Imaging metal-like monoclinic phase stabilized by surface coordination effect in vanadium dioxide nanobeam

Identification of the conductivity type of single-walled carbon nanotubes via dual-modulation dielectric force microscopy

The conductivity type is one of the most fundamental transport properties of semiconductors, which is usually identified by fabricating the field-effect transistor, the Hall-effect device, etc. However, it is challenging to obtain an Ohmic contact if the sample is down to nanometer-scale because of the small size and intrinsic heterogeneity. Noncontact dielectric force microscopy (DFM) can identify the conductivity type of the sample by applying a DC gate voltage to the tip, which is effective in tuning the accumulation or depletion of charge carriers. Here, we further developed a dual-modulation DFM, which simplified the conductivity type identification from multiple scan times under different DC gate voltages to a single scan under an AC gate voltage. Taking single-walled carbon nanotubes as testing samples, the semiconducting-type sample exhibits a more significant charge carrier accumulation/depletion under each half-period of the AC gate voltage than the metallic-type sample due to the stronger rectification effect. The charge carrier accumulation or depletion of the p-type sample is opposite to that of the n-type sample at the same half-period of the AC gate voltage because of the reversed charge carrier type.

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.

Reducing phase transition temperature of vanadium dioxide by ascorbic acid

The phase transition of vanadium dioxide brings huge change in its optical property, which is well used in thermochromic window, fixed-temperature heat sensor, laser protection and other fields. Tunable phase transition temperature is one key for its wide applications. In this paper we verified a new simple method to reduce phase transition temperature. A coordination effect of ascorbic acid on VO2powder reduces its phase transition temperature to about 32 °C. This simple method offers a new efficient option to deal with VO2, which may dramatically promote the applications of VO2.

Decoupled ultrafast electronic and structural phase transitions in photoexcited monoclinic VO2

Photoexcitation has emerged as an efficient way to trigger phase transitions in strongly correlated materials. There are great controversies about the atomistic mechanisms of structural phase transitions (SPTs) from monoclinic (M₁-) to rutile (R-) VO₂ and its association with electronic insulator-metal transitions (IMTs). Here, we illustrate the underlying atomistic processes and decoupling nature of photoinduced SPT and IMT in nonequilibrium states. The photoinduced SPT proceeds in the order of dilation of V-V pairs and increase of twisting angles after a small delay of ~40 fs. Dynamic simulations with hybrid functionals confirm the existence of isostructural IMT. The photoinduced SPT and IMT exhibit the same thresholds of electronic excitations, indicating similar fluence thresholds in experiments. The IMT is quasi-instantaneously (<10 fs) generated, while the SPT takes place with time a constant of 100 to 300 fs. These findings clarify some key controversies in the literature and provide insights into nonequilibrium phase transitions in correlated materials.

Phase management in single-crystalline vanadium dioxide beams

A systematic study of various metal-insulator transition (MIT) associated phases of VO₂, including metallic R phase and insulating phases (T, M1, M2), is required to uncover the physics of MIT and trigger their promising applications. Here, through an oxide inhibitor-assisted stoichiometry engineering, we show that all the insulating phases can be selectively stabilized in single-crystalline VO₂ beams at room temperature. The stoichiometry engineering strategy also provides precise spatial control of the phase configurations in as-grown VO₂ beams at the submicron-scale, introducing a fresh concept of phase transition route devices. For instance, the combination of different phase transition routes at the two sides of VO₂ beams gives birth to a family of single-crystalline VO₂ actuators with highly improved performance and functional diversity. This work provides a substantial understanding of the stoichiometry-temperature phase diagram and a stoichiometry engineering strategy for the effective phase management of VO₂.

Control of the phases associated with the metal-insulator transition in VO₂ underpins its applications as a phase change material. Here, the authors report phase management by means of oxide inhibitor-assisted growth and present high-performance VO₂ actuators based on asymmetric phase transition routes.

Decoupling the metal insulator transition and crystal field effects of VO2

VO2 is a highly correlated electron system which has a metal-to-insulator transition (MIT) with a dramatic change of conductivity accompanied by a first-order structural phase transition (SPT) near room temperature. The origin of the MIT is still controversial and there is ongoing debate over whether an SPT induces the MIT and whether the Tc can be engineered using artificial parameters. We examined the electrical and local structural properties of Cr- and Co-ion implanted VO2 (Cr-VO2 and Co-VO2) films using temperature-dependent resistance and X-ray absorption fine structure (XAFS) measurements at the V K edge. The temperature-dependent electrical resistance measurements of both Cr-VO2 and Co-VO2 films showed sharp MIT features. The Tc values of the Cr-VO2 and Co-VO2 films first decreased and then increased relative to that of pristine VO2 as the ion flux was increased. The pre-edge peak of the V K edge from the Cr-VO2 films with a Cr ion flux ≥ 1013 ions/cm2 showed no temperature-dependent behavior, implying no changes in the local density of states of V 3d t2g and eg orbitals during MIT. Extended XAFS (EXAFS) revealed that implanted Cr and Co ions and their tracks caused a substantial amount of structural disorder and distortion at both vanadium and oxygen sites. The resistance and XAFS measurements revealed that VO2 experiences a sharp MIT when the distance of V-V pairs undergoes an SPT without any transitions in either the VO6 octahedrons or the V 3d t2g and eg states. This indicates that the MIT of VO2 occurs with no changes of the crystal fields.

