Microstructure study of amorphous vanadium oxide thin films using raman spectroscopy

Microstructural changes of amorphous $V2O5$ films with lithium intercalation are studied using Raman-scattering measurements. The Raman spectra of as-deposited films show two broad peaks around at 520 and 650 cm⁻¹, due to the stretching modes of the $V3–O$ and $V2–O$ bonds, respectively, and a relatively sharp peak at 1027 cm⁻¹ due to the $V5+=O$ stretching mode of terminal oxygen atoms. In addition, there is a peak at 932 cm⁻¹ that we attribute to the $V4+=O$ bonds. Comparison of the Raman spectra of $V2O5$ films with different oxygen deficiencies confirms this assignment. This Raman peak due to the stretching mode of the $V4+=O$ bonds develops and shifts toward lower frequencies with increasing lithium concentration. Comparison to results from gasochromic hydrogen insertion indicates that the 932 cm⁻¹ Raman peak is not a result of vibrations which involve Li or H atoms. We propose that the $V4+=O$ bonds are created by two different mechanisms: a direct conversion from $V5+=O$ bonds and the breaking of the single oxygen bonds involving $V4+$ ions.