Dataset and publication "Design and Scalable Synthesis of Thermochromic VO2-Based Coatings for Energy-Saving Smart Windows with Exceptional Optical Performance"

Design and Scalable Synthesis of Thermochromic VO2‑Based Coatings for Energy-Saving Smart Windows with Exceptional Optical Performance Michal Kaufman, Jaroslav Vlcek,*JiríHouska, Sadoon Farrukh, and Stanislav Haviar Cite This: ACS Appl. Mater. Interfaces 2024, 16, 57268−57276 Read Online ACCESS Metrics & More Article Recommendations ABSTRACT: We report strongly thermochromic YSZ/V0.855W0.018Sr0.127O2/SiO2coatings, where YSZ is Y-stabilized ZrO2, prepared by using a scalable deposition technique on standard glass at a low substrate temperature of 320 °C and without any substrate bias voltage. The coatings exhibit a transition temperature of 22 °C with an integral luminous transmittance of 63.7% (low-temperature state) and 60.7% (high-temperature state) and a modulation of the solar energy transmittance of 11.2%. Such a combination of properties, together with the low deposition temperature, fulfills the requirements for large-scale implementation on building glass and has not been reported yet. Reactive high-power impulse magnetron sputtering with a pulsed O2flow feedback control allows us to prepare crystalline W and Sr codoped VO2of the correct stoichiometry. The W doping of VO2 decreases the transition temperature, while the Sr doping of VO2increases the luminous transmittance significantly. A coating design utilizing second-order interference in two antireflection layers is used to maximize both the integral luminous transmittance and the modulation of the solar energy transmittance. A compact crystalline structure of the bottom YSZ antireflection layer further improves the VO2crystallinity, while the top SiO2antireflection layer provides also the mechanical and environmental protection for the V0.855W0.018Sr0.127O2layer. KEYWORDS: doped vanadium dioxide, strongly thermochromic coatings, low transition temperature, scalable synthesis, low deposition temperature, smart windows 1. INTRODUCTION Global warming and the energy crisis drive a focus on energysaving materials. Buildings have been estimated to produce about 20% of all anthropogenic greenhouse gas emissions 1 and are responsible for up to 40% of the primary energy consumption 2 in the world. Approximately 50% of the total building energy is consumed for compensating the heat gains and losses via windows and glass facades, 3 which are the most energy-inefficient components of buildings. It is evident that energy-saving smart windows with adjustable throughput of solar energy can lower the energy expenditure. 4 Vanadium dioxide (VO2) exhibits a reversible phase transition from a low-temperature monoclinic VO2(M1) semiconducting phase to a high-temperature tetragonal VO2 (R) metallic phase at a transition temperature (Ttr) of approximately 68 °C for the bulk material. 5 The Ttr can be lowered using doping of VO2with other elements (such as W). 6,7 The automatic (i.e., without any switch system) response to temperature and the abrupt decrease of infrared transmittance with almost the same luminous transmittance (allowing us to utilize daylight) at the transition into the metallic state make VO2-based coatings a promising candidate for thermochromic (TC) smart windows reducing the energy consumption of buildings. Magnetron sputter deposition with its versatility and ease of scaling up to large substrate sizes is probably the most important preparation technique of TC VO2-based coatings. 7−11 Note that magnetron sputter sources are used very frequently not only in glass production lines (e.g., for deposition of low-emissivity coatings) but also in large-scale roll-to-roll deposition devices 12 producing coatings on ultrathin flexible glass or polymer foils. Moreover, the atom-byatom magnetron co-sputtering is a much simpler and much more effective method than, e.g., chemical methods, for a doping of the VO2layers with other elements. To meet the requirements for large-scale implementation on building glass (glass panes or flexible glass and polymer foils Received: April 8, 2024 Revised: August 28, 2024 Accepted: September 24, 2024 Published: October 14, 2024 Research Articlewww.acsami.org © 2024 The Authors. Published by American Chemical Society 57268 https://doi.org/10.1021/acsami.4c05696 ACS Appl. Mater. Interfaces 2024, 16, 57268−57276 This article is licensed under CC-BY 4.0 Downloaded via UNIV OF WEST BOHEMIA on December 16, 2024 at 10:11:30 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. laminated to glass panes), VO2-based coatings should satisfy the following strict criteria simultaneously: a maximum substrate temperature (Ts) during the preparation (deposition and possible postannealing) close to 300 °C or lower, 7−9,13,14 Ttr close to 25 °C or lower, 15 an integral luminous transmittance Tlum > 60%, 16−18 a modulation of the solar energy transmittance ΔTsol > 10%, 19−21 long-term environmental stability, 10,22−24 and a more appealing color 25,26 than the usual yellowish or brownish colors in transmission. The simultaneous fulfillment of these requirements has not yet been reported in the literature. A major challenge is to achieve the high Tlum and ΔTsol at a relatively low Ttr and Ts. 7−27 Reactive high-power impulse magnetron sputtering (HiPIMS) has been found to be a promising deposition technique for a low-temperature (300−350 °C) preparation of undoped TC VO2films. 