Reversible Metal-insulator Transition Research Articles

Solvethermal synthesis and thermochromic properties of CaF2@VO2-based core-shell structure composites

Vanadium oxide (VO2)-based core-shell structures exhibit unique properties due to their reversible metal–insulator transition (MIT). As a result, numerous applications on the device level have been reported in the literature. However, since VO2 are multivalent compounds, the controllable preparation of VO2 core-shell-based particles is quite challenging. Along these lines, in this work, to overcome these limitations, the growth mechanism of VO2 shells on spherical substrates was systematically investigated. More specifically, the CaF2@VO2 core-shell microspheres were fabricated successfully by applying the solvent/hydrothermal-calcination method for the first time, while the phase evolution and morphological characteristics of the preparation process were studied in detail. From the obtained results, it was demonstrated that the composite particle's morphology and phase structure strongly depend on the activator, solvent, annealing atmosphere, and the annealing temperature. In addition, the infrared transmittance of the prepared CaF2@VO2(M) coating exhibited a decrease of 34.08 % during the metal–semiconductor phase transition in the 4–12 μm wavelength. Compared with the VO2 coatings and the uncoated CaF2/VO2 coatings, the transmittance conversion performance (ΔT) of the CaF2@VO2(M) coating was also increased by 22.62 % and 21.22 %, respectively.

Variation of the metal-insulator phase transition temperature in VO2: An overview of some possible implementation methods

The great interest in VO2, which has stimulated a large number of studies and publications in recent decades, is caused by the reversible metal-insulator phase transition (MIT) that occurs at T = 68 °C and is accompanied by the transformation of a low-temperature dielectric (semiconductor) monoclinic phase into a high-temperature metallic phase with a rutile structure. Despite the ongoing discussion about the physical mechanism of this transition, the concomitant rapid change in the electrical and optical characteristics of the material by several orders of magnitude already finds numerous applications in optics, optoelectronics and sensors. At the same time, it became obvious that both the number and performance of the applications of VO2 would greatly increase, if it were possible to decrease the temperature of the phase transition without deterioration of other properties. This issue has become the subject of numerous studies. Mechanical stress and oxygen vacancies in the VO2 lattice, the concentration of free charge carriers, tuned by impurity doping or implantation, have been investigated and discussed as the main factors affecting the transition temperature. In this review, we intend to summarize and analyze the literature data on these ways, primarily those which are most efficient in influencing the transition temperature while maintaining a significant change in the modulation characteristics.

Формирование наноструктур из поликристаллических пленок диоксида ванадия с помощью сканирующей зондовой литографии

Vanadium dioxide (VO2) undergoes a reversible first-order metal-insulator phase transition near room temperature, accompanied by a structural phase transition. This causes a significant change in its electrical and optical properties, making it useful for practical applications. VO2-based nanostructures exhibit notable mechanical resistance to structural transition caused by their small size and demonstrate vivid properties during the phase transition. Obtaining VO2 nanostructures is an extremely demanded objective. This research study presents the use of scanning probe lithography techniques to fabricate nanostructures on polycrystalline VO2 films. The present study focuses on the modification of VO2 films when a positive bias is applied to the sample. The effect of the value and duration of the applied voltage, relative humidity on the quality of the formed nanolithographic pattern was analyzed. The oxidation mechanism was determined. It was found that as a result of local anodic oxidation the formed oxide structures consisting of vanadium penta-oxide (V2O5) completely dissolve in water. This process leads to the separation of the continuous polycrystalline VO2 film into individual nanostructures with precise dimensions. The presented method of forming nanostructures from crystalline VO2 films is promising for nanophotonics and nanoelectronics.

Kirkendall effect induced ultrafine VOOH nanoparticles and their transformation into VO2(M) for energy-efficient smart windows.

