Gate‐Induced Massive and Reversible Phase Transition of VO2 Channels Using Solid‐State Proton Electrolytes

The use of gate bias to control electronic phases in VO₂, an archetypical correlated oxide, offers a powerful method to probe their underlying physics, as well as for the potential to develop novel electronic devices. Up to date, purely electrostatic gating in 3‐terminal devices with correlated channel shows the limited electrostatic gating efficiency due to insufficiently induced carrier density and short electrostatic screening length. Here massive and reversible conductance modulation is shown in a VO₂ channel by applying gate bias V_G at low voltage by a solid‐state proton (H⁺) conductor. By using porous silica to modulate H⁺ concentration in VO₂, gate‐induced reversible insulator‐to‐metal (I‐to‐M) phase transition at low voltage, and unprecedented two‐step insulator‐to‐metal‐to‐insulator (I‐to‐M‐to‐I) phase transition at high voltage are shown. V_G strongly and efficiently injects H⁺ into the VO₂ channel without creating oxygen deficiencies; this H⁺‐induced electronic phase transition occurs by giant modulation (≈7%) of out‐of‐plane lattice parameters as a result of H⁺‐induced chemical expansion. The results clarify the role of H⁺ on the electronic state of the correlated phases, and demonstrate the potentials for electronic devices that use ionic/electronic coupling.     

Photoinduced Colossal Magnetoresistance under Substantially Reduced Magnetic Field

The colossal magnetoresistive insulator to metal switching of almost nine orders of magnitude under the significantly reduced magnetic field is achieved by illumination for the low bandwidth manganite thin films. Similarly, by changing the measuring bias voltage through the sample the required magnetic field for insulator–metal transition can be further fine‐tuned. By applying a magnetic field of suitable strength, the samples can also be tuned to be extra sensitive to the illumination having colossal effect on the resistivity at low temperatures. This kind of utilizing of multiple external stimulants, which together change the properties of the material, could have significant impact on the new generation of phase‐change memories working under affordable conditions.     

Mechanical Modulation of Colossal Magnetoresistance in Flexible Epitaxial Perovskite Manganite

Heteroepitaxially flexible oxide systems have been intensely developed and considered as the most promising materials for leading the creation of next‐generation flexible electronic devices. Among them, perovskite manganites have attracted significant attention with their abundant and novel properties such as colossal magnetoresistance (CMR) and metal‐insulator transition. However, the requirement of high quality samples hampers this field, not to mention the advanced nanoengineering. In this study, fluorophlogopite mica (F‐mica) is selected as a flexible substrate to fabricate heteroepitaxial Pr_0.5Ca_0.5MnO₃ (PCMO) with a nanocolumn structure. Through a precise control of thickness, different morphologies are realized to manipulate the magnetotransport properties (reduction of melting field). Moreover, thanks to the excellent flexibility of F‐mica, mechanical modulation of CMR (≈1000%) can be achieved in different flex modes while the magnetic properties remain unaffected. Detailed bending tests are performed to study the behavior of resistive change (≈30%). Through the combination of high flexibility, high quality PCMO, and well‐designed nanocolumn structure, the study exhibits the significant controllability of CMR via mechanical bending, and manifests the potential of such a heteroepitaxially flexible oxide system which can be applied on flexible magnetoresistive devices and sensors.     

The magnetoresistance of compensated Ge:As at microwave frequencies in the vicinity of the metal-insulator phase transition

The magnetoresistance of heavily doped Ge:As near the metal-insulator phase transition has been studied both on the metal and insulator sides of the transition. Measurements were made at microwave frequencies using a noncontact technique of electron spin resonance. The field and temperature dependences of the magnetoresistance derivative in metallic samples reveal two main features of the phenomenon: a negative magnetoresistance at weak fields, due to the weak localization effect, and a positive magnetoresistance at strong fields, arising from the electron-electron interaction in the diffusion channel. Only a weak negative magnetoresistance with a characteristic low-field behavior is observed in insulating samples. The results are compared with the theory of quantum corrections.     

