Electrochromic writing and erasing on tungsten oxide films in air by scanning tunneling microscopy

The scanning tunneling microscope (STM) was used for both spot coloring and bleaching on $\mathrm{\alpha -}{\mathrm{WO}}_{\text{3−x}}$ thin films. By wetting the STM tip with 1 M KOH solutions or de-ionized water before the tip approached, and keeping the humidity higher than 40%, the electrochemical reaction occurred at the tip–surface gap while the electrochromic reactions took place at the film surface. With a constant negative sample bias and grounded tip, a brown color circle or a light blue disk was formed on the surface with an enlarged diameter. The circle and disk formations were attributed to the production of alkali and hydrogen tungsten bronzes. By applying a positive sample bias with a grounded tip, the color of the hydrogen tungsten bronze could be bleached. The cathodic reduction of ${\mathrm{W}}^{\mathrm{6+}}$ forms a color center at the ${\mathrm{W}}^{\mathrm{5+}}$ site, while the anodic reaction removes an electron from the color center ${\mathrm{W}}^{\mathrm{5+}}$ state. Raman spectroscopy indicated that for the colored state, the frequency for the W–O bond stretching mode increased, while for the W=O bonds it decreased.