Vanadium dioxide (VO2) has received widespread attention for application in energy-efficient smart windows because of its distinct thermochromic property in the near-infrared region during the reversible metal-insulator phase transition. In this study, lepidocrocite VOOH ultrafine nanoparticles (NPs) with a diameter less than 30 nm were prepared by a mild and efficient hydrothermal method, and the Kirkendall effect played a vital role in the growth of the VOOH NPs. It was found that VOOH could be transformed into VO2via a subsequent annealing treatment during which the size and morphology of VOOH are well preserved even though the annealing temperature is up to 500 °C. The ultrafine VO2 NPs are crucial for achieving excellent nanothermochromic performance with a luminous transmittance (Tlum) up to 56.45% and solar modulation ability (ΔTsol) up to 14.95%. The environmental durability is well improved by coating VO2 NPs with an SiO2 shell as confirmed via progressive oxidation and acid corrosion experiments. Meanwhile, the Tlum of the VO2@SiO2 film is further increased from 56.45% to 62.29% while the ΔTsol remained unchanged. This integrated thermochromic performance presents great potential for the development of VO2-based smart windows.

Continuously tunable the phase transition behavior of VO2 by T-doping and the formation of T-T chain (T = Ta, Nb)

Vanadium dioxide (VO2), a typical strongly correlated transition metal oxide, undergoes a fascinating reversible metal-insulator transition (MIT) at 340 K, attracting increasing interest from both scientific and technological perspectives. Nevertheless, achieving a rational regulation of the phase transition temperature (Tc) and gaining a deeper understanding of the modulation mechanism, as well as clarifying the relationship between the dopants and Tc in VO2, remain significant ongoing challenges. In this work, we present a density functional theoretical study on the structural and electronic properties of T-doped (T = Ta, Nb) VO2. The results demonstrate that the introduction of T (T = Ta, Nb) atoms tends to drive the R phase structure towards a dimerized monoclinic-like structure, while the uniformly aligned V–V chains exhibit alternating short and long V–V dimerization feature. Notably, the R phase VO2 appears to be more susceptible to structural perturbations. Moreover, T (T = Ta, Nb) doping concentration, compressive strain and the numbers of T-T chain cause curious changes in the structural evolution and electronic structures of VO2, acting as effective modulation methods for tuning the phase transition temperature. In addition, the Tc of VO2 could be significantly reduced when the concentration of T (T = Ta, Nb) dopant is less than 6.3%. The current results offer valuable insights into the mechanism of the doping driven MIT in T-doped (T = Ta, Nb) VO2, and contribute to a deeper understanding of the relationship between the dopants and phase transition temperature, further providing valuable insights into its metal-insulator transition behavior.

Printed and Room Temperature Processed Nanoparticulate VO2 Thin Films Toward Memristive Device Applications

Vanadium dioxide (VO2) has received tremendous research interest in recent years for its versatile electronic application possibilities based on its reversible metal-insulator transition (MIT) behavior, which can be triggered by a large set of stimuli, such as current, electric field, or light. VO2 undergoes a phase transition from a semi-insulating monoclinic to metallic rutile phase at a low critical temperature of 68 °C, which is very close to room temperature, and compatible with any inexpensive flexible substrate. However, owing to the existence of various vanadium oxide polymorphs, the synthesis of the pure monoclinic VO2 (M) phase is a challenge. In fact, the difficulty level increases further when it is to be solution-processed. However, synthesis protocol for phase-pure VO2 (M) following wet chemical routes is essential to obtain high throughput, cost-efficient, and substrate-independent fabrication routine for the large volume of devices. In this regard, in the present study, we report a solution-based synthesis of ultra-small VO2 (M) nanoparticles that are phase-pure and the small nanoparticles may easily be used to formulate inkjet printable VO2 nanoparticulate. The printed VO2 (M) films have been found to be homogeneous, crack-free and the memristor device thus fabricated has demonstrated temperature-dependent phase transition and a resistance change of more than three orders of magnitude around 68 °C; on the other hand, retention of the metallic and insulating state has also been demonstrated for 180 minutes.

Enhanced Thermochromic Performance of VO2 Nanoparticles by Quenching Process.

