Pressure-Temperature Phase Diagram of Vanadium Dioxide

Recent Advances of VO2 in Sensors and Actuators

Vanadium dioxide (VO2) stands out for its versatility in numerous applications, thanks to its unique reversible insulator-to-metal phase transition. This transition can be initiated by various stimuli, leading to significant alterations in the material's characteristics, including its resistivity and optical properties. As the interest in the material is growing year by year, the purpose of this review is to explore the trends and current state of progress on some of the applications proposed for VO2 in the field of sensors and actuators using literature review methods. Some key applications identified are resistive sensors such as strain, temperature, light, gas concentration, and thermal fluid flow sensors for microfluidics and mechanical microactuators. Several critical challenges have been recognized in the field, including the expanded investigation of VO2-based applications across multiple domains, exploring various methods to enhance device performance such as modifying the phase transition temperature, advancing the fabrication techniques for VO2 structures, and developing innovative modelling approaches. Current research in the field shows a variety of different sensors, actuators, and material combinations, leading to different sensor and actuator performance input ranges and output sensitivities.

Recent advances in dynamic dual mode systems for daytime radiative cooling and solar heating

Thermal management, including heating and cooling, plays an important role in human productive activities and daily life. Nevertheless, in the actual environment, almost all the ambient scenarios come with the challenge that the objects are located in a quite dynamic and variable environment, which includes fluctuations in aspects such as space, time, sunlight, season, and temperature. It is imperative to develop low-energy or even zero-energy thermal-management technologies with renewable and clean energy. In this review, we summarised the latest technological advances and the prospects in this burgeoning field. First, we present the fundamental principles of the daytime passive radiative cooling (PDRC) thermal management device. Next, In the domain of dual-mode systems, they are classified into various types based on the diverse mechanisms of transitioning between cooling and heating states, including electrical responsive, mechanical responsive, temperature responsive, and solution responsive. Furthermore, we conducted an in-depth analysis of the principles and design methodologies associated with these categories, followed by a comparative assessment of their performance in radiative cooling and solar heating applications. Finally, this review presents the challenges and opportunities of dynamic dual mode thermal management, while also identifying future directions.

High-Strain-Induced Local Modification of the Electronic Properties of VO2 Thin Films

Vanadium dioxide (VO₂) is a popular candidate for electronic and optical switching applications due to its well-known semiconductor-metal transition. Its study is notoriously challenging due to the interplay of long- and short-range elastic distortions, as well as the symmetry change and the electronic structure changes. The inherent coupling of lattice and electronic degrees of freedom opens the avenue toward mechanical actuation of single domains. In this work, we show that we can manipulate and monitor the reversible semiconductor-to-metal transition of VO₂ while applying a controlled amount of mechanical pressure by a nanosized metallic probe using an atomic force microscope. At a critical pressure, we can reversibly actuate the phase transition with a large modulation of the conductivity. Direct tunneling through the VO₂-metal contact is observed as the main charge carrier injection mechanism before and after the phase transition of VO₂. The tunneling barrier is formed by a very thin but persistently insulating surface layer of the VO₂. The necessary pressure to induce the transition decreases with temperature. In addition, we measured the phase coexistence line in a hitherto unexplored regime. Our study provides valuable information on pressure-induced electronic modifications of the VO₂ properties, as well as on nanoscale metal-oxide contacts, which can help in the future design of oxide electronics.

Self-rolling of vanadium dioxide nanomembranes for enhanced multi-level solar modulation

Thermochromic window develops as a competitive solution for carbon emissions due to comprehensive advantages of its passivity and effective utilization of energy. How to further enhance the solar modulation ([Formula: see text]) of thermochromic windows while ensuring high luminous transmittance ([Formula: see text]) becomes the latest challenge to touch the limit of energy efficiency. Here, we show a smart window combining mechanochromism with thermochromism by self-rolling of vanadium dioxide (VO₂) nanomembranes to enhance multi-level solar modulation. The mechanochromism is introduced by the temperature-controlled regulation of curvature of rolled-up smart window, which benefits from effective strain adjustment in VO₂ nanomembranes upon the phase transition. Under geometry design and optimization, the rolled-up smart window with high [Formula: see text] and [Formula: see text] is achieved for the modulation of indoor temperature self-adapted to seasons and climate. Furthermore, such rolled-up smart window enables high infrared reflectance after triggered phase transition and acts as a smart lens protective cover for strong radiation. This work supports the feasibility of self-rolling technology in smart windows and lens protection, which promises broad interest and practical applications of self-adapting devices and systems for smart building, intelligent sensors and actuators with the perspective of energy efficiency.

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.

