Formation of a core-shell-like vanadium dioxide nanobeam via reduction and surface oxidation and its metal-insulator phase transition behavior

Abstract

The structural and electrical phase transition in a single core-shell-like VO2 nanobeam formed by reduction and surface oxidation is systematically investigated. The core-shell-like structures were fabricated by a two-step process through hydrogen-annealing and subsequent oxygen-annealing processes on VO2 nanobeams that were synthesized on a Si substrate with a 300-nm thick SiO2 layer. The nanobeam was a roughly rectangular shape elongated along the rutile c-axis with length of approximately 65–110 μm, width of 2–3 μm, and thickness of 200–300 nm. Temperature-dependent Raman and electrical characteristics of the as-grown VO2 nanobeams and the VO2 nanobeams treated by annealing under hydrogen and oxygen gases were compared. Specifically, temperature-dependent Raman scattering in the core-shell-like VO2 nanobeam exhibited a lower transition temperature of the structural phase without the monoclinic M2 phase than those of the as-grown VO2 nanobeams. With respect to the electrical characteristics, the core-shell-like VO2 nanobeam exhibited a more gradual change in resistance with non-hysteretic behavior and without abrupt current jumping in contrast to the as-grown VO2 nanobeams. The shell thickness of the core-shell-like nanobeam was measured to be approximately a few tens of nanometers. These results fit well with the theoretical calculations taking into account the phase coexistence of metal and insulator domains in VO2 nanobeams. Such different characteristics of the core-shell-like VO2 nanobeams are understood in terms of their core-shell-like structure with differences in oxygen stoichiometry resulting from the surface oxidation of the hydrogen-annealed VO2 nanobeams.