Printed, sub-2V ZnO electrolyte gated transistors and inverters on plastic

Novel solution-gated transistor sensor-based SnO2 epitaxial thin films grown by pulsed laser deposition for nitrite detection

A solution-gate controlled thin-film transistor with SnO2 epitaxial thin films (SnO2-SGTFT) is successfully utilized for highly sensitive detection of nitrite. The SnO2 films are deposited as channel materials on a c-plane sapphire (c-Al2O3) substrate through pulsed laser deposition (PLD), with superior crystal quality and out-of-plane atomic ordering. PtAu NPs/rGO nanocomposites are electrodeposited on a gold electrode to function as a transistor gate to further enhance the nitrite catalytic performance of the device. The change in effective gate voltage due to the electrooxidation of nitrite on the gate electrode is the primary sensing mechanism of the device. Based on the inherent amplification effect of transistors, the superior electrical properties of SnO2, and the high electrocatalytic activity of PtAu NPs/rGO, the SnO2-SGTFT sensor has a low detection limit of 0.1 nM and a wide linear detection range of 0.1 nM ~ 50 mM at VGS = 1.0 V. Furthermore, the sensor has excellent characteristics such as rapid response time, selectivity, and stability. The practicability of the device has been confirmed by the quantitative detection of nitrite in natural lake water. SnO2 epitaxial films grown by PLD provide a simple and efficient way to fabricate nitrite SnO2-SGTFT sensors in environmental monitoring and food safety, among others. It also provides a reference for the construction of other high-performance thin-film transistor sensors.

Aqueous electrolyte-gated solution-processed metal oxide transistors for direct cellular interfaces

Biocompatible field-effect-transistor-based biosensors have drawn attention for the development of next-generation human-friendly electronics. High-performance electronic devices must achieve low-voltage operation, long-term operational stability, and biocompatibility. Herein, we propose an electrolyte-gated thin-film transistor made of large-area solution-processed indium-gallium-zinc oxide (IGZO) semiconductors capable of directly interacting with live cells at physiological conditions. The fabricated transistors exhibit good electrical performance operating under sub-0.5 V conditions with high on-/off-current ratios (>107) and transconductance (>1.0 mS) over an extended operational lifetime. Furthermore, we verified the biocompatibility of the IGZO surface to various types of mammalian cells in terms of cell viability, proliferation, morphology, and drug responsiveness. Finally, the prolonged stable operation of electrolyte-gated transistor devices directly integrated with live cells provides the proof-of-concept for solution-processed metal oxide material-based direct cellular interfaces.

Tuning Threshold Voltage of Electrolyte-Gated Transistors by Binary Ion Doping

Electrolyte-gated transistors (EGTs) operating at low voltages have attracted significant attention in widespread applications, including neuromorphic devices, nonvolatile memories, chemical/biosensors, and printed electronics. To increase the practicality of the EGTs in electronic circuits, systematic control of threshold voltage (Vth), which determines the power consumption and noise margin of the circuits, is essential. In this study, we present a simple strategy for systematically tuning Vth to almost half of the operating potential range of the EGT by controlling the electrochemical doping of electrolyte ions into organic p-type semiconductors. The type of anion in the ionogel determines Vth as well as other transistor characteristics, such as the subthreshold swing and mobility, because the positive hole carriers are the majority carriers. More importantly, Vth can be finely controlled by binary anion doping using ionogels with two anions with varying molar fractions at a fixed cation. In addition, the binary anion doping successfully controls the inversion characteristics of ion-gated inverters. As unlimited combinations of ion pairs are possible for ionogels, this study opens a route for controlling the device characteristics to expand the practicality and applicability of ionogel-based EGTs for next-generation ionic/electronic devices.

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.

Sub-3 V, MHz-Class Electrolyte-Gated Transistors and Inverters

Electrolyte-gated transistors (EGTs) have emerging applications in physiological recording, neuromorphic computing, sensing, and flexible printed electronics. A challenge for these devices is their slow switching speed, which has several causes. Here, we report the fabrication and characterization of n-type ZnO-based EGTs with signal propagation delays as short as 70 ns. Propagation delays are assessed in dynamically operating inverters and five-stage ring oscillators as a function of channel dimensions and supply voltages up to 3 V. Substantial decreases in switching time are realized by minimizing parasitic resistances and capacitances that are associated with the electrolyte in these devices. Stable switching at 1-10 MHz is achieved in individual inverter stages with 10-40 μm channel lengths, and analysis suggests that further improvements are possible.

