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VO2 is one of the most studied vanadium oxides because it undergoes a reversible metal-insulator transition (MIT) upon heating with a critical temperature of around 340 K. One of the most overlooked aspects of VO2 is the band’s anisotropy in the metallic phase when the Fermi level is crossed by two bands: π* and d||. They are oriented perpendicularly in one respect to the other, hence generating anisotropy. One of the parameters tuning MIT properties is the unbalance of the electron population of π* and d|| bands that arise from their different energy position with respect to the Fermi level. In systems with reduced dimensionality, the electron population disproportion is different with respect to the bulk leading to a different anisotropy. Investigating such a system with a band-selective spectroscopic tool is mandatory. In this manuscript, we show the results of the investigation of a single crystalline 8 nm VO2/TiO2(101) film. We report on the effectiveness of linearly polarized resonant photoemission (ResPES) as a band-selective technique probing the intrinsic anisotropy of VO2.

SiO2/TiO2 composite films have been frequently used to realize the functions of self-cleaning and antireflection. Increasing the TiO2 volume ratio in SiO2/TiO2 composite film is beneficial to enhance the self-cleaning effect, while high TiO2 content leads to a strong Rayleigh scattering and depresses the antireflective performance, resulting in a bottleneck problem for the dual-functional application. Here, we have achieved a high-quality TiO2 nanotubes film with excellent antireflective and near-perfect self-cleaning performances. Ultrasound assisted pickling method has been developed to effectively prepare the well-dispersed protonated titanate nanotubes colloid. After spin-coating and annealing treatment, glass substrate coated with double-side TiO2 nanotubes film has a peak transmittance of 99.2 % and average transmittance of 97.4 % at 400−800 nm. Ultra-high porosity of TiO2 nanotubes film (80 %) and ultra-fine size of TiO2 nanotubes (8.6 nm in outer diameter) lead to excellent antireflective performance. With high UV absorptivity (80 % at 254 nm) and formal quantum efficiency of stearic acid (10.9 × 10−3), TiO2 nanotubes film shows near-perfect self-cleaning performance. A persistent anti-fogging ability is also presented. This study demonstrates the feasibility to fabricate pure TiO2 antireflective coating for glass substrate, extends application field of the classic TiO2 nanotubes, and sheds lights on the practical applications of high-powered TiO2 nanotube-based multi-functional films.

Oxygen vacancy is a common defect in metal oxides that causes appreciable damage to material properties and performance. Removing bulk defects of oxygen vacancy (VO) typically needs harsh conditions such as high-temperature annealing. Supported by first-principles simulations, we propose an effective strategy of removing VO bulk defects in metal oxides by evaporating hydrogen dopants. The hydrogen dopants not only lower the migration barrier of VO but also push VO away due to their repulsive interaction. The coevaporation mechanism was supported by a neural networks potential-based molecular dynamics simulation, which shows that the migration of hydrogen dopants from inside to surface at 400 K promotes the migration of VO as well. Our proof-of-concept study suggests an alternative and efficient way of modulating oxygen vacancies in metal oxides via reversible hydrogen doping.

As a typical correlated metal oxide, vanadium dioxide (VO2) shows specific metal-insulator transition (MIT) properties and demonstrates great potential applications in ultrafast optoelectronic switch, resistive memory, and neuromorphic devices. Effective control of the MIT process is essential for improving the device performance. In the current study, we have first proposed a photoassisted ion-doping method to modulate the phase transition of the VO2 layer based on the photovoltaic effect and electron-ion synergic doping in acid solution. Experimental results show that, for the prepared n-VO2/p-GaN nanojunction, this photoassisted strategy can effectively dope the n-VO2 layer by H+, Al3+, or Mg2+ ions under light radiation and trigger consecutive insulator-metal-insulator transitions. If combined with standard lithography or electron beam etching processes, selective doping with nanoscale size area can also be achieved. This photoassisted doping method not only shows a facile route for MIT modulation via a doping route under ambient conditions but also supplies some clues for photosensitive detection in the future.

