Solution-processed phase-change VO(2) metamaterials from colloidal vanadium oxide (VO(x)) nanocrystals

Engineering a polyvinyl butyral hydrogel as a thermochromic interlayer for energy-saving windows

Achieving mastery over light using thermochromic materials is crucial for energy-saving glazing. However, challenges such as high production costs, limited durability, and recyclability issues have hindered their widespread application in buildings. Herein, we develop a glass interlayer made of a polyvinyl butyral-based hydrogel swollen with LiCl solution. In addition to a fast, isochoric, and reversible transparency-to-opacity transition occurring as ambient temperatures exceed thermally comfortable levels, this hydrogel uniquely encompasses multiple features such as frost resistance, recyclability, scalability, and toughness. The combination of these features is achieved through a delicate balance of polyvinyl butyral's amphiphilicity and the suppression of network-forming phase separation. This design endows a nanostructured polyvinyl butyral-LiCl composite gel with swollen molecular segments linked by dispersed cross-linking sites in the form of hydrophobic nano-nodules. Upon laminating this hydrogel (a thickness of 0.3 mm), the resultant glazing product demonstrates approximately 90% luminous transmittance even at sub-zero temperatures, along with a significant modulation of solar and infrared radiation at 80.8% and 68.5%, respectively. Through simulations, we determined that windows equipped with the hydrogel could reduce energy consumption by 36% compared to conventional glass windows in warm seasons. The widespread adoption of polyvinyl butyral in construction underscores the promise of this hydrogel as a thermochromic interlayer for glazing.

Thermally Reconfigurable, 3D Chiral Optical Metamaterials: Building with Colloidal Nanoparticle Assemblies

The three-dimensional, geometric handedness of chiral optical metamaterials allows for the rotation of linearly polarized light and creates a differential interaction with right and left circularly polarized light, known as circular dichroism. These three-dimensional metamaterials enable polarization control of optical and spin excitation and detection, and their stimuli-responsive, dynamic switching widens applications in chiral molecular sensing and imaging and spintronics; however, there are few reconfigurable solid-state implementations. Here, we report all-solid-state, thermally reconfigurable chiroptical metamaterials composed of arrays of three-dimensional nanoparticle/metal bilayer heterostructures fabricated from coassemblies of phase change VO2 and metallic Au colloidal nanoparticles and thin films of Ni. These metamaterials show dynamic switching in the mid-infrared as VO2 is thermally cycled through an insulator-metal phase transition. The spectral range of operation is tailored in breadth by controlling the periodicity of the arrays and thus the hybridization of optical modes and in position through the mixing of VO2 and Au nanoparticles.

Terahertz Metasurfaces Exploiting the Phase Transition of Vanadium Dioxide

Artificially designed modulators that enable a wealth of freedom in manipulating the terahertz (THz) waves at will are an essential component in THz sources and their widespread applications. Dynamically controlled metasurfaces, being multifunctional, ultrafast, integrable, broadband, high contrasting, and scalable on the operating wavelength, are critical in developing state-of-the-art THz modulators. Recently, external stimuli-triggered THz metasurfaces integrated with functional media have been extensively explored. The vanadium dioxide (VO2)-based hybrid metasurfaces, as a unique path toward active meta-devices, feature an insulator-metal phase transition under the excitation of heat, electricity, and light, etc. During the phase transition, the optical and electrical properties of the VO2 film undergo a massive modification with either a boosted or dropped conductivity by more than four orders of magnitude. Being benefited from the phase transition effect, the electromagnetic response of the VO2-based metasufaces can be actively controlled by applying external excitation. In this review, we present recent advances in dynamically controlled THz metasurfaces exploiting the VO2 phase transition categorized according to the external stimuli. THz time-domain spectroscopy is introduced as an indispensable platform in the studies of functional VO2 films. In each type of external excitation, four design strategies are employed to realize external stimuli-triggered VO2-based THz metasurfaces, including switching the transreflective operation mode, controlling the dielectric environment of metallic microstructures, tailoring the equivalent resonant microstructures, and modifying the electromagnetic properties of the VO2 unit cells. The microstructures' design and electromagnetic responses of the resulting active metasurfaces have been systematically demonstrated, with a particular focus on the critical role of the VO2 films in the dynamic modulation processes.

