Suppression of metal-insulator transition in VO2 by electric field-induced oxygen vacancy formation

Voltage-Control of Magnetism in All-Solid-State and Solid/Liquid Magnetoelectric Composites

The control of magnetism by means of low-power electric fields, rather than dissipative flowing currents, has the potential to revolutionize conventional methods of data storage and processing, sensing, and actuation. A promising strategy relies on the utilization of magnetoelectric composites to finely tune the interplay between electric and magnetic degrees of freedom at the interface of two functional materials. Albeit early works predominantly focused on the magnetoelectric coupling at solid/solid interfaces; however, recently there has been an increased interest related to the opportunities offered by liquid-gating techniques. Here, a comparative overview on voltage control of magnetism in all-solid-state and solid/liquid composites is presented within the context of the principal coupling mediators, i.e., strain, charge carrier doping, and ionic intercalation. Further, an exhaustive and critical discussion is carried out, concerning the suitability of using the common definition of coupling coefficient α C = Δ M Δ E   to compare the strength of the interaction between electricity and magnetism among different magnetoelectric systems.

Oxygen-Valve Formed in Cobaltite-Based Heterostructures by Ionic Liquid and Ferroelectric Dual-Gating

Manipulation of oxygen vacancies via electric-field-controlled ionic liquid gating has been reported in many model systems within the emergent fields of oxide electronics and iontronics. It is then significant to investigate the oxygen vacancy formation/annihilation and migration across an additional ferroelectric layer with ionic liquid gating. Here, we report that via a combination of ionic liquid and ferroelectric gating, the remote control of oxygen vacancies and magnetic phase transition can be achieved in SrCoO_2.5 films capped with an ultrathin ferroelectric BaTiO₃ layer at room temperature. The ultrathin BaTiO₃ layer acts as an atomic oxygen valve and is semitransparent to oxygen-ion transport due to the competing interaction between vertical electron tunneling and ferroelectric polarization plus surface electrochemical changes in itself, thus resulting in the striking emergence of new mixed-phase SrCoO _x. The lateral coexistence of brownmillerite phase SrCoO_2.5 and perovskite phase SrCoO_3-δ was directly observed by transmission electron microscopy. Besides the fundamental significance of long-range interaction in ionic liquid gating, the ability to control the flow of oxygen ions across the heterointerface by the oxygen valve provides a new approach on the atomic scale for designing multistate memories, sensors, and solid-oxide fuel cells.

Low-Temperature Charging Dynamics of the Ionic Liquid and Its Gating Effect on FeSe0.5Te0.5 Superconducting Films

Ionic liquids (ILs) have been investigated extensively because of their unique ability to form the electric double layer (EDL), which induces high electrical field. For certain materials, low-temperature IL charging is needed to limit the electrochemical etching. Here, we report our investigation of the low-temperature charging dynamics in two widely used ILs-DEME-TF₂N and C₄mim-TF₂N. Results show that the formation of the EDL at ∼220 K requires several hours relative to milliseconds at room temperature, and an equivalent voltage V_e is introduced as a measure of the EDL formation during the biasing process. The experimental observation is supported by molecular dynamics simulation, which shows that the dynamics are logically a function of gate voltage, time, and temperature. To demonstrate the importance of understanding the charging dynamics, a 140 nm thick FeSe_0.5Te_0.5 film was biased using the DEME IL, showing a tunable T_c between 18 and 35 K. Notably, this is the first observation of the tunability of the T_c in thick film FeSe_0.5Te_0.5 superconductors.

Insulator-metal transition induced by electric voltage in a ruthenate Mott insulator

We report the observation of electric-voltage induced insulator-metal phase transition in a ruthenate Mott insulator Ca₃(Ru_0.9Ti_0.1)₂O₇. We show that the electric field effect dominates and leads to a sharp phase transition at measurement temperatures far below the Mott transition, whereas the thermal effect becomes more significant and broadens the phase transition as the measurement temperature approaches the insulator-metal transition. The electric field induced insulator-metal transition is presumably attributed to the avalanche breakdown of the correlated insulating state when driven out of equilibrium. This work highlights the strategy of using electric voltage to control the phase transition of this system in addition to other nonthermal parameters such as magnetic field and pressure reported previously.

A Ferrite Synaptic Transistor with Topotactic Transformation

Hardware implementation of artificial synaptic devices that emulate the functions of biological synapses is inspired by the biological neuromorphic system and has drawn considerable interest. Here, a three-terminal ferrite synaptic device based on a topotactic phase transition between crystalline phases is presented. The electrolyte-gating-controlled topotactic phase transformation between brownmillerite SrFeO_2.5 and perovskite SrFeO_{3- δ} is confirmed from the examination of the crystal and electronic structure. A synaptic transistor with electrolyte-gated ferrite films by harnessing gate-controllable multilevel conduction states, which originate from many distinct oxygen-deficient perovskite structures of SrFeO_x induced by topotactic phase transformation, is successfully constructed. This three-terminal artificial synapse can mimic important synaptic functions, such as synaptic plasticity and spike-timing-dependent plasticity. Simulations of a neural network consisting of ferrite synaptic transistors indicate that the system offers high classification accuracy. These results provide insight into the potential application of advanced topotactic phase transformation materials for designing artificial synapses with high performance.

