Flexible thermochromic window based on hybridized VO2/graphene

Thermochromic Vanadium Dioxide Nanostructures for Smart Windows and Radiative Cooling

The pursuit of energy-saving materials and technologies has garnered significant attention for their pivotal role in mitigating both energy consumption and carbon emissions. In particular, thermochromic windows in buildings offer energy-saving potential by adjusting the transmittance of solar irradiation in response to temperature changes. Radiative cooling (RC), radiating thermal heat from an object surface to the cold outer space, also offers a potential way for cooling without energy consumption. Accordingly, smart window and RC technologies based on thermochromic materials can play a crucial role in improving energy efficiency and reducing energy consumption in buildings in response to the surrounding temperature. Vanadium dioxide (VO2) is a promising thermochromic material for energy-saving smart windows and RC due to its reversible metal-to-insulator transition, accompanying large changes in its optical properties. This review provides a brief summary of synthesis methods of VO2 nanostructures based on nanoparticles and thin films. Moreover, this review emphasizes and summarizes modulation strategies focusing on doping, thermal processing, and structure manipulation to improve and regulate the thermochromic and emissivity performance of VO2 for smart window and RC applications. In last, the challenges and recent advances of VO2-based smart window and RC applications are briefly presented.

A Smart Window with Passive Radiative Cooling and Switchable Near-Infrared Light Transmittance via Molecular Engineering

Smart windows with synergetic light modulation have heightened demands for applications in smart cars and novel buildings. However, improving the on-demand energy-saving efficiency is quite challenging due to the difficulty of modulating sunlight with a broad bandwidth in an energy-saving way. Herein, a smart window with switchable near-infrared light transmittance and passive radiative cooling is prepared via a monomer design strategy and photoinduced polymerization. The effects of hydrogen bonds and fluorine groups in acrylate monomers on the electro-optical properties as well as microstructures of polymer-dispersed liquid crystal films have been systematically studied. Some films show a high contrast ratio of 90.4 or a low threshold voltage (Vth) of 2.0 V, which can be roll-to-roll processed in a large area. Besides, the film has a superior indoor temperature regulation ability due to its passive radiative cooling and controllable near-infrared light transmittance properties. Its radiative cooling efficiency is calculated to be 142.69 W/m2 and NIR transmittance could be switched to below 10%. The introduction of a carboxylic monomer and fluorinated monomer into the system endows the film with a highly efficient temperature management capability. The film has great potential for applications in fields such as flexible smart windows, camouflage materials, and so on.

Stretchable and Transparent Heaters Based on Hydrophobic Ionogels with Superior Moisture Insensitivity

Flexible and stretchable transparent heaters (THs) have been widely used in various applications, including deicing and defogging of flexible screens as well as thermotherapy pads. Ionic THs based on ionogels have emerged as a promising alternative to conventional electronic THs due to their unique advantages in terms of transparency-conductance conflict, uniform heating, and interfacial adhesion. However, the commonly used hydrophilic ionogels inevitably introduce a moisture-sensitive issue. In this work, we present a stretchable and transparent hydrophobic ionogel-based heater that utilizes ionic current-induced Joule heating under high-frequency alternating current. This ionogel-based TH exhibits exceptional multifunctional properties with low hysteresis, a fracture strain of 840%, transmittance of 93%, conductivity of 0.062 S m-1, temperature resistance up to 165 °C, voltage resistance up to 120 V, heating rate of 0.1 °C s-1, steady-state temperature at 115 °C, and uniform heating even when bent or stretched (up to 200%). Furthermore, it maintains its heating performance when it is directly exposed to water. This hydrophobic ionogel-based TH expands the range of materials available for ionic THs and paves the way for their practical applications.

Metal-insulator transition effect on Graphene/VO[Formula: see text] heterostructure via temperature-dependent Raman spectroscopy and resistivity measurement

High-quality VO[Formula: see text] films were fabricated on top of c-Al[Formula: see text]O[Formula: see text] substrates using Reactive Bias Target Ion Beam Deposition (RBTIBD) and the studies of graphene/VO[Formula: see text] heterostructure were conducted. Graphene layers were placed on top of [Formula: see text] 50 and [Formula: see text] 100 nm VO[Formula: see text]. The graphene layers were introduced using mechanical exfoliate and CVD graphene wet-transfer method to prevent the worsening crystallinity of VO[Formula: see text], to avoid the strain effect from lattice mismatch and to study how VO[Formula: see text] can affect the graphene layer. Slight increases in graphene/VO[Formula: see text] T[Formula: see text] compared to pure VO[Formula: see text] by [Formula: see text] 1.9 [Formula: see text]C and [Formula: see text] 3.8 [Formula: see text]C for CVD graphene on 100 and 50 nm VO[Formula: see text], respectively, were observed in temperature-dependent resistivity measurements. As the strain effect from lattice mismatch was minimized in our samples, the increase in T[Formula: see text] may originate from a large difference in the thermal conductivity between graphene and VO[Formula: see text]. Temperature-dependent Raman spectroscopy measurements were also performed on all samples, and the G-peak splitting into two peaks, G[Formula: see text] and G[Formula: see text], were observed on graphene/VO[Formula: see text] (100 nm) samples. The G-peak splitting is a reversible process and may originates from in-plane asymmetric tensile strain applied under the graphene layer due to the VO[Formula: see text] phase transition mechanism. The 2D-peak measurements also show large blue-shifts around 13 cm[Formula: see text] at room temperature and slightly red-shifts trend as temperature increases for 100 nm VO[Formula: see text] samples. Other electronic interactions between graphene and VO[Formula: see text] are expected as evidenced by 2D-peak characteristic observed in Raman measurements. These findings may provide a better understanding of graphene/VO[Formula: see text] and introduce some new applications that utilize the controllable structural properties of graphene via the VO[Formula: see text] phase transition.

