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 (HLW) 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 (ΔTIR) of ∼37% at a wavelength of 2000 nm. In addition, the TiO2/VO2/TiO2 sample showed the lowest HLW (3 °C) with a ΔTIR of ∼30%. The direct film fabrication on conventional
glass substrates, relatively low HLW,
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.