Adding Solvent into Ionic Liquid-Gated Transistor: The Anatomy of Enhanced Gating Performance

Most studies of ionic liquid (IL)-gated field effect transistors (FETs) focus on the extremely large electric field and capacitance induced in liquid/solid interfaces and correspondingly the significantly enhanced carrier density in semiconductors, which can appreciably improve the gating performance. However, how to boost the switching speed, another key property of gating performance of FETs, has been rarely explored. In this work, the gating performance of molybdenum disulfide (MoS₂) FETs, gated by the mixtures of IL/organic solvent (1-butyl-3-methylimidazolium tetrafluoroborate/acetonitrile, [Bmim][BF₄]/ACN) at different ion concentrations, is investigated for both dynamic and static properties by a combination of molecular dynamics simulation and resistance network analysis. Results reveal that organic solvent can speed up the IL response time by a factor of about 40 times at the optimal ion concentration of 1.94 M, which is mainly attributed to the increased ionic conductivity of IL via the addition of organic solvent. Meanwhile, the surface charge distribution of MoS₂ becomes more homogenous after the addition of organic solvent, which increases the conductivity of MoS₂ by up to 2.4 times. Surprisingly, the optimal ion concentration for increased switching speed is nearly the same as that for achieving the highest MoS₂ conductivity. Thus, our findings provide a strategy to simultaneously improve the dynamic and static gating performance of IL-gated FETs as well as a modeling technique to screen out the ideal ion concentration.

Manipulate the Electronic and Magnetic States in NiCo2 O4 Films through Electric-Field-Induced Protonation at Elevated Temperature

Ionic-liquid-gating- (ILG-) induced proton evolution has emerged as a novel strategy to realize electron doping and manipulate the electronic and magnetic ground states in complex oxides. While the study of a wide range of systems (e.g., SrCoO_2.5 , VO₂ , WO₃ , etc.) has demonstrated important opportunities to incorporate protons through ILG, protonation remains a big challenge for many others. Furthermore, the mechanism of proton intercalation from the ionic liquid/solid interface to whole film has not yet been revealed. Here, with a model system of inverse spinel NiCo₂ O₄ , an increase in system temperature during ILG forms a single but effective method to efficiently achieve protonation. Moreover, the ILG induces a novel phase transformation in NiCo₂ O₄ from ferrimagnetic metallic into antiferromagnetic insulating with protonation at elevated temperatures. This study shows that environmental temperature is an efficient tuning knob to manipulate ILG-induced ionic evolution.

Daylight-Induced Metal-Insulator Transition in Ag-Decorated Vanadium Dioxide Nanorod Arrays

Metal-insulator transition (MIT) in strongly correlated electronic materials has enormous potential with scientific and technological impacts in future oxide nanoelectronic devices. Although photo-induced MIT can provide opportunities to extend the novel functionality of strongly correlated electronic materials, there have rarely been reports on it. Here, we report MIT provoked by visible-near-infrared light in Ag-decorated VO₂ nanorod arrays (NRs) because of localized surface plasmon resonance (LSPR) and its application to broadband photodetectors. Our simulation results based on the finite-difference time-domain method show that the electric field resulting from LSPR can be generated at the interface between Ag nanoparticles and VO₂ layers under vis NIR illumination. Using high-resolution transmission electronic microscopy and Raman spectroscopy, we observe the MIT and structural phase transition in the Ag-decorated VO₂ NRs due to the LSPR effect. The optoelectronic measurements confirm that high, fast, and broad photoresponse of Ag-decorated VO₂ NRs is attributed to photo-induced MIT due to LSPR. Our study will open up a new strategy to trigger MIT in strongly correlated electronic materials through functionalization with plasmonic nanoparticles and serve as a valuable proof of concept for next-generation optoelectronic devices with fast response, low power consumption, and high performance.

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.

Pseudodoping of a metallic two-dimensional material by the supporting substrate

Charge transfers resulting from weak bondings between two-dimensional materials and the supporting substrates are often tacitly associated with their work function differences. In this context, two-dimensional materials could be normally doped at relatively low levels. Here, we demonstrate how even weak hybridization with substrates can lead to an apparent heavy doping, using the example of monolayer 1H-TaS₂ grown on Au(111). Ab-initio calculations show that sizable changes in Fermi areas can arise, while the transferred charge between substrate and two-dimensional material is much smaller than the variation of Fermi areas suggests. This mechanism, which we refer to as pseudodoping, is associated with non-linear energy-dependent shifts of electronic spectra, which our scanning tunneling spectroscopy experiments reveal for clean and defective TaS₂ monolayer on Au(111). The influence of pseudodoping on the formation of many-body states in two-dimensional metallic materials is analyzed, shedding light on utilizing pseudodoping to control electronic phase diagrams.

Weak hybridization of two-dimensional metallic materials with their substrates plays a crucial role in charge transfer and doping characteristics. Here, the authors report heavy doping of monolayer 1H-TaS2 synthesized on Au(111) by ab-initio calculations and STM/STS experiments.

Electrolyte-based ionic control of functional oxides

The use of electrolyte gating to electrically control electronic, magnetic and optical properties of materials has seen strong recent growth, driven by the potential of the many devices and applications that such control may enable. Contrary to initial expectations of a purely electrostatic response based on electron or hole doping, electrochemical mechanisms based on the motion of ions are now understood to be common, suggesting promising new electrical control concepts.

