Two-Dimensional SiO2/VO2 Photonic Crystals with Statically Visible and Dynamically Infrared Modulated for Smart Window Deployment

Micro/nanofabrication of heat management materials for energy-efficient building facades

Advanced building facades, which include windows, walls, and roofs, hold great promise for reducing building energy consumption. In recent decades, the management of heat transfer via electromagnetic radiation between buildings and outdoor environments has emerged as a critical research field aimed at regulating solar irradiation and thermal emission properties. Rapid advancements have led to the widespread utilization of advanced micro/nanofabrication techniques. This review provides the first comprehensive summary of fabrication methods for heat management materials with potential applications in energy-efficient building facades, with a particular emphasis on recent developments in fabrication processing and material property design. These methods include coating, vapor deposition, nanolithography, printing, etching, and electrospinning. Furthermore, we present our perspectives regarding their advantages and disadvantages and our opinions on the opportunities and challenges in this field. This review is expected to expedite future research by providing information on the selection, design, improvement, and development of relevant fabrication techniques for advanced materials with energy-efficient heat management capabilities.

Using Thin Films of Phase-Change Material for Active Tuning of Terahertz Waves Scattering on Dielectric Cylinders

The scattering of electromagnetic waves by isotropic dielectric cylinders can be dramatically modified by means of vanadium dioxide (VO2) thin-film coatings. Efficient dynamic control of scattering is achieved due to the variations in material parameters realizable by means of external biasing. In this paper, we study the scattering of terahertz waves in a case where the coating shells are made of VO2, a phase-change material, whose thin films may work rather as electromagnetic phase screens in the insulator material phase, but as lossy quasi-metallic components in the metallic material phase. The shells that uniformly cover the dielectric cylinders are investigated. Attention will be paid to the demonstration of the potential of VO2 in the external control of diverse scattering regimes of the dielectric-VO2 core-shell scatterer, while conductivity of VO2 corresponds to rather insignificant variations in temperature. In line with the purposes of this work, it is shown that the different resonant and nonresonant regimes have different sensitivity to the variations in VO2 conductivity. Both the total scattering cross section and field distributions inside and around the core are studied, as well as the angle-dependent scattering cross section.

Nanostructured Vanadium Dioxide Materials for Optical Sensing Applications

Vanadium dioxide (VO2) is one of the strongly correlated materials exhibiting a reversible insulator-metal phase transition accompanied by a structural transition from a low-temperature monoclinic phase to high-temperature rutile phase near room temperature. Due to the dramatic change in electrical resistance and optical transmittance of VO2, it has attracted considerable attention towards the electronic and optical device applications, such as switching devices, memory devices, memristors, smart windows, sensors, actuators, etc. The present review provides an overview of several methods for the synthesis of nanostructured VO2, such as solution-based chemical approaches (sol-gel process and hydrothermal synthesis) and gas or vapor phase synthesis techniques (pulsed laser deposition, sputtering method, and chemical vapor deposition). This review also presents stoichiometry, strain, and doping engineering as modulation strategies of physical properties for nanostructured VO2. In particular, this review describes ultraviolet-visible-near infrared photodetectors, optical switches, and color modulators as optical sensing applications associated with nanostructured VO2 materials. Finally, current research trends and perspectives are also discussed.

Self-Adhesive Self-Healing Thermochromic Ionogels for Smart Windows with Excellent Environmental and Mechanical Stability, Solar Modulation, and Antifogging Capabilities

Current thermochromic materials used in smart windows still face challenges, such as poor mechanical and environmental stability, unsatisfactory solar modulation capacity, and low transparency. Herein, the first self-adhesive self-healing thermochromic ionogels with excellent mechanical and environmental stability, antifogging capability, transparency, and solar modulation capability by loading binary ionic liquids (ILs) into rational-designed self-healing poly(urethaneurea) with acylsemicarbazide (ASCZ) moieties that have reversible and multiple hydrogen bonds are reported and their feasibility as smart windows with reliability and long service life is demonstrated. The self-healing thermochromic ionogels can switch between transparent and opaque without leakage or shrinkage, by the constrained reversible phase separation of ILs within the ionogels. The ionogels have the highest transparency and solar modulation capability among reported thermochromic materials and such excellent solar modulation capability can be well maintained after undergoing 1000 transitions, stretches, and bends, and storage at -30 °C, 60 °C, 90% RH, and vacuum environment for 2 months. The formation of high-density hydrogen bonds among the ASCZ moieties contributes to the excellent mechanical strength of the ionogels and allows the thermochromic ionogels to spontaneously heal their damages and be fully recycled at room temperature without the loss of thermochromic capabilities.

