Engineering of Active and Passive Loss in High-Quality-Factor Vanadium Dioxide-Based BIC Metasurfaces.

Active functionalities of metasurfaces are of growing interest in nanophotonics. The main strategy employed to date is spectral resonance tuning affecting predominantly the far-field response. However, this barely influences other essential resonance properties like near-field enhancement, signal modulation, quality factor, and absorbance, which are all vital for numerous applications. Here we introduce an active metasurface approach that combines temperature-tunable losses in vanadium dioxide with far-field coupling tunable symmetry-protected bound states in the continuum. This method enables exceptional precision in independently controlling both radiative and nonradiative losses. Consequently, it allows for the adjustment of both the far-field response and, notably, the near-field characteristics like local field enhancement and absorbance. We experimentally demonstrate continuous tuning from under- through critical- to overcoupling, achieving quality factors of 200 and a relative switching contrast of 78%. Our research marks a significant step toward highly tunable metasurfaces, controlling both near- and far-field properties.

Coordination Compound Guided a Novel Composite Film with Zn2V2O7 Nanoflake and VO2@Zn2V2O7 Nanoparticle for Enhanced Thermochromism Smart Window.

Thermochromic vanadium dioxide (VO2) can intelligently modulate the transmittance of indoor solar radiation to reduce the energy consumption of air conditioning in buildings. Nevertheless, it remains a great challenge to simultaneously improve the luminous transmittance (Tlum) and solar modulation ability (ΔTsol) of VO2. In this study, a novel approach is employed utilizing a coordination compound to finely tune the growth of a VO2 based composite film, yielding a hierarchical film comprising Zn2V2O7 nanoflakes and VO2@Zn2V2O7 core-shell nanoparticles. Remarkably, the resulting composite films showcase exceptional optical performance, achieving a Tlum of up to 73.0% and ΔTsol of 15.7%. These outcomes are attributed to the antireflection properties inherent in the nanoflake structure and the localized surface plasmon resonance of well-dispersed VO2 nanoparticles. In addition, the Zn2V2O7-VO2 film demonstrates remarkable environmental durability, retaining 90% of its initial ΔTsol even after undergoing aging at 100 °C under 50% relative humidity for a substantial period of 30 days - a durability equivalent to ≈20 years under ambient conditions. This work not only achieves a harmonious balance between Tlum and ΔTsol but also introduces a promising avenue for the design of distinctive composite nanostructures.

Boosting Polysulfide Redox Kinetics by Temperature-Induced Metal-Insulator Transition Effect of Tungsten-Doped Vanadium Dioxide for High-Temperature Lithium-Sulfur Batteries.

The practical application of Li-S batteries is still severely restricted by poor cyclic performance caused by the intrinsic polysulfides shuttle effect, which is even more severe under the high-temperature condition owing to the inevitable increase of polysulfides' solubility and diffusion rate. Herein, tungsten-doped vanadium dioxide (W-VO2) micro-flowers are employed with first-order metal-insulator phase transition (MIT) property as a robust and multifunctional modification layer to hamper the shuttle effect and simultaneously improve the thermotolerance of the common separator. Tungsten doping significantly reduces the transition temperature from 68 to 35 °C of vanadium dioxide, which renders the W-VO2 easier to turn from the insulating monoclinic phase into the metallic rutile phase. The systematic experiments and theoretical analysis demonstrate that the temperature-induced in-suit MIT property endows the W-VO2 catalyst with strong chemisorption against polysulfides, low energy barrier for liquid-to-solid conversion, and outstanding diffusion kinetics of Li-ion under high temperatures. Benefiting from these advantages, the Li-S batteries with W-VO2 modified separator exhibit significantly improved rate and long-term cyclic performance under 50 °C. Remarkably, even at an elevated temperature (80 °C), they still exhibit superior electrochemical performance. This work opens a rewarding avenue to use phase-changing materials for high-temperature Li-S batteries.

Crystallographic Pathways to Tailoring Metal-Insulator Transition through Oxygen Transport in VO2.