Metal Oxide Nanocomposites: A Perspective from Strain, Defect, and Interface

Vertically aligned nanocomposite thin films with ordered two phases, grown epitaxially on substrates, have attracted tremendous interest in the past decade. These unique nanostructured composite thin films with large vertical interfacial area, controllable vertical lattice strain, and defects provide an intriguing playground, allowing for the manipulation of a variety of functional properties of the materials via the interplay among strain, defect, and interface. This field has evolved from basic growth and characterization to functionality tuning as well as potential applications in energy conversion and information technology. Here, the remarkable progress achieved in vertically aligned nanocomposite thin films from a perspective of tuning functionalities through control of strain, defect, and interface is summarized.

Observation of Weakened V-V Dimers in the Monoclinic Metallic Phase of Strained VO_{2}

Emergent order at mesoscopic length scales in condensed matter can provide fundamental insight into the underlying competing interactions and their relationship with the order parameter. Using spectromicroscopy, we show that mesoscopic stripe order near the metal-insulator transition (MIT) of strained VO_{2} represents periodic modulations in both crystal symmetry and V-V dimerization. Above the MIT, we unexpectedly find the long-range order of V-V dimer strength and crystal symmetry become dissociated beyond ≈200  nm, whereas the conductivity transition proceeds homogeneously in a narrow temperature range.

Understanding Electric Double-Layer Gating Based on Ionic Liquids: from Nanoscale to Macroscale

In electric double-layer transistors (EDLTs), it is well known that the EDL formed by ionic liquids (ILs) can induce an ultrahigh carrier density at the semiconductor surface, compared to solid dielectric. However, the mechanism of device performance is still not fully understood, especially at a molecular level. Here, we evaluate the gating performance of amorphous indium gallium zinc oxide (a-IGZO) transistor coupled with a series of imidazolium-based ILs, using an approach combining of molecular dynamics simulation and finite element modeling. Results reveal that the EDL with different ion structures could produce inhomogeneous electric fields at the solid-electrolyte interface, and the heterogeneity of electric field-induced charge distributions at semiconductor surface could reduce the electrical conductance of a-IGZO during gating process. Meanwhile, a resistance network analysis was adopted to bridge the nanoscopic data with the macroscopic transfer characteristics of IL-gated transistor, and showed that our theoretical results could well estimate the gating performance of practical devices. Thereby, our findings could provide both new concepts and modeling techniques for IL-gated transistors.

Dynamic Field Modulation of the Octahedral Framework in Metal Oxide Heterostructures

Control over the oxygen octahedral framework is widely recognized as key to the design of functional properties in perovskite oxide heterostructures. Although the oxygen octahedral framework can be manipulated during synthesis, the as-grown oxygen octahedra generally remain fixed, preventing the development of adaptive behavior in electronic and ionotronic systems. Here, it is demonstrated that the oxygen octahedral framework can be dynamically and reversibly manipulated by an electric field through the coupling with oxygen vacancies. Studying model WO₃ heterostructures during ionic liquid gating with a combination of in situ X-ray scattering and spectroscopy, it is shown that large changes in electronic properties can arise due to the increased flexibility of the octahedral network at high vacancy concentrations. The results describe a generic framework for the construction of dynamic systems and devices with an array of field-tunable properties.

Isostructural metal-insulator transition in VO2

The metal-insulator transition in correlated materials is usually coupled to a symmetry-lowering structural phase transition. This coupling not only complicates the understanding of the basic mechanism of this phenomenon but also limits the speed and endurance of prospective electronic devices. We demonstrate an isostructural, purely electronically driven metal-insulator transition in epitaxial heterostructures of an archetypal correlated material, vanadium dioxide. A combination of thin-film synthesis, structural and electrical characterizations, and theoretical modeling reveals that an interface interaction suppresses the electronic correlations without changing the crystal structure in this otherwise correlated insulator. This interaction stabilizes a nonequilibrium metallic phase and leads to an isostructural metal-insulator transition. This discovery will provide insights into phase transitions of correlated materials and may aid the design of device functionalities.

Growth without Postannealing of Monoclinic VO2 Thin Film by Atomic Layer Deposition Using VCl4 as Precursor

Vanadium dioxide (VO2) is a multifunctional material with semiconductor-to-metal transition (SMT) property. Organic vanadium compounds are usually employed as ALD precursors to grow VO2 films. However, the as-deposited films are reported to have amorphous structure with no significant SMT property, therefore a postannealing process is necessary for converting the amorphous VO2 to crystalline VO2. In this study, an inorganic vanadium tetrachloride (VCl4) is used as an ALD precursor for the first time to grow VO2 films. The VO2 film is directly crystallized and grown on the substrate without any postannealing process. The VO2 film displays significant SMT behavior, which is verified by temperature-dependent Raman spectrometer and four-point-probing system. The results demonstrate that the VCl4 is suitably employed as a new ALD precursor to grow crystallized VO2 films. It can be reasonably imagined that the VCl4 can also be used to grow various directly crystallized vanadium oxides by controlling the ALD-process parameters.

Energy Materials Design for Steering Charge Kinetics

Charge kinetics is a critical factor that determines working efficiencies of energy materials in their various applications. It is governed by electronic structures of the materials of interest and can be fine-tuned via purposeful adjustment of electronic structures. Recent advances in the development of energy materials with desirable electronic structures to steering charge kinetics toward specific applications are highlighted here. Two key strategies are presented: one is through the tuning of energy states and the other is to control spatial distributions of charges. Each strategy is described by several different schemes. Finally, the challenges and perspectives in designing energy materials with fine control of charge kinetics are discussed.