13,14,18,22,28,29 In our recent paper, 30 we presented a scalable 31 deposition technique used for a lowtemperature preparation of high-performance three-layer ZrO2/V0.982W0.018O2/ZrO2coatings on soda-lime glass (SLG). The TC V0.982W0.018O2layers were deposited by a controlled HiPIMS of a V target, combined with a Figure 1. (a) Schematic energy band diagram with two gaps Eg1 and Eg2 for pure VO2(M1). Reproduced with permission. 33 Copyright 2012, AIP Publishing. (b) (αE)1/2 as a function of the photon energy E, where αis the absorption coefficient, for the YSZ(167 nm)/V0.855W0.018Sr0.127O2(71 nm) coating (denoted as VWSrO) and the YSZ (178 nm)/V0.984W0.016O2(73 nm) coating (denoted as VWO) on 1 mm-thick glass at Tms =−20 °C and Tmm = 70 °C. At −20 °C, linear fittings are performed to extract Eg1 and Eg2 as discussed in the text. Spectral dependences of the refractive index (c) and the extinction coefficient (d) measured for the same two-layer coatings at the same temperatures as in (b). Figure 2. (a) Optical model used to measure the characteristics of the coating without the top layer (glass/YSZ/V0.855W0.018Sr0.127O2, including the thick surface roughness layer on the V0.855W0.018Sr0.127O2layer) and to subsequently design the top layer (glass/YSZ/V0.855W0.018Sr0.127O2/SiO2, including the thick intermix layer�instead of the aforementioned surface roughness layer�between V0.855W0.018Sr0.127O2and SiO2). (b,c) Predicted Tlum (average of the similar Tlum values at Tms =−20 °C and Tmm = 70 °C) and ΔTsol, respectively, as a function of the thickness of the bottom YSZ (hb) and the top SiO2(ht) for the V0.855W0.018Sr0.127O2thickness h= 71 nm (value measured before the deposition of the top layer: 56 nm bulk + half of the 30 nm roughness). ACS Applied Materials & Interfaces www.acsami.org Research Article https://doi.org/10.1021/acsami.4c05696 ACS Appl. Mater. Interfaces 2024, 16, 57268−57276 57269 simultaneous pulsed DC magnetron sputtering of a W target (doping of VO2with W to reduce the Ttr to 20 °C without any degradation of TC properties), at Ts= 330 °C in an argon− oxygen gas mixture. The coatings exhibited Tlum = 49.9% (below the Ttr) and 46.0% (above the Ttr) and ΔTsol = 10.4% for a V0.982W0.018O2layer thickness of 69 nm. In this study, we report the design and scalable synthesis of strongly TC YSZ/V0.855W0.018Sr0.127O2/SiO2coatings, where YSZ is Y-stabilized ZrO2, which fulfill the aforementioned requirements for large-scale implementation on building glass. The coatings exhibit a transition temperature Ttr = 22 °C with an integral luminous transmittance Tlum = 63.7% (below the Ttr) and 60.7% (above the Ttr) and a modulation of the solar energy transmittance ΔTsol = 11.2% for a V0.855W0.018Sr0.127O2 layer thickness of 71 nm. We have modified the sputter deposition technique, based on reactive HiPIMS, to perform a controlled codoping of VO2with W, shifting the Ttr to room temperature, and with Sr, increasing the Tlum substantially. Reactive HiPIMS with a pulsed O2flow feedback control allowed us to prepare crystalline VO2of the correct stoichiometry at a low substrate temperature Ts= 320 °C and without any substrate bias voltage. An original design of a three-layer VO2-based coating utilizing second-order interference in two antireflection (AR) layers was applied to increase both the Tlum and the ΔTsol. A compact crystalline structure of the bottom YSZ AR layer further improves the VO2 crystallinity and the process reproducibility, while the top SiO2AR layer also provides mechanical and environmental protection for the TC V0.855W0.018Sr0.127O2layer. 2. RESULTS AND DISCUSSION The results presented in this section are for two TC layers: an optimized V0.855W0.018Sr0.127O2(see Section 4.1) and an optimized V0.984W0.016O2(prepared for comparative purposes with about the same layer thickness and W content but without Sr). The section is organized as follows. First, we analyze how the Sr incorporation affects the electronic structure and, in turn, optical constants (Figure 1). Second, we discuss how to translate the benefits of Sr incorporation into application potential as high as possible (Figure 2 and Table 1). Third, we present the excellent performance of the subsequently prepared optimized coating and compare it with the state of the art (Figure 3 and Table 2). 2.1. Effect of W and Sr Codoping on Optical Band Gaps and Optical Properties of a Two-Layer YSZ/ V0.855W0.018Sr0.127O2Coating. The electronic structure of the low-temperature VO2(M1) semiconducting phase is complex and includes two gaps which affect its functional properties. 