Electrolyte Gating Research Articles

Full Control of Solid-State Electrolytes for Electrostatic Gating.

Ionic gating is a powerful technique to realize field-effect transistors (FETs) enabling experiments not possible otherwise. So far, ionic gating has relied on the use of top electrolyte gates, which pose experimental constraints and make device fabrication complex. Promising results obtained recently in FETs based on solid-state electrolytes remain plagued by spurious phenomena of unknown origin, preventing proper transistor operation, and causing limited control and reproducibility. Here, a class of solid-state electrolytes for gating (Lithium-ion conducting glass-ceramics, LICGCs) is explored, the processes responsible for the spurious phenomena and irreproducible behavior are identified, and properly functioning transistors exhibiting high density ambipolar operation with gate capacitance of ≈ (depending on the polarity of the accumulated charges) are demonstrated. Using 2D semiconducting transition-metal dichalcogenides, the ability to implement ionic-gate spectroscopy to determine the semiconducting bandgap, and to accumulate electron densities above 1014 cm-2 are demostrated, resulting in gate-induced superconductivity in MoS2 multilayers. As LICGCs are implemented in a back-gate configuration, they leave the surface of the material exposed, enabling the use of surface-sensitive techniques (such as scanning tunneling microscopy and photoemission spectroscopy) impossible so far in ionic-gated devices. They also allow double ionic gated devices providing independent control of charge density and electricfield.

Tetragonal Mexican-hat dispersion and switchable half-metal state with multiple anisotropic Weyl fermions in penta-graphene

Due to the paired valence electrons configuration, all known two-dimensional (2D) carbon allotropes are intrinsically nonmagnetic. Based on the reported 2D carbon structure database and first-principles calculations, herein we demonstrate that inherent ferromagnetism can be obtained in the prominent allotrope, penta-graphene, which has a unique Mexican-hat valence band edge, giving rise to van Hove singularities and electronic instability. Induced by modest hole-doping that is achievable in electrolyte gate, the semiconducting penta-graphene can be transformed into different ferromagnetic half-metals with room-temperature stability and switchable spin directions. In particular, multiple anisotropic Weyl states, including type-I and type-II Weyl cones and hybrid quasi Weyl nodal loop, can be found in a sizable energy window of spin-down half-metal under proper strains. These findings not only identify a promising carbon allotrope to obtain the inherent magnetism for carbon-based spintronic devices, but highlight the possibility to realize different Weyl states by combining the electronic and mechanical means as well.

Synaptic behavior of Fe3O4-based artificial synapse by electrolyte gating for neuromorphic computing

Neuromorphic computing (NC) is a crucial step toward realizing power-efficient artificial intelligence systems. Hardware implementation of NC is expected to overcome the challenges associated with the conventional von Neumann computer architecture. Synaptic devices that can emulate the rich functionalities of biological synapses are emerging. Out of several approaches, electrolyte-gated synaptic transistors have attracted enormous scientific interest owing to their similar working mechanism. Here, we report a three-terminal electrolyte-gated synaptic transistor based on Fe3O4 thin films, a half-metallic spinel ferrite. We have realized gate-controllable multilevel, non-volatile, and rewritable states for analog computing. Furthermore, we have emulated essential synaptic functions by applying electrical stimulus to the gate terminal of the synaptic device. This work provides a new candidate and a platform for spinel ferrite-based devices for future NC applications.

A multilevel electrolyte-gated artificial synapse based on ruthenium-doped cobalt ferrite

Synaptic devices that emulate synchronized memory and processing are considered the core components of neuromorphic computing systems for the low-power implementation of artificial intelligence. In this regard, electrolyte-gated transistors (EGTs) have gained much scientific attention, having a similar working mechanism as the biological synapses. Moreover, compared to a traditional solid-state gate dielectric, the liquid dielectric has the key advantage of inducing extremely large modulation of carrier density while overcoming the problem of electric pinholes, that typically occurs when using large-area films gated through ultra-thin solid dielectrics. Herein we demonstrate a three-terminal synaptic transistor based on ruthenium-doped cobalt ferrite (CRFO) thin films by electrolyte gating. In the CRFO-based EGT, we have obtained multilevel non-volatile conductance states for analog computing and high-density storage. Furthermore, the proposed synaptic transistor exhibited essential synaptic behavior, including spike amplitude-dependent plasticity, spike duration-dependent plasticity, long-term potentiation, and long-term depression successfully by applying electrical pulses. This study can motivate the development of advanced neuromorphic devices that leverage simultaneous modulation of electrical and magnetic properties in the same device and show a new direction to synaptic electronics.