Dynamic Electronic Doping for Correlated Oxides by a Triboelectric Nanogenerator

The metal-insulator transition of vanadium dioxide (VO₂ ) is exceptionally sensitive to charge density and electron orbital occupancy. Thus three-terminal field-effect transistors with VO₂ channels are widely adopted to control the phase transition by external gating voltage. However, current leakage, electrical breakdown, or interfacial electrochemical reactions may be inevitable if conventional solid dielectrics or ionic-liquid layers are used, which possibly induce Joule heating or doping in the VO₂ layer and make the voltage-controlled phase transition more complex. Here, a triboelectric nanogenerator (TENG) and a VO₂ film are combined for a novel TENG-VO₂ device, which can overcome the abovementioned challenges and achieve electron-doping-induced phase modulation. By taking advantage of the TENG structure, electrons can be induced in the VO₂ channel and thus adjust the electronic states of the VO₂ , simultaneously. The modulation degree of the VO₂ resistance depends on the temperature, and the major variation occurs when the temperature is in the phase-transition region. The accumulation of electrons in the VO₂ channel also is simulated by finite element analysis, and the electron doping mechanism is verified by theoretical calculations. The results provide a promising approach to develop a novel type of tribotronic transistor and new electronic doping technology.

Voltage-Controlled ON-OFF Ferromagnetism at Room Temperature in a Single Metal Oxide Film

Electric-field-controlled magnetism can boost energy efficiency in widespread applications. However, technologically, this effect is facing important challenges: mechanical failure in strain-mediated piezoelectric/magnetostrictive devices, dearth of room-temperature multiferroics, or stringent thickness limitations in electrically charged metallic films. Voltage-driven ionic motion (magneto-ionics) circumvents most of these drawbacks while exhibiting interesting magnetoelectric phenomena. Nevertheless, magneto-ionics typically requires heat treatments and multicomponent heterostructures. Here we report on the electrolyte-gated and defect-mediated O and Co transport in a Co₃O₄ single layer which allows for room-temperature voltage-controlled ON-OFF ferromagnetism (magnetic switch) via internal reduction/oxidation processes. Negative voltages partially reduce Co₃O₄ to Co (ferromagnetism: ON), resulting in graded films including Co- and O-rich areas. Positive bias oxidizes Co back to Co₃O₄ (paramagnetism: OFF). This electric-field-induced atomic-scale reconfiguration process is compositionally, structurally, and magnetically reversible and self-sustained, since no oxygen source other than the Co₃O₄ itself is required. This process could lead to electric-field-controlled device concepts for spintronics.

Oxygen Electromigration and Energy Band Reconstruction Induced by Electrolyte Field Effect at Oxide Interfaces

Electrolyte gating is a powerful means for tuning the carrier density and exploring the resultant modulation of novel properties on solid surfaces. However, the mechanism, especially its effect on the oxygen migration and electrostatic charging at the oxide heterostructures, is still unclear. Here we explore the electrolyte gating on oxygen-deficient interfaces between SrTiO_{3} (STO) crystals and LaAlO_{3} (LAO) overlayer through the measurements of electrical transport, x-ray absorption spectroscopy, and photoluminescence spectra. We found that oxygen vacancies (O_{vac}) were filled selectively and irreversibly after gating due to oxygen electromigration at the amorphous LAO/STO interface, resulting in a reconstruction of its interfacial band structure. Because of the filling of O_{vac}, the amorphous interface also showed an enhanced electron mobility and quantum oscillation of the conductance. Further, the filling effect could be controlled by the degree of the crystallinity of the LAO overlayer by varying the growth temperatures. Our results reveal the different effects induced by electrolyte gating, providing further clues to understand the mechanism of electrolyte gating on buried interfaces and also opening a new avenue for constructing high-mobility oxide interfaces.

Elasticity Modulation Due to Polarization Reversal and Ionic Motion in the Ferroelectric Superionic Conductor KTiOPO4

The coupling between ionic degrees of freedom and ferroelectricity has received renewed attention in recent years, given that surface electrochemical processes have been shown to be intrinsically linked to ferroelectric phase stability in ultrathin ferroelectric films. However, the coupling between bulk ionic transport and local polarization switching has received less attention, as typically the bulk ionic mobilities are low for common ferroelectrics at room temperature. Here, we use the coupled band-excitation method in conjunction with site-correlated time-of-flight secondary ion mass spectrometry, to determine the coupling between ferroelectric switching and ionic motion in single crystal KTiOPO₄. The local scanning probe measurements indicate a substantial softening, as determined by resonant frequency changes, during reversal of polarization along one direction. These changes are correlated with the mass spectrometry measurements, showing a polarization-dependent accumulation of K ions at the polar surfaces, thus corroborating their role in the screening process. These studies shed light on the interplay between ionic dynamics and bulk ferroelectric switching and have implications for studies on domain wall conductivity, chemical switching, and bulk and surface-screening phenomena.