Sol-Gel Derived Tungsten Doped VO2 Thin Films on Si Substrate with Tunable Phase Transition Properties

Vanadium dioxide (VO₂) with semiconductor-metal phase transition characteristics has presented great application potential in various optoelectrical smart devices. However, the preparation of doped VO₂ film with a lower phase transition threshold on Si substrate needs more investigation for the exploration of silicon-based VO₂ devices. In this work, the VO₂ films doped with different contents of W element were fabricated on high-purity Si substrate, assisted with a post-annealing process. The films exhibited good crystallinity and uniform thickness. The X-ray diffraction and X-ray photoelectron spectroscopy characterizations illustrated that W element can be doped into the lattice of VO₂ and lead to small lattice distortion. In turn, the in situ FT-IR measurements indicated that the phase transition temperature of the VO₂ films can be decreased continuously with W doping content. Simultaneously, the doping would lead to largely enhanced conductivity in the film, which results in reduced optical transmittance. This work provides significant insights into the design of doped VO₂ films for silicon-based devices.

Advanced liquid crystal-based switchable optical devices for light protection applications: principles and strategies

With the development of optical technologies, transparent materials that provide protection from light have received considerable attention from scholars. As important channels for external light, windows play a vital role in the regulation of light in buildings, vehicles, and aircrafts. There is a need for windows with switchable optical properties to prevent or attenuate damage or interference to the human eye and light-sensitive instruments by inappropriate optical radiation. In this context, liquid crystals (LCs), owing to their rich responsiveness and unique optical properties, have been considered among the best candidates for advanced light protection materials. In this review, we provide an overview of advances in research on LC-based methods for protection against light. First, we introduce the characteristics of different light sources and their protection requirements. Second, we introduce several classes of light modulation principles based on liquid crystal materials and demonstrate the feasibility of using them for light protection. In addition, we discuss current light protection strategies based on liquid crystal materials for different applications. Finally, we discuss the problems and shortcomings of current strategies. We propose several suggestions for the development of liquid crystal materials in the field of light protection.

From Materials to Devices: Graphene toward Practical Applications

Graphene, as an emerging 2D material, has been playing an important role in flexible electronics since its discovery in 2004. The representative fabrication methods of graphene include mechanical exfoliation, liquid-phase exfoliation, chemical vapor deposition, redox reaction, etc. Based on its excellent mechanical, electrical, thermo-acoustical, optical, and other properties, graphene has made a great progress in the development of mechanical sensors, microphone, sound source, electrophysiological detection, solar cells, synaptic transistors, light-emitting devices, and so on. In different application fields, large-scale, low-cost, high-quality, and excellent performance are important factors that limit the industrialization development of graphene. Therefore, laser scribing technology, roll-to-roll technology is used to reduce the cost. High-quality graphene can be obtained through chemical vapor deposition processes. The performance can be improved through the design of structure of the devices, and the homogeneity and stability of devices can be achieved by mechanized machining means. In total, graphene devices show promising prospect for the practical fields of sports monitoring, health detection, voice recognition, energy, etc. There is a hot issue for industry to create and maintain the market competitiveness of graphene products through increasing its versatility and killer application fields.

Growth and Characterization of Ultrathin Vanadium Oxide Films on HOPG

Integration of graphene into various electronic devices requires an ultrathin oxide layer on top of graphene. However, direct thin film growth of oxide on graphene is not evident because of the low surface energy of graphene promoting three-dimensional island growth. In this study, we demonstrate the growth of ultrathin vanadium oxide films on a highly oriented pyrolytic graphite (HOPG) surface, which mimics the graphene surface, using (oxygen-assisted) molecular beam epitaxy, followed by a post-annealing. The structural properties, surface morphology, and chemical composition of the films have been systematically investigated by in situ reflection high-energy electron diffraction during the growth and by ex situ techniques, such as atomic force microscopy, scanning tunneling microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy (XPS). Crystalline monolayer vanadium oxide can be achieved on HOPG by systematically tuning the deposition time of V atoms and by subsequent annealing at 450 °C in controlled atmospheres. Increasing the partial pressure of O₂ during the deposition seems to decrease the mobility of V atoms on the graphitic surface of HOPG and promote the formation of a two-dimensional (2D) vanadium oxide. The obtained oxide layers are found to be polycrystalline with an average grain size of 15 nm and to have a mixed-valence state with mainly V⁵⁺ and V⁴⁺. Moreover, XPS valence band measurements indicate that the vanadium oxide is insulating. These results demonstrate that a 2D insulating vanadium oxide can be grown directly on HOPG and suggest vanadium oxide as a promising candidate for graphene/oxide heterostructures.

Electrospun Magnetic Ionic Liquid Based Electroactive Materials for Tissue Engineering Applications

Functional electrospun fibers incorporating ionic liquids (ILs) present a novel approach in the development of active microenviroments due to their ability to respond to external magnetic fields without the addition of magnetic particles. In this context, this work reports on the development of magnetically responsive magneto-ionic fibers based on the electroactive polymer poly(vinylidene fluoride) and the magnetic IL (MIL), bis(1-butyl-3-methylimidazolium) tetrathiocyanatocobaltate ([Bmim]₂[(SCN)₄Co]). The PVDF/MIL electrospun fibers were prepared incorporating 5, 10 and 15 wt.% of the MIL, showing that the inclusion of the MIL increases the polar β-phase content of the polymer from 79% to 94% and decreases the crystallinity of the fibers from 47% to 36%. Furthermore, the thermal stability of the fibers decreases with the incorporation of the MIL. The magnetization of the PVDF/MIL composite fibers is proportional to the MIL content and decreases with temperature. Finally, cytotoxicity assays show a decrease in cell viability with increasing the MIL content.

Sputtering Flexible VO2 Films for Effective Thermal Modulation

Flexible vanadium dioxide (VO₂) thermochromic films show great potential for large-scale fabrication and possess broader applications compared with VO₂ coatings on rigid substrates. However, the fabrication of flexible VO₂ films remains a challenge so far, leading to the scarcity of research on flexible VO₂ films for smart windows. With the aim to obtain a flexible VO₂-based films with excellent optical properties and a long service life, we designed and successfully fabricated a flexible ITO/VO₂/ITO (IVI) film on the colorless transparent polyimide substrate, which could be directly attached to glasses for indoor temperature modulation. This flexible IVI film effectively enhances the luminous transmittance (T_lum) and solar modulation ability (ΔT_sol) (15 and 68% increase relative to a VO₂ single layer), reduces the thermal emissivity (ε_T) (50.7% decrease relative to a VO₂ single layer), and exhibits better durability than previously reported structures. Such excellent comprehensive performance offers it great potential in practical applications on smart windows. This work is supposed to provide a new strategy for facile direct fabrication of flexible VO₂ films and broaden the applications of flexible VO₂ in more coatings and devices.