Vanadium Oxide: Phase Diagrams, Structures, Synthesis, and Applications

Vanadium oxides with multioxidation states and various crystalline structures offer unique electrical, optical, optoelectronic and magnetic properties, which could be manipulated for various applications. For the past 30 years, significant efforts have been made to study the fundamental science and explore the potential for vanadium oxide materials in ion batteries, water splitting, smart windows, supercapacitors, sensors, and so on. This review focuses on the most recent progress in synthesis methods and applications of some thermodynamically stable and metastable vanadium oxides, including but not limited to V₂O₃, V₃O₅, VO₂, V₃O₇, V₂O₅, V₂O₂, V₆O₁₃, and V₄O₉. We begin with a tutorial on the phase diagram of the V-O system. The second part is a detailed review covering the crystal structure, the synthesis protocols, and the applications of each vanadium oxide, especially in batteries, catalysts, smart windows, and supercapacitors. We conclude with a brief perspective on how material and device improvements can address current deficiencies. This comprehensive review could accelerate the development of novel vanadium oxide structures in related applications.

Self-evolving photonic crystals for ultrafast photonics

Ultrafast dynamics in nanophotonic materials is attracting increasing attention from the perspective of exploring new physics in fundamental science and expanding functionalities in various photonic devices. In general, such dynamics is induced by external stimuli such as optical pumping or voltage application, which becomes more difficult as the optical power to be controlled becomes larger owing to the increase in the energy required for the external control. Here, we demonstrate a concept of the self-evolving photonic crystal, where the spatial profile of the photonic band is dynamically changed through carrier-photon interactions only by injecting continuous uniform current. Based on this concept, we experimentally demonstrate short-pulse generation with a high peak power of 80 W and a pulse width of <30 ps in a 1-mm-diameter GaAs-based photonic crystal. Our findings on self-evolving carrier-photon dynamics will greatly expand the potential of nanophotonic materials and will open up various scientific and industrial applications.

Programming Thermochromic Liquid Crystal Hetero-Oligomers for Near-Infrared Reflectors: Unequal Incorporation of Similar Reactive Mesogens in Thiol-ene Oligomers

Cholesteric liquid crystal oligomers are widely researched for their interesting thermochromic properties. However, structure-property relationships to program the thermochromic properties of these oligomers have been rarely reported. In this work, we use the versatile thiol-ene click reaction to synthesize a series of hetero-oligomers and study the impact of different compositions on the thermochromic behavior of the resulting material. Characterization of the oligomers shows significantly different rates of reaction for the monomers despite their very similar structures, which leads to oligomer compositions that do not match the original reaction feed. The oligomers are then used to produce thin near-infrared reflecting coatings. The best-performing thermochromic reflector has a room-temperature reflection band that shifts a total of 510 nanometers upon heating to 120 °C. The shift is repeatable for up to 10 times with no appreciable degradation. The room temperature reflection of the coatings is shown to be tunable not only by adjusting the chiral dopant concentration but also by the ratio of the monomers. Finally, we show that the oligomers can be chemically modified by making their reactive end groups undergo a reaction with monothiol compounds. These modifications allow for further fine-tuning of liquid crystal oligomers for heat-regulating window films, for example.

