Metal-insulator Transition In VO2 Research Articles

Artificial afferent neurons based on the metal-insulator transition of VO2

Artificial afferent neurons based on the metal-insulator transition of VO2

Hidden magnetism and split off flat bands in the insulator metal transition in VO2

Transition metal d-electron oxides with an odd number of electrons per unit cell are expected to form metals with partially occupied energy bands, but exhibit in fact a range of behaviors, being either insulators, or metals, or having insulator-metal transitions. Traditional explanations involved predominantly electron-electron interactions in fixed structural symmetry. The present work focuses instead on the role of symmetry breaking local structural motifs. Viewing the previously observed V-V dimerization in VO2 as a continuous knob, reveals in density functional calculations the splitting of an isolated flat band from the broad conduction band. This leads past a critical percent dimerization to the formation of the insulating phase while lowering the total energy. In VO2 this transition is found to have a rather low energy barrier approaching the thermal energy at room temperature, suggesting energy-efficient switching in neuromorphic computing. Interestingly, sufficient V-V dimerization suppresses magnetism, leading to the nonmagnetic insulating state, whereas magnetism appears when dimerization is reduced, forming a metallic state. This study opens the way to design novel functional quantum materials with symmetry breaking-induced flat bands.

Single-site DFT+DMFT for vanadium dioxide using bond-centered orbitals

We present a combined density-functional theory and single-site dynamical mean-field theory (DMFT) study of vanadium dioxide (VO2) using an unconventional set of bond-centered orbitals as the basis of the correlated subspace. VO2 is a prototypical material undergoing a metal-insulator transition (MIT), hosting both intriguing physical phenomena and the potential for industrial applications. With our choice of correlated subspace basis, we investigate the interplay of structural dimerization and electronic correlations in VO2 in a computationally cheaper way compared to other state-of-the-art methods, such as cluster DMFT. Our approach allows us to treat the rutile and M1 monoclinic VO2 phases on an equal footing and to vary the dimerizing distortion continuously, exploring the energetics of the transition between the two phases. The choice of basis presented in this work hence offers a complementary view on the long-standing discussion of the MIT in VO2 and suggests possible future extensions to other similar materials hosting molecular-orbital-like states. Published by the American Physical Society 2024

VO2 thin films: various microstructures for hysteresis manipulations

Vanadium dioxide reveals a phase change transition at 68 °C associated to typical hysteresis curves with strong variations in physico-chemical properties. The study presented here aims to investigate the influence of the structure and the microstructure of VO2 condensed matter on optical transmittance and electrical resistivity hysteresis curves during the metal-insulator transition. The characteristics of VO2 thin films synthesized through conventional laser ablation and of VO2 nanoparticle stacks generated via a laser vaporization cluster source are investigated. This study reveals the capability to modulate the transition temperature by as much as 20 °C, the transition width within the range of 4–30 °C, and the resistivity dynamics across the semiconducting-to-metallic phase transition by more than two orders of magnitude. These effects appear to arise because of the change of microstructure, the physical percolation of nanoparticles and associated collective effects.More complex hysteresis behaviors, taking benefit of the fabrication processes presented in this paper, and coupling the two previous distinct VO2 architectures (thin film and nanoparticles stack) lead to optical transmittance curves with three transition levels. This approach offers flexibility beyond binary responses, facilitating the design of innovative optoelectronic devices with n-stages modulation capabilities. Near room temperature applications of VO2 metal-insulator transition are underscored, highlighting opportunities for tailored architectures to exploit its properties effectively.

Challenges for density functional theory in simulating metal-metal singlet bonding: A case study of dimerized VO2.