32,33 First, there is a band gap in the narrow sense of the word (Eg2 in Figure 1a) between two bands made up predominantly of V 3d orbitals: the filled lower part of the split d|| band and the empty π*band. The width of this gap is in the infrared spectral range with an often reported value of ≈0.6 eV. 34−36 The V atoms are paired and in turn, the d|| band is split into two only in VO2(M1), while at the transition to VO2(R), this gap closes. 34−36 The consequently enhanced concentration of free charge carriers has a direct effect, especially on the contribution of infrared wavelengths to ΔTsol. Second, there is a gap (Eg1 in Figure 1a) between the secondhighest filled band made up predominantly of O 2p orbitals and once again the empty π*band. The width of this gap is in the visible range. Therefore, it gives rise to the interband transitions, which have a direct effect on Tlum and the coating color, and there are worldwide efforts to improve these characteristics via controlling Eg1. The values of Eg1 and Eg2, both before (V0.984W0.016O2) and after (V0.855W0.018Sr0.127O2) Sr incorporation, are shown in Figure 1b: Tauc plot (αE)1/2 ∼E−Eg, where αis the absorption coefficient (neglecting the absorption in glass and YSZ) and the exponent 1/2 is valid for indirect 33,37 allowed transitions. The results of optical measurements are shown for both phases, and the optical gaps of phase M1 are obtained by using tangents to linear parts of the low-temperature dependencies. Indeed, the incorporation of 12.7 at. % Sr into the metal sublattice at an almost fixed W content of 1.6−1.8 at. % in the metal sublattice led to a clear widening of the visiblerange gap, increasing Eg1 from 1.51 to 1.75 eV. The trend is in agreement with the previously reported results of doping VO2 with Sr 37 or codoping VO2with W and Sr. 20 A case can be made that the aforementioned elemental compositions are averaged over crystal grains and their amorphous boundaries, i.e., that the true value of the gradient (1.75 −1.51)/12.7 = 0.019 eV/at. % Sr may be even higher. The enhancement of Eg1 is also consistent with lowering of the extinction coefficient in the whole visible range, qualitatively observable over here (at a given energy, αEis proportional to k) and quantified next. While the figure also indicates that the effect of Sr on Eg2 is opposite to that on Eg1, the effect of the infrared-range gap on visible-range properties is arguably less direct, and its narrowing does not constitute a problem as long as the coating performance (Figure 3) is high. The effect of incorporation of 12.7 at. % Sr into the metal sublattice on n(λ) and k(λ), as measured by spectroscopic ellipsometry, is shown in Figure 1c,d, respectively. On the one hand, both optical constants of both phases exhibit qualitatively similar dispersions with and without Sr. On the other hand, there are important quantitative differences. First, the Sr incorporation leads in most of the wavelength range studied to a lower n(λ) of both phases. To put a quantitative example, n550 decreased from 2.84 (R) to 3.13 (M1) without Sr to 2.38 (R) to 2.58 (M1) with Sr. This affects not only the best achievable coating performance but also the way this best performance is achieved (choice of materials for AR layers) as discussed in the next section. The observation is consistent with the usual trend: lower polarizability per unit volume and in turn lower refractive index of oxides of main group elements (n550 = 1.88 for pure SrO) 38 compared to oxides of transition metals. Second and probably most importantly, the Sr incorporation leads to a significantly lower k(λ) in the visible. To put a quantitative example, k550 decreased from 0.45 (R) to 0.47 (M1) without Sr to 0.29 (R) to 0.26 (M1) with Sr. This Table 1. Optimum Thickness of the Bottom Second-Order AR YSZ Layer, Optimum Thickness of the Top SecondOrder AR SiO2Layer, and the Tlum and ΔTsol Values Which These Layers Lead To a VWSrO (nm) YSZ (nm) SiO2(nm) Tlum (%) ΔTsol (%) 40 190 274 72.6 7.41 55 179 270 66.9 9.22 70 171 269 61.4 10.89 85 163 273 55.5 12.38 100 160 280 49.6 13.61 a The data are shown for the thickness of the TC V0.855W0.018Sr0.127O2 layer (denoted as VWSrO) from 40 to 100 nm. ACS Applied Materials & Interfaces www.acsami.org Research Article https://doi.org/10.1021/acsami.4c05696 ACS Appl. Mater. Interfaces 2024, 16, 57268−57276 57270 confirms the success of the efforts to enhance Eg1 and opens a pathway toward TC layers possessing higher Tlum (at a given thickness) or higher ΔTsol (because lower k550 allows a higher thickness) or both. The fact that the lowering of k550 of the low-temperature phase M1 is even larger than that of the hightemperature phase R is also beneficial because it can improve the contribution of visible wavelengths to ΔTsol. Third, the Sr incorporation leads to a less steeply increasing k(λ) of the metallic phase R in the infrared, i.e., to somewhat weaker TC transition compared to that exhibited by pure or only W-doped VO2. Again, this does not constitute a problem as long as it