Self-rolling of vanadium dioxide nanomembranes for enhanced multi-level solar modulation

Thermochromic window develops as a competitive solution for carbon emissions due to comprehensive advantages of its passivity and effective utilization of energy. How to further enhance the solar modulation ([Formula: see text]) of thermochromic windows while ensuring high luminous transmittance ([Formula: see text]) becomes the latest challenge to touch the limit of energy efficiency. Here, we show a smart window combining mechanochromism with thermochromism by self-rolling of vanadium dioxide (VO₂) nanomembranes to enhance multi-level solar modulation. The mechanochromism is introduced by the temperature-controlled regulation of curvature of rolled-up smart window, which benefits from effective strain adjustment in VO₂ nanomembranes upon the phase transition. Under geometry design and optimization, the rolled-up smart window with high [Formula: see text] and [Formula: see text] is achieved for the modulation of indoor temperature self-adapted to seasons and climate. Furthermore, such rolled-up smart window enables high infrared reflectance after triggered phase transition and acts as a smart lens protective cover for strong radiation. This work supports the feasibility of self-rolling technology in smart windows and lens protection, which promises broad interest and practical applications of self-adapting devices and systems for smart building, intelligent sensors and actuators with the perspective of energy efficiency.

Dual-Responsive Hydrogels with Three-Stage Optical Modulation for Smart Windows

Since room temperature management consumes a large amount of building energy, thermochromic smart windows have been extensively used for temperature regulation and energy management. However, the development of the smart window is still limited by its simple thermochromic performance, unreasonable thermochromic temperature, and the lack of additional stimulation conditions. In this work, a dual-responsive hydrogel was developed by introducing sodium dodecyl sulfate (SDS) and sodium chloride into the cross-linking network of poly(N-isopropylacrylamide) (PNIPAM) and polyacrylamide (PAM) for energy-saving and privacy protection. By controlling the temperature from low (<15 °C) to medium (15-28 °C) to high (>28 °C), the dual-responsive hydrogel achieved a reversible three-stage transition of opaque-transparent-translucent. The hydrogel exhibited a satisfactory solar modulation ability (T_lum = 80.3%, ΔT_sol,15-18°C = 72.9%, ΔT_sol,18-35°C = 42.7%) and effective IR and UV shielding at high (or low) temperatures. Moreover, compared with traditional windows, smart windows made of dual-responsive hydrogels could offer better thermal insulation and heat preservation. The electrochromic properties of the dual-responsive hydrogel presented a facile strategy to meet the needs of different situations. The dual-responsive hydrogel features energy-saving, privacy protection, three-stage optical modulation, and multistimulus responsiveness, making it an ideal smart window candidate.

Element doping: a marvelous strategy for pioneering the smart applications of VO2

Smart materials are leading the future of materials by virtue of their autonomous response behavior to external stimuli; it is widely believed their development and application will bring a new revolution. Among them, vanadium dioxide (VO₂) is a special one showing a unique multi-stimulus responsive metal-insulator transition (MIT) accompanied by a structural phase transition (SPT) with striking changes of physical properties including optical, electrical and thermal properties, etc., making it ideal for smart windows, micro-bolometers, actuators, etc. Since the attractive performances of VO₂ are rooted in MIT behavior (coupled with SPT), element doping becomes a powerful tool in tailoring VO₂ performance. Oriented on the practical requirements, element-doped VO₂ is more promising and competitive in terms of performance, prospect, and cost. Here we focus specifically on element-doped VO₂, the recent progress and potential challenges of which are discussed. We devote attention to the crucial roles of element doping in modulating the properties and driving the practicality of VO₂, aiming to inspire current research to pioneer new applications of VO₂.

Enabling Active Nanotechnologies by Phase Transition: From Electronics, Photonics to Thermotics

Phase transitions can occur in certain materials such as transition metal oxides (TMOs) and chalcogenides when there is a change in external conditions such as temperature and pressure. Along with phase transitions in these phase change materials (PCMs) come dramatic contrasts in various physical properties, which can be engineered to manipulate electrons, photons, polaritons, and phonons at the nanoscale, offering new opportunities for reconfigurable, active nanodevices. In this review, we particularly discuss phase-transition-enabled active nanotechnologies in nonvolatile electrical memory, tunable metamaterials, and metasurfaces for manipulation of both free-space photons and in-plane polaritons, and multifunctional emissivity control in the infrared (IR) spectrum. The fundamentals of PCMs are first introduced to explain the origins and principles of phase transitions. Thereafter, we discuss multiphysical nanodevices for electronic, photonic, and thermal management, attesting to the broad applications and exciting promises of PCMs. Emerging trends and valuable applications in all-optical neuromorphic devices, thermal data storage, and encryption are outlined in the end.

