Discovery of superconductivity in KTaO₃ by electrostatic carrier doping

Interface Engineering in Organic Field-Effect Transistors: Principles, Applications, and Perspectives

Heterogeneous interfaces that are ubiquitous in optoelectronic devices play a key role in the device performance and have led to the prosperity of today's microelectronics. Interface engineering provides an effective and promising approach to enhancing the device performance of organic field-effect transistors (OFETs) and even developing new functions. In fact, researchers from different disciplines have devoted considerable attention to this concept, which has started to evolve from simple improvement of the device performance to sophisticated construction of novel functionalities, indicating great potential for further applications in broad areas ranging from integrated circuits and energy conversion to catalysis and chemical/biological sensors. In this review article, we provide a timely and comprehensive overview of current efficient approaches developed for building various delicate functional interfaces in OFETs, including interfaces within the semiconductor layers, semiconductor/electrode interfaces, semiconductor/dielectric interfaces, and semiconductor/environment interfaces. We also highlight the major contributions and new concepts of integrating molecular functionalities into electrical circuits, which have been neglected in most previous reviews. This review will provide a fundamental understanding of the interplay between the molecular structure, assembly, and emergent functions at the molecular level and consequently offer novel insights into designing a new generation of multifunctional integrated circuits and sensors toward practical applications.

Enabling high performance n-type metal oxide semiconductors at low temperatures for thin film transistors

The review highlights low temperature activation processes for high performance n-type metal oxide semiconductors for TFTs.

Field-Driven Athermal Activation of Amorphous Metal Oxide Semiconductors for Flexible Programmable Logic Circuits and Neuromorphic Electronics

Despite extensive research, large-scale realization of metal-oxide electronics is still impeded by high-temperature fabrication, incompatible with flexible substrates. Ideally, an athermal treatment modifying the electronic structure of amorphous metal oxide semiconductors (AMOS) to generate sufficient carrier concentration would help mitigate such high-temperature requirements, enabling realization of high-performance electronics on flexible substrates. Here, a novel field-driven athermal activation of AMOS channels is demonstrated via an electrolyte-gating approach. Facilitating migration of charged oxygen species across the semiconductor-dielectric interface, this approach modulates the local electronic structure of the channel, generating sufficient carriers for charge transport and activating oxygen-compensated thin films. The thin-film transistors (TFTs) investigated here depict an enhancement of linear mobility from 51 to 105.25 cm² V^-1 s^-1 (ionic-gated) and from 8.09 to 14.49 cm² V^-1 s^-1 (back-gated), by creating additional oxygen vacancies. The accompanying stochiometric transformations, monitored via spectroscopic measurements (X-ray photoelectron spectroscopy) corroborate the detailed electrical (TFT, current evolution) parameter analyses, providing critical insights into the underlying oxygen-vacancy generation mechanism and clearly demonstrating field-induced activation as a promising alternative to conventional high-temperature annealing strategies. Facilitating on-demand active programing of the operation modes of transistors (enhancement vs depletion), this technique paves way for facile fabrication of logic circuits and neuromorphic transistors for bioinspired computing.

Controlling the Magnetic Properties of LaMnO3 /SrTiO3 Heterostructures by Stoichiometry and Electronic Reconstruction: Atomic-Scale Evidence

Interface-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking are of importance for fundamental physics and device applications. How interfaces affect the interplay between charge, spin, orbital, and lattice degrees of freedom is the key to boosting device performance. In LaMnO₃ /SrTiO₃ (LMO/STO) polar-nonpolar heterostructures, electronic reconstruction leads to an antiferromagnetic to ferromagnetic transition, making them viable for spin filter applications. The interfacial electronic structure plays a critical role in the understanding of the microscopic origins of the observed magnetic phase transition, from antiferromagnetic at 5 unit cells (ucs) of LMO or below to ferromagnetic at 6 ucs or above, yet such a study is missing. Here, an atomic scale understanding of LMO/STO ambipolar ferromagnetism is offered by quantifying the interface charge distribution and performing first-principles density functional theory (DFT) calculations across this abrupt magnetic transition. It is found that the electronic reconstruction is confined within the first 3 ucs of LMO from the interface, and more importantly, it is robust against oxygen nonstoichiometry. When restoring stoichiometry, an enhanced ferromagnetic insulating state in LMO films with a thickness as thin as 2 nm (5 uc) is achieved, making LMO readily applicable as barriers in spin filters.

