VO2 /TiN Plasmonic Thermochromic Smart Coatings for Room-Temperature Applications

Unveiling the Cation Effects on Electrocatalytic CO2 Reduction via Operando Surface-enhanced Raman Spectroscopy

The electrocatalytic carbon dioxide reduction reaction (CO2RR) can be significantly improved by the presence of alkali metal cations, yet the underlying mechanisms remain unclear. In this study, we developed clean Cu nanoparticles with tailored curvatures to modulate the local concentration of K+ cations and investigate their effects on CO2RR. The adjustment of particle curvature allows for direct control over cation concentrations within the electrochemical double layer, enabling broad-range modulation of cation concentration without concerns regarding solubility limitations or anionic interference. By tuning the plasmonic modes of the Cu particles, we achieved highly sensitive surface-enhanced Raman spectroscopy (SERS) under resonant conditions, facilitating in situ tracking of the short-lived intermediates in CO2RR. Our results revealed that K+ cations not only stabilize *COOH and *CO species and reduce the reaction energy barrier for C─C coupling but also increase the surface coverage of *CO, particularly for bridge *CO configurations. Furthermore, our findings suggest that the interactions between bridge *CO and atop *CO play a crucial role in facilitating the C─C coupling, offering insights for the design of electrocatalysts for CO2RR.

Optical and Plasmonic Properties of High-Electron-Density Epitaxial and Oxidative Controlled Titanium Nitride Thin Films

The present paper reports on the fabrication, detailed structural characterizations, and theoretical modeling of titanium nitride (TiN) and its isostructural oxide derivative, titanium oxynitride (TiNO) thin films that have excellent plasmonic properties and that also have the potential to overcome the limitation of noble metal and refractory metals. The TiNO films deposited at 700 °C in high vacuum conditions have the highest reflectance (R = ∼ 95%), largest negative dielectric constant (ε1 = -161), and maximal plasmonic figure of merit (FoM = -ε1/ε2) of 1.2, followed by the 600 °C samples deposited in a vacuum (R = ∼ 85%, ε1 = -145, FoM = 0.8) and 700 °C-5 mTorr sample (R = ∼ 82%, ε1 = -8, FoM = 0.3). To corroborate our experimental observations, we calculated the phonon dispersions and Raman active modes of TiNO by using the virtual crystal approximation. From the experimental and theoretical studies, a multilayer optical model has been proposed for the TiN/TiNO epitaxial thin films for obtaining individual complex dielectric functions from which many other optical parameters can be calculated. The advantages of oxide derivatives of TiN are the continuation of similar free electron density as in TiN and the acquisition of additional features such as oxygen-dependent semiconductivity with a tunable bandgap.

Phase-change VO2-based thermochromic smart windows

Thermochromic coatings hold promise in reducing building energy consumption by dynamically regulating the heat gain of windows, which are often regarded as less energy-efficient components, across different seasons. Vanadium dioxide (VO2) stands out as a versatile thermochromic material for smart windows owing to its reversible metal-to-insulator transition (MIT) alongside correlated structural and optical properties. In this review, we delve into recent advancements in the phase-change VO2-based thermochromic coatings for smart windows, spanning from the macroscopic crystal level to the microscopic structural level (including elemental doping and micro/nano-engineering), as well as advances in controllable fabrication. It is notable that hybridizing functional elements/materials (e.g., W, Mo/SiO2, TiN) with VO2 in delicate structural designs (e.g., core-shell, optical cavity) brings new degrees of freedom for controlling the thermochromic properties, including the MIT temperature, luminous transmittance, solar-energy modulation ability and building-relevant multi-functionality. Additionally, we provide an overview of alternative chromogenic materials that could potentially complement or surpass the intrinsic limitations of VO2. By examining the landscape of emerging materials, we aim to broaden the scope of possibilities for smart window technologies. We also offer insights into the current challenges and prospects of VO2-based thermochromic smart windows, presenting a roadmap for advancing this field towards enhanced energy efficiency and sustainable building design. In summary, this review innovatively categorizes doping strategies and corresponding effects of VO2, underscores their crucial NIR-energy modulation ability for smart windows, pioneers a theoretical analysis of inverse core-shell structures, prioritizes practical engineering strategies for solar modulation in VO2 films, and summarizes complementary chromogenic materials, thus ultimately advancing VO2-based smart window technologies with a fresh perspective.

Plasmonic trimers designed as SERS-active chemical traps for subtyping of lung tumors

Plasmonic materials can generate strong electromagnetic fields to boost the Raman scattering of surrounding molecules, known as surface-enhanced Raman scattering. However, these electromagnetic fields are heterogeneous, with only molecules located at the 'hotspots', which account for ≈ 1% of the surface area, experiencing efficient enhancement. Herein, we propose patterned plasmonic trimers, consisting of a pair of plasmonic dimers at the bilateral sides and a trap particle positioned in between, to address this challenge. The trimer configuration selectively directs probe molecules to the central traps where 'hotspots' are located through chemical affinity, ensuring a precise spatial overlap between the probes and the location of maximum field enhancement. We investigate the Raman enhancement of the Au@Al2O3-Au-Au@Al2O3 trimers, achieving a detection limit of 10-14 M of 4-methylbenzenethiol, 4-mercaptopyridine, and 4-aminothiophenol. Moreover, single-molecule SERS sensitivity is demonstrated by a bi-analyte method. Benefiting from this sensitivity, our approach is employed for the early detection of lung tumors using fresh tissues. Our findings suggest that this approach is sensitive to adenocarcinoma but not to squamous carcinoma or benign cases, offering insights into the differentiation between lung tumor subtypes.

