Recent Advances in Fabrication of Flexible, Thermochromic Vanadium Dioxide Films for Smart Windows

Monoclinic-phase VO2 (VO2(M)) has been extensively studied for use in energy-saving smart windows owing to its reversible insulator–metal transition property. At the critical temperature (Tc = 68 °C), the insulating VO2(M) (space group P21/c) is transformed into metallic rutile VO2 (VO2(R) space group P42/mnm). VO2(M) exhibits high transmittance in the near-infrared (NIR) wavelength; however, the NIR transmittance decreases significantly after phase transition into VO2(R) at a higher Tc, which obstructs the infrared radiation in the solar spectrum and aids in managing the indoor temperature without requiring an external power supply. Recently, the fabrication of flexible thermochromic VO2(M) thin films has also attracted considerable attention. These flexible films exhibit considerable potential for practical applications because they can be promptly applied to windows in existing buildings and easily integrated into curved surfaces, such as windshields and other automotive windows. Furthermore, flexible VO2(M) thin films fabricated on microscales are potentially applicable in optical actuators and switches. However, most of the existing fabrication methods of phase-pure VO2(M) thin films involve chamber-based deposition, which typically require a high-temperature deposition or calcination process. In this case, flexible polymer substrates cannot be used owing to the low-thermal-resistance condition in the process, which limits the utilization of flexible smart windows in several emerging applications. In this review, we focus on recent advances in the fabrication methods of flexible thermochromic VO2(M) thin films using vacuum deposition methods and solution-based processes and discuss the optical properties of these flexible VO2(M) thin films for potential applications in energy-saving smart windows and several other emerging technologies.


Introduction
To address the rapidly increasing energy demand and growing environmental concerns, the development of renewable resources and smart-energy materials is receiving widespread attention [1]. Building energy consumption is estimated to account for 30-40% of the total global energy consumption, and this proportion is expected to continue increasing [2,3]. Windows are the most energy-inefficient component of a building; in this regard, smart windows offer the potential to reduce energy consumption by reducing the air-conditioning load via modulation of solar radiation [4]. Researchers have extensively studied the development of energy-efficient materials for smart windows to address the increasing energy needs. Monoclinic-phase VO 2 (VO 2 (M)) was first reported by Morin in 1959 and is the most widely studied inorganic material owing to its switchable thermochromic properties [5]. VO 2 exhibits a first-order insulator-metal phase transition at the critical temperature (T c = 68 • C), accompanied by reversible phase-change properties in the transition from the insulating monoclinic (P21/c) phase to the metallic rutile (P42/mmm) phase [6,7]. Figure 1 shows the crystal structure and band diagram of monoclinic and rutile phase VO 2 . The vanadium ions in the monoclinic phase dimerize to form zigzag atomic chains with two V-V distances of ≈3.12 and ≈2.65 Å. Conversely, in the rutile (P42/mmm) phase [6,7]. Figure 1 shows the crystal structure and band diagram of monoclinic and rutile phase VO2. The vanadium ions in the monoclinic phase dimerize to form zigzag atomic chains with two V-V distances of ≈3.12 and ≈2.65 Å. Conversely, in the rutile phase, straight and evenly distanced vanadium chains are formed along the c-axis with ≈2.85 Å of distance and V 4+ ions surrounded by O 2-are located at the center and corner positions [8,9]. Dimerization of the vanadium ion causes the dll band to split into a filled bonding (dll) and an empty antibonding (dll*). Furthermore, the π* orbitals shift to higher energies and make a forbidden band of approximately 0.7 eV between the dll and π* [10,11]. The Fermi level is located within the forbidden band, thereby forming the insulating VO2(M). When the temperature is higher than Tc, the density of the Fermi energy states in VO2(R) is formed by a mixture of π* and dll orbitals [9,12]. The electrons at the dll state exhibit a behavior similar to that of free electrons, accomplishing a half-filled metallic state. The Fermi level form between the π* and dll bands, indicating an enhanced electrical conductivity of the VO2(R) [13]. Therefore, electrical and optical properties are considerably modulated during the phase transition. The phase transition of VO2 can be induced by different types of stimuli, such as heat [5], electric fields [14], and mechanical strain [15]. The phase change in VO2(M) has also been utilized in various emerging technologies, including optical switches [16], thermoelectrics [17], hydrogen storage [18,19], sensors [20][21][22], transistors [23][24][25], active metamaterials [26][27][28], and photoelectric devices [29]. The application of VO2(M) in smart windows was investigated in the 1980s by Jorgenson et al. [7] and Babulanam et al. [30]. When the external temperature is lower than the phasetransition temperature, which is approximately 68 °C, VO2(M) exists in the insulating phase, exhibiting high transmittance of near-infrared (NIR) wavelengths in the solar spectrum. Conversely, when the temperature is higher than the phase-transition temperature, the crystal and band structures change because of the transition from the insulator phase (VO2(M)) to the metallic phase (rutile VO2 (VO2(R))), which significantly reduces the optical transmittance of NIR wavelengths. Therefore, thermochromic smart windows can reversibly modulate their solar transmittance at different temperatures and can reduce the room temperature during hot weather conditions; this will reduce the total energy consumption of the building. VO2-based thermochromic smart windows offer characteristic advantages over other types of energy-saving windows, such as low-emissivity (low-e) glass [31,32] and electrochromic (EC) windows, owing to their ability to self-regulate solar transmission/reflection according to the external environment without utilizing an external energy supply [33][34][35]. Moreover, thermochromic windows have a relatively simple structure when compared with low-e or EC glass, thereby exhibiting potential for largearea installation and mass production for commercialization [36]. Figure 1. Schematic of the crystal structure and electronic band structure of the insulating VO 2 (M) and the metallic VO 2 (R). Adapted with permission from [13]. Copyright 2011, American Chemical Society.