Water-Induced Breaking of the Coulombic Ordering in a Room-Temperature Ionic Liquid Metal Complex

Control of ion arrangements in ionic liquids represents a major challenge owing to the presence of the predominant coulombic interactions between cationic and anionic ion species that forms the coulombic ordering. Here, water-induced ion rearrangement in a room-temperature ionic liquid (RT-IL) metal complex, (1-ethyl-3-methylimidazolium)₂ [MnN(CN)₄ ], is demonstrated through coordinative interactions between anions. Solidification occurred, which was associated with the formation of a "separated" structure consisting of cation columns and anionic cyanide-bridged one-dimensional coordination polymers. The energy diagram is in accord with the resultant RT-IL incorporating mononuclear [MnN(CN)₄ ]^2- molecules being a kinetic phase stabilized by inter-ion repulsions of the anionic divalent metal complex moieties. Water acts to decrease the coulombic interactions, including repulsion, giving rise to breaking of the coulombic ordering arising from coordination bond formation in the IL phase.

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.

Electric-field-controlled superconductor-ferromagnetic insulator transition

Superconductivity beyond electron-phonon mechanism is always twisted with magnetism. Based on a new field-effect transistor with solid ion conductor as the gate dielectric (SIC-FET), we successfully achieve an electric-field-controlled phase transition between superconductor and ferromagnetic insulator in (Li,Fe)OHFeSe. A dome-shaped superconducting phase with optimal T_c of 43 K is continuously tuned into a ferromagnetic insulating phase, which exhibits an electric-field-controlled quantum critical behavior. The origin of the ferromagnetism is ascribed to the order of the interstitial Fe ions expelled from the (Li,Fe)OH layers by gating-controlled Li injection. These surprising findings offer a unique platform to study the relationship between superconductivity and ferromagnetism in Fe-based superconductors. This work also demonstrates the superior performance of the SIC-FET in regulating physical properties of layered unconventional superconductors.

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.

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.

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.

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.

Evidence of Incoherent Carriers Associated with Resonant Impurity Levels and Their Influence on Superconductivity in the Anomalous Superconductor Pb_{1-x}Tl_{x}Te

We present a combined experimental and theoretical study of the evolution of the Fermi surface of the anomalous superconductor Pb_{1-x}Tl_{x}Te as a function of thallium concentration, drawing on a combination of magnetotransport measurements (Shubnikov-de Haas oscillations and the Hall coefficient), angle resolved photoemission spectroscopy, and density functional theory calculations of the electronic structure. Our results indicate that for Tl concentrations beyond a critical value, the Fermi energy coincides with resonant impurity states in Pb_{1-x}Tl_{x}Te, and we rule out the presence of an additional valence band maximum at the Fermi energy. A comparison to nonsuperconducting Pb_{1-x}Na_{x}Te implies that the presence of these impurity states at the Fermi energy provides the enhanced pairing interaction and thus also the anomalously high temperature superconductivity in this material.

Tuning Charge Transport in Aromatic-Ring Single-Molecule Junctions via Ionic-Liquid Gating

Achieving gate control with atomic precision, which is crucial to the transistor performance on the smallest scale, remains a challenge. Herein we report a new class of aromatic-ring molecular nanotransistors based on graphene-molecule-graphene single-molecule junctions by using an ionic-liquid gate. Experimental phenomena and theoretical calculations confirm that this ionic-liquid gate can effectively modulate the alignment between molecular frontier orbitals and the Fermi energy level of graphene electrodes, thus tuning the charge-transport properties of the junctions. In addition, with a small gate voltage (|V_G |≤1.5 V) ambipolar charge transport in electrochemically inactive molecular systems (E_G >3.5 eV) is realized. These results offer a useful way to build high-performance single-molecule transistors, thus promoting the prospects for molecularly engineered electronic devices.

Ionic-liquid-gating setup for stable measurements and reduced electronic inhomogeneity at low temperatures

The ionic-liquid-gating technique can be applied to the search for novel physical phenomena at low temperatures because of its wide controllability of the charge carrier density. Ionic-liquid-gated field-effect transistors are often fragile upon cooling, however, because of the large difference between the thermal expansion coefficients of frozen ionic liquids and solid target materials. In this paper, we provide a practical technique for setting up ionic-liquid-gated field-effect transistors for low-temperature measurements. It allows stable measurements and reduces the electronic inhomogeneity by reducing the shear strain generated in frozen ionic liquid.