Exploring and Engineering 2D Transition Metal Dichalcogenides toward Ultimate SERS Performance

Surface-enhanced Raman spectroscopy (SERS) is an ultrasensitive surface analysis technique that is widely used in chemical sensing, bioanalysis, and environmental monitoring. The design of the SERS substrates is crucial for obtaining high-quality SERS signals. Recently, 2D transition metal dichalcogenides (2D TMDs) have emerged as high-performance SERS substrates due to their superior stability, ease of fabrication, biocompatibility, controllable doping, and tunable bandgaps and excitons. In this review, a systematic overview of the latest advancements in 2D TMDs SERS substrates is provided. This review comprehensively summarizes the candidate 2D TMDs SERS materials, elucidates their working principles for SERS, explores the strategies to optimize their SERS performance, and highlights their practical applications. Particularly delved into are the material engineering strategies, including defect engineering, alloy engineering, thickness engineering, and heterojunction engineering. Additionally, the challenges and future prospects associated with the development of 2D TMDs SERS substrates are discussed, outlining potential directions that may lead to significant breakthroughs in practical applications.

Tuning the plasmonic resonance in TiN refractory metal

Plasmonic coatings can absorb electromagnetic radiation from visible to far-infrared spectrum for the better performance of solar panels and energy saving smart windows. For these applications, it is important for these coatings to be as thin as possible and grown at lower temperatures on arbitrary substrates like glass, silicon, or flexible polymers. Here, we tune and investigate the plasmonic resonance of titanium nitride thin films in lower thicknesses regime varying from ~ 20 to 60 nm. High-quality crystalline thin films of route-mean-square roughness less than ~ 0.5 nm were grown on a glass substrate at temperature of ~ 200 °C with bias voltage of - 60 V using cathodic vacuum arc deposition. A local surface-enhanced-plasmonic-resonance was observed between 400 and 500 nm, which further shows a blueshift in plasmonic frequency in thicker films due to the increase in the carrier mobility. These results were combined with finite-difference-time-domain numerical analysis to understand the role of thicknesses and stoichiometry on the broadening of electromagnetic absorption.

Tunable VO2 cavity enables multispectral manipulation from visible to microwave frequencies

Optical materials capable of dynamically manipulating electromagnetic waves are an emerging field in memories, optical modulators, and thermal management. Recently, their multispectral design preliminarily attracts much attention, aiming to enhance their efficiency and integration of functionalities. However, the multispectral manipulation based on these materials is challenging due to their ubiquitous wavelength dependence restricting their capacity to narrow wavelengths. In this article, we cascade multiple tunable optical cavities with selective-transparent layers, enabling a universal approach to overcoming wavelength dependence and establishing a multispectral platform with highly integrated functions. Based on it, we demonstrate the multispectral (ranging from 400 nm to 3 cm), fast response speed (0.9 s), and reversible manipulation based on a typical phase change material, vanadium dioxide. Our platform involves tandem VO2-based Fabry-Pérot (F-P) cavities enabling the customization of optical responses at target bands independently. It can achieve broadband color-changing capacity in the visible region (a shift of ~60 nm in resonant wavelength) and is capable of freely switching between three typical optical models (transmittance, reflectance, and absorptance) in the infrared to microwave regions with drastic amplitude tunability exceeding 0.7. This work represents a state-of-art advance in multispectral optics and material science, providing a critical approach for expanding the multispectral manipulation ability of optical systems.

Probing Oxidation Mechanisms in Plasmonic Catalysis: Unraveling the Role of Reactive Oxygen Species

Plasmon-induced oxidation has conventionally been attributed to the transfer of plasmonic hot holes. However, this theoretical framework encounters challenges in elucidating the latest experimental findings, such as enhanced catalytic efficiency under uncoupled irradiation conditions and superior oxidizability of silver nanoparticles. Herein, we employ liquid surface-enhanced Raman spectroscopy (SERS) as a real-time and in situ tool to explore the oxidation mechanisms in plasmonic catalysis, taking the decarboxylation of p-mercaptobenzoic acid (PMBA) as a case study. Our findings suggest that the plasmon-induced oxidation is driven by reactive oxygen species (ROS) rather than hot holes, holding true for both the Au and Ag nanoparticles. Subsequent investigations suggest that plasmon-induced ROS may arise from hot carriers or energy transfer mechanisms, exhibiting selectivity under different experimental conditions. The observations were substantiated by investigating the cleavage of the carbon-boron bonds. Furthermore, the underlying mechanisms were clarified by energy level theories, advancing our understanding of plasmonic catalysis.

Stimuli-fluorochromic smart organic materials

Smart materials based on stimuli-fluorochromic π-conjugated solids (SFCSs) have aroused significant interest due to their versatile and exciting properties, leading to advanced applications. In this review, we highlight the recent developments in SFCS-based smart materials, expanding beyond organometallic compounds and light-responsive organic luminescent materials, with a discussion on the design strategies, exciting properties and stimuli-fluorochromic mechanisms along with their potential applications in the exciting fields of encryption, sensors, data storage, display, green printing, etc. The review comprehensively covers single-component and multi-component SFCSs as well as their stimuli-fluorochromic behaviors under external stimuli. We also provide insights into current achievements, limitations, and major challenges as well as future opportunities, aiming to inspire further investigation in this field in the near future. We expect this review to inspire more innovative research on SFCSs and their advanced applications so as to promote further development of smart materials and devices.

Colloidal Plasmonic TiN Nanoparticles for Efficient Solar Seawater Desalination

Transferring traditional plasmonic noble metal nanomaterials from the laboratory to industrial production has remained challenging due to the high price of noble metals. The development of cost-effective non-noble-metal alternatives with outstanding plasmonic properties has therefore become essential. Herein, we report on the gram-scale production of differently shaped TiN nanoparticles with strong plasmon-enabled broadband light absorption, including differently sized TiN nanospheres, nanobipyramids, and nanorod arrays. The TiN nanospheres and nanobipyramids are further coembedded in highly porous poly(vinyl alcohol) films to function as a photothermal material for solar seawater desalination. A seawater evaporation rate of 3.8 kg m-2 h-1 is achieved, which marks the record performance among all plasmonic solar seawater desalination systems reported so far. The removal percentage of phenol reaches 98.3%, which is attributed to the joint action of the excellent photocatalytic ability and the superhydrophilicity of the porous TiN-based composite film.

