Doping and temperature dependence of optical conductivity of Nb(x)V(1-x)O2 single crystals

Vanadium dioxide (VO2) is known for its reversible transformation from an insulating monoclinic phase to a metallic rutile phase, occurring just above room temperature. This property enables various photonics applications like optical switching and modulators. By adjusting the concentration of substituents through chemical doping, it becomes possible to manipulate the transition temperature, thereby broadening the potential applications of VO2 to lower energy scales. Here, we investigate the doping dependence of the optical conductivity of Nb(x)V(1-x)O2 single crystals (x = 0.05, 0.11, 0.14, 0.24, 0.35, and 0.88) at room temperature. We measured a reflection spectrum over a wide range of frequencies for the Kramers-Kronig analysis. With the small doping of Nb, the insulator-to-metal transition temperature is significantly reduced. The Drude optical conductivity shows up at 11% Nb at room temperature, indicating the transition temperature is lower than room temperature. We also performed the temperature dependence of the optical conductivity, especially for 11% Nb. The insulator-to-metal transition shows around -45 degrees Celsius. We applied an extended Drude model to fit the optical conductivity to evaluate the metal-to-insulator or the metal-to-semiconductor transition.