is more than compensated for by the aforementioned benefits and the coating performance (Figure 3) is high. 2.2. Design of Three-Layer YSZ/V0.855W0.018Sr0.127O2/ SiO2Coatings. The design of our TC coating is shown in Figure 2a. The overall idea has been utilized 30,31,39,40 previously: the active TC layer (thickness h) is combined with a bottom AR layer (which constitutes also a crystalline template; thickness hb) and a top AR layer (which constitutes also a mechanical and environmental protection; thickness ht). In order to achieve as high efficiency of the AR layers [as high upper envelope of T(λ)] as possible, their refractive indices should be between that of glass and the TC layer (bottom AR layer) and between that of air and the TC layer (top AR layer), ideally neglecting finite hand k(λ) of the TC layer as a square root of the corresponding product. The pure ZrO2 30,31,39 or Ystabilized ZrO2, 40 used by us previously as a material for both AR layers around Sr-free W-doped VO2, possesses acceptable n(λ) and negligible k(λ) in the visible, high hardness (for an oxide), easily achievable crystallinity and controllable crystal orientation, and high potential to serve as a crystalline template (especially in the case of YSZ). In our recent work, 40 we showed that the crystal structure of the bottom YSZ layer, formed by tetragonal YSZ crystal grains, is much more Figure 3. (a) X-ray diffraction (XRD) pattern taken at Tm= 25 °C from the YSZ(167 nm)/V0.855W0.018Sr0.127O2(71 nm)/SiO2(280 nm) coating on 1 mm-thick glass. The main diffraction peaks of VO2(M1), VO2(R), and YSZ (tetragonal Y0.06Zr0.94O1.97) are marked. (b) Temperature dependence of the transmittance at λ= 2500 nm for the YSZ(167 nm)/V0.855W0.018Sr0.127O2(71 nm)/SiO2(280 nm) coating (denoted as VWSrO) and the YSZ(178 nm)/V0.984W0.016O2(73 nm)/SiO2(280 nm) coating (denoted as VWO). The transition temperatures are also given. (c) Spectral transmittance measured for the same three-layer coatings as in (b) at Tms =−20 °C and Tmm = 70 °C. The contours of the shaded areas represent the luminous sensitivity of the human eye (φlum) and the solar irradiance spectrum (φsol), normalized to maxima of 100%. (d) Average luminous transmittance and the modulation of the solar energy transmittance achieved in this work (full circle) and reported in the literature 20,21,30,40,44 for VO2-based coatings with a transition temperature Ttr ≤38 °C prepared on glass substrates using magnetron sputter deposition. Adapted with permission. 40 Copyright 2023, Elsevier. The labels denote a maximum substrate temperature during the preparation (deposition and postannealing) of the coatings and their transition temperature (both in °C). For comparison, we give an excellent result (marked with a cross) achieved recently using a hydrothermal process. 27 The shaded area represents the required values of Tlum and ΔTsol for smart-window applications. Calculated Tlum and ΔTsol (empty circles) for the V0.855W0.018Sr0.127O2thickness from 40 nm (on the right side) to 100 nm with a step of 15 nm are also presented (see Table 1). Table 2. Integral Luminous and Solar Energy Transmittance [Tlum(Tm) and Tsol(Tm), Respectively] Measured at Tms =−20 °C and Tmm = 70 °C, Together with the Corresponding Modulations ΔTlum and ΔTsol, for the YSZ(167 nm)/ V0.855W0.018Sr0.127O2(71 nm)/SiO2(280 nm) Coating (Denoted as VWSrO) and the YSZ(178 nm)/V0.984W0.016O2(73 nm)/ SiO2(280 nm) Coating (Denoted as VWO) on 1 mm-Thick Glass sample Tlum(Tms) (%) Tlum(Tmm) (%) ΔTlum (%) Tsol(Tms) (%) Tsol(Tmm) (%) ΔTsol (%) VWSrO 63.7 60.7 3.0 58.8 47.6 11.2 VWO 47.0 45.3 1.7 39.9 31.5 8.4 ACS Applied Materials & Interfaces www.acsami.org Research Article https://doi.org/10.1021/acsami.4c05696 ACS Appl. Mater. Interfaces 2024, 16, 57268−57276 57271 compact (very narrow amorphous boundary regions) than that of the bottom ZrO2layer comprising monoclinic and tetragonal ZrO2crystal grains. However, there are also new issues to consider resulting from the incorporation of Sr. First, the lowered n550 of the TC layer (2.38 to 2.58; Figure 1c) leading to a lower optimum n550 of the top AR layer (√2.38 = 1.54 to √2.58 = 1.61) makes high refractive index materials such as YSZ (in our case n550 = 2.21) rather suboptimum for the top AR layer and at the same time increases the comparative advantage of low refractive index materials such as SiO2(n550 = 1.46). This has been confirmed by a comparison of results of optical modeling for different designs (not shown). Thus, the optical modeling is shown (Figure 2b,c), and the experiments have been performed (Figure 3) for the optimum YSZ/ V0.855W0.018Sr0.127O2/SiO2coating. Second, it has been found that the surface roughness of sputtered VO2codoped with W and Sr is much higher (≈30 nm) than the almost negligible roughness of VO2doped only with W (≈10 nm) or YSZ and SiO2(≈5 nm). This roughness is not negligible anymore, and it has been included in the optical modeling of glass/YSZ/ V0.855W0.018Sr0.127O2/SiO2by inserting a 30 nm-thick intermix layer between