Vanadium dioxide (VO2) has been a promising energy-saving material due to its reversible metal-insulator transition (MIT) performance. However, the application of VO2 films has been seriously restricted due to the intrinsic low solar-energy modulation ability (ΔTsol) and low luminous transmittance (Tlum) of VO2. In order to solve the problems, the surface structure of VO2 particles was regulated by the quenching process and the VO2 dispersed films were fabricated by spin coating. Characterizations showed that the VO2 particles quenched in deionized water or ethanolreserved VO2(M) phase structure and they were accompanied by surface lattice distortion compared to the pristine VO2. Such distortion structure contributed to less aggregation and highly individual dispersion of the quenched particles in nanocomposite films. The corresponding film of VO2 quenched in water exhibited much higher ΔTsol with an increment of 42.5% from 8.8% of the original VO2 film, because of the significant localized surface plasmon resonance (LSPR) effect. The film fabricated from the VO2 quenched in ethanol presented enhanced thermochromic properties with 15.2% of ΔTsol and 62.5% of Tlum. It was found that the excellent Tlum resulted from the highly uniform dispersion state of the quenched VO2 nanoparticles. In summary, the study provided a facile way to fabricate well-dispersed VO2 nanocomposite films and to facilitate the industrialization development of VO2 thermochromic films in the smart window field.

Integrated Electromagnetic Device with On‐Off Heterointerface for Intelligent Switching Between Wave‐Absorption and Wave‐Transmission

AbstractIntelligent electromagnetic wave absorbers (IEAs) are in high demand due to their dynamic electromagnetic parameters that can adapt to the complex and volatile application environments of the current 5G era. Despite of this, there is currently a lack of research on the convertible electromagnetic wave (EMW) absorption mode (switching between wave‐absorption and wave‐transmission) and their integrated design with external physical stimulations, so that the electromagnetic device will realize intelligent switching in any conditions. In this work, a V2C‐VO2(M) heterostructure that exhibits a reversible metal–insulator transition at the temperature of 62 °C is fabricated via oxidation of MXene. The heterostructures demonstrate near wave‐transmission characteristics at 25 °C while wave‐absorption behavior with wide effective absorption bandwidth (4.04 GHz) at 70 °C. VO2(M) exhibits stronger intrinsic conductivity after phase transition, and the “on‐off” heterostructure between V2C and VO2 lead to poor/strong local conductive network and interfacial polarization in 25/70 °C, thus creating “quantized” dielectric loss. Furthermore, a multilayered electromagnetic functional device is developed to facilitate the absorber's phase transition temperature at a voltage of 17.5 V. This work presents promising opportunities of the V2C‐VO2(M) heterostructure for various applications, including radar stealth, portable stealth suits, signal regulation, and deicing.

Assembly of CQDs/mesoporous SiO2/VO2 composites with wide optical response and abnormal phase transition temperature

The rapid reversible metal-insulator transition (MIT) of vanadium dioxide (VO2) between the monoclinic and rutile phase brings about an abrupt change in optical properties. Until now, most researches focused on regulating near-infrared spectra and lowering phase transition temperature (Tc) while the optical response in the region of UV–vis and high Tc for VO2 are ignored. Herein, carbon quantum dots (CQDs) were introduced into mesoporous SiO2 microspheres and then CQDs/mesoporous SiO2/VO2 composites were synthesized in situ to expand the optical response and Tc range of VO2. Water bath time and annealing temperature have important influences on the morphologies and properties of as-prepared composites. The prolongation of hydrothermal time could promote the formation of VO2 nanoparticles. After annealing, the Tc for as-prepared CQDs/mesoporous SiO2/VO2 composites was in the range of 80.5–93.6 °C. The CQDs/mesoporous SiO2/VO2 composites exhibited broad and strong UV–vis absorption in the range of 200–1200 nm, and the sample prepared under the conditions of a water bath time of 3 h and annealing temperature of 550 °C had the maximum luminous transmittance (Tlum = 64.665 %) as well as the highest solar modulating ability (ΔTsol = 2.42 %). Therefore, the wide range of optical response and abnormal Tc besides typical phase transition behaviors for CQDs/mesoporous SiO2/VO2 composites have a potential application in the field of optical devices related to UV–vis region and high temperature environments.