Energy Saving and Energy Generation Smart Window with Active Control and Antifreezing Functions

Windows are the least energy efficient part of the buildings, as building accounts for 40% of global energy consumption. Traditional smart windows can only regulate solar transmission, while all the solar energy on the window is wasted. Here, for the first time, the authors demonstrate an energy saving and energy generation integrated smart window (ESEG smart window) in a simple way by combining louver structure solar cell, thermotropic hydrogel, and indium tin oxides (ITO) glass. The ESEG smart window can achieve excellent optical properties with ≈90% luminous transmission and ≈54% solar modulation, which endows excellent energy saving performance. The outstanding photoelectric conversion efficiency (18.24%) of silicon solar cells with louver structure gives the smart window excellent energy generation ability, which is more than 100% higher than previously reported energy generation smart window. In addition, the solar cell can provide electricity to for ITO glass to turn the transmittance of hydrogel actively, as well as the effect of antifreezing. This work offers an insight into the design and preparation together with a disruptive strategy of easy fabrication, good uniformity, and scalability, which opens a new avenue to realize energy storage, energy saving, active control, and antifreezing integration in one device.

New focus of the cloud point/Krafft point of nonionic/cationic surfactants as thermochromic materials for smart windows

A nonionic poly(oxyethylene) monoalkyl ether (C12(EO)6) and a cationic hexadecylpyridinium bromide (HPB) were used to achieve warm/cool transparency transition.

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.

Kerr-nonlinearity-assisted NIR nonreciprocal absorption in a VO2-based core-shell composite integrated with 1D nonlinear multilayers

We theoretically investigate the nonreciprocal optical response of a one-dimensional multilayer possessing nonlinear (NL) Kerr dielectrics hybridized with a VO₂-based core-shell structure. As a consequence of parameter optimization, it is found that semiconductor-to-metallic reconfiguring of relatively thin VO₂ nanoinclusions with a core-shell radius ratio of 0.95 is accompanied by enhanced multispectral near-infrared absorption of the system for both forward and backward incidences of light. However, increasing intensity of the incident wave bends the resonant wavelengths due to the NL response of Kerr dielectrics. When the incident light is well set up for an appropriate non-resonant wavelength, the absorption contrast between two directions of incidence enhances in some ranges of intensities due to the NL Kerr effect. There is also the possibility of reaching S-shaped bistable absorption. These features make the modeled system suitable for designing near-infrared absorptive diodes or isolators.

Information-Providing Flexible and Transparent Smart Window Display

A smart window, which can easily adjust light transmittance, can provide barrier functions, such as improvement in energy efficiency, glare prevention, and privacy protection. However, a smart window that can selectively provide real-time information and display various colorful characters and images at a desired location has not been developed. In this study, a novel smart window capable of real-time information conversion is developed by advancing the light transmittance control of the existing smart windows. A transparent and flexible window display is fabricated by synthesizing poly(N-isopropylacrylamide) (pNIPAM)-N,N-methylenebisacrylamide-crosslinked hydrogels (NBcH) and near-infrared (NIR) absorption-heating films sandwiched between two plastic substrates. When the NIR laser irradiates the window display panel surface, the temperature rises rapidly, as the NIR absorption-heating film absorbs the NIR wavelength. The generated heat is transferred to pNIPAM in contact with the NIR absorption-heating film, and an image forms in real time. In addition, if the NIR laser and projector simultaneously irradiate the window display panel surface, various colorful images can be displayed. The smart window for real-time information provision proposed in this study acts like a glass curtain that can selectively make a desired location transparent or opaque by controlling the transmittance of light and acts as a display that can present various colorful characters and images in real time. Therefore, it is expected to be highly convenient for users.

