Hydrogen Diffusion and Stabilization in Single-Crystal
VO<sub>2</sub> Micro/Nanobeams by Direct Atomic Hydrogenation
- Publication date
- Publisher
Abstract
We
report measurements of the diffusion of atomic hydrogen in single
crystalline VO<sub>2</sub> micro/nanobeams by direct exposure to atomic
hydrogen, without catalyst. The atomic hydrogen is generated by a
hot filament, and the doping process takes place at moderate temperature
(373 K). Undoped VO<sub>2</sub> has a metal-to-insulator phase transition
at ∼340 K between a high-temperature, rutile, metallic phase
and a low-temperature, monoclinic, insulating phase with a resistance
exhibiting a semiconductor-like temperature dependence. Atomic hydrogenation
results in stabilization of the metallic phase of VO<sub>2</sub> micro/nanobeams
down to 2 K, the lowest point we could reach in our measurement setup.
Optical characterization shows that hydrogen atoms prefer to diffuse
along the <i>c</i> axis of rutile (<i>a</i> axis
of monoclinic) VO<sub>2</sub>, along the oxygen “channels”.
Based on observing the movement of the hydrogen diffusion front in
single crystalline VO<sub>2</sub> beams, we estimate the diffusion
constant for hydrogen along the <i>c</i> axis of the rutile
phase to be 6.7 × 10<sup>–10</sup> cm<sup>2</sup>/s at
approximately 373 K, exceeding the value in isostructural TiO<sub>2</sub> by ∼38×. Moreover, we find that the diffusion
constant along the <i>c</i> axis of the rutile phase exceeds
that along the equivalent <i>a</i> axis of the monoclinic
phase by at least 3 orders of magnitude. This remarkable change in
kinetics must originate from the distortion of the “channels”
when the unit cell doubles along this direction upon cooling into
the monoclinic structure. Ab initio calculation results are in good
agreement with the experimental trends in the relative kinetics of
the two phases. This raises the possibility of a switchable membrane
for hydrogen transport