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Design of Antireflection and Enhanced Thermochromic Properties of TiO2/VO2 Thin Films

Design of Antireflection and Enhanced Thermochromic Properties of TiO2/VO2 Thin Films IntroductionAs a typical thermochromic material, vanadium dioxide (VO2) can undergo reversible structural phase transition (SPT) from monoclinic phase (M1) to rutile phase (R) at a critical temperature (Tc) of 68 °C. Meanwhile, metal‐insulator phase transition (MIT) occurs,[1] resulting in sudden change of the electrical and optical properties.[2,3] At the temperature lower than Tc, VO2 is an insulator with high optical transmittance; but will be transformed into a metal with higher optical reflectance at the temperature above Tc. The phase transition in VO2 can also be triggered by force, light and electricity.[4–6] The phase transition endows VO2 with potential applications in spacecraft thermal radiation device,[7] supercapacitor,[8] electrode material,[9,10] optical modulator,[11] optical switch,[12] microwave passive electronic device,[13] phase change memory,[14] field effect transistor,[15] micro and nano oscillator,[16] sensor,[17] THz device,[18] and smart glass.[19–21] VO2 thin films on glass can sense the environmental temperature, and switch the transmittance of sunlight in the infrared wavelength range at high and room temperatures, and adjust the input of solar radiation energy and indoor temperature spontaneously. Accordingly, the energy consumption in the buildings could be substantially reduced. However, there are still some obstacles to be overcome for the industrialization of VO2 smart glass. Firstly, the phase transition temperature of VO2 is still too high to be utilized in residence.[21] Secondly, it is difficult to achieve satisfactory luminous transmittance (Tlum) and solar modulation (ΔTsol) at the same time.[19] Thirdly, the preparation of VO2 thin films with good thermochromic properties is difficult: the chemical process is complex with low yield rate, while the physical methods are often carried out at high temperature.[22]A great number of strategies, such as, doping with high valence elements,[23] exerting lattice stress and strain,[24] and manufacturing oxygen vacancies,[25] and so on, have been exploited to lower the Tc value. For example, the Tc value can be lowered by 23 °C if VO2 is doped with 1 at% W6+.[26] Mg2+, Ti4+, Sn4+ and Zr4+ doping can improve the Tlum, but with reduced solar modulation.[27–32] Thin films prepared by colloidal lithography have the Tlum value up to 70.2%, but with ΔTsol value of only 7.9%.[33] The porous thin films prepared by freeze‐drying showed the better thermochromic properties with Tlum of 50% and ΔTsol of 14.7%, because the filling air reduces the refractive loss.[34] However, the above methods are still complex with poor controllability. The antireflective (AR) multilayer thin films might improve the Tlum of VO2 films significantly but often degrade ΔTsol. For example, Hyun Koo et al.[35] used CeO2 as AR film, and improved the Tlum of VO2 up to 67.5%, but with ΔTsol of only 5.4%. Liu et al. demonstrated that SnO2/VO2/SnO2 sandwiched thin films had the Tlum value of 50.2% and the ΔTsol of 15.3%.[36] However, the mechanism for the enhanced modulation by the AR films is still not well known.Herein, TiO2/VO2 AR thin films are designed through optical simulations, the influences of the film stacking configuration on the Tlum and ΔTsol values are discussed. Accordingly, VO2 thin films with optimized thickness are prepared through deposition of V thin films by magnetron sputtering at room temperature and then thermal annealing in oxygen atmosphere, while TiO2 thin films with designed thickness are prepared by reaction magnetron sputtering. 200 nm TiO2/ 100 nm VO2 thin films show excellent overall thermochromic properties with the Tlum value of 46.29% and the ΔTsol value of 16.03%, which is consistent with the simulation results. The Tlum and ΔTsol values of TiO2/VO2/TiO2 sandwiched thin films can be up to 50.49% and 20.11%, respectively. The design, preparation, and improvement mechanism of the films are discussed.Results and DiscussionDesign of Thin Films by Theoretical SimulationsUsing the optical characteristic matrix and the optical constants, the Tlum and ΔTsol values of VO2 thin films with different thicknesses are calculated according to the Equations (1) and (2), and Figure 1a shows the results. The Tlum value of VO2 thin films is reduced with increasing film thickness. However, the ΔTsol value increases firstly and then decreases with increasing film thickness, owing to the enhanced absorption and scattering of sunlight.[37] Therefore, the thickness of VO2 thin films should be optimized to achieve both high Tlum and ΔTsol. Herein, the optimized thickness of VO2 thin films is 100 nm, for which the Tlum and ΔTsol can be well balanced with the values of 33.53% and 14.81%, respectively. Figure 1b shows the optical transmittance of the VO2 thin film with a thickness of 100 nm. A large gap exists in the transmittance spectra of infrared band at 90 °C and 20 °C. As shown in Figure S1 (Supporting Information), the suitable refractive index (n) of the AR film for VO2 film is 2.0–2.5. Because the refractive index of the as‐prepared TiO2 films is 2.1–2.2, and the preparation of TiO2 is simple and economical, TiO2 is selected as the AR film. Figure 1c illustrates the optical properties of TiO2/VO2 thin films. As the thickness of TiO2 is increased from 0 to 500 nm, the ΔTsol value increases firstly, and then remains at a constant value, that is, it is feasible to improve the solar modulation property of VO2 thin films through adjusting the thickness of TiO2. As displayed in Figure S2 (Supporting Information), the Tlum value changes periodically with increasing thickness of TiO2 due to the interference effect. Referencing to Figure 1c, the highest Tlum can be obtained at the TiO2 film thickness of around 50 nm. However, if both Tlum and ΔTsol are taken into account, the optimized thickness of TiO2 should be at about 200 nm. So TiO2 antireflective films with the thickness of 50–200 nm are involved in the following to study the effects of AR film thickness.1FigureThe calculation results: a) Tlum and ΔTsol of VO2 films. b) Transmittance