Controllable Design of Bifunctional VO2 Coatings with Superhydrophobic and Thermochromic Performances

The structure and functions of natural organisms provide great inspirational sources for designing and manufacturing bionic coatings, which hold a distinguished scientific promise to tackle challenges facing humans. In this work, we report a facile and controllable approach to prepare various hexagonal periodic array VO₂ thin films by simply manipulating the speed of the dip-coating operation. The hexagonal cellular-structured VO₂ surface delivered the best thermochromic performance with a T_lum of 79.34% and a ΔT_sol of 9.87%. Impressively, superhydrophobic and thermochromic properties could be integrated into hexagonal semi-dome thin films (with a T_lum of 70.9%, a ΔT_sol of 9.3%, and a water contact angle of 150°) without any post-treatment by low-surface-energy chemicals, which hold considerable potential for application in multifunctional smart windows. Moreover, based on the Cassie-Baxter mode and finite-difference time-domain calculations, the dependence of the thermochromic and wettability performances on the VO₂ structure has been investigated in this study.

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.

Femtosecond Laser-Induced Vanadium Oxide Metamaterial Nanostructures and the Study of Optical Response by Experiments and Numerical Simulations

Surface patterning is a popular approach to produce photonic metasurfaces that are tunable when electro-optic, thermo-optic, or magneto-optic materials are used. Vanadium oxides (V_yO_x) are well-known phase change materials with many applications, especially when used as tunable metamaterial photonic structures. Particularly, VO₂ is a well-known thermochromic material for its near-room-temperature phase transition from the insulating to the metallic state. One-dimensional (1D) VO₂ nanograting structures are studied by numerical simulation, and the simulation results reveal that the VO₂ nanograting structures could enhance the luminous transmittance (T_lum) compared with a pristine flat VO₂ surface. It is worth mentioning that T_lum is also polarization-dependent, and both larger grating height and smaller grating periodicity give enhanced T_lum, particularly at TE polarization in both insulating (20 °C) and metallic (90 °C) states of VO₂. Femtosecond laser-patterned VO₂ films exhibiting nanograting structures with an average periodicity of ≈500-700 nm have been fabricated for the first time to enhance thermochromic properties. Using X-ray photoelectron spectroscopy, it is shown that at the optimum laser processing conditions, VO₂ dominates the film composition, while under extra processing, the existence of other vanadium oxide phases such as V₂O₃ and V₂O₅ increases. Such structures show enhanced transmittance in the near-infrared (NIR) region, with an improvement in NIR and solar modulation abilities (ΔT_NIR = 10.8%, ΔT_sol = 10.9%) compared with a flat VO₂ thin film (ΔT_NIR = 8%, ΔT_sol = 10.2%). The slight reduction in transmittance in the visible region is potentially due to the scattering caused by the imperfect nanograting structures. This new patterning approach helps understand the polarization-dependent optical response of VO₂ thin films and opens a new gateway for smart devices.

Bio-inspired TiO2 nano-cone antireflection layer for the optical performance improvement of VO2 thermochromic smart windows

Vanadium dioxide (VO₂) is a promising material for thermochromic glazing. However, VO₂ thermochromic smart windows suffer from several problems that prevent commercialization: low luminous transmittance (T_lum) and low solar modulation ability (ΔT_sol). The solution to these problems can be sought from nature where the evolution of various species has enabled them to survive. Investigations into the morphology of moths eyes has shown that their unique nanostructures provide an excellent antireflection optical layer that helps moths sharply capture the light in each wavelength from a wide angle. Inspired by this mechanism, a VO₂ thermochromic smart window coated with a TiO₂ antireflection layer with a novel nano-cone structure, is presented in this study to achieve high T_lum and ΔT_sol. Optimization for the key structure parameters is summarized based on the FDTD numerical simulations. The optimized structure exhibits a T_lum of 55.4% with ΔT_sol of 11.3%, an improvement of about 39% and 72% respectively compared to the VO₂ window without an antireflection layer. Furthermore, wide-angle antireflection and polarization independence are also demonstrated by this nano-cone coating. This work provides an alternative method to enhance the optical performance of VO₂ smart windows.