The utilization of clean hydrogen energy is becoming more feasible for the sustainable development of this society. Considering the safety issues in the hydrogen production, storage, and utilization, a sensitive hydrogen sensor for reliable detection is essential and highly important. Though various gas sensor devices are developed based on tin oxide, tungsten trioxide, or other oxides, the relatively high working temperature, unsatisfactory response time, and detection limitation still affect the extensive applications. In the current study, an amorphous tungsten trioxide (a-WO

The electrochromic WO

Optical control of exotic properties in strongly correlated electron materials is very attractive owing to their potential applications in optical and electronic devices. Herein, we demonstrate a vertical heterojunction made of a correlated electron oxide thin film VO2 and a conductive 0.05 wt% Nb-doped TiO2 single crystal, whose metal-insulator transition (MIT) across the nanoscale heterointerface can be efficiently modulated by visible light irradiation. The magnitude of the MIT decreases from ~350 in the dark state to ~7 in the illuminated state, obeying a power law with respect to the light power density. The junction resistance is switched in a reversible and synchronous manner by turning light on and off. The optical tunability of it is also exponentially proportional to the light power density, and a 320-fold on/off ratio is achieved with an irradiance of 65.6 mW cm−2 below the MIT temperature. While the VO2 thin film is metallic above the MIT temperature, the optical tunability is remarkably weakened, with a one-fold change remaining under light illumination. These results are co-attributed to a net reduction (~15 meV) in the apparent barrier height and the photo-carrier-injection-induced metallization of the VO2 heterointerface through a photovoltaic effect, which is induced by deep defect level transition upon the visible light irradiance at low temperature. Additionally, the optical tunability is minimal, resulting from the quite weak modulation of the already metallic band structure in the Schottky-type junction above the MIT temperature. This work enables a remotely optical scheme to manipulate the MIT, implying potential uncooled photodetection and photoswitch applications.

The sun (∼6,000 K) and outer space (∼3 K) are two significant renewable thermodynamic resources for human beings on Earth. The solar thermal conversion by photothermal (PT) and harvesting the coldness of outer space by radiative cooling (RC) have already attracted tremendous interest. However, most of the PT and RC approaches are static and monofunctional, which can only provide heating or cooling respectively under sunlight or darkness. Herein, a spectrally self-adaptive absorber/emitter (SSA/E) with strong solar absorption and switchable emissivity within the atmospheric window (i.e., 8 to 13 μm) was developed for the dynamic combination of PT and RC, corresponding to continuously efficient energy harvesting from the sun and rejecting energy to the universe. The as-fabricated SSA/E not only can be heated to ∼170 °C above ambient temperature under sunshine but also be cooled to 20 °C below ambient temperature, and thermal modeling captures the high energy harvesting efficiency of the SSA/E, enabling new technological capabilities.

Oxygen vacancies (VO), a common type of point defects in metal oxides materials, play important roles on the physical and chemical properties. To obtain stoichiometric oxide crystal, the pre-existing VO is always removed via careful post-annealing treatment at high temperature in air or oxygen atmosphere. However, the annealing conditions is difficult to control and the removal of VO in bulk phase is restrained due to high energy barrier of VO migration. Here, we selected VO2 crystal film as the model system and developed an alternative annealing treatment aided by controllable hydrogen doping, which can realizes effective removal of VO defects in VO2-{\delta} crystal at lower temperature. This finding is attributed to the hydrogenation accelerated oxygen vacancies recovery in VO2-{\delta} crystal. Theoretical calculations revealed that the H-doping induced electrons are prone to accumulate around the oxygen defects in VO2-{\delta} film, which facilitates the diffusion of VO 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., Comment: 14 pages, 4 figures

As a typical correlated metal oxide, vanadium dioxide (VO

Charge doping is an effective way to induce metal-insulate transition (MIT) in correlated materials for many important utilizations, which is however practically limited by problem of low stability. In this study, we have achieved pronounced phase modulation and stabilized the metallic state of monoclinic vanadium dioxide (VO2) at room temperature, via a novel electron-proton co-doping mechanism driven by surface absorption of self-assembled L-ascorbic acid (AA) molecules. The ionized AA- species in solution donate effective electrons to the adsorbed VO2 surface, which then electrostatically attract surrounding protons to penetrate, and eventually results in stable hydrogen-doped metallic VO2. The variations of phase and electronic structures as well as the electron occupancy of V-3d/O-2p hybrid orbitals were examined by synchrotron characterizations and first-principle theoretical simulations, which explain the formation of stable metallic state. Importantly, the adsorbed molecules protect hydrogen dopants from escaping out of lattice and thereby stabilize the metallic phase for VO2. Such an electron-proton co-doping mechanism driven by suitable molecules absorption would open a new door for engineering properties of correlated oxide materials., 25 pages, 5 figures

