect band gap in hexagonal structure. For the same wire the band gap in hexagonal structure is smaller than that in wurtzite structure. The origin of the band gap transition from direct to indirect one is discussed.