Cooperative coupling of anisotropic phonon modes intensifies visible thermochromism in layered α-MoO3

Applications that provide versatile, high temperature warnings require the development of thermochromic materials based on solid-state oxides. To boost the visible thermochromic properties, a fundamental approach to reveal the unclear roles of local structure on band structure modulation should be considered by scrutinizing the thermal motion of phonon modes. Herein, we demonstrate that selective coupling of intra-layer phonon modes intensifies the visible thermochromism of layered oxides α-MoO₃. As a result of thermally induced band gap reduction in α-MoO₃, the observed color reversibly changes from white at 25 °C to yellow at 300 °C owing to a red shift of the absorption edge with an increase of temperature. This high-temperature thermochromism is attributed to the anisotropic change of layered α-MoO₃ crystal structures characterized by synchrotron X-ray diffraction. Notably, quantitative characterizations combined with theoretical calculations reveal that the cooperative coupling of active Raman modes in intra-layer [MoO₆] octahedra are responsible for the band gap reduction at high temperature; this defies the general belief regarding the origin of visible thermochromism in layered oxides as the modulation of a van der Waals inter-layer distance. These original results can aid the development of a new strategy to further intensify high-temperature thermochromism by anion doping for highly sensitive temperature-indicating applications.

A dual-mode laser-textured ice-phobic slippery surface: low-voltage-powered switching transmissivity and wettability for thermal management

Smart windows that dynamically fine-tune the solar energy gain are promising candidates for alleviating the global energy crisis. However, current smart surfaces easily deteriorate when rain or frozen ice dwells on the surface structure, heavily hindering their applications. Here, we report an electric-powered dual-mode slippery lubricant-impregnated porous surface (DM-SLIPS) developed by integrating paraffin wax and laser-ablated polytetrafluoroethylene (LA-PTFE) along with a silver nanowire thin-film heater. Owing to its fast electrical response, DM-SLIPS can be switched to repel surface-dwelling liquids within 20 s by applying an ultra-low voltage of 6 V. Simultaneously, light irradiated on DM-SLIPS can be finely-tuned between a "lock mode" and "release mode" in response to the solidification/liquidation of paraffin. Owing to homogeneous Joule heating, the DM-SLIPS surface can remove surface-frozen ice within 4 min in situ. As a proof-of-concept, the temperature of an indoor object shielded with electric-actuated DM-SLIPS could be reversibly switched between 34 °C and 29 °C, realizing controllable solar energy input. In comparison with previously reported surfaces, the present water-repellent, ice-phobic and transparency-switchable DM-SLIPS can be more useful for thermal management in extreme climates.

Wafer-scale freestanding vanadium dioxide film

Vanadium dioxide (VO₂), with well-known metal-to-insulator phase transition, has been used to realize intriguing smart functions in photodetectors, modulators, and actuators. Wafer-scale freestanding VO₂ (f-VO₂) films are desirable for integrating VO₂ with other materials into multifunctional devices. Unfortunately, their preparation has yet to be achieved because the wafer-scale etching needs ultralong time and damages amphoteric VO₂ whether in acid or alkaline etchants. Here, we achieved wafer-scale f-VO₂ films by a nano-pinhole permeation-etching strategy in 6 min, far less than that by side etching (thousands of minutes). The f-VO₂ films retain their pristine metal-to-insulator transition and intrinsic mechanical properties and can be conformably transferred to arbitrary substrates. Integration of f-VO₂ films into diverse large-scale smart devices, including terahertz modulators, camouflageable photoactuators, and temperature-indicating strips, shows advantages in low insertion loss, fast response, and low triggering power. These f-VO₂ films find more intriguing applications by heterogeneous integration with other functional materials.

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 (T_c = 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 T_c, 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.

A smart material built upon the photo-thermochromic effect and its use for managing indoor temperature

We demonstrate a material by dispersing a thermochromic mixture of leuco dye, developer, and solvent as microspheres in a polymer matrix to improve the efficiency of building energy management. The smart, photo-thermochromic film can automatically switch between a colored and colorless state in response to climate temperature and light to realize photothermal heating and cooling.

Graphene-Based Environmental Sensors: Electrical and Optical Devices

In this review paper, we summarized the recent progress of using graphene as a sensing platform for environmental applications. Especially, we highlight the electrical and optical sensing devices developed based on graphene and its derivatives. We discussed the role of graphene in these devices, the sensing mechanisms, and the advantages and disadvantages of specific devices. The approaches to improve the sensitivity and selectivity are also discussed.

Recent Progress on Vanadium Dioxide Nanostructures and Devices: Fabrication, Properties, Applications and Perspectives

Vanadium dioxide (VO₂) is a typical metal-insulator transition (MIT) material, which changes from room-temperature monoclinic insulating phase to high-temperature rutile metallic phase. The phase transition of VO₂ is accompanied by sudden changes in conductance and optical transmittance. Due to the excellent phase transition characteristics of VO₂, it has been widely studied in the applications of electric and optical devices, smart windows, sensors, actuators, etc. In this review, we provide a summary about several phases of VO₂ and their corresponding structural features, the typical fabrication methods of VO₂ nanostructures (e.g., thin film and low-dimensional structures (LDSs)) and the properties and related applications of VO₂. In addition, the challenges and opportunities for VO₂ in future studies and applications are also discussed.

3D mesoporous structure assembled from monoclinic M-phase VO2 nanoflakes with enhanced thermochromic performance

Herein, 3D mesoporous structures assembled from monoclinic M-phase VO₂ nanoflakes were successfully synthesized for enhanced thermochromic performance.

Influence of micro-structure on modulation properties in VO2 composite terahertz memory metamaterials

We have grown VO₂ films and combined with terahertz metamaterials to manipulate the memory effect during the insulator-to-metal transition. The temperature-dependent resonant frequency of hybrid structure shows a thermal hysteresis accompanied with frequency shift and bandwidth variation due to the presence of a VO₂ dielectric layer. This frequency memory effect significantly depends on the metallic micro-structure. Further theoretical calculation demonstrates this phenomenon mainly originates from the different coupling strength between VO₂ and metallic structures. Our findings could facilitate the application of VO₂ films in the smart window and dynamical terahertz modulators.