curve of VO2 thin films with thickness of 100 nm. c) Optical properties of TiO2/VO2 thin films as a function of the TiO2 thickness. d) Transmittance curves of TiO2/VO2 bilayer thin films.Figure 1d shows the calculated transmittance curves of the TiO2/VO2 thin films with the TiO2 thickness of 0, 50, 100, 150, and 200 nm, at 20 °C. The transmittance curves in visible light are consistent with the above analysis. For the TiO2 film thickness of 50 nm and 200 nm, the transmittance curves have a peak at around 550 nm, accordingly these two film systems have higher Tlum. Moreover, a transmittance peak is evidenced in the near‐infrared light band, and it will be shifted to higher band with increasing TiO2 thickness. The transmittance to infrared radiation is increased at 20 °C, but not at 90 °C (Figure S3, Supporting Information), due to the temperature dependent refractive index of VO2. Hence, the TiO2/VO2 thin films can be designed to play an AR role at 20 °C, but not at 90 °C. So, the modulation ability of VO2 to infrared radiation is improved by the thicker TiO2 antireflection layer. The Tlum and ΔTsol values of the TiO2/VO2 thin films with the TiO2 film thickness of 200 nm can be up to 52.5% and 20.2% respectively, which is higher than that of VO2 films by 56.58% and 36.39%.Structure of the As‐Prepared VO2 and TiO2/VO2 Thin FilmsFigure 2a shows the X‐ray diffraction (XRD) patterns of the V thin films after thermal annealing at 550 °C but at different oxygen partial pressure for 2 hours. When the thermal annealing is done at the oxygen partial pressure of 10 Pa, the XRD peaks can be ascribed to V2O3 (JCPDS no.71‐0345) and VO2 (M) (JCPDS no.75‐0514).[23] The XRD peaks of VO2 (M) are significantly enhanced at the oxygen partial pressure of 15 Pa. However, at the oxygen partial pressure of 20 Pa, the XRD peaks at 30.94° and 34.16° are ascribed to V2O5 (JCPDS no.89‐0611), owing to excessive oxygen. Based on this, the oxygen partial pressure of 15 Pa is adopted in the following experiments, and the annealing time is elongated appropriately. Figure 2b shows the XRD pattern of the film annealed at 550 °C for 4 h, the XRD peaks of VO2(M) are substantially enhanced, indicating the formation of highly crystalline VO2 (M). Figure 2c,d shows the survey X‐ray photoelectron spectroscopy (XPS) spectra and the XPS spectra of V 2p and O 1s. O and V are from the VO2 films, but C is from the surface contamination. Two splitting energy levels of V2p are evidenced, and the binding energy difference between V 2p3/2 and O 1s is 13.87 eV, confirming the formation of VO2 (M).[38,39] Figure S4 (Supporting Information) shows the XPS spectra along the depth. The close position of V indicates the uniform composition in the films.2Figurea) X‐ray diffraction (XRD) patterns of VOx films annealed at different oxygen fluxes. b–d) XRD patterns and X‐ray photoelectron spectroscopy (XPS) spectra of VO2(M) after annealing under 15 Pa O2 partial pressure at 550 °C for 4 h. b) XRD patterns, c) XPS survey spectra, d) XPS spectra of V 2p and O 1s.Figure 3a displays the scanning electron microscopy (SEM) images of the thin films, and Figure 3b shows the particle size distribution. The grain size in the films increases with the oxygen partial pressure and annealing time gradually. As displayed in the inset of Figure 3a, the transmittance of the films is improved with increasing oxygen partial pressure. When the V film is annealed under 20 Pa oxygen partial pressure, the film becomes light gray rather than the yellowish brown of VO2 film. Figure 3c shows the element distribution in the annealed thin films, and V and O are uniformly distributed. Figure 3d shows the SEM image of the cross‐section of the thin film, confirming that the thickness of the VO2 thin film is about 100 nm.3Figurea1–a4) SEM images of the thin films annealed at different conditions: a1) 10 Pa O2‐550 °C ‐2 h, a2) 15 Pa O2‐550 °C ‐2 h, a3) 20 Pa O2‐550 °C ‐2 h, a4) 15 Pa O2‐550 °C ‐4 h, the insets are the photographs. b1—b4) particle size distribution in (a1–a4). c1–c3) Elemental mapping in the selected area in (a4). d) SEM image of the cross‐section of the thin film (a4).Figure 4a,b displays the survey XPS spectra and the XPS spectra of Ti 2p. The peaks of Ti 2p, Ti 2s, and O 1s orbitals are evidenced, and the binding energy difference of the two splitting levels of Ti 2p is 5.7 eV, indicating the existence of Ti4+. Figure 4c shows the thickness of TiO2 thin films as a function of sputtering time, and the deposition rate is calculated to be 90 nm h−1. Figure 4d,e shows the SEM image and the elemental distribution in the TiO2 thin films. The film surface is very dense, and Ti and O are uniformly distributed, indicating the uniform composition in the film.4Figurea,b) X‐ray photoelectron spectroscopy (XPS) spectra of TiO2 thin films; a) XPS survey spectra, b) XPS spectra of Ti2p. c) The thickness of TiO2 thin films as a function of sputtering time. d) SEM images of the TiO2 thin film; e1,e2) elemental mapping of the selected area in (d).Performances of VO2 and TiO2/VO2 Thin FilmsAs shown in Figure 5a, the transmittance curves of the thin films at 20 °C and 90 °C show great difference in the infrared band, owing to the phase transition. The Tlum value of the VO2 thin film with thickness of 100 nm is 29.03%, and the ΔTsol is 15.8%, which is in good agreement with the calculated results. Figure 5b shows the transmittance curves of TiO2/VO2 thin films with the TiO2 thickness of 50, 60, and 70 nm and Figure 5c displays the magnified curves in the visible band. The transmittance curves of both TiO2/VO2 thin films and VO2 thin films show a peak in the visible band, while there is little difference in the infrared band. However, the transmittance of TiO2/VO2 thin films is significantly increased in the visible band due to the strongly suppressed reflectance, and the peak value of the visible transmittance becomes nearer to the peak position of the standard spectral sensitivity of the light‐adapted eye at 555 nm (Figure S5, Supporting Information), contributing to the Tlum enhancement effect. If the thickness of TiO2 AR layer is increased up to 60 nm, the Tlum of TiO2/VO2 thin films is up to 47.06%, which is higher than that of VO2 thin film by 62.11%, with the ΔTsol value of 14.36%. If the 200 nm thick