Climate change, environmental pollution, and energy scarcity have prompted society to prioritize energy conservation and swiftly transition to renewable energy sources. Buildings typically exhibit high energy consumption, especially owing to temperature control [1]. Energy consumption in buildings can be reduced by reducing the heat or infrared spectrum radiating through windows while simultaneously enhancing visible light transmittance. For this, a promising approach is the application of smart thin film coatings on windows and glass surfaces of buildings to enable the modulation of solar and thermal radiation based on specific requirements. Smart windows made of thermochromic materials are attracting attention worldwide [2–4]. Among various oxides, vanadium dioxide (VO₂) has emerged as a promising thermochromic material [5–7]. VO₂ uniquely undergoes a phase shift from monoclinic M1 to a rutile phase at a critical temperature (T_c) of 68 °C; correspondingly, it changes from an insulator to a metal, respectively [8–10]. The monoclinic insulating phase is transparent to near-infrared (NIR) radiation, while the rutile metallic phase is opaque to NIR radiation [5,10]. Therefore, windows made of VO₂ can work like smart windows as they allow and block the passingthrough of NIR radiation below and above T_c, respectively [10,11]. Since NIR solar radiation causes heating effects, VO₂ smart windows can help maintain the internal temperature of buildings. However, the commercial usage of VO₂ in smart windows is limited by its higher T_c and low visible light transmittance. Numerous studies have been conducted to tune the T_c of VO₂ to room temperature (~27 °C) by controlling deposition parameters, managing strain/stress, selecting appropriate doping, ion implantation, hydrogenation, or controlling oxygen vacancies [5,7,12–16]. Compared to T_c tuning, the challenge of low visible light transmittance in VO₂ is yet to be satisfactorily resolved, and it needs further study. Some recent studies integrated VO₂ with an antireflecting layer to overcome the low visible light transmittance [17–19]. Building upon these efforts, in this study, we explore the integration of Ti-doped ZnO (TZO, 5 %) as an antireflective layer in combination with VO₂ to enhance the visible light transmittance.

In our present study, we deposited single-layer VO₂ and bilayer TZO/VO₂ thin-film samples on c-Al₂O₃ substrates using radio frequency (RF) sputtering. The structural phase transition and optical properties of these films were thoroughly investigated. Interestingly, we observed a higher visible light transmittance in the TZO/VO₂ sample compared to that in the VO₂ sample, highlighting the suitability of this antireflective layer for applications in VO₂-based smart windows. Our research contributes to ongoing efforts by exploring the integration of TZO as an antireflective layer with VO₂ for realizing improved luminous transmittance.

This study provides valuable insights into the structural phase transition and optical properties of the integrated samples and paves the way for further advancements in energy-efficient smart window technologies.

VO₂ and TZO thin films were prepared using RF magnetron sputtering. Commercially purchased VO₂ and TZO targets were used to deposit the respective layers. First, chemically cleaned c-Al₂O₃ substrates were loaded in the RF deposition chamber, and the deposition chamber was evacuated to ~1 × 10⁻⁶ Torr. Before the deposition of each layer, the respective target was presputtered for 5 min to produce a fresh target surface. For the deposition of both VO₂ and TZO layers, Ar was used as a sputtering gas. First, an approximately 110-nm-thick VO₂ layer was simultaneously grown on two c-Al₂O₃ substrate pieces with a sputtering power of 125 W, substrate temperature of 500 °C, and Ar sputtering gas pressure of 28 mTorr. After VO₂ layer deposition, one piece was removed (hereafter called the VO₂ thin film sample) and the TZO layer was deposited on top of the VO₂ layer on the other piece (hereafter called the TZO/VO₂ thin sample). TZO layer deposition was performed at room temperature with a deposition power of 105 W and Ar sputtering gas pressure of 33 mTorr. The thickness of the TZO layer was kept at ~60 nm.