The performance of VO 2 for smart windows is evaluated in terms of the luminous transmittance (T lum ) and solar modulation ability. Luminous transmittance refers to the Nanomaterials 2021, 11, 2674 3 of 22 integrated optical amount of visible-light transmittance, which is determined from the following equation: T lum = Φ lum (λ)Tdλ/ Φ lum (λ)dλ, (380 to 780 nm) T(λ) and Φ lum characterize the transmittance of the wavelength λ and photopic luminous efficiency function in the visible region, respectively [30,37]. Solar-energy modulation ability (∆T sol ) is also a critical feature for determining the energy-saving capability of material. ∆T sol is defined as the difference in the solar-energy transmittance (T sol ) values before and after phase transition in the 240 to 2500 nm spectrum, which is estimated using the follow equations [38]: ∆T sol = T sol,low temperature − T sol,high temperature where Φ sol denotes the solar irradiance spectrum for an air mass of 1.5, which is equivalent to the presence of the sun at an angle of 37 • from the horizon [37]; moreover, T sol,low temperature and T sol,high temperature represent the solar transmittance of VO 2 films at a low temperature in the monoclinic phase and at a high temperature in the rutile phase, respectively. T lum should be greater than 40% to indicate the requirement for daylight across windows, and ∆T sol should be sufficiently high, at least 10%, for energy saving [39]. Furthermore, the phase-transition temperature of VO 2 (T c = 68 • C) should be reduced from 68 • C for efficient regulation of solar energy during daytime [40]. Therefore, a reduced phase-transition temperature (T c ), high luminous transmittance (T lum ), and strong solar-energy modulation ability (∆T sol ) are important characteristics for energy-efficient smart windows. To fulfill the demand for practical applications of energy-saving smart windows, VO 2 -based thermochromic thin films should possess the following features: the phase-transition temperature (T c ) should be reduced to near-ambient temperature, and a high luminous transmittance (T lum > 40%) accompanied by a strong solar-energy modulation ability (∆T sol > 10%) should be available [41,42]. Several studies have been conducted to improve the energy-saving performance of VO 2 -based smart windows. For example, reductions in T c have been achieved by doping with metal ions [43][44][45], or by utilizing nonstoichiometric compounds [46], strains [47], and nano-size effects [48]. Among the aforementioned methods, doping with metal ions, such as W 6+ [49], Al 3+ [50], Mg 2+ [51], Sn 4+ [52], and Mo 6+ [53,54], is considered the most efficient. However, an increase in the dopant content results in the deterioration of phase-transition behaviors, such as a reduction in ∆T sol and a broadened phase-transition temperature range [55,56]. High values of T lum and ∆T sol are also required to accomplish high-energy modulation efficiency for smart windows; however, these parameters involve a tradeoff, and thus, it is difficult to enhance them simultaneously [57]. Various strategies have been suggested to improve T lum and ∆T sol simultaneously, such as doping with Mg 2+ [56] and F − [55], or utilizing nano-size thermochromic materials [58], photonic crystals [59], antireflective overcoating [60], porous films [60], and multilayered structures [60,61]. However, the fabrication of VO 2 (M) films with high T lum (> 40%) and ∆T sol (>10%) values as well as a sufficiently reduced T c remains challenging, which limits the utilization of VO 2 (M) in practical applications [56,57,62] Recently, the fabrication of flexible VO 2 (M) films has attracted widespread attention [39,56]. Flexible thermochromic films demonstrate significant potential for large-scale fabrication and commercialization [63][64][65][66]. For example, flexible VO 2 (M) films can be instantly applied to the windows of existing buildings and easily integrated onto curved surfaces, such as automobile windows. Moreover, flexible VO 2 (M) thin films show the potential for application in actuators and optical switches for future optical and electronic devices [63,67]. Thus far, high-quality VO 2 (M) thin films have been fabricated using vacuum-chamber-based techniques, such as chemical vapor deposition (CVD) [68][69][70], physical vapor deposition [56], radiofrequency (RF) magnetron sputtering [71], and pulsed laser deposition [72]. These deposition methods provide high-quality and highly crystalline VO 2 (M) films; however, they often require high-temperature deposition conditions or an additional thermal annealing process to yield phase-pure crystalline VO 2 (M) films [63]. The deposition temperature is typically higher than 400 • C, which exceeds the thermal resistance of most flexible polymeric substrates [51,[73][74][75]. Therefore, chamber-based deposition processes are predominantly performed on rigid inorganic substrates with high thermal resistance, which limits the fabrication of crystalline VO 2 (M) films on flexible substrates. Flexible VO 2 (M) films can also be obtained via colloidal deposition using VO 2 (M) nanoparticles (NPs) [56,65,76]. Colloidal dispersion of VO 2 (M) enables solutionbased deposition onto polymeric substrates or the formation of flexible composite films through mixing in a polymer matrix. Hydrothermal synthesis of colloidal VO 2 (M) NPs was reported in a recent study, which demonstrated the feasibility of producing flexible VO 2 (M) films through the solution-based deposition of NPs at room temperature [77,78]. However, for colloidal VO 2 (M) NPs synthesized hydrothermally, lowering T c while maintaining favorable optical properties, such as a high T lum and ∆T sol , remains difficult [79,80]. Therefore, the fabrication of flexible VO 2 thin films using colloidal VO 2 (M) NPs with a reduced T c , high T lum , and high ∆T sol is still significantly challenging. In this review, we focus on the recent advances in the fabrication methods for flexible thermochromic VO 2 (M) thin films. We systematically review the fabrication process, including chamber-based vacuum deposition on flexible substrates that possess high thermal resistance. In addition, we introduce film-transfer techniques used to transfer VO 2 (M) layers deposited on rigid substrates onto flexible polymer substrates. Finally, we introduce the solution-based deposition process using colloidal VO 2 (M) NPs. The optical properties and phase transition behaviors are discussed to investigate the potential of flexible VO 2 (M) films for application in energy-saving smart windows and other emerging technologies.

Fabrication of Flexible Monoclinic-Phase VO 2 (VO 2 (M)) Films via Chamber-Based Deposition
As discussed, the fabrication of stoichiometric and highly crystalline VO 2 films using vacuum deposition requires high-temperature conditions or an additional calcination process [81]. Therefore, rigid inorganic substrates, which have high thermal stability, such as SiO 2 [82], MgF 2 [83], and Al 2 O 3 [84], are generally used for growing VO 2 (M) films. The fabrication of flexible VO 2 (M) films through chamber-based deposition of VO 2 (M) films has also demonstrated using flexible substrates with high thermal stability. For example, muscovite sheets were first used as substrates for the fabrication of VO 2 (M) films because such sheets possess a high thermal stability of over 500 • C and superior chemical resistance, which enable the formation of highly crystalline VO 2 (M) films through high-temperature sintering. High-quality, single-phase VO 2 (M) films can be grown epitaxially on (001) muscovite substrates with high crystallinity, leading to superior phase transition behaviors in terms of resistivity and infrared (IR) transmittance [85]. Li et al. also developed a process for depositing a VO 2 film directly on a flexible muscovite substrate [86]. First, V 2 O 5 films were deposited on a native muscovite substrate through pulsed-laser deposition for 20 min; then, the films were annealed at 650 • C under a 5 mTorr oxygen atmosphere to obtain highly crystalline VO 2 (M) films (Figure 2a). The electrical resistance of the VO 2 (M) thin films was measured under various bending radii. During the phase transition, the electrical resistance of the films varied by an order of 10 3 or more (∆R/R > 10 3 ), and the change in luminous transmittance was higher than 50% (∆T r > 50%) (Figure 2b). Owing to the intrinsic transparency and flexibility of muscovite sheets, the VO 2 /muscovite heterogeneous structures also exhibited superior flexibility and visible-light transparency. The electrical resistance of the VO 2 /muscovite films remained the same even after the films were bent 1000 times; this confirmed the high mechanical stability of the films (Figure 2c). Thus, considering their enhanced electrical, thermal, optical, and mechanical properties, VO 2 /muscovite films demonstrate considerable potential for application in flexible electronic devices, especially optical switches. tion, the electrical resistance of the films varied by an order of 10 3 or more (ΔR/R > 10 3 ), and the change in luminous transmittance was higher than 50% (ΔTr > 50%) (Figure 2b). Owing to the intrinsic transparency and flexibility of muscovite sheets, the VO2/muscovite heterogeneous structures also exhibited superior flexibility and visible-light transparency. The electrical resistance of the VO2/muscovite films remained the same even after the films were bent 1000 times; this confirmed the high mechanical stability of the films (Figure 2c). Thus, considering their enhanced electrical, thermal, optical, and mechanical properties, VO2/muscovite films demonstrate considerable potential for application in flexible electronic devices, especially optical switches. VO2(M) thin films grown on substrates, such as TiO2, Al2O3, diamond, and SiO2, have strong chemical bonds (ionic or covalent) between the VO2(M) layers and the substrates. Thus, the VO2 lattice is constrained, which is known as the substrate-clamping effect; this complicates the lattice rearrangement during phase transition [82,87,88]. Therefore, VO2 films deposited on inorganic substrates typically require a higher energy to drive the metal-insulator transition (MIT). Conversely, VO2(M) films deposited on mica sheets typically have weak van der Waals (vdW) bonds (0.1-10 kJ mol -1 ) between VO2(M) and the mica layer, which is 2-3 times weaker than the aforementioned ionic or covalent bonds (100-1000 kJ mol -1 ) [89]. This weak vdW bonding between the VO2 film and the mica sheet does not induce any significant lattice strain in the VO2 layer. Therefore, the VO2 film behaves as a nearly freestanding film on the mica sheet, which enables MIT with exceedingly low energy stimuli [90]. Moreover, owing to the weak vdW bonding between adjacent mica sheets, the thin mica sheet can be peeled off from the substrate, creating transparent and flexible VO2(M)/mica sheets. Wang et al. also employed a mica sheet as a support for VO2(M) to fabricate a mechanically flexible and electrically tunable flexible phase-change material for IR absorption [91]. First, 100-nm-thick Au thin films were deposited on a mica sheet through magnetron sputtering. Then, a 100-nm-thick vanadium film was deposited on the Au film via electron-beam evaporation and was thermally annealed in an oxygen atmosphere at temperatures of 430-470 °C. Au and mica sheet can withstand high-temperature annealing conditions. Finally, graphene thin films were transferred onto the VO2 thin film to deposit the conductive electrode that induces the phase transition of the VO2(M) thin films through Joule heating (Figure 3a). The IR absorption of this device can be continuously adjusted from 20% to 90% by changing the current applied to the graphene film. Moreover, this structure exhibited superior bending durability when it was bent up to 1500 times, without any noticeable deterioration in the optical properties (Figure 3b). Such tunable and flexible VO2 devices have various application prospects in flexible photodetectors and active wearable devices. VO 2 (M) thin films grown on substrates, such as TiO 2 , Al 2 O 3 , diamond, and SiO 2 , have strong chemical bonds (ionic or covalent) between the VO 2 (M) layers and the substrates. Thus, the VO 2 lattice is constrained, which is known as the substrate-clamping effect; this complicates the lattice rearrangement during phase transition [82,87,88]. Therefore, VO 2 films deposited on inorganic substrates typically require a higher energy to drive the metalinsulator transition (MIT). Conversely, VO 2 (M) films deposited on mica sheets typically have weak van der Waals (vdW) bonds (0.1-10 kJ mol -1 ) between VO 2 (M) and the mica layer, which is 2-3 times weaker than the aforementioned ionic or covalent bonds (100-1000 kJ mol -1 ) [89]. This weak vdW bonding between the VO 2 film and the mica sheet does not induce any significant lattice strain in the VO 2 layer. Therefore, the VO 2 film behaves as a nearly freestanding film on the mica sheet, which enables MIT with exceedingly low energy stimuli [90]. Moreover, owing to the weak vdW bonding between adjacent mica sheets, the thin mica sheet can be peeled off from the substrate, creating transparent and flexible VO 2 (M)/mica sheets. Wang et al. also employed a mica sheet as a support for VO 2 (M) to fabricate a mechanically flexible and electrically tunable flexible phase-change material for IR absorption [91]. First, 100-nm-thick Au thin films were deposited on a mica sheet through magnetron sputtering. Then, a 100-nm-thick vanadium film was deposited on the Au film via electron-beam evaporation and was thermally annealed in an oxygen atmosphere at temperatures of 430-470 • C. Au and mica sheet can withstand high-temperature annealing conditions. Finally, graphene thin films were transferred onto the VO 2 thin film to deposit the conductive electrode that induces the phase transition of the VO 2 (M) thin films through Joule heating (Figure 3a). The IR absorption of this device can be continuously adjusted from 20% to 90% by changing the current applied to the graphene film. Moreover, this structure exhibited superior bending durability when it was bent up to 1500 times, without any noticeable deterioration in the optical properties ( Figure 3b). Such tunable and flexible VO 2 devices have various application prospects in flexible photodetectors and active wearable devices. Chen et al. fabricated a flexible VO2(M) thin film on a muscovite (mica) sheet directly through RF-plasma-assisted oxide molecular beam epitaxy (rf-OMBE) [92]. First, the VO2 layer was grown using rf-OMBE on the (001) plane of mica sheets at 550 °C. Then, a layered single-walled carbon nanotube (SWNT) films was deposited using CVD on the highquality VO2/mica thin film. The SWNT layer exhibited superior conductivity and flexibility and can be employed as an efficient heater when a current/bias voltage is applied. The almost freestanding SWNTs/VO2/mica (SVM) film was fabricated by peeling off the thinlayered SVM film from the substrates. Two Au electrodes were deposited on the flexible SVM thin film to provide a two-terminal electrode. The MIT process of the flexible VO2(M) thin film can be easily controlled by heating SVM films with a bias current on Au electrodes, thereby enabling reversible modulation of IR transmission. When a bias current was applied, the transmittance decreased sharply from 70% and maintained an almost constant value of approximately 30% thereafter. When the input current was turned off, the transmittance quickly returned to its highest value of 70%; this confirms that direct modulation of the transmittance by applying a current is possible (Figure 4a  (c) Resistance-dependent temperature curve for VO2/mica thin film (the inset shows the differential curves during phase transition). Reproduced with permission from [92]. Copyright 2017, Elsevier.
In addition to mica sheets, carbon-based substrates, such as graphene sheets and networks of carbon nanotubes (CNTs), have also been utilized as flexible substrates for VO2 deposition owing to their high thermal resistance. Xiao et al. reported the fabrication of VO2/graphene/CNT (VGC) flexible thin films [93]. First, the graphene/CNT thin film was prepared by depositing graphene on a Cu substrate via low-pressure CVD. Then, the Chen et al. fabricated a flexible VO 2 (M) thin film on a muscovite (mica) sheet directly through RF-plasma-assisted oxide molecular beam epitaxy (rf-OMBE) [92]. First, the VO 2 layer was grown using rf-OMBE on the (001) plane of mica sheets at 550 • C. Then, a layered single-walled carbon nanotube (SWNT) films was deposited using CVD on the high-quality VO 2 /mica thin film. The SWNT layer exhibited superior conductivity and flexibility and can be employed as an efficient heater when a current/bias voltage is applied. The almost freestanding SWNTs/VO 2 /mica (SVM) film was fabricated by peeling off the thin-layered SVM film from the substrates. Two Au electrodes were deposited on the flexible SVM thin film to provide a two-terminal electrode. The MIT process of the flexible VO 2 (M) thin film can be easily controlled by heating SVM films with a bias current on Au electrodes, thereby enabling reversible modulation of IR transmission. When a bias current was applied, the transmittance decreased sharply from 70% and maintained an almost constant value of approximately 30% thereafter. When the input current was turned off, the transmittance quickly returned to its highest value of 70%; this confirms that direct modulation of the transmittance by applying a current is possible (Figure 4a  Chen et al. fabricated a flexible VO2(M) thin film on a muscovite (mica) sheet directly through RF-plasma-assisted oxide molecular beam epitaxy (rf-OMBE) [92]. First, the VO2 layer was grown using rf-OMBE on the (001) plane of mica sheets at 550 °C. Then, a layered single-walled carbon nanotube (SWNT) films was deposited using CVD on the highquality VO2/mica thin film. The SWNT layer exhibited superior conductivity and flexibility and can be employed as an efficient heater when a current/bias voltage is applied. The almost freestanding SWNTs/VO2/mica (SVM) film was fabricated by peeling off the thinlayered SVM film from the substrates. Two Au electrodes were deposited on the flexible SVM thin film to provide a two-terminal electrode. The MIT process of the flexible VO2(M) thin film can be easily controlled by heating SVM films with a bias current on Au electrodes, thereby enabling reversible modulation of IR transmission. When a bias current was applied, the transmittance decreased sharply from 70% and maintained an almost constant value of approximately 30% thereafter. When the input current was turned off, the transmittance quickly returned to its highest value of 70%; this confirms that direct modulation of the transmittance by applying a current is possible (Figure 4a  (c) Resistance-dependent temperature curve for VO2/mica thin film (the inset shows the differential curves during phase transition). Reproduced with permission from [92]. Copyright 2017, Elsevier.
In addition to mica sheets, carbon-based substrates, such as graphene sheets and networks of carbon nanotubes (CNTs), have also been utilized as flexible substrates for VO2 deposition owing to their high thermal resistance. Xiao et al. reported the fabrication of VO2/graphene/CNT (VGC) flexible thin films [93]. First, the graphene/CNT thin film was prepared by depositing graphene on a Cu substrate via low-pressure CVD. Then, the (c) Resistance-dependent temperature curve for VO 2 /mica thin film (the inset shows the differential curves during phase transition). Reproduced with permission from [92]. Copyright 2017, Elsevier.
