Enhanced Thermochromic Properties of Vanadium Dioxide (VO₂)/Glass Heterostructure by Inserting a Zr-Based Thin Film Metallic Glasses (Cu₅₀Zr₅₀) Buffer Layer

Featured Application

Smart windows and other transferable infrared devices.

Abstract

Vanadium dioxide (VO₂) with reversible metal–insulator transition (MIT) is one of the most promising energy-efficient materials. Especially for VO₂-based smart windows, the visible transmittance and solar modulation ability are the most critical parameters. However, VO₂ thin films that are directly deposited onto glass substrates are of poor crystallinity and MIT performance, limiting the practical applications of VO₂/glass heterostructures. In this paper, a buffer layer of Cu₅₀Zr₅₀ was introduced to build a novel Zr-based thin film metallic glass (VO₂/Cu₅₀Zr₅₀/glass) with multilayer structures for thermochromic applications. It is observed that the insertion of a Cu₅₀Zr₅₀ buffer layer with appropriate thickness results in a clear enhancement of crystalline quality and MIT performance in the VO₂/Cu₅₀Zr₅₀/glass thin films, compared with the single-layer VO₂/glass thin films. Moreover, the VO₂/Cu₅₀Zr₅₀/glass bi-layer films exhibit better optical performance with enhanced solar modulation ability (ΔT_sol = 14.3%) and a high visible transmittance (T_vis = 52.3%), which represents a good balance between ΔT_sol and T_vis for smart window applications.

1. Introduction

In response to a large energy-consuming and environmentally deteriorating condition, developing energy-saving materials and sustainable energy has aroused wide attention. Vanadium dioxide (VO₂) thin film is a first-order phase-changed material with superfast reaction speed that is near the critical temperature, 340 K [1,2]. This phase transformation is called a metal–insulator transition (MIT), and involves significant changes in electrical and optical characteristics. Its phase switching behavior makes VO₂ a hopeful candidate for a variety of applications, such as smart windows [3], sensor devices [4], ultrafast switches [5], Mott field effect transistors [4,6], etc.

Various techniques have been developed to form VO₂ thin films, for instance, molecular beam epitaxy [7], pulsed laser deposition [8], magnetron sputtering [9], and chemical vapor deposition [10]. Over the past decades, high-quality VO₂ thin films with the single crystalline characteristics have been seeking because of the impact on MIT features. It is known that the characteristics of the chosen substrates play a major role in the thermochromic properties of VO₂ thin films [11,12]. A lot of single crystals have been employed to act as the growth substrates, such as titanium dioxide (TiO₂), silica (SiO₂), sapphire (Al₂O₃), and magnesium fluoride (MgF₂). The employ of single crystals results in a high expense for the device preparations and therefore immensely limits practical manufacture based on VO₂ thin films. As an alternative approach, common glass substrates with very low cost and good optical transmittance are potentially promising for the production of smart windows and other transferable infrared devices. However, VO₂ thin films that are directly grown onto conventional glass substrates are generally of poor crystallinity. Hence, single-layer VO₂/glass thin films have worse MIT and photoelectric properties [13,14]. In particular, the luminous transmittance and solar modulation ability are often especially bad in single-layer VO₂/glass structures, which limits the practical manufacture of VO₂-based smart windows. Thus, it is the need of the immediate to find a way to optimize the crystallinity and photoelectric properties in VO₂/glass heterostructures.

One way to do this is to design and fabricate a multilayered structure. Multilayered VO₂ thin films have been completed on different substrates by use of “buffer layers” of materials, such as TiO₂, SnO₂, ZnO [15,16,17], conductive oxides (Aluminum-doped Zinc Oxied (AZO), Fluorine Tin Oxide (FTO), Indium Tin Oxide (ITO), etc.) [18,19,20], metals (Ag, Cu, etc.), thin films [21], etc. Recently, thin film metallic glasses (TFMGs) were introduced and showed better surface roughness [22] and good optical properties [23,24]. In particular, Zr-based TFMG have received much scientific research attention for potential applications, since their good mechanical, tribological and fatigue properties, and their corrosion resistance and excellent adhesion [25,26,27,28]. Recently, Zong found that the Zr-based thin film metallic glasses (Cu₅₀Zr₅₀) have excellent near-infrared transmission (larger than 80%) [28]. Cu₅₀Zr₅₀, which can be prepared at room temperature, is a good candidate for template layers. The excellent near-infrared transmission and the appearance of the surface plasmon polaritons (SPPs) is anticipated to be favorable for the optical property of VO₂ thin films.

