Electrolyte-gated graphene Schottky barrier transistors

Advanced Neuromorphic Applications Enabled by Synaptic Ion-Gating Vertical Transistors

Bioinspired synaptic devices have shown great potential in artificial intelligence and neuromorphic electronics. Low energy consumption, multi-modal sensing and recording, and multifunctional integration are critical aspects limiting their applications. Recently, a new synaptic device architecture, the ion-gating vertical transistor (IGVT), has been successfully realized and timely applied to perform brain-like perception, such as artificial vision, touch, taste, and hearing. In this short time, IGVTs have already achieved faster data processing speeds and more promising memory capabilities than many conventional neuromorphic devices, even while operating at lower voltages and consuming less power. This work focuses on the cutting-edge progress of IGVT technology, from outstanding fabrication strategies to the design and realization of low-voltage multi-sensing IGVTs for artificial-synapse applications. The fundamental concepts of artificial synaptic IGVTs, such as signal processing, transduction, plasticity, and multi-stimulus perception are discussed comprehensively. The contribution draws special attention to the development and optimization of multi-modal flexible sensor technologies and presents a roadmap for future high-end theoretical and experimental advancements in neuromorphic research that are mostly achievable by the synaptic IGVTs.

Graphene barristors for de novo optoelectronics

Graphene-based vertical Schottky-barrier transistors (SBTs), renowned as graphene barristors, have emerged as a feasible candidate to fundamentally expand the horizon of conventional transistor technology. The remote tunability of graphene's electronic properties could endorse multi-stimuli responsive functionalities for a broad range of electronic and optoelectronic applications of transistors, with the capability of incorporating nanochannel architecture with dramatically reduced footprints from the vertical integrations. In this Feature Article, we provide a comprehensive overview of the progress made in the field of SBTs over the last 10 years, starting from the operating principles, materials evolution, and processing developments. Depending on the types of stimuli such as electrical, optical, and mechanical stresses, various fields of applications from conventional digital logic circuits to sensory technologies are highlighted. Finally, more advanced applications toward beyond-Moore electronics are discussed, featuring recent advancements in neuromorphic devices based on SBTs.

Performance Improvement with an Ultrathin p-Type Interfacial Layer in n-Type Vertical Organic Field-Effect Transistors Based on Reduced Graphene Oxide Electrode

Vertical organic field-effect transistors (VOFETs) with a large current on/off ratio and easy fabrication process are highly desirable for future organic electronics. In this paper, we proposed an ultrathin p-type copper (II) phthalocyanine (CuPc) interfacial layer in reduced graphene oxide (rGO)-based VOFETs. The CuPc interfacial layer was sandwiched between the rGO electrode and the N,N'-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) organic layer. The introduced CuPc interfacial layer not only decreased the off-current density of the device but also slightly enhanced the on-current density. The threshold voltage of the device was also effectively improved and stabilized at around 0 V. The obtained device exhibited a current on/off ratio exceeding 106, which is the largest value reported for rGO-based VOFETs. The vertical electron mobility of the PTCDI-C8 layer estimated by the space-charge-limited current technique was 1.14 × 10-3 cm2/(V s). However, it was not the main limiting factor for the current density in this device. We totally fabricated 48 devices, and more than 75% could work. Besides, the device was stable with little performance degradation after 1 month. The use of low-cost, solution-processable rGO as work-function-tunable electrode and the application of an ultrathin CuPc interfacial layer in VOFETs may open up opportunities for future organic electronics.

Monolithic Tandem Multicolor Image Sensor Based on Electrochromic Color-Radix Demultiplexing

Optical data acquisition has been set as one of the milestones to testify the developments aimed at harnessing the full potential of the spatial and temporal data processing capabilities of the advanced semiconductor technology. A highly promising approach to drive the level of acquisition beyond the current technological node is the vertical integration of multiple photodetectors. However, vertical integration so far requires the same level of circuit complexity as lateral integration from the incapability of monolithic integration. Here, an electrochromic device architecture is introduced that enables realization of a monolithic tandem multicolor photodetector. The device, composed of vertically stacked p-type and n-type graphene barristors, is demonstrated to be capable of regulating the balanced charge transport under any desired illumination wavelengths. It exhibits variable anti-ambipolar charge transport behavior, which yields sensitive voltage-controlled photoconductive gain spectra. These electrical behaviors are utilized to fabricate an optoelectronic logic sensor that can demultiplex the desired color coordinate or wavelength in the constituent array with high color accuracy.

