Protonation-induced change on optical, electrical, and structural properties of epitaxial WO3 films

Protonation and deprotonation processes of c-axis oriented monoclinic WO
₃
epitaxial films were studied by fabricating a solution-gate device operated in H
₂
SO
₄
aqueous solution. The protonation process by positive gate biasing brought about an optical transmittance decrease in visible (VIS) and near-infrared (NIR) regions. The deprotonation process by negative gate biasing recovered the decrease in VIS region while that in NIR one remained. The decrease in NIR region was stable at room temperature in air over several weeks. By analyzing the data on X-ray diffraction and Hall effect measurements, the decrease in VIS region was related to the formation of tetragonal H
_x
WO
₃
tungsten bronze and that in NIR region was related to the formation of orthorhombic WO
₃
·0.33H
₂
O tungsten hydrate.