Flexible complementary circuits operating at sub-0.5 V via hybrid organic-inorganic electrolyte-gated transistors

Electrolyte-gated transistors (EGTs) hold great promise for next-generation printed logic circuitry, biocompatible integrated sensors, and neuromorphic devices. However, EGT-based complementary circuits with high voltage gain and ultralow driving voltage (<0.5 V) are currently unrealized, because achieving balanced electrical output for both the p- and n-type EGT components has not been possible with current materials. Here we report high-performance EGT complementary circuits containing p-type organic electrochemical transistors (OECTs) fabricated with an ion-permeable organic semiconducting polymer (DPP-g2T) and an n-type electrical double-layer transistor (EDLT) fabricated with an ion-impermeable inorganic indium-gallium-zinc oxide (IGZO) semiconductor. Adjusting the IGZO composition enables tunable EDLT output which, for In:Ga:Zn = 10:1:1 at%, balances that of the DPP-g2T OECT. The resulting hybrid electrolyte-gated inverter (HCIN) achieves ultrahigh voltage gains (>110) under a supply voltage of only 0.7 V. Furthermore, NAND and NOR logic circuits on both rigid and flexible substrates are realized, enabling not only excellent logic response with driving voltages as low as 0.2 V but also impressive mechanical flexibility down to 1-mm bending radii. Finally, the HCIN was applied in electrooculographic (EOG) signal monitoring for recording eye movement, which is critical for the development of wearable medical sensors and also interfaces for human-computer interaction; the high voltage amplification of the present HCIN enables EOG signal amplification and monitoring in which a small ∼1.5 mV signal is amplified to ∼30 mV.

Electrolyte-gated transistors for enhanced performance bioelectronics

Electrolyte-gated transistors (EGTs), capable of transducing biological and biochemical inputs into amplified electronic signals and stably operating in aqueous environments, have emerged as fundamental building blocks in bioelectronics. In this Primer, the different EGT architectures are described with the fundamental mechanisms underpinning their functional operation, providing insight into key experiments including necessary data analysis and validation. Several organic and inorganic materials used in the EGT structures and the different fabrication approaches for an optimal experimental design are presented and compared. The functional bio-layers and/or biosystems integrated into or interfaced to EGTs, including self-organization and self-assembly strategies, are reviewed. Relevant and promising applications are discussed, including two-dimensional and three-dimensional cell monitoring, ultra-sensitive biosensors, electrophysiology, synaptic and neuromorphic bio-interfaces, prosthetics and robotics. Advantages, limitations and possible optimizations are also surveyed. Finally, current issues and future directions for further developments and applications are discussed.

UV-Responsive Screen-Printed Porous ZnO Nanostructures on Office Paper for Sustainable and Foldable Electronics

The fabrication of low-cost, flexible, and recyclable electronic devices has been the focus of many research groups, particularly for integration in wearable technology and the Internet of Things (IoT). In this work, porous zinc oxide (ZnO) nanostructures are incorporated as a UV sensing material into the composition of a sustainable water-based screen-printable ink composed of carboxymethyl cellulose (CMC). The formulated ink is used to fabricate flexible and foldable UV sensors on ubiquitous office paper. The screen-printed CMC/ZnO UV sensors operate under low voltage (≤2 V) and reveal a stable response over several on/off cycles of UV light exposure. The devices reach a response current of 1.34 ± 0.15 mA and a rise and fall time of 8.2 ± 1.0 and 22.0 ± 2.3 s, respectively. The responsivity of the sensor is 432 ± 48 mA W−1, which is the highest value reported in the literature for ZnO-based UV sensors on paper substrates. The UV-responsive devices display impressive mechanical endurance under folding, showing a decrease in responsivity of only 21% after being folded 1000 times. Their low-voltage operation and extreme folding stability indicate a bright future for low-cost and sustainable flexible electronics, showing potential for low-power wearable applications and smart packaging.

Printable hexagonal boron nitride ionogels

We describe aerosol jet-printable ionogels with high ionic conductivities and mechanical strengths using exfoliated hexagonal boron nitride (hBN) nanoplatelets as the solid matrix.

