Hydrogen diffusion and stabilization in single-crystal VO2 micro/nanobeams by direct atomic hydrogenation

Effects of oxygen vacancies and interfacial strain on the metal-insulator transition of VO2 nanobeams

The role of oxygen vacancies and interfacial strain on the metal-insulator transition (MIT) behavior of high-quality VO2 nanobeams (NBs) synthesized on SiO2/Si substrates employing V2O5 as a precursor has been investigated in this research. Selective oxygen vacancies have been generated by argon plasma irradiation. The MIT is progressively suppressed as the duration of plasma processing increases; in addition, the temperature of MIT (TMIT) drops by up to 95 K relative to the pristine VO2 NBs. Incorporating oxygen vacancies into VO2 may increase its electron concentration, which might shift the Fermi levels upward, strengthen the electronic orbital overlap of the V-V chains, and further stabilize the metallic phase at lower temperatures, based on first-principles calculations. Furthermore, in order to evaluate the influence of substrate-induced strain in our situation, the MIT in two distinct types of VO2 NB samples is examined without metal contacts by using the distinctive light scattering characteristics of the metal (M) and insulator (I) phases (i.e., M/I domains) by optical microscopy. It is found that the domain structures in the "clamped" NBs persisted up to ∼453 K, while the "released" NBs (transferred to a new substrate) did not exhibit any domain structures and turned into an entirely M phase with a dark contrast above ∼348 K. When combined with first-principles calculations, the electronic orbital occupancy in the rutile phase contributes to explaining the interfacial strain-induced modulation of MIT. The current findings shed light on how interfacial strain and oxygen vacancies impact MIT behavior. It also suggests several types of control strategies for MIT in VO2 NBs, which are essential for a broader spectrum of VO2 NB applications.

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.

Revealing the Role of Hydrogen in Electron-Doping Mottronics for Strongly Correlated Vanadium Dioxide

Hydrogen-associated electron-doping Mottronics for d-band correlated oxides (e.g., VO₂) opens up a new paradigm to regulate the electronic functionality via directly manipulating the orbital configuration and occupancy. Nevertheless, the role of hydrogen in the Mottronic transition of VO₂ is yet unclear because opposite orbital reconfigurations toward either the metallic or highly insulating states were both reported. Herein, we demonstrate the root cause for such hydrogen-induced multiple electronic phase transitions by ¹H quantification using nuclear reaction analysis. A low hydrogenation temperature is demonstrated to be vital in achieving a large hydrogen concentration (n_H ≈ 10²² cm^-3) that further enhances the t_2g orbital occupancy to trigger electron localizations. In contrast, elevating the hydrogenation temperatures surprisingly reduces n_H to ∼10²¹ cm^-3 but forms more stable metallic H_0.06VO₂. This leads to the recognition of a weaker hydrogen interaction that triggers electron localization within VO₂ via Mottronically enhancing the orbital occupancies.

Element doping: a marvelous strategy for pioneering the smart applications of VO2

Smart materials are leading the future of materials by virtue of their autonomous response behavior to external stimuli; it is widely believed their development and application will bring a new revolution. Among them, vanadium dioxide (VO₂) is a special one showing a unique multi-stimulus responsive metal-insulator transition (MIT) accompanied by a structural phase transition (SPT) with striking changes of physical properties including optical, electrical and thermal properties, etc., making it ideal for smart windows, micro-bolometers, actuators, etc. Since the attractive performances of VO₂ are rooted in MIT behavior (coupled with SPT), element doping becomes a powerful tool in tailoring VO₂ performance. Oriented on the practical requirements, element-doped VO₂ is more promising and competitive in terms of performance, prospect, and cost. Here we focus specifically on element-doped VO₂, the recent progress and potential challenges of which are discussed. We devote attention to the crucial roles of element doping in modulating the properties and driving the practicality of VO₂, aiming to inspire current research to pioneer new applications of VO₂.