AR TiO2 layer is utilized, the transmittance of the TiO2/VO2 thin films changes greatly. As shown in Figure 5d, the transmittance of TiO2/VO2 thin films is significantly increased in the visible band, and a peak emerges in the near infrared band at 20 °C, but changes little at 90 °C. Figure 5e depicts the enlarged transmittance curve in the visible band. For the 200 nm thick TiO2 AR layer, the transmittance peak appears at about 600 nm in visible range. The Tlum and ΔTsol values of TiO2/VO2 (200/100 nm) thin films can be up to 46.29% and 16.03%, respectively, which are substantially improved, as compared with VO2 thin films. As shown in Figure 5f, the measured Tlum and ΔTsol values of the TiO2/VO2 thin films change almost with the same trend as that of the simulation results.5Figurea) The transmittance curve of VO2 (100 nm). b) The transmittance curve of the TiO2/VO2 thin films with TiO2 thickness of 0, 50, 60, 70 nm. c) Amplified curves in the visible band. d) The transmittance curve of the TiO2/VO2 thin films with the TiO2 thickness of 100, 150, 200, and 250 nm. e) Amplified curves in the visible band. f) Comparison of the calculated and experimental results.Optical Properties of TiO2/VO2/TiO2 Sandwiched Thin FilmsBased on the above results, if the AR TiO2 films are deposited on both sides of VO2 thin films, forming TiO2/VO2/TiO2 sandwiched thin films, the optical properties might be further improved. The eigenmatrix method is adopted to predict the performances of the film systems. Figure 6a,b shows the calculated Tlum and ΔTsol values of the films, respectively. The optical properties of the TiO2/VO2/TiO2 sandwiched thin films depend on the TiO2 thickness, which is similar to that of TiO2/VO2. For thinner TiO2 thin films, the TiO2/VO2/TiO2 sandwiched thin films show better antireflection but degraded modulation. As shown in Figure 6c, the AR effect of the TiO2/VO2/TiO2 sandwiched thin films with both the upper and lower TiO2 layers of 50 nm is the best, with Tlum up to 69.49%, which is higher than that of VO2 by 107.25%, but the ΔTsol is only 11.49%. The modulation can be improved if thicker TiO2 films are utilized. Figure 6d shows the transmittance curves of the TiO2/VO2/TiO2 sandwiched thin films with the TiO2 film thickness of 100, 150, and 200 nm. Two peaks appear in the visible and near‐infrared band. For the film system with the TiO2 film thickness of 200 nm, the Tlum and ΔTsol can be up to 60.20% and 18.72%, respectively, which are higher than that of VO2 by 79.54% and 26.40%.6Figurea–d) Calculation results of TiO2/VO2/TiO2 films. a) Optical modulation capability and b) visible light transmittance as the thickness of TiO2 films changes. c) Transmittance curves of TiO2 (50 nm) /VO2(100 nm)/TiO2(50 nm). d) The transmittance curves of the TiO2/VO2/TiO2 sandwiched thin films with TiO2 thickness of 100, 150, and 200 nm, respectively.The TiO2/VO2/TiO2 sandwiched thin films are prepared by magnetron sputtering. Figure 7a,b displays the SEM images of TiO2/VO2/TiO2 thin films. Figure 7c shows the SEM images of the cross‐section of TiO2 (200 nm), VO2/TiO2 (100/200 nm), and TiO2/VO2/TiO2 (200/100/200 nm) thin films, and the interfaces between thin films are clear. Figure 7d shows the optical photographs of the naked glass (d1), VO2 thin film (d2), TiO2/VO2 films (d3‐d4), and TiO2/VO2/TiO2 sandwiched films (d5‐d6). Compared with VO2 thin film, the visible light transmittance of TiO2/VO2 thin films is improved substantially, particularly, the TiO2/VO2/TiO2 (50/100/50 nm) thin film sample.7FigureSEM images of TiO2/VO2/TiO2 thin films with thicknesses of a) 50/100/50 nm and b) 200/100/200 nm. The cross‐section SEM images of c1) TiO2 (200 nm), c2) VO2/TiO2 (100/200 nm), and c3) TiO2/VO2/TiO2 (200/100/200 nm) thin films. d) Optical photographs of d1) naked glass, d2) VO2 (100 nm) film, d3) TiO2/VO2 (50/100 nm) film, d4) TiO2/VO2 (200/100 nm) film, d5) TiO2/VO2/TiO2 (50/100/50 nm) film, and d6) TiO2/VO2/TiO2 (200/100/200 nm) film at 20 °C.Figure 8a shows the XRD patterns of TiO2, VO2, and TiO2/VO2/TiO2 sandwiched thin films for comparison. The peaks of TiO2 (A) and VO2 (M) are evidenced in the TiO2/VO2/TiO2 thin films. The TiO2 thin film under VO2 is in anatase structure, but the TiO2 thin film on VO2 is amorphous if no annealing is done. The refractive indices of both polycrystalline and amorphous TiO2 films in this work are in the range of 2.1–2.2, and thus affect the antireflection little. The infrared (IR) transmittance at 2500 nm against temperature is measured for the VO2 and TiO2/VO2/TiO2 thin films, and the results are shown in Figure 8b. The phase transition temperatures of the VO2 and TiO2/VO2/TiO2 thin films are 68.5 °C and 72 °C, respectively. As compared with VO2 (M), the phase transition temperature of TiO2/VO2/TiO2 thin films is slightly elevated with widened hysteresis, which might be ascribed to two reasons. Firstly, since the thermal expansion coefficient of VO2 is larger than that of TiO2, the thermal compressive stress is generated in the VO2 thin films during heating process, and hinders the phase transition in VO2.[21] Secondly, the interdiffusion at the interface between VO2 and TiO2 affects the phase transition temperature. Since the radius of Ti4+ is smaller than that of V4+, substitution doping will occur when Ti4+ ions are diffused into VO2, resulting in slightly shortened V–O bonds in VO2 and more difficult phase transition. This leads to slightly elevated phase transition temperature and widened hysteresis of the TiO2/VO2/TiO2 thin films.[40] Figure 8c displays the transmittance curves of the TiO2/VO2/TiO2 thin films, and Figure 8d summarizes the optical properties for comparison. The Tlum and ΔTsol values of TiO2/VO2/TiO2 (50/100/50 nm) sandwiched thin films are 50.49% and 17.49%, while those of TiO2/VO2/TiO2 (200/100/200 nm) sandwiched thin films are 47.65% and 20.11%, which are better than the reported results of TiO2/VO2/TiO2 (100/150/190 nm) thin film with Tlum of 30.1% and ΔTsol of 10.2%.