Synaptic transistors and neuromorphic systems based on carbon nano-materials

Carbon-based materials possessing a nanometer size and unique electrical properties perfectly address the two critical issues of transistors, the low power consumption and scalability, and are considered as a promising material in next-generation synaptic devices. In this review, carbon-based synaptic transistors were systematically summarized. In the carbon nanotube section, the synthesis of carbon nanotubes, purification of carbon nanotubes, the effect of architecture on the device performance and related carbon nanotube-based devices for neuromorphic computing were discussed. In the graphene section, the synthesis of graphene and its derivative, as well as graphene-based devices for neuromorphic computing, was systematically studied. Finally, the current challenges for carbon-based synaptic transistors were discussed.

Color-Selective Schottky Barrier Modulation for Optoelectric Logic

The limitation on signal processes implementable using conventional semiconductor circuits based on electric signals necessitates a revolutionary change in device structures such that they can exploit photons or light. Herein, we introduce optoelectric logic circuits that convert optical signals with different wavelengths corresponding to different colors into binary electric signals. Such circuits are assembled using unit devices in which the electric current through the semiconductor channel is effectively gated by lights of different colors. Color-selective optical modulation of the device is cleverly achieved using graphene decorated with different organic dyes as the electrode of a Schottky diode structure. The drastic change in the electrode work function under illumination induces a change in the height of the Schottky barrier formed at the electrode/semiconductor junction and consequent modulation of the electric current; we term the developed device a photonic barristor. We construct logic circuits using an array of photonic barristors and demonstrate that they execute the functions of conventional NAND and NOR gates from optical input signals.

Graphene Hybrid Structures for Integrated and Flexible Optoelectronics

Graphene (Gr) has many unique properties including gapless band structure, ultrafast carrier dynamics, high carrier mobility, and flexibility, making it appealing for ultrafast, broadband, and flexible optoelectronics. To overcome its intrinsic limit of low absorption, hybrid structures are exploited to improve the device performance. Particularly, van der Waals heterostructures with different photosensitive materials and photonic structures are very effective for improving photodetection and modulation efficiency. With such hybrid structures, Gr hybrid photodetectors can operate from ultraviolet to terahertz, with significantly improved R (up to 10⁹ A W^-1 ) and bandwidth (up to 128 GHz). Furthermore, integration of Gr with silicon (Si) complementary metal-oxide-semiconductor (CMOS) circuits, the human body, and soft tissues is successfully demonstrated, opening promising opportunities for wearable sensors and biomedical electronics. Here, the recent progress in using Gr hybrid structures toward high-performance photodetectors and integrated optoelectronic applications is reviewed.

2D-Organic Hybrid Heterostructures for Optoelectronic Applications

The unique properties of hybrid heterostructures have motivated the integration of two or more different types of nanomaterials into a single optoelectronic device structure. Despite the promising features of organic semiconductors, such as their acceptable optoelectronic properties, availability of low-cost processes for their fabrication, and flexibility, further optimization of both material properties and device performances remains to be achieved. With the emergence of atomically thin 2D materials, they have been integrated with conventional organic semiconductors to form multidimensional heterostructures that overcome the present limitations and provide further opportunities in the field of optoelectronics. Herein, a comprehensive review of emerging 2D-organic heterostructures-from their synthesis and fabrication to their state-of-the-art optoelectronic applications-is presented. Future challenges and opportunities associated with these heterostructures are highlighted.