Surface/Interface Chemistry Engineering of Correlated-Electron Materials: From Conducting Solids, Phase Transitions to External-Field Response

Correlated electronic materials (CEMs) with strong electron-electron interactions are often associated with exotic properties, such as metal-insulator transition (MIT), charge density wave (CDW), superconductivity, and magnetoresistance (MR), which are fundamental to next generation condensed matter research and electronic devices. When the dimension of CEMs decreases, exposing extremely high specific surface area and enhancing electronic correlation, the surface states are equally important to the bulk phase. Therefore, surface/interface chemical interactions provide an alternative route to regulate the intrinsic properties of low-dimensional CEMs. Here, recent achievements in surface/interface chemistry engineering of low-dimensional CEMs are reviewed, using surface modification, molecule-solid interaction, and interface electronic coupling, toward modulation of conducting solids, phase transitions including MIT, CDW, superconductivity, and magnetism transition, as well as external-field response. Surface/interface chemistry engineering provides a promising strategy for exploring novel properties and functional applications in low-dimensional CEMs. Finally, the current challenge and outlook of the surface/interface engineering are also pointed out for future research development.

Understanding the Phase Transition Evolution Mechanism of Partially M2 Phased VO2 Film by Hydrogen Incorporation

Studies on the hydrogen incorporated M1 phase of VO₂ film have been widely reported. However, there are few works on an M2 phase of VO₂. Recently, the M2 phase in VO₂ has received considerable attention due to the possibility of realizing a Mott transition field-effect transistor. By varying the postannealing environment, systematic variations of the M2 phase in (020)-oriented VO₂ films grown on Al₂O₃(0001) were observed. The M2 phase converted to the metallic M1 phase at first and then to the metallic rutile phase after hydrogen annealing (i.e., for H₂/N₂ mixture and H₂ environments). From the diffraction and spectroscopy measurements, the transition is attributed to suppressed electron interactions, not structural modification caused by hydrogen incorporation. Our results suggest the understanding of the phase transition process of the M2 phase by hydrogen incorporation and the possibility of realization of the M2 phased-based Mott transition field-effect transistor.

Rational Design of Spinel Cobalt Vanadate Oxide Co2 VO4 for Superior Electrocatalysis

Electrochemical energy devices, such as fuel cells and metal-air batteries, convert chemical energy directly into electricity without adverse environmental impact. Attractive alternatives to expensive noble metals used in these renewable energy technologies are earth-abundant transition metal oxides. However, they are often limited by catalytic and conductive capabilities. Here reported is a spinel oxide, Co₂ VO₄ , by marrying metallic vanadium atomic chains with electroactive cobalt cations for superior oxygen reduction reaction (ORR)-a key process for fuel cells, metal-air batteries, etc. The experimental and simulated electron energy-loss spectroscopy analyses reveal that Co²⁺ cations at the octahedral sites take the low spin state with one e_g electron ( t 2 g 6 e g 1 ) , favoring advantageous ORR energetics. Measurement of actual electrical conductivity confirms that Co₂ VO₄ has several orders of magnitude increase when compared with benchmark cobalt oxides. As a result, a zinc-air battery with new spinel cobalt vanadate oxide as the ORR catalyst shows excellent performance, together with a record-high discharge peak power density of 380 mW cm^-2 . Crucially, this is superior to state-of-the-art Pt/C-based device and is greatest among zinc-air batteries assembled with metal, metal oxide, and carbon catalysts. The findings present a new design strategy for highly active and conductive oxide materials for a wide range of electrocatalytic applications, including ORR, oxygen evolution, and hydrogen evolution reactions.

Water adsorption on vanadium oxide thin films in ambient relative humidity

In this work, ambient pressure x-ray photoelectron spectroscopy (APXPS) is used to study the initial stages of water adsorption on vanadium oxide surfaces. V 2p, O 1s, C 1s, and valence band XPS spectra were collected as a function of relative humidity in a series of isotherm and isobar experiments. Experiments were carried out on two VO₂ thin films on TiO₂ (100) substrates, prepared with different surface cleaning procedures. Hydroxyl and molecular water surface species were identified, with up to 0.5 ML hydroxide present at the minimum relative humidity, and a consistent molecular water adsorption onset occurring around 0.01% relative humidity. The work function was found to increase with increasing relative humidity, suggesting that surface water and hydroxyl species are oriented with the hydrogen atoms directed away from the surface. Changes in the valence band were also observed as a function of relative humidity. The results were similar to those observed in APXPS experiments on other transition metal oxide surfaces, suggesting that H₂O-OH and H₂O-H₂O surface complex formation plays an important role in the oxide wetting process and water dissociation. Compared to polycrystalline vanadium metal, these vanadium oxide films generate less hydroxide and appear to be more favorable for molecular water adsorption.