Stabilization of the VO2(M2) Phase and Change in Lattice Parameters at the Phase Transition Temperature of WXV1-XO2 Thin Films

Various methods have been used to fabricate vanadium dioxide (VO2) thin films exhibiting polymorph phases and an identical chemical formula suited to different applications. Most fabrication techniques require post-annealing to convert the amorphous VO2 thin film into the VO2 (M1) phase. In this study, we provide a temperature-dependent XRD analysis that confirms the change in lattice parameters responsible for the metal-to-insulator transition as the structure undergoes a monoclinic to the tetragonal phase transition. In our study, we deposited VO2 and W-doped VO2 thin films onto silica substrates using a high repetition rate (10 kHz) fs-PLD deposition without post-annealing. The XRD patterns measured at room temperature revealed stabilization of the monoclinic M2 phase by W6+ doping VO2. We developed an alternative approach to determine the phase transition temperatures using temperature-dependent X-ray diffraction measurements to evaluate the a and b lattice parameters for the monoclinic and rutile phases. The a and b lattice parameters versus temperature revealed phase transition temperature reduction from ∼66 to 38 °C when the W6+ concentration increases. This study provides a novel unorthodox technique to characterize and evaluate the structural phase transitions seen on VO2 thin films.

Chemical and Physical Properties of Photonic Noble-Metal Nanomaterials

Colloidal noble metal nanoparticles (NPs) are composed of metal cores and organic or inorganic ligand shells. These NPs support size- and shape-dependent plasmonic resonances. They can be assembled from dispersions into artificial metamolecules which have collective plasmonic resonances originating from coupled bright and dark optical electric and magnetic modes that form depending on the size and shape of the constituent NPs and their number, arrangement, and interparticle distance. NPs can also be assembled into extended 2D and 3D metamaterials that are glassy thin films or ordered thin films or crystals, also known as superlattices and supercrystals. The metamaterials have tunable optical properties that depend on the size, shape, and composition of the NPs, and on the number of NP layers and their interparticle distance. Interestingly, strong light-matter interactions in superlattices form plasmon polaritons. Tunable interparticle distances allow designer materials with dielectric functions tailorable from that characteristic of an insulator to that of a metal, and serve as strong optical absorbers or scatterers, respectively. In combination with lithography techniques, these extended assemblies can be patterned to create subwavelength NP superstructures and form large-area 2D and 3D metamaterials that manipulate the amplitude, phase, and polarization of transmitted or reflected light.

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.

Local structure elucidation of tungsten-substituted vanadium dioxide (V[Formula: see text]W[Formula: see text]O[Formula: see text])

Initially, vanadium dioxide seems to be an ideal first-order phase transition case study due to its deceptively simple structure and composition, but upon closer inspection there are nuances to the driving mechanism of the metal-insulator transition (MIT) that are still unexplained. In this study, a local structure analysis across a bulk powder tungsten-substitution series is utilized to tease out the nuances of this first-order phase transition. A comparison of the average structure to the local structure using synchrotron x-ray diffraction and total scattering pair-distribution function methods, respectively, is discussed as well as comparison to bright field transmission electron microscopy imaging through a similar temperature-series as the local structure characterization. Extended x-ray absorption fine structure fitting of thin film data across the substitution-series is also presented and compared to bulk. Machine learning technique, non-negative matrix factorization, is applied to analyze the total scattering data. The bulk MIT is probed through magnetic susceptibility as well as differential scanning calorimetry. The findings indicate the local transition temperature ([Formula: see text]) is less than the average [Formula: see text] supporting the Peierls-Mott MIT mechanism, and demonstrate that in bulk powder and thin-films, increasing tungsten-substitution instigates local V-oxidation through the phase pathway VO[Formula: see text] V[Formula: see text]O[Formula: see text] V[Formula: see text]O[Formula: see text].