Quantum interference mediated vertical molecular tunneling transistors

Molecular transistors operating in the quantum tunneling regime represent potential electronic building blocks for future integrated circuits. However, due to their complex fabrication processes and poor stability, traditional molecular transistors can only operate stably at cryogenic temperatures. Here, through a combined experimental and theoretical investigation, we demonstrate a new design of vertical molecular tunneling transistors, with stable switching operations up to room temperature, formed from cross-plane graphene/self-assembled monolayer (SAM)/gold heterostructures. We show that vertical molecular junctions formed from pseudo-p-bis((4-(acetylthio)phenyl)ethynyl)-p-[2,2]cyclophane (PCP) SAMs exhibit destructive quantum interference (QI) effects, which are absent in 1,4-bis(((4-acetylthio)phenyl)ethynyl)benzene (OPE3) SAMs. Consequently, the zero-bias differential conductance of the former is only about 2% of the latter, resulting in an enhanced on-off current ratio for (PCP) SAMs. Field-effect control is achieved using an ionic liquid gate, whose strong vertical electric field penetrates through the graphene layer and tunes the energy levels of the SAMs. The resulting on-off current ratio achieved in PCP SAMs can reach up to ~330, about one order of magnitude higher than that of OPE3 SAMs. The demonstration of molecular junctions with combined QI effect and gate tunability represents a critical step toward functional devices in future molecular-scale electronics.

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.

Tunnelling spectroscopy of gate-induced superconductivity in MoS2

The ability to gate-induce superconductivity by electrostatic charge accumulation is a recent breakthrough in physics and nanoelectronics. With the exception of LaAlO₃/SrTiO₃ interfaces, experiments on gate-induced superconductors have been largely confined to resistance measurements, which provide very limited information about the superconducting state. Here, we explore gate-induced superconductivity in MoS₂ by performing tunnelling spectroscopy to determine the energy-dependent density of states (DOS) for different levels of electron density n. In the superconducting state, the DOS is strongly suppressed at energy smaller than the gap Δ, which is maximum (Δ ~2 meV) for n of ~1 × 10¹⁴ cm^-2 and decreases monotonously for larger n. A perpendicular magnetic field B generates states at E < Δ that fill the gap, but a 20% DOS suppression of superconducting origin unexpectedly persists much above the transport critical field. Conversely, an in-plane field up to 10 T leaves the DOS entirely unchanged. Our measurements exclude that the superconducting state in MoS₂ is fully gapped and reveal the presence of a DOS that vanishes linearly with energy, the explanation of which requires going beyond a conventional, purely phonon-driven Bardeen-Cooper-Schrieffer mechanism.

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.

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.

Gate-Induced Metal-Insulator Transition in MoS2 by Solid Superionic Conductor LaF3

Electric-double-layer (EDL) gating with liquid electrolyte has been a powerful tool widely used to explore emerging interfacial electronic phenomena. Due to the large EDL capacitance, a high carrier density up to 10¹⁴ cm^-2 can be induced, directly leading to the realization of field-induced insulator to metal (or superconductor) transition. However, the liquid nature of the electrolyte has created technical issues including possible side electrochemical reactions or intercalation, and the potential for huge strain at the interface during cooling. In addition, the liquid coverage of active devices also makes many surface characterizations and in situ measurements challenging. Here, we demonstrate an all solid-state EDL device based on a solid superionic conductor LaF₃, which can be used as both a substrate and a fluorine ionic gate dielectric to achieve a wide tunability of carrier density without the issues of strain or electrochemical reactions and can expose the active device surface for external access. Based on LaF₃ EDL transistors (EDLTs), we observe the metal-insulator transition in MoS₂. Interestingly, the well-defined crystal lattice provides a more uniform potential distribution in the substrate, resulting in less interface electron scattering and therefore a higher mobility in MoS₂ transistors. This result shows the powerful gating capability of LaF₃ solid electrolyte for new possibilities of novel interfacial electronic phenomena.

Full superconducting dome of strong Ising protection in gated monolayer WS2

Many recent studies show that superconductivity not only exists in atomically thin monolayers but can exhibit enhanced properties such as a higher transition temperature and a stronger critical field. Nevertheless, besides being unstable in air, the weak tunability in these intrinsically metallic monolayers has limited the exploration of monolayer superconductivity, hindering their potential in electronic applications (e.g., superconductor-semiconductor hybrid devices). Here we show that using field effect gating, we can induce superconductivity in monolayer WS₂ grown by chemical vapor deposition, a typical ambient-stable semiconducting transition metal dichalcogenide (TMD), and we are able to access a complete set of competing electronic phases over an unprecedented doping range from band insulator, superconductor, to a reentrant insulator at high doping. Throughout the superconducting dome, the Cooper pair spin is pinned by a strong internal spin-orbit interaction, making this material arguably the most resilient superconductor in the external magnetic field. The reentrant insulating state at positive high gating voltages is attributed to localization induced by the characteristically weak screening of the monolayer, providing insight into many dome-like superconducting phases observed in field-induced quasi-2D superconductors.

