## 2 Materials and Methods  
Vanadium oxide thin films were deposited using the gas impulse magnetron sputtering method using a high purity metallic vanadium target (99.99%). The pressure during deposition was kept constant at ca. 5\(\cdot\)10\({}^{-3}\) mbar. The target-substrate distance was equal to 160 mm. The gas was injected directly over the target area using a fast valve (Festo) with a frequency of 10 Hz and each gas pulse lasted for 30 ms. This in turn caused repeatable ignition of the glow discharge because the powering of the magnetron source was electronically synchronized with each gas pulse. After each pulse, the power supply was cut, causing a 70 ms break of the glow discharge. The frequency of the gas impulse and the pulse width were precisely set at these values to ensure the stability of the sputtering process and a reasonable deposition rate of the thin films. For the sputtering process, a medium frequency power supply (165 kHz) was used with a voltage amplitude of 1.8 kV. Seven sets of vanadium oxide thin films were deposited and each set was prepared in different Ar:O\({}_{2}\) gas ratios ranging from 2:1 to 8:1. The duration of the sputtering process was adjusted to obtain thin films with a similar thickness, i.e., in the range from 310 nm to 340 nm. The thin films were deposited on various substrates, including polished silicon, fused silica, and alumina substrates. The alumina substrates were purchased from BVT Company (Czech Republic) and were composed of corundum ceramic base and interdigitated structures of electrodes made of platinum-gold alloy applied on their surface. The substrate dimensions were as follows: 25 mm in length, 7 mm in width, and 0.6 mm thickness. The microstructure of the thin films was evaluated based on X-ray diffraction measurements performed in the grazing incidence mode using the Empyrean PANalytical X-ray diffractometer equipped with a PIXel3D detector and CuK\(\alpha\) X-ray source with a wavelength of 1.5406 A. It is worth mentioning that XRD measurements were conducted in a grazing incidence mode, which is more useful for very thin samples and is more surface sensitive since wave penetration is limited. The measurements were carried out in the 20 range of 30 to 80\({}^{\circ}\) with a step of 0.05\({}^{\circ}\) and a time per step of 5 s with an incidence angle of X-ray radiation of 3\({}^{\circ}\) relative to the sample surface. For such low incidence angles, it is possible to measure the structural properties of very thin layers at the surface. The surface morphology and cross-section studies were performed using the SEM/Xe-PFIB FEI Helios scanning electron microscope equipped with an EDAX energy dispersive spectroscopy detector, which was used for the qualitative analysis of the material composition of the thin films. The optical properties were assessed on the basis of the light transmission spectra measured in the wavelength range of 300-1000 nm using an Ocean Optics QE65000 spectrophotometer and a Mikropack DH-BAL 2000 light source. The fundamental absorption edge and the optical energy band gap were determined for each set of thin films. The current-voltage characteristics and Arrhenius plots were obtained using the Keith-ley SCS4200 characterization system and a M100 Cascade Microtech probe station, and were used to calculate the sheet resistance and resistivity of as-deposited thin films. The temperature-dependent resistivity measurements were performed in the temperature range of 300 K to 350 K and were used for the calculation of the activation energy. Based on the obtained optical and electrical data, the deposited vanadium oxide thin films were compared using the figure of merit (\(Q\)) coefficient calculated according to the following equation:  
\[Q=\frac{T^{10}}{\rho} \tag{1}\]  
where \(T\) is the average optical transmission in the visible wavelength range, and \(\rho\) is the resistivity at room temperature (\(\Omega\)cm).