Deterministic, Reversible, and Nonvolatile Low-Voltage Writing of Magnetic Domains in Epitaxial BaTiO3/Fe3O4 Heterostructure

The ability to electrically write magnetic bits is highly desirable for future magnetic memories and spintronic devices, though fully deterministic, reversible, and nonvolatile switching of magnetic moments by electric field remains elusive despite extensive research. In this work, we develop a concept to electrically switch magnetization via polarization modulated oxygen vacancies, and we demonstrate the idea in a multiferroic epitaxial heterostructure of BaTiO₃/Fe₃O₄ fabricated by pulsed laser deposition. The piezoelectricity and ferroelectricity of BaTiO₃ have been confirmed by macro- and microscale measurements, for which Fe₃O₄ serves as the top electrode for switching the polarization. X-ray absorption spectroscopy and X-ray magnetic circular dichroism spectra indicate a mixture of Fe²⁺ and Fe³⁺ at O _h sites and Fe³⁺ at T _d sites in Fe₃O₄, while the room-temperature magnetic domains of Fe₃O₄ are revealed by microscopic magnetic force microscopy measurements. It is demonstrated that the magnetic domains of Fe₃O₄ can be switched by not only magnetic fields but also electric fields in a deterministic, reversible, and nonvolatile manner, wherein polarization reversal by electric field modulates the oxygen vacancy distribution in Fe₃O₄, and thus its magnetic state, making it attractive for electrically written magnetic memories.

Magnetic irreversibility in VO2/Ni bilayers

We studied the temperature dependence of the magnetic properties of VO₂/Ni bilayers. The Ni films were deposited on either monoclinic or rutile phase VO₂. The temperature induced VO₂ transformation from a monoclinic to a rutile structure induces strain in the Ni film. Due to an inverse magnetoelastic effect the coercivity of the Ni films is strongly modified. Both Ni films show strong enhancement of the coercivity near the transition temperature. The coercivity enhancement of Ni is associated with the phase coexistence observed in the VO₂ first order phase transition. Above the transition temperature, Ni deposited on monoclinic VO₂ shows a coercivity enhancement whereas Ni deposited on rutile VO₂ shows suppression of the coercivity. The samples were cycled several times to check if the changes in coercivity were reversible. While samples with Ni deposited on rutile VO₂ show reversibility, samples with Ni deposited on monoclinic VO₂ shown an irreversibility after the first structural phase transition. This irreversibility can be associated with cracking of the VO₂ layer as it relieves stress due to the transition and has implications for the resistance versus temperature behavior of the VO₂.

Manipulating Behaviors from Heavy Tungsten Doping on Interband Electronic Transition and Orbital Structure Variation of Vanadium Dioxide Films

Vanadium dioxide (VO₂) with a metal-insulator transition (MIT) has been supposed as a candidate for optoelectronic devices. However, the MIT temperature ( T_MIT) above room temperature limits its application scope. Here, high-quality V_{1- x}W _xO₂ films have been prepared by pulsed laser deposition. On the basis of temperature-dependent transmittance and Raman spectra, it was found that T_MIT increases from 241 to 279 K, when increasing the doping concentration in the range of 0.16 ≤ x ≤ 0.20. The interband electronic transitions and orbital structures of V_{1- x}W _xO₂ films have been investigated via fitting transmittance spectra. Moreover, with the aid of first-principles calculations, an effective orbital theory has been proposed to explain the unique phenomenon. When the W doping concentration increases, the π* and d_II orbitals shift toward the π orbital. Meanwhile, the energy gap between the π* and d_II orbitals decreases at the insulator state. It indicates that the bandwidth is narrowed, which impedes MIT. In addition, the overlap of the π* and d_II orbitals increases at the metal state, and more doping electrons occupy the π* orbital induced by increasing W doping concentration. It manifests that the Mott insulating state becomes more stable, which further improves T_MIT. The present work provides a feasible approach to tune T_MIT via orbital variation and can be helpful in developing the potential VO₂-based optoelectronic devices.

Electrochemical Tuning of Metal Insulator Transition and Nonvolatile Resistive Switching in Superconducting Films

Modulation of carrier concentration in strongly correlated oxides offers the unique opportunity to induce different phases in the same material, which dramatically change their physical properties, providing novel concepts in oxide electronic devices with engineered functionalities. This work reports on the electric manipulation of the superconducting to insulator phase transition in YBa2Cu3O7-δ thin films by electrochemical oxygen doping. Both normal state resistance and the superconducting critical temperature can be reversibly manipulated in confined active volumes of the film by gate-tunable oxygen diffusion. Vertical and lateral oxygen mobility may be finely modulated, at the micro- and nano-scale, by tuning the applied bias voltage and operating temperature thus providing the basis for the design of homogeneous and flexible transistor-like devices with loss-less superconducting drain-source channels. We analyze the experimental results in light of a theoretical model, which incorporates thermally activated and electrically driven volume oxygen diffusion.

Large Intercalation Pseudocapacitance in 2D VO2 (B): Breaking through the Kinetic Barrier

VO2 (B) features two lithiation/delithiation processes, one of which is kinetically facile and has been commonly observed at 2.5 V versus Li/Li+ in various VO2 (B) structures. In contrast, the other process, which occurs at 2.1 V versus Li/Li+ , has only been observed at elevated temperatures due to large interaction energy barrier and extremely sluggish kinetics. Here, it is demonstrated that a rational design of atomically thin, 2D nanostructures of VO2 (B) greatly lowers the interaction energy and Li+ -diffusion barrier. Consequently, the kinetically sluggish step is successfully enabled to proceed at room temperature for the first time ever. The atomically thin 2D VO2 (B) exhibits fast charge storage kinetics and enables fully reversible uptake and removal of Li ions from VO2 (B) lattice without a phase change, resulting in exceptionally high performance. This work presents an effective strategy to speed up intrinsically sluggish processes in non-van der Waals layered materials.