In-Plane Amorphous Oxide Ionotronic Devices and Circuits with Photochemically Enabled Favorable Interfaces

Here, we demonstrate a side-gated in-plane structure of solution-processed amorphous oxide semiconductor ionotronic devices and logic circuits enabled by ion gel gate dielectrics with a monolithically integrated nanoscale passivation architecture. The large capacitance of the electric double layer (EDL) in the ion gel allows a device structure to be a side gate geometry, forming an in-plane structured amorphous In-Ga-Zn-O (a-IGZO) ionotronic transistor, which can be translated into a simplified logic gate configuration with a low operation voltage. Particularly, the monolithic passivation of the coplanar electrodes offers advantages over conventional inhomogeneous passivation, mitigating unintentional parasitic leakage current through the ion gel dielectric layer. More importantly, the monolithically integrated passivation over electrodes was readily obtained with a complementary metal-oxide semiconductor-compatible photochemical process by employing a controlled ultraviolet light manipulation under ozone ambient, which introduced not only much enhanced electrical characteristics but also a scalable device architecture. We investigated various electrical behaviors of the side-gated a-IGZO ionotronic transistor based on EDL, which is called an electric double layer transistor (EDLT), and logic circuits enabled by photochemically realized monolithic aluminum oxide (AlO_X) passivation comparing to the native or polymerized passivation layer, which reveals that the photoassisted AlO_X secures high-performance a-IGZO EDLTs with a low off current (<5.23 × 10^-8 A), high on/off ratio (>1.87 × 10⁵), and exceptional high carrier mobility (>14.5 cm² V^-1 s^-1). Owing to the significantly improved electrical characteristics, an inverter circuit was successfully achieved with broad operation voltages from an ultralow V_DD of 1 mV to 1.5 V, showing a fully logical voltage transfer characteristic with a gain of more than 4 V V^-1.

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.

Microfluidic opportunities in printed electrolyte-gated transistor biosensors

Printed electrolyte-gated transistors (EGTs) are an emerging biosensor platform that leverage the facile fabrication engendered by printed electronics with the low voltage operation enabled by ion gel dielectrics. The resulting label-free, nonoptical sensors have high gain and provide sensing operations that can be challenging for conventional chemical field effect transistor architectures. After providing an overview of EGT device fabrication and operation, we highlight opportunities for microfluidic enhancement of EGT sensor performance via multiplexing, sample preconcentration, and improved transport to the sensor surface.

Effect of semiconductor surface homogeneity and interface quality on electrical performance of inkjet-printed oxide field-effect transistors

In semiconductor technology, the crystallite size of semiconductors is often directly correlated with their superior intrinsic and device mobility. However, when solution-processed, large crystals may bring in higher surface roughness and layer inhomogeneity, which can deteriorate the interface quality and device performance. Along this line, a thorough study on printed oxide field-effect transistors (FETs) has been performed, where the relative significance of crystallite size, surface roughness and spatial homogeneity are evaluated. The comprehensive investigations suggest the spatial homogeneity to be more important than crystallite size in solution processed/printed devices. It is demonstrated that the addition of a small amount of high boiling point polyol in the precursor ink can create large nucleation sites, resulting in reduced average crystallite size, superior inter-particle neck formation, and high spatial homogeneity. Interestingly, carefully estimated device mobility of these polyol-derived In₂O₃ FETs (∼50-55 cm² V^-1 s^-1) is found to be larger than the FETs prepared without polyols, although the crystallite size of the former is an order of magnitude smaller. The high spatial homogeneity and the large mobility values of the polyol-derived In₂O₃ transistors, as compared to the amorphous oxide FETs, lowers the importance of the latter, at least within the solution-processed/printed electronics domain.

Ion buffering and interface charge enable high performance electronics with organic electrochemical transistors

Organic electrochemical transistors rely on ionic-electronic volumetric interaction to provide a seamless interface between biology and electronics with outstanding signal amplification. Despite their huge potential, further progress is limited owing to the lack of understanding of the device fundamentals. Here, we investigate organic electrochemical transistors in a wide range of experimental conditions by combining electrical analyses and device modeling. We show that the measurements can be quantitatively explained by nanoscale ionic-electronic charge interaction, giving rise to ion buffering and interface charge compensation. The investigation systematically explains and unifies a wide range of experiments, providing the rationale for the development of high-performance electronics. Unipolar inverters — universal building blocks for electronics — with gain larger than 100 are demonstrated. This is the highest gain ever reported, enabling the design of devices and circuits with enhanced performance and opening opportunities for the next-generation integrated bioelectronics and neuromorphic computing.

