Insulator-to-Metal Transition in Electron-doped Vanadium Dioxides Nb_xV_1-xO₂ Single Crystals

Vanadium dioxide (VO₂) is well known for its reversible insulator-to-metal transition (MIT) occurring near room temperature. The transition temperature is strongly influenced by chemical substitutions. Here, we study the Nb-doped VO₂ system to examine how electron doping affects the electronic structure and charge-carrier behavior across the MIT. Broadband optical reflectance spectra were measured from 10,000 cm^-1 to 200 cm^-1 using a Fourier Transform Infrared spectrometer with an infrared microscope, a variable-angle reflectometer, and a temperature-controlled stage compatible with the microscope. Temperature-dependent optical conductivity was obtained by using Kramers-Kronig analysis. We observed systematic MIT trends, indicating a transition from a gapped insulator to a Drude-like metal. The MIT temperature decreased to 30 °C at 5% Nb doping and shifted to -45 °C at 11% Nb. At higher Nb concentrations, no transition was observed within the accessible temperature range. At low Nb dopings, the scattering rate in the metallic phase was smaller than that of undoped VO₂. The study demonstrates the high sensitivity of VO₂ to electron doping and reveals distinct electronic structure changes between Nb-substituted compounds.