Highly conductive tungsten suboxide nanotubes

We demonstrate a high electron conductivity (>10(2) S/cm and up to 10(3) S/cm) of tungsten suboxide W18O52.4-52.9 (or equivalently WO2.91-2.94) nanotubes (2-3 nm in diameter, similar to mu m long). The conductivity is measured in the temperature range of 120-300 K by a four-probe scanning tunneling microscope in ultrahigh vacuum. The nanotubes are synthesized by a low-temperature and low-cost solvothermal method. They self-assemble in bundles of hundreds of nanotubes forming nanowires (similar to mu m long, few tens nm wide). We observe a large anisotropy of the conductivity with a ratio (longitudinal conductivity/perpendicular conductivity) of similar to 10(5). A large fraction of them (similar to 65%-95%) shows a metallic-like, thermal activation-less electron transport behavior. Few of them, with a lower conductivity from 10 to 10(2) S/cm, display a variable range hopping behavior. In this latter case, a hopping barrier energy of similar to 0.24 eV is inferred in agreement with the calculated energy level of the oxygen vacancy below the conduction band. This result is in agreement with a relative average concentration of oxygen vacancies of similar to 3%, for which a semiconductor-to-metal transition was theoretically predicted. These tungsten suboxide nanostructures are prone to a wide range of applications in nanoelectronics.