Chiral materials can exhibit different levels of transmission for opposite propagation directions of the same electromagnetic wave. Here we demonstrate thermal switching of asymmetric transmission of linearly polarized terahertz waves. The effect is observed in a terahertz metamaterial containing 3D-chiral metallic inclusions and achiral vanadium dioxide inclusions. The chiral structure exhibits pronounced asymmetric transmission at room temperature when vanadium dioxide is in its insulator phase. As the metamaterial is heated, the insulator-to-metal phase transition of vanadium dioxide effectively renders the structure achiral and the transmission asymmetry vanishes. We demonstrate the effect numerically and experimentally, describe it analytically and explain the underlying physical mechanism based on simulated surface current distributions. Potential applications include directionally asymmetric active devices as well as intensity and polarization modulators for electromagnetic waves.

Radiative cooling is a natural phenomenon in which earthbound objects emit energy into outer space through thermal radiation. Emitters should have high spectral reflectivity in the solar radiation band and high spectral absorptivity (emissivity) in the infrared band, especially the atmospheric window band (i.e., 0.3–3 and 8–13 µm), to efficiently utilize radiative cooling. In this work, we proposed two types of surfaces to achieve such spectral properties based on the analysis of selective and broadband radiator cooling performance. One type is a specular surface made by ultra-white glass-plated silver (Ag). The other type is a diffuse surface made by spraying zinc phosphate sodium (NaZnPO4) particles on aluminum (Al) substrate. Sample tests showed that both surfaces can effectively reflect sunlight and strongly exchange heat with outer space via thermal radiation. A radiative cooling apparatus with simple structure was built, and the equilibrium temperatures of the surfaces were tested. Both surfaces achieved consecutive 24 h sub-ambient radiative cooling effect. The equilibrium temperatures of the specular and diffuse surfaces at nighttime were lower than those of ambient air by approximately 9.8 °C and 7.3 °C, respectively. When solar irradiance exceeded 430 W m−2 at noontime, the two surfaces reached equilibrium temperatures of 2.5 °C and 1.5 °C lower than those of the surroundings. Both surfaces showed daytime radiative cooling characteristics and were superior to existing radiative coolers in terms of cost and fabrication.

The Metal-Insulator transition (MIT) in VO2 is characterized by the complex interplay among lattice, electronic and orbital degrees of freedom. In this contribution we investigated the strain-modulation of the orbital hierarchy and the influence over macroscopic properties of the metallic phase of VO2 such as Fermi Level (FL) population and metallicity, i.e., the material ability to screen an electric field, by means of temperature-dependent X-ray Absorption Near Edge Structure (XANES) and Resonant Photoemission spectroscopy (ResPES). We demonstrate that the MIT in strained VO2 is of the Filling Control type, hence it is generated by electron correlation effects. In addition, we show that the MIT in Nanostructured (NS) disordered VO2, where the structural phase transition is quenched, is driven by electron correlation. Therefore a fine tuning of the correlation could lead to a precise control of the transition features.

The sun (~6000 K) and outer space (~3 K) are two significant renewable thermodynamic resources for human beings on Earth. The solar thermal conversion by photothermal (PT) and harvesting the coldness of outer space by radiative sky cooling (RSC) have already attracted tremendous interest. However, most of the approaches for PT and RSC are static and mono-functional, which can only provide heating or cooling under sunlight or darkness. Here, we develop a spectrally self-adaptive absorber/emitter (SSA/E) with strong solar absorption and switchable emissivity within the atmospheric window (i.e., 8-13μm) for the dynamic combination of PT and RSC, corresponding to continuously efficient energy harvesting from the sun and universe. The as-fabricated SSA/E not only can be heated to ~170°C above ambient temperature under sunshine and but also be cooled to 20°C below ambient temperature, and thermal modeling captures the high energy harvesting efficiency of the SSA/E, enabling new technological capabilities.