[41]8Figurea) X‐ray diffraction (XRD) patterns of VO2, TiO2, and TiO2/VO2/TiO2 thin films. b) The transmittance of thin films at the wavelength of 2500 nm at different temperatures. c) The transmittance curves of the TiO2/VO2/TiO2 films with TiO2 thickness of 0, 50, and 200 nm. d) Comparison of the calculated and experimental results.Figure 9 summarizes the calculated and experimental results, as well as those reported in references, for comparison. Generally, the thin films reported previously could not have high Tlum and large ΔTsol simultaneously. The thin films designed and prepared in this work can overcome this issue. The Tlum and ΔTsol values of the TiO2/VO2 (200/100 nm) thin films can be up to 46.29% and 16.03%, and can be further improved up to 47.65% and 20.11%, respectively, for TiO2/VO2/TiO2 (200/100/200 nm) sandwiched thin films, which are far beyond the available data. Therefore, TiO2/VO2/TiO2 sandwiched thin films exhibit potential applications in smart windows.9FigureComparison of the results in this work with other research.[25,37,41–62]ConclusionIn this work, the optical properties of TiO2/VO2 thin films are studied by optical simulation and experimental measurements. It is found that TiO2 thin films could effectively modulate the optical properties of VO2 owing to the enhanced antireflection. The Tlum value of the TiO2/VO2 thin films with the TiO2 thickness of 200 nm can be up to 46.29%, which is increased by 59.4%, as compared with that of VO2, the ΔTsol value is also improved up to 16.03%. The optical properties of TiO2/VO2/TiO2 sandwiched thin films can be further improved. The Tlum and ΔTsol of the TiO2/VO2/TiO2 (200/100/200 nm) sandwiched thin films can be up to 47.65% and 20.11%, respectively, owing to the enhanced reflection of light at the multiple interfaces. The results provide an idea for design of high‐performance thermochromic thin films.Experimental SectionSimulation Model and MethodThe optical properties of VO2 thin film and TiO2/VO2 multilayer thin films were simulated using the transfer matrix method.[63] The optical constants (n and k) of VO2, TiO2, and the soda‐lime glass were from the experimental results.[23] The thicknesses of VO2 and TiO2 thin films were changed in the ranges of 0–300 nm and 0–500 nm, respectively. In the calculations, wavelength, film thickness, and refractive index were input variables. For the multilayer thin films and for given optical characteristic matrix of each layer, the equivalent thin film characteristic matrix could be obtained by multiplying the matrices. Based on the calculated transmittance spectra, the luminous (lum, 380–780 nm) and solar (sol, 300–2500 nm) properties of the thin films were evaluated as:1Tlum(sol)=∫φlum(sol) (λ)T(λ)dλ/∫φlum(sol)(λ)dλ\[\begin{array}{*{20}{c}}{{T_{{\rm{lum}}\left( {sol} \right)}} = \smallint {\varphi _{{\rm{lum}}\left( {sol} \right)}}\;\left( \lambda \right)T\left( \lambda \right)d\lambda /\smallint {\varphi _{{\rm{lum}}\left( {sol} \right)}}\left( \lambda \right)d\lambda }\end{array}\]in which φlum(λ) denotes the standard spectral sensitivity of the light‐adapted eye, as shown in Figure S5 (Supporting Information); φsol(λ) represents the solar irradiance spectrum for air mass 1.5 corresponding to the sun standing 37° above the horizon, as shown in Figure S6 (Supporting Information);[64] and T(λ) represents the transmittance of the thin film at wavelength λ. Accordingly, ΔTsol could be calculated from the transmittance at 20 °C and 90 °C, as:2Δ Tsol=Tsol,20 ∘C −Tsol,90 ∘C\[\begin{array}{*{20}{c}}{\Delta \;{T_{sol}} = {T_{sol,20{\;^ \circ }{\rm{C}}}}\; - {T_{sol,90{\;^ \circ }{\rm{C}}}}}\end{array}\]Preparation and Characterizations of the Thin FilmsThe TiO2/VO2 multilayer thin films were prepared on the ordinary soda‐lime glass by magnetron sputtering. Prior to the film deposition, the deposition chamber was evacuated to 2 × 10−4 Pa, and 30 sccm pure Ar (99.9995%) was introduced to a pressure of 0.75 Pa. Firstly, V thin films were prepared by radio frequency (RF) sputtering of a vanadium target (d = 50 mm, 99.99% purity) with a power of 100 W for half an hour, and then the V thin films were thermally annealed in O2 atmosphere, resulting in the formation of VO2 thin films. Finally, TiO2 thin films were deposited on the VO2 thin films by reactive sputtering of Ti target (d = 50 mm, 99.99%) in Ar‐O2 mixture with 20% O2 partial pressure. The preparation process of TiO2/VO2 films is presented in Scheme 1, and the preparation conditions for VO2 and TiO2 are listed in Table S1 (Supporting Information). When preparing TiO2/VO2/TiO2 sandwiched films, TiO2 thin film was deposited by magnetron sputtering firstly and then V film. The TiO2/V thin films were annealed in oxygen, and then another TiO2 thin film was deposited. The samples were not taken out from the magnetron sputtering cavity, ensuring the cleanliness between films.1SchemeSchematic diagram of the preparation procedure for TiO2/VO2 films.During the sputtering process, a quartz oscillator was used to monitor the film thickness, and an ellipsometer was used to accurately determine the film thickness. The cross‐section morphology of the thin films was analyzed by the field emission scanning electron microscopy (FESEM, JSM‐7000F). The elemental composition and phase of the thin films were characterized by XPS, equipped with a monochromatic Al Kα source, operating at 12.5 kV/16 mA) and XRD (Bruker D8 Advance X‐ray diffractometer with Cu Kα radiation at 1.542 Å). The surface morphology of the thin films was observed by FESEM, and the element distribution in the thin films was detected by the energy dispersive X‐ray spectroscopy. The absolute spectral transmittance was measured by a Hitachi U‐4100 spectrometer, in the wavelength range of 300–2500 nm, at 20 °C and 90 °C, corresponding to VO2(M) and VO2(R), respectively. The optical transition behavior was studied by measuring the IR transmittance at 2500 nm against the temperature with a heating‐cooling rate of 2 °C min−1, and the phase transition temperature (Tc) is defined as the average of the half‐maximum temperature on the heating and cooling curves, as described previously.[63]AcknowledgementsThis work was jointly supported by National Natural Science Foundation of China (Grant No. 52271136), Natural Science Foundation of Shaanxi Province (Nos. 2021JC‐06 and 2019TD‐020). The authors also acknowledge Research Fellow, Wei Wang and Yanhuai Li of Xi'an Jiaotong University for the help with carrying out SEM and XRD analyses.Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available from the corresponding author upon reasonable request.K. Liu, S. Lee, S. Yang, O. Delaire, J. Wu, Mater. Today 2018, 21, 875.J. Cao, Y. Gu, W. Fan, L. Q. Chen, D. F. 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Sci. 2018, 8, 1751. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Advanced Materials Interfaces Wiley

