Electronic structure and insulating gap in epitaxial VO₂ polymorphs

Determining the origin of the insulating gap in the monoclinic V O₂(M1) is a long-standing issue. The difficulty of this study arises from the simultaneous occurrence of structural and electronic transitions upon thermal cycling. Here, we compare the electronic structure of the M1 phase with that of single crystalline insulating V O₂(A) and V O₂(B) thin films to better understand the insulating phase of VO₂. As these A and B phases do not undergo a structural transition upon thermal cycling, we comparatively study the origin of the gap opening in the insulating VO₂ phases. By x-ray absorption and optical spectroscopy, we find that the shift of unoccupied t_2g orbitals away from the Fermi level is a common feature, which plays an important role for the insulating behavior in VO₂ polymorphs. The distinct splitting of the half-filled t_2g orbital is observed only in the M1 phase, widening the bandgap up to ∼0.6 eV. Our approach of comparing all three insulating VO₂ phases provides insight into a better understanding of the electronic structure and the origin of the insulating gap in VO₂.