VO2 is renowned for its electric transition from an insulating monoclinic (M1) phase, characterized by V-V dimerized structures, to a metallic rutile (R) phase above 340K. This transition is accompanied by a magnetic change: the M1 phase exhibits a non-magnetic spin-singlet state, while the R phase exhibits a state with local magnetic moments. Simultaneous simulation of the structural, electric, and magnetic properties of this compound is of fundamental importance, but the M1 phase alone has posed a significant challenge to the density functional theory (DFT). In this study, we show none of the commonly used DFT functionals, including those combined with on-site Hubbard U to treat 3d electrons better, can accurately predict the V-V dimer length. The spin-restricted method tends to overestimate the strength of the V-V bonds, resulting in a small V-V bond length. Conversely, the spin-symmetry-breaking method exhibits the opposite trends. Each of these two bond-calculation methods underscores one of the two contentious mechanisms, i.e., Peierls lattice distortion or Mott localization due to electron-electron repulsion, involved in the metal-insulator transition in VO2. To elucidate the challenges encountered in DFT, we also employ an effective Hamiltonian that integrates one-dimensional magnetic sites, thereby revealing the inherent difficulties linked with the DFT computations.

Toward an Insulator-to-Metal Transition (IMT) in VOx Deposited by ALD: Huge Effect on Large IR Transmission Range

Vanadium dioxide (VO2) exhibits an insulating-to-metallic transition (IMT) from an insulating monoclinic to a metallic rutile phase at a IMT temperature (TIMT) of about 68 °C. This transition is accompanied by a decrease in electrical resistivity of 5 orders of magnitude and a drastic change in optical properties. Until recently, several works described the fundamental nature of the phase transition in a contradictory way, attributing it either to a structure-induced electron-phonon interaction: the Peierls transition [1], or to a strong electron-electron correlation: the Mott transition [2]. Nowadays, theoretical and experimental results [3-6] tend to show that the phase transition is affected by the crystal and electronic structures according to a mechanism combining the two opposed mechanisms and thus called collaborative Mott-Peierls transition.In order to achieve the growth of high-quality and conformal VO2 thin films and with a low thermal budget, Atomic Layer Deposition (ALD) is one of the most suitable ones, among physical and chemical vapor deposition techniques [7]. In this aim, vanadium oxide (VOx) films were deposited on both silicon (100) and glass substrates (figure with and without VOx) at 240°C using vanadium tri-isopropoxide (VTIP) asV-precursor and water as oxydant reagent. Deposited films, with thicknesses ranging from 30 nm to 120 nm depending on the number of ALD cycles, were annealed during one hour under a reducing atmosphere (forming gas) at four temperatures ranging from 400°C to 550°C with 50°C steps. Thanks to X-ray diffraction (XRD), and transmission electron microscopy (TEM), unannealed films were found amorphous independently of the substrate. Annealed films were found polycrystalline with a mixture of both VO2 and V2O5 (or V6O13) phases with crystallite lateral size up to 300 nm. RBS analysis showed how the chemical stoichiometry evolves with annealing temperature. The V content remains constant with annealing temperature while the O content and the film density were both decreasing under reducing conditions. By means of XPS measurements, at a temperature below TIMT, different valence states of V element were identified with a majority of V4+ ions and a minority of V3+ and V5+ ions. At a temperature above TIMT, a valence state modification occurs with a reduction of the V4+ ion concentration in favor of the V3+ and V5+ones. Cooling back under TIMT led almost to the same valence states initial reorganization with a majority of V4+.The variation of electrical and optical properties of VOx films occurring during IMT was also investigated. It was found that the resistivity r decreased down to 10-1 Ω.cm with a rising temperature above TIMT, and the r rose again with decreasing temperature with a hysteresis by contrast to the first drop. Dramatically, the optical transmittances of VOx films on Si wafer, measured with FTIR, displayed an important reduction during IMT of almost 70% in a broad spectral range from 2 to 20 µm wavelength. The optical properties of VOx on a glass substrate in UV-visible spectral range at IMT evidenced a modification of dielectric constant at IMT. Raman spectroscopy of VOx films around IMT showed the disappearance of low-temperature VO2 peaks, characteristic for this transition.In conclusion, crystalline VOx films were successfully produced by an ALD process and subsequent annealing at 500°C leading to films with reversible and reproducible IMT occurring at ~70°C. The important variation of some electrical or optical properties in infrared range paves the way for the light and heat monitoring in many applications including thermal stealth, heat management, tunable thermochromic or electrochromic windows.[1] Goodenough, J. B. The two components of the crystallographic transition in VO2.[2] Zylbersztejn, A. & Mott, N. F. Metal–insulator transition in vanadium dioxide. Phys. Rev. B 11, 4383–4395 (1975).[3] Cocker, T. L.; Titova, L. V.; Fourmaux, S.; Holloway, G.; Bandulet, H.-C.; Brassard, D.; Kieffer, J.-C.; El Khakani, M. A.; Hegmann, F. A. Phase Diagram of the Ultrafast Photoinduced, Insulator-Metal Transition in Vanadium Dioxide. Phys. Rev. B 2012,85, No. 155120.[4] Yao, T. et al. Understanding the Nature of the Kinetic Process in a VO2 Metal-Insulator Transition. Phys. Rev. Lett. 2010, 105, No. 226405.[5] Weber, C. et al. Vanadium dioxide: A Peierls–Mott insulator stable against disorder. Phys. Rev. Lett. 108, 256402 (2012).[6] Koethe, T. et al. Transfer of spectral weight and symmetry across the metal–insulator transition in VO2. Phys. Rev. Lett. 97, 116402(2006).[7] Prasadam, V. P. et al. Atomic layer deposition of vanadium oxides: process and application review. Mater. Today Chem. 12, 396–423 (2019). Figure 1