High-pressure monoclinic-monoclinic transition in fergusonite-type HoNbO4

In this paper we perform a high-pressure (HP) study of fergusonite-type HoNbO₄. Powder x-ray diffraction experiments andab initiodensity-functional theory (DFT) simulations provide evidence of a phase transition at 18.9(1.1) GPa from the monoclinic fergusonite-type structure (space group I2/a) to another monoclinic polymorph described by space group P2₁/c. The phase transition is reversible and the HP structural behavior is different than the one previously observed in related niobates. The HP phase remains stable up to 29 GPa. The observed transition involves a change in the Nb coordination number from 4 to 6, and it is driven by mechanical instabilities. We have determined the pressure dependence of unit-cell parameters of both phases and calculated their room-temperature equation of state. For the fergusonite-phase we have also obtained the isothermal compressibility tensor. In addition to the HP studies, we report ambient-pressure Raman and infrared (IR) spectroscopy measurements. We have been able to identify all the active modes of fergusonite-type HoNbO₄, which have been assigned based upon DFT calculations. These simulations also provide the elastic constants of the different structures and the pressure dependence of the Raman and IR modes of the two phases of HoNbO₄. According toab initiocalculations, the reported phase transition is related to a mechanical instability and a phonon softening.

On-off near-infrared absorbance based on thermal-responsive plasmonic coupling in vanadium dioxide arrays for thermochromic windows

Vanadium dioxide (VO₂) emerges as an attractive plasmonic material due to its unique reversible thermal-responsive phase transition and the promising application in energy-saving smart windows. Here, by optimizing the geometry of VO₂ nano-cylinder arrays, we demonstrate a significant performance enhancement for energy-efficient thermochromic windows. Such a performance enhancement relies on the on-off behavior of plasmonic resonance in the extremely high packing density of VO₂ nano-cylinder arrays. Different from the typical plasmonic material, silver, VO₂ nano-cylinders are characterized to have strong absorbance in near-infrared spectrum with significantly weaker plasmonic coupling to their neighbors, making them suitable to be arranged with a high packing density. The VO₂ nano-cylinder arrays exhibit a 160% luminous transmittance increment, comparing to a flat film with the same solar modulation of ∼10%. The work provides a better understanding of the plasmonic behavior on phase-change VO₂ and an efficient method to enhance smart window performance.

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

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

Temperature effects on VO2 thin films deposited by RF sputtering for the degradation by photocatalysis of methylene blue and naproxen

The temperature effect on VO2 thin films synthesized by RF sputtering on their morphological, optical, electrical properties and their activity in the photocatalytic degradation of methylene blue and naproxen was studied. Characterization results presented microcrystallinity for VO2 films treated at 50 and 100 °C. Nevertheless, the untreated films and films treated at 200 °C revealed characteristic peaks of monoclinic and tetragonal phases. SEM micrographs with elemental mapping of VO2 films showed granular morphology and a good oxygen dispersion along the film surface, possibly due to a restructuring on the film occasioned by particle coalescence and vanadium oxide island conformation. The electronic transmittance spectra showed the d–d transition characteristic for the square-pyramidal stereochemistry of vanadium (IV) ion, where the optical band interval was high for films treated at 50 °C. Raman spectroscopy results presented an increment in the V = O/V–O ratio as a function of temperature, probably related to superficial vanadium species formation. X-ray spectroscopy results showed the Onon-lattice/Olattice ratio values higher for films treated at 50 °C than the other films, related to an oxide character. The V 2p fit results presented V4+, V5+ regions and satellites for VO2 films thermal treated at 50 °C. The electrical resistivity on the VO2 films decreased as a function of temperature. Finally, the VO2 films thermal treated at 50 °C had higher photocatalytic activity in the degradation of methylene blue and naproxen compared to the other VO2 films, possibly associated with high electron mobility between the surface and the bulk, where the oxygen vacancies act as recombination sites for the e−/h+ pairs during photocatalytic degradation.

Giant nonvolatile resistive switching in a Mott oxide and ferroelectric hybrid

Controlling the electronic properties of oxides that feature a metal-insulator transition (MIT) is a key requirement for developing a new class of electronics often referred to as "Mottronics." A simple, controllable method to switch the MIT properties in real time is needed for practical applications. Here we report a giant, nonvolatile resistive switching (ΔR/R > 1,000%) and strong modulation of the MIT temperature (ΔT_c > 30 K) in a voltage-actuated V₂O₃/PMN-PT [Pb(Mg,Nb)O₃-PbTiO₃] heterostructure. This resistive switching is an order of magnitude larger than ever encountered in any other similar systems. The control of the V₂O₃ electronic properties is achieved using the transfer of switchable ferroelastic strain from the PMN-PT substrate into the epitaxially grown V₂O₃ film. Strain can reversibly promote/hinder the structural phase transition in the V₂O₃, thus advancing/suppressing the associated MIT. The giant resistive switching and strong T_c modulation could enable practical implementations of voltage-controlled Mott devices and provide a platform for exploring fundamental electronic properties of V₂O₃.