Transmissive terahertz metasurfaces with vanadium dioxide split-rings and grids for switchable asymmetric polarization manipulation

Metasurfaces containing arrays of thermally tunable metal-free (double-)split-ring meta-atoms and metal-free grids made of vanadium dioxide (VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2), a phase-change material can deliver switching between (1) polarization manipulation in transmission mode as well as related asymmetric transmission and (2) other functionalities in the terahertz regime, especially when operation in the transmission mode is needed to be conserved for both phases of VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2. As the meta-atom arrays function as arrays of metallic subwavelength resonators for the metallic phase of VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2, but as transmissive phase screens for the insulator phase of VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2, numerical simulations of double- and triple-array metasurfaces strongly indicate extreme scenarios of functionality switching also when the resulting structure comprises only VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2 meta-atoms and VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2 grids. More switching scenarios are achievable when only one meta-atom array or one grid is made of VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2 components. They are enabled by the efficient coupling of the geometrically identical resonator arrays/grids that are made of the materials that strongly differ in terms of conductivity, i.e. Cu and VO\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$_2$$\end{document}2 in the metallic phase.

Multilevel Absorbers via the Integration of Undoped and Tungsten-Doped Multilayered Vanadium Dioxide Thin Films

Reconfigurable light absorbers have attracted much attention by providing additional optical responses and expanding the number of degrees of freedom in security applications. Fabry-Pèrot absorbers based on phase change materials with tunable properties can be implemented over large scales without the need for additional steps such as lithography, while exhibiting reconfigurable optical responses. However, a fundamental limitation of widely used phase change materials such as vanadium dioxide and germanium-antimony-tellurium-based chalcogenide glasses is that they have only two distinct phases; therefore, only two different states of optical properties are available. Here, we experimentally demonstrate active multilevel absorbers that are tuned by controlling the external temperature. This is produced by creating large-scale lithography-free multilayer structures with both undoped and tungsten-doped solution-processed monoclinic-phase vanadium dioxide thin films. The doping of vanadium dioxide with tungsten allows for the modulation of the phase-transition temperature, which results in an extra degree of freedom and therefore an additional step for the tunable properties. The proposed multilevel absorber is designed and characterized both numerically and experimentally. Such large-scale multilevel tunable absorbers realized with nanoparticle-based solution fabrication techniques are expected to open the way for advanced thermo-optical cryptographic devices based on tunable reflective coloration and near-infrared absorption.

Influence of water concentration on the solvothermal synthesis of VO2(B) nanocrystals

Phase and length control of VO2(B) nanocrystals afforded by manipulating the ratio of toluene to water.

Self-adaptive control of infrared emissivity in a solution-processed plasmonic structure

Active control of optical properties, particularly in the infrared (IR) regime, is critical for the regulation of thermal emission. However, most photonic structures and devices are based on a sophisticated design, making the dynamic control of their IR properties challenging. Here, we demonstrate self-adaptive control of IR absorptivity/emissivity in a simple stacked structure that consists of an oxide plasmonic nanocrystal layer and a phase change material (VO₂) layer, both fabricated via a solution process. The resonance wavelength and emission intensity for this structure depend on the phase of the VO₂. This has potential applications for thermal emission structures (e.g., self-adaptive radiative cooling and IR camouflage). The proposed structure is a candidate low-cost and scalable active photonic platform.

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.

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.

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

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

Heating-up synthesis of cesium bismuth bromide perovskite nanocrystals with tailored composition, morphology, and optical properties

This study represents the heating-up synthesis of lead-free cesium bismuth bromide perovskite nanocrystals (NCs). CsBr and BiBr₃ precursors are used to synthesize uniform and phase-pure cesium bismuth bromide NCs, and the reaction is performed via an injection-free, heating-up method in the presence of a solvent mixture with a high boiling point. The size and composition of cesium bismuth bromide NCs are readily controlled by changing the reaction time, temperature, and amount of surfactant added to the reaction mixture. Upon heating, sequential phase evolution occurs, resulting in the formation of kinetically stable BiOBr in the early reaction stages, which transformed into the thermodynamically stable Cs₃BiBr₆ and Cs₃Bi₂Br₉ with an increase in either the reaction time or the reaction temperature. Furthermore, the absorption and photoluminescence properties of Cs₃BiBr₆ and Cs₃Bi₂Br₉ NCs are characterized to investigate their composition-dependent optical properties. This work provides the potential to synthesize various types of lead-free perovskite NCs by tailoring the size and compositions.