We present and discuss the role of nanoparticles size and stoichiometry over the local atomic environment of nanostructured VOx films. The samples have been characterized in situ using X-ray absorption near-edge structure (XANES) spectroscopy identifying the stoichiometry-dependent fingerprints of disordered atomic arrangement. In vanadium oxides, the ligand atoms arrange according to a distorted octahedral geometry depending on the oxidation state, e.g. trigonal distortion in V2O3 and tetragonal distortion in bulk VO2. We demonstrate, taking VO2 as a case study, that as a consequence of the nanometric size of the nanoparticles, the original ligands symmetry of the bulk is broken resulting in the coexistence of a continuum of distorted atomic conformations. The resulting modulation of the electronic structure of the nanostructured VOx as a function of the oxygen content reveals a stoichiometry-dependent increase of disorder in the ligands matrix. This work shows the possibility to produce VOx nanostructured films accessing new disordered phases and provides a unique tool to investigate the complex matter.

Searching for cost-effective photothermal material that can harvest the full solar spectrum is critically important for solar-driven water evaporation. Metal oxides are cheap materials but cannot cover the full solar spectrum. Here we prepared a hydrogenated metal oxide (H

Hydrogen doping of metal oxide semiconductors is promising for manipulation of their properties toward various applications. Yet it is quite challenging because it requires harsh reaction conditions and expensive metal catalysts. Meanwhile, acids as a cheap source of protons have long been unappreciated. Here, we develop a sophisticated acid-metal treatment for tunable hydrogenation of metal oxides at ambient conditions. Using first-principles simulations, we first show that, with proper work function difference between the metal and metal oxide, H-diffusion into negatively charged metal oxide can be well controlled, resulting in tunable H-doping of metal oxides with quasi-metal characteristics. This has been verified by proof-of-concept experiments that achieved the controllable hydrogenation of WO3 using Cu and hydrochloric acid at ambient conditions. Further, H-doping of other metal oxides (TiO2/Nb2O5/MoO3) has been achieved by metal-acid treatment and induced a change in properties. Our work provides a promising way to tailor metal oxides via tunable H-doping.

The VO2 is a 3d1 electron system that undergoes a reversible metal&ndash, insulator transition (MIT) triggered by temperature and characterized by an interplay between orbital, charge and lattice degrees of freedom. The characterization of the MIT features are therefore extremely challenging and powerful investigation tools are required. In this work, we demonstrate how a combination of resonant photoemission and constant initial state (CIS) spectroscopy can be used as an orbital selective probe of the MIT studying three different VO2/TiO2(001) strained films. The CIS spectra of the V 3d and V 3p photo-electrons shows sensitivity to different orbital contribution and the presence of a spin polarized band close to the Fermi level.

Doping is an effective way to tune the property of metal oxides1-5, for achieving functional oxide electronics6-8. Previously we developed a controllable hydrogen doping technology at ambient conditions by use of electron-proton synergistic doping strategy, which enables one to get rid of high-temperature/pressure treatments required by traditional technologies9. Here, based on this facile doping route, we achieve a visual and reversible insulator-metal transition (MIT) for tungsten trioxide (WO3) film. Its outstanding spatial selection is comparable to standard UV lithography, which shows the potential of becoming a viable way for rewritable WO3 grating device fabrication. Furthermore, the period of the obtained WO3 structural grating can also be easily changed for requirement by doping area selection. This advanced doping technology opens up alternative approaches for developing not only optical devices, but also rewritable ions devices and integrated circuits for various oxide electronics., Comment: 16 pages, 4 figures

The use of electric-field to manipulate the transport properties of correlated oxides is promising for achieving nano-functional electronic devices. As a typical correlated oxide material, vanadium dioxide (VO2) displays a special metal-insulator transition (MIT) at about 340 K. In this work, we select a water-free electrolyte gel, Li+/propylene carbonate, to achieve a non-volatile and reversible Li-doping and phase modulation in nano-VO2 crystal film under gating voltages. Synchrotron characterizations indicate the originally insulating VO2 undergoes an effective structure change and an increased V 3d-O 2p hybrid t2g-orbital occupancy, which is responsible for the metallic state formation as proved by first-principle calculation. The field-driving Li-doping shows pronounced facet dependence, indicating the existence of energetically favored channels for Li-ions diffusion along V-V chains as proved by dynamic stimulation. Our current findings will supply opportunities for the electric-field control of atomic sieve, correlated ionitronics and synaptic transistor.