Reversible metal-insulator transition in SrIrO3 ultrathin layers by field effect control of inversion symmetry breaking

SrIrO3 is a correlated semimetal with narrow t2g d-bands of strong mixed orbital character resulting from the interplay of the spin-orbit interaction due to heavy iridium atoms and the band folding induced by the lattice structure. In ultrathin layers, inversion symmetry breaking, occurring naturally due to the presence of the substrate, opens new orbital hopping channels, which in presence of spin-orbit interaction causes deep modifications in the electronic structure. Here, we show that in SrIrO3 ultrathin films the effect of inversion symmetry breaking on the band structure can be externally manipulated in a field effect experiment. We further prove that the electric field toggles the system reversibly between a metallic and an insulating state with canted antiferromagnetism and an emergent anomalous Hall effect. This is achieved through the spin-orbit driven coupling of the electric field generated in an ionic liquid gate to the electronic structure, where the electric field controls the band structure rather than the usual band filling, thereby enabling electrical control of the effective role of electron correlations. The externally tunable antiferromagnetic insulator, rooted in the strong spin-orbit interaction of iridium, may inspire interesting applications in spintronics.

Coexistent VO2 (M) and VO2 (B) Polymorphous Thin Films with Multiphase-Driven Insulator-Metal Transition.

Reversible insulator-metal transition (IMT) and structure phase change in vanadium dioxide (VO2) remain vital and challenging with complex polymorphs. It is always essential to understand the polymorphs that coexist in desired VO2 materials and their IMT behaviors. Different electrical properties and lattice alignments in VO2 (M) and VO2 (B) phases have enabled the creation of versatile functional devices. Here, we present polymorphous VO2 thin films with coexistent VO2 (M) and VO2 (B) phases and phase-dependent IMT behaviors. The presence of VO2 (B) phases may induce lattice distortions in VO2 (M). The plane spacing of (011)M in the VO2 (M) phase becomes widened, and the V-V and V-O vibrations shift when more VO2 (B) phase exists in the VO2 (M) matrix. Significantly, the coexisting VO2 (B) phases promote the IMT temperature of the polymorphous VO2 thin films. We expect that such coexistent polymorphs and IMT variations would help us to understand the microstructures and IMT in the desired VO2 materials and contribute to advanced electronic transistors and optoelectronic devices.

Investigating the Intrinsic Anisotropy of VO2(101) Thin Films Using Linearly Polarized Resonant Photoemission Spectroscopy

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.

Full-scale simulation and experimental verification of the phase-transition temperature of a VO2 nanofilm as smart window materials

Vanadium dioxide (VO2), a highly important thermotropic phase-transition material, undergoes a reversible metal-insulator transition near temperatures of 68 °C and has potential applications in the energy-efficient smart window field. Recently, it has been found that the phase-transition temperature of VO2 films with nanometer thickness approached the ideal of room temperature (30 °C); however, deciphering this nanoscale phase-transition mechanism is still difficult in experiments and computer simulations. Here, a full-scale computational model was carefully constructed using molecular dynamics and first principles based on the artificial neural net algorithm and fully validated via accurate atomic layer deposition experiments. Overall, the machine-learning training results indicated that experimental observations and numerical simulation were in good agreement, demonstrating the feasibility and accuracy of our full-scale nanothermodynamics model. The computational simulations showed that the surface 3-layer atomic occupancy of the VO2 nanomaterials was a critical regulator of the phase transition temperature, implying as an important strategy for phase transition behavior in large-scale VO2 nanofilms designed on first principles. Moreover, the precise atomic layer deposition results agree with the simulation calculations by 98%, revealing that the computational model effectively and accurately combines with the experiments, and thus can accurately regulate the phase transition temperature of the VO2 nanofilm. This work was of crucial importance for potential applications of VO2 film in smart windows.