Design of Antireflection and Enhanced Thermochromic Properties of TiO2/VO2 Thin Films

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Wiley
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© 2023 Wiley‐VCH GmbH
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2196-7350
DOI
10.1002/admi.202202506
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Abstract

IntroductionAs a typical thermochromic material, vanadium dioxide (VO2) can undergo reversible structural phase transition (SPT) from monoclinic phase (M1) to rutile phase (R) at a critical temperature (Tc) of 68 °C. Meanwhile, metal‐insulator phase transition (MIT) occurs,[1] resulting in sudden change of the electrical and optical properties.[2,3] At the temperature lower than Tc, VO2 is an insulator with high optical transmittance; but will be transformed into a metal with higher optical reflectance at the temperature above Tc. The phase transition in VO2 can also be triggered by force, light and electricity.[4–6] The phase transition endows VO2 with potential applications in spacecraft thermal radiation device,[7] supercapacitor,[8] electrode material,[9,10] optical modulator,[11] optical switch,[12] microwave passive electronic device,[13] phase change memory,[14] field effect transistor,[15] micro and nano oscillator,[16] sensor,[17] THz device,[18] and smart glass.[19–21] VO2 thin films on glass can sense the environmental temperature, and switch the transmittance of sunlight in the infrared wavelength range at high and room temperatures, and adjust the input of solar radiation energy and indoor temperature spontaneously. Accordingly, the energy consumption in the buildings could be substantially reduced. However, there are still some obstacles to be overcome for the industrialization of VO2 smart glass. Firstly, the phase transition temperature of VO2 is still too high to be utilized in residence.[21] Secondly, it is difficult to achieve satisfactory luminous transmittance (Tlum) and solar modulation (ΔTsol) at the same time.[19] Thirdly, the preparation of VO2 thin films with good thermochromic properties is difficult: the chemical process is complex with low yield rate, while the physical methods are often carried out at high temperature.[22]A great number of strategies, such as, doping with high valence elements,[23] exerting lattice stress and strain,[24] and manufacturing oxygen vacancies,[25] and so on, have been exploited to lower the Tc value. For example, the Tc value can be lowered by 23 °C if VO2 is doped with 1 at% W6+.[26] Mg2+, Ti4+, Sn4+ and Zr4+ doping can improve the Tlum, but with reduced solar modulation.[27–32] Thin films prepared by colloidal lithography have the Tlum value up to 70.2%, but with ΔTsol value of only 7.9%.[33] The porous thin films prepared by freeze‐drying showed the better thermochromic properties with Tlum of 50% and ΔTsol of 14.7%, because the filling air reduces the refractive loss.[34] However, the above methods are still complex with poor controllability. The antireflective (AR) multilayer thin films might improve the Tlum of VO2 films significantly but often degrade ΔTsol. For example, Hyun Koo et al.[35] used CeO2 as AR film, and improved the Tlum of VO2 up to 67.5%, but with ΔTsol of only 5.4%. Liu et al. demonstrated that SnO2/VO2/SnO2 sandwiched thin films had the Tlum value of 50.2% and the ΔTsol of 15.3%.[36] However, the mechanism for the enhanced modulation by the AR films is still not well known.Herein, TiO2/VO2 AR thin films are designed through optical simulations, the influences of the film stacking configuration on the Tlum and ΔTsol values are discussed. Accordingly, VO2 thin films with optimized thickness are prepared through deposition of V thin films by magnetron sputtering at room temperature and then thermal annealing in oxygen atmosphere, while TiO2 thin films with designed thickness are prepared by reaction magnetron sputtering. 200 nm TiO2/ 100 nm VO2 thin films show excellent overall thermochromic properties with the Tlum value of 46.29% and the ΔTsol value of 16.03%, which is consistent with the simulation results. The Tlum and ΔTsol values of TiO2/VO2/TiO2 sandwiched thin films can be up to 50.49% and 20.11%, respectively. The design, preparation, and improvement mechanism of the films are discussed.Results and DiscussionDesign of Thin Films by Theoretical SimulationsUsing the optical characteristic matrix and the optical constants, the Tlum and ΔTsol values of VO2 thin films with different thicknesses are calculated according to the Equations (1) and (2), and Figure 1a shows the results. The Tlum value of VO2 thin films is reduced with increasing film thickness. However, the ΔTsol value increases firstly and then decreases with increasing film thickness, owing to the enhanced absorption and scattering of sunlight.[37] Therefore, the thickness of VO2 thin films should be optimized to achieve both high Tlum and ΔTsol. Herein, the optimized thickness of VO2 thin films is 100 nm, for which the Tlum and ΔTsol can be well balanced with the values of 33.53% and 14.81%, respectively. Figure 1b shows the optical transmittance of the VO2 thin film with a thickness of 100 nm. A large gap exists in the transmittance spectra of infrared band at 90 °C and 20 °C. As shown in Figure S1 (Supporting Information), the suitable refractive index (n) of the AR film for VO2 film is 2.0–2.5. Because the refractive index of the as‐prepared TiO2 films is 2.1–2.2, and the preparation of TiO2 is simple and economical, TiO2 is selected as the AR film. Figure 1c illustrates the optical properties of TiO2/VO2 thin films. As the thickness of TiO2 is increased from 0 to 500 nm, the ΔTsol value increases firstly, and then remains at a constant value, that is, it is feasible to improve the solar modulation property of VO2 thin films through adjusting the thickness of TiO2. As displayed in Figure S2 (Supporting Information), the Tlum value changes periodically with increasing thickness of TiO2 due to the