Colossal negative magnetoresistance from hopping in insulating ferromagnetic semiconductors

Ferromagnetic semiconductor Ga_1–xMn_xAs_1–yP_y thin films go through a metal–insulator transition at low temperature where electrical conduction becomes driven by hopping of charge carriers. In this regime, we report a colossal negative magnetoresistance (CNMR) coexisting with a saturated magnetic moment, unlike in the traditional magnetic semiconductor Ga_1–xMn_xAs. By analyzing the temperature dependence of the resistivity at fixed magnetic field, we demonstrate that the CNMR can be consistently described by the field dependence of the localization length, which relates to a field dependent mobility edge. This dependence is likely due to the random environment of Mn atoms in Ga_1–xMn_xAs_1–yP_y which causes a random spatial distribution of the mobility that is suppressed by an increasing magnetic field.     

The Dynamic Phase Transition Modulation of Ion‐Liquid Gating VO2 Thin Film: Formation,Diffusion, and Recovery of Oxygen Vacancies

Electrolyte gating with ionic liquids (IL) on correlated vanadium dioxide (VO₂) nanowires/beams is effective to modulate the metal‐insulator transition (MIT) behavior. While for macrosize VO₂ film, the gating treatment shows different phase modulation process and the intrinsic mechanism is still not clear, though the oxygen‐vacancy diffusion channel is always adopted for the explanation. Herein, the dynamic phase modulation of electrolyte gated VO₂ films is investigated and the oxygen vacancies formation, diffusion, and recovery at the IL/oxide interface are observed. As a relatively slow electrochemical reaction, the gating effect gradually permeates from surface to the inside of VO₂ film, along with an unsynchronized changes of integral electric, optical, and structure properties. First‐principles‐based theoretical calculation reveals that the oxygen vacancies can not only cause the structural deformations in monoclinic VO_2, but also account for the MIT transition by inducing polarization charges and thereby adjusting the d‐orbital occupancy. The findings not only clarify the oxygen vacancies statement of electrolyte gated VO₂ film, but also can be extended to other ionic liquid/oxide systems for better understanding of the surface electrochemical stability and electronic properties modulation.     

Hydrogenation Dynamics of Electrically Controlled Metal–Insulator Transition in Proton‐Gated Transparent and Flexible WO3 Transistors

Electrolyte gating is widely adopted to electrically control the physical properties of materials, leading to numerous intriguing phenomena and various applications. However, the carrier modulation mechanism remains heavily controversial. Herein, using natural mica pieces as substrates and ionic gel as the dielectric layer, all‐transparent and flexible WO₃ transistor configuration is designed to in situ monitor the dynamic doping process of electrolyte gating. A reversible and vacuum‐dominant volatile/nonvolatile metal–insulator transition (MIT) is observed in electrolyte‐gated WO₃ thin films. In situ X‐ray diffraction experiments, together with first‐principles calculations, reveal an abrupt and symmetric structural evolution through two distinct hydrogenated metastable phases and phase separation progress. The fast volatility is assigned to a spontaneous dehydrogenation process. A prototype of a flexible vacuum meter is demonstrated on the basis of the unique vacuum‐dependent MIT, exhibiting a measurement range down to 1.0 × 10^?6 mbar and no injury of electromagnetic radiation. These findings bring new insights into hydrogenation dynamics, paving a feasible way for the realization of user‐friendly flexible electronics.     

Engineering Large Anisotropic Magnetoresistance in La0.7Sr0.3MnO3 Films at Room Temperature

The magnetoresistance (MR) effect is widely used in technologies that pervade the world, from magnetic reading heads to sensors. Diverse contributions to MR, such as anisotropic, giant, tunnel, colossal, and spin‐Hall, are revealed in materials depending on the specific system and measuring configuration. Half‐metallic manganites hold promise for spintronic applications but the complexity of competing interactions has not permitted the understanding and control of their magnetotransport properties to enable the realization of their technological potential. This study reports on the ability to induce a dominant switchable magnetoresistance in La_0.7Sr_0.3MnO₃ epitaxial films at room temperature (RT). By engineering an extrinsic magnetic anisotropy, a large enhancement of anisotropic magnetoresistance (AMR) is achieved which at RT leads to signal changes much larger than the other contributions such as the colossal magnetoresistance. The dominant extrinsic AMR exhibits large variation in the resistance in low field region, showing high sensitivity to applied low magnetic fields. These findings have a strong impact on the real applications of manganite‐based devices for the high‐resolution low field magnetic sensors or spintronics.     