Lead-free cesium bismuth bromide perovskite nanocrystals are synthesized via the heating-up method with tailored morphology and optical properties.

Nanotechnology Facets of the Periodic Table of Elements

The 150th anniversary of the periodic table of elements highlights its tremendous role in chemistry, physics, biology, astronomy, philosophy, and engineering as a shining scientific breakthrough, shedding light on the fundamental laws of nature. Nanoscience and nanotechnology are multidisciplinary, focusing on nanoscale materials and processes, in which a variety of elements are used and single atoms are often manipulated. In this Perspective, we present a new viewpoint on what the renown periodic table can offer to researchers working on nanomaterials.

Highly Efficient Active All-Dielectric Metasurfaces Based on Hybrid Structures Integrated with Phase-Change Materials: From Terahertz to Optical Ranges

Recently, all-dielectric metasurfaces (AMs) have emerged as a promising platform for high-efficiency devices ranging from the terahertz to optical ranges. However, active and fast tuning of their properties, such as amplitude, phase, and operating frequency, remains challenging. Here, a generic method is proposed for obtaining high-efficiency active AMs from the terahertz to optical ranges by using "hybrid structures" integrated with phase-change materials. Various phase-change mechanisms including metal-insulator phase change, nonvolatile phase change, and ferroelectric phase change are investigated. We first experimentally demonstrate several high-efficiency active AMs operating in the terahertz range based on hybrid structures composed of free-standing silicon microstructures covered with ultrathin phase-change nanofilms (thickness d ≪ λ). We show that both the frequencies and the strength of the Mie resonances can be efficiently tuned, resulting in unprecedented modulation depth. Furthermore, detailed analyses of available phase-change materials and their properties are provided to offer more options for active AMs. Finally, several feasible hybrid structures for active AMs in the optical range are proposed and confirmed numerically. The broad platform built in this work for active manipulation of waves from the terahertz to optical ranges may have numerous potential applications in optical devices including switches, modulators, and sensors.

Manipulating Behaviors from Heavy Tungsten Doping on Interband Electronic Transition and Orbital Structure Variation of Vanadium Dioxide Films

Vanadium dioxide (VO₂) with a metal-insulator transition (MIT) has been supposed as a candidate for optoelectronic devices. However, the MIT temperature ( T_MIT) above room temperature limits its application scope. Here, high-quality V_{1- x}W _xO₂ films have been prepared by pulsed laser deposition. On the basis of temperature-dependent transmittance and Raman spectra, it was found that T_MIT increases from 241 to 279 K, when increasing the doping concentration in the range of 0.16 ≤ x ≤ 0.20. The interband electronic transitions and orbital structures of V_{1- x}W _xO₂ films have been investigated via fitting transmittance spectra. Moreover, with the aid of first-principles calculations, an effective orbital theory has been proposed to explain the unique phenomenon. When the W doping concentration increases, the π* and d_II orbitals shift toward the π orbital. Meanwhile, the energy gap between the π* and d_II orbitals decreases at the insulator state. It indicates that the bandwidth is narrowed, which impedes MIT. In addition, the overlap of the π* and d_II orbitals increases at the metal state, and more doping electrons occupy the π* orbital induced by increasing W doping concentration. It manifests that the Mott insulating state becomes more stable, which further improves T_MIT. The present work provides a feasible approach to tune T_MIT via orbital variation and can be helpful in developing the potential VO₂-based optoelectronic devices.