As a typical strong correlated material, monoclinic vanadium dioxide (VO2) shows a distinct metal–insulator transition (MIT) near room temperature. The MIT-induced phase separations or other emergent phenomena are closely associated with the grain size and boundaries, especially within nanoscale range. Until now, the pure monoclinic VO2 nanoclusters with uniform grain size and high dispersion have rarely been reported because of the synthesis difficulty and nanocrystal agglomeration. In the current study, we have successfully fabricated well-dispersed monoclinic VO2 nanoclusters with diameters less than 5 nm on various substrates by gas-phase cluster beam deposition. Typical MIT properties including the resistance change and infrared switching effect are observed for the obtained crystal VO2 nanoclusters. In addition, the dispersed VO2 nanoclusters demonstrated excellent infrared response and the response becomes more pronounced as decreasing the spacing of interdigital electrodes, which should be promisi...

The holes doping effect on the metal–insulator transition (MIT) behavior of VO2 film is investigated via the tetrafluorotetracyanoquinodimethane (F4TCNQ) molecules adsorption induced surface charge transfer. Comparing with the MIT process of pristine VO2 film, a critical temperature decrease of about 4 °C for the F4TCNQ covered VO2 sample is observed. The MIT depression mechanism is deeply investigated based on detailed experiments including synchrotron radiation photon electronic spectroscopy (SRPES), X-ray absorption near-edge structure (XANES) spectroscopy and variable temperature Raman spectroscopy. Results indicate that the electronic structures of F4TCNQ covered VO2 sample are changed clearly due to the effective holes doping. In addition, the doped holes also change the V 3d orbital occupancy and weaken the electron–electron correlation as well, lowering the crystalline stability energy. Both of the above effects are in favor of triggering the earlier occurrence of MIT, resulting in the decrease of critical temperature.

Vanadium dioxide (VO2) nanowires/microbeams have attracted great interest recently because of their pronounced single-domain metal–insulator phase transition (MIT) behavior, which is promising for various device applications and deeper mechanism investigation. It is known that monoclinic VO2 nanostructures can be effectively prepared by a simple thermal evaporation method, while the growth of dense and ordered VO2 micro/nanowires with controlled cross sections is still a challenge. In the current study, we have selected different crystal facets as the growth template and achieved controllable growth of well-aligned VO2 micro/nanowire arrays. Combined with the crystal growth simulations, it is clarified that the epitaxial growth of VO2 micro/nanowires on TiO2 substrates is mainly dominated by both the initial crystallization mechanism and confinement of the crystal facets. The current studies not only show the potentials of fabricating VO2 micro/nanowire array-based devices but also demonstrate a facile st...

Surface morphological control of the metal-insulator transition behaviors of VO 2 epitaxial thin films is achieved by annealing substrates of (0001)-Al 2 O 3 single crystals. The well-defined terraces of the (0001)-Al 2 O 3 substrates are formed by annealing in air at 1200 °C. Correspondingly, the surface roughness dramatically decreases in the VO 2 epitaxial thin films on the annealed substrates, compared with that on the unannealed substrates. The order of magnitude of the resistivity change ratio (~ 10 2 ) of annealed samples across the metal-insulator transition (MIT) decreases by a factor of one, compared with that (~ 10 3 ) in unannealed samples. This result is ascribed to grain size effect in the VO 2 epitaxial thin films. Moreover, the MIT temperature is reduced in the annealed samples with various thickness, compared with the unannealed ones. A reduction of 14.4 K of the MIT temperature is observed in the thinnest VO 2 films on the annealed substrates, compared with the unannealed samples. This behavior results from a compressive strain along the V-V atom chains in the annealed samples, which modifies the orbital occupancy of the V 4+ ions. While increasing the film thickness, the MIT change ratio keeps on the order of magnitude 10 2 , and the MIT temperatures of the VO 2 films on the annealed substrates becomes closer and closer to those of the unannealed samples due to the weakened substrate effect. This work suggests a promising approach to decrease the MIT temperature and still maintain a moderate change ratio for the MIT, potentially enabling room-temperature electronic devices based on VO 2 thin films.

Vanadium dioxide (VO2) material shows a distinct metal–insulator transition (MIT) at the critical temperature of ∼340 K. Similar to other correlated oxides, the MIT properties of VO2 is always sensitive to those crystal defects such as oxygen vacancies. In this study, we investigated the oxygen vacancies related phase transition behavior of VO2 crystal film and systematically examined the effect of oxygen vacancies from the optical constant measurements. The results indicated that the oxygen vacancies changed not only the electron occupancy on V 3d–O 2p hybrid-orbitals, but also the electron–electron correlation energy and the related band gap, which modulated the MIT behavior and decreased the critical temperature resultantly. Our work not only provided a facile way to modulate the MIT behavior of VO2 crystal film, but also revealed the effects of the oxygen vacancies on the electronic inter-band transitions as well as the electronic correlations in driving this MIT process.