All-Inkjet-Printed Vertical Heterostructure for Wafer-Scale Electronics

In this study, we fabricated an array of all-inkjet-printed vertical Schottky barrier (SB) transistors and various logic gates on a large-area substrate. All of the electronic components, including the indium-gallium-zinc-oxide (IGZO) semiconductor, reduced graphene oxide (rGO), and indium-tin-oxide (ITO) electrodes, and the ion-gel gate dielectric, were directly and uniformly printed onto a 4 in. wafer. The vertical SB transistors had a vertically stacked structure, with the inkjet-printed IGZO semiconductor layer placed between the rGO source electrode and the ITO drain electrode. The ion-gel gate dielectric was also inkjet-printed in a coplanar gate geometry. The channel current was controlled by adjusting the SB height at the rGO/IGZO heterojunction under application of an external gate voltage. The high intrinsic capacitance of the ion-gel gate dielectric facilitated modulation of the SB height at the source/channel heterojunction to around 0.5 eV at a gate voltage lower than 2 V. The resulting vertical SB transistors exhibited a high current density of 2.0 A·cm^-2, a high on-off current ratio of 10⁶, and excellent operational and environmental stabilities. The simple device structure of the vertical SB transistors was beneficial for the fabrication of all-inkjet-printed low-power logic circuits such as the NOT, NAND, and NOR gates on a large-area substrate.

Remote Gating of Schottky Barrier for Transistors and Their Vertical Integration

This paper introduces a strategy to modulate a Schottky barrier formed at a graphene-semiconductor heterojunction. The modulation is performed by controlling the work function of graphene from a gate that is placed laterally away from the graphene-semiconductor junction, which we refer to as the remote gating of a Schottky barrier. The remote gating relies on the sensitive work function of graphene, whose local variation induced by locally applied field effect affects the change in the work function of the entire material. Using Kelvin probe force microscopy analysis, we directly visualize how this local variation in the work function propagates through graphene. These properties of graphene are exploited to assemble remote-gated vertical Schottky barrier transistors (v-SBTs) in an unconventional device architecture. Furthermore, a vertical complementary circuit is fabricated by simply stacking two remote-gated v-SBTs (pentacene layer as the p-channel and indium gallium zinc oxide layer as the n-channel) vertically. We consider that the remote gating of graphene and the associated device architecture presented herein facilitate the extendibility of graphene-based v-SBTs in the vertical assembly of logic circuits.

Gelatin Hydrogel-Based Organic Electrochemical Transistors and Their Integrated Logic Circuits

We suggest gelatin hydrogel as an electrolyte and demonstrate organic electrochemical transistors (OECTs) based on a sheet of gelatin. We also modulate electrical characteristics of the OECT with respect to pH condition of the gelatin hydrogel from acid to base and analyze its characteristics based on the electrochemical theory. Moreover, we extend the gelatin-based OECT to electrochemical logic circuits, for example, NOT, NOR, and NAND gates.

Organic-Single-Crystal Vertical Field-Effect Transistors and Phototransistors

Organic vertical field-effect transistors (VFETs) have attracted significant attention over the past years due to their unique characteristics of high output currents, low operation voltages, high working frequency, and promising high-density integration for circuits. However, most currently reported VFETs demonstrate poor performance, e.g., with low on/off ratio and current density. Here, the first organic-single-crystal vertical field-effect transistors (SC-VFETs) and phototransistors are constructed from 2,6-diphenyl anthracene (DPA) through a modified method. The devices exhibit high on/off ratio of 10⁶ and a high current density of 100 mA cm^-2 under a small voltage of -5 V, which are proved to be one of the best performances for organic VFETs. Furthermore, superior photoresponse performance with photoresponsivity of 110 A W^-1 and detectivity of 10¹³ Jones is obtained under light illumination for vertical phototransistors. These results confirm the control of the intrinsic Schottky barrier height at the graphene-DPA junction along with good interfacial contact effectively suppressing the dark current to realize a large on/off ratio and high light detectivity. This vertical integration of graphene with organic single crystals via simple, effective fabrication processes opens up new opportunities to realize high-performance integrated organic vertical electronic and optoelectronic devices.