Magnetic field sensors using VO2/Fe3O4 nanoparticle devices * *This work was supported in part by the USTAR Program at the University of Utah.

We combined the metal-insulator transition (MIT) properties of VO2 and the magnetic properties of Fe3O4 to realize a magnetometer with very large nonlinearity and switching characteristics. VO2, Fe3O4 nanoparticles, and a conductive binder (silver paint) were mixed and drop-casted onto two-terminal gap junction devices. The device’s current–voltage characteristics exhibited current-switching behavior related to MIT in VO2 which changed with the external magnetic field. The magnetoresistance and magnetostriction in Fe3O4 both contributed to the field sensitivity of the sensor. Sensitivities as high as 1 A nT−1 (or 50.8 V T−1 with a current bias) were observed near the MIT voltage. The resulting minimum detectable signal was 20 pT/SQRT(Hz).

Zero-dimensionality of a scaled-down VO2 metal-insulator transition via high-resolution electrostatic gating

An understanding of the phase transitions at the nanoscale is essential in state-of-the-art engineering1–5, instead of simply averaging the heterogeneous domains formed during phase transitions6,7. However, as materials are scaled down, the steepness of the phase transition rapidly increases8–13 and requires extremely high precision in the control method. Here, a three-terminal device, which could precisely control the phase transition electrically14–19, was applied for the first time to a scaled-down metal-insulator transition material VO2. The crossover from continuous to binary transitions with the scaled-down material was clarified, and the critical channel length was successfully elucidated via phase boundary energy. Notably, below the critical channel length, the spatial degrees of freedom degenerated, and the impact of drain voltage application disappeared in the phase transition, indicating zero-dimensionality of the VO2 channel. This zero-dimensionality could be the fundamental property in the scaled-down phase transition and have a significant impact on various fields that need nanoscale engineering.

Nonvolatile Control of Metal-Insulator Transition in VO2 and Its Applications

With the advent of the 4th industrial revolution era, there has been a high demand for high-performance electronic devices capable of collecting, storing, and calculating vast amounts of data. Vanadium dioxide (VO<sub>2</sub>) is considered an attractive candidate for next-generation electronic devices as a prototypical strongly correlated material exhibiting a metal-insulator transition (MIT) accompanied by huge electrical resistivity changes in a few nanoseconds. The nonvolatile control of the MIT in VO<sub>2</sub> has recently been the subject of intensive research. In this report, we review recent advancements in the field of nonvolatile control of MIT in VO<sub>2</sub>, using electrochemical redox reactions, inverse piezoelectric effect, and ferroelectric polarization, and their potential to develop high-performance next-generation electronic devices.