In addition to mica sheets, carbon-based substrates, such as graphene sheets and networks of carbon nanotubes (CNTs), have also been utilized as flexible substrates for VO 2 deposition owing to their high thermal resistance. Xiao et al. reported the fabrication of VO 2 /graphene/CNT (VGC) flexible thin films [93]. First, the graphene/CNT thin film was prepared by depositing graphene on a Cu substrate via low-pressure CVD. Then, the aligned CNT thin films were stacked on graphene substrates, followed by etching of the Cu substrate to form flexible graphene/CNT flexible thin films. The VO x thin film was deposited on the graphene/CNT film through DC magnetron sputtering and was then thermally annealed at 450 • C in a low-pressure oxygen environment to obtain crystalline VO 2 (M) thin films (Figure 5a). The phase transition of the VGC freestanding thin film can be induced by applying a current. The VGC films exhibited fast switching with low power consumption and highly reliable phase transition (Figure 5b,c). The drastic change in IR transmittance during the phase transition can potentially enable the application of VGC films in IR thermal camouflage, cloaking, and thermal optical modulators. aligned CNT thin films were stacked on graphene substrates, followed by etching of the Cu substrate to form flexible graphene/CNT flexible thin films. The VOx thin film was deposited on the graphene/CNT film through DC magnetron sputtering and was then thermally annealed at 450 °C in a low-pressure oxygen environment to obtain crystalline VO2(M) thin films (Figure 5a). The phase transition of the VGC freestanding thin film can be induced by applying a current. The VGC films exhibited fast switching with low power consumption and highly reliable phase transition (Figure 5b,c). The drastic change in IR transmittance during the phase transition can potentially enable the application of VGC films in IR thermal camouflage, cloaking, and thermal optical modulators.  [95]. Therefore, Cr2O3 can act as a buffer layer owing to the similarity of its lattice constants with those of VO2(R). Therefore, highly crystalline VO2/Cr2O3 films can be successfully fabricated even under relatively low deposition conditions from 250 to 350 °C. Moreover, the refractive index of Cr2O3 is 2.2-2.3; hence, Cr2O3 behaves as an antireflective coating on top of the VO2(M) layers, leading to a higher optical performance with Tlum and ΔTsol. The VO2 film fabricated at 275 °C showed 42.4% of Tlum and 0.4% of ΔTsol; in contrast, the VO2 film deposited with a 60-nm Cr2O3 buffer layer exhibits a high ΔTsol value of 6.7% at a similar Tlum (43.7%). To fabricate flexible VO2/Cr2O3/PI films, thin Cr2O3 layers were deposited on colorless PI films through magnetron sputtering; then, the VO2 layers were directly deposited on Cr2O3/PI films using magnetron sputtering (Figure 6b). VO2/Cr2O3/PI films exhibit minimal strain owing to the similar lattice parameters of the two layers. Therefore, flexible VO2(M) films have a narrow and sharp hysteresis loop. The VO2/Cr2O3/PI films exhibited superior IR modulation properties, i.e., approximately 60% variation at 2500 nm, when the VO2 film thickness was approximately 80 nm (Figure 6c). The Tc values of the films calculated during the heating and cooling cycles were 71.8 and 71.3 °C, respectively, and the transition width of the hysteresis loop was approximately 0.5 °C, which is significantly low for a phase transition  [94]. The Cr 2 O 3 layer allows an epitaxial growth of the VO 2 (M) layer, typically at approximately 300 • C, which enables the deposition of VO 2 (M) on the PI polymer substrate at a relatively lower temperature (Figure 6a). The lattice constants for Cr 2 O 3 are a = 0.496 nm, b = 0.496 nm, and c = 1.359 nm, and those for VO 2 (R) are a = 0.455 nm, b = 0.455 nm, and c = 0.286 nm [95]. Therefore, Cr 2 O 3 can act as a buffer layer owing to the similarity of its lattice constants with those of VO 2 (R). Therefore, highly crystalline VO 2 /Cr 2 O 3 films can be successfully fabricated even under relatively low deposition conditions from 250 to 350 • C. Moreover, the refractive index of Cr 2 O 3 is 2.2-2.3; hence, Cr 2 O 3 behaves as an antireflective coating on top of the VO 2 (M) layers, leading to a higher optical performance with T lum and ∆T sol . The VO 2 film fabricated at 275 • C showed 42.4% of T lum and 0.4% of ∆T sol ; in contrast, the VO 2 film deposited with a 60-nm Cr 2 O 3 buffer layer exhibits a high ∆T sol value of 6.7% at a similar T lum (43.7%). To fabricate flexible VO 2 /Cr 2 O 3 /PI films, thin Cr 2 O 3 layers were deposited on colorless PI films through magnetron sputtering; then, the VO 2 layers were directly deposited on Cr 2 O 3 /PI films using magnetron sputtering (Figure 6b). VO 2 /Cr 2 O 3 /PI films exhibit minimal strain owing to the similar lattice parameters of the two layers. Therefore, flexible VO 2 (M) films have a narrow and sharp hysteresis loop. The VO 2 /Cr 2 O 3 /PI films exhibited superior IR modulation properties, i.e., approximately 60% variation at 2500 nm, when the VO 2 film thickness was approximately 80 nm (Figure 6c). The T c values of the films calculated during the heating and cooling cycles were 71.8 and 71.3 • C, respectively, and the transition width of the hysteresis loop was approximately 0.5 • C, which is significantly low for a phase transition (Figure 6d,e). Furthermore, the resistivity decreased by more than two orders of magnitude during the phase transition, indicating the high crystallinity of VO 2 (M) films. However, the deposition temperature of >250 • C is still higher than the temperature that typical polymer films can withstand, which limits the utilization of various flexible polymeric substrates other than PI. ( Figure 6d,e). Furthermore, the resistivity decreased by more than two orders of magnitude during the phase transition, indicating the high crystallinity of VO2(M) films. However, the deposition temperature of >250 °C is still higher than the temperature that typical polymer films can withstand, which limits the utilization of various flexible polymeric substrates other than PI. Reproduced with permission from [94]. Copyright 2021, Elsevier.
Although direct deposition of VO2(M) on substrates with high thermal resistance is a simple single-step process, only a limited number of substrates can be used under hightemperature deposition conditions. In contrast, the film-transfer process offers opportunities to utilize various types of polymeric substrates for the fabrication of flexible films [96]. In this process, VO2(M) films are deposited on rigid substrates via high-temperature vacuum deposition and thermal annealing; then, the VO2(M) thin films are transferred onto flexible polymeric substrates using the film-transfer process. As the VO2(M) films are deposited under high-temperature conditions, they become highly crystalline, achieving enhanced optical properties (high Tlum and ΔTsol) and improved stability under ambient conditions that persists for several months [97]. Moreover, polymer supports can impart enhanced mechanical stability and flexibility to films. The fabrication of flexible VO2(M) films using the film-transfer process was first performed by Kim et al. [98]. In this process, an atomically thin, flexible graphene film was used to deposit a VO2(M) layer for the transfer process. An amorphous VOx layer was first deposited on a graphene/Cu substrate through RF magnetron sputtering. Then, the VOx film on the graphene/Cu substrate was thermally annealed at 500 °C to transform VOx into crystalline VO2 films. The Cu substrate was selectively etched, and the remaining VO2(M)/graphene film was transferred to a polyethylene terephthalate (PET) film to fabricate flexible VO2(M)/graphene/PET films. Because of the deposition on polymer films, the VO2(M)/graphene/PET films exhibited high mechanical stability and flexibility while maintaining their reversible phase-transition property. These flexible VO2/graphene/PET films exhibited a transmittance of 65.4% at a 550-nm wavelength; moreover, the variation in the transmittance during phase transition reached 53% at a wavelength of 2500 nm, with the transition band width being 9.8 °C (Figure 7a,b). The VO2(M)/graphene/PET was integrated onto glass in a model house to Although direct deposition of VO 2 (M) on substrates with high thermal resistance is a simple single-step process, only a limited number of substrates can be used under hightemperature deposition conditions. In contrast, the film-transfer process offers opportunities to utilize various types of polymeric substrates for the fabrication of flexible films [96]. In this process, VO 2 (M) films are deposited on rigid substrates via high-temperature vacuum deposition and thermal annealing; then, the VO 2 (M) thin films are transferred onto flexible polymeric substrates using the film-transfer process. As the VO 2 (M) films are deposited under high-temperature conditions, they become highly crystalline, achieving enhanced optical properties (high T lum and ∆T sol ) and improved stability under ambient conditions that persists for several months [97]. Moreover, polymer supports can impart enhanced mechanical stability and flexibility to films. The fabrication of flexible VO 2 (M) films using the film-transfer process was first performed by Kim et al. [98]. In this process, an atomically thin, flexible graphene film was used to deposit a VO 2 (M) layer for the transfer process. An amorphous VO x layer was first deposited on a graphene/Cu substrate through RF magnetron sputtering. Then, the VO x film on the graphene/Cu substrate was thermally annealed at 500 • C to transform VO x into crystalline VO 2 films. The Cu substrate was selectively etched, and the remaining VO 2 (M)/graphene film was transferred to a polyethylene terephthalate (PET) film to fabricate flexible VO 2 (M)/graphene/PET films. Because of the deposition on polymer films, the VO 2 (M)/graphene/PET films exhibited high mechanical stability and flexibility while maintaining their reversible phase-transition property. These flexible VO 2 /graphene/PET films exhibited a transmittance of 65.4% at a 550-nm wavelength; moreover, the variation in the transmittance during phase transition reached 53% at a wavelength of 2500 nm, with the transition band width being 9.8 • C (Figure 7a,b). The VO 2 (M)/graphene/PET was integrated onto glass in a model house to investigate its ability to regulate the indoor temperature when functioning as a smart window. The VO 2 /graphene films reduced the indoor room temperature by 5.8 • C compared with bare glass, thereby exhibiting the potential to function as an energy-efficient smart window (Figure 7c,d).