Spinodal decomposition introduces strain-enhanced thermochromism in polycrystalline V1-xTixO2 thin films

Processes of self-organization play a key role in the development of innovative functional nanocomposites, allowing, in particular, the transformation of metastable solid solutions into multilayers by activating spinodal decomposition instead of layer-by-layer film growth. We report the formation of strained layered (V,Ti)O₂ nanocomposites in thin polycrystalline films using a spinodal decomposition. Already during the growth of V_0.65Ti_0.35O₂ films, spinodal decomposition was detected while producing atomic-scale disordered V- and Ti-rich phases. Post-growth annealing enhances compositional modulation, arranges the local atomic structures of the phases, and yields periodically layered nanostructures that resemble superlattices. The coherent interfacing of the V- and Ti-rich layers results in the compression of the V-rich phase along the c-axis of the rutile structure and enables strain-enhanced thermochromism. The latter is characterized by a simultaneous decrease in the temperature and width of the metal-insulator transition in the V-rich phase. Our results provide proof-of-concept for an alternative strategy to develop VO₂-based thermochromic coatings by introducing strain-enhanced thermochromism into polycrystalline thin films.

Nanowire-based smart windows combining electro- and thermochromics for dynamic regulation of solar radiation

Smart window is an attractive option for efficient heat management to minimize energy consumption and improve indoor living comfort owing to their optical properties of adjusting sunlight. To effectively improve the sunlight modulation and heat management capability of smart windows, here, we propose a co-assembly strategy to fabricate the electrochromic and thermochromic smart windows with tunable components and ordered structures for the dynamic regulation of solar radiation. Firstly, to enhance both illumination and cooling efficiency in electrochromic windows, the aspect ratio and mixed type of Au nanorods are tuned to selectively absorb the near-infrared wavelength range of 760 to 1360 nm. Furthermore, when assembled with electrochromic W₁₈O₄₉ nanowires in the colored state, the Au nanorods exhibit a synergistic effect, resulting in a 90% reduction of near-infrared light and a corresponding 5 °C cooling effect under 1-sun irradiation. Secondly, to extend the fixed response temperature value to a wider range of 30-50 °C in thermochromic windows, the doping amount and mixed type of W-VO₂ nanowires are carefully regulated. Last but not the least, the ordered assembly structure of the nanowires can greatly reduce the level of haze and enhance visibility in the windows.

Multi-Stimuli Dually-Responsive Intelligent Woven Structures with Local Programmability for Biomimetic Applications

This work focuses on multi-stimuli-responsive materials with distinctive abilities, that is, color-changing and shape-memory. Using metallic composite yarns and polymeric/thermochromic microcapsule composite fibers, processed via a melt-spinning technique, an electrothermally multi-responsive fabric is woven. The resulting smart-fabric transfers from a predefined structure to an original shape while changing color upon heating or applying an electric field, making it appealing for advanced applications. The shape-memory and color-changing features of the fabric can be controlled by rationally controlling the micro-scale design of the individual fibers in the structure. Thus, the fibers' microstructural features are optimized to achieve excellent color-changing behavior along with shape fixity and recovery ratios of 99.95% and 79.2%, respectively. More importantly, the fabric's dual-response by electric field can be achieved by a low voltage of 5 V, which is smaller than the previously reported values. Above and beyond, the fabric is able to be meticulously activated by selectively applying a controlled voltage to any part of the fabric. The precise local responsiveness can be bestowed upon the fabric by readily controlling its macro-scale design. A biomimetic dragonfly with the shape-memory and color-changing dual-response ability is successfully fabricated, broadening the design and fabrication horizon of groundbreaking smart materials with multiple functions.

Emerging Materials and Strategies for Passive Daytime Radiative Cooling

In recent decades, the growing demands for energy saving and accompanying heat mitigation concerns, together with the vital goal for carbon neutrality, have drawn human attention to the zero-energy-consumption cooling technique. Recent breakthroughs in passive daytime radiative cooling (PDRC) might be a potent approach to combat the energy crisis and environmental challenges by directly dissipating ambient heat from the Earth to the cold outer space instead of only moving the heat across the Earth's surface. Despite significant progress in cooling mechanisms, materials design, and application exploration, PDRC faces potential functionalization, durability, and commercialization challenges. Herein, emerging materials and rational strategies for PDRC devices are reviewed. First, the fundamental physics and thermodynamic concepts of PDRC are examined, followed by a discussion on several categories of PDRC devices developed to date according to their implementation mechanism and material properties. Emerging strategies for performance enhancement and specific functions of PDRC are discussed in detail. Potential applications and possible directions for designing next-generation high-efficiency PDRC are also discussed. It is hoped that this review will contribute to exciting advances in PDRC and aid its potential applications in various fields.

Flexible plasmonic nanocavities: a universal platform for the identification of molecular orientations

The molecular orientation provides fundamental images to understand molecular behaviors in chemistry. Herein, we propose and demonstrate sandwich plasmonic nanocavities as a surface-selection ruler to illustrate the molecular orientations by surface-enhanced Raman spectroscopy (SERS). The field vector in the plasmonic nanocavity presents a transverse spinning feature under specific excitations, allowing the facile modulation of the field polarizations to selectively amplify the Raman modes of the target molecules. It does not require the knowledge of the Raman spectrum of a bare molecule as a standard and thus can be extended as a universal ruler for the identification of molecular orientations. We investigated the most widely used Raman probe, Rhodamine 6G (R6G) on the Au surface and tried to clarify the arguments about its orientations from our perspectives. The experimental results suggest concentration-dependent adsorption configurations of R6G: it adsorbs on Au primarily via an ethylamine group with the xanthene ring lying flatly on the metal surface at low concentrations, and the molecular orientation gradually changes from "flat" to "upright" with the increase of molecular concentrations.