The crystallographic properties of the grown thin-film samples were studied at the BL5A beamline of Pohang Light Source (PLS II), South Korea. X-ray diffraction (XRD) measurements were performed in the θ-2θ mode to understand the crystal structure of the VO₂ and TZO/VO₂ thin-film samples at room temperature. Temperature-dependent grazing incidence X-ray diffraction (GIXRD) measurements were performed at an incidence angle of 0.5°. Diffraction data were recorded using X-rays with an energy of 11.57 keV, and the obtained GIXRD data were converted to the Cu Kα wavelength. The surface morphology of the films was characterized by field emission scanning electron microscopy (FE-SEM; Jeol JSM-6700F). An ultraviolet-visible (UV-Vis) spectrophotometer (S-3100, Scinco) in the transmission mode was used to record transmittance spectra in the wavelength range of 200-800 nm.

Room-temperature XRD data of VO₂ and TZO/VO₂ thin-film samples prepared on the c-Al₂O₃ substrate are shown in Fig. 1(a). In the VO₂ thin-film sample, the intense diffraction peaks appearing at 2θ values of ~27.9, 39.86, and 85.86° respectively correspond to the (011), (020), and (040) planes of the monoclinic M1 phase of VO₂. No peaks corresponding to another phase were observed, indicating single-phase growth. The high intensity of the (020) and (040) plane peaks indicate the preferential growth of the monoclinic M1 phase of VO₂ along this direction. In the TZO/VO₂ thin-film sample, only one additional diffraction peak at ~33.4° corresponding to the (002) plane of the hexagonal structure of ZnO was seen [20]. The integration of the TZO layer did not result in an additional peak of the monoclinic M1 phase of VO₂ or any other phase. The top TZO layer was deposited at room temperature without any additional treatment. The observed diffraction results of the bilayer sample suggested that the preparation of the top TZO layer did not impact the significant structural changes occurring in the underlying VO₂ layer. The surface morphology of the VO₂ and TZO/VO₂ thin film samples was studied using SEM at room temperature, and the obtained SEM images are shown in Figs. 1(b) and 1(c). These images reveal different types of surface structures in the VO₂ and TZO layers. Random-sized grains mixed with a flat surface were seen in the SEM image of the single-layer VO₂, and uniformly distributed spherical grains were seen in the SEM image of the bilayer TZO/VO₂ thin-film samples. Histograms of the grain size distribution of the VO₂ and TZO/VO₂ thin-film samples are shown in Figs. 1(d) and 1(e), respectively.

Figure 1. (a) Room-temperature XRD data measured in θ-2θ mode for VO₂ and TZO/VO₂ thin films grown on c-Al2O3 substrate. Here, V: VO₂ M1 (JCPDS:0431051), Z: ZnO (JCPDS :0011136), S: c-Al2O3 substrate. (b) and (c) SEM images of VO₂ and TZO/VO₂ thin-film samples, respectively, at room temperature. (d) and (e) Histograms of grain size distributions of VO₂ and TZO/VO₂ thin-film samples estimated from SEM images, respectively.

To study the monoclinic to rutile structural phase transition, temperature dependent GIXRD measurements of VO₂ and TZO/VO₂ thinfilm samples were performed from room temperature to 100 °C around the (011) plane of the monoclinic M1 phase of VO₂. The V_M1 (011) plane diffraction peak transforms into a V_R (110) diffraction peak during the structural phase transition during heating, and therefore, it can be used to track the monoclinic M1 to rutile structural phase transition in VO₂ and TZO/VO₂ thin-film samples [5]. GIXRD data were recorded during heating and cooling cycles; the data are shown in Fig. 2. When the VO₂ thin-film sample was heated, the V_M1 (011) diffraction peak remained stable from room temperature to ~60 °C [Fig. 2(b)], started to shift toward lower 2θ values on further heating, and finally transformed to a V_R (110) diffraction peak. The V_R (110) diffraction peak reverted to a V_M1 (011) plane diffraction peak upon cooling from 100 °C to room temperature [Fig. 2(a)], highlighting the reversible nature of the structural phase transition. This nature was also observed in TZO/VO₂ thin-film samples during heating [Fig. 2(d)] and cooling [Fig. 2(c)].

Figure 2. Temperature-dependent GIXRD data of VO₂ during (a) cooling and (b) heating and of TZO/VO₂ during (c) cooling and (d) heating.