Three-Layered Hollow Nanospheres Based Coatings with Ultrahigh-Performance of Energy-Saving, Antireflection, and Self-Cleaning for Smart Windows

In this study, a well-controlled interfacial engineering method for the synthesis of SiO₂ /TiO₂ /VO₂ three-layered hollow nanospheres (TLHNs) and TLHNs-based multifunctional coatings is reported. The as-prepared coatings allow for an outstanding integration of thermochromism from the outer VO₂ (M) layer, photocatalytic self-cleaning capability from the middle TiO₂ (A) layer, and antireflective property from internal SiO₂ HNs. The TLHNs coatings exhibit excellent optical performance with ultrahigh luminous transmittance (T_lum-l = 74%) and an improved solar modulation ability (ΔT_sol = 12%). To the best knowledge, this integrated optical performance is the highest ever reported for TiO₂ /VO₂ -based thermochromic coatings. An ingenious computation model is proposed, which allows the n_eff of nanostructured coatings to be rapidly obtained. The experimental and calculated results reveal that the unique three-layered structure significantly reduces the refractive index (from 2.25 to 1.33 at 600 nm) and reflectance (Rave, from 22.3 to 5.3%) in the visible region as compared with dense coatings. Infrared thermal imaging characterization and self-cleaning tests provide valid evidence of SiO₂ /TiO₂ /VO₂ TLHNs coatings' potential for energy-saving and self-cleaning smart windows. The exciting inexpensive and universal fabrication process for well-defined structures may inspire various developments in processable and multifunctional devices.

Localized Surface Plasmon Resonance in Semiconductor Nanocrystals

Localized surface plasmon resonance (LSPR) in semiconductor nanocrystals (NCs) that results in resonant absorption, scattering, and near field enhancement around the NC can be tuned across a wide optical spectral range from visible to far-infrared by synthetically varying doping level, and post synthetically via chemical oxidation and reduction, photochemical control, and electrochemical control. In this review, we will discuss the fundamental electromagnetic dynamics governing light matter interaction in plasmonic semiconductor NCs and the realization of various distinctive physical properties made possible by the advancement of colloidal synthesis routes to such NCs. Here, we will illustrate how free carrier dielectric properties are induced in various semiconductor materials including metal oxides, metal chalcogenides, metal nitrides, silicon, and other materials. We will highlight the applicability and limitations of the Drude model as applied to semiconductors considering the complex band structures and crystal structures that predominate and quantum effects that emerge at nonclassical sizes. We will also emphasize the impact of dopant hybridization with bands of the host lattice as well as the interplay of shape and crystal structure in determining the LSPR characteristics of semiconductor NCs. To illustrate the discussion regarding both physical and synthetic aspects of LSPR-active NCs, we will focus on metal oxides with substantial consideration also of copper chalcogenide NCs, with select examples drawn from the literature on other doped semiconductor materials. Furthermore, we will discuss the promise that LSPR in doped semiconductor NCs holds for a wide range of applications such as infrared spectroscopy, energy-saving technologies like smart windows and waste heat management, biomedical applications including therapy and imaging, and optical applications like two photon upconversion, enhanced luminesence, and infrared metasurfaces.

Metamaterials based on the phase transition of VO2

In this article, we present a comprehensive review on recent research progress in design and fabrication of active tunable metamaterials and devices based on phase transition of VO₂. Firstly, we introduce mechanisms of the metal-to-insulator phase transition (MIPT) in VO₂ investigated by ultrafast THz spectroscopies. By analyzing the THz spectra, the evolutions of MIPT in VO₂ induced by different external excitations are described. The superiorities of using VO₂ as building blocks to construct highly tunable metamaterials are discussed. Subsequently, the recently demonstrated metamaterial devices based on VO₂ are reviewed. These metamaterials devices are summarized and described in the categories of working frequency. In each working frequency range, representative metamaterials based on VO₂ with different architectures and functionalities are reviewed and the contributions of the MIPT of VO₂ are emphasized. Finally, we conclude the recent reports and provide a prospect on the strategies of developing future tunable metamaterials based on VO₂.

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.