Gate-Induced Interfacial Superconductivity in 1T-SnSe2

Layered metal chalcogenide materials provide a versatile platform to investigate emergent phenomena and two-dimensional (2D) superconductivity at/near the atomically thin limit. In particular, gate-induced interfacial superconductivity realized by the use of an electric-double-layer transistor (EDLT) has greatly extended the capability to electrically induce superconductivity in oxides, nitrides, and transition metal chalcogenides and enable one to explore new physics, such as the Ising pairing mechanism. Exploiting gate-induced superconductivity in various materials can provide us with additional platforms to understand emergent interfacial superconductivity. Here, we report the discovery of gate-induced 2D superconductivity in layered 1T-SnSe₂, a typical member of the main-group metal dichalcogenide (MDC) family, using an EDLT gating geometry. A superconducting transition temperature T_c ≈ 3.9 K was demonstrated at the EDL interface. The 2D nature of the superconductivity therein was further confirmed based on (1) a 2D Tinkham description of the angle-dependent upper critical field B_c2, (2) the existence of a quantum creep state as well as a large ratio of the coherence length to the thickness of superconductivity. Interestingly, the in-plane B_c2 approaching zero temperature was found to be 2-3 times higher than the Pauli limit, which might be related to an electric field-modulated spin-orbit interaction. Such results provide a new perspective to expand the material matrix available for gate-induced 2D superconductivity and the fundamental understanding of interfacial superconductivity.

ON the Nature of Ionic Liquid Gating of La2−xSrxCuO4

Ionic liquids have recently been used as means of modulating the charge carrier properties of cuprates. The mechanism behind it, however, is still a matter of debate. In this paper we report experiments on ionic liquid gated ultrathin La_2−xSr_xCuO₄ films. Our results show that the electrostatic part of gating has limited influence in the conductance of the cuprate in the gate voltage range of 0 to −2 V. A non-electrostatic mechanism takes over for gate voltages below −2 V. This mechanism most likely changes the oxygen concentration of the film. The results presented are in line with previous X-ray based studies on ionic liquid gating induced oxygenation of the cuprate materials YBa₂Cu₃O_7−x and La_2−xSr_xCuO₄.

Strain effects on oxygen vacancy energetics in KTaO3

Due to lattice mismatch between epitaxial films and substrates, in-plane strain fields are produced in the thin films, with accompanying structural distortions, and ion implantation can be used to controllably engineer the strain throughout the film. Because of the strain profile, local defect energetics are changed. In this study, the effects of in-plane strain fields on the formation and migration of oxygen vacancies in KTaO₃ are investigated using first-principles calculations. In particular, the doubly positive charged oxygen vacancy (V) is studied, which is considered to be the main charge state of the oxygen vacancy in KTaO₃. We find that the formation energies for oxygen vacancies are sensitive to in-plane strain and oxygen position. The local atomic configuration is identified, and strong relaxation of local defect structure is mainly responsible for the formation characteristics of these oxygen vacancies. Based on the computational results, formation-dependent site preferences for oxygen vacancies are expected to occur under epitaxial strain, which can result in orders of magnitude differences in equilibrium vacancy concentrations on different oxygen sites. In addition, all possible migration pathways, including intra- and inter-plane diffusions, are considered. In contrast to the strain-enhanced intra-plane diffusion, the diffusion in the direction normal to the strained plane is impeded under the epitaxial strain field. These anisotropic diffusion processes can further enhance site preferences.

Field-, strain- and light-induced superconductivity in organic strongly correlated electron systems

Stimulated by the discovery of high-temperature superconductivity in 1986, band-filling control of strongly correlated electron systems has been a persistent challenge over the past three decades in condensed matter science. In particular, recent efforts have been focused on electrostatic carrier doping of these materials, utilising field-effect transistor (FET) structures to find novel superconductivity. Our group found the first field-induced superconductivity in an organic-based material in 2013 and has been developing various types of superconducting organic FETs. In this perspective, we summarise our recent results on the development of novel superconducting organic FETs. In addition, this perspective describes novel functionality of superconducting FETs, such as strain- and light-responsivity. We believe that the techniques and knowledge described here will contribute to advances in future superconducting electronics as well as the understanding of superconductivity in strongly correlated electron systems.

