Sub-50 nm Channel Vertical Field-Effect Transistors using Conventional Ink-Jet Printing

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.

Research Progress of Vertical Channel Thin Film Transistor Device

Thin film transistors (TFTs) as the core devices for displays, are widely used in various fields including ultra-high-resolution displays, flexible displays, wearable electronic skins and memory devices, especially in terms of sensors. TFTs have now started to move towards miniaturization. Similarly to MOSFETs problem, traditional planar structure TFTs have difficulty in reducing the channel's length sub-1μm under the existing photolithography technology. Vertical channel thin film transistors (V-TFTs) are proposed. It is an effective solution to overcome the miniaturization limit of traditional planar TFTs. So, we summarize the different aspects of VTFTs. Firstly, this paper introduces the structure types, key parameters, and the impact of different preparation methods in devices of V-TFTs. Secondly, an overview of the research progress of V-TFTs' active layer materials in recent years, the characteristics of V-TFTs and their application in examples has proved the enormous application potential of V-TFT in sensing. Finally, in addition to the advantages of V-TFTs, the current technical challenge and their potential solutions are put forward, and the future development trend of this new structure of V-TFTs is proposed.

Conductive Inks Based on Melamine Intercalated Graphene Nanosheets for Inkjet Printed Flexible Electronics

With the growing number of flexible electronics applications, environmentally benign ways of mass-producing graphene electronics are sought. In this study, we present a scalable mechanochemical route for the exfoliation of graphite in a planetary ball mill with melamine to form melamine-intercalated graphene nanosheets (M-GNS). M-GNS morphology was evaluated, revealing small particles, down to 14 nm in diameter and 0.4 nm thick. The M-GNS were used as a functional material in the formulation of an inkjet-printable conductive ink, based on green solvents: water, ethanol, and ethylene glycol. The ink satisfied restrictions regarding stability and nanoparticle size; in addition, it was successfully inkjet printed on plastic sheets. Thermal and photonic post-print processing were evaluated as a means of reducing the electrical resistance of the printed features. Minimal sheet resistance values (5 kΩ/sq for 10 printed layers and 626 Ω/sq for 20 printed layers) were obtained on polyimide sheets, after thermal annealing for 1 h at 400 °C and a subsequent single intense pulsed light flash. Lastly, a proof-of-concept simple flexible printed circuit consisting of a battery-powered LED was realized. The demonstrated approach presents an environmentally friendly alternative to mass-producing graphene-based printed flexible electronics.

High Operation Frequency and Strain Tolerance of Fully Printed Oxide Thin Film Transistors and Circuits on PET Substrates

The major limitations of solution-processed oxide electronics include high process temperatures and the absence of necessary strain tolerance that would be essential for flexible electronic applications. Here, a combination of low temperature (<100 °C) curable indium oxide nanoparticle ink and a conductive silver nanoink, which are used to fabricate fully-printed narrow-channel thin film transistors (TFTs) on polyethylene terephthalate (PET) substrates, is proposed. The metal ink is printed onto the In₂ O₃ nanoparticulate channel to narrow the effective channel lengths down to the thickness of the In₂ O₃ layer and thereby obtain near-vertical transport across the semiconductor layer. The TFTs thus prepared show On/Off ratio ≈10⁶ and simultaneous maximum current density of 172 µA µm^-1 . Next, the depletion-load inverters fabricated on PET substrates demonstrate signal gain >200 and operation frequency >300 kHz at low operation voltage of V_DD = 2 V. In addition, the near-vertical transport across the semiconductor layer is found to be largely strain tolerant with insignificant change in the TFT and inverter performance observed under bending fatigue tests performed down to a bending radius of 1.5 mm, which translates to a strain value of 5%. The devices are also found to be robust against atmospheric exposure when remeasured after a month.

Wetting transition and phase separation on flat substrates and in porous structures

In contrast to the wetting phenomenon of pure substance phase, we here concentrate on the wetting behavior of immiscible fluids with two components via numerical simulations. We investigate the energetic contribution, the wall energy and the surface excess energy, to the wetting behavior of liquid solutions varying with temperature. This investigation is in accordance with Cahn's wetting transition theory, where the surface composition plays a vital role. By analyzing the energetic contributions, we reveal two different physical mechanisms of complete wetting: (i) surface tension driven complete wetting, where the wetting microstructure is achieved via the outward spreading of the triple junction, and (ii) diffusion induced complete wetting, where the wetting film is achieved through a direct deposition of the solute on the substrate. The former one is consistent with the classic theory of Young's law, and the latter one is an alternative mechanism. To indicate the broad multiplicity of the microstructural arrangements, we take porous structures to exemplarily elucidate the formation of alternative perfect wetting structures. Differing from the wetting on a flat substrate, we show that the surface composition varying with temperature leads to a distinct wetting phenomenon in porous structures. The present findings provide an alternative interpretation for complete wetting and are expected to be exploited for designing more effectively and efficiently superhydrophilic structures.