Frontispiz: Reversible Insulator–Metal Transition by Chemical Doping and Dedoping of a Mott Insulator

Leitfähige Materialien. In ihrer Zuschrift (e202206428) berichten Masaki Matsuda et al. über einen reversiblen Isolator-Metall-Übergang durch chemische Dotierung und Dedotierung eines Mott-Isolators. Die Ergebnisse weisen den Weg zu einer neuen Klasse stark korrelierter Materialien.

Reversible Insulator–Metal Transition by Chemical Doping and Dedoping of a Mott Insulator

AbstractThe chemical carrier doping of molecular Mott insulators has been poorly investigated to date due to its difficulty. In this study, iodine doping of a molecular Mott insulator, lithium phthalocyanine crystallized in the x‐form (x‐LiPc), was performed to obtain metallic x‐LiPcI. Crystal structure analysis revealed that iodine atoms penetrated channels of x‐LiPc and formed one‐dimensional chains. The Raman spectroscopy of x‐LiPcI indicated the existence of linear I5−, demonstrating a transition from a half‐filled band of the Mott insulating state to a 2/5‐filled band of the metallic state. Electrical resistivity measurements confirmed the metallic nature of x‐LiPcI, whereas a thermally activated behavior was observed for pristine x‐LiPc. Furthermore, the x‐LiPc Mott insulator was reproduced by dedoping iodine from x‐LiPcI, suggesting that the electronic state can be reversibly tuned between the Mott insulating and metallic states by chemical doping and dedoping.

Reversible Insulator-Metal Transition by Chemical Doping and Dedoping of a Mott Insulator.

The chemical carrier doping of molecular Mott insulators has been poorly investigated to date due to its difficulty. In this study, iodine doping of a molecular Mott insulator, lithium phthalocyanine crystallized in the x-form (x-LiPc), was performed to obtain metallic x-LiPcI. Crystal structure analysis revealed that iodine atoms penetrated channels of x-LiPc and formed one-dimensional chains. The Raman spectroscopy of x-LiPcI indicated the existence of linear I5 - , demonstrating a transition from a half-filled band of the Mott insulating state to a 2/5-filled band of the metallic state. Electrical resistivity measurements confirmed the metallic nature of x-LiPcI, whereas a thermally activated behavior was observed for pristine x-LiPc. Furthermore, the x-LiPc Mott insulator was reproduced by dedoping iodine from x-LiPcI, suggesting that the electronic state can be reversibly tuned between the Mott insulating and metallic states by chemical doping and dedoping.

The Origin of the Thermochromic Property Changes in Doped Vanadium Dioxide.

Vanadium dioxide is a promising material for novel smart window applications due to its reversible metal-insulator transition which is accompanied by a change in its optical properties. The transition temperature (TMIT) can be controlled via elemental doping, but the reduction of TMIT is generally coupled with a decrease of the optical contrast between the two phases. To better understand how the contrast is fundamentally connected to TMIT, the thermochromic properties of doped VO2 were theoretically investigated across the metal-insulator transition from first principles. Different dopants and their interaction with the VO2 host structure as well as different modes of doping were studied in detail. It was found that the transition temperature change is mainly related to the stabilization of the high-temperature metallic phase due to lattice deformations which are caused by the presence of the dopant ion. Inherent limitations to the thermochromic performance of VO2 substitutionally doped by the replacement of vanadium cations with other species were found, and alternative approaches were proposed. Specifically, a charge-neutral substitution of oxygen or an oxygen substitution in combination with interstitial doping without net charge transfer between the dopant atoms and VO2 were identified as promising avenues to ensure a low TMIT and no loss of optical contrast in vanadia-based smart window materials.