V0.855W0.018Sr0.127O2and SiO2. The properties of this intermix layer are given by 50 vol % of each of these two materials (Bruggeman effective medium approximation), and the layer contributes by 15 nm to both hand ht. After candidate materials for AR layers were chosen, it is necessary to identify optimum thicknesses of these layers. This has been done by optical modeling using our own code based on a transfer matrix formalism, and the results are shown in Figure 2b,c for the thickness of the TC layer measured before the design and deposition of the top layer: h= 56 nm bulk + half of the 30 nm roughness = 71 nm. Figure 2b shows the firstand second-order interference maxima of Tlum. While the heights of all these maxima are comparable, it is necessary to choose one that combines high Tlum with high ΔTsol. After complementing Tlum in Figure 2b with ΔTsol in Figure 2c, it can be seen that their best combination is associated with two second-order (three-quarter wavelength) AR layers, specifically hb= 170 nm of YSZ and ht= 269 nm of SiO2. The reason 39 is the following: while the first-order maximum in the visible does not lead to anything special in the infrared, the second-order maximum in the visible leads to a first-order maximum in the infrared (at ≈3×longer wavelength). The enhanced transmittance in the infrared leads also to enhanced transmittance modulation in the infrared and, in turn, to enhanced contribution of infrared wavelengths to ΔTsol. Thus, this is the optimized design used in our experiments. Furthermore, while the presented recommendation hb= 170 nm of YSZ and ht= 269 nm is valid for h= 71 nm, it is worth to investigate a possible negative correlation between a chosen hand optimum hband ht. The results of this investigation are shown in Table 1. On the one hand, because of very different refractive indices of V0.855W0.018Sr0.127O2and SiO2, the correlation of hand optimum htis very weak. On the other hand, because of relatively similar refractive indices of V0.855W0.018Sr0.127O2and YSZ, the interference in the corresponding bilayer becomes important and leads to a considerable negative correlation of hincreasing from 40 to 100 nm and optimum hbdecreasing from 190 to 160 nm. In parallel, Table 1 quantifies the h-dependent trade-off between achievable Tlum and ΔTsol: increasing hfrom 40 to 100 nm combined with optimum hband htleads to decreasing predicted Tlum from 72.6% to 49.6% and to increasing predicted ΔTsol from 7.41% to 13.61%. 2.3. Structure and TC Properties of Three-Layer YSZ/ V0.855W0.018Sr0.127O2/SiO2Coatings. We have followed the presented design and prepared TC coatings YSZ(167 nm)/ V0.855W0.018Sr0.127O2(71 nm)/SiO2(280 nm) and (for comparative purposes) YSZ(178 nm)/V0.984W0.016O2(73 nm)/ SiO2(280 nm) with thicknesses of second-order AR layers very close to the optima given in the previous section. The crystalline phases identified by room-temperature XRD in the former coating can be seen in Figure 3a. The YSZ layer contributes by strong diffraction peaks close to the positions reported for tetragonal Y0.06Zr0.94O1.97 (PDF no. 04-0219607), 41 confirming its good crystallinity and its ability to serve as a crystalline template. 40 The TC layer contributes by diffraction peaks very close to the positions of both VO2(M1) (PDF no. 04-003-2035) and VO2(R) (PDF no. 01-073-2362). These two desired phases are difficult to distinguish, let alone prone to be present simultaneously, due to the Ttr (see Figure 3b) very close to the XRD measurement temperature. A weak contribution of the amorphous SiO2layer cannot be distinguished from that of the glass substrate. The most important piece of information in Figure 3a is the absence of any other peaks: there are no fingerprints of non-TC (in the Tmrange of interest) stoichiometries such as V2O3or V4O9or polymorphs such as VO2(P) or VO2(B). This confirms the success of the pulsed O2flow feedback process control and our sputter deposition technique in general. The TC transition temperature was examined by measuring the temperature dependence of T2500. The obtained hysteresis curves, both with and without Sr, are shown in Figure 3b. It can be seen that the doping with 1.6−1.8 at. % W in the metal sublattice (that is, destabilization of the low-temperature semiconducting phase by the larger size and extra valence electron of W compared to V) allowed us to lower Ttr from ≈57 °C (HiPIMS deposition of pure VO2) 28 to desired 21−22 °C. This role of W is not only qualitatively but also almost quantitatively independent of the presence of Sr, leading to a slightly narrower hysteresis curve. Let us emphasize that (contrary to some other results or even generalizing statements in the literature) 42−44 the sputter deposition technique used allowed us to lower Ttr at preserved strongly TC behavior. The TC behavior is quantified in Figure 3c in terms of lowand high-temperature spectral transmittance, once again both with and without Sr. First, the figure captures the role of AR layers: there are second-order maxima of T(λ) at ≈600 nm (intentionally not at 550 nm because the absorption around these maxima is lower at higher λ) as well as first-order maxima around ≈1700 nm. Second, the figure captures the role of Sr, leading, in agreement with the presented enhancement of Eg1 in Figure 1b and lowering of k(λ) in Figure 1d, to significantly enhanced T(λ) (at about the same h) at both measurement temperatures. While the transmittance enhancement is arguably most important in the visible region, it takes place in the whole λrange investigated. Third, while both coatings exhibit T(λ) modulation in the infrared, it is welcomed that the Sr incorporation led to a stronger modulation at the shortest infrared wavelengths (where it is multiplied by higher φsol) at a cost of weaker modulation at longer wavelengths (where it is multiplied by lower φsol). The spectral transmittance was used to calculate the integral transmittances Tlum and Tsol and their modulations. The results are provided in Figure 3d and Table 2. The table shows that ACS Applied Materials & Interfaces www.acsami.org Research Article https://doi.org/10.1021/acsami.4c05696 ACS Appl. Mater. Interfaces 2024, 16, 57268−57276 57272 the Sr incorporation led to a significant enhancement of both key quantities: Tlum increased from 45.3% to 60.7% (hightemperature state) or even from 47.0% to 63.7% (lowtemperature state) and ΔTsol increased from 8.4% to 11.2%. The enhancement of Tlum is due to the Sr-induced lowering of k(λ) in the visible, while the enhancement of ΔTsol is due to the contribution of both visible wavelengths [because the Srinduced lowering of k(λ) is larger in the low-temperature state, see the enhancement of ΔTlum from 1.7% to 3.0%] and the shortest infrared wavelengths [because of the Sr-induced stronger modulation of T(λ)]. Thus, the Sr incorporation allowed us to fulfill all quantitative criteria for large-scale implementation of these energy-saving TC coatings (Section 1): not only Tsclose to 300 °C and Ttr close to 25 °C but also Tlum > 60% and ΔTsol > 10%. Let us emphasize that the criteria have been fulfilled not only in terms of average Tlum (used by most available papers) but also in terms of minimum Tlum (which is arguably more relevant). The comparison with the literature in Figure 3d proves that the criteria of success were fulfilled for the first time. Note that this has been allowed not only by the proper coating design, deposition technique, and elemental composition of the TC layer but also by the proper thickness of the TC layer. The dashed line in Figure 3d (visualization of the data from Table 1) in a close vicinity of the experimental data point not only confirms the correctness of the ellipsometric measurements and optical modeling but also shows that h= 71 nm is close to the middle of the narrow hrange where both Tlum and ΔTsol are sufficiently high. On the contrary, all VO2-based coatings reported in the literature fail to meet the requirement for at least one of the quantities from the quadruplet Ts,Ttr,Tlum, and ΔTsol (let alone reports and coating comparisons that do not even mention some of them). A case can be made that there is a coating recently prepared 27 using a rather complicated hydrothermal synthesis which would possibly almost fulfill these conditions (at Ttr = 30 °C) in a case of different hchoice, albeit the scalability of the process to large deposition devices is (contrary to magnetron sputter deposition of VO2-based coatings) 31 yet to be demonstrated. As expected (see Figure 3c), no visible change in the transparency and color of the strongly TC YSZ/ V0.855W0.018Sr0.127O2/SiO2coating is observed in Figure 4 when its temperature increased from 5 to 55 °C. 3. CONCLUSIONS Strongly TC energy-saving YSZ/V0.855W0.018Sr0.127O2/SiO2 coatings have been prepared by using a scalable deposition technique on conventional glass. All quantitative criteria for large-scale implementation of these coatings have been fulfilled simultaneously for the first time: Ts= 320 °C without any substrate bias voltage, Ttr = 22 °C, Tlum = 60.7% (hightemperature state) to 63.7% (low-temperature state) and ΔTsol = 11.2%. The success has been achieved by a combination: (i) full utilization of the advantages of reactive HiPIMS deposition with the effective pulsed O2flow feedback control, (ii) coating design with second-order AR layers, (iii) proper choice of the materials for AR layers (YSZ and SiO2), (iv) optimum level of doping the metal sublattice of VO2with W (1.8 at. % in order to destabilize the low-temperature phase and lower Ttr), and (v) optimum level of doping the metal sublattice of VO2with Sr (12.7 at. % in order to widen the visible-range optical gap). This moves us closer to reducing the energy consumption of buildings by applying this kind of coating on windows and glass facades. 