Manipulating the strain induced poly-domains and phase transition in correlated oxide material are important for high performance devices fabrication. Though the electronic transport in the strained oxide film at macroscopic scales can be directly measured, the anisotropic electronic state and the controllable phase separation cross the insulator-to-metal transition within nanoscale size are still elusive. Here, we selected VO2 crystal film as a prototypical oxide and achieved the manipulation of anisotropy electronic phase separation via injecting He+ nanobeam into VO2 film at room temperature. In addition, this nanoscale phase separation was directly visualized by infrared near-field imaging measurements, showing the pronounced and unique cR-axis dependent anisotropy on VO2 surface. Our results offered new insights towards understanding the anisotropic nanoscale phase separation in strained metal oxide films., 15 pages,5 figures

Vanadium dioxide (VO 2 ) with reversible metal-insulator transition (MIT) is a promising energy-saving material for next-generation smart windows and infrared devices. However, the specific applications are largely limited by the relatively high critical temperature as well as the non-transferable grown-substrate. Herein, we report such limitations can be overcome by directly growing VO 2 on layered mica sheets and integrating with high transparent single-walled carbon nanotube (SWNT) films. The SWNTs/VO 2 /mica hierarchical films can be peeled-off to form a free-standing ultra-thin optical window and can further be transferred to other substrates with high flexibility and transparency. By heating the SWNTs/VO 2 layer with a bias current, the MIT process of VO 2 film can be facilely modulated, achieving the reversible and dynamical regulation of the infrared transmission. Furthermore, by adjusting the bias current, it is possible to change the starting local temperature and shift the initial situation close to the “phase transition boundary”, resulting in the decreased energy barrier to trigger the MIT behavior. This fascinating strategy overcomes the high critical temperature limit of VO 2 and avoids the bottle-neck problem in practical applications of VO 2 material, which demonstrates wide applications of this kind of device in the future.

The growth of wafer-scale and uniform monoclinic VO2 film was a challenge if considering the multivalent vanadium atom and the various phase structures of VO2 compound. Directly oxidizing metallic vanadium film in oxygen gas seemed to be an easy way, while the oxidation parameters were extremely sensitive due to the critical preparation window. Here we proposed a facile thermal oxidation by water-vapor to produce wafer-scale VO2 films with high quality. Results indicated that by using the water-vapor oxidant, the temperature window for VO2 growth was greatly broadened. In addition, the obtained wafer-size VO2 film showed very uniform surface and sharp resistance change. The chemical reaction routes with water-vapor were calculated, which favored the VO2 film growth. Our results not only demonstrated that the water-vapor could be used as a modest oxidizing agent, but also showed the unique advantage for large size VO2 film preparation., 21 pages, 6 figures

Charge doping is an effective way to induce the metal-insulator transition (MIT) in correlated materials for many important utilizations, which is however practically limited by problem of low stability. An electron-proton co-doping mechanism is used to achieve pronounced phase modulation of monoclinic vanadium dioxide (VO

VO2 material is promising for developing energy-saving "smart window", owing to its thermochromic property induced by metal-insulator transition (MIT). However, its practical application is greatly limited by the relatively high critical transition temperature (~68oC), low luminous transmittance (, 19 pages, 5 figures

Hydrogenation of tungsten trioxide (WO3) can effectively modulate its material property and induce the insulator–metal transition. Due to the insertion of H atoms, a transparent WO3 film will normally change to deep-blue color because of its pronounced electrochromism. More importantly, the electronic structure will also convert from an insulator to a metallic state, showing greatly improved conductivity with an enhancement of up to six orders of magnitude. Combined with the traditional lithography route, selective hydrogenation of WO3 in microscale size is achieved based on the electron–proton synergistic effect in an acid solution. This metal-like conductive H-doped WO3 can be used as an erasable microcircuit since it will be recovered to a WO3 insulator by annealing in air at an appropriate temperature. The current study demonstrates a facile way to fabricate erasable WO3 conducting micro/nanowires, or even microcircuits, by selective hydrogenation under ambient conditions, which should be extended to other oxide materials for functional applications.