Data for direct chemical deposition of PbS on chemical vapor deposition grown-graphene for high performance photovoltaic infrared photo-detectors

Over the past decades, graphene has attracted much attention from the scientific community due to its broad applications in the optoelectronics industries [1]. Owing to graphene's high transmission and high electrical conductivity, diverse functional materials/graphene hybridized heterostructures and interfaces are under extensive investigation to satisfy the increasing interest in the need for bendable, flexible and high performance optoelectronic devices [2]. Due to the good atomic lattice structure of graphene, varying heterostructures have been formed by depositing different functional materials directly on graphene [3], [4], [5]. We fabricated a vertical photovoltaic type G/PbS/Ti device by making use of the Ti/PbS Schottky junction and discussed the photocurrent transient characteristics. Lead sulfide (PbS) was deposited directly on large area CVD (Chemical vapor deposition) graphene by CBD (Chemical bath deposition). Temperature dependent photocurrent spectra of our G/PbS/Ti photovoltaic devices were measured by a Fourier transformed infrared (FTIR) set-up. In this paper, we present the experimental procedures and the raw experimental data for the direct chemical deposition of PbS on CVD-graphene for high performance photovoltaic infrared photo-detectors. The manuscript is already available [6].

Unveiling the optical parameters of vanadium dioxide in the phase transition region: a hybrid modeling approach

The phase change behavior of vanadium dioxide (VO₂) has been widely explored in a variety of optical and photonic applications. Commonly, its optical parameters have been studied in two extreme regimes: hot (metallic) and cold (insulating) states. However, in the transition temperatures, VO₂ acts like an inherent metamaterial with mixed metallic-insulating character. In this range, the portions of metallic and insulating inclusions are tuned by temperature, and therefore a gradual change of optical parameters can be achieved. In this paper, a universal hybrid modeling approach is developed to model VO₂ in the intermediate region. For this aim, the measured reflectivity data, is analyzed and matched through the transfer matrix method (TMM) simulations where an effective medium theory (EMT) is employed. Based on the findings of this approach, not only the relative portions of inclusions are tailored but also their grain shapes are significantly altered in the transition range. Finally, the modeling approach is testified by experimental findings through dynamic device applications operating at short and mid infrared wavelengths. In addition, the hysteretic behaviors on electrical, optical, and structural parameters of the VO₂ film along the heating and cooling cycles are demonstrated by the experiments and scrutinized by the simulations.

Transparent Wood Composites Fabricated by Impregnation of Epoxy Resin and W-Doped VO2 Nanoparticles for Application in Energy-Saving Windows

Two types of transparent wood composites with anisotropic structure for energy-saving windows were successfully fabricated by infiltration of epoxy resin dispersion containing tungsten-doped vanadium dioxide nanoparticles (W-doped VO₂ NPs) into the delignified wood template and subsequent polymerization. The well integration of the epoxy resin, W-doped VO₂ NPs, and the pore-structured wood endowed the anisotropic composites with high visible transmittance (68.2% for the composite prepared from longitudinally cut trees (L-composite), 73.3% for the composite prepared from radically cut trees (R-composite)), obviously different mechanical performance (fracture stress of 74.57 MPa (L-composite) and 56.14 MPa (R-composite) and modulus of 1.47 GPa (L-composite) and 1.23 GPa (R-composite)), and low thermal conductivity (0.20 W·m^-1 K^-1 (L-composite) and 0.32 W·m^-1 K^-1 (R-composite)). Moreover, these two kinds of W/VO₂ transparent wood composites both show an outstanding thermoregulation ability when they are used as windows. A significant amount of heat (from a simulated light source) was reflected by VO₂ NPs, and as a result, the indoor temperature of a demo system had a significant slower temperature increase rate when compared with that for a similar system with a common glass panel applied. Novel transparent wood composites combining a low thermal conductivity wood template and thermochromic VO₂ NPs provide a potential solution for replacement of heavy, high thermal conductivity, and infrared transparent glass but still meet indoor occupancy view perception.

A bottom-up strategy toward a flexible vanadium dioxide/silicon nitride composite film with infrared sensing performance

Vanadium dioxide (VO2) attracts great attention due to its well-known metal-to-insulator transition. However, traditional VO2 films grown on rigid substrates are inflexible, which limits their applications. In this work, we successfully prepared VO2/silicon nitride (VO2/SN) composite films by a simple template method. The VO2/SN film shows high flexibility, strong infrared absorption, and drastic resistance change (>103) induced by the phase transition. The application of the VO2/SN film is presented by infrared sensing, which shows a high responsivity (720 V W-1) and short response time (409 ms).

Comparative Building Energy Simulation Study of Static and Thermochromically Adaptive Energy-Efficient Glazing in Various Climate Regions

The building sector contributes approximately one third of the total energy consumption worldwide. A large part of this energy is used for the heating and cooling of buildings, which can be drastically reduced by use of energy-efficient glazing. In this study, we performed building energy simulations on a prototypical residential building, and compared commercially available static (low-e, solar IR blocking) to newly developed adaptive thermochromic glazing systems for various climate regions. The modeling results show that static energy-efficient glazing is mainly optimized for either hot climates, where low solar heat gain can reduce cooling demands drastically, or cold climates, where low-e properties have a huge influence on heating demands. For intermediate climates, we demonstrate that adaptive thermochromic glazing in combination with a low-e coating is perfectly suited. The newly developed thermochromic glazing can lead to annual energy consumption improvement of up to 22% in comparison to clear glass, which exceeds all other glazing systems. Furthermore, we demonstrate that in the Netherlands the use of this new glazing system can lead to annual cost savings of EU 638 per dwelling (172 m2, 25% window façade), and to annual nationwide CO2 savings of 4.5 Mt. Ergo, we show that further development of thermochromic smart windows into market-ready products can have a huge economic, ecological and societal impact on all intermediate climate region in the northern hemisphere.

Nonvolatile ferroelectric field effect transistor based on a vanadium dioxide nanowire with large on- and off-field resistance switching

We fabricate a ferroelectric field effect transistor (FeFET) based on a semiconducting vanadium dioxide (VO₂) nanowire (NW), and we investigate its electron transport characteristics modulated by the ferroelectric effects.