High Performance and Enhanced Durability of Thermochromic Films Using VO2@ZnO Core-Shell Nanoparticles

For VO₂-based thermochromic smart windows, high luminous transmittance (T_lum) and solar regulation efficiency (ΔT_sol) are usually pursued as the most critical issues, which have been discussed in numerous researches. However, environmental durability, which has rarely been considered, is also so vital for practical application because it determines lifetime and cycle times of smart windows. In this paper, we report novel VO₂@ZnO core-shell nanoparticles with ultrahigh durability as well as improved thermochromic performance. The VO₂@ZnO nanoparticles-based thermochromic film exhibits a robust durability that the ΔT_sol keeps 77% (from 19.1% to 14.7%) after 10³ hours in a hyperthermal and humid environment, while a relevant property of uncoated VO₂ nanoparticles-based film badly deteriorates after 30 h. Meanwhile, compared with the uncoated VO₂-based film, the VO₂@ZnO-based film demonstrates an 11.0% increase (from 17.2% to 19.1%) in ΔT_sol and a 31.1% increase (from 38.9% to 51.0%) in T_lum. Such integrated thermochromic performance expresses good potential for practical application of VO₂-based smart windows.

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.

Ultrafast all-optical tuning of direct-gap semiconductor metasurfaces

Optical metasurfaces are regular quasi-planar nanopatterns that can apply diverse spatial and spectral transformations to light waves. However, metasurfaces are no longer adjustable after fabrication, and a critical challenge is to realise a technique of tuning their optical properties that is both fast and efficient. We experimentally realise an ultrafast tunable metasurface consisting of subwavelength gallium arsenide nanoparticles supporting Mie-type resonances in the near infrared. Using transient reflectance spectroscopy, we demonstrate a picosecond-scale absolute reflectance modulation of up to 0.35 at the magnetic dipole resonance of the metasurfaces and a spectral shift of the resonance by 30 nm, both achieved at unprecedentedly low pump fluences of less than 400 μJ cm^–2. Our findings thereby enable a versatile tool for ultrafast and efficient control of light using light.

Metasurfaces are not adjustable after fabrication, and a critical challenge is to realise a technique of tuning their optical properties that is both fast and efficient. Here, Shcherbakov et al. realise an ultrafast tunable metasurface with picosecond-scale large absolute reflectance modulation at low pump fluences.

Hierarchical Materials Design by Pattern Transfer Printing of Self-Assembled Binary Nanocrystal Superlattices

We demonstrate the fabrication of hierarchical materials by controlling the structure of highly ordered binary nanocrystal superlattices (BNSLs) on multiple length scales. Combinations of magnetic, plasmonic, semiconducting, and insulating colloidal nanocrystal (NC) building blocks are self-assembled into BNSL membranes via the liquid-interfacial assembly technique. Free-standing BNSL membranes are transferred onto topographically structured poly(dimethylsiloxane) molds via the Langmuir-Schaefer technique and then deposited in patterns onto substrates via transfer printing. BNSLs with different structural motifs are successfully patterned into various meso- and microstructures such as lines, circles, and even three-dimensional grids across large-area substrates. A combination of electron microscopy and grazing incidence small-angle X-ray scattering (GISAXS) measurements confirm the ordering of NC building blocks in meso- and micropatterned BNSLs. This technique demonstrates structural diversity in the design of hierarchical materials by assembling BNSLs from NC building blocks of different composition and size by patterning BNSLs into various size and shape superstructures of interest for a broad range of applications.

Colloidal metal oxide nanocrystals as charge transporting layers for solution-processed light-emitting diodes and solar cells

This review bridges the chemistry of colloidal oxide nanocrystals and their application as charge transporting interlayers in solution-processed optoelectronics.

Electrochemically Induced Transformations of Vanadium Dioxide Nanocrystals

Vanadium dioxide (VO₂) undergoes significant optical, electronic, and structural changes as it transforms between the low-temperature monoclinic and high-temperature rutile phases. Recently, alternative stimuli have been utilized to trigger insulator-to-metal transformations in VO₂, including electrochemical gating. Here, we prepare and electrochemically reduce mesoporous films of VO₂ nanocrystals, prepared from colloidally synthesized V₂O₃ nanocrystals that have been oxidatively annealed, in a three-electrode electrochemical cell. We observe a reversible transition between infrared transparent insulating phases and a darkened metallic phase by in situ visible-near-infrared spectroelectrochemistry and correlate these observations with structural and electronic changes monitored by X-ray absorption spectroscopy, X-ray diffraction, Raman spectroscopy, and conductivity measurements. An unexpected reversible transition from conductive, reduced monoclinic VO₂ to an infrared-transparent insulating phase upon progressive electrochemical reduction is observed. This insulator-metal-insulator transition has not been reported in previous studies of electrochemically gated epitaxial VO₂ films and is attributed to improved oxygen vacancy formation kinetics and diffusion due to the mesoporous nanocrystal film structure.

