Structure, electric, and optical properties of single-crystalline domain composed thin VO₂ films on glass

VO2 films prepared on amorphous substrates are generally nanocrystalline and have low semiconductor-metal transition (SMT) properties. The current research reported that both the undoped and Nb-doped VO2 films, composed of differently-orientated single-crystalline domains, can be prepared on amorphous quartz substrates through facile annealing of sputtered precursors, with their structures, electric properties, optical properties, and thermochromism being systematically studied. The results showed that the resistance change of the undoped VO2 films across the SMT was similar to 3000 times. The SMT critical temperature (T-c) of the sc-domain VO2 films can be decreased to room temperature by the Nb doping, and the resistance change across the SMT could remain approximately two orders of magnitude higher when the T-c was decreased to 27 degrees C. It was seen that both the semiconductor-phase undoped and Nb-doped VO2 films featured defect-activated conductances, with the Nb donor ionization energy being 0.14-0.17 eV. The absorption coefficients of the VO2 films were obtained by fitting the transmittance and reflectance optical spectra with the Drude-Lorentz dispersion model, and the Nb doping widened and merged the d(||) and pi* bands, with both the optical and physical gaps being decreased due to the band widening. The thermochromic performances were also studied, and the result showed that the solar light modulation could remain 90% of the undoped film when the T-c was decreased to 27 degrees C by the Nb doping, so the Nb-doped VO2 films can be a good candidate for preparing thermochromic energy-saving coatings.