Revealing the nature of the graphene on-off control of the electrolyte gating effect in WO3 film phase transition

Electrolyte gating-induced metallization has been broadly observed in various transition metal oxides for decades, while the gating mechanism is still under debate. In this work, we realize the Insulator-Metal Transition (IMT) control of WO3 film by inserting a few-layer graphene. It's found that a sharp IMT process in WO3 film emerges if applying an electrolyte gating treatment, while this IMT behavior will be strikingly suppressed if inserting a graphene layer between the WO3 film and ionic liquid. Interestingly, the pronounced IMT behavior can be always observed if substituting the ionic liquid by acid solution, regardless of graphene layer insertion or not. According to the first-principles calculations, the diffusion energy barriers for the hydrogen or oxygen propagating graphene layer are extracted, and then the mechanism of electrolyte gating in WO3 films is proposed. Moreover, the roles of hydrogen and oxygen vacancy on the WO3 IMT process are further discussed based on the band structure calculations. Our current study will boost the development of electrolyte gating technique and bring deeper understanding of the electrolyte gating treatment for other transition metal oxides.

Ionic Liquid-Gated Near-Infrared Polymer Phototransistors and Their Persistent Photoconductivity Application in Optical Memory.

Organic phototransistors (OPTs) based on polymers have attracted substantial attention due to their excellent signal amplification, significant noise reduction, and solution process. Recently, the near-infrared (NIR) detection becomes urgent for OPTs with the increased demand for biomedicine, medical diagnostics, and health monitoring. To achieve this goal, a low working voltage of the OPTs is highly desirable. Therefore, the traditional dielectric gate can be replaced by an electrolyte gate to form electrolyte-gated organic phototransistors (EGOPTs), which are not only able to work at voltages below 1.0 V but also are biocompatible. PCDTPT, one of the most popular narrow band gap donor-acceptor copolymer, has been rarely studied in EGOPTs. In this work, an organic NIR-sensitive EGOPT based on PCDTPT is demonstrated with the detectivity of 7.08 × 1011 Jones and the photoresponsivity of 3.56 A/W at a low operating voltage. In addition, an existing persistent photoconductivity (PPC) phenomenon was also observed when the device was exposed to air. The PPC characteristic of the EGOPT in air has been used to achieve a phototransistor memory, and the gate bias can directly eliminate the PPC as an erasing operation. This work reveals the underlying mechanism of the electrolyte-gated organic phototransistor memories and broadens the application of the EGOPTs.

Two-dimensional multiband superconductivity of the optimally and uniformly Li-intercalated FeSe nanoflakes

We investigated the electrical transport properties of optimally Li+-intercalated FeSe nanoflakes. Using solid electrolyte gating, we obtained high and uniform electron doping up to ∼0.55 electrons per Fe sites in the FeSe nanoflakes, which exhibit a sharp superconducting transition at Tc ∼45.0 K. The temperature and magnetic-field dependent current-voltage characteristics clearly revealed the two-dimensional nature of the superconductivity in the intercalated FeSe nanoflakes. From the self-field critical current measurements, we found two distinct isotropic superconducting gaps with different gap ratios. These findings suggest that the highly electron-doped FeSe hosts two interband pairing channels between the hybridized electron pockets, in contrast to the case of pristine FeSe with electron and hole pockets.