Hydrothermal One-Step Synthesis of Highly Dispersed M-Phase VO2 Nanocrystals and Application to Flexible Thermochromic Film

Preparation of ultrafine highly dispersed VO₂(M) nanoparticles that are essential materials to fabricate thermochromic flexible films remains a challenge, preventing effective use of their promising properties. Here, we report an original hydrothermal approach by controlling oxidizing atmosphere of reaction with hydrogen peroxide to prepare ultrafine VO₂(M) nanoparticles free from annealing. Hydrogen peroxide is separated from precursor solution in a reactor, which creates a moderate oxygenation environment, enabling the formation of stoichiometric VO₂(M) nanoparticles. The obtained VO₂(M) nanoparticles are well-dispersed, highly uniform, and single-phase, with an average particle size ∼30 nm. The flexible thermochromic films fabricated with the VO₂(M) nanoparticles exhibit excellent thermochromic performance with a solar modulation efficiency of 12.34% and luminous transmittance of 54.26%. While the films prepared with annealed nanoparticles show reduced transmittance due to light scattering of the large size particles resulting from agglomeration and growth during annealing. This work demonstrates a promising technique to realize moderate oxidizing atmosphere by hydrothermal process for preparing well-dispersed stoichiometric nano-oxides.

Direct imaging of structural changes induced by ionic liquid gating leading to engineered three-dimensional meso-structures

The controlled transformation of materials, both their structure and their physical properties, is key to many devices. Ionic liquid gating can induce the transformation of thin-film materials over long distances from the gated surface. Thus, the mechanism underlying this process is of considerable interest. Here we directly image, using in situ, real-time, high-resolution transmission electron microscopy, the reversible transformation between the oxygen vacancy ordered phase brownmillerite SrCoO_2.5 and the oxygen ordered phase perovskite SrCoO₃. We show that the phase transformation boundary moves at a velocity that is highly anisotropic, traveling at speeds ~30 times faster laterally than through the thickness of the film. Taking advantage of this anisotropy, we show that three-dimensional metallic structures such as cylinders and rings can be realized. Our results provide a roadmap to the construction of complex meso-structures from their exterior surfaces.

Local and reversible oxidation is used to exploit the very different properties of oxygen and vacancy ordered oxides. Here the authors directly image and make use of anisotropic migration velocities of oxygen in SrCoO_x to create 3D meso-structures of those two phases by ionic liquid gating.

Integrated Circuits Comprising Patterned Functional Liquids

Solid-state heterostructures are the cornerstone of modern electronics. To enhance the functionality and performance of integrated circuits, the spectrum of materials used in the heterostructures is being expanded by an increasing number of compounds and elements of the periodic table. While the integration of liquids and solid-liquid interfaces into such systems would allow unique and advanced functional properties and would enable integrated nanoionic circuits, solid-state heterostructures that incorporate liquids have not been considered thus far. Here solid-state heterostructures with integrated liquids are proposed, realized, and characterized, thereby opening a vast, new phase space of materials and interfaces for integrated circuits. Devices containing tens of microscopic capacitors and field-effect transistors are fabricated by using integrated patterned NaCl aqueous solutions. This work paves the way to integrated electronic circuits that include highly integrated liquids, thus yielding a wide array of novel research and application opportunities based on microscopic solid/liquid systems.

Single-Crystal Pt-Decorated WO3 Ultrathin Films: A Platform for Sub-ppm Hydrogen Sensing at Room Temperature

Hydrogen-related technologies are rapidly developing, driven by the necessity of efficient and high-density energy storage. This poses new challenges to the detection of dangerous gases, in particular the realization of cheap, sensitive, and fast hydrogen sensors. Several materials are being studied for this application, but most present critical bottlenecks, such as high operational temperature, low sensitivity, slow response time, and/or complex fabrication procedures. Here, we demonstrate that WO₃ in the form of single-crystal, ultrathin films with a Pt catalyst allows high-performance sensing of H₂ gas at room temperature. Thanks to the high electrical resistance in the pristine state, this material is able to detect hydrogen concentrations down to 1 ppm near room temperature. Moreover, the high surface-to-volume ratio of WO₃ ultrathin films determines fast sensor response and recovery, with characteristic times as low as 1 s when the concentration exceeds 100 ppm. By modeling the hydrogen (de)intercalation dynamics with a kinetic model, we extract the energy barriers of the relevant processes and relate the doping mechanism to the formation of oxygen vacancies. Our results reveal the potential of single-crystal WO₃ ultrathin films toward the development of sub-ppm hydrogen detectors working at room temperature.