Charge Disproportionation and Voltage‐Induced Metal–Insulator Transitions Evidenced in β‐PbxV2O5 Nanowires

The roster of materials exhibiting metal–insulator transitions with sharply discontinuous switching of electrical conductivity close to room temperature remains rather sparse, despite the fundamental interest in the electronic instabilities manifested in such materials and the plethora of potential technological applications ranging from frequency‐agile metamaterials to electrochromic coatings and Mott field‐effect transistors. Here, unprecedented, pronounced metal‐insulator transitions induced by application of a voltage are demonstrated for nanowires of a vanadium oxide bronze with intercalated divalent cations, β‐Pb_xV₂O₅ (x ≈ 0.33). The induction of the phase transition through application of an electric field at room temperature makes this system particularly attractive and viable for technological applications. A mechanistic basis for the phase transition is proposed based on charge disproportionation evidenced at room temperature in near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy measurements, ab initio density functional theory calculations of the band structure, and electrical transport data, suggesting that transformation to the metallic state is induced by melting of specific charge localization and ordering motifs extant in these materials.     

Observation of magnetic domain structure in phase-separated manganites by lorentz electron microscopy

Magnetic domain structure in manganites was investigated by Lorentz electron microscopy, in order to understand some unusual physical properties, such as a colossal magnetoresistance (CMR) effect and a metal-to-insulator (MI) transition. In particular, we examined the spatial distribution of the charge/orbital ordered (CO/OO) insulator state and the ferromagnetic (FM) metallic state in phase-separated manganites, (La5/8-xPrx)Ca3/8MnO3 for x = 0.375, by obtaining both the dark-field and Lorentz images. We found an unusual coexistence of the CO/OO and FM metallic states with micrometer size below a MI transition temperature of 60 K. Our experimental findings provide direct evidence of the phase separation found in CMR manganites.     

Destruction of an orbitally ordered and spin-polarized state: Colossal magnetoresistance in Ca<Subscript>3</Subscript>Ru<Subscript>2</Subscript>O<Subscript>7</Subscript>

The Ca₃Ru₂O₇ with a Mott-like transition at 48 K and a Neel temperature at 56 K features different in-plane anisotropies of magnetization and magnetoresistance. Applying the magnetic field along the magnetic easy axis precipitates a spin-polarized state via a first-order metamagnetic transition but does not lead to full suppression of the Mott state, whereas applying a magnetic field along the magnetic hard axis does, causing a resistivity reduction of three orders of magnitude. The colossal magnetoresistivity is attributed to the collapse of a novel, orbitally ordered and spin-polarized state. This new phenomenon is striking in that the spin polarization, which is a fundamental driving force for all other magnetoresistive systems, is detrimental to the colossal magnetoresistance (CMR) in this 4d-based electron system. Evidence for a density wave is also presented.     

Versatile and Highly Efficient Controls of Reversible Topotactic Metal–Insulator Transitions through Proton Intercalation

The ability to tailor a new crystalline structure and associated functionalities with a variety of stimuli is one of the key issues in material design. Developing synthetic routes to functional materials with partially absorbed nonmetallic elements (i.e., hydrogen and nitrogen) can open up more possibilities for preparing novel families of electronically active oxide compounds. Fast and reversible uptake and release of hydrogen in epitaxial ABO₃ manganite films through an adapted low‐frequency inductively coupled plasma technology is introduced. Compared with traditional dopants of metallic cations, the plasma‐assisted hydrogen implantations not only produce reversibly structural transformations from pristine perovskite (PV) phase to a newly found protonation‐driven brownmillerite one but also regulate remarkably different electronic properties driving the material from a ferromagnetic metal to a weakly ferromagnetic insulator for a range of manganite (La_1?xSr_xMnO₃) thin films. Moreover, a reversible perovskite‐brownmillerite‐perovskite transition is achieved at a relatively low temperature (T ≤ 350 °C), enabling multifunctional modulations for integrated electronic systems. The fast, low‐temperature control of structural and electronic properties by the facile hydrogenation/dehydrogenation treatment substantially widens the space for exploring new possibilities of novel properties in proton‐based multifunctional materials.     