Protonation induced high-Tc phases in iron-based superconductors evidenced by NMR and magnetization measurements

Chemical substitution during growth is a well-established method to manipulate electronic states of quantum materials, and leads to rich spectra of phase diagrams in cuprate and iron-based superconductors. Here we report a novel and generic strategy to achieve nonvolatile electron doping in series of (i.e. 11 and 122 structures) Fe-based superconductors by ionic liquid gating induced protonation at room temperature. Accumulation of protons in bulk compounds induces superconductivity in the parent compounds, and enhances the T_c largely in some superconducting ones. Furthermore, the existence of proton in the lattice enables the first proton nuclear magnetic resonance (NMR) study to probe directly superconductivity. Using FeS as a model system, our NMR study reveals an emergent high-T_c phase with no coherence peak which is hard to measure by NMR with other isotopes. This novel electric-field-induced proton evolution opens up an avenue for manipulation of competing electronic states (e.g. Mott insulators), and may provide an innovative way for a broad perspective of NMR measurements with greatly enhanced detecting resolution.

Role of Oxygen in Ionic Liquid Gating on Two-Dimensional Cr2Ge2Te6: A Non-oxide Material

Ionic liquid gating can markedly modulate a material's carrier density so as to induce metallization, superconductivity, and quantum phase transitions. One of the main issues is whether the mechanism of ionic liquid gating is an electrostatic field effect or an electrochemical effect, especially for oxide materials. Recent observation of the suppression of the ionic liquid gate-induced metallization in the presence of oxygen for oxide materials suggests the electrochemical effect. However, in more general scenarios, the role of oxygen in the ionic liquid gating effect is still unclear. Here, we perform ionic liquid gating experiments on a non-oxide material: two-dimensional ferromagnetic Cr₂Ge₂Te₆. Our results demonstrate that despite the large increase of the gate leakage current in the presence of oxygen, the oxygen does not affect the ionic liquid gating effect on  the channel resistance of Cr₂Ge₂Te₆ devices (<5% difference), which suggests the electrostatic field effect as the mechanism on non-oxide materials. Moreover, our results show that ionic liquid gating is more effective on the modulation of the channel resistances compared to the back gating across the 300 nm thick SiO₂.

First-principles studies of polar perovskite KTaO3 surfaces: structural reconstruction, charge compensation, and stability diagram

First-principles calculations predict a surface phase stability diagram for the polar perovskite KTaO₃.

Tunable electronic and magnetic properties of graphene-like XYBe3 (XY = BN, AlN, SiC, GeC) nanosheets with carrier doping: a first-principles study

Graphyne-like ternary beryllide nanosheets are found to be promising host materials because of their carrier-induced tunable magnetism and half-metallicity.

Superconductivity at the Polar-Nonpolar Phase Boundary of SnP with an Unusual Valence State

Structural, magnetic, and electrical characterizations reveal that SnP with an unusual valence state (nominally Sn^{3+}) undergoes a ferroelectriclike structural transition from a simple NaCl-type structure to a polar tetragonal structure at approximately 250 K at ambient pressure. First-principles calculations indicate that the experimentally observed tetragonal distortion enhances the charge transfer from Sn to P, thereby making the polar tetragonal phase energetically more stable than the nonpolar cubic phase. Hydrostatic pressure is found to promptly suppress the structural phase transition in SnP, leading to the emergence of bulk superconductivity in a phase-competitive manner. These findings suggest that control of ferroelectriclike instability in a metal can be a promising way for creating novel superconductors.

Exceptionally High Electric Double Layer Capacitances of Oligomeric Ionic Liquids

Electric double layer (EDL) capacitors are promising as next-generation energy accumulators if their capacitances and operation voltages are both high. However, only few electrolytes can simultaneously fulfill these two requisites. Here we report that an oligomeric ionic liquid such as IL4^TFSI with four imidazolium ion units in its structure provides a wide electrochemical window of ∼5.0 V, similar to monomeric ionic liquids. Furthermore, electrochemical impedance measurements using Au working electrodes demonstrated that IL4^TFSI exhibits an exceptionally high EDL capacitance of ∼66 μF/cm², which is ∼6 times as high as those of monomeric ionic liquids so far reported. We also found that an EDL-based field effect transistor (FET) using IL4^TFSI as a gate dielectric material and SrTiO₃ as a channel material displays a very sharp transfer curve with an enhanced carrier accumulation capability of ∼64 μF/cm², as determined by Hall-effect measurements.

Activity Dependent Synaptic Plasticity Mimicked on Indium-Tin-Oxide Electric-Double-Layer Transistor

Ion coupling has provided an additional method to modulate electric properties for solid-state materials. Here, phosphorosilicate glass (PSG)-based electrolyte gated protonic/electronic coupled indium-tin-oxide electric-double-layer (EDL) transistors are fabricated. The oxide transistor exhibits good electrical performances due to an extremely strong proton gating behavior for the electrolyte. With interfacial electrochemical doping, channel conductances of the oxide EDL transistor can be regulated to different levels, corresponding to different initial synaptic weights. Thus, activity dependent synaptic responses such as excitatory postsynaptic current, paired-pulse facilitation, and high-pass filtering are discussed in detail. The proposed proton conductor gated oxide EDL synaptic transistors with activity dependent synaptic plasticities may act as fundamental building blocks for neuromorphic system applications.