Phase-Changing Polymer Film for Smart Windows with Highly Adaptive Solar Modulation

Adjusting sunlight and thermal radiation from windows is important in efficient energy-saving applications. A high solar modulation (ΔT_sol) capability as well as a high luminous transmittance (T_lum) are the ultimate aim of smart windows. In this study, hydroxypropyl cellulose (HPC), W-doped VO₂, and poly-N-iso-propylacrylamide (PNIPAm) composite hydrogel films were produced. The sample was prepared between two glasses to constitute a smart window with a sandwich construction structure, which exhibits a high T_lum of 87.16%, a ΔT_sol of 65.71%, and a lower critical solution temperature (LCST) of 29 °C. The practical applications of conventional PNIPAm hydrogels are limited by the volume contraction of phase transition. Here, this challenge is addressed by the simple method of combining with HPC. The PNIPAm-2.5 wt % HPC hydrogels possess thermo-responsive contractility with a volume shrinkage rate of 8.5%. Even after 100 high- and low-temperature cyclic durability tests, the smart windows still exhibit a high solar modulation capability.

Investigation of the Plasmon-Activated C-C Coupling Reactions by Liquid-State SERS Measurement

The implementation of plasmonic materials in heterogeneous catalysis was limited due to the lack of experimental access in managing the plasmonic hot carriers. Herein, we propose a liquid-state surface-enhanced Raman scattering (SERS) technique to manipulate and visualize heterogeneous photocatalysis with transparent plasmonic chips. The liquid-state measurement conquers the difficulties that arise from the plasmon-induced thermal effects, and thus the plasmon based strategies can be extended to investigate a wider range of catalytic reactions. We demonstrated the selection of reaction products by modulating the plasmonic hot carriers and explored the mechanisms in several typical C-C coupling reactions with 4-bromothiophenol (4-BTP) as reactants. The real-time experimental results suggest brand new mechanisms of the formation of C-C bonds on plasmonic metal nanoparticles (NPs): the residue of 4-BTP, but not thiophenol (TP), is responsible for the C-C coupling. Furthermore, this technique was extended to study the evolution of the Suzuki-Miyaura reaction on nonplasmonic palladium metals by establishing the charge transfer channels between palladium and Au NPs. The cleavage and formation of chemical bonds in each individual reaction step were discerned, and the corresponding working mechanisms were clarified.

Switchable trifunctional terahertz absorber for both broadband and narrowband operations

In this paper, we proposed a multilayer terahertz absorber composed of hybrid graphene and vanadium dioxide (VO2). Based on electrical controlling of graphene and thermal tuning of VO2, three different switchable absorption states are achieved in one structure. When VO2 is in the metal phase and the Fermi level of graphene is set as 0eV, high-frequency broadband (bandwidth, 5.45THz) absorption from 4.5 to 9.95THz is demonstrated. While VO2 is switched to the insulator state, absorption states depend on the Fermi energy of graphene. As the Fermi level changes from 1eV to 0eV, the absorption can be switched from low-frequency broadband (bandwidth, 2.86THz) to dual-frequency absorption. The effect of geometric parameters and fabrication tolerance on the robustness of the absorption properties is explored. The proposed absorber has three switchable states through modulation of graphene and VO2, which is expected to realize potential applications in modulating, filtering, detecting, and other fields.

Thermochromic Smart Windows Assisted by Photothermal Nanomaterials

Thermochromic smart windows are optical devices that can regulate their optical properties actively in response to external temperature changes. Due to their simple structures and as they do not require other additional energy supply devices, they have great potential in building energy-saving. However, conventional thermochromic smart windows generally have problems with high response temperatures and low response rates. Owing to their great effect in photothermal conversion, photothermal materials are often used in smart windows to assist phase transition so that they can quickly achieve the dual regulation of light and heat at room temperature. Based on this, research progress on the phase transition of photothermal material-assisted thermochromic smart windows is summarized. In this paper, the phase transition mechanisms of several thermochromic materials (VO₂, liquid crystals, and hydrogels) commonly used in the field of smart windows are introduced. Additionally, the applications of carbon-based nanomaterials, noble metal nanoparticles, and semiconductor (metal oxygen/sulfide) nanomaterials in thermochromic smart windows are summarized. The current challenges and solutions are further indicated and future research directions are also proposed.

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

VO2 metasurface smart thermal emitter with high visual transparency for passive radiative cooling regulation in space and terrestrial applications

Smart radiative cooling devices based on thermochromic materials such as vanadium dioxide (VO2) are of practical interest for temperature regulation and artificial homeostasis, i.e., maintaining stable equilibrium conditions for survival, both in terrestrial and space applications. In traditional solar reflector configurations, solar absorption in the VO2 layer is a performance limiting factor due to the multiple reflections of sunlight in the stack. Here, we demonstrate a visually transparent, smart radiator panel with reduced solar absorption. An Al-doped ZnO transparent conducting oxide layer acts as a frequency selective infrared back-reflector with high transmission of solar radiation. In this study we make use of high-quality VO2 thin films deposited using atomic layer deposition and optimized annealing process. Patterning of the VO2 layer into a metasurface results in a further reduction of the solar absorption parameter α to around 0.3, while exhibiting a thermal emissivity contrast Δε of 0.26 by exploiting plasmonic enhancement effects. The VO2 metasurface provides a visual spectrum transmission of up to 62%, which is of interest for a range of applications requiring visual transparency. The transparent smart metasurface thermal emitter offers a new approach for thermal management in both space and terrestrial radiative cooling scenarios.