The rationale design of optimized organic electrochemical transistors (OECTs) for next-generation bioelectronics requires further exploration of the underlying device physics. Here, the authors report the role of ionic-electronic charge interactions on OECTs and high-performance unipolar inverters.

Fully Printed Zinc Oxide Electrolyte-Gated Transistors on Paper

Fully printed and flexible inorganic electrolyte gated transistors (EGTs) on paper with a channel layer based on an interconnected zinc oxide (ZnO) nanoparticle matrix are reported in this work. The required rheological properties and good layer formation after printing are obtained using an eco-friendly binder such as ethyl cellulose (EC) to disperse the ZnO nanoparticles. Fully printed devices on glass substrates using a composite solid polymer electrolyte as gate dielectric exhibit saturation mobility above 5 cm² V⁻¹ s⁻¹ after annealing at 350 °C. Proper optimization of the nanoparticle content in the ink allows for the formation of a ZnO channel layer at a maximum annealing temperature of 150 °C, compatible with paper substrates. These devices show low operation voltages, with a subthreshold slope of 0.21 V dec⁻¹, a turn on voltage of 1.90 V, a saturation mobility of 0.07 cm² V⁻¹ s⁻¹ and an I_on/I_off ratio of more than three orders of magnitude.

Low-Cost Flexible ZnO Microwires Array Ultraviolet Photodetector Embedded in PAVL Substrate

With the advent of wearable device technology, fabrication of inorganic semiconductor devices on flexible organic substrates is of great interest. In this paper, a fascinating method and a low-cost flexible substrate material polyvinyl alcohol (PVAL) have been utilized to embed ZnO microwire (MW) array to produce ultraviolet (UV) photodetector (PD) with decent photoresponsivity. The flexible PVAL substrate is relatively cheap and has better bendability as compared to polyethylene terephthalate (PET) and other traditional flexible substrate materials, which makes it unique in comparison to traditional devices. The device shows a current photoresponsivity of 29.6 A/W in the UV spectral range (350 to 380 nm) and maintains an excellent detection performance with even a bending angle of 180°. In dark, a low current of 1.4 μA at 5 V bias and response time of 4.27 ms was observed. In addition to the excellent device performance at wide bending angles, the fabricated device also performs well with the bending radii close to 0. Therefore, ZnO MW array PD has a great potential for the real-time monitoring of harmful UV exposure to warn the users for the appropriate arrangement avoidance.

High-Performance All-Printed Amorphous Oxide FETs and Logics with Electronically Compatible Electrode/Channel Interface

Oxide semiconductors typically show superior device performance compared to amorphous silicon or organic counterparts, especially when they are physical vapor deposited. However, it is not easy to reproduce identical device characteristics when the oxide field-effect transistors (FETs) are solution-processed/printed; the level of complexity further intensifies with the need to print the passive elements as well. Here, we developed a protocol for designing the most electronically compatible electrode/channel interface based on the judicious material selection. Exploiting this newly developed fabrication schemes, we are now able to demonstrate high-performance all-printed FETs and logic circuits using amorphous indium-gallium-zinc oxide (a-IGZO) semiconductor, indium tin oxide (ITO) as electrodes, and composite solid polymer electrolyte as the gate insulator. Interestingly, all-printed FETs demonstrate an optimal electrical performance in terms of threshold voltages and device mobility and may very well be compared with devices fabricated using sputtered ITO electrodes. This observation originates from the selection of electrode/channel materials from the same transparent semiconductor oxide family, resulting in the formation of In-Sn-Zn-O (ITZO)-based-diffused a-IGZO-ITO interface that controls doping density while ensuring high electrical performance. Compressive spectroscopic studies reveal that Sn doping-mediated excellent band alignment of IGZO with ITO electrodes is responsible for the excellent device performance observed. All-printed n-MOS-based logic circuits have also been demonstrated toward new-generation portable electronics.

Continuous Low-Bias Switching of Superconductivity in a MoS2 Transistor

Engineering the properties of quantum electron systems, e.g., tuning the superconducting phase using low driving bias within an easily accessible temperature range, is of great interest for exploring exotic physical phenomena as well as achieving real applications. Here, the realization of continuous field-effect switching between superconducting and non-superconducting states in a few-layer MoS₂ transistor is reported. Ionic-liquid gating induces the superconducting state close to the quantum critical point on the top surface of the MoS₂ , and continuous switching between the super/non-superconducting states is achieved by HfO₂ back gating. The superconducting transistor works effectively in the helium-4 temperature range and requires a gate bias as low as ≈10 V. The dual-gate device structure and strategy presented here can be easily generalized to other systems, opening new opportunities for designing high-performance 2D superconducting transistors.

