Effect of substrate-induced compressive strain on protonation of SrCoO2.5 epitaxial films

Electrochemical protonation, which can be realized in an electric-field-effect transistor with a gate layer consisting of a proton-conducting electrolyte Nafion membrane, offers a simple way of electrically tuning the physical properties of materials. In this study, we grew (010)-oriented epitaxial films of brownmillerite-structured SrCoO2.5 on various substrates whose lattice mismatch against SCO ranged from 0% to -2.9% by pulsed laser deposition and investigated the effect of substrate-induced strain on their protonation in field effect transistor structures with gate layers consisting of Nafion membranes. We found that the H concentration of the SCO films that were fully compressive-strained by up to 1.3% was similar to 1.7 and that it was almost independent of the magnitude of the substrate-induced strain. We also found that the H content of the strain-partially-relaxed film with a residual compressive strain of 1.3% was lower, similar to 1.3. These results indicate that lattice deformations arising from substrate-induced strain have insignificant effects on protonation, while lattice defects and dislocations introduced upon strain relaxation, which hinders proton diffusion in the films' lattices, dominantly affect protonation in SrCoO2.5 films.