Vanadium oxide metal-insulator phase transition in different types of one-dimensional photonic microcavities

The optical properties of vanadium dioxide (VO2) can be tuned via metal-insulator transition. In this work, different types of one-dimensional photonic structure-based microcavities that embed vanadium dioxide have been studied in the spectral range between 900 nm and 2000 nm. In particular, VO2 has been sandwiched between: i) two photonic crystals made of SiO2 and ZrO2; ii) two aperiodic structures made of SiO2 and ZrO2 that follow the Thue-Morse sequence; iii) two disordered photonic structures, made of SiO2 and ZrO2 in which the disorder is introduced either by a random sequence of the two materials or by a random variation of the thicknesses of the layers; iv) two four material-based photonic crystals made of SiO2, Al2O3, Y2O3, and ZrO2. The ordered structures i and iv show, respectively, one and two intense transmission valleys with defect modes, while the aperiodic and disordered structures ii and iii show a manifold of transmission valleys due to their complex layered configurations. The metal-insulator transition of VO2, controlled by temperature, results in a modulation of the optical properties of the microcavities.

Evaluation of V<sub>1</sub><sub>-<i>x</i></sub>Cr<i><sub>x</sub></i>O<sub>2</sub> Thin Films Fabricated by MOD

Cr-doped vanadium dioxide, V1-xCrxO2 (x = 0-0.20), thin films were fabricated by metal-organic decomposition. From X-ray diffraction patterns of the films at room temperature, VO2(M2)(201) and VO2(M2)(201) diffraction peaks were observed for x = 0.03-0.20, while a VO2(M1)(011) peak was observed for x = 0-0.01, where M1 and M2 represent different monoclinic structures of VO2. Moreover, all the fabricated films showed a p-type conductivity, and the hole concentration in the films increased with increasing x. From R-T characteristics of the films at the temperature range from 20 to 90℃, abrupt resistance changes caused by the metal-insulator transition in VO2 with a hysteresis loop were gradually suppressed with increasing x; especially x≥0.15, they were completely suppressed, since the transition temperature of the film shifted to a temperature above 140℃. Relatively flat temperature dependence of the temperature coefficients of resistance (TCR) as high as -3.9%/K in the range of 20-90℃ was obtained for the V0.85Cr0.15O2 thin film. The TCR value of the film is almost same as that of a V0.75Ti0.25O2 thin film, whereas a resistivity of the former at room temperature is one order lower than that of the latter.

An active tunable terahertz functional metamaterial based on hybrid-graphene vanadium dioxide.

In this paper, we propose a switchable and tunable functional metamaterial device based on hybrid graphene-vanadium dioxide (VO2). Using the properties of the metal-insulator transition in VO2, the proposed metamaterials can enable switching between tunable circular dichroism (CD) and dual-band perfect absorption in the terahertz region. When VO2 is in the insulator state, a polarization-selective single-band perfect absorption can be achieved for circularly polarized waves, thus resulting in a strong CD response with a maximum value of 0.84. When VO2 acts as a metal, there is a tunable dual-band perfect absorption for the designed metamaterial device under the illumination of x-polarization waves. The operation mechanism behind the phenomena can be explained by utilizing the electric field distribution and the coupled mode theory. Moreover, the influences of the Fermi energy of graphene and geometrical parameters on the CD and absorption spectra are discussed in detail. Our proposed switchable and tunable metamaterial can provide a platform for designing versatile functional devices in the terahertz region.

Harnessing the Metal-Insulator Transition of VO2 in Neuromorphic Computing.

Future-generation neuromorphic computing seeks to overcome the limitations of von Neumann architectures by colocating logic and memory functions, thereby emulating the function of neurons and synapses in the human brain. Despite remarkable demonstrations of high-fidelity neuronal emulation, the predictive design of neuromorphic circuits starting from knowledge of material transformations remains challenging. VO2 is an attractive candidate since it manifests a near-room-temperature, discontinuous, and hysteretic metal-insulator transition. The transition provides a nonlinear dynamical response to input signals, as needed to construct neuronal circuit elements. Strategies for tuning the transformation characteristics of VO2 based on modification of material properties, interfacial structure, and field couplings, are discussed. Dynamical modulation of transformation characteristics through in situ processing is discussed as a means of imbuing synaptic function. Mechanistic understanding of site-selective modification; external, epitaxial, and chemical strain; defect dynamics; and interfacial field coupling in modifying local atomistic structure, the implications therein for electronic structure, and ultimately, the tuning of transformation characteristics, is emphasized. Opportunities are highlighted for inverse design and for using design principles related to thermodynamics and kinetics of electronic transitions learned from VO2 to inform the design of new Mott materials, as well as to go beyond energy-efficient computation to manifest intelligence.