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.

High- k Gate Dielectrics for Emerging Flexible and Stretchable Electronics

Recent advances in flexible and stretchable electronics (FSE), a technology diverging from the conventional rigid silicon technology, have stimulated fundamental scientific and technological research efforts. FSE aims at enabling disruptive applications such as flexible displays, wearable sensors, printed RFID tags on packaging, electronics on skin/organs, and Internet-of-things as well as possibly reducing the cost of electronic device fabrication. Thus, the key materials components of electronics, the semiconductor, the dielectric, and the conductor as well as the passive (substrate, planarization, passivation, and encapsulation layers) must exhibit electrical performance and mechanical properties compatible with FSE components and products. In this review, we summarize and analyze recent advances in materials concepts as well as in thin-film fabrication techniques for high- k (or high-capacitance) gate dielectrics when integrated with FSE-compatible semiconductors such as organics, metal oxides, quantum dot arrays, carbon nanotubes, graphene, and other 2D semiconductors. Since thin-film transistors (TFTs) are the key enablers of FSE devices, we discuss TFT structures and operation mechanisms after a discussion on the needs and general requirements of gate dielectrics. Also, the advantages of high- k dielectrics over low- k ones in TFT applications were elaborated. Next, after presenting the design and properties of high- k polymers and inorganic, electrolyte, and hybrid dielectric families, we focus on the most important fabrication methodologies for their deposition as TFT gate dielectric thin films. Furthermore, we provide a detailed summary of recent progress in performance of FSE TFTs based on these high- k dielectrics, focusing primarily on emerging semiconductor types. Finally, we conclude with an outlook and challenges section.

Flexible integrated diode-transistor logic (DTL) driving circuits based on printed carbon nanotube thin film transistors with low operation voltage

Fabrication and application of hybrid functional circuits have become a hot research topic in the field of printed electronics. In this study, a novel flexible diode-transistor logic (DTL) driving circuit is proposed, which was fabricated based on a light emitting diode (LED) integrated with printed high-performance single-walled carbon nanotube (SWCNT) thin-film transistors (TFTs). The LED, which is made of AlGaInP on GaAs, is commercial off-the-shelf, which could generate free electrical charges upon white light illumination. Printed top-gate TFTs were made on a PET substrate by inkjet printing high purity semiconducting SWCNTs (sc-SWCNTs) ink as the semiconductor channel materials, together with printed silver ink as the top-gate electrode and printed poly(pyromellitic dianhydride-co-4,4'-oxydianiline) (PMDA/ODA) as gate dielectric layer. The LED, which is connected to the gate electrode of the TFT, generated electrical charge when illuminated, resulting in biased gate voltage to control the TFT from "ON" status to "OFF" status. The TFTs with a PMDA/ODA gate dielectric exhibited low operating voltages of ±1 V, a small subthreshold swing of 62-105 mV dec^-1 and ON/OFF ratio of 10⁶, which enabled DTL driving circuits to have high ON currents, high dark-to-bright current ratios (up to 10⁵) and good stability under repeated white light illumination. As an application, the flexible DTL driving circuit was connected to external quantum dot LEDs (QLEDs), demonstrating its ability to drive and to control the QLED.

Flexible top-emitting organic light emitting diodes with a functional dielectric reflector on a metal foil substrate

We demonstrated flexible OLEDs with a DBR serving as an optical reflector and electrical passivation on a metal foil substrate.

Tin Dioxide Electrolyte-Gated Transistors Working in Depletion and Enhancement Modes

Metal oxide semiconductors are interesting for next-generation flexible and transparent electronics because of their performance and reliability. Tin dioxide (SnO₂) is a very promising material that has already found applications in sensing, photovoltaics, optoelectronics, and batteries. In this work, we report on electrolyte-gated, solution-processed polycrystalline SnO₂ transistors on both rigid and flexible substrates. For the transistor channel, we used both unpatterned and patterned SnO₂ films. Since decreasing the SnO₂ area in contact with the electrolyte increases the charge-carrier density, patterned transistors operate in the depletion mode, whereas unpatterned ones operate in the enhancement mode. We also fabricated flexible SnO₂ transistors that operate in the enhancement mode that can withstand moderate mechanical bending.