interference effect. Referencing to Figure 1c, the highest Tlum can be obtained at the TiO2 film thickness of around 50 nm. However, if both Tlum and ΔTsol are taken into account, the optimized thickness of TiO2 should be at about 200 nm. So TiO2 antireflective films with the thickness of 50–200 nm are involved in the following to study the effects of AR film thickness.1FigureThe calculation results: a) Tlum and ΔTsol of VO2 films. b) Transmittance curve of VO2 thin films with thickness of 100 nm. c) Optical properties of TiO2/VO2 thin films as a function of the TiO2 thickness. d) Transmittance curves of TiO2/VO2 bilayer thin films.Figure 1d shows the calculated transmittance curves of the TiO2/VO2 thin films with the TiO2 thickness of 0, 50, 100, 150, and 200 nm, at 20 °C. The transmittance curves in visible light are consistent with the above analysis. For the TiO2 film thickness of 50 nm and 200 nm, the transmittance curves have a peak at around 550 nm, accordingly these two film systems have higher Tlum. Moreover, a transmittance peak is evidenced in the near‐infrared light band, and it will be shifted to higher band with increasing TiO2 thickness. The transmittance to infrared radiation is increased at 20 °C, but not at 90 °C (Figure S3, Supporting Information), due to the temperature dependent refractive index of VO2. Hence, the TiO2/VO2 thin films can be designed to play an AR role at 20 °C, but not at 90 °C. So, the modulation ability of VO2 to infrared radiation is improved by the thicker TiO2 antireflection layer. The Tlum and ΔTsol values of the TiO2/VO2 thin films with the TiO2 film thickness of 200 nm can be up to 52.5% and 20.2% respectively, which is higher than that of VO2 films by 56.58% and 36.39%.Structure of the As‐Prepared VO2 and TiO2/VO2 Thin FilmsFigure 2a shows the X‐ray diffraction (XRD) patterns of the V thin films after thermal annealing at 550 °C but at different oxygen partial pressure for 2 hours. When the thermal annealing is done at the oxygen partial pressure of 10 Pa, the XRD peaks can be ascribed to V2O3 (JCPDS no.71‐0345) and VO2 (M) (JCPDS no.75‐0514).[23] The XRD peaks of VO2 (M) are significantly enhanced at the oxygen partial pressure of 15 Pa. However, at the oxygen partial pressure of 20 Pa, the XRD peaks at 30.94° and 34.16° are ascribed to V2O5 (JCPDS no.89‐0611), owing to excessive oxygen. Based on this, the oxygen partial pressure of 15 Pa is adopted in the following experiments, and the annealing time is elongated appropriately. Figure 2b shows the XRD pattern of the film annealed at 550 °C for 4 h, the XRD peaks of VO2(M) are substantially enhanced, indicating the formation of highly crystalline VO2 (M). Figure 2c,d shows the survey X‐ray photoelectron spectroscopy (XPS) spectra and the XPS spectra of V 2p and O 1s. O and V are from the VO2 films, but C is from the surface contamination. Two splitting energy levels of V2p are evidenced, and the binding energy difference between V 2p3/2 and O 1s is 13.87 eV, confirming the formation of VO2 (M).[38,39] Figure S4 (Supporting Information) shows the XPS spectra along the depth. The close position of V indicates the uniform composition in the films.2Figurea) X‐ray diffraction (XRD) patterns of VOx films annealed at different oxygen fluxes. b–d) XRD patterns and X‐ray photoelectron spectroscopy (XPS) spectra of VO2(M) after annealing under 15 Pa O2 partial pressure at 550 °C for 4 h. b) XRD patterns, c) XPS survey spectra, d) XPS spectra of V 2p and O 1s.Figure 3a displays the scanning electron microscopy (SEM) images of the thin films, and Figure 3b shows the particle size distribution. The grain size in the films increases with the oxygen partial pressure and annealing time gradually. As displayed in the inset of Figure 3a, the transmittance of the films is improved with increasing oxygen partial pressure. When the V film is annealed under 20 Pa oxygen partial pressure, the film becomes light gray rather than the yellowish brown of VO2 film. Figure 3c shows the element distribution in the annealed thin films, and V and O are uniformly distributed. Figure 3d shows the SEM image of the cross‐section of the thin film, confirming that the thickness of the VO2 thin film is about 100 nm.3Figurea1–a4) SEM images of the thin films annealed at different conditions: a1) 10 Pa O2‐550 °C ‐2 h, a2) 15 Pa O2‐550 °C ‐2 h, a3) 20 Pa O2‐550 °C ‐2 h, a4) 15 Pa O2‐550 °C ‐4 h, the insets are the photographs. b1—b4) particle size distribution in (a1–a4). c1–c3) Elemental mapping in the selected area in (a4). d) SEM image of the cross‐section of the thin film (a4).Figure 4a,b displays the survey XPS spectra and the XPS spectra of Ti 2p. The peaks of Ti 2p, Ti 2s, and O 1s orbitals are evidenced, and the binding energy difference of the two splitting levels of Ti 2p is 5.7 eV, indicating the existence of Ti4+. Figure 4c shows the thickness of TiO2 thin films as a function of sputtering time, and the deposition rate is calculated to be 90 nm h−1. Figure 4d,e shows the SEM image and the elemental distribution in the TiO2 thin films. The film surface is very dense, and Ti and O are uniformly distributed, indicating the uniform composition in the film.4Figurea,b) X‐ray photoelectron spectroscopy (XPS) spectra of TiO2 thin films; a) XPS survey spectra, b) XPS spectra of Ti2p. c) The thickness of TiO2 thin films as a function of sputtering time. d) SEM images of the TiO2 thin film; e1,e2) elemental mapping of the selected area in (d).Performances of VO2 and TiO2/VO2 Thin FilmsAs shown in Figure 5a, the transmittance curves of the thin films at 20 °C and 90 °C show great difference in the infrared band, owing to the phase transition. The Tlum value of the VO2 thin film with thickness of 100 nm is 29.03%, and the ΔTsol is 15.8%, which is in good agreement with the calculated results. Figure 5b shows the transmittance curves of TiO2/VO2 thin films with the TiO2 thickness of 50, 60, and 70 nm and Figure 5c displays the magnified curves in the visible band. The transmittance curves of both TiO2/VO2 thin films and VO2 thin films show a peak in the visible band, while there is little difference in the infrared band. However, the transmittance of TiO2/VO2 thin films is significantly increased in the visible band due to the strongly suppressed reflectance, and