Oxygen Incorporation and Release in Metastable Bixbyite V2O3 Nanocrystals

A new, metastable polymorph of V2O3 with a bixbyite structure was recently stabilized in colloidal nanocrystal form. Here, we report the reversible incorporation of oxygen in this material, which can be controlled by varying temperature and oxygen partial pressure. Based on X-ray diffraction (XRD) and thermogravimetric analysis, we find that oxygen occupies interstitial sites in the bixbyite lattice. Two oxygen atoms per unit cell can be incorporated rapidly and with minimal changes to the structure while the addition of three or more oxygen atoms destabilizes the structure, resulting in a phase change that can be reversed upon oxygen removal. Density functional theory (DFT) supports the reversible occupation of interstitial sites in bixbyite by oxygen, and the 1.1 eV barrier to oxygen diffusion predicted by DFT matches the activation energy of the oxidation process derived from observations by in situ XRD. The observed rapid oxidation kinetics are thus facilitated by short diffusion paths through the bixbyite nanocrystals. Due to the exceptionally low temperatures of oxidation and reduction, this earth-abundant material is proposed for use in oxygen storage applications.

Plasmonic-Field Interactions at Nanoparticle Interfaces for Infrared Thermal-Shielding Applications Based on Transparent Oxide Semiconductors

This paper describes infrared plasmonic responses in three-dimensional (3D) assembled films of In2O3:Sn nanoparticles (NPs). The introduction of surface modifications to NPs can facilitate the production of electric-field interactions between NPs due to the creation of narrow crevices in the NP interfaces. In particular, the electric-field interactions along the in-plane and out-of-plane directions in the 3D assembled NP films allow for resonant splitting of plasmon excitations to the quadrupole and dipole modes, thereby realizing selective high reflections in the near- and mid-infrared range, respectively. The origins of these plasmonic properties were revealed from electric-field distributions calculated by electrodynamic simulations that agreed well with experimental results. The interparticle gaps and their derived plasmon couplings play an important role in producing high reflective performances in assembled NP films. These 3D assemblies of NPs can be further extended to produce large-size flexible films with high infrared reflectance, which simultaneously exhibit microwave transmittance essential for telecommunications. This study provides important insights for harnessing infrared optical responses using plasmonic technology for the fabrication of infrared thermal-shielding applications.

Active Optical Metasurfaces Based on Defect-Engineered Phase-Transition Materials

Active, widely tunable optical materials have enabled rapid advances in photonics and optoelectronics, especially in the emerging field of meta-devices. Here, we demonstrate that spatially selective defect engineering on the nanometer scale can transform phase-transition materials into optical metasurfaces. Using ion irradiation through nanometer-scale masks, we selectively defect-engineered the insulator-metal transition of vanadium dioxide, a prototypical correlated phase-transition material whose optical properties change dramatically depending on its state. Using this robust technique, we demonstrated several optical metasurfaces, including tunable absorbers with artificially induced phase coexistence and tunable polarizers based on thermally triggered dichroism. Spatially selective nanoscale defect engineering represents a new paradigm for active photonic structures and devices.

Vanadium Dioxide Nanoparticle-based Thermochromic Smart Coating: High Luminous Transmittance, Excellent Solar Regulation Efficiency, and Near Room Temperature Phase Transition

An annealing-assisted preparation method of well-crystallized VxW1-xO2(M)@SiO2 core-shell nanoparticles for VO2-based thermochromic smart coatings (VTSC) is presented. The additional annealing process reduces the defect density of the initial hydrothermally prepared VxW1-xO2(M) nanoparticles and enhances their crystallinity so that the thermochromic film based on VxW1-xO2(M)@SiO2 nanoparticles can exhibit outstanding thermochromic performance with balanced solar regulation efficiency (ΔTsol) of 17.3%, luminous transmittance (Tlum) up to 52.2%, and critical phase transition temperature (Tc) around 40.4 °C, which is very promising for practical application. Furthermore, it makes great progress in reducing Tc of VTSC to near room temperature (25.2 °C) and simutaneously maintaining excellent optical properties (ΔTsol = 14.7% and Tlum = 50.6%). Such thermochromic performance is good enough to make VTSC applicable to practical architecture.

