Dielectric spectroscopy of strongly correlated electronic states of vanadium dioxide

The thermal evolution of the conductivity of a VO$_2$ film and database-obtained band gap $E_g$ of film nanocrystallites is traced in the temperature range of -196$^{\circ}$C $<T<$ 100$^{\circ}$C (77 K $<T<$ 273 K); the level position of donor impurity centers is determined to be $E_d$ = 0.04 eV. It is shown that energy $E_g$ decreases from 0.8 to $\sim$0 eV with an increase in temperature in the range of 273 K $<T<$ 300 K, which is caused by the narrowing of the energy gap due to correlation effects and considered as the temperature-extended Mott “insulator–metal” electron phase transition with the monoclinic lattice symmetry retained. The subsequent jump in the symmetry from monoclinic to tetragonal with a further increase in temperature is considered as the Peierls structural phase transition, the temperature of which is in the vicinity of 340 K and determined by the size effects, nonstoichiometry of VO$_2$ film nanocrystallites, and degree of their adhesion to the substrate.