the peak value of the visible transmittance becomes nearer to the peak position of the standard spectral sensitivity of the light‐adapted eye at 555 nm (Figure S5, Supporting Information), contributing to the Tlum enhancement effect. If the thickness of TiO2 AR layer is increased up to 60 nm, the Tlum of TiO2/VO2 thin films is up to 47.06%, which is higher than that of VO2 thin film by 62.11%, with the ΔTsol value of 14.36%. If the 200 nm thick AR TiO2 layer is utilized, the transmittance of the TiO2/VO2 thin films changes greatly. As shown in Figure 5d, the transmittance of TiO2/VO2 thin films is significantly increased in the visible band, and a peak emerges in the near infrared band at 20 °C, but changes little at 90 °C. Figure 5e depicts the enlarged transmittance curve in the visible band. For the 200 nm thick TiO2 AR layer, the transmittance peak appears at about 600 nm in visible range. The Tlum and ΔTsol values of TiO2/VO2 (200/100 nm) thin films can be up to 46.29% and 16.03%, respectively, which are substantially improved, as compared with VO2 thin films. As shown in Figure 5f, the measured Tlum and ΔTsol values of the TiO2/VO2 thin films change almost with the same trend as that of the simulation results.5Figurea) The transmittance curve of VO2 (100 nm). b) The transmittance curve of the TiO2/VO2 thin films with TiO2 thickness of 0, 50, 60, 70 nm. c) Amplified curves in the visible band. d) The transmittance curve of the TiO2/VO2 thin films with the TiO2 thickness of 100, 150, 200, and 250 nm. e) Amplified curves in the visible band. f) Comparison of the calculated and experimental results.Optical Properties of TiO2/VO2/TiO2 Sandwiched Thin FilmsBased on the above results, if the AR TiO2 films are deposited on both sides of VO2 thin films, forming TiO2/VO2/TiO2 sandwiched thin films, the optical properties might be further improved. The eigenmatrix method is adopted to predict the performances of the film systems. Figure 6a,b shows the calculated Tlum and ΔTsol values of the films, respectively. The optical properties of the TiO2/VO2/TiO2 sandwiched thin films depend on the TiO2 thickness, which is similar to that of TiO2/VO2. For thinner TiO2 thin films, the TiO2/VO2/TiO2 sandwiched thin films show better antireflection but degraded modulation. As shown in Figure 6c, the AR effect of the TiO2/VO2/TiO2 sandwiched thin films with both the upper and lower TiO2 layers of 50 nm is the best, with Tlum up to 69.49%, which is higher than that of VO2 by 107.25%, but the ΔTsol is only 11.49%. The modulation can be improved if thicker TiO2 films are utilized. Figure 6d shows the transmittance curves of the TiO2/VO2/TiO2 sandwiched thin films with the TiO2 film thickness of 100, 150, and 200 nm. Two peaks appear in the visible and near‐infrared band. For the film system with the TiO2 film thickness of 200 nm, the Tlum and ΔTsol can be up to 60.20% and 18.72%, respectively, which are higher than that of VO2 by 79.54% and 26.40%.6Figurea–d) Calculation results of TiO2/VO2/TiO2 films. a) Optical modulation capability and b) visible light transmittance as the thickness of TiO2 films changes. c) Transmittance curves of TiO2 (50 nm) /VO2(100 nm)/TiO2(50 nm). d) The transmittance curves of the TiO2/VO2/TiO2 sandwiched thin films with TiO2 thickness of 100, 150, and 200 nm, respectively.The TiO2/VO2/TiO2 sandwiched thin films are prepared by magnetron sputtering. Figure 7a,b displays the SEM images of TiO2/VO2/TiO2 thin films. Figure 7c shows the SEM images of the cross‐section of TiO2 (200 nm), VO2/TiO2 (100/200 nm), and TiO2/VO2/TiO2 (200/100/200 nm) thin films, and the interfaces between thin films are clear. Figure 7d shows the optical photographs of the naked glass (d1), VO2 thin film (d2), TiO2/VO2 films (d3‐d4), and TiO2/VO2/TiO2 sandwiched films (d5‐d6). Compared with VO2 thin film, the visible light transmittance of TiO2/VO2 thin films is improved substantially, particularly, the TiO2/VO2/TiO2 (50/100/50 nm) thin film sample.7FigureSEM images of TiO2/VO2/TiO2 thin films with thicknesses of a) 50/100/50 nm and b) 200/100/200 nm. The cross‐section SEM images of c1) TiO2 (200 nm), c2) VO2/TiO2 (100/200 nm), and c3) TiO2/VO2/TiO2 (200/100/200 nm) thin films. d) Optical photographs of d1) naked glass, d2) VO2 (100 nm) film, d3) TiO2/VO2 (50/100 nm) film, d4) TiO2/VO2 (200/100 nm) film, d5) TiO2/VO2/TiO2 (50/100/50 nm) film, and d6) TiO2/VO2/TiO2 (200/100/200 nm) film at 20 °C.Figure 8a shows the XRD patterns of TiO2, VO2, and TiO2/VO2/TiO2 sandwiched thin films for comparison. The peaks of TiO2 (A) and VO2 (M) are evidenced in the TiO2/VO2/TiO2 thin films. The TiO2 thin film under VO2 is in anatase structure, but the TiO2 thin film on VO2 is amorphous if no annealing is done. The refractive indices of both polycrystalline and amorphous TiO2 films in this work are in the range of 2.1–2.2, and thus affect the antireflection little. The infrared (IR) transmittance at 2500 nm against temperature is measured for the VO2 and TiO2/VO2/TiO2 thin films, and the results are shown in Figure 8b. The phase transition temperatures of the VO2 and TiO2/VO2/TiO2 thin films are 68.5 °C and 72 °C, respectively. As compared with VO2 (M), the phase transition temperature of TiO2/VO2/TiO2 thin films is slightly elevated with widened hysteresis, which might be ascribed to two reasons. Firstly, since the thermal expansion coefficient of VO2 is larger than that of TiO2, the thermal compressive stress is generated in the VO2 thin films during heating process, and hinders the phase transition in VO2.[21] Secondly, the interdiffusion at the interface between VO2 and TiO2 affects the phase transition temperature. Since the radius of Ti4+ is smaller than that of V4+, substitution doping will occur when Ti4+ ions are diffused into VO2, resulting in slightly shortened V–O bonds in VO2 and more difficult phase transition. This leads to slightly elevated phase transition temperature and widened hysteresis of the TiO2/VO2/TiO2 thin films.[40] Figure 8c displays the transmittance curves of the TiO2/VO2/TiO2 thin films, and Figure 8d summarizes the optical properties for comparison. The Tlum and ΔTsol values of TiO2/VO2/TiO2 (50/100/50 nm) sandwiched thin films are 50.49% and 17.49%, while those of TiO2/VO2/TiO2 (200/100/200 nm) sandwiched thin films are 47.65% and 20.11%, which are better than the reported results of TiO2/VO2/TiO2 (100/150/190 nm) thin film with Tlum of 30.1% and ΔTsol of 10.2%.