Mechano-thermo-chromic device with supersaturated salt hydrate crystal phase change

Active control of transparency/color is the key to many functional optoelectric devices. Applying an electric field to an electrochromic or liquid crystal material is the typical approach for optical property control. In contrast to the conventional electrochromic method, we developed a new concept of smart glass using new driving mechanisms (based on mechanical stimulus and thermal energy) to control optical properties. This mechano-thermo-chromic smart glass device with an integrated transparent microheater uses a sodium acetate solution, which shows a unique marked optical property change under mechanical impact (mechanochromic) and heat (thermochromic). Such mechano-thermo-chromic devices may provide a useful approach in future smart window applications that could be operated by external environment conditions.

Self-Template Synthesis of Nanoporous VO2-Based Films: Localized Surface Plasmon Resonance and Enhanced Optical Performance for Solar Glazing Application

Poly(tetrafluoroethylene) (Teflon) has been selected as the self-template structural material in the preparation of VO₂ films using a reactive magnetron sputtering method and post-annealing process. VO₂ films with spontaneous random nanoporous structures growing on quartz glasses have been deliberately established via bottom-up processing through this novel and facile approach. The nanoporous VO₂ films exhibit an excellent optical performance based on the localized surface plasmon resonance, with ultrahigh luminous transmittance ( T_lum-L) up to 78.0% and the promoted solar modulation ability (Δ T_sol) of 14.1%. Meanwhile, the ingenious microstructure of the film provides an antireflection function from multiple perspectives on visible light and indicates the potential of the windshield on vehicles for smart solar modulation. The nanoporous films expand the practical application of thermochromic VO₂ to a fire-new field, breaking the optical performance envelope of the single-layer dense VO₂ film away, and offering a universal method to prepare homogeneous nanoporous structures for thin films.

Spectral-Selective Plasmonic Polymer Nanocomposites Across the Visible and Near-Infrared

State-of-the-art commercial light-reflecting glass is coated with a metalized film to decrease the transmittance of electromagnetic waves. In addition to the cost of the metalized film, one major limitation of such light-reflecting glass is the lack of spectral selectivity over the entire visible and near-infrared (NIR) spectrum. To address this challenge, we herein effectively harness the transmittance, reflectance, and filtration of any wavelength across the visible and NIR, by judiciously controlling the planar orientation of two-dimensional plasmonic silver nanoplates (AgNPs) in polymer nanocomposites. In contrast to conventional bulk polymer nanocomposites where plasmonic nanoparticles are randomly mixed within a polymer matrix, our thin-film polymer nanocomposites comprise a single layer, or any desired number of multiple layers, of planarly oriented AgNPs separated by tunable spacings. This design employs a minimal amount of metal and yet efficiently manages light across the visible and NIR. The thin-film plasmonic polymer nanocomposites are expected to have a significant impact on spectral-selective light modulation, sensing, optics, optoelectronics, and photonics.

Daylight-Induced Metal-Insulator Transition in Ag-Decorated Vanadium Dioxide Nanorod Arrays

Metal-insulator transition (MIT) in strongly correlated electronic materials has enormous potential with scientific and technological impacts in future oxide nanoelectronic devices. Although photo-induced MIT can provide opportunities to extend the novel functionality of strongly correlated electronic materials, there have rarely been reports on it. Here, we report MIT provoked by visible-near-infrared light in Ag-decorated VO₂ nanorod arrays (NRs) because of localized surface plasmon resonance (LSPR) and its application to broadband photodetectors. Our simulation results based on the finite-difference time-domain method show that the electric field resulting from LSPR can be generated at the interface between Ag nanoparticles and VO₂ layers under vis NIR illumination. Using high-resolution transmission electronic microscopy and Raman spectroscopy, we observe the MIT and structural phase transition in the Ag-decorated VO₂ NRs due to the LSPR effect. The optoelectronic measurements confirm that high, fast, and broad photoresponse of Ag-decorated VO₂ NRs is attributed to photo-induced MIT due to LSPR. Our study will open up a new strategy to trigger MIT in strongly correlated electronic materials through functionalization with plasmonic nanoparticles and serve as a valuable proof of concept for next-generation optoelectronic devices with fast response, low power consumption, and high performance.

Gate-controlled VO2 phase transition for high-performance smart windows

Vanadium dioxide (VO₂) is a promising material for developing energy-saving "smart windows," owing to its infrared thermochromism induced by metal-insulator transition (MIT). However, its practical application is greatly limited by its relatively high critical temperature (~68°C), low luminous transmittance (<60%), and poor solar energy regulation ability (<15%). Here, we developed a reversible and nonvolatile electric field control of the MIT of a monoclinic VO₂ film. With a solid electrolyte layer assisting gating treatment, we modulated the insertion/extraction of hydrogen into/from the VO₂ lattice at room temperature, causing tristate phase transitions that enable control of light transmittance. The dramatic increase in visible/infrared transmittance due to the phase transition from the metallic (lightly H-doped) to the insulating (heavily H-doped) phase results in an increased solar energy regulation ability up to 26.5%, while maintaining 70.8% visible luminous transmittance. These results break all previous records and exceed the theoretical limit for traditional VO₂ smart windows, making them ready for energy-saving utilization.

Growth of vanadium dioxide thin films on hexagonal boron nitride flakes as transferrable substrates

Vanadium dioxide (VO2) is an archetypal metal-insulator transition (MIT) material, which has been known for decades to show an orders-of-magnitude change in resistivity across the critical temperature of approximately 340 K. In recent years, VO2 has attracted increasing interest for electronic and photonic applications, along with advancement in thin film growth techniques. Previously, thin films of VO2 were commonly grown on rigid substrates such as crystalline oxides and bulk semiconductors, but the use of transferrable materials as the growth substrates can provide versatility in applications, including transparent and flexible devices. Here, we employ single-crystalline hexagonal boron nitride (hBN), which is an insulating layered material, as a substrate for VO2 thin film growth. VO2 thin films in the polycrystalline form are grown onto hBN thin flakes exfoliated onto silicon (Si) with a thermal oxide, with grains reaching up-to a micrometer in size. The VO2 grains on hBN are orientated preferentially with the (110) surface of the rutile structure, which is the most energetically favorable. The VO2 film on hBN shows a MIT at approximately 340 K, across which the resistivity changes by nearly three orders of magnitude, comparable to VO2 films grown on common substrates such as sapphire and titanium dioxide. The VO2/hBN stack can be picked up from the supporting Si and transferred onto arbitrary substrates, onto which VO2 thin films cannot be grown directly. Our results pave the way for new possibilities for practical and versatile applications of VO2 thin films in electronics and photonics.