Nanomaterials 2021, 11, x FOR PEER REVIEW 9 of 23 investigate its ability to regulate the indoor temperature when functioning as a smart window. The VO2/graphene films reduced the indoor room temperature by 5.8 °C compared with bare glass, thereby exhibiting the potential to function as an energy-efficient smart window (Figure 7c,d). The fabrication of flexible VO2(M) thin films with a reduced Tc is still challenging owing to the difficulty in doping during the deposition process. Chae et al. reported a solution-based process to deposit VO2(M) films using W-doped colloidal NPs, followed by film transfer, to fabricate flexible W-doped VO2(M) films [74]. Colloidal VOx NPs were synthesized via high-temperature thermal decomposition with vanadium precursors, which were used for the deposition of VO2(M) layers [99]. During the synthesis, W precursors were added into the reaction mixture for efficient doping of W during the formation of VOx NPs. Then, VOx NPs were deposited on mica substrates using the solutionbased process and thermally annealed to form highly crystalline VO2(M) films. Subsequently, the VO2(M)/mica films were transferred onto polymeric substrates using adhesive-coated PET films. During the transfer process, a thin layer of mica sheet was peeled off and transferred to the polymer film to form transparent and flexible mica/VO2(M)/PET films. As mica sheets are brittle, the polymer substrate can provide high mechanical strength and ensure reliable bending (Figure 8a). The W dopants were effectively doped into the VO2(M) thin films, which resulted in the systematic reduction in Tc depending on the different possible doping concentrations. The Tc of flexible mica/VO2(M)/PET films can be easily controlled at 25.6 °C when 1 at% W doping is used (Figure 8b). These flexible films exhibit superior optical properties-a Tlum of 53% and a ΔTsol of 10%-at a Tc of 29 °C when 1.3 at% Tungsten (W) doping is used. Thus, such films can be viable for use in energy-saving smart windows (Figure 8c,d). The fabrication of flexible VO 2 (M) thin films with a reduced T c is still challenging owing to the difficulty in doping during the deposition process. Chae et al. reported a solution-based process to deposit VO 2 (M) films using W-doped colloidal NPs, followed by film transfer, to fabricate flexible W-doped VO 2 (M) films [74]. Colloidal VO x NPs were synthesized via high-temperature thermal decomposition with vanadium precursors, which were used for the deposition of VO 2 (M) layers [99]. During the synthesis, W precursors were added into the reaction mixture for efficient doping of W during the formation of VO x NPs. Then, VO x NPs were deposited on mica substrates using the solution-based process and thermally annealed to form highly crystalline VO 2 (M) films. Subsequently, the VO 2 (M)/mica films were transferred onto polymeric substrates using adhesive-coated PET films. During the transfer process, a thin layer of mica sheet was peeled off and transferred to the polymer film to form transparent and flexible mica/VO 2 (M)/PET films. As mica sheets are brittle, the polymer substrate can provide high mechanical strength and ensure reliable bending (Figure 8a). The W dopants were effectively doped into the VO 2 (M) thin films, which resulted in the systematic reduction in T c depending on the different possible doping concentrations. The T c of flexible mica/VO 2 (M)/PET films can be easily controlled at 25.6 • C when 1 at% W doping is used (Figure 8b). These flexible films exhibit superior optical properties-a T lum of 53% and a ∆T sol of 10%-at a T c of 29 • C when 1.3 at% Tungsten (W) doping is used. Thus, such films can be viable for use in energy-saving smart windows (Figure 8c,d). Nanomaterials 2021, 11, x FOR PEER REVIEW 10 of 23

Fabrication of Flexible VO2(M) Films through Solution-Based Deposition Process
Although the vacuum chamber-based deposition and film-transfer processes are highly effective for the fabrication of crystalline VO2(M) films on flexible substrates, these processes are significantly complex, involving multiple deposition steps and often requiring an etching process, which can potentially limit large-scale fabrication and commercialization [100]. In contrast, the solution-based process enables simple, low-cost, and large-area fabrication of flexible VO2(M) films [101]. Early examples of solution-processed VO2(M) films were demonstrated via a sol-gel process [102]. Speck et al. were the first to demonstrate the sol-gel deposition of VO2(M) films using molecular vanadium precursors [103]. In general, the sol-gel process of VO2(M) thin films have been performed on thermally stable substrates, such as quartz, mica, or silicon wafers, owing to the high temperature thermal annealing process, typically above 400 °C [104]. Recent literature demonstrates that low temperature sol-gel deposition of VO2(M) film processes can be achieved using deep ultraviolet photoactivation chemistry, which enable the fabrication of flexible VO2(M)/Al2O3/PI films at 250 °C [105]. Not only has the sol-gel process been widely studied for the fabrication of flexible smart windows, but solution-based deposition using colloidal VO2(M) NPs has too. Among a variety of synthetic methods based on colloidal VO2(M) NPs, hydrothermal synthesis has attracted considerable attention owing to the high phase purity of the as-synthesized VO2(M) NPs [38]. Hydrothermal synthesis involves a chemical reaction that yields high-quality crystals in a sealed pressurized reactor under high pressure and temperature. Hydrothermal growth of VO2(M) films on the substrates has also been reported in the literature [106,107]. For example, VO2(M) films have been fabricated via hydrothermal reactions by placing r-Al2O3 substrates in a hydrothermal reactor containing a solution mixture of ammonium metavanadate and oxalic acid

Fabrication of Flexible VO 2 (M) Films through Solution-Based Deposition Process
Although the vacuum chamber-based deposition and film-transfer processes are highly effective for the fabrication of crystalline VO 2 (M) films on flexible substrates, these processes are significantly complex, involving multiple deposition steps and often requiring an etching process, which can potentially limit large-scale fabrication and commercialization [100]. In contrast, the solution-based process enables simple, low-cost, and large-area fabrication of flexible VO 2 (M) films [101]. Early examples of solution-processed VO 2 (M) films were demonstrated via a sol-gel process [102]. Speck et al. were the first to demonstrate the sol-gel deposition of VO 2 (M) films using molecular vanadium precursors [103]. In general, the sol-gel process of VO 2 (M) thin films have been performed on thermally stable substrates, such as quartz, mica, or silicon wafers, owing to the high temperature thermal annealing process, typically above 400 • C [104]. Recent literature demonstrates that low temperature sol-gel deposition of VO 2 (M) film processes can be achieved using deep ultraviolet photoactivation chemistry, which enable the fabrication of flexible VO 2 (M)/Al 2 O 3 /PI films at 250 • C [105]. Not only has the sol-gel process been widely studied for the fabrication of flexible smart windows, but solution-based deposition using colloidal VO 2 (M) NPs has too. Among a variety of synthetic methods based on colloidal VO 2 (M) NPs, hydrothermal synthesis has attracted considerable attention owing to the high phase purity of the as-synthesized VO 2 (M) NPs [38]. Hydrothermal synthesis involves a chemical reaction that yields high-quality crystals in a sealed pressurized reactor under high pressure and temperature. Hydrothermal growth of VO 2 (M) films on the substrates has also been reported in the literature [106,107]. For example, VO 2 (M) films have been fabricated via hydrothermal reactions by placing r-Al 2 O 3 substrates in a hydrothermal reactor containing a solution mixture of ammonium metavanadate and oxalic acid [108]. The self-organized VO 2 (M) films were formed with T lum of 65% and ∆T sol of~11.82% [109]. However, for the direct hydrothermal deposition of VO 2 (M) films on flexible substrates, the substrates should have high thermal and chemical resistance to ensure that they can withstand hydrothermal reaction conditions and calcination temperature [110][111][112]. Therefore, the use of VO 2 (M) NPs for film depositions could have potential for large-area fabrication by mass-production processes using various substrate types. The single-step hydrothermal synthesis of VO 2 (M) NPs was first demonstrated by Théobald et al. using a V 2 O 3 -V 2 O 5 -H 2 O system, and the reaction was performed at a temperature of 20-400 • C under supercritical pressure [113]. There exist several stable vanadium oxide structures, such as VO 2 , V 2 O 5 , V 2 O 3 , V 5 O 9 , V 6 O 13 , and V 6 O 11 , with various nonstoichiometric compounds [114]. Even in the stoichiometric compound, i.e., VO 2 , several polymorphs exist, such as VO 2 (A) [115], VO 2 (B) [59], VO 2 (D) [116], VO 2 (P) [117], and VO 2 (M) [118]. Therefore, hydrothermal synthesis of phase-pure and highly crystalline VO 2 (M) is significantly challenging. Strong phase transition behaviors and favorable optical properties, including high values of T lum and ∆T sol , can be obtained using high-purity VO 2 (M) NPs, in the absence of nonstoichiometry and impurities of metastable polymorphs. Therefore, careful control of synthetic procedures, including the hydrothermal reaction conditions, types of metal precursors, solvents, and additives, is a prerequisite for obtaining phase-pure VO 2 (M) NPs.