Plasmonic Nanoparticle Film for Low-Power NIR-Enhanced Photocatalytic Reaction

Plasmonic metal nanostructures offer the unique ability to effectively enhance sunlight harvesting by localized surface plasmon resonance (LSPR), which can induce direct photocatalytic reactions. However, only metal nanoparticles with a relatively low magnitude of electromagnetic field enhancement usually require a high illumination intensity to ensure the catalytic performance, which greatly limits the solar photocatalytic efficiency. Herein, we designed plasmonic Au nanoparticle film with high electromagnetic field enhancement to achieve high-efficiency catalytic activity under low-power NIR light illumination. This work minimized the influence of the photothermal effect on the reaction by using a low illumination intensity and further revealed the main contribution of plasmon-excited hot electrons to the photochemical reaction. This study provides important insights into the study of the mechanism of LSPR in photocatalytic reactions and further improves the efficiency of solar energy utilization.

A Universal Approach To Achieve High Luminous Transmittance and Solar Modulating Ability Simultaneously for Vanadium Dioxide Smart Coatings via Double-Sided Localized Surface Plasmon Resonances

Vanadium dioxide (VO₂)-based thermochromic coatings has attracted considerable attention in the application of smart windows as a result of their intriguing property of metal-insulator transition at moderate temperatures. However, the practical requirements of smart windows, i.e., the high luminous transmittance of T_lum > 60% and large solar modulating ability of ΔT_sol > 10%, are competing to a large extent and hardly satisfied simultaneously. Here, we proposed a facile and universal method to prepare VO₂ coatings for exceeding the criteria above using double-sided localized surface plasmon resonances (LSPRs), which are excited by the VO₂ nanoparticles dispersed evenly on both surfaces of the fused silica substrate. With subtle engineering of the sol-gel and heat treatment processes, the morphology of as-prepared VO₂ nanoparticles and corresponding LSPRs are controlled to achieve a high luminous transmittance (T_lum = 68.2%) and solar modulating ability (ΔT_sol = 11.7%) simultaneously. Further simulation suggests that the double-sided LSPRs can collectively enhance the performance of VO₂ smart coatings. Moreover, the double-sided VO₂ nanoparticle coatings demonstrate stable performance with no more than 1% degradation of T_lum and ΔT_sol after 1500 cycles. This study provides an alternative strategy to obtain high-quality VO₂ (M) solar modulating coatings.

Self-Assembled VO2 Mesh Film-Based Resistance Switches with High Transparency and Abrupt ON/OFF Ratio

Vanadium dioxide, a well-known phase transition material with abrupt resistance change during its transition temperature, is herein used to fabricate the transparent mesh film onto a glass slide through self-assembly mesh printing. A record high ON/OFF ratio up to 10⁴ is achieved together with high visible transmittance of 86% compared to the normal glass slide with visible transmittance at 88%. The high transparent properties make the resistive switches applicable for next-generation electronics, such as see-through computing device and beyond. A simple and scalable mesh printing approach-integrated phase change material may provide a promising way to fabricate transparent resistance switches for next-generation electronics.

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.

Tunable filter and modulator with controlled bandwidth and wide dynamic range based on planar thin films structure

Tunable narrowband spectral filters with high throughput and wide dynamic range are in high demand for many applications, however, a cost is usually associated with the filter narrowing either in the dynamic range, in the throughput or the manufacturability. Here it is shown for the first time that using the coupling between waveguide and lossy modes (LMs) in lossy ultrathin films through thin film coupling layer it is possible to obtain a reflection peak with controllable width (sub-Angstroms till tens of nm) and tunability over wide spectral range (>500nm in the visible and near infrared). The excitation of broadband LM is enabled using an ultrathin absorptive layer with high imaginary to real part ratio of the dielectric constant (i.e 6nm of Cr). The wider dynamic range and higher contrast are observed more with TE polarization than TM. The tuning is achieved by incidence angle scan of few degrees or by modulating the waveguide layer from the visible till the near infrared and in principle it can be designed to operate in any spectral range. Such a thin waveguide layer can allow tuning at ultrahigh speed using conventional electrooptic, magnetooptic, piezoelectric or thermooptic materials using relatively low external fields. The tuning sensitivity and range depend strongly on the waveguide layer thickness and the refractive index mismatch between the waveguide and the coupling layer. Under small index mismatch new peaks are seen via Rabi type splitting with gaps as high as 700nm or more, thus exhibiting ultrahigh tuning with negative sensitivity.