Multifunctional La0.67Sr0.33MnO3 (LSMO) Thin Films Integrated on Mica Substrates toward Flexible Spintronics and Electronics

Integrating oxide thin films on flexible substrates is a critical step toward future applications of multifunctional oxides for flexible electronics and spintronic devices. As a demonstration, multifunctional La_0.67Sr_0.33MnO₃ (LSMO) thin films have been deposited on flexible mica substrates. The crystallinity and microstructure of the films have been characterized to show the good epitaxial quality of the films. The LSMO thin films on mica present excellent ferromagnetic and magnetoresistance properties (such as saturation magnetization M_s of 125-400 emu/cm³ at 10 K and a high MR value of ∼45% at 5 K under 1 T for the 50 mTorr deposited sample), which is even better than the ones on conventional rigid single-crystal oxide substrates. More interestingly, no deterioration of the properties is observed under mechanically bending condition, which demonstrates the good mechanical stretchability and property stability of the LSMO thin films on mica. The demonstration of functional oxides integrated on flexible mica substrates paves a route toward future flexible spintronics and electronics.

Enhanced Efficiency of Functional Smart Window with Solar Wavelength Conversion Phosphor-Photochromic Hybrid Film

Here, the high sensitivity and enhanced photochromic efficiency of the hybrid film of solar wavelength conversion phosphor and photochromic film for functional smart window have been achieved by fabricating a double-layered hybrid structure of wavelength conversion phosphor and photochromic film. Y₂SiO₅:Pr³⁺ phosphor nanoparticles, which upconvert visible light to UV light, were synthesized by simple hydrothermal method. The synthesized Y₂SiO₅:Pr³⁺ nanoparticle was coated as layered structured film on photochromic H₃PW₁₂O₄₀ film. The Y₂SiO₅:Pr³⁺/H₃PW₁₂O₄₀ hybrid film showed an enhanced sensitivity and efficiency of photochromic process with solar light irradiation due to the increased UV portion of solar light through upconversion process of visible light by wavelength conversion phosphor layer. The increased UV portion by upconversion process of Y₂SiO₅:Pr³⁺ layer through excited-state absorption and energy transfer upconversion process, contributed to an enhancement of coloration rate of photochromic H₃PW₁₂O₄₀ film by 7 times with 50 min of 1 sun irradiation due to fast conversion of W⁶⁺ state to W⁵⁺ state in H₃PW₁₂O₄₀ film with 5 times enhanced photochromic sensitivity compared with pristine photochromic film.

Heat-Shielding and Self-Cleaning Smart Windows: Near-Infrared Reflective Photonic Crystals with Self-Healing Omniphobicity via Layer-by-Layer Self-Assembly

Bioinspired photonic crystals that can be used to precisely control the optical reflection of light of a specific wavelength by varying their thickness and refractive index have attracted much attention. Among them, photonic crystals that can reflect near-infrared light have attracted attention owing to their potential applications including window coating with heat-shielding property. However, photonic crystals with an optical function in practical use sometimes lose their function because of contamination. Here, a near-infrared reflection coating film with self-healing omniphobicity was designed and prepared by layer-by-layer assembly and an instant liquid phase omniphobization method. The fabricated films had a self-cleaning thermal shielding effect. The films were visually transparent and could be used to control the reflection peak of the near-infrared light (range of 700-1000 nm) by adjusting the film thickness, which prevented the increase in temperature in enclosed spaces. After omniphobization, the films had self-cleaning properties of their surface and retained their optical properties. These functions are promising for practical application on windows as heat-shielding.

Ambiguous Role of Growth-Induced Defects on the Semiconductor-to-Metal Characteristics in Epitaxial VO2/TiO2 Thin Films

Controlling the semiconductor-to-metal transition temperature in epitaxial VO₂ thin films remains an unresolved question both at the fundamental as well as the application level. Within the scope of this work, the effects of growth temperature on the structure, chemical composition, interface coherency and electrical characteristics of rutile VO₂ epitaxial thin films grown on TiO₂ substrates are investigated. It is hereby deduced that the transition temperature is lower than the bulk value of 340 K. However, it is found to approach this value as a function of increased growth temperature even though it is accompanied by a contraction along the V⁴⁺-V⁴⁺ bond direction, the crystallographic c-axis lattice parameter. Additionally, it is demonstrated that films grown at low substrate temperatures exhibit a relaxed state and a strongly reduced transition temperature. It is suggested that, besides thermal and epitaxial strain, growth-induced defects may strongly affect the electronic phase transition. The results of this work reveal the difficulty in extracting the intrinsic material response to strain, when the exact contribution of all strain sources cannot be effectively determined. The findings also bear implications on the limitations in obtaining the recently predicted novel semi-Dirac point phase in VO₂/TiO₂ multilayer structures.

A high-response transparent heater based on a CuS nanosheet film with superior mechanical flexibility and chemical stability

Transparent heaters are widely used in technologies such as window defrosting/defogging, displays, gas sensing, and medical equipment. Apart from mechanical robustness and electrical and optical reliabilities, outstanding chemical stability is also critical to the application of transparent heaters. In this regard, we first present a highly flexible and large-area CuS transparent heater fabricated by a colloidal crackle pattern method with an optimized sheet resistance (Rs) as low as 21.5 Ω sq-1 at a ∼80% transmittance. The CuS transparent heater exhibits remarkable mechanical robustness during bending tests as well as high chemical stability against acid and alkali environments. In the application as a transparent heater, the CuS heater demonstrates a high thermal resistance of 197 °C W-1 cm2 with a fast switching time (<30 s), requiring low input voltages (<4.5 V) to achieve uniform temperatures of ∼110 °C across large areas. The temperature of the wearable CuS heater, which is stuck on the skin, can be real-time controlled through a Bluetooth device in a cell phone wirelessly. Based on the wireless control system, we demonstrated an application of the CuS heater in snow removal for solar panels. These CuS network TCEs with high flexibility, transparency, conductivity, and chemical stability could be widely used in wearable electronic products.