Electromagnetic Mechanisms or Chemical Mechanisms? Role of Interfacial Charge Transfer in the Plasmonic Metal/Semiconductor Heterojunction

The plasmonic metal/semiconductor heterojunction provides a unique paradigm for manipulating light to improve the efficiency of plasmonic materials. Previous studies suggest that the improvement originates from the enhanced carrier exchanges between the plasmonic component of the heterojunction and molecules. This viewpoint, known as the chemical mechanism, is reasonable but insufficient, because the construction of the heterojunction will lead to a charge redistribution in the plasmonic component and cause changes in its physical characteristics. Herein, we will try to clarify that these changes are decisive factors in specific applications by investigating the surface-enhanced Raman scattering (SERS) behavior of a typical Ag/TiO₂ heterojunction. We observed significant changes in SERS spectra by modulating the band alignment of the heterojunction in a loop. Identical trends in SERS spectra were observed despite the fact that the charge transfer from the heterojunction to molecules was blocked, suggesting that the major SERS enhancement originates from electromagnetic mechanisms rather than chemical ones.

A simulation of diffractive liquid crystal smart window for privacy application

Using a single substrate, we demonstrate a simple two-dimensional (2-D) phase grating cell with an octothorp electrode. Owing to the large spatial phase difference in any direction, the proposed grating cell has a high haze value in the opaque state (76.7%); Moreover, it has the advantages of a one-dimensional (1-D) phase grating cell, such as high fabricability, fast response time, and low operating voltage. Furthermore, the proposed grating cell has a faster response time than the 2-D grating cell (comparable to a 1-D grating cell). All the electro-optic parameters have been calculated using a commercial modeling tool. Consequently, we expect our proposed grating cell to find applications in virtual reality (VR)/augmented reality (AR) systems or window displays with fast response times.

Dynamic Refractive Index-Matching for Adaptive Thermoresponsive Smart Windows

Thermoresponsive smart windows (TRSWs) take great advantages in energy-efficient buildings and on-demand devices owing to their self-adaptiveness and external energy consumption-free nature. Currently used TRSWs largely rely on thermal-induced phase transitions in single-material systems, however, the intrinsic characteristics of which may not be suited for practical window utilization, such as poor luminous transparency and fixed critical temperature (T_c ). Herein, an adaptive TRSW based on dynamic refractive index (RI) matching between two phases is demonstrated, which is facilely fabricated by embedding ethylene glycol solution microdroplets into polydimethylsiloxane (PDMS) via a one-step emulsification approach, realizing a smart temperature response in PDMS. The TRSW presents high transparency (≈92%) and bidirectional transparency-temperature response (≈20% at 73 °C, ≈40% at 8 °C). Moreover, the RI dispersion generates a unique effect of wavelength selectivity with temperature. Notably, the effective optical-temperature response with variable T_c could be tuned over a wide range of 13-68 °C by adjusting the EGS concentration. The proposed strategy with dynamic RI matching allows TRSW construction to extend beyond phase transitional materials and greatly broadens the applicable scope of TRSWs, which is promising in the fields of smart optical devices such as smart windows, anti-counterfeiting, optical switches, and optical selection.

Photothermal Dual Passively Driven Liquid Crystal Smart Window

Photochromic or thermochromic liquid crystal (LC) smart windows have attracted wide attention due to their spontaneous transmittance modulation under different environments. There remains a challenge for the LC smart windows that can be modulated with light and temperature simultaneously owing to the difficulty in selecting photothermal molecules. Herein, we selected a photothermal molecule, isobutyl-substituted diimmonium borate (IDI), which shows excellent characteristics of a photothermal material used in smart windows, such as transparency in the visible light range with a slight brown color, good compatibility with the LC system, and excellent photothermal effect compared with common photothermal materials. Thus, a photothermal dual-driven smart window is developed by doping IDI into chiral LC mixtures, which can efficiently modulate the transmittance at different temperatures (or light intensities) by varying the phase state from the homeotropically oriented smectic phase (transparent) to the focal conic cholesteric phase (opaque). The transmittance is high (70%) when the ambient temperature is low and the light intensity is weak, allowing more sunlight to enter the room. The transmittance is low (20%) when the ambient temperature is high and the light intensity is strong, which prevents sunlight from entering the room. The proposed smart window will have a promising application in terms of energy saving and personalized privacy protection.

Deformable Thermo-Responsive Smart Windows Based on a Shape Memory Polymer for Adaptive Solar Modulations

Thermo-responsive smart windows that control solar transmission are expected to be the promising solution to excessive building energy consumption and overheating of solar cell devices. The two performance indices, namely, the luminous transmission (T_lum) and the solar modulation (ΔT_sol), are often intrinsically limited by conventional thermo-responsive materials, which restrict their applications in smart windows. Alternatively, constructing a deformable surface morphology of smart windows can be an effective strategy to modulate the solar transmission. Here, we report a new category of thermo-responsive smart windows with a deformable surface morphology, which can be custom designed to achieve both desirable ΔT_sol and T_lum according to the sunlight incident angles of actual applications. This design is based on a thermo-responsive shape memory polymer and an optical coating, which is termed the butterfly-wing-like smart window (BSW). The BSW reversibly transforms from a temporary shape of flat topography to a predefined original shape of tilted configuration upon heating. It is demonstrated that the BSW has a high ΔT_sol of 32.6% and an excellent T_lum(average) of 64.5%. This work provides a new design strategy and mechanism for thermo-responsive smart windows.