Chiral media exhibit optical activity, which manifests itself as differential retardation and attenuation of circularly polarized electromagnetic waves of opposite handedness. This effect can be described by different refractive indices for left- and right-handed waves and yields a negative index in extreme cases. Here we demonstrate active control of chirality, optical activity and refractive index. These phenomena are observed in a terahertz (THz) metamaterial based on three-dimensionally (3D) chiral metallic resonators and achiral vanadium dioxide inclusions. The chiral structure exhibits pronounced optical activity and a negative refractive index at room temperature when vanadium dioxide is in its insulating phase. Upon heating, the insulator-to-metal phase transition of vanadium dioxide effectively renders the structure achiral, resulting in absence of optical activity and a positive refractive index. The origin of the structure’s chiral response is traced to magnetic coupling between front and back of the structure, while the temperature-controlled chiral-to-achiral transition is found to correspond to a transition from magnetic to electric dipole excitations. The use of a fourfold rotationally symmetric design avoids linear birefringence and dichroism, allowing such a structure to operate as tunable polarization rotator, adjustable linear polarization converter and switchable circular polarizer.

In this work, high-quality VO2 epitaxial films were prepared on high-conductivity n-GaN (0001) crystal substrates via an oxide molecular beam epitaxy method. By fabricating a two-terminal VO2/GaN film device, we observed that the infrared transmittance and resistance of VO2 films could be dynamically controlled by an external bias voltage. Based on the hysteretic switching effect of VO2 in infrared range, an optoelectronic memory device was achieved. This memory device was operated under the “electrical writing–optical reading” mode, which shows promising applications in VO2-based optoelectronic device in the future.

High quality VO2 films with different thickness were epitaxially grown on MgF2 (001) substrates by oxide molecular beam epitaxy method. The evolution of interfacial strain was investigated by synchrotron based grazing incidence X-ray diffraction. By adjusting the incidence angles, the penetration depth of X-ray in VO2 film could be controlled and the thickness-depend lattice distortion in the epitaxial VO2 film was investigated. Due to the lattice mismatching, the pronounced tensile strain was observed in ultra-thin VO2 film. As the film thickness increasing, the interfacial strain relaxed gradually and became fully relaxed for thick VO2 films. Combined with the electric transport measurement, it was revealed that the phase transition temperature of ultra-thin VO2 film decreased greatly. The effect of interfacial strain induced phase transition modulation and the intrinsic mechanism was systematically discussed.

Herein, we report a bottom-up solvothermal route to synthesize a flexible, highly efficient MoS2@SWNT electrocatalyst for hydrogen evolution reactions (HER). Characterization revealed that branch-like MoS2 nanosheets containing sulfurrich sites were in situ uniformly dispersed on free-standing single-walled carbon nanotube (SWNT) film, which could expose more unsaturated sulfur atoms, allowing excellent electrical contact with active sites. The flexible catalyst exhibited excellent HER performance with a low overpotential (~150 mV at 10 mA/cm2) and small Tafel slope (41 mV/dec). To further explain the improved performance, the local electronic structure was investigated by X-ray absorption near-edge structure (XANES) analysis, proving the presence of unsaturated sulfur atoms and strong electronic coupling between MoS2 and SWNT. This study provides an in-situ synthetic route to create new multifunctional flexible hybridized catalysts and useful insights into the relationships among the catalyst microstructure, electronic structure, and properties.

Electrolyte gating with ionic liquids (IL) on correlated vanadium dioxide (VO2) nanowires/beams is effective to modulate the metal-insulator transition (MIT) behavior. While for macrosize VO2 film, the gating treatment shows different phase modulation process and the intrinsic mechanism is still not clear, though the oxygen-vacancy diffusion channel is always adopted for the explanation. Herein, the dynamic phase modulation of electrolyte gated VO2 films is investigated and the oxygen vacancies formation, diffusion, and recovery at the IL/oxide interface are observed. As a relatively slow electrochemical reaction, the gating effect gradually permeates from surface to the inside of VO2 film, along with an unsynchronized changes of integral electric, optical, and structure properties. First-principles-based theoretical calculation reveals that the oxygen vacancies can not only cause the structural deformations in monoclinic VO2, but also account for the MIT transition by inducing polarization charges and thereby adjusting the d-orbital occupancy. The findings not only clarify the oxygen vacancies statement of electrolyte gated VO2 film, but also can be extended to other ionic liquid/oxide systems for better understanding of the surface electrochemical stability and electronic properties modulation.

