Enhanced visible transmittance and reduced transition temperature for VO₂ thin films modulated by index-tunable SiO₂ anti-reflection coatings

Abstract

The low visible transmission is one of the bottleneck problems for the application of vanadium dioxide films since the high refractive index (RI) of VO₂ films results in strong reflection in the visible wavelength. To address this problem, in this paper, high-purity VO₂ films were deposited on fused silica by DC reactive magnetron sputtering at low temperature of 320 °C. Silica sol–gel coatings with tunable refractive index (RI) coated onto VO₂ films have been fabricated to enhance visible transmittance with the potential application in the field of smart windows. SiO₂ coatings with tunable RI (1.16–1.42 at λ = 700 nm) were prepared by sol–gel dip-coating technique. The double structure SiO₂/VO₂ films were characterized through several techniques, including X-ray diffraction, UV-VIS-NIR spectrophotometry and scanning electron microscopy. Compared with the single-layer VO₂ film (ΔT_sol of 6.25% and T_lum of 38.58%), the three kinds of SiO₂/VO₂ bilayer films had higher T_lum (41.93–50.44%) and larger ΔT_sol (8.15–8.51%) simultaneously due to significantly decreased reflectance. Moreover, the crystallization properties of VO₂ films are essentially unchanged by applying a SiO₂ top layer, while the phase transition temperature and thermal hysteresis width of sample S116 are lower than those of pure VO₂ film. The presented RI-tunable SiO₂ coatings, can regulate optical properties continuously for various VO₂ substrates, paving the way for practical applications of VO₂ films in the field of smart windows or others.

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1. Introduction

Vanadium dioxide (VO₂) is the best thermochromic material for energy-efficient smart windows.^1,2 At the critical temperature (∼68 °C), a reversible semiconductor-to-metal transition occurs in bulk VO₂.³ Accompanied with the crystal structure transitioning from rutile phase (R) to monoclinic phase (M), VO₂ exhibits a drastic change in electrical conductivity and NIR optical transmittance.⁴ Such specific features make VO₂ a potential candidate in the application of thermochromic energy-efficient smart windows, uncooled infrared (IR) microbolometers, laser protection and optical storage devices.^5–8 High-purity VO₂ films can be fabricated by many preparation methods, including the pulsed laser deposition, sol–gel method, chemical vapor deposition (CVD), thermal evaporation, atomic layer deposition and magnetron sputtering.^9–14 Among these, magnetron sputtering technology shows great advantages for the application of VO₂ films in smart windows due to the good uniformity and potential for industrialization.¹⁵

Up till now, there has been no successful application of vanadium dioxide in the field of smart windows. To realize an available solution, several challenges needed to be overcame: low solar modulating ability, unattractive color, high transition temperature and especially low visible transmittance. Generally, the low transmission of single 80 nm VO₂ film can only reach 40% in the visible region (380–780 nm).¹⁶ There have been various popular methods to enhance the visible transmittance of VO₂ films: metal-ion doping,¹⁷ multilayer structure,¹⁸ porous texture,¹⁹ etc. Moreover, VO₂ films with different nanostructures can behaver enhanced T_lum and ΔT_sol, such as moth eye structure,²⁰ photonic structure,²¹ plasmonic structure²² and grided structure²³. Additionally, applying antireflection coatings (ARCs) on the film surface can also be a promising approach to improve the visible transmittance of VO₂ films through TiO₂, ZnO, ZrO₂ or SiO₂, etc.,²⁴ which is useful in industrial applications. The anti-reflective devise of double-layer films should be enhanced as the high RI and reflectivity of VO₂ films in the visible region. However, most of the overcoats are usually with high RI, expensive and energy-consuming. It is worth noting that compared with other antireflection coatings, the SiO₂ coatings with low RI prepared by low-cost sol–gel methods behaved some advantages of large-area, widespread used and anti-oxidation.^25,26 Much research has been devoted to the fabrication of SiO₂/VO₂ double-layer structure to improve the visible transmission of VO₂ films significantly. Yu et al. increased the T_lum of VO₂ film from 37.6% to 47.7% via SiO₂/VO₂ composite structure, while ΔT_sol exhibited an obvious decrease trend from 8.06% to 7.62%.²⁷ Zhang et al. introduced SiO₂ with RI of 1.299 as an antireflection coating coated on the VO₂ film, which improved the visible transmission from 69.8% to 80.0% significantly, whereas the NIR control capability was also slightly decreased from 12.6% to 10.2%.²⁸