The metal-insulator (MI) transition of vanadium dioxide (VO2) is effectively modulated by oxygen vacancies, which decrease the transition temperature and insulating resistance. Oxygen vacancies in thin films can be driven by oxygen transport using electrochemical potential. This study delves into the role of crystallographic channels in VO2 in facilitating oxygen transport and the subsequent tuning of electrical properties. A model system is designed with two types of VO2 thin films: (100)- and (001)-oriented, where channels align parallel and perpendicular to the surface, respectively. Growing an oxygen-deficient TiO2 layer on these VO2 films prompted oxygen transport from VO2 to TiO2. Notably, in (001)-VO2 film, where oxygen ions move along the open channels, the oxygen migration deepens the depleted region beyond that in (100)-VO2, leading to more pronounced changes in metal-insulator transition behaviors. The findings emphasize the importance of understanding the intrinsic crystal structure, such as channel pathways, in controlling ionic defects and customizing electrical properties for applications.

Reversible Switching Between Microwave Absorption and EMI Shielding of VO2 Composite Foam.

Developing lightweight composite with reversible switching between microwave (MW) absorption and electromagnetic interference (EMI) shielding is promising yet remains highly challenging due to the completely inconsistent attenuation mechanism for electromagnetic (EM) radiation. Here, a lightweight vanadium dioxide/expanded polymer microsphere composites foam (VO2/EPM) is designed and fabricated with porous structures and 3D VO2 interconnection, which possesses reversible switching function between MW absorption and EMI shielding under thermal stimulation. The VO2/EPM exhibits MW absorption with a broad effective absorption bandwidth of 3.25 GHz at room temperature (25 °C), while provides EMI shielding of 23.1 dB at moderately high temperature (100 °C). This reversible switching performance relies on the porous structure and tunability of electrical conductivity, complex permittivity, and impedance matching, which are substantially induced by the convertible crystal structure and electronic structure of VO2. Finite element simulation is employed to qualitatively investigate the change in interaction between EM waves and VO2/EPM before and after the phase transition. Moreover, the application of VO2/EPM is demonstrated with a reversible switching function in controlling wireless transmission on/off, showcasing its excellent cycling stability. This kind of smart material with a reversible switching function shows great potential in next-generation electronic devices.

Thermochromic Vanadium Dioxide Nanostructures for Smart Windows and Radiative Cooling.

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

Plasmonic switches based on VO2as the phase change material.

In this paper, a comprehensive review of the recent advancements in the design and development of plasmonic switches based on vanadium dioxide (VO2) is presented. Plasmonic switches are employed in applications such as integrated photonics, plasmonic logic circuits and computing networks for light routing and switching, and are based on the switching of the plasmonic properties under the effect of an external stimulus. In the last few decades, plasmonic switches have seen a significant growth because of their ultra-fast switching speed, wide spectral tunability, ultra-compact size, and low losses. In this review, first, the mechanism of the semiconductor to metal phase transition in VO2is discussed and the reasons for employing VO2over other phase change materials for plasmonic switching are described. Subsequently, an exhaustive review and comparison of the current state-of-the-art plasmonic switches based on VO2proposed in the last decade is carried out. As the phase transition in VO2can be activated by application of temperature, voltage or optical light pulses, this review paper has been categorized into thermally-activated, electrically-activated, and optically-activated plasmonic switches based on VO2operating in the visible, near-infrared, infrared and terahertz frequency regions.

Photothermally Activated Artificial Neuromorphic Synapses.

Biological nervous systems rely on the coordination of billions of neurons with complex, dynamic connectivity to enable the ability to process information and form memories. In turn, artificial intelligence and neuromorphic computing platforms have sought to mimic biological cognition through software-based neural networks and hardware demonstrations utilizing memristive circuitry with fixed dynamics. To incorporate the advantages of tunable dynamic software implementations of neural networks into hardware, we develop a proof-of-concept artificial synapse with adaptable resistivity. This synapse leverages the photothermally induced local phase transition of VO2 thin films by temporally modulated laser pulses. Such a process quickly modifies the conductivity of the film site-selectively by a factor of 500 to "activate" these neurons and store "memory" by applying varying bias voltages to induce self-sustained Joule heating between electrodes after activation with a laser. These synapses are demonstrated to undergo a complete heating and cooling cycle in less than 120 ns.

