Restructuring tungsten thin films into nanowires and hollow square cross-section microducts

We report on the growth of nanowires and unusual hollow microducts of tungsten oxide by thermal treatment of tungsten films in a radio frequency H-2/Ar plasma at temperatures between 550 and 620 degrees C. Nanowires with diameters of 10-30 nm and lengths between 50 and 300 nm were formed directly from the tungsten film, while under certain specific operating conditions hollow microducts having edge lengths similar to 0.5 mu m and lengths between 10 and 200 mu m were observed. Presence of a reducing gas such as H-2 was crucial in growing these nanostructures as were trace quantities of oxygen, which was necessary to form a volatile tungsten species. Preferential restructuring of the film surface into nanowires or microducts appeared to be influenced significantly by the rate of mass transfer of gas-phase species to the surface. Nanowires were also observed to grow on tungsten wires under similar conditions. A surface containing nanowires, annealed at 500 degrees C in air, exhibited the capability of sensing trace quantities of nitrous oxides (NOx).