Polarized Raman scattering of epitaxial vanadium dioxide films with low-temperature monoclinic phase

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A polarized Raman scattering study was carried out on epitaxial VO2 thin films on MgF2(001) and (110) substrates to investigate the Raman symmetry and tensor elements of the phonon modes of the films in a low-temperature monoclinic phase. From the polarization angular dependence of the Raman intensity, we assigned the phonon modes at 137, 194, 310, 340, 499, 612, and 663 cm−1 to Ag symmetry and the phonon modes at 143, 262, 442, 480, 582, and 820 cm−1 to Bg symmetry. The angular-dependence measurements also revealed that two phonon modes with Ag and Bg symmetries are present at about 224 and 393 cm−1, although only a single peak was observed in the Raman spectra at around these wavenumbers. On the basis of the experimental results, we evaluated the Raman tensors of the identified phonon modes. From the Raman tensors, we found that the atomic displacements of the 194 and 340 cm−1 phonon modes are approximately perpendicular and parallel, respectively, to the V–V dimer direction. This is consistent with a previous theoretical prediction, i.e., these modes are attributable to the tilting motion and the stretching vibration of the V–V dimers, respectively.