Antenna-coupled microbolometer based on VO2's non-linear properties across the metal–insulator transition region

This paper presents an antenna-coupled non-linear vanadium dioxide (VO2) microbolometer operating in the non-linear metal–insulator transition (MIT) region with an ultra-high responsivity of 6.55 × 104 V/W. Sputtered VO2 films used in this device exhibit 104 times change in resistivity between the dielectric and conductive states. The VO2 microbolometer is coupled to a wideband dipole antenna operating at 31–55 GHz and a coplanar waveguide for probed measurement. To enhance the sensitivity, the sensor is suspended in air by micro-electro-mechanical systems process. The large thermal coefficient of resistance of VO2 is utilized by DC biasing the device in the MIT region. Measurements for the fabricated sensor were performed, and a high responsivity was demonstrated, owing to non-linear conductivity change in the transition region. The measured sensitivity is >102 times higher than the state-of-the-art sensors. In addition, the concept of utilizing the proposed VO2 sensor in a mmWave imager was demonstrated by the radiation pattern measurement of a 4 × 4 (16 elements) antenna-coupled VO2 sensor array. The results presented in this work reveal the initial step to employ VO2's MIT for a hyper-sensitive sensor in future mmWave sensing and imaging applications.

A near-field study of VO2/(100)TiO2 film and its crack-induced strain relief

Temperature-induced metal–insulator transition (MIT) in vanadium dioxide (VO2) has been under intense research interest for decades both theoretically and experimentally. Due to the complex nature of electron correlations, the underlying physics behind the MIT in VO2 has yet to be fully grasped. In this work, we utilize the fine resolution of the scattering-type scanning near-field optical microscope to investigate the MIT in an epitaxial VO2 thin film on the (100)R TiO2 substrate with mid-infrared light. Bidirectional tweed-like metal–insulator phase coexistence patterns are observed and understood under the Landau free energy paradigm. More interestingly, delayed metallic nucleation is observed near the surface cracks due to local strain relief. This research proposes ideas in investigating the temperature–pressure phase diagram and tuning the interplay between local strain and MIT in oxide thin films.

The Role of Local DFT+U Minima in the First-Principles Modeling of the Metal-Insulator Transition in Vanadium Dioxide.

The DFT+U method is frequently employed to improve the first-principles description of strongly correlated materials. However, it is prone to deliver metastable electronic minima. While these local minima of the DFT+U method are often considered to be computational artifacts, their physical meaning and relationship to true excited states remains unclear. In this work, the possibility of theoretically modeling transformations in the solid state that require thermal or optical excitations of electrons is explored, taking into account the metastable states of the computationally undemanding DFT+U formalism. For this purpose, we choose to examine the example of the VO2 metal-insulator transition. Metastable states that are located on different electronic potential energy surfaces are found to correspond to experimentally observed VO2 phases. The identified metastable electronic states can be used to model the collapse of the VO2 band gap at elevated temperatures and upon photoexcitation as well as other monoclinic-monoclinic phase transformations. The results suggest that local DFT+U minima can indeed carry physical meaning, while they remain under-reported in theoretical literature on transition metal oxides like VO2.

Unveiling the Role of VO2 (B) Polymorph in the Insulator–Metal Transition of VO2 (M1) Thin Films

Characteristic first‐order insulator–metal transition (IMT) in VO2 (M1) phase finds a place in ample practical applications such as switches, transistors, memory devices, and energy‐saving windows. A polymorph VO2 (B) phase which may appear as an additional phase can strongly influence IMT in VO2 (M1) and not much is known about it. Here, the electrical conductivity and the electronic structure studies on the VO2 thin films on R‐ and C‐cut sapphire and silicon (Si) substrates are reported. X‐ray diffraction (XRD) and Raman spectra confirm the growth of VO2 (B) as well as VO2 (M1) polymorphs on the sapphire substrates and the pure VO2 (M1) phase on the Si substrate. The first‐order IMT is observed in all the thin films, however, due to the presence of the semimetallic VO2 (B) phase in the thin film on sapphire substrate, a decrease in the IMT strength and higher electrical conductivity have been observed. The electronic structure studies, by utilizing soft X‐ray absorption spectroscopy (SXAS) measurements, reveal that the presence of VO2 (B) phase not only causes an overall increase in the crystal field energy but also significantly affects the orbital occupancies around the Fermi level.

