Photoluminescence study of ZnO nanotubes under hydrostatic pressure

Photoluminescence of ZnO single crystal nanotubes grown on sapphire substrate by metal organic chemical vapor deposition has been studied as a function of applied hydrostatic pressure using the diamond-anvil-cell technique. The photoluminescence spectra of the ZnO nanotubes at atmospheric pressure are dominated by strong near-band-edge Gamma(FX) and Gamma(BX) excitonic emission lines accompanied by a weak broad deep-level (DL) emission band. The pressure-induced shifts of all observed emission lines are followed up to 15 Gpa, when ZnO nanotubes undergo a phase transition from a direct-gap wurtzite structure to an indirect-gap rocksalt structure. The Gamma(FX) emission is found to shift toward higher energy with applied pressure at a rate of 29.6 meV/GPa, which provides a method to measure the pressure coefficient of the direct Gamma band gap in the wurtzite ZnO nanotubes. The Gamma(BX) emission has a pressure coefficient of 21.6 meV/GPa, about 30% smaller than that of the ZnO band gap, which suggests that it might originate from the radiative recombination of the excitons bound to donorlike deep centers rather than shallow donors. The pressure coefficient of the broad DL emission band in ZnO tube is 16.8 meV/GPa, which indicates that the initial states involved in the emission process are deep localized states.