Oxygen Vacancy in WO3 Film-based FET with Ionic Liquid Gating

Ionic liquid gating is a versatile method for inducing a metal-insulator transition in field-effect transistor device structures. The mechanism of carrier doping in metal oxide films is under debate. Ionic liquid gating of a WO₃ film-based field effect transistor is discussed in this report. Flat and relatively smooth WO₃ films were deposited on SrTiO₃ substrates by pulsed laser deposition. Swept and constant gate voltage characteristics are measured in both argon and oxygen atmospheres. The results show a clear dependence on the oxygen pressure of the experimental chamber. Metallic behavior in the films is attributed to oxygen vacancy formation in the WO₃ layer induced by the high electric field at the oxide-ionic liquid interface. The density of states of a monoclinic supercell of oxygen deficient WO₃ was studied by density functional theory (DFT). Calculated W and O partial densities of states verify metallic behavior even at dilute oxygen vacancy concentrations and show the role of W and O orbitals in the conductivity.

N-Type Superconductivity in an Organic Mott Insulator Induced by Light-Driven Electron-Doping

The presence of interface dipoles in self-assembled monolayers (SAMs) gives rise to electric-field effects at the device interfaces. SAMs of spiropyran derivatives can be used as photoactive interface dipole layer in field-effect transistors because the photochromism of spiropyrans involves a large dipole moment switching. Recently, light-induced p-type superconductivity in an organic Mott insulator, κ-(BEDT-TTF)₂ Cu[N(CN)₂ ]Br (κ-Br: BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene) has been realized, thanks to the hole carriers induced by significant interface dipole variation in the spiropyran-SAM. This report explores the converse situation by designing a new type of spiropyran monolayer in which light-induced electron-doping into κ-Br and accompanying n-type superconducting transition have been observed. These results open new possibilities for novel electronics utilizing a photoactive SAMs, which can design not only the magnitude but also the direction of photoinduced electric-fields at the device interfaces.

Electrical tuning of a quantum plasmonic resonance

Surface plasmon (SP) excitations in metals facilitate confinement of light into deep-subwavelength volumes and can induce strong light-matter interaction. Generally, the SP resonances supported by noble metal nanostructures are explained well by classical models, at least until the nanostructure size is decreased to a few nanometres, approaching the Fermi wavelength λ_F of the electrons. Although there is a long history of reports on quantum size effects in the plasmonic response of nanometre-sized metal particles, systematic experimental studies have been hindered by inhomogeneous broadening in ensemble measurements, as well as imperfect control over size, shape, faceting, surface reconstructions, contamination, charging effects and surface roughness in single-particle measurements. In particular, observation of the quantum size effect in metallic films and its tuning with thickness has been challenging as they only confine carriers in one direction. Here, we show active tuning of quantum size effects in SP resonances supported by a 20-nm-thick metallic film of indium tin oxide (ITO), a plasmonic material serving as a low-carrier-density Drude metal. An ionic liquid (IL) is used to electrically gate and partially deplete the ITO layer. The experiment shows a controllable and reversible blue-shift in the SP resonance above a critical voltage. A quantum-mechanical model including the quantum size effect reproduces the experimental results, whereas a classical model only predicts a red shift.

Endeavor of Iontronics: From Fundamentals to Applications of Ion-Controlled Electronics

Iontronics is a newly emerging interdisciplinary concept which bridges electronics and ionics, covering electrochemistry, solid-state physics, electronic engineering, and biological sciences. The recent developments of electronic devices are highlighted, based on electric double layers formed at the interface between ionic conductors (but electronically insulators) and various electronic conductors including organics and inorganics (oxides, chalcogenide, and carbon-based materials). Particular attention is devoted to electric-double-layer transistors (EDLTs), which are producing a significant impact, particularly in electrical control of phase transitions, including superconductivity, which has been difficult or impossible in conventional all-solid-state electronic devices. Besides that, the current state of the art and the future challenges of iontronics are also reviewed for many applications, including flexible electronics, healthcare-related devices, and energy harvesting.