Roll-to-roll redox-welding and embedding for silver nanowire network electrodes

We developed a continuous roll-to-roll redox-welding and embedding method for the fabrication of electrodes of silver nanowire (AgNWs) networks. The roll-to-roll welding method involved a sequence of oxidation and reduction reactions in an aqueous solution. The redox-welding significantly decreased the sheet resistance of the AgNW film owing to the strong fusion and interlocking at the nanowire junction, while the optical transmittance was maintained. The first oxidation step using HNO₃ generated ionized silver (Ag⁺) which got re-deposited onto the nanowire junctions via an autocatalytic reaction. The oxide layers, which formed on the nanowire surface by both air exposure and the first step of oxidation, were removed by the second reduction step using NaBH₄. The redox-welded AgNW electrodes exhibited a sheet resistance of 11.3 Ω sq^-1 at the optical transmittance of 90.5% at 550 nm. Furthermore, redox-welding of the AgNWs significantly enhanced their mechanical robustness compared to that of the as-coated AgNWs. The redox-welded AgNWs embedded in a UV curable resin, using a roll-to-roll embedding process, were successfully applied as anode electrodes for large-area and flexible organic light emitting diodes (OLEDs). The device performance is superior to that of a device based on the as-coated AgNW electrode, and is also comparable to that of a device using commercial ITO as the electrode. The redox-welding and embedding processes provide a facile and reliable method for fabricating large-area transparent flexible electrodes for next-generation flexible optoelectronic devices.

Integrated Circuits Comprising Patterned Functional Liquids

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

Five-minute synthesis of silver nanowires and their roll-to-roll processing for large-area organic light emitting diodes

Silver (Ag) nanowires (NWs) are promising building blocks for flexible transparent electrodes, which are key components in fabricating soft electronic devices such as flexible organic light emitting diodes (OLEDs). Typically, Ag NWs have been synthesized using a polyol method, but it still remains a challenge to produce high-aspect-ratio Ag NWs via a simple and rapid process. In this work, we developed a modified polyol method and newly found that the addition of propylene glycol to ethylene glycol-based polyol synthesis facilitated the growth of Ag NWs, allowing the rapid production of long Ag NWs with high aspect ratios of about 2000 in a high yield (∼90%) within 5 min. Transparent electrodes fabricated with our Ag NWs exhibited performance comparable to that of an indium tin oxide-based electrode. With these Ag NWs, we successfully demonstrated the fabrication of a large-area flexible OLED with dimensions of 30 cm × 15 cm using a roll-to-roll process.

Nanopatched Graphene with Molecular Self-Assembly Toward Graphene-Organic Hybrid Soft Electronics

Increasing the mechanical durability of large-area polycrystalline single-atom-thick materials is a necessary step toward the development of practical and reliable soft electronics based on these materials. Here, it is shown that the surface assembly of organosilane by weak epitaxy forms nanometer-thick organic patches on a monolayer graphene surface and dramatically increases the material's resistance to harsh postprocessing environments, thereby increasing the number of ways in which graphene can be processed. The nanopatched graphene with the improved mechanical durability enables stable operation when used as transparent electrodes of wearable strain sensors. Also, the nanopatched graphene applied as an electrode modulates the molecular orientation of deposited organic semiconductor layers, and yields favorable nominal charge injection for organic transistors. These results demonstrate the potential for use of self-assembled organic nanopatches in graphene-based soft electronics.