To enhance phase purity and crystallinity, a two-step hydrothermal synthesis process to synthesize VO 2 (M) NPs has widely studied. In this process, metastable VO 2 NPs are first synthesized hydrothermally and then thermally annealed for the conversion into the VO 2 (M) phase. Phase-pure VO 2 (M) NPs are obtained from various types of metastable VO 2 NPs and under different annealing conditions. Xie et al. first reported the hydrothermal synthesis of VO 2 (D) with a size of 1-2 µm, using NH 4 VO 3 and H 2 C 2 O 4 . Hydrothermal synthesis was performed at 210 • C for 24 h, followed by a calcination process to transform the VO 2 (D) into VO 2 (M) [116]. Calcination of VO 2 (D) was performed at temperatures as low as 300 • C for 2 h under a flow of high-purity nitrogen to obtain VO 2 (R) NPs. These NPs also exhibit MIT near 68 • C. A two-step hydrothermal synthesis using VO 2 (B) NPs has also been reported; however, the phase transformation from VO 2 (B) to VO 2 (M) occurs at a significantly higher annealing temperature, typically higher than 500 • C [119]. Corr et al. also studied the hydrothermal synthesis of VO 2 (B) nanorods using V 2 O 5 and formaldehyde solution at 180 • C for two days [120]. Then, thermal annealing was performed to convert VO 2 (B) to VO 2 (R) at 700 • C for 1 h in an argon atmosphere. Sun et al. reported the hydrothermal synthesis of VO 2 (P) using VO(OC 3 H 7 ) 3 and oleylamine at 220 • C for 48 h; then, they obtained VO 2 (M) after thermal annealing at 400 • C for 40 or 60 s in a nitrogen or air atmosphere [121]. The size-dependent MIT property of VO 2 (M) NPs was demonstrated through in situ variable-temperature IR spectroscopy. The authors observed that the variation in the transmittance of single-domain VO 2 (M) NPs during phase transition systematically increased with a reduction in the size of the VO 2 (M) NPs. Zhong et al. reported star-shaped VO 2 (M) NPs that were hydrothermally synthesized using NH 4 VO 3 and formic acid for two days at 200 • C. Then, the as-synthesized NPs were thermally annealed at 300-450 • C for 1 h to obtain VO 2 (M) NPs. The VO 2 (M) NP thin films were 325 nm thick and exhibited a T lum and ∆T sol of 44.18% and 7.32%, respectively [122]. Song et al. reported the hydrothermal synthesis of VO 2 (D) using NH 4 VO 3 and H 2 C 2 O 4 ·2H 2 O at~220 • C for~18 h, followed by thermal annealing of VO 2 (D) at 250-600 • C for 3 h, to obtain VO 2 (M) nanoaggregates [123]. The as-synthesized VO 2 (M) exhibited a low T c of approximately 41.0 • C and a thermal hysteresis width of approximately 6.6 • C. Li et al. demonstrated the electrothermochromicity of VO 2 (M) NPs/Ag nanowire (NW) thin films deposited on glass and flexible PET substrates [124]. VO 2 (M) NPs were hydrothermally synthesized using V 2 O 5 and an oxalic acid dehydrate via at 220 • C for 36 h, followed by additional thermal annealing at 400 • C for 1 h in a vacuum chamber. The VO 2 (M) NPs were deposited on top of Ag NW heaters. The optical response of the VO 2 (M) NP films was then dynamically modulated by applying voltage on Ag NW. The infrared (IR) transmittance variation of the films from 0 V to 8 V of applied voltage is approximately 50% at 1500 nm. Li et al. demonstrated the two-step hydrothermal synthesis of VO 2 (M) NPs using V 2 O 5 , H 2 C 2 O 4 , and polyvinyl alcohol precursors [125]. The hydrothermal synthesis was performed at 220 • C for over 36 h, and the calcination was performed at 300-450 • C under vacuum. The VO 2 (M) film had a thickness of 463 nm and exhibited a high T lum of over 70% at 700 nm; moreover, its IR transmittances at 1500 nm were approximately 89.5% and 53.8% before and after phase transition, respectively. The IR modulation exceeded 35%, which represents favorable optical properties for application in smart windows.