Topochemical Transformation of Two-Dimensional VSe2 into Metallic Nonlayered VO2 for Water Splitting Reactions in Acidic and Alkaline Media

The engineering of the structural and morphological properties of nanomaterials is a fundamental aspect to attain desired performance in energy storage/conversion systems and multifunctional composites. We report the synthesis of room temperature-stable metallic rutile VO₂ (VO₂ (R)) nanosheets by topochemically transforming liquid-phase exfoliated VSe₂ in a reductive Ar-H₂ atmosphere. The as-produced VO₂ (R) represents an example of two-dimensional (2D) nonlayered materials, whose bulk counterparts do not have a layered structure composed by layers held together by van der Waals force or electrostatic forces between charged layers and counterbalancing ions amid them. By pretreating the VSe₂ nanosheets by O₂ plasma, the resulting 2D VO₂ (R) nanosheets exhibit a porous morphology that increases the material specific surface area while introducing defective sites. The as-synthesized porous (holey)-VO₂ (R) nanosheets are investigated as metallic catalysts for the water splitting reactions in both acidic and alkaline media, reaching a maximum mass activity of 972.3 A g^-1 at -0.300 V vs RHE for the hydrogen evolution reaction (HER) in 0.5 M H₂SO₄ (faradaic efficiency = 100%, overpotential for the HER at 10 mA cm^-2 = 0.184 V) and a mass activity (calculated for a non 100% faradaic efficiency) of 745.9 A g^-1 at +1.580 V vs RHE for the oxygen evolution reaction (OER) in 1 M KOH (overpotential for the OER at 10 mA cm^-2 = 0.209 V). By demonstrating proof-of-concept electrolyzers, our results show the possibility to synthesize special material phases through topochemical conversion of 2D materials for advanced energy-related applications.

The mechanism of semiconductor to metal transition in the hydrogenation of VO2: A density functional theory study

VO2 is a glamorous material with specific metal-semiconductor-transition (MST). The hydrogenation of VO2 could make it a promising material applying in the ambient environment. In this work, we reveal the...

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

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

Hydrogen Control of Double Exchange Interaction in La0.67 Sr0.33 MnO3 for Ionic-Electric-Magnetic Coupled Applications

The dynamic tuning of ion concentrations has attracted significant attention for creating versatile functionalities of materials, which are impossible to reach using classical control knobs. Despite these merits, the following fundamental questions remain: how do ions affect the electronic bandstructure, and how do ions simultaneously change the electrical and magnetic properties? Here, by annealing platinum-dotted La_0.67 Sr_0.33 MnO₃ films in hydrogen and argon at a lower temperature of 200 °C for several minutes, a reversible change in resistivity is achieved by three orders of magnitude with tailored ferromagnetic magnetization. The transition occurs through the tuning of the double exchange interaction, ascribed to an electron-doping-induced and/or a lattice-expansion-induced modulation, along with an increase in the hydrogen concentration. High reproducibility, long-term stability, and multilevel linearity are appealing for ionic-electric-magnetic coupled applications.

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.

Long-range propagation of protons in single-crystal VO2 involving structural transformation to HVO2

Vanadium dioxide (VO₂) is a strongly correlated electronic material with a metal-insulator transition (MIT) near room temperature. Ion-doping to VO₂ dramatically alters its transport properties and the MIT temperature. Recently, insulating hydrogenated VO₂ (HVO₂) accompanied by a crystal structure transformation from VO₂ was experimentally observed. Despite the important steps taken towards realizing novel applications, essential physics such as the diffusion constant of intercalated protons and the crystal transformation energy between VO₂ and HVO₂ are still lacking. In this work, we investigated the physical parameters of proton diffusion constants accompanied by VO₂ to HVO₂ crystal transformation with temperature variation and their transformation energies. It was found that protons could propagate several micrometers with a crystal transformation between VO₂ and HVO₂. The proton diffusion speed from HVO₂ to VO₂ was approximately two orders higher than that from VO₂ to HVO_2. The long-range propagation of protons leads to the possibility of realizing novel iontronic applications and energy devices.