Non-catalytic hydrogenation of VO2 in acid solution

Hydrogenation is an effective way to tune the property of metal oxides. It can conventionally be performed by doping hydrogen into solid materials with noble-metal catalysis, high-temperature/pressure annealing treatment, or high-energy proton implantation in vacuum condition. Acid solution naturally provides a rich proton source, but it should cause corrosion rather than hydrogenation to metal oxides. Here we report a facile approach to hydrogenate monoclinic vanadium dioxide (VO₂) in acid solution at ambient condition by placing a small piece of low workfunction metal (Al, Cu, Ag, Zn, or Fe) on VO₂ surface. It is found that the attachment of a tiny metal particle (~1.0 mm) can lead to the complete hydrogenation of an entire wafer-size VO₂ (>2 inch). Moreover, with the right choice of the metal a two-step insulator–metal–insulator phase modulation can even be achieved. An electron–proton co-doping mechanism has been proposed and verified by the first-principles calculations.

Hydrogenation is an effective way to tune the property of metal oxides. Here, the authors report a simple approach to hydrogenate VO₂ in acid solution under ambient conditions by placing a small piece of low workfunction metal on VO₂ surface.

Dual-Band Modulation of Visible and Near-Infrared Light Transmittance in an All-Solution-Processed Hybrid Micro-Nano Composite Film

Smart windows with controllable visible and near-infrared light transmittance can significantly improve the building's energy efficiency and inhabitant comfort. However, most of the current smart window technology cannot achieve the target of ideal solar control. Herein, we present a novel all-solution-processed hybrid micronano composite smart material that have four optical states to separately modulate the visible and NIR light transmittance through voltage and temperature, respectively. This dual-band optical modulation was achieved by constructing a phase-separated polymer framework, which contains the microsized liquid crystals domains with a negative dielectric constant and tungsten-doped vanadium dioxide (W-VO₂) nanocrystals (NCs). The film with 2.5 wt % W-VO₂ NCs exhibits transparency at normal condition, and the passage of visible light can be reversibly and actively regulated between 60.8% and 1.3% by external applied voltage. Also, the transmittance of NIR light can be reversibly and passively modulated between 59.4% and 41.2% by temperature. Besides, the film also features easy all-solution processability, fast electro-optical (E-O) response time, high mechanical strength, and long-term stability. The as-prepared film provides new opportunities for next-generation smart window technology, and the proposed strategy is conductive to engineering novel hybrid inorganic-organic functional matters.

Highly Flexible, Multipixelated Thermosensitive Smart Windows Made of Tough Hydrogels

In a cold night, a clear window that will become opaque while retaining the indoor heat is highly desirable for both privacy and energy efficiency. A thermally responsive material that controls both the transmittance of solar radiance (predominantly in the visible and near-infrared wavelengths) and blackbody radiation (mainly in the mid-infrared) can realize such windows with minimal energy consumption. Here, we report a smart coating made from polyampholyte hydrogel (PAH) that transforms from a transparency state to opacity to visible radiation and strengthens opacity to mid-infrared when lowering the temperature as a result of phase separation between the water-rich and polymer-rich phases. To match a typical temperature fluctuation during the day, we fine-tune the phase transition temperature between 25 and 55 °C by introducing a small amount of relatively hydrophobic monomers (0.1 to 0.5 wt % to PAH). To further demonstrate an actively controlled, highly flexible, and high-contrast smart window, we build in an array of electric heaters made of printed elastomeric composite. The multipixelated window offers rapid switching, ∼70 s per cycle, whereas the device can withstand high strain (up to 80%) during operations.

Hydrothermal Synthesis of VO2 Polymorphs: Advantages, Challenges and Prospects for the Application of Energy Efficient Smart Windows

Vanadium dioxide (VO₂ ) is a widely studied inorganic phase change material, which has a reversible phase transition from semiconducting monoclinic to metallic rutile phase at a critical temperature of τ_c ≈ 68 °C. The abrupt decrease of infrared transmittance in the metallic phase makes VO₂ a potential candidate for thermochromic energy efficient windows to cut down building energy consumption. However, there are three long-standing issues that hindered its application in energy efficient windows: high τ_c , low luminous transmittance (T_lum ), and undesirable solar modulation ability (ΔT_sol ). Many approaches, including nano-thermochromism, porous films, biomimetic surface reconstruction, gridded structures, antireflective overcoatings, etc, have been proposed to tackle these issues. The first approach-nano-thermochromism-which is to integrate VO₂ nanoparticles in a transparent matrix, outperforms the rest; while the thermochromic performance is determined by particle size, stoichiometry, and crystallinity. A hydrothermal method is the most common method to fabricate high-quality VO₂ nanoparticles, and has its own advantages of large-scale synthesis and precise phase control of VO₂ . This Review focuses on hydrothermal synthesis, physical properties of VO₂ polymorphs, and their transformation to thermochromic VO₂ (M), and discusses the advantages, challenges, and prospects of VO₂ (M) in energy-efficient smart windows application.

Facile and Low-Temperature Fabrication of Thermochromic Cr2O3/VO2 Smart Coatings: Enhanced Solar Modulation Ability, High Luminous Transmittance and UV-Shielding Function

In the pursuit of energy efficient materials, vanadium dioxide (VO₂) based smart coatings have gained much attention in recent years. For smart window applications, VO₂ thin films should be fabricated at low temperature to reduce the cost in commercial fabrication and solve compatibility problems. Meanwhile, thermochromic performance with high luminous transmittance and solar modulation ability, as well as effective UV shielding function has become the most important developing strategy for ideal smart windows. In this work, facile Cr₂O₃/VO₂ bilayer coatings on quartz glasses were designed and fabricated by magnetron sputtering at low temperatures ranging from 250 to 350 °C as compared with typical high growth temperatures (>450 °C). The bottom Cr₂O₃ layer not only provides a structural template for the growth of VO₂ (R), but also serves as an antireflection layer for improving the luminous transmittance. It was found that the deposition of Cr₂O₃ layer resulted in a dramatic enhancement of the solar modulation ability (56.4%) and improvement of luminous transmittance (26.4%) when compared to single-layer VO₂ coating. According to optical measurements, the Cr₂O₃/VO₂ bilayer structure exhibits excellent optical performances with an enhanced solar modulation ability (ΔT_sol = 12.2%) and a high luminous transmittance (T_lum,lt = 46.0%), which makes a good balance between ΔT_sol and T_lum for smart windows applications. As for UV-shielding properties, more than 95.8% UV radiation (250-400 nm) can be blocked out by the Cr₂O₃/VO₂ structure. In addition, the visualized energy-efficient effect was modeled by heating a beaker of water using infrared imaging method with/without a Cr₂O₃/VO₂ coating glass.