Metal–Insulator–Semiconductor Coaxial Microfibers Based on Self‐Organization of Organic Semiconductor:Polymer Blend for Weavable,Fibriform Organic Field‐Effect Transistors

With the increasing importance of electronic textiles as an ideal platform for wearable electronic devices, requirements for the development of functional electronic fibers with multilayered structures are increasing. In this paper, metal–polymer insulator–organic semiconductor (MIS) coaxial microfibers using the self‐organization of organic semiconductor:insulating polymer blends for weavable, fibriform organic field‐effect transistors (FETs) are demonstrated. A holistic process for MIS coaxial microfiber fabrication, including surface modification of gold microfiber thin‐film coating on the microfiber using a die‐coating system, and the self‐organization of organic semiconductor–insulator polymer blend is presented. Vertical phase‐separation of the organic semiconductor:insulating polymer blend film wrapping the metal microfibers provides a coaxial bilayer structure of gate dielectric (inside) and organic semiconductor (outside) with intimate interfacial contact. It is determined that the fibriform FETs based on MIS coaxial microfiber exhibit good charge carrier mobilities that approach the values of typical devices with planar substrate. It additionally exhibits electrical property uniformity over the entire fiber surface and improved bending durability. Fibriform organic FET embedded in a textile is demonstrated by weaving MIS coaxial microfibers with cotton and conducting threads, which verifies the feasibility of MIS coaxial microfiber for use in electronic textile applications.     

Broad Range Tuning of Phase Transition Property in VO2 Through Metal‐Ceramic Nanocomposite Design

Vanadium dioxide (VO₂) is a well‐studied Mott‐insulator because of the very abrupt physical property switching during its semiconductor‐to‐metal transition (SMT) around 341 K (68 °C). In this work, through novel oxide‐metal nanocomposite designs (i.e., Au:VO₂ and Pt:VO₂), a very broad range of SMT temperature tuning from ≈ 323.5 to ≈ 366.7 K has been achieved by varying the metallic secondary phase in the nanocomposites (i.e., Au:VO₂ and Pt:VO₂ thin films, respectively). More surprisingly, the SMT T_c can be further lowered to ≈ 301.8 K (near room temperature) by reducing the Au particle size from 11.7 to 1.7 nm. All the VO₂ nanocomposite thin films maintain superior phase transition performance, i.e., large transition amplitude, very sharp transition, and narrow width of thermal hysteresis. Correspondingly, a twofold variation of the complex dielectric function has been demonstrated in these metal‐VO₂ nanocomposites. The wide range physical property tuning is attributed to the band structure reconstruction at the metal‐VO₂ phase boundaries. This demonstration paved a novel approach for tuning the phase transition property of Mott‐insulating materials to near room temperature transition, which is important for sensors, electrical switches, smart windows, and actuators.     

Ionic Liquid Activation of Amorphous Metal‐Oxide Semiconductors for Flexible Transparent Electronic Devices

Amorphous metal‐oxide semiconductors offer the high carrier mobilities and excellent large‐area uniformity required for high performance, transparent, flexible electronic devices; however, a critical bottleneck to their widespread implementation is the need to activate these materials at high temperatures which are not compatible with flexible polymer substrates. The highly controllable activation of amorphous indium gallium zinc oxide semiconductor channels using ionic liquid gating at room temperature is reported. Activation is controlled by electric field‐induced oxygen migration across the ionic liquid‐semiconductor interface. In addition to activation of unannealed devices, it is shown that threshold voltages of a transistor can be linearly tuned between the enhancement and depletion modes. Finally, the first ever example of transparent flexible thin film metal oxide transistor on a polyamide substrate created using this simple technique is demonstrated. This study demonstrates the potential of field‐induced activation as a promising alternative to traditional postdeposition thermal annealing which opens the door to wide scale implementation into flexible electronic applications.     

