Vertically aligned ZnO nanowires prepared by thermal oxidation of RF magnetron sputtered metallic zinc films

ZnO nanowires have been successfully grown by thermal oxidation of metallic zinc films at 430 °C. Polycrystalline zinc films were deposited on Si (100) substrates by RF magnetron sputtering utilizing discharge power from 70 to 180 W. Experimental results show that 70 W discharge power results in the formation of porous zinc nanoparticles that prevent zinc atom from diffusion and thus does not result in the formation of ZnO nanowires by subsequent thermal oxidation. By increasing discharge power to 120 W the zinc film transforms to Zone II with a columnar structure, while further increase in discharge power to 180 W results in re-crystallization and formation of micron-sized hexagonal structures on the surface. Vertically aligned ZnO nanowires can only be obtained by thermal oxidation of columnar zinc films that exhibit a field emission threshold of 5.3 V/μm (at a current density of 10 μA/cm2) with a field enhancement factor of 1834. A target current density of 0.75 mA/cm2 is achieved with a bias field less than 10 V/μm.