The novel preparation method of high-performance thermochromic vanadium dioxide thin films by thermal oxidation of vanadium-stainless steel co-sputtered films

The preparation of high-performance thermochromic vanadium dioxide films and the assurance of its batch-to-batch uniformity is technically difficult since the oxidation number of vanadium ranges from +2 to +5 under various reducing or oxidizing atmospheres at elevated temperatures. In this work, the vanadium dioxide films were fabricated by thermal-oxidation of vanadium and stainless-steel (mainly containing Fe, Cr, Ni) co-sputtered thin films at elevated temperature under air atmosphere. The as-prepared vanadium oxide films were further characterized by four-point probe measurement, ellipsometry, grazing incidence X-ray diffraction (XRD) and X-ray photoelectron spectroscopic (XPS) analysis. Our experiments demonstrated that with the use of an SS target we could efficiently deposit the magnetic atoms (iron and nickel) and nonmagnetic Cr elements into vanadium films in stable plasma conditions. The shift of the main peak of XRD patterns to smaller angles showed that Fe, Cr, and Ni elements were successfully doped into vanadium oxide films. These dopants depressed the vanadium atoms from over-oxidization during the heat treatment. As a result, a noticeable amount of VO2(M) crystalline phase (higher V(+4)/V(+5) ratios) was obtained from the as-prepared films accompanied with the increment of the SS target voltage. Consequently, the formation of V6O13, V3O7, and V2O5 vanadium oxides crystallites was significantly minimized after the thermal-oxidation stage, which gave the as-prepared films outstanding thermochromic-responsive properties. The proposed scheme is proved to be a novel fabrication technique to prepare high-performance thermochromic vanadium dioxide films with the assurance of batch-to-batch uniformity.