Electrolyte-gate-driven carrier density modulation and metal–insulator transition in semiconducting epitaxial CdO films

CdO has drawn much recent interest as a high-room-temperature-mobility oxide semiconductor with exciting potential for mid-infrared photonics and plasmonics. Wide-range modulation of carrier density in CdO is of interest both for fundamental reasons (to explore transport mechanisms in single samples) and for applications (in tunable photonic devices). Here, we thus apply ion-gel-based electrolyte gating to ultrathin epitaxial CdO(001) films, using transport, x-ray diffraction, and atomic force microscopy to deduce a reversible electrostatic gate response from −4 to +2 V, followed by rapid film degradation at higher gate voltage. Further advancing the mechanistic understanding of electrolyte gating, these observations are explained in terms of low oxygen vacancy diffusivity and high acid etchability in CdO. Most importantly, the 6-V-wide reversible electrostatic gating window is shown to enable ten-fold modulation of the Hall electron density, a striking voltage-induced metal–insulator transition, and 15-fold variation of the electron mobility. Such modulations, which are limited only by unintentional doping levels in ultrathin films, are of exceptional interest for voltage-tunable devices.

Gate‐tunable spin valve effect in Fe3GeTe2‐based van der Waals heterostructures

AbstractMagnetic tunnel junctions (MTJs), a prominent type of spintronic device based on the spin valve effect, have facilitated the development of numerous spintronic applications. The technical appeal for the next‐generation MTJ devices has been proposed in two directions: improving device performance by utilizing advanced two‐dimensional (2D) ferromagnetic materials or extending device functionalities by exploring the gate‐tunable magnetic properties of ferromagnets. Based on the recent development of 2D magnets with the ease of external stimuli, such as electric field, due to their reduced dimensions, reliable prospects for gate‐tunable MTJ devices can be achieved, shedding light on the great potential of next‐generation MTJs with multiple functionalities for various application environments. While the electrical gate‐tunable MTJ device is highly desirable for practical spintronic devices, it has not yet been demonstrated. Here, we demonstrate the experimental realization of a spin valve device by combining a vertical Fe3GeTe2/h‐BN/Fe3GeTe2 MTJ with an electrolyte gate. The magnetoresistance ratio (MR ratio) of 36% for the intrinsic MTJ confirms the good performance of the device. By electrolyte gating, the tunneling MR ratio of Fe3GeTe2/h‐BN/Fe3GeTe2 MTJ can be elevated 2.5 times, from 26% to 65%. Importantly, the magnetic fields at which the magnetoresistance switches for the MTJ can be modulated by electrical gating, providing a promising method to control the magnetization configuration of the MTJ. Our work demonstrates a gate‐tunable MTJ device toward the possibility for gate‐controlled spintronic devices, paving the way for performing 2D magnetism manipulations and exploring innovative spintronic applications.image

A comparative study of electrochemical and electrostatic doping modulation of magnetism in Fe3O4 via ultracapacitor structure

Electric field control of magnetism can boost energy efficiency and have brought revolutionary breakthroughs in the development of widespread applications in spintronics. Electrolyte gating plays an important role in magnetism modulation. In this work, reversible room-temperature electric field control of saturation magnetization in Fe3O4 via a supercapacitor structure is demonstrated with three types of traditional gate electrolytes for comparison. Different magnetization response and responsible mechanisms are revealed by Operando magnetometry PPMS/VSM and XPS characterization. The main mechanism in Na2SO4, KOH aqueous electrolytes is electrochemical effect, while both electrochemical and electrostatic effects were found in LiPF6 organic electrolyte. This work offers a kind of reference basis for selecting appropriate electrolyte in magnetism modulation by electrolyte-gating in the future, meanwhile, paves its way towards practical use in magneto-electric actuation, voltage-assisted magnetic storage, facilitating the development of high-performance spintronic devices.

Time-Dependent Sensitivity Tunable pH Sensors Based on the Organic-Inorganic Hybrid Electric-Double-Layer Transistor.