Research Progress of Photo-/Electro-Driven Thermochromic Smart Windows

Thermochromic smart windows can automatically control solar radiation according to the ambient temperature. Compared with photochromic and electrochromic smart windows, they have a stronger applicability and lower energy consumption, and have a wide range of application prospects in the field of building energy efficiency. At present, aiming at the challenge of the high transition temperature of thermochromic smart windows, a large amount of innovative research has been carried out via the principle that thermochromic materials can be driven to change their optical performance by photothermal or electrothermal effects at room temperature. Based on this, the research progress of photo- and electro-driven thermochromic smart windows is summarized from VO₂-based composites, hydrogels and liquid crystals, and it is pointed out that there are two main development trends of photo-/electro-driven thermochromic smart windows. One is exploring the diversified combination methods of photothermal materials and thermochromic materials, and the other is developing low-cost large-area heating electrodes.

Voltage triggered near-infrared light modulation using VO2 thin film

Development of compact and fast modulators of infrared light has garnered strong research interests in recent years due to their potential applications in communication, imaging, and sensing. In this study, electric field induced fast modulation near-infrared light caused by phase change in VO₂ thin films grown on GaN suspended membranes has been reported. It was observed that metal insulator transition caused by temperature change or application of electric field, using an interdigitated finger geometry, resulted in 7% and 14% reduction in transmitted light intensity at near-infrared wavelengths of 790 and 1550 nm, respectively. Near-infrared light modulation has been demonstrated with voltage pulse widths down to 300 µs at 25 V magnitude. Finite element simulations performed on the suspended membrane modulator indicate a combination of the Joule heating and electric field is responsible for the phase transition.

Smart Materials for Dynamic Thermal Radiation Regulation

Thermal radiation in the mid-infrared region profoundly affects human lives in various fields, including thermal management, imaging, sensing, camouflage, and thermography. Due to their fixed emissivities, radiance features of conventional materials are usually proportional to the quadruplicate of surface temperature, which set the limit, that one type of material can only present a single thermal function. Therefore, it is necessary and urgent to design materials for dynamic thermal radiation regulations to fulfill the demands of the age of intelligent machines. Recently, the ability of some smart materials to dynamically regulate thermal radiation has been evaluated. These materials are found to be competent enough for various commands, thereby, providing better alternatives and tremendously promoting the commercial potentials. In this review, the dynamic regulatory mechanisms and recent progress in the evaluation of these smart materials are summarized, including thermochromic materials, electrochromic materials, mechanically and humidity responsive materials, with the potential applications, insufficient problems, and possible strategies highlighted.

Programmed topographical features generated on command in confined electroactive films

This work describes a method to create dynamic pre-programmed surface textures by an alternating electric field on coatings that consist of a silicon oxide reinforced viscoelastic siloxane network. The finite element method is developed to predict the complex deformation figures and time-resolved experimental topographical surface analysis is used to confirm them.

Switchable terahertz metamaterial absorber with broadband absorption and multiband absorption

Based on the phase-transition property of vanadium dioxide (VO₂), a terahertz bifunctional absorber is proposed with switchable functionalities of broadband absorption and multiband absorption. When VO₂ is metal, the system is regarded as a broadband absorber, which is composed of VO₂ patch, topas spacer, and VO₂ film with metallic disks inserted. The system obtains a broadband absorption with absorptance >90% from 3.25 THz to 7.08 THz. Moreover, the designed broadband absorber has a stable performance within the incident angle range of 50°. When VO₂ is dielectric, multiband absorption with six peaks is realized in the designed system. Graphene and the metallic disk-shaped array play the dominant role in the mechanism of multiband absorption. Through changing the Fermi energy level of graphene, the performance of multiband absorption can be dynamically adjusted. Because of the switchable functionalities, the proposed design may have potential application in the fields of intelligent absorption and terahertz switch.

Manipulating atomic defects in plasmonic vanadium dioxide for superior solar and thermal management

Vanadium dioxide (VO₂) is a unique active plasmonic material due to its intrinsic metal-insulator transition, remaining less explored. Herein, we pioneer a method to tailor the VO₂ surface plasmon by manipulating its atomic defects and establish a universal quantitative understanding based on seven representative defective VO₂ systems. Record high tunability is achieved for the localized surface plasmon resonance (LSPR) energy (0.66-1.16 eV) and transition temperature range (40-100 °C). The Drude model and density functional theory reveal that the charge of cations plays a dominant role in the numbers of valence electrons to determine the free electron concentration. We further demonstrate their superior performances in extensive unconventional plasmonic applications including energy-saving smart windows, wearable camouflage devices, and encryption inks.

Rapid Nitriding of Titanium Alloy with Fine Grains at Room Temperature

Multifunctional surfaces are required to design safe engineering products for human lives. Heating in a nitrogen atmosphere (nitriding) improves the tribological properties but reduces the strength of titanium (Ti) alloys owing to grain coarsening. A rapid nitriding method for Ti alloys forms the nitrided layer on the surface of a Ti alloy by bombarding with commercially pure Ti fine particles with a nitrided phase at room temperature within a short period. Furthermore, fine grains of Ti alloy are formed in the nitrided layer because of the impact of the Ti particles. These results reveal that this room-temperature method resolves the trade-off between the rapid formation of a nitrided layer and the suppression of grain coarsening for Ti alloys.

The formation of TiO2/VO2 multilayer structure via directional cationic diffusion

The alternative VO2/TiO2 nanostructure is a potential candidate for application in optical or electrical devices. A promising and straightforward route to form tunable alternative VO2/TiO2 nanostructure is in high demand. Herein, we demonstrate that the VO2/TiO2 nanostructure could be self-assembled from the VO2 film/TiO2 substrate via directional cationic migration, characterizing Ti-rich nano-lamellas with nanoscale spacing along the c-axis. Through aberration-corrected high-resolution transmission electron microscopy, it has been shown that the realization of directional cationic migration is assisted by the interstitial position inside the VO2 lattice. Non-equilibrium cationic diffusion could even retain these interstitial atoms in the form of incoherent strain lines, which affect the local electronic structure as validated by theoretical calculation. Due to Ti-rich nano-lamellas and incoherent strain lines, the phase transition temperature decreased (∼10 °C). The idea of tailoring the elemental distribution by directional cationic diffusion significantly broadens the functional application of VO2 films.