Therefore, in this study, SiO₂ anti-reflection coatings with tunable refraction index were coated on VO₂ films to enhance ΔT_sol and T_lum simultaneously and continuously. Furthermore, the RI tunability of top SiO₂ coatings can be applied to not only the VO₂ continuous thin films but also the nanocomposites and micro-patterned VO₂ films to improve the nanothermochromism effect.²⁹ In addition, an anti-oxidation overcoat is also necessary for the unstable VO₂ films. Index-tunable anti-reflection SiO₂ coatings optimized by sol–gel method prepared on the surface of VO₂ films by sol–gel dip-coating technique. The effect of RI tunability of SiO₂ coatings on the thermochromism and optical properties of VO₂ films was investigated.

2. Experimental details

VO₂ films were deposited on the fused silica by reactive magnetron sputtering technique. The deposition chamber was pumped to a base pressure of 4 × 10⁻⁴ Pa by using a turbo molecular pump, and pre-cleaned in Ar gases for 10 min to remove contamination from the target surface. The vanadium target (Φ60 mm) of purity 99.999% was used and the power density of V target was 4.2 W cm⁻². The distance of target-substrate was approximately 100 mm. Pure oxygen gas and argon gas were introduced to chamber at 1.8 sccm and 40 sccm, respectively. The deposition pressure and substrate temperature were maintained at 0.5 Pa and 320 °C, respectively. The deposition time was 30 min and the thickness of VO₂ films was controlled accurately to 150 nm. A schematic of DC magnetron sputtering is shown in Fig. 1(a). For low refractive silica (n = 1.16), the SiO₂ sols were prepared by base catalysis of ammonia, mixed with the teraethoxysilane (TEOS), H₂O, NH₃ and ethanol (C₂H₅OH). The molar ratio of TEOS, H₂O, C₂H₅OH and NH₃ was 1 : 2 : 34.2 : 0.9.³⁰ High refractive silica (n = 1.42) was synthesized by mixing teraethoxysilane (TEOS), H₂O, HCl and ethanol (C₂H₅OH). The molar ratio of TEOS, H₂O, C₂H₅OH and HCl was 1 : 4 : 20 : 0.05.³¹ The SiO₂ coating with refractive index of 1.34 was prepared by mixing low refractive silica and high refractive silica with a molar ratio of 7 : 3.

Fig. 1 Schematic diagrams of procedure of preparation of (a) VO₂ films and (b) SiO₂ sols. (c) Schematic illustration of the structure of SiO₂/VO₂ double-layer film.

The above resulting chemical reagents were stirred for 5 h and placed into an oven to age for 4 days. VO₂ films (Φ30 mm, d = 3 mm) were coated with the three kinds of silicas by a dip coating process with a withdrawal speed of 7 cm min⁻¹, and then were baked at 180 °C for 24 h. The thickness of SiO₂ antireflection coating was about 110 nm. The schematics of preparation process of SiO₂ sols and dip coating process are shown Fig. 1(b). The SiO₂/VO₂ bilayer films coated by SiO₂ coatings with refractive indices of 1.16, 1.34 and 1.42 are denoted as S116, S134 and S142, respectively. And the VO₂ film without SiO₂ antireflection is marked as PVO₂. Fig. 1(c) illustrates the structure of VO₂ film on fused quartz with a SiO₂ antireflection coating. The optical transmittance of the SiO₂/VO₂ bilayer films were studied with a PerkinElmer Lambda 950 UV-VIS-NIR spectrophotometer from 250 to 2500 nm. A spectroscopic ellipsometer (Horiba uvisel 2) was used to characterize the refractive indices and physics thickness of SiO₂ coatings. The microstructure characterization of pure VO₂ and SiO₂/VO₂ bilayer films was measured by X-ray diffraction (Rigaku Ultima IV) with a 2θ geometry. In addition, the optical transition behavior was studied by Thin film phase transition measurement system (PERFECT PTM1700) to collect the IR transmittance of films at a fixed wavelength (1550 nm) against temperature with an interval of 1.0 °C. The surface morphologies of the films were observed using a field-emission scanning electron microscope (Zeiss Supra 5S). The calculation of integral visible transmittance (T_lum, 380–780 nm) and solar transmittance (T_sol, 250–2500 nm) can refer to our previous work.¹⁶