In this study, we propose tunable pH sensors based on the electric-double-layer transistor (EDLT) with time-dependent sensitivity characteristics. The EDLT is able to modulate the drain current by using the mobile ions inside the electrolytic gate dielectric. This property allows the implementation of a device with sensitivity characteristics that are simply adjusted according to the measurement time. An extended gate-type, ion-sensitive, field-effect transistor consisting of a chitosan/Ta2O5 hybrid dielectric EDLT transducer, and an SnO2 sensing membrane, were fabricated to evaluate the sensing behavior at different buffer pH levels. As a result, we were able to achieve tunable sensitivity by only adjusting the measurement time by using a single EDLT and without additional gate electrodes. In addition, to demonstrate the unique sensing behavior of the time-dependent tunable pH sensors based on organic–inorganic hybrid EDLT, comparative sensors consisting of a normal FET with a SiO2 gate dielectric were prepared. It was found that the proposed pH sensors exhibit repeatable and stable sensing operations with drain current deviations <1%. Therefore, pH sensors using a chitosan electrolytic EDLT are suitable for biosensor platforms, possessing tunable sensitivity and high-reliability characteristics.

Compressive-Strain-Facilitated Fast Oxygen Migration with Reversible Topotactic Transformation in La0.5Sr0.5CoOx via All-Solid-State Electrolyte Gating

Modifying the crystal structure and corresponding functional properties of complex oxides by regulating their oxygen content has promising applications in energy conversion and chemical looping, where controlling oxygen migration plays an important role. Therefore, finding an efficacious and feasible method to facilitate oxygen migration has become a critical requirement for practical applications. Here, we report a compressive-strain-facilitated oxygen migration with reversible topotactic phase transformation (RTPT) in La0.5Sr0.5CoOx films based on all-solid-state electrolyte gating modulation. With the lattice strain changing from tensile to compressive strain, significant reductions in modulation duration (∼72%) and threshold voltage (∼70%) for the RTPT were observed, indicating great promotion of RTPT by compressive strain. Density functional theory calculations verify that such compressive-strain-facilitated efficient RTPT comes from significant reduction of the oxygen migration barrier in compressive-strained films. Further, ac-STEM, EELS, and sXAS investigations reveal that varying strain from tensile to compressive enhances the Co 3d band filling, thereby suppressing the Co-O hybrid bond in oxygen vacancy channels, elucidating the micro-origin of such compressive-strain-facilitated oxygen migration. Our work suggests that controlling electronic orbital occupation of Co ions in oxygen vacancy channels may help facilitate oxygen migration, providing valuable insights and practical guidance for achieving highly efficient oxygen-migration-related chemical looping and energy conversion with complex oxides.

Cationic effects on solid polymer electrolyte-gated organic transistors

Unlike the conventional organic field effect transistors using traditional gate dielectric layer, the electrolyte-gated organic transistors (EGOTs) use electrolytes as gate media, resulting in a tunable highly conductive channel. Due to their unique advantages, such as: higher gate-capacitance and lower operating voltages, the EGOTs are subject to extensive research. In a solid polymer electrolyte gate, the solvated cations and anions are initially evenly distributed in the ion-conducting polymer, under gate bias, the counter ions then drift towards the electrolyte/active layer interface and under high enough bias they might insert into the active layer and dope the polymer. Given the above mechanism, the anions and cations play the crucial role for gate operation in EGOTs. Although how the anions significantly affect the organic electrochemical transistors using the liquid electrolyte media was investigated, the ionic effect in the solid electrolyte-gated transistors has been barely reported. In this work, the effects of cation on the device performance of solid polymer electrolyte-gated organic transistors have been systematically studied. In the devices made with alkali perchlorates, a systematic shift in threshold voltage, current hysteresis, and response time with increasing cation size/mass has been observed. A lower threshold voltage is obtained with the smaller cation which also displays smaller hysteresis and faster response due to faster migration of smaller cations in polymer electrolyte. This study suggests the smaller cations should be used in the fabrication of solid polymer EGOTs. • Cationic effects on the solid electrolyte (PEO:alkali perchlorate salt)-gated organic transistors (EGOTs) are studied. • The smaller cation based device displays lower threshold voltage, smaller hysteresis, and faster response. • The cation motion in polymer electrolyte is key for performances of EGOTs.