Hydroxypropylmethyl Cellulose Modified with Carbon Dots Exhibits Light-Responsive and Reversible Optical Switching

A light-responsive optical switching material is reported, which was obtained by incorporating carbon dots (CDs) into thermochromic hydroxypropylmethyl cellulose (HPMC). The ultrasmall size of CDs guarantees the considerable transparency of CDs/HPMC. Under illumination, CDs/HPMC shows rapid and reversible optical switching between transparent and opaque states due to the remarkable photothermal effect of CDs. Moreover, the interaction between CDs and HPMC enhances the light absorption and boosts the nonradiative recombination of photoexcited charge carriers that further promote the photothermal conversion of CDs, and also ensures the structural stability of the composite. The obtained CDs/HPMC with good reversibility and high sensitivity which can dynamically switch their transparency in response to weather conditions exhibits excellent solar modulation ability.

Multicolor and Multistage Response Electrochromic Color-Memory Wearable Smart Textile and Flexible Display

Electrochromic materials have great application in soft displays and devices, but the application of ideal electrochromic textiles still faces challenges owing to the inconvenience of a continuous power supply. Here, electrochromic color-memory microcapsules (ECM-Ms-red, -yellow, and -blue) with a low drive voltage (2.0 V), coloration efficiency (921.6 cm² C^-1), a practical response rate (34.4 s^-1), multistage response discoloration, and good color-memory performance (>72 h) and reversibility (≥1000 cycles) are developed. The color-memory performance is controlled by the energy difference of oxidation-reduction reactions. A multicolor and multistage response electrochromic color-memory wearable smart textile and flexible display are developed that are convenient and energy-efficient for application. The design philosophy of color-memory based on controllable energy difference of reactions has great potential application in sensors and smart textiles.

On-off near-infrared absorbance based on thermal-responsive plasmonic coupling in vanadium dioxide arrays for thermochromic windows

Vanadium dioxide (VO₂) emerges as an attractive plasmonic material due to its unique reversible thermal-responsive phase transition and the promising application in energy-saving smart windows. Here, by optimizing the geometry of VO₂ nano-cylinder arrays, we demonstrate a significant performance enhancement for energy-efficient thermochromic windows. Such a performance enhancement relies on the on-off behavior of plasmonic resonance in the extremely high packing density of VO₂ nano-cylinder arrays. Different from the typical plasmonic material, silver, VO₂ nano-cylinders are characterized to have strong absorbance in near-infrared spectrum with significantly weaker plasmonic coupling to their neighbors, making them suitable to be arranged with a high packing density. The VO₂ nano-cylinder arrays exhibit a 160% luminous transmittance increment, comparing to a flat film with the same solar modulation of ∼10%. The work provides a better understanding of the plasmonic behavior on phase-change VO₂ and an efficient method to enhance smart window performance.

Photothermal behaviour of titanium nitride nanoparticles evaluated by transient X-ray diffraction

The photothermal properties of metal nitrides have recently received significant attention owing to diverse applications in solar energy conversion, photothermal therapies, photoreactions, and thermochromic windows. Here, the photothermal response of titanium nitride nanoparticles is examined using transient X-ray diffraction, in which optical excitation is synchronized with X-ray pulses to characterize dynamic changes in the TiN lattice. Photoinduced diffraction data is quantitatively analyzed to determine increases in the TiN lattice spacing, which are furthermore calibrated against static, temperature-dependent diffraction patterns of the same samples. Measurements of 20 nm and 50 nm diameter TiN nanoparticles reveal transient lattice heating from room temperature up to ∼175 °C for the highest pump fluences investigated here. Increasing excitation intensity drives sublinear increases in lattice temperature, due to increased heat capacity at the higher effective temperatures achieved at higher powers. Temporal dynamics show that higher excitation intensity drives not only higher lattice temperatures, but also unexpectedly slower cooling of the TiN nanoparticles, which is attributed to heating of the solvent proximal to the nanoparticle surface.

Smart Window Based on Angular-Selective Absorption of Solar Radiation with Guest–Host Liquid Crystals

In this study, we analyzed angular-selective absorption in a guest–host liquid crystal (GHLC) cell for its application in smart windows. For reducing the energy consumption, angular-selective absorption is desired because the light transmitted through windows during the daytime is predominantly incident obliquely from direct sunlight. Owing to the absorption anisotropy of guest dichroic dyes, a GHLC cell can absorb the obliquely incident light, while allowing people to see through windows in a normal view. Therefore, the cell can provide a comfortable environment for occupants, and reduce the energy required for cooling by blocking the solar heat incident from the oblique direction. The GHLC cell can be switched between the transparent and opaque states for a normal view. The rising (falling) time was 6.1 (80.5) ms when the applied voltage was 10 V.

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.

Temperature-programmed nitridation of monodispersed VOx nanoparticles into nanocrystalline superconducting oxygen-doped vanadium nitride

A two-stage synthesis process was employed to prepare high-quality nanocrystalline vanadium nitride (VN) for superconducting applications.

Titanium Nitride Nanodonuts Synthesized from Natural Ilmenite Ore as a Novel and Efficient Thermoplasmonic Material

Nanostructures of titanium nitride (TiN) have recently been considered as a new class of plasmonic materials that have been utilized in many solar energy applications. This work presents the synthesis of a novel nanostructure of TiN that has a nanodonut shape from natural ilmenite ore using a low-cost and bulk method. The TiN nanodonuts exhibit strong and spectrally broad localized surface plasmon resonance absorption in the visible region centered at 560 nm, which is well suited for thermoplasmonic applications as a nanoscale heat source. The heat generation is investigated by water evaporation experiments under simulated solar light, demonstrating excellent solar light harvesting performance of the nanodonut structure.