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.

Quasi-Two-Dimensional Metal Oxide Semiconductors Based Ultrasensitive Potentiometric Biosensors

Ultrasensitive field-effect transistor-based biosensors using quasi-two-dimensional metal oxide semiconductors were demonstrated. Quasi-two-dimensional low-dimensional metal oxide semiconductors were highly sensitive to electrical perturbations at the semiconductor-bio interface and showed competitive sensitivity compared with other nanomaterial-based biosensors. Also, the solution process made our platform simple and highly reproducible, which was favorable compared with other nanobioelectronics. A quasi-two-dimensional In₂O₃-based pH sensor showed a small detection limit of 0.0005 pH and detected the glucose concentration at femtomolar levels. Detailed electrical characterization unveiled how the device's parameters affect the biosensor sensitivity, and lowest detectable charge was extrapolated, which was consistent with the experimental data.

Enhanced Performance in Al-Doped ZnO Based Transparent Flexible Transparent Thin-Film Transistors Due to Oxygen Vacancy in ZnO Film with Zn-Al-O Interfaces Fabricated by Atomic Layer Deposition

Highly conductive and optical transparent Al-doped ZnO (AZO) thin film composed of ZnO with a Zn-Al-O interface was fabricated by thermal atomic layer deposition (ALD) method. The as-prepared AZO thin film exhibits excellent electrical and optical properties with high stability and compatibility with temperature-sensitive flexible photoelectronic devices; film resistivity is as low as 5.7 × 10^-4 Ω·cm, the carrier concentration is high up to 2.2 × 10²¹ cm^-3. optical transparency is greater than 80% in a visible range, and the growth temperature is below 150 °C on the PEN substrate. Compared with the conventional AZO film containing by a ZnO-Al₂O₃ interface, we propose that the underlying mechanism of the enhanced electrical conductivity for the current AZO thin film is attributed to the oxygen vacancies deficiency derived from the free competitive growth mode of Zn-O and Al-O bonds in the Zn-Al-O interface. The flexible transparent transistor based on this AZO electrode exhibits a favorable threshold voltage and I_on/I_off ratio, showing promising for use in high-resolution, fully transparent, and flexible display applications.

All-inkjet-printed flexible ZnO micro photodetector for a wearable UV monitoring device

Fabrication of small-sized patterns of inorganic semiconductor onto flexible substrates is a major concern when manufacturing wearable devices for measuring either biometric or environmental parameters. In this study, micro-sized flexible ZnO UV photodetectors have been thoroughly prepared by a facile inkjet printing technology and followed with heat treatments. A simple ink recipe of zinc acetate precursor solution was investigated. It is found that the substrate temperature during zinc precursor ink depositing has significant effects on ZnO pattern shape, film morphology, and crystallization. The device fabricated from the additive manufacturing approach has good bendability, Ohmic contact, short response time as low as 0.3 s, and high on/off ratio of 3525. We observed the sensor's dependence of response/decay time by the illuminating UV light intensity. The whole process is based on additive manufacturing which has many benefits such as rapid prototyping, saving material, being environmentally friendly, and being capable of creating high-resolution patterns. In addition, this method can be applied to flexible substrates, which makes the device more applicable for applications requiring flexibility such as wearable devices. The proposed all-inkjet-printing approach for a micro-sized ZnO UV photodetector would significantly simplify the fabrication process of micro-sized inorganic semiconductor-based devices. A potential application is real-time monitoring of UV light exposure to warn users about unsafe direct sunlight to implement suitable avoidance solutions.

Three-dimensional microarchitected materials and devices using nanoparticle assembly by pointwise spatial printing

Three-dimensional (3D) hierarchical materials are important to a wide range of emerging technological applications. We report a method to synthesize complex 3D microengineered materials, such as microlattices, with nearly fully dense truss elements with a minimum diameter of approximately 20 μm and having high aspect ratios (up to 20:1) without using any templating or supporting materials. By varying the postprocessing conditions, we have also introduced an additional control over the internal porosity of the truss elements to demonstrate a hierarchical porous structure with an overall void size and feature size control of over five orders of magnitudes in length scale. The method uses direct printing of nanoparticle dispersions using the Aerosol Jet technology in 3D space without templating or supporting materials followed by binder removal and sintering. In addition to 3D microlattices, we have also demonstrated directly printed stretchable interconnects, spirals, and pillars. This assembly method could be implemented by a variety of microdroplet generation methods for fast and large-scale fabrication of the hierarchical materials for applications in tissue engineering, ultralight or multifunctional materials, microfluidics, and micro-optoelectronics.