Electrochemical mechanism of ionic-liquid gating in antiferromagnetic Mott-insulating NiS2 single crystals

We explore the effect of ionic-liquid gating in the antiferromagnetic Mott insulator ${\mathrm{NiS}}_{2}$. Through temperature- and gate-voltage-dependent electronic transport measurements, a gating-induced three-dimensional metallic state is observed at positive gate bias on single-crystal surfaces. Based on transport, energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy, atomic force microscopy, and other techniques, we deduce an $\mathit{electrochemical}$ gating mechanism involving a substantial decrease in the S:Ni ratio over hundreds of nanometers, which is both nonvolatile and irreversible. Such findings are in striking contrast to the reversible, volatile, two-dimensional $\mathit{electrostatic}$ gate effect previously seen in pyrite ${\mathrm{FeS}}_{2}$. We attribute this stark difference in electrochemical vs electrostatic gating response in ${\mathrm{NiS}}_{2}$ and ${\mathrm{FeS}}_{2}$ to the much larger S diffusion coefficient in ${\mathrm{NiS}}_{2}$. The gating irreversibility, on the other hand, is associated with the lack of atmospheric S, in contrast to the better understood oxide case, where electrolysis of atmospheric ${\mathrm{H}}_{2}\mathrm{O}$ provides an O reservoir. The present study of ${\mathrm{NiS}}_{2}$ thus provides insight into electrolyte gating mechanisms in functional materials, in a relatively unexplored limit.

Flexible VO2 Films for In‐Sensor Computing with Ultraviolet Light

AbstractWith their unique advantages in portability, shape adaptability, and human friendly surfaces, flexible electronics pave the way for the implementation of wearable electronic textiles and human–machine interfaces. Although organic materials are promising for flexible devices because of the low‐cost manufacturing and inherent flexibility, they meet challenges in harsh environments such as ultraviolet (UV) irradiation, which limits their applicability in UV sensors. Here, a flexible UV neuromorphic sensor is presented using inorganic vanadium dioxide (VO2) films grown on mica substrates. The flexible device shows UV photoinduced nonvolatile phase transition, and can be reversibly modulated using electrolyte gating. The optical responses remain almost unchanged after 10 000 bending cycles or at small bending radius, exhibiting high tolerance to the bending deformation. Besides, the variations in image recognition accuracy under different bending conditions keep within 1.6%, indicating that the device can be adapted to various deformation conditions. By constructing near‐/in‐sensor computing architectures using the flexible VO2 neuromorphic sensors with photoinduced nonvolatile phase transition, both static image processing and motion detection are realized without redundant and massive information transfer. This result lays the foundation for the development of flexible UV neuromorphic sensors.

Flexible Mott Synaptic Transistor on Polyimide Substrate for Physical Neural Networks

AbstractFlexible synaptic devices that emulate biological synapses are the key to developing wearable intelligent equipment. However, there are still challenges to achieving multiple synaptic functions in flexible synaptic devices and realizing large‐scale production of synaptic devices. Herein, a flexible synaptic transistor on polyimide substrate is designed and fabricated using a solid‐state electrolyte gate and a VO2 Mott insulator thin film channel. Four orders conductance modulation is obtained in the VO2 thin film channel across the metal‐insulator transition. Several essential synaptic behaviors, including potentiation, depression, and short‐term/long‐term plasticity, are successfully demonstrated in this flexible synaptic device. More importantly, this synaptic transistor exhibits high cycling stability and good tolerance to bending deformation. A simulated artificial neural network built from this synaptic device achieves high recognition accuracy of 98% in the flat state and 92% in the bending state. In addition, this flexible device is fabricated on polyimide substrate by magnetron sputtering method, which has low cost and is compatible with modern semiconductor technology. This work can pave the way for large‐scale production of flexible synaptic Mott transistor devices.

Solid‐State Electrolyte Gate Transistor with Ion Doping for Biosignal Classification of Neuromorphic Computing

AbstractAs the core component of an intelligent neuromorphic computer system, reliable synaptic devices process vast amounts of data with high computing speed and low energy consumption. In this work, the ion‐doped eco‐friendly solution‐processed indium oxide (InOx)/aluminum oxide (AlOx) electrolyte gate transistors (EGTs) with typical and reliable synaptic behavior are proposed. The lithium ions doped into the AlOx solid‐state layer to facilitate the generation of electrical double layers and doped into InOx to improve the stability of long‐term potentiation/depression cyclic update and enhance the synaptic plasticity. Finally, an artificial neural network simulator is well designed to electrocardiogram signal recognition based on the Gmax/Gmin ratio and nonlinearity of weight update curve. According to the results, the device possesses tremendous potential for biosignal prediction and neural intervention. Moreover, for the first time, the recognition accuracy of the abnormality of the cardiovascular can reach over 94.8% obtained from the confusion matrix. Consequently, this research article presents a stable and robust neuromorphic device for biosignal recognition based on solid‐state EGTs via the synaptic long‐term plasticity.

