Restructuring tungsten thin films into nanowires and hollow square cross-section microducts
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Restructuring tungsten thin films into nanowires and hollow square cross-section microducts Prahalad M. Parthangal University of Maryland, College Park, Maryland 20742; and National Institute of Standards and Technology, Gaithersburg, Maryland 20899
Richard E. Cavicchi, Christopher B. Montgomery, and Shirley Turner National Institute of Standards and Technology, Gaithersburg, Maryland 20899
Michael R. Zachariaha) University of Maryland, College Park, Maryland 20742; and National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 3 February 2005; accepted 7 June 2005)
We report on the growth of nanowires and unusual hollow microducts of tungsten oxide by thermal treatment of tungsten films in a radio frequency H2/Ar plasma at temperatures between 550 and 620 °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 ∼0.5 m and lengths between 10 and 200 m were observed. Presence of a reducing gas such as H2 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 °C in air, exhibited the capability of sensing trace quantities of nitrous oxides (NOx ). Growth of nanostructures, such as nanowires,1,2 nanotubes,3,4 nanobelts,5 etc., has attracted tremendous research interest recently. Due to their nearly onedimensional structure, they possess unique electrical, thermal, optical, and mechanical properties, which may be exploited in a variety of applications. Advances in the growth of such high aspect ratio structures have been hampered by difficulties in growing them with controllable dimensions, morphology, and phase purity. The most common techniques to fabricate these nanostructures include e-beam lithography,6 solution-phase synthesis,7 vapor phase evaporation/condensation,8 and templatedirected synthesis.9,10 In the case of nanowires, transition metals like tungsten find applications in electronic devices, sensors, and magnetic recording devices.11,12 Very recently, Lee et al.13 reported the growth of tungsten nanowires of less than 100 nm in diameter and about 1 m in length by thermal treatment of tungsten films in the presence of H2, and demonstrated excellent a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0373 J. Mater. Res., Vol. 20, No. 11, Nov 2005
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field-emission properties. Gu et al.14 grew tungsten oxide nanowires on metal tungsten tips (prepared by electrochemical etching of tungsten wires) heated in argon. They observed tungsten oxide nanowires b
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