4. EXPERIMENTAL SECTION 4.1. Coating Preparation. The coatings were deposited onto 1 mm-thick SLG substrates in argon−oxygen gas mixtures at the argon partial pressure pAr = 1 Pa, corresponding to the argon flow rate of 60 sccm, in an ultrahigh-vacuum multimagnetron sputter device (ATC 2200-V AJA International Inc.) equipped by unbalanced magnetrons with planar targets (diameter of 50 mm and thickness of 6 mm in all cases). The base pressure before deposition was below 10−4Pa. The rotating (20 rpm) substrates at a distance of 145 mm from the targets were at a floating potential. The V0.855W0.018Sr0.127O2layer was deposited by controlled HiPIMS of a single V−W (4.0 wt % corresponding to 1.14 at. %) target (99.95% purity), combined with a simultaneous pulsed DC magnetron sputtering of a Sr target (99.8% purity), at the substrate surface temperature Ts= 320 °C (Figure 5). The total oxygen flow rate (ΦOd 2) in two to-substrate O2inlets, injecting oxygen in front of the V−W target, was not fixed but alternating between 2.0 and 3.0 sccm. This resulted in oscillations of Figure 4. View of a YSZ(167 nm)/V0.855W0.018Sr0.127O2(71 nm)/ SiO2(280 nm) coating with Ttr = 22 °C on 1 mm-thick glass at Tm= 5 and 55 °C, attached to a window to illustrate its transparency and color. Figure 5. Schematic diagram of the deposition system with four magnetron targets showing two magnetrons with V−W and Sr targets, located in opposite positions, which were used for the deposition of the V0.855W0.018Sr0.127O2layer. Two O2inlets were placed 20 mm from the V−W target surface and oriented to the substrate. Positions of the pressure sensor and the Ar inlet at the wall of the vacuum chamber are also shown. An increased local value of the oxygen partial pressure due to the O2injection is denoted as pO2 . ACS Applied Materials & Interfaces www.acsami.org Research Article https://doi.org/10.1021/acsami.4c05696 ACS Appl. Mater. Interfaces 2024, 16, 57268−57276 57273 the oxygen partial pressure pOd 2between 26 and 55 mPa. The moments of switching of the ΦOd 2pulses were determined during the deposition by a programmable logic controller using a preselected critical value of the discharge current on the V−W target. The basic principles of this effective pulsed oxygen flow control are presented in our recent papers. 28,30,40 The magnetron with the V−W target was driven by a unipolar high-power pulsed DC power supply (TruPlasma Highpulse 4002 TRUMPF Huettinger). The voltage pulse duration was 80 μs at a repetition frequency of 500 Hz (duty cycle of 4%) and the deposition-averaged target power density (spatially averaged over the total target area) was 14.9 W cm−2. The magnetron with the Sr target was driven by a unipolar pulsed DC power supply (IAP-1010 EN Technologies Inc.). To minimize arcing on the Sr target surface at an increased Sr target power density and to control the Sr content in the layers easily, we used short 7 μs voltage pulses at a relatively high repetition frequency of 50 kHz (duty cycle of 35%) during the depositions with a preselected deposition-averaged target power density in the range from 0.1 to 3.3 W cm−2. The optimized TC V0.855W0.018Sr0.127O2layer with a thickness of 71 nm was deposited onto a 167 nm-thick YSZ layer on a 1 mm-thick SLG substrate at a deposition-averaged target power density of 1.9 W cm−2. It exhibited (without a top SiO2AR layer) a high Tlum = 56.8% (low-temperature state) and the highest achieved ΔTsol = 8.3%. A further increase in the Sr content resulted in a higher Tlum and a lower ΔTsol. Details of the developed sputter deposition technique and the effect of an increasing Sr content in the W and Sr codoped VO2films on their electronic and crystal structure and time-dependent optical and electrical properties will be presented elsewhere. The V0.984W0.016O2layer was deposited by controlled HiPIMS of a V target (99.9% purity), combined with a simultaneous pulsed DC magnetron sputtering of a W target (99.9% purity), at Ts= 320 °C. The ΦOd 2and pOd 2oscillated between 1.5 and 1.9 sccm and between 23 and 72 mPa, respectively, during the deposition performed using the same power supplies. For the V target (HiPIMS), the voltage pulse duration was 80 μs at a repetition frequency of 500 Hz and the deposition-averaged target power density was 14.2 W cm−2. For the W target, the voltage pulse duration was 16 μs at a repetition frequency of 5 kHz and the deposition-averaged target power density was 25 mW cm−2. The YSZ layers were deposited by controlled HiPIMS of a single Zr−Y (9.0 wt % corresponding to 9.2 at. %) target (99.9% purity) at Ts= 320 °C. The ΦOd 2and pOd 2oscillated between 1.9 and 2.4 sccm and 25 and 73 mPa, respectively, during the depositions performed using the aforementioned high-power pulsed DC power supply. The voltage pulse duration was 80 μs at a repetition frequency of 500 Hz, and the deposition-averaged target power density was 14.9 W cm−2. The SiO2 layers were deposited by midfrequency bipolar dual magnetron sputtering of two Si (99.999% purity) targets at Ts≤35 °C (without any external heating). The ΦOd 2= 17 sccm and pOd 2= 0.2 Pa were used during the depositions performed using a bipolar dual power supply (TruPlasma Bipolar 4010 TRUMPF Huettinger). The voltage pulse duration was 10 μs at a repetition frequency of 50 kHz, and the deposition-averaged target power density was approximately 8 W cm−2. 