Advanced Functional Carbon Nitride by Implanting Semi-Isolated VO2 Active Sites for Photocatalytic H2 Production and Organic Pollutant Degradation.

It is critical to facilitate surface interaction for liquid-solid two-phase photocatalytic reactions. This study demonstrates more advanced, efficient, and rich molecular-level active sites to extend the performance of carbon nitride (CN). To achieve this, semi-isolated vanadium dioxide is obtained by controlling the growth of non-crystalline VO2 anchored into sixfold cavities of the CN lattice. As a proof-of-concept, the experimental and computational results solidly corroborate that this atomic-level design has potentially taken full advantage of two worlds. The photocatalyst comprises the highest dispersion of catalytic sites with the lowest aggregation, like single-atom catalysts. It also demonstrates accelerated charge transfer with the boosted electron-hole pairs, mimicking heterojunction photocatalysts. Density functional theory calculations show that single-site VO2 anchored into the sixfold cavities significantly elevates the Fermi level, compared with the typical heterojunction. The unique features of semi-isolated sites result in a high visible-light photocatalytic H2 production of 645 µmol h-1 g-1 with only 1 wt% Pt. They also represent an excellent photocatalytic degradation for rhodamine B as well as tetracycline, surpassing the activities obtained from many conventional heterojunctions. This study presents exciting opportunities for the design of new heterogeneous metal oxide for a variety of reactions.

Chemical Modulation of Metal-Insulator Transition toward Multifunctional Applications in Vanadium Dioxide Nanostructures.

The metal-insulator transition (MIT) of vanadium dioxide (VO2 ) has been of great interest in materials science for both fundamental understanding of strongly correlated physics and a wide range of applications in optics, thermotics, spintronics, and electronics. Due to the merits of chemical interaction with accessibility, versatility, and tunability, chemical modification provides a new perspective to regulate the MIT of VO2 , endowing VO2 with exciting properties and improved functionalities. In the past few years, plenty of efforts have been devoted to exploring innovative chemical approaches for the synthesis and MIT modulation of VO2 nanostructures, greatly contributing to the understanding of electronic correlations and development of MIT-driven functionalities. Here, this comprehensive review summarizes the recent achievements in chemical synthesis of VO2 and its MIT modulation involving hydrogen incorporation, composition engineering, surface modification, and electrochemical gating. The newly appearing phenomena, mechanism of electronic correlation, and structural instability are discussed. Furthermore, progresses related to MIT-driven applications are presented, such as the smart window, optoelectronic detector, thermal microactuator, thermal radiation coating, spintronic device, memristive, and neuromorphic device. Finally, the challenges and prospects in future research of chemical modulation and functional applications of VO2 MIT are also provided.

Nanostructured Vanadium Dioxide Materials for Optical Sensing Applications.

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

Actively Tunable Metasurfaces via Plasmonic Nanogap Cavities with Sub-10-nm VO2 Films.

Actively tunable optical materials integrated with engineered subwavelength structures could enable novel optoelectronic devices, including reconfigurable light sources and tunable on-chip spectral filters. The phase-change material vanadium dioxide (VO2) provides a promising solid-state solution for dynamic tuning; however, previous demonstrations have been limited to thicker and often rough VO2 films or require a lattice-matched substrate for growth. Here, sub-10-nm-thick VO2 films are realized by atomic layer deposition (ALD) and integrated with plasmonic nanogap cavities to demonstrate tunable, spectrally selective absorption across 1200 nm in the near-infrared (NIR). Upon inducing the phase transition via heating, the absorption resonance is blue-shifted by as much as 60 nm. This process is reversible upon cooling and repeatable over more than ten temperature cycles. Dynamic, ultrathin VO2 films deposited by ALD, as demonstrated here, open up new potential architectures and applications where VO2 can be utilized to provide reconfigurability including three-dimensional, flexible and large-area structures.

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

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

NH4 + Deprotonation at Interfaces Induced Reversible H3 O+ /NH4 + Co-insertion/Extraction.