Nanoscale Control of Oxygen Defects and Metal-Insulator Transition in Epitaxial Vanadium Dioxides

Strongly correlated vanadium dioxide (VO₂) is one of the most promising materials that exhibits a temperature-driven, metal-insulator transition (MIT) near room temperature. The ability to manipulate the MIT at nanoscale offers both insight into understanding the energetics of phase transition and a promising potential for nanoelectronic devices. In this work, we study nanoscale electrochemical modifications of the MIT in epitaxial VO₂ thin films using a combined approach with scanning probe microscopy (SPM) and theoretical calculations. We find that applying electric voltages of different polarity through an SPM tip locally changes the contact potential difference and conductivity on the surface of VO₂ by modulating the oxygen stoichiometry. We observed nearly 2 orders of magnitude change in resistance between positive and negative biased-tip written areas of the film, demonstrating the electric field modulated MIT behavior at the nanoscale. Density functional theory calculations, benchmarked against more accurate many-body quantum Monte Carlo calculations, provide information on the formation energetics of oxygen defects that can be further manipulated by strain. This study highlights the crucial role of oxygen vacancies in controlling the MIT in epitaxial VO₂ thin films, useful for developing advanced electronic and iontronic devices.

Ion Migration Studies in Exfoliated 2D Molybdenum Oxide via Ionic Liquid Gating for Neuromorphic Device Applications

The formation of an electric double layer in ionic liquid (IL) can electrostatically induce charge carriers and/or intercalate ions in and out of the lattice which can trigger a large change of the electronic, optical, and magnetic properties of materials and even modify the crystal structure. We present a systematic study of ionic liquid gating of exfoliated 2D molybdenum trioxide (MoO₃) devices and correlate the resultant electrical properties to the electrochemical doping via ion migration during the IL biasing process. A nearly 9 orders of magnitude modulation of the MoO₃ conductivity is obtained for the two types of ionic liquids that are investigated. In addition, notably rapid on/off switching was realized through a lithium-containing ionic liquid whereas much slower modulation was induced via oxygen extraction/intercalation. Time of flight-secondary ion mass spectrometry confirms the Li intercalation. Density functional theory (DFT) calculations have been carried out to examine the underlying metallization mechanism. Results of short-pulse tests show the potential of these MoO₃ devices as neuromorphic computing elements due to their synaptic plasticity.

Antisite-disorder engineering in La-based oxide heterostructures via oxygen vacancy control

It has been realized lately that disorder, primarily in the form of oxygen vacancies, cation stoichiometry, atomic inter-diffusion and antisite defects, has a major effect on the electronic and transport properties of a 2D electron liquid at oxide hetero-interfaces - the first and the last being the two key players. In order to delineate the roles of these two key factors, we have investigated the effect of oxygen vacancies on the antisite disorder at a large number of interfaces separating two La-based transition metal oxides, using density functional theory. The oxygen vacancy is found to suppress antisite disorder in some heterostructures thereby stabilizing the ordered structure, while in some others, it tends to favor disorder, opening up the possibility of using it to control the order. Our calculations show that the oxygen vacancy offers an opportunity to generate new magnetic states by manipulating the inter-site coupling. Moreover, it can be used to control the electrical transport. The oxygen vacancy and antisite disorder are intrinsic to oxide heterostructures and it is therefore incumbent to engineer the latter and tune the magnetic and transport properties by controlling the oxygen partial pressure during growth.

Nature of the metal-insulator transition in few-unit-cell-thick LaNiO3 films

The nature of the metal-insulator transition in thin films and superlattices of LaNiO₃ only a few unit cells in thickness remains elusive despite tremendous effort. Quantum confinement and epitaxial strain have been evoked as the mechanisms, although other factors such as growth-induced disorder, cation non-stoichiometry, oxygen vacancies, and substrate-film interface quality may also affect the observable properties of ultrathin films. Here we report results obtained for near-ideal LaNiO₃ films with different thicknesses and terminations grown by atomic layer-by-layer laser molecular beam epitaxy on LaAlO₃ substrates. We find that the room-temperature metallic behavior persists until the film thickness is reduced to an unprecedentedly small 1.5 unit cells (NiO₂ termination). Electronic structure measurements using X-ray absorption spectroscopy and first-principles calculation suggest that oxygen vacancies existing in the films also contribute to the metal-insulator transition.

Revealing the role of oxygen vacancies on the phase transition of VO2 film from the optical-constant measurements

Vanadium dioxide (VO₂) material shows a distinct metal–insulator transition (MIT) at the critical temperature of ∼340 K. Similar to other correlated oxides, the MIT properties of VO₂ is always sensitive to those crystal defects such as oxygen vacancies. In this study, we investigated the oxygen vacancies related phase transition behavior of VO₂ crystal film and systematically examined the effect of oxygen vacancies from the optical constant measurements. The results indicated that the oxygen vacancies changed not only the electron occupancy on V 3d–O 2p hybrid-orbitals, but also the electron–electron correlation energy and the related band gap, which modulated the MIT behavior and decreased the critical temperature resultantly. Our work not only provided a facile way to modulate the MIT behavior of VO₂ crystal film, but also revealed the effects of the oxygen vacancies on the electronic inter-band transitions as well as the electronic correlations in driving this MIT process.

Optical conductivity spectroscopy was performed to reveal the role of oxygen vacancies during VO₂ metal–insulator transition.

Ambipolar ferromagnetism by electrostatic doping of a manganite

Complex-oxide materials exhibit physical properties that involve the interplay of charge and spin degrees of freedom. However, an ambipolar oxide that is able to exhibit both electron-doped and hole-doped ferromagnetism in the same material has proved elusive. Here we report ambipolar ferromagnetism in LaMnO₃, with electron-hole asymmetry of the ferromagnetic order. Starting from an undoped atomically thin LaMnO₃ film, we electrostatically dope the material with electrons or holes according to the polarity of a voltage applied across an ionic liquid gate. Magnetotransport characterization reveals that an increase of either electron-doping or hole-doping induced ferromagnetic order in this antiferromagnetic compound, and leads to an insulator-to-metal transition with colossal magnetoresistance showing electron-hole asymmetry. These findings are supported by density functional theory calculations, showing that strengthening of the inter-plane ferromagnetic exchange interaction is the origin of the ambipolar ferromagnetism. The result raises the prospect of exploiting ambipolar magnetic functionality in strongly correlated electron systems.