4.2. Coating Characterization. The W and Sr contents in the metal sublattice of V0.984W0.016O2and V0.855W0.018Sr0.127O2, i.e., 1.6 ± 0.3 at. % of W and 1.8 ±0.2 at. % of W and 12.7 ±1.8 at. % of Sr, respectively, were measured on a dedicated 440 nm-thick layer on a Si(100) substrate in a scanning electron microscope (SU-70, Hitachi) using wave-dispersive spectroscopy (MagnaRay, Thermo Scientific) at a low primary electron energy of 7.5 keV. Standard reference samples of pure V, W, Fe2O3, and SrSO4(Astimex Scientific Ltd.) were utilized. The room-temperature (25 °C) crystal structure of coatings was characterized by XRD using a PANalytical X’Pert PRO diffractometer working with Cu Kα(40 kV, 40 mA) radiation at a glancing incidence of 1°. The thickness and optical constants (refractive index, n, and extinction coefficient, k) of individual layers were measured by spectroscopic ellipsometry using the J.A. Woollam Co., Inc. VASE instrument equipped by an Instec heat/cool stage. The measurements of n(λ) and k(λ) were performed in the wavelength range of 300−2000 nm at the angles of incidence of 55, 60, and 65°in reflection for Tms =−20 °C (semiconducting state below Ttr) and Tmm = 70 °C (metallic state above Ttr). YSZ was described by the Cauchy dispersion formula, and V0.984W0.016O2and V0.855W0.018Sr0.127O2were represented by a combination of the Cody− Lorentz oscillator, Lorentz oscillators, and (in the case of the metallic phase) Drude oscillator. The coating transmittance (T) and reflectance (R) were measured by spectrophotometry using the Agilent CARY 7000 instrument with an in-house-made heat/cool cell. The measurements were performed in the wavelength range of 300− 2500 nm at the angles of incidence of 0°(T) and 7°(R) for Tms = −20 °C and Tmm = 70 °C. Hysteresis curves were measured for Tat λ = 2500 nm in the temperature range Tm=−20 to 70 °C. The coating performance is quantified by means of integral luminous transmittance (Tlum), integral solar energy transmittance (Tsol), and their modulations (ΔTlum and ΔTsol). The quantities are defined as T T T T ( ) ( ) ( ) ( , ) d ( ) ( ) d lum m 380 780 lum sol m 380 780 lum sol = T T T T T( ) ( ) lum lum ms lum mm = T T T T ( ) ( ) ( , ) d ( ) d sol m 300 2500 sol m 300 2500 sol = T T T T T( ) ( ) sol sol ms sol mm = where φlum is the luminous sensitivity of the human eye and φsol is the solar irradiance spectrum at an air mass of 1.5. 45 The average luminous transmittance is defined as Tlum = [Tlum(Tms) + Tlum(Tmm)]/ 2. The optical band gaps Eg1 and Eg2 were determined from Tauc plots by utilizing the relation (αE)1/2 ∼E−Eg, 33,37 where Eis the photon energy and αis the absorption coefficient calculated as α=−[ln(T/1 −R)]/h, 33 where his the thickness of the TC layer. ■ASSOCIATED CONTENT Data Availability Statement The data that support the findings of this study are available from the corresponding author upon reasonable request. ■AUTHOR INFORMATION Corresponding Author Jaroslav Vlcek −Department of Physics and NTIS-European Centre of Excellence, University of West Bohemia, 30100 Plzen, Czech Republic; orcid.org/0000-0003-2627-2074; Email: [email protected] Authors Michal Kaufman −Department of Physics and NTISEuropean Centre of Excellence, University of West Bohemia, 30100 Plzen, Czech Republic; orcid.org/0009-00011733-2998 Jirí Houska −Department of Physics and NTIS-European Centre of Excellence, University of West Bohemia, 30100 Plzen, Czech Republic; orcid.org/0000-0002-4809-4128 Sadoon Farrukh −Department of Physics and NTISEuropean Centre of Excellence, University of West Bohemia, 30100 Plzen, Czech Republic Stanislav Haviar −Department of Physics and NTISEuropean Centre of Excellence, University of West Bohemia, 30100 Plzen, Czech Republic Complete contact information is available at: https://pubs.acs.org/10.1021/acsami.4c05696 ACS Applied Materials & Interfaces www.acsami.org Research Article https://doi.org/10.1021/acsami.4c05696 ACS Appl. Mater. Interfaces 2024, 16, 57268−57276 57274 Notes The authors declare no competing financial interest. ■ACKNOWLEDGMENTS This work was supported by the Czech Science Foundation under project no. 21-28277S and by the project Quantum materials for applications in sustainable technologies (QM4ST), funded as project no. CZ.02.01.01/00/22_008/ 0004572 by Programme Johannes Amos Comenius, call Excellent Research. ■REFERENCES (1) Ritchie, H.; Roser, M.; Rosado, P. CO2and Greenhouse Gas Emissions, Our World in Data. 2020, https://ourworldindata.org/ co2-and-other-greenhouse-gas-emissions. (2) Omer, A. M. Energy, Environment and Sustainable Development. Renewable Sustainable Energy Rev. 2008,12, 2265−2300. (3) Ke, Y.; Zhou, C.; Zhou, Y.; Wang, S.; Chan, S. H.; Long, Y. Emerging Thermal-Responsive Materials and Integrated Techniques Targeting the Energy-Efficient Smart Window Application. Adv. Funct. Mater. 2018,28, 1800113. 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