Fully Inkjet-Printed Mesoporous SnO2-Based Ultrasensitive Gas Sensors for Trace Amount NO2 Detection

Printed sensors are among the most successful groups of devices within the domain of printed electronics, both in terms of their application versatility and the emerging market share. However, reports on fully printed gas sensors are rare in the literature, even though it can be an important development toward fully printed multisensor platforms for diagnostics, process control, and environmental safety-related applications. In this regard, here, we present the traditional tin oxide-based completely inkjet-printed co-continuous and mesoporous thin films with an extremely large surface-to-volume ratio and then investigate their NO₂ sensing properties at low temperatures. A method known as evaporation-induced self-assembly (EISA) has been mimicked in this study using pluronic F127 (PEO₁₀₆-PPO₇₀-PEO₁₀₆) as the soft templating agent and xylene as the micelle expander to obtain highly reproducible and spatially homogeneous co-continuous mesoporous crystalline SnO₂ with an average pore diameter of the order of 15-20 nm. The fully printed SnO₂ gas sensors thus produced show high linearity for NO₂ detection, along with extremely high average response of 11,507 at 5 ppm NO₂. On the other hand, the sensors show an ultralow detection limit of the order of 20 ppb with an easy to amplify response of 31. While the excellent electronic transport properties along such co-continuous, mesoporous structures are ensured by their well-connected (co-continuous) ligaments and pores (thereby ensuring high surface area and high mobility transport at the same time) and may actually be responsible for the outstanding sensor performance that has been observed, the use of an industrial printing technique ascertains the possibility of high-throughput manufacturing of such sensor units toward inexpensive and wide-range applications.

Inkjet-printed co-continuous mesoporous oxides for high-current power transistors

Limited printing resolution has always been a major hindrance for printed electronics; irrespective of the high mobility demonstrated by solution-processed semiconductors, long-channel printed field-effect transistors (FETs) have demonstrated low On-state conductance and switching speeds. Although various concepts have been proposed to obtain narrow-channel printed FETs, the actual demonstration of high On-currents/channel conductance has been rare. In this context, herein, we report a general recipe to print co-continuous mesoporous structures with high surface-to-volume ratios for the first time for a large range of metallic and semiconducting oxides, both n- and p-type; next, by exploiting an innovative transistor architecture by printing an additional silver layer on top of the printed porous channel, we reduced the necessary length of electronic transport through the semiconductor material to a short vertical distance of the order of a few tens of nanometres. Basically, when a composite solid polymer electrolyte was used as a gate insulator, we essentially obtained channel length-independent transport with the unprecedented On-current of 67 μA μm^-1 and transconductance of 143 μS μm^-1 at the supply voltage of only 0.5 V. Among others, one may foresee the usage of these devices in high power switches and for drawing power from batteries in all-printed electronic circuits.

Gravure printing for mesoporous film preparation

This study presents gravure printing as a new strategy for rapid printing of ceramic mesoporous thin films and highlights its advantages over conventional mesoporous film preparation using evaporation induced self-assembly together with dip-coating.

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.

Printed Electronics Based on Inorganic Semiconductors: From Processes and Materials to Devices

Following the ever-expanding technological demands, printed electronics has shown palpable potential to create new and commercially viable technologies that will benefit from its unique characteristics, such as, large-area and wide range of substrate compatibility, conformability and low-cost. Through the last few decades, printed/solution-processed field-effect transistors (FETs) and circuits have witnessed immense research efforts, technological growth and increased commercial interests. Although printing of functional inks comprising organic semiconductors has already been initiated in early 1990s, gradually the attention, at least partially, has been shifted to various forms of inorganic semiconductors, starting from metal chalcogenides, oxides, carbon nanotubes and very recently to graphene and other 2D semiconductors. In this review, the entire domain of printable inorganic semiconductors is considered. In fact, thanks to the continuous development of materials/functional inks and novel design/printing strategies, the inorganic printed semiconductor-based circuits today have reached an operation frequency up to several hundreds of kilohertz with only a few nanosecond time delays at the individual FET/inverter levels; in this regard, often circuits based on hybrid material systems have been found to be advantageous. At the end, a comparison of relative successes of various printable inorganic semiconductor materials, the remaining challenges and the available future opportunities are summarized.

Self-Assembled Porous Alumina Based Organic Nanotriode Arrays

We utilize ordered mesoporous alumina templates for solution processable electronics and demonstrate massively parallel organization of connected three-terminal vertical transistors. The vertical transistor device consists of a connected organic nanotriode array obtained using porous anodized alumina membranes of pore density ≈ 10⁹ pores/cm². In this structure, a collector-emitter diode gives rise to a space charge limited current, which can be controlled by a third intermediate porous base electrode to give transistor-like characteristics. We study the response characteristics along with 2D device simulations of this novel structure to indicate key parameters involved in the underlying mechanism. Device operation at single transistor level is verified by conductive atomic force microscopy, and the inherent short switching time scales of the vertical structure device is also demonstrated.

Graphene as an electrode for solution-processed electron-transporting organic transistors

Organic field-effect transistors (OFETs) are fundamental building blocks for plastic electronics such as organic photovoltaics or bendable displays with organic light emitting diodes, and radio-frequency identification (RFID) tags. A key part in the performance of OFET is the organic material constituting the channel. OFETs based on solution-processed polymers represent a new class of organic electronic devices. Recent developments in upscale solution-processed polymers have advanced towards high throughput, low-cost, and environmentally friendly materials for high-performance applications. Together with the integration of high performance materials, another enduring challenge in OFET development is the improvement and control of the injection of charge carriers. Graphene, a two-dimensional layer of covalently bonded carbon atoms, is steadily making progress into applications relying on van der Waals heterointerfaces with organic semiconductors. Here, we demonstrate the versatile operation of solution-processed organic transistors both in lateral and vertical geometries by exploiting the weak-screening effect and work function modulation properties of graphene electrodes. Our results demonstrate a general strategy for overcoming traditional noble metal electrodes and to integrate graphene with solution-processed Polyera ActiveInk™ N2200 polymer transistors for high-performance devices suitable for future plastic electronics.