Solution processable broadband transparent mixed metal oxide nanofilm optical coatings via substrate diffusion doping

Devices composed of transparent materials, particularly those utilizing metal oxides, are of significant interest due to increased demand from industry for higher fidelity transparent thin film transistors, photovoltaics and a myriad of other optoelectronic devices and optics that require more cost-effective and simplified processing techniques for functional oxides and coatings. Here, we report a facile solution processed technique for the formation of a transparent thin film through an inter-diffusion process involving substrate dopant species at a range of low annealing temperatures compatible with processing conditions required by many state-of-the-art devices. The inter-diffusion process facilitates the movement of Si, Na and O species from the substrate into the as-deposited vanadium oxide thin film forming a composite fully transparent V0.0352O0.547Si0.4078Na0.01. Thin film X-ray diffraction and Raman scattering spectroscopy show the crystalline component of the structure to be α-NaVO3 within a glassy matrix. This optical coating exhibits high broadband transparency, exceeding 90-97% absolute transmission across the UV-to-NIR spectral range, while having low roughness and free of surface defects and pinholes. The production of transparent films for advanced optoelectronic devices, optical coatings, and low- or high-k oxides is important for planar or complex shaped optics or surfaces. It provides opportunities for doping metal oxides to ternary, quaternary or other mixed metal oxides on glass, encapsulants or other substrates that facilitate diffusional movement of dopant species.

Assembly and Photocarrier Dynamics of Heterostructured Nanocomposite Photoanodes from Multicomponent Colloidal Nanocrystals

Multicomponent oxides and their heterostructures are rapidly emerging as promising light absorbers to drive oxidative chemistry. To fully exploit their functionality, precise tuning of their composition and structure is crucial. Here, we report a novel solution-based route to nanostructured bismuth vanadate (BiVO4) that facilitates the assembly of BiVO4/metal oxide (TiO2, WO3, and Al2O3) nanocomposites in which the morphology of the metal oxide building blocks is finely tailored. The combination of transient absorption spectroscopy-spanning from picoseconds to second time scales-and photoelectrochemical measurements reveals that the achieved structural tunability is key to understanding and directing charge separation, transport, and efficiency in these complex oxide heterostructured films.

VO₂ nanorods for efficient performance in thermal fluids and sensors

VO2 (B) nanorods with average width ranging between 50-100 nm are synthesized via a hydrothermal method and the post hydrothermal treatment drying temperature is found to be influential in their overall phase and growth morphology evolution. The nanorods with unusually high optical bandgap for a VO2 material are effective in enhancing the thermal performance of ethylene glycol nanofluids over a wide temperature range as is indicated by the temperature dependent thermal conductivity measurements. Humidity and LPG sensors fabricated using the VO2 (B) nanorods bear testament to their efficient sensing performance, which can be partially attributed to the mesoporous nature of the nanorods.

In situ synthesis of VO2for tunable mid-infrared photonic devices

Tunable PhCs were fabricated by inclusion of VO₂particles in high aspect ration MIR microtube arrays.

Surface adsorption and electrochemical reduction of 2,4,6-trinitrotoluene on vanadium dioxide

The electrochemical reduction of 2,4,6-trinitrotoluene (TNT) was investigated using films of vanadium dioxide. Three distinct reduction peaks were observed in the potential range of -0.50 to -0.90 V (vs an Ag/AgCl reference electrode), corresponding to the electrochemical reduction of the three nitro-groups on the TNT molecule. Adsorptive stripping voltammetry was performed to achieve detection down to 1 μg/L (4.4 nM), revealing a linear response to TNT concentration. These results are the first describing the use of VO2 films as an electrochemical sensor and open new avenues for further electrochemical research using this unique material.