Motivated by the above demands for the thermochromic application of VO₂/glass thin films, in the present study we constructed a Zr-based thin film metallic glass (VO₂/Cu₅₀Zr₅₀/glass) with a multilayer structure, consisting of Cu₅₀Zr₅₀ TFMG as a buffer layer on amorphous glass substrates. The thickness dependence of the microstructure and optoelectronic properties of the VO₂/Cu₅₀Zr₅₀/glass multilayers was investigated. The achievements may be more comprehensive to understand the role of Cu₅₀Zr₅₀ buffer layers in the formation of high-crystalline VO₂ thin films on glass substrates, and thus pave a way towards heightening the photoelectrical properties of VO₂ thin film-based devices.

2. Experimental

2.1. Method for Film Deposition

VO₂/glass monolayer and VO₂/Cu₅₀Zr₅₀/glass bilayer structures (different Cu₅₀Zr₅₀ buffer layer thicknesses) were prepared by pulsed laser deposition (wavelength 248 nm). The targets are V (99.95% purity, 25 mm diameter, 3 mm thick) and Cu₅₀Zr₅₀ alloy (99.99% purity, 40 mm diameter, 5 mm thick). Cleaned the amorphous glass substrates (BF33) with an ethanol/acetone solution, rinsed with distilled water, and finally blown with pure nitrogen. Prior to deposition, the base pressure was controlled at 1.0 × 10⁻⁴ Pa and the target-substrate distance was 6 cm. During the deposition, the laser operated at repetition rate of 5 Hz and output pulse energy of 200 mJ for entire deposition, the target and substrate were rotated at a rate of 18 rpm. The deposition conditions for the VO₂ film and buffer layer were 500 °C with P_O2 = 0.9 Pa for VO₂ film, room temperature without oxygen inlet for Cu₅₀Zr₅₀. The oxygen flow rate was 25 sccm for VO₂. The film thickness was controlled by deposition time and corrected by step profiler (DektakXT, Bruker, Karlsruhe, Germany). The Cu₅₀Zr₅₀ buffer layers had thicknesses of 40 nm, 80 nm, and 160 nm in the three multilayer VO₂/Cu₅₀Zr₅₀/glass thin films. For comparison, the single-layer VO₂ thin films were grown directly on an amorphous glass substrate under the same conditions as the VO₂/Cu₅₀Zr₅₀/glass thin films. The thickness of all the VO₂ thin films was 60 nm. The four thin films were marked as VO₂(60 nm)/glass, VO₂(60 nm)/Cu₅₀Zr₅₀(40 nm)/glass, VO₂(60 nm)/Cu₅₀Zr₅₀(80 nm)/glass, and VO₂(60 nm)/Cu₅₀Zr₅₀(160 nm)/glass.

2.2. Film Characterization

The crystallographic properties of the sample was characterized by X-ray diffraction (XRD) using a instrument modeled LabXRD-6000 (Shimadzu, Kyushu, Japan) (λ = 0.15406 nm). Using atomic force microscopy (AFM), the surface morphology of the samples was studied with a profilometer (Dektak 150, Bruker, Karlsruhe, Germany)). Images were additionally acquired by scanning electron microscopy (SEM) (Zeiss Supa 50VP, Jena Germany). The temperature-driven MIT properties were measured during the heating and cooling process within the temperature range 30–120 °C by the Hall Effect Measurement System (HMS-5300, Ecopia, Pyeongchang, South Korea). A double beam spectrophotometer (UV-3600, Shimadzu, Kyushu, Japan) with a spectral range of 200–2650 nm was used to record the transmittance spectra of the films. The temperature was controlled in situ by a heater.