Bifunctional Template-Induced VO2@SiO2 Dual-Shelled Hollow Nanosphere-Based Coatings for Smart Windows

Thermochromic vanadium dioxide (VO₂) as one of the most promising candidates for smart windows has attracted widespread attention in recent years. Excellent optical performances (luminous transmittance, T_lum, and solar modulation efficiency, Δ T_sol) of VO₂-based coatings are usually pursued as crucial issues. In the current work, we report an ingenious approach for the synthesis of VO₂@SiO₂ dual-shell hollow nanospheres (DSHNs) and the preparation of DSHNs thermochromic coatings. A sequential bifunctional template-induced mechanism for the formation of DSHNs was proposed. Because of the unique hollow-core and dual-shell structure, the as-prepared VO₂@SiO₂ DSHNs coatings exhibited appealing optical performances with enhanced luminous transmittance of 61.8% and solar modulation efficiency of 12.6%, compared with continuous and dense VO₂ coatings. It has been proved that the improvement of visible transmittance could be ascribed to the effective reduction of refractive index (from 2.6 to 1.6 at 630 nm). In addition, its excellent thermochromic performance has been confirmed by the model cubes measurements, expressing a great potential as energy-efficient smart windows in high-rise buildings. The bifunctional template-induced synthetic strategy may inspire more facile, efficient and inexpensive processes for development of well-defined multishelled hollow nanostructures for varied applications.

Tunable THz generalized Weyl points

Weyl points, as linearly double degenerated point of band structures, have been extensively researched in electronic and classical wave systems. However, Weyl points' realization is always accompanied with delicate "lattice structures". In this work, frequency-tunable terahertz (THz) generalized Weyl points inside the parameter space have been investigated and displayed by a specially designed photonic crystal with polydimethylsiloxane (PDMS) immersed in 4-cyano'-pentylbipenyl (5CB) liquid crystals (LCs). The reflective phase vortices as a signature of the generalized Weyl points are observed through our numerically simulations. Besides, interface states between photonic crystals and any reflective substrates are fulfilled too. Meanwhile, we could also change the orientation of LC molecule by the external magnetic field so as to tune the frequency of the first two bands' Weyl point from 0.27698THz to 0.30013THz. This band lies in the short-range wireless communication. Thus, our proposal may be beneficial to the investigation and application of Weyl points' properties and strongly localized states.

Supramolecular strategy for smart windows

Supramolecular strategy-based materials are outlined and their applications for fabricating smart windows are summarized for future exploration of ideal smart windows.

Highly Sensitive, Temperature-Independent Oxygen Gas Sensor Based on Anatase TiO2 Nanoparticle Grafted, 2D Mixed Valent VO x Nanoflakelets

Herein, we report a facile approach for the synthesis of TiO₂ nanoparticles tethered on 2D mixed valent vanadium oxide (VO _x/TiO₂) nanoflakelets using a thermal decomposition assisted hydrothermal method and investigation of its temperature-independent performance enhancement in oxygen-sensing properties. The material was structurally characterized using XRD, TEM, Raman, DSC, and XPS analysis. The presence of mixed valent states, such as V₂O₅ and VO₂ in VO _x, and the metastable properties of VO₂ have been found to play crucial roles in the temperature-independent electrical conductivity of VO _x/TiO₂ nanoflakelets. Though pristine VO _x exhibited characteristic semiconductor-to-metal transition of monoclinic VO₂, pure VO _x nanoflakelets exhibited poor sensitivity toward sensing oxygen. VO _x/TiO₂ nanoflakelets showed a very low temperature coefficient of resistance above 150 °C with improved sensitivity (35 times higher than VO _x for 100 ppm) toward oxygen gas. VO _x/TiO₂ nanoflakelets exhibited much higher response, faster adsorption and desorption toward oxygen as compared to pristine VO _x beyond 100 °C, which endowed the sensor with excellent temperature-independent sensor properties within 150-500 °C. The faster adsorption and desorption after 100 °C led to shorter response time (3-5 s) and recovery time (7-9 s). The results suggest that 2D VO _x/TiO₂ can be a promising candidate for temperature-independent oxygen sensor applications.

