Low temperature growth of amorphous VO2 films on flexible polyimide substrates with a TiO2 buffer layer

Amorphous VO2 thin films were grown on anatase TiO2-buffered polyimide (PI) films using radio-frequency magnetron sputtering deposition with a VO2 target as low as at 175 degrees C. For comparison, the authors grew VO2 films on TiO2-buffered SiO2/Si substrates. The structural and morphological properties of the VO2 films were evaluated by x-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. VO2 films grown on TiO2/SiO2/Si were crystalline at 200 and 250 degrees C and were amorphous at 175 degrees C. VO2 films grown on TiO2/PI were amorphous. No peak corresponding to the monoclinic phase of VO2 appeared in the Raman spectra of VO2/TiO2/PI films grown at 175 or 200 degrees C. The chemical compositions of VO2 and the binding energy spectra of V and O atoms were probed by x-ray photoelectron spectroscopy. The authors discussed the multivalence states of V atoms and oxygen vacancies based on the x-ray photoemission spectroscopy of crystalline and amorphous VO2 films. The authors obtained the hysteresis curves of the resistivity as a function of temperature for both VO2/TiO2/SiO2/Si and VO2/TiO2/PI films. In addition, the authors measured the reflectivity of VO2/TiO2/PI films below and above the metal-insulator transition temperature using spectroscopic ellipsometry. The reflectivity changed substantially and was comparable to the literature values of well-crystallized VO2 films, even though the ratio of the switching resistivity values was as low as sixty. This work demonstrates that VO2 films grown on plastic films grown at temperatures as low as 175 degrees C can be applicable as flexible thermochromic films for use in energy-saving windows. Published by the AVS.