Influence of hydrogen incorporation on conductivity and work function of VO2 nanowires

We report improved conductance by reducing the work function via incorporation of hydrogen into VO2 nanowires. The VO2 nanowires were prepared using the chemical vapor deposition method with V2O5 powder on silicon substrates at 850 °C. Hydrogenation was carried out using the high-pressure hydrogenation method. Raman spectroscopy confirmed that the incorporated hydrogen atoms resulted in a change in the lattice constant of the VO2 nanowires (NWs). To quantitatively measure the work function of the nanowires, Kelvin probe force microscopy (KPFM) was employed at ambient conditions. We found that the work function decreased with increasing H2 pressure, which also resulted in increased conductance. This is associated with hydrogen diffused into the VO2 that acts as a donor to elevate the Fermi level, which was also confirmed by KPFM. From these results, tuning of the reversible electrical properties of VO2 NWs, including the conductance and work function, can be achieved by incorporating hydrogen at relatively moderate temperatures.

Gate-controlled VO2 phase transition for high-performance smart windows

Vanadium dioxide (VO₂) is a promising material for developing energy-saving "smart windows," owing to its infrared thermochromism induced by metal-insulator transition (MIT). However, its practical application is greatly limited by its relatively high critical temperature (~68°C), low luminous transmittance (<60%), and poor solar energy regulation ability (<15%). Here, we developed a reversible and nonvolatile electric field control of the MIT of a monoclinic VO₂ film. With a solid electrolyte layer assisting gating treatment, we modulated the insertion/extraction of hydrogen into/from the VO₂ lattice at room temperature, causing tristate phase transitions that enable control of light transmittance. The dramatic increase in visible/infrared transmittance due to the phase transition from the metallic (lightly H-doped) to the insulating (heavily H-doped) phase results in an increased solar energy regulation ability up to 26.5%, while maintaining 70.8% visible luminous transmittance. These results break all previous records and exceed the theoretical limit for traditional VO₂ smart windows, making them ready for energy-saving utilization.

Size and crystallinity control of dispersed VO2 particles for modulation of metal–insulator transition temperature and hysteresis

Growth mechanism of VO₂ particles with size dependent crystallinity: a solid-state dewetting and pyrolysis synergistic effect. Crystallinity, strain and defects optimize and modulate the MIT behavior of VO₂ particles.

Non-catalytic hydrogenation of VO2 in acid solution

Hydrogenation is an effective way to tune the property of metal oxides. It can conventionally be performed by doping hydrogen into solid materials with noble-metal catalysis, high-temperature/pressure annealing treatment, or high-energy proton implantation in vacuum condition. Acid solution naturally provides a rich proton source, but it should cause corrosion rather than hydrogenation to metal oxides. Here we report a facile approach to hydrogenate monoclinic vanadium dioxide (VO₂) in acid solution at ambient condition by placing a small piece of low workfunction metal (Al, Cu, Ag, Zn, or Fe) on VO₂ surface. It is found that the attachment of a tiny metal particle (~1.0 mm) can lead to the complete hydrogenation of an entire wafer-size VO₂ (>2 inch). Moreover, with the right choice of the metal a two-step insulator–metal–insulator phase modulation can even be achieved. An electron–proton co-doping mechanism has been proposed and verified by the first-principles calculations.

Hydrogenation is an effective way to tune the property of metal oxides. Here, the authors report a simple approach to hydrogenate VO₂ in acid solution under ambient conditions by placing a small piece of low workfunction metal on VO₂ surface.

