Electrochromic properties of single-crystalline tungsten trioxide films grown by molecular beam epitaxy

C-axis oriented single-crystalline monoclinic WO3 films were grown by molecular beam epitaxy, and studied their properties by fabricating solution-gate electrochromic devices in H2SO4 aqueous solution. These devices exhibited reversible change in the following three states; a colored state with low electrical resistivity after biasing a positive gate voltage, a transparent state with low electrical resistivity after biasing a negative gate voltage, and another transparent state with high electrical resistivity after thermal annealing. At the former two states, the c-axis length of the monoclinic WO3 films was found to expand slightly compared with that of the latter state after annealing. These results were consistently explained in terms of the protonation and hydration of WO3.