What controls electrostatic vs electrochemical response in electrolyte-gated materials? A perspective on critical materials factors

Electrolyte-gate transistors are a powerful platform for control of material properties, spanning semiconducting behavior, insulator-metal transitions, superconductivity, magnetism, optical properties, etc. When applied to magnetic materials, for example, electrolyte-gate devices are promising for magnetoionics, wherein voltage-driven ionic motion enables low-power control of magnetic order and properties. The mechanisms of electrolyte gating with ionic liquids and gels vary from predominantly electrostatic to entirely electrochemical, however, sometimes even in single material families, for reasons that remain unclear. In this Perspective, we compare literature ionic liquid and ion gel gating data on two rather different material classes—perovskite oxides and pyrite-structure sulfides—seeking to understand which material factors dictate the electrostatic vs electrochemical gate response. From these comparisons, we argue that the ambient-temperature anion vacancy diffusion coefficient (not the vacancy formation energy) is a critical factor controlling electrostatic vs electrochemical mechanisms in electrolyte gating of these materials. We, in fact, suggest that the diffusivity of lowest-formation-energy defects may often dictate the electrostatic vs electrochemical response in electrolyte-gated inorganic materials, thereby advancing a concrete hypothesis for further exploration in a broader range of materials.

Photo-induced non-volatile VO2 phase transition for neuromorphic ultraviolet sensors

In the quest for emerging in-sensor computing, materials that respond to optical stimuli in conjunction with non-volatile phase transition are highly desired for realizing bioinspired neuromorphic vision components. Here, we report a non-volatile multi-level control of VO2 films by oxygen stoichiometry engineering under ultraviolet irradiation. Based on the reversible regulation of VO2 films using ultraviolet irradiation and electrolyte gating, we demonstrate a proof-of-principle neuromorphic ultraviolet sensor with integrated sensing, memory, and processing functions at room temperature, and also prove its silicon compatible potential through the wafer-scale integration of a neuromorphic sensor array. The device displays linear weight update with optical writing because its metallic phase proportion increases almost linearly with the light dosage. Moreover, the artificial neural network consisting of this neuromorphic sensor can extract ultraviolet information from the surrounding environment, and significantly improve the recognition accuracy from 24% to 93%. This work provides a path to design neuromorphic sensors and will facilitate the potential applications in artificial vision systems.

Emulation of Synaptic Plasticity on a Cobalt-Based Synaptic Transistor for Neuromorphic Computing.

Neuromorphic computing (NC), which emulates neural activities of the human brain, is considered for the low-power implementation of artificial intelligence. Toward realizing NC, fabrication, and investigations of hardware elements─such as synaptic devices and neurons─are crucial. Electrolyte gating has been widely used for conductance modulation by massive carrier injections and has proven to be an effective way of emulating biological synapses. Synaptic devices, in the form of synaptic transistors, have been studied using various materials. Despite the remarkable progress, the study of metallic channel-based synaptic transistors remains massively unexplored. Here, we demonstrated a three-terminal electrolyte gating-modulated synaptic transistor based on a metallic cobalt thin film to emulate biological synapses. We have realized gating-controlled, non-volatile, and distinct multilevel conductance states in the proposed device. The essential synaptic functions demonstrating both short-term and long-term plasticity have been emulated in the synaptic device. A transition from short-term to long-term memory has been realized by tuning the gate pulse parameters, such as amplitude and duration. The crucial cognitive behavior, including learning, forgetting, and re-learning, has been emulated, showing a resemblance to the human brain. Beyond that, dynamic filtering behavior has been experimentally implemented in the synaptic device. These results provide an insight into the design of metallic channel-based synaptic transistors for NC.