Electrons-Donating Derived Dual-Resistant Crust of VO2 Nano-Particles via Ascorbic Acid Treatment for Highly Stable Smart Windows Applications

Traditional vanadium dioxide (VO₂) material faced severe challenges of low stability in acid, humid, and oxygenic environments, which hinder its real applications. Here, we report a facile improving process which can enhanced the stability of VO₂ nanocrystals in the environments above. Ascorbic acid (AA), as an important antioxidant for organism in medicine and biology, was ingeniously used for enhancing the antioxidation abilities of inorganic material. At the same time, the AA could generate the hydrogen doping occurred on the surface of VO₂ nanocrystals, which enhanced their Antiacid abilities simultaneously. The AA treated VO₂ nanocrystals retain stable in H₂SO₄ and H₂O₂ solution and exhibit high durability in hyperthermal (60 °C) and humid (90%) environment. Characterizations and first-principles theoretical calculations confirmed that the coordination of ascorbic acid molecules on VO₂ nanocrystals induced charge-carrier density reorganization and protons transferring electrostatically. Then the formed H_xVO₂ provides an enhancing formation energy for oxygen vacancy and protects the particles from corrosion. This work is beneficial to the VO₂ nanoparticles coated and decorated processes and exhibit good potential for practical application of VO₂-based smart windows.

Thermophysical characterisation of VO2 thin films hysteresis and its application in thermal rectification

Hysteresis loops exhibited by the thermophysical properties of VO2 thin films deposited on either a sapphire or silicon substrate have been experimentally measured using a high frequency photothermal radiometry technique. This is achieved by directly measuring the thermal diffusivity and thermal effusivity of the VO2 films during their heating and cooling across their phase transitions, along with the film-substrate interface thermal boundary resistance. These thermal properties are then used to determine the thermal conductivity and volumetric heat capacity of the VO2 films. A 2.5 enhancement of the VO2 thermal conductivity is observed during the heating process, while its volumetric heat capacity does not show major changes. This sizeable thermal conductivity variation is used to model the operation of a conductive thermal diode, which exhibits a rectification factor about 30% for small temperature differences (≈70 °C) on its terminals. The obtained results grasp thus new insights on the control of heat currents.

Manipulation of Charge Transport by Metallic V13 O16 Decorated on Bismuth Vanadate Photoelectrochemical Catalyst

Conductive metal oxides represent a new category of functional material with vital importance for many modern applications. The present work introduces a new conductive metal oxide V₁₃ O₁₆ , which is synthesized via a simplified photoelectrochemical procedure and decorated onto the semiconducting photocatalyst BiVO₄ in controlled mass percentages ranging from 25% to 37%. Owing to its excellent conductivity and good compatibility with oxide materials, the metallic V₁₃ O₁₆ -decorated BiVO₄ hybrid catalyst shows a high photocurrent density of 2.2 ± 0.2 mA cm^-2 at 1.23 V versus reversible hydrogen electrode (RHE). Both experimental characterization and density functional theory calculations indicate that the superior photocurrent derives from enhanced charge separation and transfer, resulting from ohmic contact at the interface of mixed phases and superior electrical conductivity from V₁₃ O₁₆ . A Co-Pi coating on BiVO₄ -V₁₃ O₁₆ further increases the photocurrent to 5.0 ± 0.5 mA cm^-2 at 1.23 V versus RHE, which is among the highest reported for BiVO₄ -based photoelectrodes. Surface photovoltage and transient photocurrent measurements suggest a charge-transfer model in which photocurrents are enhanced by improved surface passivation, although the barrier at the Co-Pi/electrolyte interface limits the charge transfer.

Functional Scanning Force Microscopy for Energy Nanodevices

Energy nanodevices, including energy conversion and energy storage devices, have become a major cross-disciplinary field in recent years. These devices feature long-range electron and ion transport coupled with chemical transformation, which call for novel characterization tools to understand device operation mechanisms. In this context, recent developments in functional scanning force microscopy techniques and their application in thin-film photovoltaic devices and lithium batteries are reviewed. The advantages of scanning force microscopy, such as high spatial resolution, multimodal imaging, and the possibility of in situ and in operando imaging, are emphasized. The survey indicates that functional scanning force microscopy is making significant contributions in understanding materials and interfaces in energy nanodevices.