In Situ Visualization of Fast Surface Ion Diffusion in Vanadium Dioxide Nanowires

We investigate in situ ion diffusion in vanadium dioxide (VO₂) nanowires (NWs) by using photocurrent imaging. Alkali metal ions are injected into a NW segment via ionic liquid gating and are shown to diffuse along the NW axis. The visualization of ion diffusion is realized by spatially resolved photocurrent measurements, which detect the charge carrier density change associated with the ion incorporation. Diffusion constants are determined to be on the order of 10^-10 cm²/s for both Li⁺ and Na⁺ ions at room temperature, while H⁺ diffuses much slower. The ion diffusion is also found to occur mainly at the surface of the NWs, as metal contacts can effectively block the ion diffusion. This novel method of visualizing ion distribution is expected to be applied to study ion diffusion in a broad range of materials, providing key insights on phase transition electronics and energy storage applications.

Evolution of Structural and Electrical Properties of Oxygen-Deficient VO2 under Low Temperature Heating Process

Structural stability and functional performances of vanadium dioxide (VO₂) are strongly influenced by oxygen vacancies. However, the mechanism of metal-insulator transition (MIT) influenced by defects is still under debate. Here, we study the evolution of structure and electrical property of oxygen-deficient VO₂ by a low temperature annealing process (LTP) based on a truss-structured VO₂ nanonet. The oxygenation process of the oxygen-deficient VO₂ is greatly prolonged, which enables us to probe the gradual change of properties of the oxygen-deficient VO₂. A continuous lattice reduction is observed during LTP. No recrystallization and structural collapse of the VO₂ nanonet can be found after LTP. The valence-band X-ray photoelectron spectroscopy (XPS) measurements indicate that the oxygen deficiency strongly affects the energy level of the valence band edge. Correspondingly, the resistance changes of the VO₂ films from 1 to 4.5 orders of magnitude are achieved by LTP. The effect of oxygen vacancy on the electric field driven MIT is investigated. The threshold value of voltage triggering the MIT decreases with increasing the oxygen vacancy concentration. This work demonstrates a novel and effective way to control the content of oxygen vacancies in VO₂ and the obvious impact of oxygen vacancy on MIT, facilitating further research on the role of oxygen vacancy in structure and MIT of VO₂, which is important for the deep understanding of MIT and exploiting innovative functional application of VO₂.

Mesoscopic quantum effects in a bad metal, hydrogen-doped vanadium dioxide

The standard treatment of quantum corrections to semiclassical electronic conduction assumes that charge carriers propagate many wavelengths between scattering events, and succeeds in explaining multiple phenomena (weak localization magnetoresistance (WLMR), universal conductance fluctuations, Aharonov-Bohm oscillations) observed in polycrystalline metals and doped semiconductors in various dimensionalities. We report apparent WLMR and conductance fluctuations in H _x VO₂, a poor metal (in violation of the Mott-Ioffe-Regel limit) stabilized by the suppression of the VO₂ metal-insulator transition through atomic hydrogen doping. Epitaxial thin films, single-crystal nanobeams, and nanosheets show similar phenomenology, though the details of the apparent WLMR seem to depend on the combined effects of the strain environment and presumed doping level. Self-consistent quantitative analysis of the WLMR is challenging given this and the high resistivity of the material, since the quantitative expressions for WLMR are derived assuming good metallicity. These observations raise the issue of how to assess and analyze mesoscopic quantum effects in poor metals.

Evidence for Ionic Liquid Gate-Induced Metallization of Vanadium Dioxide Bars over Micron Length Scales

It has recently been shown that the metal-insulator transition in vanadium dioxide epitaxial films can be suppressed and the material made metallic to low temperatures by ionic liquid gating due to migration of oxygen. The gating is only possible on certain crystal facets where volume channels along the VO₂'s rutile c-axis intersect the surface. Here, we fabricate bars with the c-axis in plane and oriented parallel to or perpendicular to the length of the bars. We show that only bars with the c-axis perpendicular to the bars, for which the volume channels are accessible from the sides of the bar, can be metallized by ionic liquid gating. Moreover, we find that bars up to at least 0.5 μm wide can be fully gated, demonstrating the possibility of the electric field induced migration of oxygen over very long distances, ∼5 times longer than previously observed.