A single-step hydrothermal synthesis without a calcination process has also been reported. This method is a potentially simple, convenient, and low-cost process because it involves no additional post-annealing to obtain phase-pure VO 2 (M) NPs [126]. An additional thermal annealing processes induces grain growth in VO 2 (M) films. Size dependence of VO 2 (M) NPs on thermochromic properties have also been reported. Notably, a decrease in the size of VO 2 (M) NPs improves T lum and ∆T sol values [80]. Narayan et al. reported a phase-transition model in which the hysteresis width is directly proportional to the grain boundary area per unit volume [127]. Therefore, the hysteresis width is inversely correlated to the particle radius, and as the particle size increases, the phase transition temperature reduces, and the hysteresis width decreases. The smaller the nanoparticle size, the wider the hysteresis, and the VO 2 thermochromic performance is improved [128,129]. Therefore, single-step hydrothermal synthesis is more preferable to prevent particle coarsening by an additional thermal annealing process, hence sustaining a high thermochromic performance [130,131]. Gao et al. first demonstrated single-step hydrothermal synthesis of W-doped snowflake-shaped VO 2 (R) using V 2 O 5 and H 2 C 2 O 4 . The reaction was performed for seven days at 240 • C, and VO 2 (M) NPs were synthesized without a thermal annealing step [78].  (Figure 9a,b). The as-prepared VO 2 (M) NPs exhibited a transmittance change of approximately 50% at a wavelength of 2000 nm [134]. Moreover, as the concentration of the W dopant increased from 0% to 1%, the T c of the VO 2 (M) NPs decreased from 55.5 to 37.1 • C (Figure 9c) [43]. The reaction was performed at 250 • C for 12 h to 7 days. W doping (0.90%) led to remarkable modulation of the T c of VO 2 (M) films, from 68.0 to 33.8 • C. Shen et al. demonstrated that Zr doping significantly enhances optical properties while reducing T c . [118]. Moreover, Zr doping of VO 2 (M) reduces T c while improving T lum and ∆T sol . However, T c is only reduced from 68.6 to 64.3 • C with 9.8% Zr doping; conversely, Zr-doped VO 2 flexible films exhibit high values of T lum (60.4%) and T sol (14.1%). The optical bandgap, which is 1.59 eV for undoped VO 2 (M), increases to 1.89 eV after 9.8% Zr doping, resulting in a change in the apparent color of the VO 2 (M) films. Accordingly, the color of the Zr-doped VO 2 (M) flexible films is affected; the brown-yellow color of flexible VO 2 (M) film is brightened, along with an increase in T lum . In addition, T c is further reduced to 28.6 • C, and T lum and ∆T sol values of 48.6% and 4.9%, respectively, are achieved through W-Zr-co-doping. with an increase in Tlum. In addition, Tc is further reduced to 28.6 °C, and Tlum and ΔTsol values of 48.6% and 4.9%, respectively, are achieved through W-Zr-co-doping. Several studies have been conducted to optimize the conditions for single-step hydrothermal synthesis to enhance the phase purity of VO2(M) NPs and their optical properties, including Tlum and ΔTsol. Guo et al. performed a one-step hydrothermal synthesis process using VOSO4 and N2H4·H 2O in the presence of H2O2 [77]. H2O2, a strong oxidizing agent, is separated after the reaction with the vanadium solution in a hydrothermal autoclave reactor. Then, H2O2 decomposes and evaporates at 150 °C to provide a moderately oxidizing environment. This facilitates the synthesis of stoichiometric and highly crystalline VO2(M) NPs. The as-synthesized VO2(M) NPs exhibited an average size of ~30 nm, with significant size uniformity (Figure 10a,b). For the preparation of flexible VO2(M) films, the VO2(M) NPs were dispersed in N,N-dimethylformamide with polyacrylonitrile polymers. Then, the solution was deposited on a flexible PET substrate. The flexible VO2(M) films attained favorable optical properties, with a Tlum of 54.26% and a ΔTsol of 12.34% (Figure 10c,d). In addition to optimizing the hydrothermal reaction conditions, the enhancement in the purity of vanadium precursors also produces VO2(M) NPs with improved optical properties.  Several studies have been conducted to optimize the conditions for single-step hydrothermal synthesis to enhance the phase purity of VO 2 (M) NPs and their optical properties, including T lum and ∆T sol . Guo [77]. H 2 O 2 , a strong oxidizing agent, is separated after the reaction with the vanadium solution in a hydrothermal autoclave reactor. Then, H 2 O 2 decomposes and evaporates at 150 • C to provide a moderately oxidizing environment. This facilitates the synthesis of stoichiometric and highly crystalline VO 2 (M) NPs. The as-synthesized VO 2 (M) NPs exhibited an average size of 30 nm, with significant size uniformity (Figure 10a,b). For the preparation of flexible VO 2 (M) films, the VO 2 (M) NPs were dispersed in N,N-dimethylformamide with polyacrylonitrile polymers. Then, the solution was deposited on a flexible PET substrate. The flexible VO 2 (M) films attained favorable optical properties, with a T lum of 54.26% and a ∆T sol of 12.34% (Figure 10c,d). In addition to optimizing the hydrothermal reaction conditions, the enhancement in the purity of vanadium precursors also produces VO 2 (M) NPs with improved optical properties. with an increase in Tlum. In addition, Tc is further reduced to 28.6 °C, and Tlum and ΔTsol values of 48.6% and 4.9%, respectively, are achieved through W-Zr-co-doping. Several studies have been conducted to optimize the conditions for single-step hydrothermal synthesis to enhance the phase purity of VO2(M) NPs and their optical properties, including Tlum and ΔTsol. Guo et al. performed a one-step hydrothermal synthesis process using VOSO4 and N2H4·H 2O in the presence of H2O2 [77]. H2O2, a strong oxidizing agent, is separated after the reaction with the vanadium solution in a hydrothermal autoclave reactor. Then, H2O2 decomposes and evaporates at 150 °C to provide a moderately oxidizing environment. This facilitates the synthesis of stoichiometric and highly crystalline VO2(M) NPs. The as-synthesized VO2(M) NPs exhibited an average size of ~30 nm, with significant size uniformity (Figure 10a,b). For the preparation of flexible VO2(M) films, the VO2(M) NPs were dispersed in N,N-dimethylformamide with polyacrylonitrile polymers. Then, the solution was deposited on a flexible PET substrate. The flexible VO2(M) films attained favorable optical properties, with a Tlum of 54.26% and a ΔTsol of 12.34% (Figure 10c,d). In addition to optimizing the hydrothermal reaction conditions, the enhancement in the purity of vanadium precursors also produces VO2(M) NPs with improved optical properties. Kim et al. demonstrated single-step hydrothermal synthesis of VO 2 (M) NPs using phase-pure vanadium precursors [136]. After mixing the vanadium precursors, size-selective purification was performed to enhance the phase purity of the precursors, resulting in the formation of VO 2 (M) NPs with enhanced optical properties. The obtained phase-pure VO 2 (M) NPs exhibited an enhanced T lum (55%) and ∆T sol (18%), and the ∆T sol value is one of the highest reported for hydrothermally synthesized VO 2 (M). Furthermore, W-doped VO 2 (M) NPs have been reported to exhibit superior phase-transition behaviors, while T c is systematically reduced depending on the W doping concentration (Figure 11a,b). Flexible VO 2 (M) films were fabricated and deposited on PET polymer substrates over a large area using a spray coater (15 cm × 15 cm) (Figure 11c,d). In model house experiments under daytime solar irradiation, the W-doped VO 2 (M) films applied onto glass provided a significant reduction in the indoor temperature; thus, these films are potentially viable for practical applications. Kim et al. demonstrated single-step hydrothermal synthesis of VO2(M) NPs using phase-pure vanadium precursors [136]. After mixing the vanadium precursors, size-selective purification was performed to enhance the phase purity of the precursors, resulting in the formation of VO2(M) NPs with enhanced optical properties. The obtained phasepure VO2(M) NPs exhibited an enhanced Tlum (55%) and ΔTsol (18%), and the ΔTsol value is one of the highest reported for hydrothermally synthesized VO2(M). Furthermore, Wdoped VO2(M) NPs have been reported to exhibit superior phase-transition behaviors, while Tc is systematically reduced depending on the W doping concentration (Figure  11a,b). Flexible VO2(M) films were fabricated and deposited on PET polymer substrates over a large area using a spray coater (15 cm × 15 cm) (Figure 11c,d). In model house experiments under daytime solar irradiation, the W-doped VO2(M) films applied onto glass provided a significant reduction in the indoor temperature; thus, these films are potentially viable for practical applications.  [139]. Applying a current along the ITO layer induced ohmic heating, which resulted in the phase transition of VO2(M) layers and a change in IR transmittance. The obtained film showed well-controlled IR switching properties upon changing the input voltage, as well as superior thermochromic properties (Tlum of 57.3% and ΔTsol of 13.8%). Under ohmic heating, the IR con- Colloidal NPs enable convenient, large-scale fabrication of flexible VO 2 (M) film through the solution process, which is beneficial considering the expected requirement for large-scale fabrication techniques [63,65,76,137]. For the fabrication of flexible VO 2 (M) films, VO 2 (M) NPs have been coated on flexible polymer films or embedded into a polymer matrix [138]. Shen et al. reported a process for blade coating of VO 2 (M) NPs on indium tin oxide (ITO)-coated PET substrates to form flexible VO 2 (M) films [139]. Applying a current along the ITO layer induced ohmic heating, which resulted in the phase transition of VO 2 (M) layers and a change in IR transmittance. The obtained film showed wellcontrolled IR switching properties upon changing the input voltage, as well as superior thermochromic properties (T lum of 57.3% and ∆T sol of 13.8%). Under ohmic heating, the IR conversion properties did not show any evident deterioration, even after 10,000 bending cycles, which indicates superior stability and flexibility. Chen [140]. Mg-doped VO 2 (M) composite foils were prepared by mixing NPs with PU solutions and were deposited on a PET substrate using a roll-coater. The flexible composite foils exhibited a high T lum and ∆T sol of 54.2% and 10.6%, respectively. Liang et al. reported the bar-coating of W-doped VO 2 (M) nanorods; the nanorods were prepared via one-step hydrothermal synthesis for 48 h at 240 • C using V 2 O 5 , C 4 H 6 O 6 , and ammonium tungstate (Figure 12a,b) [49]. The nanorods were then mixed with the tetraethyl orthosilicate and poly(ethyl methacrylate) solution. The solution mixture was cast on PET substrates using a stainless-steel coating bar to fabricate large-area, flexible VO 2 (M) films (Figure 12c). The T c of the flexible VO 2 (M) films could be systematically modulated by approximately 24.52 • C for 1 at% of W doping, and the mid-infrared transmission could be modulated by 31% at a T c of 37.3 • C. Inkjet printing has also been widely utilized as a useful direct-write technology to fabricate high-resolution, low-cost, large-area, and uniformsurface films on flexible substrates [141,142]. Haining et al. reported the fabrication of VO 2 (M) smart windows via inkjet printing using hydrothermally synthesized VO 2 (M) NPs [143,144]. Large-area VO 2 (M) films were fabricated on polyethylene substrates with a T lum of 56.96% and a ∆T sol of 5.21%. version properties did not show any evident deterioration, even after 10,000 bending cycles, which indicates superior stability and flexibility. Chen et al. demonstrated the preparation of VO2(M)/polymer composite films by embedding VO2(M) NPs into a polymeric matrix. VO2(M) NPs were synthesized hydrothermally using V2O5 and N2H4 at approximately 180-400 °C for 15 h [80]. The size of the VO2(M) NPs ranged from approximately 25 to 45 nm. The synthesized VO2(M) NPs were dispersed in polyurethane (PU) and coated onto PET to form flexible VO2(M) films. These films achieved high optical performance, with a ΔTsol of 22.3% and a Tlum of 45.6%. Similarly, Zhou et al. reported the rollcoating of Mg-doped VO2(M) NPs that were hydrothermally synthesized using V2O5 and H2C2O4 [140]. Mg-doped VO2(M) composite foils were prepared by mixing NPs with PU solutions and were deposited on a PET substrate using a roll-coater. The flexible composite foils exhibited a high Tlum and ΔTsol of 54.2% and 10.6%, respectively. Liang et al. reported the bar-coating of W-doped VO2(M) nanorods; the nanorods were prepared via one-step hydrothermal synthesis for 48 h at 240 °C using V2O5, C4H6O6, and ammonium tungstate (Figure 12a,b) [49]. The nanorods were then mixed with the tetraethyl orthosilicate and poly(ethyl methacrylate) solution. The solution mixture was cast on PET substrates using a stainless-steel coating bar to fabricate large-area, flexible VO2(M) films (Figure 12c). The Tc of the flexible VO2(M) films could be systematically modulated by approximately 24.52 °C for 1 at% of W doping, and the mid-infrared transmission could be modulated by 31% at a Tc of 37.3 °C. Inkjet printing has also been widely utilized as a useful direct-write technology to fabricate high-resolution, low-cost, large-area, and uniformsurface films on flexible substrates [141,142]. Haining et al. reported the fabrication of VO2(M) smart windows via inkjet printing using hydrothermally synthesized VO2(M) NPs [143,144]. Large-area VO2(M) films were fabricated on polyethylene substrates with a Tlum of 56.96% and a ΔTsol of 5.21%. The chemical instability of VO2(M) NPs can potentially limit their long-term usage as smart windows in real-world environments [145]. To enhance the chemical stability of VO2(M) NPs, core-shell structures, in which VO2(M) NPs are overcoated with chemically inert shells, have been developed. Gao et al. reported a core-shell structure with VO2@SiO2 NPs. VO2(M) was synthesized through a hydrothermal reaction, and SiO2 shells were overcoated using the Stöber method [56]. SiO2 is chemically inert and optically transparent, which is ideal for protecting VO2(M) NPs. VO2@SiO2 NPs exhibit improved chemical resistance to oxidation. The SiO2 shell of VO2 NPs serves as an oxygen diffusion barrier layer, which can prevent the VO2 from changing to V2O5. This phenomenon was confirmed through experiments conducted with VO2 NPs and VO2@SiO2 NPs after annealing in an air atmosphere for 2 h at 300 °C. Flexible films were fabricated by embedding VO2@SiO2 NPs into a PU matrix; then, the VO2@SiO2 NPs/PU cast on a PET matrix were dispersed to fabricate flexible VO2@SiO2/PU composite films. These films exhibited a high Tlum (55.3%) and ΔTsol (7.5%). In addition to SiO2, various types of oxides, such as ZnS [146], TiO2 [11], The chemical instability of VO 2 (M) NPs can potentially limit their long-term usage as smart windows in real-world environments [145]. To enhance the chemical stability of VO 2 (M) NPs, core-shell structures, in which VO 2 (M) NPs are overcoated with chemically inert shells, have been developed. Gao et al. reported a core-shell structure with VO 2 @SiO 2 NPs. VO 2 (M) was synthesized through a hydrothermal reaction, and SiO 2 shells were overcoated using the Stöber method [56]. SiO 2 is chemically inert and optically transparent, which is ideal for protecting VO 2 (M) NPs. VO 2 @SiO 2 NPs exhibit improved chemical resistance to oxidation. The SiO 2 shell of VO 2 NPs serves as an oxygen diffusion barrier layer, which can prevent the VO 2 from changing to V 2 O 5 . This phenomenon was confirmed through experiments conducted with VO 2 NPs and VO 2 @SiO 2 NPs after annealing in an air atmosphere for 2 h at 300 • C. Flexible films were fabricated by embedding VO 2 @SiO 2 NPs into a PU matrix; then, the VO 2 @SiO 2 NPs/PU cast on a PET matrix were dispersed to fabricate flexible VO 2 @SiO 2 /PU composite films. These films exhibited a high T lum (55.3%) and ∆T sol (7.5%). In addition to SiO 2 , various types of oxides, such as ZnS [146], TiO 2 [11], and ZrO 2 [147], have been utilized for overcoating to prepare core-shell NPs. Saini et al. demonstrated an approach to improve the thermal stability and thermochromic properties of VO 2 (M) NPs by overcoating with CeO 2 [148]. VO 2 (M)@CeO 2 NPs were observed to be thermally stable for up to 320 • C in air, which confirmed the enhancement in stability after overcoating.

Perspectives
Flexible VO 2 (M) films offer significant potential for the integration of energy-saving smart windows in existing buildings, as well as for application in novel flexile devices, such as sensors and actuators. Various methods for fabricating flexible thermochromic thin films based on the vacuum deposition and solution-based process have been reported; these methods are potentially suitable for commercialization. However, certain issues still remain to be resolved before VO 2 -based smart windows can be utilized in practice. For example, flexible VO 2 films fabricated using vacuum deposition and film-transfer techniques show high T lum and ∆T sol values; however, these methods are still limited in terms of large-area and mass-production capabilities. In addition, deposition methods with uniform doping should be developed further to systematically reduce T c while maintaining favorable phasechange optical properties. Conversely, the annealing-free, solution-based process offers advantages such as convenient, low-cost, large-area deposition of phase-change VO 2 (M) on flexible substrates. Particularly, hydrothermal synthesis yields highly crystalline VO 2 (M) NPs with colloidal stability and moderately useful phase-change behaviors. However, it is still challenging to prepare flexible VO 2 (M) films with high T lum and ∆T sol values as well as a reduced T c . The optical properties of the representative flexible VO 2 films fabricated using deposition and solution-based processes are summarized in Figure 13, which displays the opportunities for utilizing flexible VO 2 (M) films in energy-saving smart windows. Therefore, large-scale, high-throughput, mass-production capabilities for the fabrication and commercialization of high-performance VO 2 (M) films should be realized. Finally, certain limitations in terms of the intrinsic properties of VO 2 (M) should be overcome to utilize flexible VO 2 films. First, phase-change VO 2 films show an inherent brown color, which is not desirable for window applications. Therefore, it is highly recommended to develop fabrication methods that can enable control of the apparent colors of VO 2 (M) while ensuring a high T lum and ∆T sol and low T c . Moreover, vanadium oxide has various stable phases and a stable stoichiometry; consequently, VO 2 (M) films are easily oxidized into other phases under exposure in ambient conditions. Therefore, processes to prevent VO 2 (M) from being oxidized, for example, overcoating of VO 2 (M) films or using NPs with protective layers, should be developed to enable long-term usage of the films.  [139], (c) [56], (d) [136], (e) [118], (f) [149], (g) [74], (h), [64], (i) [143], (j) [109].