Thermochromic vanadium dioxide smart coatings grown on Kapton substrates by reactive pulsed laser deposition

Thermochromic undoped and metal (Ti and W)-doped VO2 smart coatings were achieved on Kapton HN by reactive pulsed laser deposition. The optimization of the deposition was conducted with Si (100) substrates. The coatings were deposited at relatively low deposition temperatures (250, 300, and 350°C), which are compatible with the characteristics of Kapton. The stoichiometry of the VO2-coated Kapton was confirmed by x-ray photoelectron spectroscopy analysis of the vanadium and oxygen bands. Moreover, the single phase VO2 was confirmed by x-ray diffraction of VO2∕Si synthesized at 300°C. Unlike VO2/Kapton, the VO2∕Si exhibited the well-known semiconductor-to-metallic transition, as shown by the temperature dependence of the infrared transmittance. This coating exhibited a similar transition temperature to that of VO2 single crystal (≈68°C), but a small transmittance switching (about 7%) at 2.5μm. The temperature dependence of the electrical resistivity of all coatings on Kapton was investigated by means of the standard four-point probe technique. The resistivity decreased with increasing temperature. No abrupt semiconductor-to-metallic transition was observed either for undoped or for metal-doped VO2 coatings. It was found that Ti and W dopants have an antagonistic effect on the resistivity. The resistivity was enhanced by the Ti dopant, whereas it was decreased for W-doped VO2 coatings. These results show that the tunability of the resistivity can be tailored either by controlling the deposition temperature or by adjusting the concentration of Ti and W dopants. In addition, at room temperature a much higher temperature coefficient of resistance of −3.29%∕°C was achieved in W(0.5%)-doped VO2/Kapton. Finally, these VO2 smart coatings are promising materials for the IR sensing and sunshield applications.