Highly Reduced Phase Transition Hysteresis of Vanadium Dioxide Thin Films in Multilayer Structure with Titanium Dioxide

Herein, we present the electrical, structural, and optical characteristics of pristine VO₂, VO₂/TiO₂, and TiO₂/VO₂/TiO₂ thin films deposited on a conventional glass substrate via magnetron sputtering. To obtain a crystallized structure, the as-deposited films were annealed in a tube furnace at 450 and 550 °C in an oxygen atmosphere at 20-25 mTorr for 90 min. The prepared films were characterized by four-point probe resistivity, X-ray diffraction, X-ray photoelectron spectroscopy, ultraviolet-visible-near-infrared spectrophotometry, and field-emission transmission electron microscopy. The microstructural analyses revealed that using TiO₂ as a buffer and the TiO₂/VO₂/TiO₂ sandwich structure contributed to the improvement in VO₂ crystallinity. In particular, the (011) diffraction peak parameters of VO₂, such as crystallite size, increased when the d-spacing and microstrain of the films decreased. The atomic fraction of the VO₂ phase in the TiO₂/VO₂/TiO₂ sample increased from 11 to 19 at. % after annealing at 450 °C. In addition, the multilayer film exhibited relatively increased optical transmittance near the infrared region and showed a reduction in the hysteresis loop width (H_LW) from 21 to 10 °C at a transition temperature of 65 °C in relation to those of pure VO₂ and bilayer VO₂/TiO₂ films. Upon increasing the annealing temperature to 550 °C, the bilayer film showed the highest temperature-dependent infrared transmittance variation (ΔT_IR) of ∼37% at a wavelength of 2000 nm. In addition, the TiO₂/VO₂/TiO₂ sample showed the lowest H_LW (3 °C) with a ΔT_IR of ∼30%. The direct film fabrication on conventional glass substrates, relatively low H_LW, and increase in optical transmittance in the near-infrared region can contribute to the production of cost-effective, fine-tuned, energy-saving smart windows and infrared switches.