When 2D Materials Meet Molecules: Opportunities and Challenges of Hybrid Organic/Inorganic van der Waals Heterostructures

van der Waals heterostructures, composed of vertically stacked inorganic 2D materials, represent an ideal platform to demonstrate novel device architectures and to fabricate on-demand materials. The incorporation of organic molecules within these systems holds an immense potential, since, while nature offers a finite number of 2D materials, an almost unlimited variety of molecules can be designed and synthesized with predictable functionalities. The possibilities offered by systems in which continuous molecular layers are interfaced with inorganic 2D materials to form hybrid organic/inorganic van der Waals heterostructures are emphasized. Similar to their inorganic counterpart, the hybrid structures have been exploited to put forward novel device architectures, such as antiambipolar transistors and barristors. Moreover, specific molecular groups can be employed to modify intrinsic properties and confer new capabilities to 2D materials. In particular, it is highlighted how molecular self-assembly at the surface of 2D materials can be mastered to achieve precise control over position and density of (molecular) functional groups, paving the way for a new class of hybrid functional materials whose final properties can be selected by careful molecular design.

Halide Welding for Silver Nanowire Network Electrode

We developed a method of chemically welding silver nanowires (AgNWs) using an aqueous solution containing sodium halide salts (NaF, NaCl, NaBr, or NaI). The halide welding was performed simply by immersing the as-coated AgNW film into the sodium halide solution, and the resulting material was compared with those obtained using two typical thermal and plasmonic welding techniques. The halide welding dramatically reduced the sheet resistance of the AgNW electrode because of the strong fusion among nanowires at each junction while preserving the optical transmittance. The dramatic decrease in the sheet resistance was attributed to the autocatalytic addition of dissolved silver ions to the nanowire junction. Unlike thermal and plasmonic welding methods, the halide welding could be applied to AgNW films with a variety of deposition densities because the halide ions uniformly contacted the surface or junction regions. The optimized AgNW electrodes exhibited a sheet resistance of 9.3 Ω/sq at an optical transmittance of 92%. The halide welding significantly enhanced the mechanical flexibility of the electrode compared with the as-coated AgNWs. The halide-welded AgNWs were successfully used as source-drain electrodes in a transparent and flexible organic field-effect transistor (OFET). This simple, low-cost, and low-power consumption halide welding technique provides an innovative approach to preparing transparent electrodes for use in next-generation flexible optoelectronic devices.

One-Transistor-One-Transistor (1T1T) Optoelectronic Nonvolatile MoS2 Memory Cell with Nondestructive Read-Out

Taking advantage of the superlative optoelectronic properties of single-layer MoS₂, we developed a one-transistor-one-transistor (1T1T)-type MoS₂ optoelectronic nonvolatile memory cell. The 1T1T memory cell consisted of a control transistor (CT) and a memory transistor (MT), in which the drain electrode of the MT was connected electrically to the gate electrode of the CT, whereas the source electrode of the CT was connected electrically to the gate electrode of the MT. Single-layer MoS₂ films were utilized as the channel materials in both transistors, and gold nanoparticles acted as the floating gates in the MT. This 1T1T device architecture allowed for a nondestructive read-out operation in the memory because the writing (programming or erasing) and read-out processes were operated separately. The switching of the CT could be controlled by light illumination as well as the applied gate voltage due to the strong light absorption induced by the direct band gap of single-layer MoS₂ (∼1.8 eV). The resulting MoS₂ 1T1T memory cell exhibited excellent memory performance, including a large programming/erasing current ratio (over 10⁶), multilevel data storage (over 6 levels), cyclic endurance (200 cycles), and stable retention (10³ s).

Large-Area CVD-Grown Sub-2 V ReS2 Transistors and Logic Gates

We demonstrated the fabrication of large-area ReS₂ transistors and logic gates composed of a chemical vapor deposition (CVD)-grown multilayer ReS₂ semiconductor channel and graphene electrodes. Single-layer graphene was used as the source/drain and coplanar gate electrodes. An ion gel with an ultrahigh capacitance effectively gated the ReS₂ channel at a low voltage, below 2 V, through a coplanar gate. The contact resistance of the ion gel-gated ReS₂ transistors with graphene electrodes decreased dramatically compared with the SiO₂-devices prepared with Cr electrodes. The resulting transistors exhibited good device performances, including a maximum electron mobility of 0.9 cm²/(V s) and an on/off current ratio exceeding 10⁴. NMOS logic devices, such as NOT, NAND, and NOR gates, were assembled using the resulting transistors as a proof of concept demonstration of the applicability of the devices to complex logic circuits. The large-area synthesis of ReS₂ semiconductors and graphene electrodes and their applications in logic devices open up new opportunities for realizing future flexible electronics based on 2D nanomaterials.