For better visualization of the reversible nature of the structural phase transition in VO₂ and TZO/VO₂ thin-film samples and to identify the structural phase transition temperatures, diffraction intensity values obtained from the temperature-dependent GIXRD data at V_M1 (011) and V_R (110) peak positions are plotted in Figs. 3(a) and 3(b). The decrease (increase) in the diffraction intensity of the V_M1 (011) peak [V_R (110) peak] was clearly evident during heating around the structural phase transition temperature for the VO₂ thin-film sample [Fig. 3(a)]. The estimated values of the structural transition temperatures for the VO₂ thin-film sample were ∼73 and 67 °C during the heating and cooling cycles, respectively. The hysteresis in the heating and cooling curves indicates the first-order-type nature of the structural phase transition in VO₂ [21]. A significant broader hysteresis was observed in the TZO/VO₂ thin-film sample [Fig. 3(b)] compared to that in the VO₂ thin-film sample [Fig. 3(a)]. The estimated structural transition temperatures for TZO/VO₂ thin-film samples were ∼72 and 63 °C during heating and cooling cycles, respectively. The slightly lower structural transition temperatures in the bilayer TZO/VO₂ thin-film sample than that in the single-layer VO₂ thin-film sample are likely associated with the formation of defects such as oxygen vacancies in the underlying VO₂ layer during the growth of the top TZO layer. A bilayer sample containing an Al-doped ZnO layer on top of VO₂ also shows a reduction in transition temperature [22]. Moreover, when the VO₂ layer was grown on top of the TZO layer, the bilayer VO₂/TZO sample showed an enhanced transition temperature compared to that of the single-layer VO₂ [23]. In the bilayer samples, defects likely form during the growth of the TZO layer on top of the VO₂ layer compared to that in the case of the growth of the VO₂ layer on top of the TZO layer.

Figure 3. GIXRD intensity plot of VO₂ M1 and VO₂ R peaks during heating and cooling cycles for (a) VO₂ and (b) TZO/VO₂ thin-film samples.

After VO₂ and TZO/VO₂ thin-film samples were subjected to structural characterization and surface morphological analysis, their optical properties were examined by UV-Vis spectroscopy at room temperature in the transmission mode. Figure 4 shows the UV-Vis spectra of the VO₂ and TZO/VO₂ thin-film samples in the range of 200−800 nm. The results reveal that the growth of the TZO layer on top of the VO₂ layer significantly modifies the optical transmittance of the TZO/VO₂ thin-film sample compared to that of the VO₂ thin-film sample. The transmittance in the UV region was minimal in the TZO/VO₂ thinfilm sample. Moreover, the transmittance in the visible region was significantly higher for the TZO/VO₂ bilayer film than for the VO₂ single-layer film, likely owing to the antireflection property of the TZO layer. The antireflection layer helps reduce light loss by taking advantage of the phase change and reflectivity reliance on the index of refraction. The reduced light loss eventually leads to improved transmittance.

Figure 4. Room-temperature UV-Vis spectroscopy spectra of VO₂ and TZO/VO₂ thin-film samples prepared on c-Al2O3 substrate.

We have presented the results of single-layer VO₂ and bilayer TZO/ VO₂ bilayer thin films grown on c-Al₂O₃ substrates using an RF sputtering physical vapor deposition technique. The monoclinic M1 phase of VO₂ was confirmed through XRD in both single-layer VO₂ and bilayer TZO/VO₂ thin-film samples. Out-of-plane oriented growth with a hexagonal ZnO crystal structure was established for the TZO layer in the bilayer sample. The structural phase transition in both singlelayer VO₂ and bilayer TZO/VO₂ thin-film samples was examined using temperature-dependent GIXRD measurements around the V_M1 (011) plane diffraction peak during heating and cooling cycles. The bilayer TZO/VO₂ sample showed a slightly lower structural transition temperature and broader hysteresis. Further, in the bilayer structure, the transmission results obtained using UV-vis spectroscopy revealed a substantial enhancement in the optical transmission that was probably due to the antireflecting property of the TZO layer. The present study establishes TZO as an effective antireflective material to address the low visible transmittance of VO₂-based smart windows. However, the transition temperature in the presently studied bilayer sample is still on a higher side. So, more efforts are needed to achieve more control on the transition temperature in the integrated structure.

This work was supported throughout by the Council of Scientific Research (CSIR) and Frontiers in Science and Technology (FIST). Chirag Saharan is grateful to CSIR Delhi for providing research grants as JRF and SRF.

The authors declare no conflict of interest.