Formation of hydrogen tungsten bronze by proton implantation

Proton ions were implanted into polycrystalline (c-) and amorphous (a-)WO3 thin films at an acceleration energy of 60 keV to fluences from 1 × 1017 cm−2 to 1 × 1018 cm−2. The dc electrical conductivity and optical transmission spectra were measured on the thin films before and after the implantations. Upon the implantation, conductivity in c-WO3 thin films at 300 K was significantly increased from 10−5 Scm−1 to 10−1 (for 1 × 1017 cm−2), 101 (3 × 1017 cm−2) or 102 Scm−1 (1 × 1018 cm−2). The temperature dependence of conductivity changed from thermal-activation type (activation energy ∼0.13 eV) for the 1 × 1017 cm−2 sample to degenerate type beyond the fluence of 2 × 1017 cm−2. A similar conductivity increase was observed in the implanted a-WO3 thin films; however, this increase was two orders of magnitude smaller than that in the corresponding c-WO3 thin films. A broad optical absorption band peaking at ∼1 eV was observed for all the implanted specimens. This induced band is similar to that arising from plasma oscillation due to conduction electrons in hydrogen tungsten bronze. Therefore, it is concluded that hydrogen tungsten bronze was formed by implanting protons into polycrystalline and amorphous WO3 thin films.