Ion insertions always involve electrode-electrolyte interface process, desolvation for instance, which determines the electrochemical kinetics. However, it's still a challenge to achieve fast ion insertion and investigate ion transformation at interface. Herein, the interface deprotonation of NH4 + and the introduced dissociation of H2 O molecules to provide sufficient H3 O+ to insert into materials' structure for fast energy storages are revealed. Lewis acidic ion-NH4 + can, on one hand provide H3 O+ itself via deprotonation, and on the other hand hydrolyze with H2 O molecules to produce H3 O+ . In situ attenuated total reflection-Fourier transform infrared ray method probed the interface accumulation and deprotonation of NH4 + , and density functional theory calculations manifested that NH4 + tend to thermodynamically adsorb on the surface of monoclinic VO2 , and deprotonate to provide H3 O+ . In addition, the inserted NH4 + has a positive effect for stabilizing the VO2 (B) structure. Therefore, high specific capacity (>300 mAh g-1 ) and fast ionic insertion/extraction (<20 s) can be realized in VO2 (B) anode. This interface derivation proposes a new path for designing proton ion insertion/extraction in mild electrolyte.

Liquid Precursor-Guided Phase Engineering of Single-Crystal VO2 Beams.

The study of VO2 flourishes due to its rich competing phases induced by slight stoichiometry variations. However, the vague mechanism of stoichiometry manipulation makes the precise phase engineering of VO2 still challenging. Here, stoichiometry manipulation of single-crystal VO2 beams in liquid-assisted growth is systematically studied. Contrary to previous experience, oxygen-rich VO2 phases are abnormally synthesized under a reduced oxygen concentration, revealing the important function of liquid V2 O5 precursor: It submerges VO2 crystals and stabilizes their stoichiometric phase (M1) by isolating them from the reactive atmosphere, while the uncovered crystals are oxidized by the growth atmosphere. By varying the thickness of liquid V2 O5 precursor and thus the exposure time of VO2 to the atmosphere, various VO2 phases (M1, T, and M2) can be selectively stabilized. Furthermore, this liquid precursor-guided growth can be used to spatially manages multiphase structures in single VO2 beams, enriching their deformation modes for actuation applications.

Unlocking the High Capacity Ammonium-Ion Storage in Defective Vanadium Dioxide.

Aqueous ammonium-ion storage has been considered a promising energy storage competitor to meet the requirements of safety, affordability, and sustainability. However, ammonium-ion storage is still in its infancy in the absence of reliable electrode materials. Here, defective VO2 (d-VO) is employed as an anode material for ammonium-ion batteries with a moderate transport pathway and high reversible capacity of ≈200 mAh g-1 . Notably, an anisotropic or anisotropic behavior of structural change of d-VO between c-axis and ab planes depends on the state of charge (SOC). Compared with potassium-ion storage, ammonium-ion storage delivers a higher diffusion coefficient and better electrochemical performance. A full cell is further fabricated by d-VO anode and MnO2 cathode, which delivers a high energy density of 96 Wh kg-1 (based on the mass of VO2 ), and a peak energy density of 3254 W kg-1 . In addition, capacity retention of 70% can be obtained after 10 000 cycles at a current density of 1 A g-1 . What's more, the resultant quasi-solid-state MnO2 //d-VO full cell based on hydrogel electrolyte also delivers high safety and decent electrochemical performance. This work will broaden the potential applications of the ammonium-ion battery for sustainable energy storage.

Phytotoxicity of VO2 nanoparticles with different sizes to pea seedlings.