3. Results and Discussion

3.1. Microstructural Properties

Figure 1 shows the XRD diffraction patterns of the VO₂/glass film and VO₂/Cu₅₀Zr₅₀/glass films. The generalized diffraction with the 2θ ranging from 15° to 30° indicates the glass properties of the substrates. For the thin films, the peaks that were located at 27.80° are attributed to the VO₂ (011) peak (JCPDS No. 43-1051), which is the feature diffraction peak for M1-phase VO₂ thin films [29]. All of the VO₂ thin films are strongly oriented along the [011] direction, whether or not buffer layers were inserted on the glass substrate. The diffraction peak located at 31.47° belongs to the ZrO₂ (111) peak (JCPDS No. 65-1024), implying the monoclinic symmetry characteristics of the ZrO₂. The peak located at 42.42° is assigned to the Cu₂O (200) peak (JCPDS No. 77-0199). The appearance of ZrO₂ and Cu₂O may be due to the incorporation of Zr and Cu ions with oxygen during the VO₂ thin film growth. In addition, as the thickness of the buffer layer increases, the intensity of VO₂ (011) peaks (in units of counts per second (cps)) initially increases and then decreases, indicating that the crystal quality of VO₂ thin films initially improves and then worsens. The characteristic peaks of the VO₂ phase disappear in the 160 nm-thick VO₂/Cu₅₀Zr₅₀/glass thin films, which clearly indicates that thicker buffer layers make VO₂ amorphous.

Figure 2 shows the AFM micrographs of the surfaces of the VO₂/glass film and VO₂/Cu₅₀Zr₅₀/glass films. The surfaces of the VO₂/Cu₅₀Zr₅₀/glass films are smoother and more uniform than the VO₂/glass film. The measured roughnesses of the VO₂/Cu₅₀Zr₅₀/glass films without a buffer layer, and with buffer layers with thicknesses of 40 nm, 80 nm, and 160 nm, are 2.86 nm, 1.32 nm, 1.51 nm, and 2.12 nm, respectively, indicating that the surface morphology of VO₂ films that are grown on glass substrates can be optimized effectively by embedding a Cu₅₀Zr₅₀ buffer layer. What calls for special attention is a suitable thickness needs that the Cu₅₀Zr₅₀ buffer layer need to select, since excess thickness of Cu₅₀Zr₅₀ layers will generate rougher films.

To further show the effects of the buffer layer thickness on the microstructures, the SEM images of the surface morphology of the VO₂/Cu₅₀Zr₅₀/glass thin films with different buffer layer thicknesses are shown in Figure 3. The surfaces of all the films are found to be continuous and dense. The bright white segments that are visible on the surface of films are identified as amorphous metal V particles, according to the elemental analysis illustrated in the small red circles of Figure 3c,d by Energy-dispersive X-ray spectroscopy (EDS). After embedding an alloy buffer layer, the number of the amorphous metal V particles increases clearly, resulting in poor crystallinity of the VO₂ thin films on the thick buffer layer.

By combining the above analysis, the following can be conjectured. When the buffer layer thickness is relatively thin (~40 nm), the incorporation of Zr atoms with O atoms during deposition forms monoclinic ZrO_2. The ZrO₂ interfacial layer serves as a template to improve the crystallinity of the upper VO₂ thin films. This result is highly consistent with the case of the VO₂/Y-doped ZrO₂/Si thin films [30], where the Y-doped ZrO₂ served as the buffer layer. As the buffer layer thickness increases to 80 nm, the high roughness and impure phase of Cu₂O deteriorates the crystal quality of the VO₂ thin films. When the buffer layer thickness increases to 160 nm, much more oxygen is consumed and too many amorphous metal V particles are present on the film surface, which reduces the crystalline quality of the VO₂(M) thin film.