Fine structure of metal-insulator transition in EuO resolved by doping engineering

Metal-insulator transitions (MITs) offer new functionalities for nanoelectronics. However, ongoing attempts to control the resistivity by external stimuli are hindered by strong coupling of spin, charge, orbital and lattice degrees of freedom. This difficulty presents a quest for materials which exhibit MIT caused by a single degree of freedom. In the archetypal ferromagnetic semiconductor EuO, magnetic orders dominate the MIT. Here we report a new approach to take doping under control in this material on the nanoscale: formation of oxygen vacancies is strongly suppressed to exhibit the highest MIT resistivity jump and magnetoresistance among thin films. The nature of the MIT is revealed in Gd doped films. The critical doping is determined to be more than an order of magnitude lower than in all previous studies. In lightly doped films, a remarkable thermal hysteresis in resistivity is discovered. It extends over 100 K in the paramagnetic phase reaching 3 orders of magnitude. In the warming mode, the MIT is shown to be a two-step process. The resistivity patterns are consistent with an active role of magnetic polarons-formation of a narrow band and its thermal destruction. High-temperature magnetic polaron effects include large negative magnetoresistance and ferromagnetic droplets revealed by x-ray magnetic circular dichroism. Our findings have wide-range implications for the understanding of strongly correlated oxides and establish fundamental benchmarks to guide theoretical models of the MIT.

Natural and induced growth of VO2 (M) on VO2 (B) ultrathin films

This work examines the synthesis of single phase VO₂ (B) thin films on LaAlO₃ (100) substrates, and the naturally-occurring and induced subsequent growth of VO₂ (M) phase on VO₂ (B) films. First, the thickness (t) dependence of structural, morphological and electrical properties of VO₂ films is investigated, evidencing that the growth of VO₂ (B) phase is progressively replaced by that of VO₂ (M) when t > ~11 nm. This change originates from the relaxation of the substrate-induced strain in the VO₂ (B) films, as corroborated by the simultaneous increase of surface roughness and decrease of the c-axis lattice parameter towards that of bulk VO₂ (B) for such films, yielding a complex mixed-phase structure composed of VO₂ (B)/VO₂ (M) phases, accompanied by the emergence of the VO₂ (M) insulator-to-metal phase transition. Second, the possibility of inducing this phase conversion, through a proper surface modification of the VO₂ (B) films via plasma treatment, is demonstrated. These natural and induced VO₂ (M) growths not only provide substantial insights into the competing nature of phases in the complex VO₂ polymorphs system, but can also be further exploited to synthesize VO₂ (M)/VO₂ (B) heterostructures at the micro/nanoscale for advanced electronics and energy applications.

Ambiguous Role of Growth-Induced Defects on the Semiconductor-to-Metal Characteristics in Epitaxial VO2/TiO2 Thin Films

Controlling the semiconductor-to-metal transition temperature in epitaxial VO₂ thin films remains an unresolved question both at the fundamental as well as the application level. Within the scope of this work, the effects of growth temperature on the structure, chemical composition, interface coherency and electrical characteristics of rutile VO₂ epitaxial thin films grown on TiO₂ substrates are investigated. It is hereby deduced that the transition temperature is lower than the bulk value of 340 K. However, it is found to approach this value as a function of increased growth temperature even though it is accompanied by a contraction along the V⁴⁺-V⁴⁺ bond direction, the crystallographic c-axis lattice parameter. Additionally, it is demonstrated that films grown at low substrate temperatures exhibit a relaxed state and a strongly reduced transition temperature. It is suggested that, besides thermal and epitaxial strain, growth-induced defects may strongly affect the electronic phase transition. The results of this work reveal the difficulty in extracting the intrinsic material response to strain, when the exact contribution of all strain sources cannot be effectively determined. The findings also bear implications on the limitations in obtaining the recently predicted novel semi-Dirac point phase in VO₂/TiO₂ multilayer structures.

Electric-field Control of Electronic States in WS2 Nanodevices by Electrolyte Gating

A method of carrier number control by electrolyte gating is demonstrated. We have obtained WS2 thin flakes with atomically flat surface via scotch tape method or individual WS2 nanotubes by dispersing the suspension of WS2 nanotubes. The selected samples have been fabricated into devices by the use of the electron beam lithography and electrolyte is put on the devices. We have characterized the electronic properties of the devices under applying the gate voltage. In the small gate voltage region, ions in the electrolyte are accumulated on the surface of the samples which leads to the large electric potential drop and resultant electrostatic carrier doping at the interface. Ambipolar transfer curve has been observed in this electrostatic doping region. When the gate voltage is further increased, we met another drastic increase of source-drain current which implies that ions are intercalated into layers of WS2 and electrochemical carrier doping is realized. In such electrochemical doping region, superconductivity has been observed. The focused technique provides a powerful strategy for achieving the electric-filed-induced quantum phase transition.