3. Results and discussion

The refractive index of SiO₂ films with wavelength were measured by using ellipsometry and fitted by Cauchy ellipsometry model. Fig. 2 shows the refractive indices of SiO₂ antireflection coatings dipped by base catalyzed silica, acid catalyzed silica and mixed of them on the fused quartz. The refractive index of the SiO₂ film prepared by base catalyzed silica was measured to be ∼1.16 at 700 nm, considerably lower than that prepared by acid catalyzed silica (∼1.42) or that prepared by mixed of base and acid catalyzed silica (∼1.34). On the other hand, the wavelength dispersions of the refractive index and extinction coefficient of pure VO₂ films below and above phase transition temperature were also measured and fitted by ellipsometry, which can be found from our previous work.⁵

Fig. 2 Dispersion curves of the refractive indices of the SiO₂ AR coatings prepared on the fused quartz.

Fig. 3(a) shows the XRD patterns of both bottom-layer and double-layer thin films on quartz substrates, including the standard pattern of monoclinic (M) VO₂ (PDF#82-0661). All the diffraction peaks of bottom-layer and double-layer films are similar, which correspond with the characteristic pattern of the VO₂ M-phase. It is worth noting that no other impurity peaks (such as V₂O₃, V₂O₅ or V₄O₉) can be observed in the XRD patterns, indicating the formation of high-purity single crystal VO₂ films. These reveal that high-quality VO₂ films were obtained through DC reactive magnetron sputtering at low substrate temperature and the crystalline of VO₂ in double-layer film was unchanged after coated with sol–gel silica anti-reflection coatings. Additionally, the broad peaks in the range of 10° to 25° were assigned to fused quartz substrates. The fine patterns of VO₂ (011) peaks is shown in Fig. 3(b). It can be obvious seen that the values of two film samples are both slightly smaller than 27.827° of VO₂ powder diffraction, and the value of intense diffraction peak position of the double-layer SiO₂/VO₂ film sample is slightly smaller than 27.805° of the pure VO₂ film sample. The stress in films could be calculated by the formula:¹⁵

where d and d₀ are the lattice distances in the strained and unstrained states, respectively. E and v are Young's modulus and Poisson's ratio of the film, respectively. The lattice coefficient could be obtained according to Bragg formula:

2d sin θ = kλ (2)

where θ is the X-ray diffraction angle, and λ is the X-ray wavelength. The compressive stress in double-layer SiO₂/VO₂ film sample is larger than that of pure VO₂ film after calculation from the above formulas.

Fig. 3 XRD spectra of the pure VO₂ film and SiO₂/VO₂ film: (a) normalized patterns; (b) fine patterns of VO₂ (011) peaks.

Fig. 4 shows the top-view SEM images of pure VO₂ and SiO₂/VO₂ bilayer films. For the pure VO₂ film, rice-like nanoparticles scattered on the surface, indicating the good crystallinity of the sample. The morphology is porous for the sample S116, while the sample S142 behaved a denser structure without any pores on the surface. The surface morphology of sample S134 shows both characteristic mentioned above. Moreover, the cross-sectional SEM image of sample S116 is shown in the inset image of Fig. 4, in which an obvious interface between the 110 nm SiO₂ antireflective layer and 150 nm VO₂ film can be observed.

Fig. 4 Top-view SEM images of pure VO₂ and SiO₂/VO₂ bilayer films.