Vanadium dioxide nanoparticles (VO2 NPs) have been massively produced due to their excellent metal-insulator transition characteristics for various applications. Pilot studies indicated the toxicity of VO2 NPs to bacteria and mammalian cells, but the environmental hazards of VO2 NPs to plants have been unrevealed to date. In this study, we reported the inhibitive effects of VO2 NPs to the growth and photosynthesis of pea seedlings. Laboratory synthesized monoclinic VO2 NPs (N-VO2), commercial nanosized VO2 NPs (S-VO2), and commercial microsized VO2 particles (M-VO2) were carefully characterized for environmental toxicity evaluations. VO2 particles were supplemented to culture medium for seed germination and seedling growth. All three VO2 samples did not affect the germination rates of pee seeds, while serious growth inhibition of pea seedlings was observed at 10 mg/L for S-VO2 and N-VO2, and 100 mg/L for M-VO2. VO2 particles had no impact on the chlorophyll contents, but the photosynthesis of leaf was significantly decreased following the consequence of N-VO2 > S-VO2 > M-VO2. The inhibition of photosynthesis was attributed to the damage of acceptor side of photosystem II by VO2 particles at high concentrations. Abundant bioaccumulations of vanadium in roots aroused oxidative damage and changed the root structure. Our results collectively indicated that the phytotoxicity of VO2 NPs was related to the concentration, size and crystalline degree.

Nanotextured Dynamics of a Light-Induced Phase Transition in VO2.

We investigate transient nanotextured heterogeneity in vanadium dioxide (VO2) thin films during a light-induced insulator-to-metal transition (IMT). Time-resolved scanning near-field optical microscopy (Tr-SNOM) is used to study VO2 across a wide parameter space of infrared frequencies, picosecond time scales, and elevated steady-state temperatures with nanoscale spatial resolution. Room temperature, steady-state, phonon enhanced nano-optical contrast reveals preexisting "hidden" disorder. The observed contrast is associated with inequivalent twin domain structures. Upon thermal or optical initiation of the IMT, coexisting metallic and insulating regions are observed. Correlations between the transient and steady-state nano-optical textures reveal that heterogeneous nucleation is partially anchored to twin domain interfaces and grain boundaries. Ultrafast nanoscopic dynamics enable quantification of the growth rate and bound the nucleation rate. Finally, we deterministically anchor photoinduced nucleation to predefined nanoscopic regions by locally enhancing the electric field of pump radiation using nanoantennas and monitor the on-demand emergent metallicity in space and time.

A Novel Route for the Easy Production of Thermochromic VO2 Nanoparticles.

In this work, a simple, fast and dry method for the fabrication of a thermochromic product with a high load of VO2 (M1) consisting of the controlled heat treatment of pure vanadium nanoparticles in air is presented. After a complete design of experiments, it is concluded that the most direct way to attain the maximum transformation of V into VO2 (M1) consists of one cycle with a fast heating ramp of 42 °C s-1 , followed by keeping 700 °C for 530-600 seconds, and a subsequent cooling at 0.05 °C s-1 . Careful examination of these results lead to a second optimum, even more suitable for industrial production (quicker and less energy-intensive because of its lower temperatures and shorter times), consisting of subjecting V to two consecutive cycles of temperatures and times (625 °C for 5 minutes) with similar preheating (42 °C s-1 ) but a much faster postcooling (∼ 8 °C s-1 ). These green reactions only use the power for heating a tube open to atmosphere and a vanadium precursor; without assistance of reactive gases or catalysts, and no special vacuum or pressure requirements. The best products present similar thermochromic properties but higher thermal stability than commercial VO2 particles. These methods can be combined with VO2 doping.

Nanoscale axial position and orientation measurement of hexagonal boron nitride quantum emitters using a tunable nanophotonic environment.

Color centers in hexagonal boron nitride (hBN) have emerged as promising candidates for single-photon emitters (SPEs) due to their bright emission characteristics at room temperature. In contrast to mono- and few-layeredhBN, color centers in multi-layered flakes show superior emission characteristics such as higher saturation counts and spectral stability. Here, we report a method for determining both the axial position and three-dimensional dipole orientation of SPEs in thickhBN flakes by tuning the photonic local density of states using vanadium dioxide (VO2), a phase change material. Quantum emitters under study exhibit a strong surface-normal dipole orientation, providing some insight on the atomic structure ofhBN SPEs, deeply embedded in thick crystals. Next, we optimized a hot pickup technique to reproducibly transfer thehBN flake from VO2/sapphire substrate onto SiO2/Si substrate and relocated the same emitters. Our approach serves as a practical method to systematically characterize SPEs inhBN prior to integration in quantum photonics systems.