QMCPACK: an open source ab initio quantum Monte Carlo package for the electronic structure of atoms, molecules and solids

QMCPACK is an open source quantum Monte Carlo package for ab initio electronic structure calculations. It supports calculations of metallic and insulating solids, molecules, atoms, and some model Hamiltonians. Implemented real space quantum Monte Carlo algorithms include variational, diffusion, and reptation Monte Carlo. QMCPACK uses Slater-Jastrow type trial wavefunctions in conjunction with a sophisticated optimizer capable of optimizing tens of thousands of parameters. The orbital space auxiliary-field quantum Monte Carlo method is also implemented, enabling cross validation between different highly accurate methods. The code is specifically optimized for calculations with large numbers of electrons on the latest high performance computing architectures, including multicore central processing unit and graphical processing unit systems. We detail the program's capabilities, outline its structure, and give examples of its use in current research calculations. The package is available at http://qmcpack.org.

Reconfigurable Vanadium Dioxide Nanomembranes and Microtubes with Controllable Phase Transition Temperatures

Two additional structural forms, free-standing nanomembranes and microtubes, are reported and added to the vanadium dioxide (VO₂) material family. Free-standing VO₂ nanomembranes were fabricated by precisely thinning as-grown VO₂ thin films and etching away the sacrificial layer underneath. VO₂ microtubes with a range of controllable diameters were rolled-up from the VO₂ nanomembranes. When a VO₂ nanomembrane is rolled-up into a microtubular structure, a significant compressive strain is generated and accommodated therein, which decreases the phase transition temperature of the VO₂ material. The magnitude of the compressive strain is determined by the curvature of the VO₂ microtube, which can be rationally and accurately designed by controlling the tube diameter during the rolling-up fabrication process. The VO₂ microtube rolling-up process presents a novel way to controllably tune the phase transition temperature of VO₂ materials over a wide range toward practical applications. Furthermore, the rolling-up process is reversible. A VO₂ microtube can be transformed back into a nanomembrane by introducing an external strain. Because of its tunable phase transition temperature and reversible shape transformation, the VO₂ nanomembrane-microtube structure is promising for device applications. As an example application, a tubular microactuator device with low driving energy but large displacement is demonstrated at various triggering temperatures.

Correlated High-Pressure Phase Sequence of VO2 under Strong Compression

Understanding how the structures of a crystal behave under compression is a fundamental issue both for condensed matter physics and for geoscience. Traditional description of a crystal as the stacking of a unit cell with special symmetry has gained much success on the analysis of physical properties. Unfortunately, it is hard to reveal the relationship between the compressed phases. Taking the family of metal dioxides (MO₂) as an example, the structural evolution, subject to fixed chemical formula and highly confined space, often appears as a set of random and uncorrelated events. Here we provide an alternative way to treat the crystal as the stacking of the coordination polyhedron and then discover a unified structure transition pattern, in our case VO₂. X-ray diffraction (XRD) experiments and first-principles calculations show that the coordination increase happens only at one apex of the V-centered octahedron in an orderly fashion, leaving the base plane and the other apex topologically intact. The polyhedron evolves toward increasing their sharing, indicating a general rule for the chemical bonds of MO₂ to give away the ionicity in exchange for covalency under pressure.

Ab initio study of Li, Mg and Al insertion into rutile VO2: fast diffusion and enhanced voltages for multivalent batteries

Vanadium oxides are among the most promising materials that can be used as electrodes in rechargeable metal-ion batteries. In this work, we systematically investigate thermodynamic, electronic, and kinetic properties associated with the insertion of Li, Mg and Al atoms into rutile VO₂. Using first-principles calculations, we systematically study the structural evolution and voltage curves of Li_xVO₂, Mg_xVO₂ and Al_xVO₂ (0 < x < 1) compounds. The calculated lithium intercalation voltage starts at 3.50 V for single-atom insertion and decreases to 2.23 V for full lithiation, to the LiVO₂ compound, which agrees well with the experimental results. The Mg insertion features a plateau about 1.6 V up to Mg_0.5VO₂ and then another plateau-like region at around 0.5 V up to Mg₁VO₂. The predicted voltage curve for Al insertion starts at 1.98 V, followed by two plateaus at 1.48 V and 1.17 V. The diffusion barrier of Li, Mg and Al in the tunnel structure of VO₂ is 0.06, 0.33 and 0.50 eV, respectively. The demonstrated excellent Li, Mg and Al mobility, high structural stability and high specific capacity suggest promising potential of rutile VO₂ electrodes especially for multivalent batteries.