Fig. 5(a) shows the temperature-dependent transmittance of the pure VO₂ film and S116 bilayer film sample at λ = 1550 nm. Thermal hysteresis loop is evident for both film samples and the semiconductor-to-metal transition occurred, indicating that the double-layer SiO₂/VO₂ film sample maintained the phase transition properties. Compared with the pure VO₂ film, the enhancement of transmittance at λ = 1550 nm is obvious, and the highest value can reach 72% for the sample S116. The tunable-RI SiO₂ coatings can significantly suppress the optical reflection losses and improve light transmission.

Fig. 5 (a) Transmittance hysteresis loops of pure VO₂ film and SiO₂/VO₂ bilayer films at λ = 1550 nm and (b) corresponding dT/dt vs. temperature curve for film samples PVO₂ and S116.

Fig. 5(b) shows the Standard Gauss fitting derivative logarithmic plots, which were obtained by taking the derivative of the logarithmic transmittance with respect to the temperature of film sample. Phase transition temperature can be defined as T_c = (T_l + T_h)/2, and the thermal hysteresis width was calculated by ΔH = T_h − T_l. The values of phase transition temperatures for pure VO₂ and S116 film sample are 63.5 °C and 60 °C, respectively, which are lower than 68 °C of the bulk VO₂. Moreover, the phase transition temperature of sample S116 is lower than that of pure VO₂ film. The main reason is that the larger compressive stress existed in the double-layer film sample compared with the pure VO₂ film sample, which has been discussed in the XRD analysis. Our previous work³² reported that the compressive stress changed the phase transition temperature of VO₂ film samples by applying TiO₂ buffer layers. The width of the hysteresis loop of sample S116 is 14.5 °C, which is also smaller than that of pure VO₂ film sample (18.5 °C). These results are different from other reports, which usually revealed that adding SiO₂ antireflection coatings would increase the T_c and ΔH to a certain extent due to the existence of SiO₂ as thermal insulation.^24,28 The main reason of our experimental results may be the special porous morphology for the sample S116 with low refractive index.

To investigate the effect of RI-fixed SiO₂ coatings on the optical properties of the VO₂ films, the transmittance spectra were examined as shown in Fig. 6(a). The visible transmittance (at λ = 700 nm) and IR transmittance modulation (at λ = 2500 nm) of the film samples were calculated, as shown in Fig. 6(b). The transmittance at λ = 700 nm for the samples with SiO₂ antireflection coatings is higher than that of the single-layer VO₂ film, in which the sample with SiO₂ refractive index of 1.42 behaved the best. The visible transmittance at λ = 700 nm increased from 47% to 66% as the refractive index of SiO₂ antireflection coating increased from 1.16 to 1.42.

Fig. 6 (a)Transmittance spectra of pure VO₂ film and bilayer films. Solid lines represent the transmittance recorded at 25 °C and dotted lines measured at 80 °C. (b) T_vis at λ = 700 nm and ΔT_IR at λ = 2500 nm for the samples.

To fully display the superior optical properties of single-layer VO₂ and SiO₂/VO₂ double-layer films, the luminous transmittance (T_lum) and solar spectral transmittance (T_sol) of film samples were calculated based on the measured transmittance using the following eqn (3) and (4):

here, T(λ) represents the transmittance, φ_sol is the solar spectral irradiance for mass of 1.5. The angle between the sun and the horizon is above 37° and the wavelength is 280–2500 nm. φ_lum is the standard luminous efficiency function of photonic vision and the wavelength for the φ_lum is 380–780 nm. The values of ΔT_sol and T_lum(avg) can be obtained as follows:

ΔT_sol = T_sol,l − T_sol,h (5)

T_lum(avg) = (T_lum,l + T_lum,h)/2 (6)

here, T_sol,l indicates the solar transmittance in semiconductor state and T_sol,h indicates the solar transmittance in metal state. T_lum,l and T_lum,h mean the luminous transmittance in the low temperature state and high temperature state, respectively.

