Correlation between structural properties and electrochemical proton insertion in (001) VO₂ epitaxial films

We epitaxially grew rutile-structured VO₂ films with various out-of-plane lattice constants on (001) TiO₂ substrates by pulsed laser deposition and investigated their protonation by electrochemically injecting protons to the films in transistor structures with gate layers of proton conducting Nafion membranes. We found that VO₂ films with out-of-plane lattice expansion are less protonated. On the basis of the experimental results, we discuss the correlation between the out-of-plane lattice expansion and protonation of (001) VO₂ epitaxial films and highlight that reducing lattice defects is key to promoting the protonation of VO₂ films.