Thin film transistors based on two dimensional graphene and graphene/semiconductor heterojunctions

During the past few years, two-dimensional (2D) layered materials have emerged as the most fundamental building blocks of a wide variety of optoelectronic devices.

Parasitic Capacitance Effect on Dynamic Performance of Aerosol-Jet-Printed Sub 2 V Poly(3-hexylthiophene) Electrolyte-Gated Transistors

Printed, low-voltage poly(3-hexylthiophene) (P3HT) electrolyte-gated transistors (EGTs) have favorable quasi-static characteristics, including sub 2 V operation, carrier mobility (μ) of 1 cm²/(V s), ON/OFF current ratio of 10⁶, and static leakage current density of 10^-6 A/cm². Here we study the dynamic performance of P3HT EGTs in which the semiconductor, dielectric, and gate electrode were deposited using aerosol-jet printing; the source and drain electrodes were patterned by conventional microlithography. With a source-to-drain separation of 2.5 μm, the highest theoretical achievable switching frequency is ∼10 MHz, assuming the movement of charge through the semiconductor is the limiting step. However, the measured maximum switching frequency of P3HT EGTs to date is ∼1 kHz, implying that another process is slowing the response. By systematically varying the device geometry, we show that the frequency is limited by the capacitance between the gate and drain (i.e., parasitic capacitance). The traditional scaling of switching time with the square of channel length (L) does not hold for P3HT EGTs. Rather, minimizing the size of the drain electrode increases the maximum switching speed. We achieve 10 kHz for P3HT EGTs with source/drain electrode dimensions of 2.5 μm × 50 μm and channel dimensions of 2.5 μm × 50 μm. Further improvements will require additional shrinkage of electrode dimensions as well as consideration of other factors such as ion gel thickness and carrier mobility.

Wafer-Scale Microwire Transistor Array Fabricated via Evaporative Assembly

One-dimensional (1D) nano/microwires have attracted significant attention as promising building blocks for various electronic and optical device applications. The integration of these elements into functional device networks with controlled alignment and density presents a significant challenge for practical device applications. Here, we demonstrated the fabrication of wafer-scale microwire field-effect transistor (FET) arrays based on well-aligned inorganic semiconductor microwires (indium-gallium-zinc-oxide (IGZO)) and organic polymeric insulator microwires fabricated via a simple and large-area evaporative assembly technique. This microwire fabrication method offers a facile approach to precisely manipulating the channel dimensions of the FETs. The resulting solution-processed monolithic IGZO microwire FETs exhibited a maximum electron mobility of 1.02 cm(2) V(-1) s(-1) and an on/off current ratio of 1 × 10(6). The appropriate choice of the polymeric microwires used to define the channel lengths enabled fine control over the threshold voltages of the devices, which were employed to fabricate high-performance depletion-load inverters. Low-voltage-operated microwire FETs were successfully fabricated on a plastic substrate using a high-capacitance ion gel gate dielectric. The microwire fabrication technique involving evaporative assembly provided a facile, effective, and reliable method for preparing flexible large-area electronics.

An Organic Vertical Field-Effect Transistor with Underside-Doped Graphene Electrodes

High-performance vertical field-effect transistors are developed, which are based on graphene electrodes doped using the underside doping method. The underside doping method enables effective tuning of the graphene work function while maintaining the surface properties of the pristine graphene.

Low-Voltage Complementary Electronics from Ion-Gel-Gated Vertical Van der Waals Heterostructures

Low-voltage complementary circuits comprising n-type and p-type van der Waals heterojunction vertical field-effect transistors (VFETs) are demonstrated. The resulting VFETs possess high on-state current densities (>3000 A cm(-2) ) and on/off current ratios (>10(4) ) in a narrow voltage window (<3 V).