Optical properties of rectangular cross-sectional ZnS nanowires

ZnS nanowaveguides with rectangular cross-section (∼50 × 50 nm²) and tens of microns in length have been synthesized by pulsed laser vaporization of ZnS/10% Au targets in a flow of Ar/5% H₂. The highly crystalline filaments exhibit the wurtzite structure, growing mainly along the [001] or [100] directions. Photoluminescence at room temperature shows strong near-edge luminescence doublets (∼3.75 eV and 3.68 eV) and a weak defect luminescence structure attributed to stoichiometric defects and possibly to Au impurities. Optical absorption (OA) at room temperature shows a strong broadening of the fundamental direct absorption edge identified with stoichoimetric defects. Two peaks (3.75 and 3.85 eV) in the OA are also observed. We believe that the structure in the photoluminescence and optical absorption (3.68, 3.75, 3.85 eV) are from direct transitions between the conduction band and the spin-orbit/crystal field splitting of the valance bands. Theoretical results are also presented that show the size-dependence of the band gap in ZnS nanowires.