## 3 Results and Discussion
### Optical Properties  
The optical parameters of VO\({}_{2}\)/glass and VO\({}_{2}\)/Cu\({}_{50}\)Zr\({}_{50}\)/glass thin films is measured with in situ varying temperature. Figure 5 depicts the thermochromic transmittance curves of VO\({}_{2}\)/glass \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Thickness of Cu\({}_{50}\)Zr\({}_{50}\)** & **0 nm** & **40 nm** & **80 nm** \\
**Buffer Layer (nm)** & & & \\ \hline Transition temperature, _Tc_ & 80 \({}^{\circ}\)C & 68 \({}^{\circ}\)C & 72 \({}^{\circ}\)C \\ Resistance change, \(\Delta R\) & 2 \(\times\) 10\({}^{2}\) & 0.7 \(\times\) 10\({}^{2}\) & 1.2 \(\times\) 10\({}^{2}\) \\ Hysteresis width, \(\Delta H\) & 19 \({}^{\circ}\)C & 7 \({}^{\circ}\)C & 8 \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Thermochromic transition characteristics of the multilayer VO\({}_{2}\)/Cu\({}_{50}\)Zr\({}_{50}\)/glass thin films. Figure 4: Temperature-dependent sheet resistance of the VO\({}_{2}\)/Cu\({}_{50}\)Zr\({}_{50}\)/glass thin films with different buffer layer thicknesses. (**a**) No film; (**b**) 40 nm thickness; (**c**) 80 nm thickness; and, (**d**) 160 nm thickness. The insets d(lgR)/dT \(\sim\)T are also shown to determine metal–insulator transition (MIT) temperatures. Temperature ( \({}^{\circ}\)C )  
and VO\({}_{2}\)/Cu\({}_{50}\)Zr\({}_{50}\)/glass films in response to preset temperatures (from room temperature to 100 \({}^{\circ}\)C). Objectively speaking, the transmittance of all the samples in the visible region is almost constant. However, the transmittance of VO\({}_{2}\)(60 nm)/glass, VO\({}_{2}\)(60 nm)/Cu\({}_{50}\)Zr\({}_{50}\)(40 nm)/glass, and VO\({}_{2}\)(60 nm)/Cu\({}_{50}\)Zr\({}_{50}\)(80 nm)/glass decreases clearly with the temperature increases in the infrared region, as shown in Figure 5a-c, while in the VO\({}_{2}\)(60 nm)/Cu\({}_{50}\)Zr\({}_{50}\)(160 nm)/glass thin films the transmittance changes relatively little (Figure 5d). The optical modulation properties of the VO\({}_{2}\) thin film are investigated to evaluate its potential in smart windows. In order to realize the applications of VO\({}_{2}\) in smart windows, technological challenges need to be addressed, including improving the maximum visible transmittance (\(T_{vis}\)), maintaining high solar modulating efficiency (\(\Delta T_{sol}\)), and undergoing more than 10,000 cycles without any degradation [36]. The \(T_{vis}\), \(\Delta T_{sol}\), and near-infrared (NIR) switching efficiency (\(\Delta T_{2500\mathrm{nm}}\)) of all the samples were obtained by the calculation of the transmittance spectra and are displayed in Figure 6. The solar transmittance (\(T_{sol}\), 300-2500 nm) and the \(\Delta T_{sol}\) values are derived from the following formulas:  
\[T_{sol}=\frac{\int\varphi sol(\lambda)T(\lambda)\mathrm{d}\lambda}{\int\varphi sol (\lambda)\mathrm{d}\lambda} \tag{1}\]  
\[\Delta T_{sol}=T_{sol(30\ ^{\circ}\mathrm{C})}-T_{sol(100\ ^{\circ}\mathrm{C})} \tag{2}\]  
where \(T(\lambda)\) is defined as the transmittance at wavelength \(\lambda\) and \(\varphi_{sol}\) is the solar irradiance spectrum for air mass 1.5 (corresponding to the sun standing 37\({}^{\circ}\) above the horizon) [37]. As shown in Figure 6, the \(T_{vis}\), \(\Delta T_{sol}\), and \(\Delta T_{2500\mathrm{nm}}\) values of the monolayer VO\({}_{2}\) film are 44.6%, 7.2% and 42.7%, respectively. Therefore, the good thermochromic properties with enhanced luminous transmittance is obtained by introducing Cu\({}_{50}\)Zr\({}_{50}\) as buffer layer, possibly because Cu\({}_{50}\)Zr\({}_{50}\) can act as an anti-reflection layer (AR) for VO\({}_{2}\) films.