3.2. Electrical Properties

Figure 4 shows the square resistance variation with temperature in the heating and cooling cycles of the VO₂ thin films with various thicknesses of Cu₅₀Zr₅₀ inserting buffer layers. In Figure 4a, the VO₂/glass thin films show a resistance–temperature curve with a wide transformation hysteresis. In the cases of the 40- and 80-nm-thick buffer layers, the VO₂ thin films both exhibit sharper decreases in sheet resistance with increasing temperature, as illustrated in Figure 4b,c, strongly implying the occurrence of an MIT. However, no MIT occurs when the buffer layer thickness attains to 160 nm. This result can be attributed to poor crystallization in the VO₂/Cu₅₀Zr₅₀/glass thin films, which is good corresponding to the XRD analysis.

To better comparing, the observed electrical properties of VO₂ films with various buffer layer thicknesses are summarized in

Table 1. Thermochromic transition characteristics of the multilayer VO₂/Cu₅₀Zr₅₀/glass thin films.

Thickness of Cu50Zr50 Buffer Layer (nm)	0 nm	40 nm	80 nm
Transition temperature, Tc	80 °C	68 °C	72 °C
Resistance change, ΔR	2 × 102	0.7 × 102	1.2 × 102
Hysteresis width, ΔH	19 °C	7 °C	8 °C

. The transition temperature (T_c) is defined as the center of the derivative curve of the heating curve in the insets of Figure 3. Here, we defined the amplitude of MIT (ΔR) as the relative resistance change ratio between room temperature and a temperature of 100 °C. The hysteresis width (ΔH) is the difference between the T_c values that were measured during the heating and cooling cycles. From Table 1, the single-layer VO₂ thin films have higher T_c than the buffered VO₂ thin films. After introducing the buffer layer (40 nm and 80 nm in thickness), the T_c values of VO₂ thin films are similar to that of the bulk VO₂ single crystal (68 °C), which is generally due to the release of stress [31]. The ΔR values of the VO₂/glass thin films are slightly larger than those of the 40- and 80-nm-thick VO₂/Cu₅₀Zr₅₀/glass thin films. This could be due to the larger crystal size in the non-buffered VO₂ thin films [32]. The samples with smaller or larger ΔH values can be applied in different fields and the ΔH value is closely connected with the quality of crystallization and crystallite dimension in the VO₂ thin films [33,34,35]. From Table 1, it can be seen that the ΔH values of the buffered VO₂ thin films are much smaller than the non-buffered one. The buffer layer serves as a good template for VO₂ thin film growth and it thus results in a better crystallinity in the 40- and 80-nm-thick VO₂/Cu₅₀Zr₅₀/glass thin films. These conclusions point out that the electrical characteristic of VO₂ films can be significantly enhanced by inserting Cu₅₀Zr₅₀ buffer layer of appropriate thickness.

3.3. Optical Properties

The optical parameters of VO₂/glass and VO₂/Cu₅₀Zr₅₀/glass thin films is measured with in situ varying temperature. Figure 5 depicts the thermochromic transmittance curves of VO₂/glass and VO₂/Cu₅₀Zr₅₀/glass films in response to preset temperatures (from room temperature to 100 °C). Objectively speaking, the transmittance of all the samples in the visible region is almost constant. However, the transmittance of VO₂(60 nm)/glass, VO₂(60 nm)/Cu₅₀Zr₅₀(40 nm)/glass, and VO₂(60 nm)/Cu₅₀Zr₅₀(80 nm)/glass decreases clearly with the temperature increases in the infrared region, as shown in Figure 5a–c, while in the VO₂(60 nm)/Cu₅₀Zr₅₀(160 nm)/glass thin films the transmittance changes relatively little (Figure 5d).