The solar switching efficiency (ΔT_sol) and visible transmittance (T_lum) of the pure VO₂ film and bilayer films are shown on Table 1. It is obvious that T_lum and ΔT_sol of the SiO₂/VO₂ films were all higher than the pure VO₂ film. Furthermore, the T_lum,l and T_lum,h of these samples both increased obviously as the refractive indices of SiO₂ coatings increased from 1.16 to 1.42, and the values of ΔT_sol also exhibited a slightly increasing trend at the same time. The maximum values of T_lum and ΔT_sol for film sample S142 can reach 50.44% and 8.51%, respectively. This result was better than the values of reported VO₂@SiO₂ arrays prepared by rapid thermal annealing of sputtered vanadium films,²⁶ SiO₂/VO₂ double-layered films obtained vis plasma enhanced chemical vapor deposition,²⁷ α-SiO₂/VO₂/α-SiO₂ multi-layered films and TiO₂/VO₂ films deposited by radio frequency reactive magnetron sputtering.^33,34 Although the TiO₂(R)/VO₂(M)/TiO₂(A)³⁵ multi-layered films made through radio frequency reactive magnetron sputtering and VO₂/SiO₂/TiO₂ (ref. 36) films fabricated vis atmospheric-pressure chemical vapor deposition behaved higher ΔT_sol than our result, the values of their T_lum(avg) were lower. More detailed results about recent studies were summarized in Table 1. In this work, the formation of smoother and denser film surface after SiO₂ coating can decrease the light scattering of the sample and enhance the visible transmittance. These results indicate that applying SiO₂ antireflection coatings with tunable refraction index can be an efficient way to enhance the visible transmittance of VO₂ films simultaneously and continuously.

Table 1 Solar switching efficiency (ΔT_sol) and luminous transmittance (T_lum) of the pure VO₂ film and SiO₂/VO₂ bilayer films compared with the reported studies

Samples	Tsol,l [%]	Tsol,h [%]	ΔTsol [%]	Tlum,l [%]	Tlum,h [%]	Tlum(avg) [%]	ΔTlum [%]
PVO2 (this work)	44.57	38.32	6.25	37.06	40.10	38.58	3.04
S116 (this work)	50.71	42.43	8.28	40.81	43.04	41.93	2.23
S134 (this work)	54.14	45.99	8.15	47.77	48.37	48.07	0.60
S142 (this work)	57.83	49.32	8.51	50.11	50.76	50.44	0.65
VO2@SiO2 arrays^26	—	—	6.70	—	—	43.50	—
SiO2/VO2 (ref. 27)	—	—	7.62	—	—	47.70	—
α-SiO2/VO2/α-SiO2 (ref. 33)	36.50	32.00	4.50	35.20	34.80	35.00	0.40
TiO2/VO2 (ref. 34)	46.25	39.26	6.99	49.15	45.05	47.10	4.10
TiO2(R)/VO2(M)/TiO2(A)^35	33.80	23.60	10.20	30.10	27.80	28.95	2.30
Moth-eye structure^20	52.10	45.00	7.10	43.60	45.30	44.50	−1.70
VO2/SiO2/TiO2 coating^36	28.83	13.55	15.29	17.81	18.23	18.02	−0.42

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4. Conclusion

In this work, SiO₂/VO₂ bilayer films were synthesized successfully by magnetron sputtering and sol–gel method. The crystallization properties of VO₂ films is essentially unchanged by applying SiO₂ top layer. The bilayer films can behave high transmittance and good thermochromic performance when compared with the single-layer pure VO₂ film. For the sample S142, the maximum value of T_lum can reach 50.44%. For the sample S116, the width of hysteresis loop and transition temperature are 14.5 °C and 60 °C, respectively, which are smaller than those of pure VO₂ film sample (18.5 °C and 63.5 °C). Moreover, the T_lum of these bilayer films increased obviously as the refractive index of SiO₂ antireflective layer increased from 1.16 to 1.42, and the ΔT_sol exhibited a slightly increasing trend. Hence, the enhancement of T_lum and ΔT_sol simultaneously and continuously provides a meaningful solution for VO₂-based thermochromic energy-efficient smart windows.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Shenzhen Science & Technology Project (JCYJ20150529164656098, ZDSY20170228105421966), National Nature Science Foundation of China (Grant No. 11705259) and the Research & Development Fund of Guangdong/ Shenzhen (2016B090930011, JCYJ20160226192609015).

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