Exceptionally Bright Visible-wavelength Luminescence from Sulfur-doped ZnO Nanowires

Sulfur-doped ZnO nanowires have been grown by the vapor-liquid-solid technique using ZnS and carbon as starting materials. The broadband, visible-wavelength emission of unprecedented brightness from these nanowires is characterized by steady-state and time-resolved spectroscopy. Energy transfer is explored by simultaneously studying the fast ($<$50 ps) decay of band edge emission and the slow ($>$5 ns) decay of the visible-wavelength emission as a function of temperature and excitation intensity. The contributions of sulfur doping and nanostructuring to the generation of this intense visible-wavelength emission are clarified by characterizing ZnO nanowires and micropowders of different morphologies and dopant concentrations. The results can be understood in terms of a physically motivated rate equation model, for which several of the key parameters are experimentally constrained.