Atomic Origin for Hydrogenation Promoted Bulk Oxygen Vacancies Removal in Vanadium Dioxide

Oxygen vacancies (V_O), a common type of point defect in metal oxides materials, play important roles in the physical and chemical properties. To obtain stoichiometric oxide crystal, the pre-existing V_O is always removed via careful post-annealing treatment at high temperature in an air or oxygen atmosphere. However, the annealing conditions are difficult to control, and the removal of V_O in the bulk phase is restrained because of the high energy barrier of V_O migration. Here, we selected VO₂ crystal film as the model system and developed an alternative annealing treatment aided by controllable hydrogen doping, which can realize effective removal of V_O defects in the VO_2-δ crystal at a lower temperature. This finding is attributed to the hydrogenation accelerated oxygen vacancies recovery in the VO_2-δ crystal. Theoretical calculations revealed that the H-doping-induced electrons are prone to accumulate around the oxygen defects in the VO_2-δ film, which facilitates the diffusion of V_O and thus makes it easier to be removed. The methodology is expected to be applied to other metal oxides for oxygen-related point defects control.

Self-assembled nitride-metal nanocomposites: recent progress and future prospects

Two-phase nanocomposites have gained significant research interest because of their multifunctionalities, tunable geometries and potential device applications. Different from the previously demonstrated oxide-oxide 2-phase nanocomposites, coupling nitrides with metals shows high potential for building alternative hybrid plasmonic metamaterials towards chemical sensing, tunable plasmonics, and nonlinear optics. Unique advantages, including distinct atomic interface, excellent crystalline quality, large-scale surface coverage and durable solid-state platform, address the high demand for new hybrid metamaterial designs for versatile optical material needs. This review summarizes the recent progress on nitride-metal nanocomposites, specifically targeting bottom-up self-assembled nanocomposite thin films. Various morphologies including vertically aligned nanocomposites (VANs), self-organized nanoinclusions, and nanoholes fabricated by additional chemical treatments are introduced. Starting from thin film nucleation and growth, the prerequisites of successful strain coupling and the underlying growth mechanisms are discussed. These findings facilitate a better control of tunable nanostructures and optical functionalities. Future research directions are proposed, including morphological control of the secondary phase to enhance its homogeneity, coupling nitrides with magnetic phase for the magneto-optical effect and growing all-ceramic nanocomposites to extend functionalities and anisotropy.

Durable and Controllable Smart Windows Based on Thermochromic Hydrogels

In recent years, the use of smart windows to adjust sunlight to achieve energy conversion has received increasing attention. In this paper, a novel smart window was easily prepared by using thermochromic hydrogels as an interlayer and indium tin oxide films as an electric heating layer. The shielding transmission rates of visible and near-infrared light reached 88.3 and 85.4% at the temperature of 25 °C, respectively. However, the transmittance at a light wavelength of 550 nm was greater than 70% after applying voltage. The smart windows with different components could possess thermochromic temperature ranging from 28 to 35 °C, which was suitable for daily life. The smart window could maintain a stable reversible thermochromic transition. Importantly, the time of light transition and the demand of energy efficiency could be adjusted by controlling the magnitude of the output voltage, which benefited the development of energy-efficient materials.

Achieving broadband absorption and polarization conversion with a vanadium dioxide metasurface in the same terahertz frequencies

We present the bifunctional design of a broadband absorber and a broadband polarization converter based on a switchable metasurface through the insulator-to-metal phase transition of vanadium dioxide. When vanadium dioxide is metal, the designed switchable metasurface behaves as a broadband absorber. This absorber is composed of a vanadium dioxide square, silica spacer, and vanadium dioxide film. Calculated results show that in the frequency range of 0.52-1.2 THz, the designed system can absorb more than 90% of the energy, and the bandwidth ratio is 79%. It is insensitive to polarization due to the symmetry, and can still work well even at large incident angles. When vanadium dioxide is an insulator, a terahertz polarizer is realized by a simple anisotropic metasurface. Numerical calculation shows that efficient conversion between two orthogonal linear polarizations can be achieved. Reflectance of a cross-polarized wave can reach 90% from 0.42 THz to 1.04 THz, and the corresponding bandwidth ratio is 85%. This cross-polarized converter has the advantages of wide angle, broad bandwidth, and high efficiency. So our design can realize bifunctionality of broadband absorption and polarization conversion between 0.52 THz and 1.04 THz. This architecture could provide one new way to develop switchable photonic devices and functional components in phase change materials.

Terahertz bifunctional absorber based on a graphene-spacer-vanadium dioxide-spacer-metal configuration

A terahertz bifunctional absorber is presented with broadband and narrowband absorbing properties in a graphene-spacer-vanadium dioxide-spacer-metal configuration. Carrier relaxation time of graphene τ = 1.0ps (τ = 0.1ps) is chosen for narrowband (broadband) absorption. When vanadium dioxide is in the conducting state, the design behaves as a narrowband absorber, and it is composed of a square-shaped graphene, topas spacer, and metallic vanadium dioxide film. There is an absorption band with 100% absorptance at the frequency of 1.37 THz. Narrowband absorption is caused by the localized magnetic resonance. When vanadium dioxide is in the insulating state, the design behaves as a broadband absorber composed of a square-shaped graphene, topas layer, vanadium dioxide film, and metal film. It has a broadband absorption in the frequency range of 1.05-2.35 THz, and the corresponding absorptance is more than 90%. The merging of two resonances with overlapping region ensures broadband performance of the designed absorber. The working bandwidth and intensity of narrowband absorption and broadband absorption can be dynamically adjusted by changing the Fermi energy level of graphene. The influences of structure parameters are discussed on absorption performance. In addition, the designed absorber is not sensitive to incident angle. Because of the simple structure, our design can be applied to many promising fields in intelligent absorption and terahertz switch.