In situ Raman spectroscopy study of metal-enhanced hydrogenation and dehydrogenation of VO2

Vanadium dioxide (VO2) has a phase transition from insulator to metal at 340 K, and this transition can be strongly modified by hydrogenation. In this work, two dimensional (2D) VO2 sheets have been grown on Si(1 1 1) surfaces through chemical vapor deposition, and metal (Au, Pt) thin films were deposited on VO2 surfaces by sputtering. The hydrogenation and dehydrogenation of VO2 and metal-decorated VO2 structures in H2 and in air were in situ studied by Raman. We found that hydrogenation and dehydrogenation temperatures have been significantly decreased with the VO2 surface decorated by Au and Pt. The enhanced hydrogenation and dehydrogenation reactions can be attributed to catalytic dissociation of H2 and O2 molecules on metal surfaces and subsequent spillover of dissociated H and O atoms to the oxide surfaces.

The structures of interstitial hydrogen centers in VO2 in the dilute limit from their vibrational properties and theory

The introduction of a large concentration of H into VO2 is known to suppress the insulating phase of the metal-insulator transition that occurs upon cooling below 340 K. We have used infrared spectroscopy and complementary theory to study the properties of interstitial H and D in VO2 in the dilute limit to determine the vibrational frequencies, thermal stabilities, and equilibrium positions of isolated interstitial H and D centers. The vibrational lines of several OH and OD centers were observed to have thermal stabilities similar to that of the hydrogen that suppresses the insulating phase. Theory associates two of the four possible OH configurations for Hi in the insulating VO2 monoclinic phase with OH lines seen by experiment. Furthermore, theory predicts the energies and vibrational frequencies for configurations with Hi trapped near a substitutional impurity and suggests such defects as candidates for additional OH centers that have been observed.

Reversible phase modulation and hydrogen storage in multivalent VO2 epitaxial thin films

Hydrogen, the smallest and the lightest atomic element, is reversibly incorporated into interstitial sites in vanadium dioxide (VO2), a correlated oxide with a 3d(1) electronic configuration, and induces electronic phase modulation. It is widely reported that low hydrogen concentrations stabilize the metallic phase, but the understanding of hydrogen in the high doping regime is limited. Here, we demonstrate that as many as two hydrogen atoms can be incorporated into each VO2 unit cell, and that hydrogen is reversibly absorbed into, and released from, VO2 without destroying its lattice framework. This hydrogenation process allows us to elucidate electronic phase modulation of vanadium oxyhydride, demonstrating two-step insulator (VO2)-metal (HxVO2)-insulator (HVO2) phase modulation during inter-integer d-band filling. Our finding suggests the possibility of reversible and dynamic control of topotactic phase modulation in VO2 and opens up the potential application in proton-based Mottronics and novel hydrogen storage.

Visualization of one-dimensional diffusion and spontaneous segregation of hydrogen in single crystals of VO2

Hydrogen intercalation in solids is common, complicated, and very difficult to monitor. In a new approach to the problem, we have studied the profile of hydrogen diffusion in single-crystal nanobeams and plates of VO2, exploiting the fact that hydrogen doping in this material leads to visible darkening near room temperature connected with the metal-insulator transition at 65 °C. We observe hydrogen diffusion along the rutile c-axis but not perpendicular to it, making this a highly one-dimensional diffusion system. We obtain an activated diffusion coefficient, [Formula: see text] applicable in metallic phase. In addition, we observe dramatic supercooling of the hydrogen-induced metallic phase and spontaneous segregation of the hydrogen into stripes implying that the diffusion process is highly nonlinear, even in the absence of defects. Similar complications may occur in hydrogen motion in other materials but are not revealed by conventional measurement techniques.

First-principles study of the effect of oxygen vacancy and strain on the phase transition temperature of VO2

The calculated oxygen-vacancy diffusion barrier indicates that the existence of oxygen-vacancy could stabilize the rutile phase at a low temperature.