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.

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

Index-tunable anti-reflection SiO₂ coatings prepared on the surface of VO₂ films by sol–gel dip-coating technique to enhance the visible and infrared transmittance of SiO₂/VO₂ films.

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.

Controllable Fabrication of Two-Dimensional Patterned VO2 Nanoparticle, Nanodome, and Nanonet Arrays with Tunable Temperature-Dependent Localized Surface Plasmon Resonance

A universal approach to develop various two-dimensional ordered nanostructures, namely nanoparticle, nanonet and nanodome arrays with controllable periodicity, ranging from 100 nm to 1 μm, has been developed in centimeter-scale by nanosphere lithography technique. Hexagonally patterned vanadium dioxide (VO₂) nanoparticle array with average diameter down to sub-100 nm as well as 160 nm of periodicity is fabricated, exhibiting distinct size-, media-, and temperature-dependent localized surface plasmon resonance switching behaviors, which fits well with the predication of simulations. We specifically explore their decent thermochromic performance in an energy saving smart window and develop a proof-of-concept demo which proves the effectiveness of patterned VO₂ film to serve as a smart thermal radiation control. This versatile and facile approach to fabricate various ordered nanostructures integrated with attractive phase change characteristics of VO₂ may inspire the study of temperature-dependent physical responses and the development of smart devices in extensive areas.

Facile Phase Control of Multivalent Vanadium Oxide Thin Films (V2O5 and VO2) by Atomic Layer Deposition and Postdeposition Annealing

Atomic layer deposition was adopted to deposit VO_x thin films using vanadyl tri-isopropoxide {VO[O(C₃H₇)]₃, VTIP} and water (H₂O) at 135 °C. The self-limiting and purge-time-dependent growth behaviors were studied by ex situ ellipsometry to determine the saturated growth conditions for atomic-layer-deposited VO_x. The as-deposited films were found to be amorphous. The structural, chemical, and optical properties of the crystalline thin films with controlled phase formation were investigated after postdeposition annealing at various atmospheres and temperatures. Reducing and oxidizing atmospheres enabled the formation of pure VO₂ and V₂O₅ phases, respectively. The possible band structures of the crystalline VO₂ and V₂O₅ thin films were established. Furthermore, an electrochemical response and a voltage-induced insulator-to-metal transition in the vertical metal-vanadium oxide-metal device structure were observed for V₂O₅ and VO₂ films, respectively.

Synthesis and Thermochromic Properties of Cr-Doped Al₂O₃ for a Reversible Thermochromic Sensor

An inorganic thermochromic material based on Cr-doped Al₂O₃ is synthesized using a solid-state method. The crystal structure, chemical composition, and morphology of the synthesized material are analyzed using X-ray diffraction, scanning electron microscopy coupled with an energy-dispersive X-ray spectrometer, and Fourier transform infrared (FT-IR) spectroscopy. The color performances of the synthesized material are analyzed using a UV-VIS spectrometer. Finally, the thermochromism exhibited by the powdered samples at high temperatures is investigated. The material exhibits exceptional thermochromic property, transitioning from pink to gray or green in a temperature range of 25–600 °C. The change in color is reversible and is dependent on the surrounding temperature and chromium concentration; however, it is independent of the exposure time. This novel property of Cr-doped Al₂O₃ can be potentially employed in reversible thermochromic sensors that could be used not only for warning users of damage due to overheating when the environmental temperature exceeds certain limits, but also for detecting and monitoring the temperature of various devices, such as aeronautical engine components, hotplates, and furnaces.

Hybrid films of VO2 nanoparticles and a nickel(ii)-based ligand exchange thermochromic system: excellent optical performance with a temperature responsive colour change

This work reports a VO₂/NLETS hybrid film with a 127% increase in ΔT_sol and an evident temperature-responsive colour change.