Modulation of the metal-insulator transition in VO2 nanoparticles with nRu+W (n=1–4) codoping: Implications for energy-saving smart windows

Vanadium dioxide (VO2), a remarkably popular thermochromic material, has attracted considerable attention due to its unique reversible metal-insulator phase transition characteristic, which can be used in practical applications such as energy-saving smart windows. However, the advance on enhancing the visible light transmittance while obtaining a useable phase transition temperature remains unresolved issue. Herein, the atomic structures and electronic-optical properties of nRu+W-codoped (n =1–4) VO2 were investigated by the first-principles calculations. The results show that the nRu + W-codoped VO2 is thermally and mechanically stable. The desirable nRu + W co-doped VO2 can be easily prepared under V-rich conditions, and that some V atoms were replaced by Ru and W atoms in the codoping structures, with the [RuO6] and [WO6] octahedra cosharing an edge or oxygen atoms. Furthermore, a lower phase transition temperature and the near-infrared modulation capacity of VO2 could be obtained in theoretical calculations due to the synergistic effect of nRu+W codoping. That is, the introduction of Ru atoms could widen the optical band gap of VO2, increasing the luminous transmittance; Simultaneously, the Fermi levels for nRu+W configurations gradually entered one of the narrow bands, decreasing the electronic band gap and further reducing the phase transition temperature due to the incorporation of the W dopant, which donates extra electrons to the codoping systems due to its higher ion valence. The current work provides an approach for improving the thermochromic performance of VO2 films in energy-saving smart windows, thereby providing new insights into the relationship between overall performance improvement and structural variations induced by elements codoping.

Unraveling Structural Phase Transformation by Simultaneously Determining the Lattice Constants and Mismatch Angle in VO2/Al2O3 Epitaxial Thin Films

As a prototype of a strongly correlated electron system, bulk vanadium dioxide (VO2) exhibits a large and reversible metal–insulator transition (MIT) near 340 K, concomitantly accompanied by a monoclinic–rutile structural phase transformation (SPT). In this study, we systematically investigated the SPT across the MIT in a (010)-VO2/(0001)-Al2O3 epitaxial thin film by simultaneously determining three lattice constants (a, b, and c) and the mismatch angle (Δβ) using high-resolution X-ray diffraction. The lattice constants a, b, and c were approximately 5.723, 4.521, and 5.393 Å, respectively, at room temperature, and the mismatch angle was approximately 122.02°. As the temperature increased, the lattice constants and mismatch angle did not change significantly until the temperature reached the MIT point. Then, a, b, and c suddenly increased to approximately 5.689 Å, 4.538 Å, and 5.411 Å, respectively, and retained this value up to nearly 90°C. However, the mismatch angle first slightly increased and then sharply decreased to 122.00°. Additionally, the lattice constants and mismatch angle were almost reproducible with decreasing temperature, except for hysteresis in the MIT region. These results verify that VO2 undergoes an MIT, simultaneously accompanied by SPT, in thicker films with small strain and weak substrate constraints, analogous to bulk VO2. This was further confirmed by in-situ varying-temperature Raman characterization. These findings provide insights into the SPT and reveal an angular parameter for judging the SPT in VO2 systems.

A liquid approach for the synthesis of M phase VO2 nanocrystals: Facile synthesis, characterization and reaction mechanism

Vanadium dioxide is considered to be a promising material in most fields because of its special reversible metal-insulator transition (MIT) near ambient temperature. Pure VO2(M) is usually synthesized by a hydrothermal method combined with a subsequent annealing process or one-step hydrothermal synthesis. Both synthetic routes have the disadvantages of long reaction times. In this report, a new synthesis method of VO2(M) nanoparticles was established under liquid conditions. First, the V2O4·2H2O precursor was prepared successfully by using N2H4·2HCl-reduced V2O5 under thermostatic stirring without any reagent auxiliary. Second, the V2O4·2H2O suspension in water transformed into VO2(M) nanocrystals under hydrothermal synthesis. The intermediate and final products were characterized by XRD, SEM, HRTEM, XPS and DSC. The morphology of prepared VO2(M) exhibits a snowflake-like structure (approximately 1.0–4.0 μm in length, 0.5–3.0 μm in width and 50–200 nm in thickness). The phase-transition temperature is 65.49 °C. In addition, the reaction route of "V2O4·2H2O →V2O4·xH2O → VO2(B) → VO2(M) transformation" and reaction mechanism were established by testing the precipitate at different times at the hydrothermal stage.