Unraveling the role of V-V dimer on the vibrational properties of VO2 by first-principles simulations and Raman spectroscopic analysis

We investigate the vibrational properties of VO2, particularly the low temperature M1 phase using the density functional theory as well as Raman spectroscopy. We perform the structural optimization using SCAN meta-GGA functional and obtain the optimized crystal structures not only with the correct energy hierarchy for the rutile and M1 phases, but also phonon properties in great agreement with previous neutron scattering measurement as well as Raman spectroscopic measurements presented in this work. Based on the harmonic approximation around the optimized structures at zero temperature, we reproduce the phonon softening in the rutile phase as well as the phonon stiffening in the M1 phase. In addition, we perform Raman experiments on VO2 thin films which stimulate enormous interests in the field of thin-film engineering for transition metal oxides. We demonstrate from the comparison between theoretical calculations and Raman measurements that the subtle change of the V-V dimer due to the strain effect can be reliably detected by the Raman spectroscopy, which could be a new framework to determine the subtle change of crystal structure for transition metal oxide thin films.