Electrochromic mechanism in a-WO3−y thin films

The electrochromic mechanism in amorphous tungsten oxide films is studied using Raman scattering measurements. The Raman spectra of as-deposited films show two strong peaks at 770 and 950 ${\mathrm{cm}}^{\mathrm{-1}}$ due to vibrations of the ${\mathrm{W}}^{\mathrm{6+}}$ –O and ${\mathrm{W}}^{\mathrm{6+}}$ =O bonds, respectively, and a weaker peak at 220 ${\mathrm{cm}}^{\mathrm{-1}}$ that we attribute to the ${\mathrm{W}}^{\mathrm{4+}}$ –O bonds. When lithium or hydrogen ions and electrons are inserted, extra Raman peaks due to ${\mathrm{W}}^{\mathrm{5+}}$ –O and ${\mathrm{W}}^{\mathrm{5+}}$ =O bonds appear at 330 and 450 ${\mathrm{cm}}^{\text{−1}},$ respectively. Comparison of the Raman spectra of sputtered isotopic $\mathrm{a-}{\mathrm{W}}^{\mathrm{16}}{\mathrm{O}}_{\text{3−y}}$ and $\mathrm{a-}{\mathrm{W}}^{\mathrm{18}}{\mathrm{O}}_{\text{3−y}}$ films confirms these assignments. We conclude that the as-deposited films contain mainly the ${\mathrm{W}}^{\mathrm{4+}}$ and ${\mathrm{W}}^{\mathrm{6+}}$ states, and the ${\mathrm{W}}^{\mathrm{5+}}$ states are generated as a result of reduction of the ${\mathrm{W}}^{\mathrm{6+}}$ states when lithium or hydrogen ions and electrons are inserted. We propose that the optical absorption in the colored films is caused by transitions between the ${\mathrm{W}}^{\mathrm{6+}}$ and ${\mathrm{W}}^{\mathrm{5+}},$ and ${\mathrm{W}}^{\mathrm{5+}}$ and ${\mathrm{W}}^{\mathrm{4+}}$ states.