Mirrors are widely used for redirection of electromagnetic waves in optical systems, making them arguably the most irreplaceable optical component. Metamaterial-based chiral mirrors, composed of a 2D-chiral planar metallic structure backed by a conventional mirror, reflect one circular polarization without changing its handedness, while absorbing the other. Here, three types of switchable chiral mirror are demonstrated. Switching from a chiral mirror to either a conventional mirror, a handedness-preserving mirror, or a chiral mirror of opposite handedness, is realized. These advances are underpinned by switching the handedness of 2D-chiral metamaterial and the associated effect of circular conversion dichroism, which is reported here for the first time. Switching is achieved by exploiting the temperature-activated dielectric-to-metal phase transition of vanadium dioxide to modify the symmetry and chirality of the metamaterial's resonators. Current distributions explain the temperature-controlled optical properties by handedness-selective excitation of reflective electric dipole and absorbing magnetic dipole modes.

The metal-insulator transition of vanadium dioxide (VO2 ) is exceptionally sensitive to charge density and electron orbital occupancy. Thus three-terminal field-effect transistors with VO2 channels are widely adopted to control the phase transition by external gating voltage. However, current leakage, electrical breakdown, or interfacial electrochemical reactions may be inevitable if conventional solid dielectrics or ionic-liquid layers are used, which possibly induce Joule heating or doping in the VO2 layer and make the voltage-controlled phase transition more complex. Here, a triboelectric nanogenerator (TENG) and a VO2 film are combined for a novel TENG-VO2 device, which can overcome the abovementioned challenges and achieve electron-doping-induced phase modulation. By taking advantage of the TENG structure, electrons can be induced in the VO2 channel and thus adjust the electronic states of the VO2 , simultaneously. The modulation degree of the VO2 resistance depends on the temperature, and the major variation occurs when the temperature is in the phase-transition region. The accumulation of electrons in the VO2 channel also is simulated by finite element analysis, and the electron doping mechanism is verified by theoretical calculations. The results provide a promising approach to develop a novel type of tribotronic transistor and new electronic doping technology.

Vanadium dioxide (VO

Hydrogenation is an effective way to tune material property1-5. Traditional techniques for doping hydrogen atoms into solid materials are very costly due to the need for noble metal catalysis and high-temperature/pressure annealing treatment or even high energy proton implantation in vacuum condition5-8. Acid solution contains plenty of freely-wandering protons, but it is difficult to act as a proton source for doping, since the protons always cause corrosions by destroying solid lattices before residing into them. Here we achieve a facile way to hydrogenate monoclinic vanadium dioxide (VO2) with protons in acid solution by attaching suitable metal to it. Considering the Schottky contact at the metal/VO2 interface, electrons flow from metal to VO2 due to workfunction difference and simultaneously attract free protons in acid solution to penetrate, forming the hydrogens dopants inside VO2 lattice. This metal-acid treatment constitutes an electron-proton co-doping strategy, which not only protects the VO2 lattice from corrosion, but also causes pronounced insulator-to-metal transitions. In addition, the metal-acid induced hydrogen doping behavior shows a ripple effect, and it can spread contagiously up to wafer-size area (>2 inch) even triggered by a tiny metal particle attachment (~1.0mm). This will stimulate a new way of simple and cost-effective atomic doping technique for some other oxide materials.

Three-dimensional (3D) nanoarchitectures can improve the performance of electrical energy storage systems. In this paper, combining an improved solvothermal method with calcination treatment, thin layer carbon-coated MoO 2 nanoparticles with the size of 20-50 nm were uniformly dispersed in 3D graphene aerogel (abbreviated as MoO 2 @C-Gas). The hybrid materials were subsequently used as a new-structured anode for lithium-ion batteries (LIBs), yielding high reversible capacity with excellent stable cyclability. We found that the battery with MoO 2 @C-Gas electrode could deliver a discharge capacity of 1515 mAh g −1 at a current density of 80 mA g −1 in the first cycle, and remain at 1113 mAh g −1 over 60 cycles. Furthermore, with the increase of current density, it also exhibited a high rate of capability and good cycling performance. The outstanding performance of the hybrid electrode were attributed to the unique 3D aerogel morphology and the close combination between C@MoO 2 and graphene layers, which could significantly improve the anode's electronic conductivity.