Determination of the effective thermal conductivity of particulate composites based on VO2 and SiO2

The effective thermal conductivity of composites made up of VO2 (SiO2) spherical particles randomly distributed and embedded in a SiO2 (VO2) matrix are numerically studied in a range of temperatures around the metal-insulator transition of VO2. This is done by means of three-dimensional finite element simulations for different concentrations and sizes of the particles as well as various interface thermal resistances. Our results are validated against the Mori-Tanaka analytical model. In addition, we develop a numerical method to calculate the heat storage capacity for composites with VO2 particles dispersed into a SiO2 matrix. It is shown that: i) The effective thermal conductivity of VO2/SiO2 composites increases with the VO2 particles' size, while the one of SiO2/VO2 composites is pretty much independent of the SiO2 particles' radius. ii) At the VO2 transition temperature (342.5 K), the effective thermal conductivity of VO2/SiO2 composites increases significantly at a rate of 2.7 × 10−3 Wm−1K−2, such that its value doubles up the SiO2 matrix thermal conductivity at the particle concentration of 40.2%. By contrast, the effective thermal conductivity of SiO2/VO2 composites decreases at a rate of 8.6 × 10−3 Wm−1K−2. iii) The effective thermal conductivity is strongly affected by the thermal resistance in VO2/SiO2 composites, by contrast the resistance effect does not play an important role for particle volume fractions of SiO2 up to 34.1% in SiO2/VO2 composites. The Mori-Tanaka model and our simulations predict the same trend of the effective thermal conductivity values of VO2/SiO2 composites. However, the analytical model fails when the matrix is made up of VO2 and the volumetric fraction of SiO2 exceeds 34.1%. The latent heat storage capacity of VO2/SiO2 composites increases with the VO2 particles’ concentration, such that at 40.2%, it takes the value of 24553 J kg−1 (486.7 cal mol−1), which is about half that of the pure VO2.

Reversible modulation of metal–insulator transition in VO2 via chemically induced oxygen migration

Metal-insulator transition (MIT), an intriguing correlated phenomenon induced by the subtle competition of the electrons' repulsive Coulomb interaction and kinetic energy, is of great potential use for electronic applications due to the dramatic change in resistivity. Here, we demonstrate a reversible control of MIT in VO2 films via oxygen stoichiometry engineering. By facilely depositing and dissolving a water-soluble yet oxygen-active Sr3Al2O6 capping layer atop VO2 at room temperature, oxygen ions can reversibly migrate between VO2 and Sr3Al2O6, resulting in a gradual suppression and a complete recovery of MIT in VO2. The migration of the oxygen ions is evidenced in a combination of transport measurement, structural characterization, and first-principles calculations. This approach of chemically induced oxygen migration using a water-dissolvable adjacent layer could be useful for advanced electronic and iontronic devices and studying oxygen stoichiometry effects on the MIT.

Tunable dual-band and high-quality-factor perfect absorption based on VO2-assisted metasurfaces.

Perfect absorbers with high quality factors (Q-factors) are of great practical significance for optical filtering and sensing. Moreover, tunable multiwavelength absorbers provide a multitude of possibilities for realizing multispectral light intensity manipulation and optical switches. In this study, we demonstrate the use of vanadium dioxide (VO2)-assisted metasurfaces for tunable dual-band and high-quality-factor perfect absorption in the mid-infrared region. In addition, we discuss the potential applications of these metasurfaces in reflective intensity manipulation and optical switching. The Q-factors of the dual-band perfect absorption in the proposed metasurfaces are greater than 1000, which can be attributed to the low radiative loss induced by the guided-mode resonances and low intrinsic loss from the constituent materials. By utilizing the insulator-metal transition in VO2, we further proved that a continuous tuning of the reflectance with a large modulation depth (31.8 dB) can be realized in the designed metasurface accompanied by a dual-channel switching effect. The proposed VO2-assisted metasurfaces have potential applications in dynamic and multifunctional optical devices, such as tunable multiband filters, mid-infrared biochemical sensors, optical switches, and optical modulators.