Electrochemically Induced Insulator-Metal-Insulator Transformations of Vanadium Dioxide Nanocrystal Films

Vanadium dioxide (VO₂) undergoes a metal-insulator transformation (MIT) around 68C. Alternative stimuli besides temperature have been explored to trigger the MIT, including electrochemical gating. This effect depends on film geometry, orientation and strain, and recent efforts have studied the gating transformation in epitaxial VO₂ thin films. A heterostructure of VO₂ has been prepared through solution deposition of colloidal V₂O₃ nanocrystals into a mesoporous thin film, followed by oxidative annealing. The electrochromic behavior of nanocrystalline VO₂ films is explored with in situ optical transmission measurements in a temperature-controlled electrochemical cell. As expected from prior gating experiments, electrochemical reduction causes a reversible transition from the insulating monoclinic state to a reduced IR darkened state with higher conductivity and minor structural distortions. However, an unexpected transition from this darkened state to a separate insulating phase occurs upon further reduction, and the structure of this phase is explored. This sequential insulator–metal–insulator transition has not been reported in previous studies of gated epitaxial VO₂ films, and demonstrates a unique functionality attributable to the film’s nanocrystalline mesostructure.