2D Hexagonal Covalent Organic Radical Frameworks as Tunable Correlated Electron Systems

Quantum materials hold huge technological promise but challenge the fundamental understanding of complex electronic interactions in solids. The Mott metal–insulator transition on half-filled lattices is an archetypal demonstration of how quantum states can be driven by electronic correlation. Twisted bilayers of 2D materials provide an experimentally accessible means to probe such transitions, but these seemingly simple systems belie high complexity due to the myriad of possible interactions. Herein, it is shown that electron correlation can be simply tuned in experimentally viable 2D hexagonally ordered covalent organic radical frameworks (2D hex-CORFs) based on single layers of half-filled stable radical nodes. The presented carefully procured theoretical analysis predicts that 2D hex-CORFs can be varied between a correlated antiferromagnetic Mott insulator state and a semimetallic state by modest out-of-plane compressive pressure. This work establishes 2D hex-CORFs as a class of versatile single-layer quantum materials to advance the understanding of low dimensional correlated electronic systems.     

CMR锰氧化物在激光及光探测方面的应用进展

混合价态钙钛矿锰氧化物在外界温度变化和磁场作用下表现出巨大的磁电阻 (Colossalmagnetoresistance ,简称CMR)效应 ,引起了人们的广泛关注。由于CMR材料在传感器、探测器以及硬盘读出磁头等应用器件研发上极具潜力 ,科学家们对其进行了大量研究。本文在简单介绍CMR材料结构和机理的基础上 ,着重报道了我们利用CMR薄膜的激光感生热电电势 (LITV)制备激光功率 能量探测器和利用电阻在室温附近的巨大变化研制的光热辐射探测器 (Bolometer)方面的新进展     

A Noble Metal Dichalcogenide for High‐Performance Field‐Effect Transistors and Broadband Photodetectors

2D layered materials are an emerging class of low‐dimensional materials with unique physical and structural properties and extensive applications from novel nanoelectronics to multifunctional optoelectronics. However, the widely investigated 2D materials are strongly limited in high‐performance electronics and ultrabroadband photodetectors by their intrinsic weaknesses. Exploring the new and narrow bandgap 2D materials is very imminent and fundamental. A narrow‐bandgap noble metal dichalcogenide (PtS₂) is demonstrated in this study. The few‐layer PtS₂ field‐effect transistor exhibits excellent electronic mobility exceeding 62.5 cm² V^?1 s^?1 and ultrahigh on/off ratio over 10⁶ at room temperature. The temperature‐dependent conductance and mobility of few‐layer PtS₂ transistors show a direct metal‐to‐insulator transition and carrier scattering mechanisms, respectively. Remarkably, 2D PtS₂ photodetectors with broadband photodetection from visible to mid‐infrared and a fast photoresponse time of 175 µs at 830 nm illumination for the first time are obtained at room temperature. Our work opens an avenue for 2D noble‐metal dichalcogenides to be applied in high‐performance electronic and mid‐infrared optoelectronic devices.     

Strain Accommodation during Phase Transformations in Olivine‐Based Cathodes as a Materials Selection Criterion for High‐Power Rechargeable Batteries

High energy lithium‐ion batteries have improved performance in a wide variety of mobile electronic devices. A new goal in portable power is the achievement of safe and durable high‐power batteries for applications such as power tools and electric vehicles. Towards this end, olivine‐based positive electrodes are amongst the most important and technologically enabling materials. While certain lithium metal phosphate olivines have been shown to be promising, not all olivines demonstrate beneficial properties. The mechanisms allowing high power in these compounds have been extensively debated. Here we show that certain high rate capability olivines are distinguished by having extended lithium nonstoichiometry (up to ca. 20 %), with which is correlated a reduced lattice misfit as the material undergoes an electrochemically driven, reversible, first‐order phase transformation. The rate capability in several other intercalation oxides can also be correlated with lattice strain, and suggests that nanomechanics plays an important and previously unrecognized role in determining battery performance.     

Ionic‐Liquid Gating of InAs Nanowire‐Based Field‐Effect Transistors

Here, the operation of a field‐effect transistor based on a single InAs nanowire gated by an ionic liquid is reported. Liquid gating yields very efficient carrier modulation with a transconductance value 30 times larger than standard back gating with the SiO₂/Si₊₊ substrate. Thanks to this wide modulation, the controlled evolution from semiconductor to metallic‐like behavior in the nanowire is shown. This work provides the first systematic study of ionic‐liquid gating in electronic devices based on individual III–V semiconductor nanowires: this architecture opens the way to a wide range of fundamental and applied studies from the phase transitions to bioelectronics.