Molecularly-thin anatase field-effect transistors fabricated through the solid state transformation of titania nanosheets

We demonstrate the field-effect transistor (FET) operation of a molecularly-thin anatase phase produced through solid state transformation from Ti_0.87O₂ nanosheets. A monolayer Ti_0.87O₂ nanosheet with a thickness of 0.7 nm is a two-dimensional oxide insulator in which Ti vacancies are incorporated, rather than oxygen vacancies. Since the fabrication method, in general, largely affects the film quality, the anatase films derived from the Ti_0.87O₂ nanosheets show interesting characteristics, such as no photocurrent peak at ∼2 eV, which is related to oxygen vacancies, and a larger band gap of 3.8 eV. The 10 nm thick anatase FETs exhibit superior transport characteristics with a maximum mobility of ∼1.3 cm² V^-1 s^-1 and a current on/off ratio of ∼10⁵ at room temperature. The molecularly-thin anatase FET may provide new functionalities, such as field-effect control of catalytic properties.

In operando evidence of deoxygenation in ionic liquid gating of YBa2Cu3O7-X

Field-effect experiments on cuprates using ionic liquids have enabled the exploration of their rich phase diagrams [Leng X, et al. (2011) Phys Rev Lett 107(2):027001]. Conventional understanding of the electrostatic doping is in terms of modifications of the charge density to screen the electric field generated at the double layer. However, it has been recently reported that the suppression of the metal to insulator transition induced in VO₂ by ionic liquid gating is due to oxygen vacancy formation rather than to electrostatic doping [Jeong J, et al. (2013) Science 339(6126):1402-1405]. These results underscore the debate on the true nature, electrostatic vs. electrochemical, of the doping of cuprates with ionic liquids. Here, we address the doping mechanism of the high-temperature superconductor YBa₂Cu₃O_7-X (YBCO) by simultaneous ionic liquid gating and X-ray absorption experiments. Pronounced spectral changes are observed at the Cu K-edge concomitant with the superconductor-to-insulator transition, evidencing modification of the Cu coordination resulting from the deoxygenation of the CuO chains, as confirmed by first-principles density functional theory (DFT) simulations. Beyond providing evidence of the importance of chemical doping in electric double-layer (EDL) gating experiments with superconducting cuprates, our work shows that interfacing correlated oxides with ionic liquids enables a delicate control of oxygen content, paving the way to novel electrochemical concepts in future oxide electronics.

Ambipolar transport and magneto-resistance crossover in a Mott insulator, Sr2IrO4

Electric field effect (EFE) controlled magnetoelectric transport in thin films of undoped and La-doped Sr₂IrO₄ (SIO) is investigated using ionic liquid gating. The temperature dependent resistance measurements exhibit insulating behavior in chemically and EFE doped samples with the band filling up to 10%. The ambipolar transport across the Mott gap is demonstrated by EFE tuning of the channel resistance and chemical doping. We observe a crossover from high temperature negative to low temperature positive magnetoresistance around  ∼80-90 K, irrespective of the filling. This temperature and magnetic field dependent crossover is discussed in the light of conduction mechanisms of SIO, especially variable range hopping (VRH), and its relevance to the insulating ground state of SIO.

Recent Advances in Doping of Molybdenum Disulfide: Industrial Applications and Future Prospects

Owing to their excellent physical properties, atomically thin layers of molybdenum disulfide (MoS₂ ) have recently attracted much attention due to their nonzero-gap property, exceptionally high electrical conductivity, good thermal stability, and excellent mechanical strength, etc. MoS₂ -based devices exhibit great potential for applications in optoelectronics and energy harvesting. Here, a comprehensive review of various doping strategies is presented, including wet doping and dry doping of atomically crystalline MoS₂ thin layers, and the progress made so far for their doping-based prospective applications is also discussed. Finally, several significant research issues for the prospects of doped-MoS₂ in industry, as a guide for 2D material community, are also provided.

Lithium-ion-based solid electrolyte tuning of the carrier density in graphene

We have developed a technique to tune the carrier density in graphene using a lithium-ion-based solid electrolyte. We demonstrate that the solid electrolyte can be used as both a substrate to support graphene and a back gate. It can induce a change in the carrier density as large as 1 × 10¹⁴ cm⁻², which is much larger than that induced with oxide-film dielectrics, and it is comparable with that induced by liquid electrolytes. Gate modulation of the carrier density is still visible at 150 K, which is lower than the glass transition temperature of most liquid gating electrolytes.

Proton Conducting Graphene Oxide/Chitosan Composite Electrolytes as Gate Dielectrics for New-Concept Devices

New-concept devices featuring the characteristics of ultralow operation voltages and low fabrication cost have received increasing attention recently because they can supplement traditional Si-based electronics. Also, organic/inorganic composite systems can offer an attractive strategy to combine the merits of organic and inorganic materials into promising electronic devices. In this report, solution-processed graphene oxide/chitosan composite film was found to be an excellent proton conducting electrolyte with a high specific capacitance of ~3.2 μF/cm² at 1.0 Hz, and it was used to fabricate multi-gate electric double layer transistors. Dual-gate AND logic operation and two-terminal diode operation were realized in a single device. A two-terminal synaptic device was proposed, and some important synaptic behaviors were emulated, which is interesting for neuromorphic systems.