[41]8Figurea) X‐ray diffraction (XRD) patterns of VO2, TiO2, and TiO2/VO2/TiO2 thin films. b) The transmittance of thin films at the wavelength of 2500 nm at different temperatures. c) The transmittance curves of the TiO2/VO2/TiO2 films with TiO2 thickness of 0, 50, and 200 nm. d) Comparison of the calculated and experimental results.Figure 9 summarizes the calculated and experimental results, as well as those reported in references, for comparison. Generally, the thin films reported previously could not have high Tlum and large ΔTsol simultaneously. The thin films designed and prepared in this work can overcome this issue. The Tlum and ΔTsol values of the TiO2/VO2 (200/100 nm) thin films can be up to 46.29% and 16.03%, and can be further improved up to 47.65% and 20.11%, respectively, for TiO2/VO2/TiO2 (200/100/200 nm) sandwiched thin films, which are far beyond the available data. Therefore, TiO2/VO2/TiO2 sandwiched thin films exhibit potential applications in smart windows.9FigureComparison of the results in this work with other research.[25,37,41–62]ConclusionIn this work, the optical properties of TiO2/VO2 thin films are studied by optical simulation and experimental measurements. It is found that TiO2 thin films could effectively modulate the optical properties of VO2 owing to the enhanced antireflection. The Tlum value of the TiO2/VO2 thin films with the TiO2 thickness of 200 nm can be up to 46.29%, which is increased by 59.4%, as compared with that of VO2, the ΔTsol value is also improved up to 16.03%. The optical properties of TiO2/VO2/TiO2 sandwiched thin films can be further improved. The Tlum and ΔTsol of the TiO2/VO2/TiO2 (200/100/200 nm) sandwiched thin films can be up to 47.65% and 20.11%, respectively, owing to the enhanced reflection of light at the multiple interfaces. The results provide an idea for design of high‐performance thermochromic thin films.Experimental SectionSimulation Model and MethodThe optical properties of VO2 thin film and TiO2/VO2 multilayer thin films were simulated using the transfer matrix method.[63] The optical constants (n and k) of VO2, TiO2, and the soda‐lime glass were from the experimental results.[23] The thicknesses of VO2 and TiO2 thin films were changed in the ranges of 0–300 nm and 0–500 nm, respectively. In the calculations, wavelength, film thickness, and refractive index were input variables. For the multilayer thin films and for given optical characteristic matrix of each layer, the equivalent thin film characteristic matrix could be obtained by multiplying the matrices. Based on the calculated transmittance spectra, the luminous (lum, 380–780 nm) and solar (sol, 300–2500 nm) properties of the thin films were evaluated as:1Tlum(sol)=∫φlum(sol) (λ)T(λ)dλ/∫φlum(sol)(λ)dλ\[\begin{array}{*{20}{c}}{{T_{{\rm{lum}}\left( {sol} \right)}} = \smallint {\varphi _{{\rm{lum}}\left( {sol} \right)}}\;\left( \lambda \right)T\left( \lambda \right)d\lambda /\smallint {\varphi _{{\rm{lum}}\left( {sol} \right)}}\left( \lambda \right)d\lambda }\end{array}\]in which φlum(λ) denotes the standard spectral sensitivity of the light‐adapted eye, as shown in Figure S5 (Supporting Information); φsol(λ) represents the solar irradiance spectrum for air mass 1.5 corresponding to the sun standing 37° above the horizon, as shown in Figure S6 (Supporting Information);[64] and T(λ) represents the transmittance of the thin film at wavelength λ. Accordingly, ΔTsol could be calculated from the transmittance at 20 °C and 90 °C, as:2Δ Tsol=Tsol,20 ∘C −Tsol,90 ∘C\[\begin{array}{*{20}{c}}{\Delta \;{T_{sol}} = {T_{sol,20{\;^ \circ }{\rm{C}}}}\; - {T_{sol,90{\;^ \circ }{\rm{C}}}}}\end{array}\]Preparation and Characterizations of the Thin FilmsThe TiO2/VO2 multilayer thin films were prepared on the ordinary soda‐lime glass by magnetron sputtering. Prior to the film deposition, the deposition chamber was evacuated to 2 × 10−4 Pa, and 30 sccm pure Ar (99.9995%) was introduced to a pressure of 0.75 Pa. Firstly, V thin films were prepared by radio frequency (RF) sputtering of a vanadium target (d = 50 mm, 99.99% purity) with a power of 100 W for half an hour, and then the V thin films were thermally annealed in O2 atmosphere, resulting in the formation of VO2 thin films. Finally, TiO2 thin films were deposited on the VO2 thin films by reactive sputtering of Ti target (d = 50 mm, 99.99%) in Ar‐O2 mixture with 20% O2 partial pressure. The preparation process of TiO2/VO2 films is presented in Scheme 1, and the preparation conditions for VO2 and TiO2 are listed in Table S1 (Supporting Information). When preparing TiO2/VO2/TiO2 sandwiched films, TiO2 thin film was deposited by magnetron sputtering firstly and then V film. The TiO2/V thin films were annealed in oxygen, and then another TiO2 thin film was deposited. The samples were not taken out from the magnetron sputtering cavity, ensuring the cleanliness between films.1SchemeSchematic diagram of the preparation procedure for TiO2/VO2 films.During the sputtering process, a quartz oscillator was used to monitor the film thickness, and an ellipsometer was used to accurately determine the film thickness. The cross‐section morphology of the thin films was analyzed by the field emission scanning electron microscopy (FESEM, JSM‐7000F). The elemental composition and phase of the thin films were characterized by XPS, equipped with a monochromatic Al Kα source, operating at 12.5 kV/16 mA) and XRD (Bruker D8 Advance X‐ray diffractometer with Cu Kα radiation at 1.542 Å). The surface morphology of the thin films was observed by FESEM, and the element distribution in the thin films was detected by the energy dispersive X‐ray spectroscopy. The absolute spectral transmittance was measured by a Hitachi U‐4100 spectrometer, in the wavelength range of 300–2500 nm, at 20 °C and 90 °C, corresponding to VO2(M) and VO2(R), respectively. 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Journal

Advanced Materials Interfaces – Wiley

Published: May 1, 2023

Keywords: antireflective TiO 2 /VO 2 thin films; magnetron sputtering; thermochromic properties; transfer matrix method

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