The electro-optic mechanism and infrared switching dynamic of the hybrid multilayer VO2/Al:ZnO heterojunctions

Active and widely controllable phase transition optical materials have got rapid applications in energy-efficient electronic devices, field of meta-devices and so on. Here, we report the optical properties of the vanadium dioxide (VO₂)/aluminum-doped zinc oxide (Al:ZnO) hybrid n-n type heterojunctions and the corresponding electro-optic performances of the devices. Various structures are fabricated to compare the discrepancy of the optical and electrical characteristics. It was found that the reflectance spectra presents the wheel phenomenon rather than increases monotonically with temperature at near-infrared region range. The strong interference effects was found in the hybrid multilayer heterojunction. In addition, the phase transition temperature decreases with increasing the number of the Al:ZnO layer, which can be ascribed to the electron injection to the VO₂ film from the Al:ZnO interface. Affected by the double layer Al:ZnO, the abnormal Raman vibration mode was presented in the insulator region. By adding the external voltage on the Al₂O₃/Al:ZnO/VO₂/Al:ZnO, Al₂O₃/Al:ZnO/VO₂ and Al₂O₃/VO₂/Al:ZnO thin-film devices, the infrared optical spectra of the devices can be real-time manipulated by an external voltage. The main effect of joule heating and assistant effect of electric field are illustrated in this work. It is believed that the results will add a more thorough understanding in the application of the VO₂/transparent conductive film device.

Microscopic Evaluation of Electrical and Thermal Conduction in Random Metal Wire Networks

Ideally, transparent heaters exhibit uniform temperature, fast response time, high achievable temperatures, low operating voltage, stability across a range of temperatures, and high optical transmittance. For metal network heaters, unlike for uniform thin-film heaters, all of these parameters are directly or indirectly related to the network geometry. In the past, at equilibrium, the temperature distributions within metal networks have primarily been studied using either a physical temperature probe or direct infrared (IR) thermography, but there are limits to the spatial resolution of these cameras and probes, and thus, only average regional temperatures have typically been measured. However, knowledge of local temperatures within the network with a very high spatial resolution is required for ensuring a safe and stable operation. Here, we examine the thermal properties of random metal network thin-film heaters fabricated from crack templates using high-resolution IR microscopy. Importantly, the heaters achieve predominantly uniform temperatures throughout the substrate despite the random crack network structure (e.g., unequal sized polygons created by metal wires), but the temperatures of the wires in the network are observed to be significantly higher than the substrate because of the significant thermal contact resistance at the interface between the metal and the substrate. Last, the electrical breakdown mechanisms within the network are examined through transient IR imaging. In addition to experimental measurements of temperatures, an analytical model of the thermal properties of the network is developed in terms of geometrical parameters and material properties, providing insights into key design rules for such transparent heaters. Beyond this work, the methods and the understanding developed here extend to other network-based heaters and conducting films, including those that are not transparent.

Synthesis of graphene-transition metal oxide hybrid nanoparticles and their application in various fields

Single layer graphite, known as graphene, is an important material because of its unique two-dimensional structure, high conductivity, excellent electron mobility and high surface area. To explore the more prospective properties of graphene, graphene hybrids have been synthesised, where graphene has been integrated with other important nanoparticles (NPs). These graphene-NP hybrid structures are particularly interesting because after hybridisation they not only display the individual properties of graphene and the NPs, but also they exhibit further synergistic properties. Reduced graphene oxide (rGO), a graphene-like material, can be easily prepared by reduction of graphene oxide (GO) and therefore offers the possibility to fabricate a large variety of graphene-transition metal oxide (TMO) NP hybrids. These hybrid materials are promising alternatives to reduce the drawbacks of using only TMO NPs in various applications, such as anode materials in lithium ion batteries (LIBs), sensors, photocatalysts, removal of organic pollutants, etc. Recent studies have shown that a single graphene sheet (GS) has extraordinary electronic transport properties. One possible route to connecting those properties for application in electronics would be to prepare graphene-wrapped TMO NPs. In this critical review, we discuss the development of graphene-TMO hybrids with the detailed account of their synthesis. In addition, attention is given to the wide range of applications. This review covers the details of graphene-TMO hybrid materials and ends with a summary where an outlook on future perspectives to improve the properties of the hybrid materials in view of applications are outlined.

Low temperature fabrication of thermochromic VO2 thin films by low-pressure chemical vapor deposition

High-performance thermochromic VO₂ films were fabricated by LPCVD, and the mechanism for their low transition temperatures was studied using first-principles calculations.

Visibly Transparent Heaters

Heater plates or sheets that are visibly transparent have many interesting applications in optoelectronic devices such as displays, as well as in defrosting, defogging, gas sensing and point-of-care disposable devices. In recent years, there have been many advances in this area with the advent of next generation transparent conducting electrodes (TCE) based on a wide range of materials such as oxide nanoparticles, CNTs, graphene, metal nanowires, metal meshes and their hybrids. The challenge has been to obtain uniform and stable temperature distribution over large areas, fast heating and cooling rates at low enough input power yet not sacrificing the visible transmittance. This review provides topical coverage of this important research field paying due attention to all the issues mentioned above.

Switchable Materials for Smart Windows

This article reviews the basic principles of and recent developments in electrochromic, photochromic, and thermochromic materials for applications in smart windows. Compared with current static windows, smart windows can dynamically modulate the transmittance of solar irradiation based on weather conditions and personal preferences, thus simultaneously improving building energy efficiency and indoor human comfort. Although some smart windows are commercially available, their widespread implementation has not yet been realized. Recent advances in nanostructured materials provide new opportunities for next-generation smart window technology owing to their unique structure-property relations. Nanomaterials can provide enhanced coloration efficiency, faster switching kinetics, and longer lifetime. In addition, their compatibility with solution processing enables low-cost and high-throughput fabrication. This review also discusses the importance of dual-band modulation of visible and near-infrared (NIR) light, as nearly 50% of solar energy lies in the NIR region. Some latest results show that solution-processable nanostructured systems can selectively modulate the NIR light without affecting the visible transmittance, thus reducing energy consumption by air conditioning, heating, and artificial lighting.