Gold-catalyzed low-temperature growth of cadmium oxide nanowires by vapor transport.

Ultralong cadmium oxide nanowires were synthesized in high yield on gold-coated silicon substrates by using a vapor transport process. Cadmium vapor generated by the carbothermal reduction of CdO powder in a tube furnace heated to 500 degrees C was carried to the substrate zone by an argon flow with a trace amount of oxygen. The CdO nanowires grew via a vapor-liquid-solid growth mechanism. The diameters of the nanowires are approximately 40-80 nm, and can reach lengths of 30-50 mum. Because the nanowire formation was gold particle catalyzed, patterned nanowire growth on substrates can be achieved. These nanowires grew along the [111] direction and have slightly rough surfaces due to the presence of crystalline CdO shells formed via a physical vapor deposition process. Interesting CdO nanowires with a necklace-like morphology were also observed in a small region of the substrate, where the oxygen supply may be ample to facilitate the lateral growth of rhombohedron-shaped crystals over the straight wires. Electron diffraction and high-resolution TEM results suggest that these side crystals should grow epitaxially on the wire surfaces. The band gap of the CdO nanowires with smoother surfaces was determined to be approximately 2.53 eV. These nanowires exhibit a relatively weak emission band centered at approximately 550 nm.