Metallization of Epitaxial VO2 Films by Ionic Liquid Gating through Initially Insulating TiO2 Layers

Ionic liquid gating has been shown to metallize initially insulating layers formed from several different oxide materials. Of these vanadium dioxide (VO2) is of especial interest because it itself is metallic at temperatures above its metal-insulator transition. Recent studies have shown that the mechanism of ionic liquid gated induced metallization is entirely distinct from that of the thermally driven metal-insulator transition and is derived from oxygen migration through volume channels along the (001) direction of the rutile structure of VO2. Here we show that it is possible to metallize the entire volume of 10 nm thick layers of VO2 buried under layers of rutile titanium dioxide (TiO2) up to 10 nm thick. Key to this process is the alignment of volume channels in the respective oxide layers, which have the same rutile structure with clamped in-plane lattice constants. The metallization of the VO2 layers is accompanied by large structural expansions of up to ∼6.5% in the out-of-plane direction, but the structure of the TiO2 layer is hardly affected by gating. The TiO2 layers become weakly conducting during the gating process, but in contrast to the VO2 layers, the conductivity disappears on exposure to air. Indeed, even after air exposure, X-ray photoelectron spectroscopy studies show that the VO2 films have a reduced oxygen content after metallization. Ionic liquid gating of the VO2 films through initially insulating TiO2 layers is not consistent with conventional models that have assumed the gate induced carriers are of electrostatic origin.

Evolution of Insulator-Metal Phase Transitions in Epitaxial Tungsten Oxide Films during Electrolyte-Gating

An interface between an oxide and an electrolyte gives rise to various processes as exemplified by electrostatic charge accumulation/depletion and electrochemical reactions such as intercalation/decalation under electric field. Here we directly compare typical device operations of those in electric double layer transistor geometry by adopting A-site vacant perovskite WO3 epitaxial thin films as a channel material and two different electrolytes as gating agent. In situ measurements of X-ray diffraction and channel resistance performed during the gating revealed that in both the cases WO3 thin film reaches a new metallic state through multiple phase transitions, accompanied by the change in out-of-plane lattice constant. Electrons are electrostatically accumulated from the interface side with an ionic liquid, while alkaline metal ions are more uniformly intercalated into the film with a polymer electrolyte. We systematically demonstrate this difference in the electrostatic and electrochemical processes, by comparing doped carrier density, lattice deformation behavior, and time constant of the phase transitions.

Oxygen Displacement in Cuprates under Ionic Liquid Field-Effect Gating

We studied structural changes in a 5 unit cell thick La_1.96Sr_0.04CuO₄ film, epitaxially grown on a LaSrAlO₄ substrate with a single unit cell buffer layer, when ultra-high electric fields were induced in the film by applying a gate voltage between the film (ground) and an ionic liquid in contact with it. Measuring the diffraction intensity along the substrate-defined Bragg rods and analyzing the results using a phase retrieval method we obtained the three-dimensional electron density in the film, buffer layer, and topmost atomic layers of the substrate under different applied gate voltages. The main structural observations were: (i) there were no structural changes when the voltage was negative, holes were injected into the film making it more metallic and screening the electric field; (ii) when the voltage was positive, the film was depleted of holes becoming more insulating, the electric field extended throughout the film, the partial surface monolayer became disordered, and equatorial oxygen atoms were displaced towards the surface; (iii) the changes in surface disorder and the oxygen displacements were both reversed when a negative voltage was applied; and (iv) the c-axis lattice constant of the film did not change in spite of the displacement of equatorial oxygen atoms.

Flexible Proton-Gated Oxide Synaptic Transistors on Si Membrane

Ion-conducting materials have received considerable attention for their applications in fuel cells, electrochemical devices, and sensors. Here, flexible indium zinc oxide (InZnO) synaptic transistors with multiple presynaptic inputs gated by proton-conducting phosphorosilicate glass-based electrolyte films are fabricated on ultrathin Si membranes. Transient characteristics of the proton gated InZnO synaptic transistors are investigated, indicating stable proton-gating behaviors. Short-term synaptic plasticities are mimicked on the proposed proton-gated synaptic transistors. Furthermore, synaptic integration regulations are mimicked on the proposed synaptic transistor networks. Spiking logic modulations are realized based on the transition between superlinear and sublinear synaptic integration. The multigates coupled flexible proton-gated oxide synaptic transistors may be interesting for neuroinspired platforms with sophisticated spatiotemporal information processing.