The electrochromic phenomenon is described as the ability of a material to reversibly change its opacity and its color once an electric field is applied. Electrochromic oxides are suitable materials for optical applications due to the association of their wide temperature and voltage working range with their ability to change color once an electric field is applied. In this work, we explore the variation in the band gap of two-dimensional $\alpha \text{−}{\mathrm{MoO}}_3$ nanobelts after the localized application of intense electric fields using scanning tunneling microscopy and spectroscopy. A deterministic change of electronic band gap in ${\mathrm{MoO}}_3$ is observed for negative applied biases. These changes are fully reversible and robust with respect to successive cycles for a range of applied bias compatible with common applications. In our experiments the band-gap modifications are ascribed to changes in the surface stoichiometry due to the local electric field created by the tunneling tip. Our result points to the possibility of writing and reading surface electronic analogical information, with optical access to the material state, indicating a potential use in technological areas.

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• Crystal stoichiometry
• Electrical conductivity
• 2-dimensional systems
• Wide band gap systems
• Scanning tunneling spectroscopy