The optical modulation properties of the VO₂ thin film are investigated to evaluate its potential in smart windows. In order to realize the applications of VO₂ in smart windows, technological challenges need to be addressed, including improving the maximum visible transmittance (T_vis), maintaining high solar modulating efficiency (ΔT_sol), and undergoing more than 10,000 cycles without any degradation [36]. The T_vis, ΔT_sol, and near-infrared (NIR) switching efficiency (ΔT_2500nm) 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 ΔT_sol values are derived from the following formulas:

$$T_{sol}=\frac{\int \phi sol(\lambda )T(\lambda )\mathrm{d}\lambda }{\int \phi sol(\lambda )\mathrm{d}\lambda }$$

(1)

$$\Delta T_{sol}=T_{sol({30}^{\circ }\text{C})}-T_{sol({100}^{\circ }\text{C})}$$

(2)

where T(λ) is defined as the transmittance at wavelength λ and φ_sol is the solar irradiance spectrum for air mass 1.5 (corresponding to the sun standing 37° above the horizon) [37]. As shown in Figure 6, the T_vis, ΔT_sol, and ΔT_2500nm values of the monolayer VO₂ film are 44.6%, 7.2% and 42.7%, respectively. Therefore, the good thermochromic properties with enhanced luminous transmittance is obtained by introducing Cu₅₀Zr₅₀ as buffer layer, possibly because Cu₅₀Zr₅₀ can act as an anti-reflection layer (AR) for VO₂ films. When compared with the VO₂(60 nm)/glass, the samples of VO₂(60 nm)/Cu₅₀Zr₅₀(40 nm)/glass and VO₂(60 nm)/Cu₅₀Zr₅₀(80 nm)/glass have higher ΔT_sol and Δ T_2500nm values. According to [28], Cu₅₀Zr₅₀ possesses a metallic property in the infrared region. For the sandwich structure VO₂/Cu₅₀Zr₅₀/glass, p-polarized SPPs are supported on the metallic film interfaces in the corresponding range. The appearance of the SPPs probably effectively modulates the optical properties. This mechanism will be further studied in the future [38,39]. When increasing the Cu₅₀Zr₅₀ buffer layer thickness to 160 nm, the ΔT_sol and ΔT_2500nm values clearly decreased. The main reason for this is the effect of crystallization quality, which corresponds to the above XRD data analysis. Favorable thermochromic properties are achieved in the sample of VO₂(60 nm)/Cu₅₀Zr₅₀(40 nm)/glass. The sample of VO₂(60 nm)/Cu₅₀Zr₅₀(40 nm)/glass shows better optical thermochromic performance, and the ΔT_sol value is as high as ~14.3% with the T_vis value up to 52.3% and the ΔT_2500nm value up to 60.2%, which represents a good balance between ΔT_sol and T_vis for smart window applications. These results can be compared with previous experimental results, such as those for periodic and aperiodic porous VO₂(M1) films that were prepared through complex chemical and physical processes in multiple layers of TiO₂(or SiO₂)/VO₂/substrate films [40,41,42] and VO₂-based composite thin films [43,44]. All in all, optimizing optical properties of VO₂ films had indeed been enhanced by the introduction of Cu₅₀Zr₅₀ buffer layer.

4. Conclusions

To conclude, VO₂ thin films with monoclinic crystal phase are here successfully grown on glass substrates by PLD (Plused Laser Deposition) techniques. Cu₅₀Zr₅₀ buffer layers of various thicknesses were introduced to modulate the MIT and optical properties. It is observed that a pronounced smaller ΔH across the MIT is achieved in the optimized VO₂ (60 nm)/Cu₅₀Zr₅₀(40 nm)/glass film. The ΔT_sol value is as high as ~14.3%, with the T_vis value up to 52.3%. A ΔT_2500nm value of up to 60.2% is achieved in the VO₂ (60 nm)/Cu₅₀Zr₅₀(40 nm)/glass film. The present study suggests that the introduction of a Cu₅₀Zr₅₀ buffer layer of appropriate thickness on a glass substrate is beneficial to the improvement of crystalline quality and thermochronic properties of VO₂ thin films, which makes VO₂ a candidate material for use in photoelectronic devices and smart windows.