Top-down synthesis of zinc oxide based inks for inkjet printing

A top-down approach for synthesizing pure and doped inkjet printable metal oxide inks.

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

A printed vertical field-effect transistor is demonstrated, which decouples critical device dimensions from printing resolution. A printed mesoporous semiconductor layer, sandwiched between vertically stacked drive electrodes, provides <50 nm channel lengths. A polymer-electrolyte-based gate insulator infiltrates the percolating pores of the mesoporous channel to accumulate charge carriers at every semiconductor domain, thereby, resulting in an unprecedented current density of MA cm^-2 .

Excitatory Post-Synaptic Potential Mimicked in Indium-Zinc-Oxide Synaptic Transistors Gated by Methyl Cellulose Solid Electrolyte

The excitatory postsynaptic potential (EPSP) of biological synapses is mimicked in indium-zinc-oxide synaptic transistors gated by methyl cellulose solid electrolyte. These synaptic transistors show excellent electrical performance at an operating voltage of 0.8 V, I_on/off ratio of 2.5 × 10⁶, and mobility of 38.4 cm²/Vs. After this device is connected to a resistance of 4 MΩ in series, it exhibits excellent characteristics as an inverter. A threshold potential of 0.3 V is achieved by changing the gate pulse amplitude, width, or number, which is analogous to biological EPSP.

Temperature-Dependent Performance of Printed Field-Effect Transistors with Solid Polymer Electrolyte Gating

Printable, physical, and air-stable composite solid polymer electrolytes (CSPEs) with high ionic conductivity have been established as a suitable alternative to standard dielectric gate insulators for printed field-effect transistors (FETs) and logics. We have performed a stress and temperature stability study involving several CSPEs. Mechanical tensile and shear tests have been performed to determine the physical condition of CSPEs. A comprehensive temperature dependent study has been conducted within the working temperature range which electric double layer (EDL) capacitors or CSPE-gated FETs may typically experience during their lifetime. Moreover, calorimetric measurements have been performed to investigate the CSPEs stability, especially at low temperatures. Mechanical characterizations have shown tensile strength and shear modulus of the material that is typical for solid polymer electrolytes while DSC measurements show no change in the physical state within the measured temperature range. An expected increase in ionic conductivity of the CSPEs of nearly 1 order of magnitude has been observed with an increase in temperature, while an anomalous positive temperature relationship to EDL capacitance has also been noticed. Interestingly, the transistor performance characteristics, namely, on-current and threshold voltage, are found to be quite independent of the temperature, thus ensuring a large and stable operation temperature window for CSPE-gated FETs. The other parameters, subthreshold slope and the device mobility, have varied following the classical semiconductor behavior. In fact, the present study not only provides a detailed understanding of temperature dependence of the CSPE-gated FETs but also offers an insight into the physical and electrical properties of the CSPEs itself. Therefore, these results may very well help to comprehend and improve EDL capacitors, supercapacitors, and other devices that use CSPEs as the active material.

High-resolution electrohydrodynamic inkjet printing of stretchable metal oxide semiconductor transistors with high performance

As demands for high pixel densities and wearable forms of displays increase, high-resolution printing technologies to achieve high performance transistors beyond current amorphous silicon levels and to allow low-temperature solution processability for plastic substrates have been explored as key processes in emerging flexible electronics. This study describes electrohydrodynamic inkjet (e-jet) technology for direct printing of oxide semiconductor thin film transistors (TFTs) with high resolution (minimum line width: 2 μm) and superb performance, including high mobility (∼230 cm² V^-1 s^-1). Logic operations of the amplifier circuits composed of these e-jet-printed metal oxide semiconductor (MOS) TFTs demonstrate their high performance. Printed In₂O TFTs with e-jet printing-assisted high-resolution S/D electrodes were prepared, and the direct printing of passivation layers on these channels enhanced their gate-bias stabilities significantly. Moreover, low process temperatures (<250 °C) enable the use of thin plastic substrates; highly flexible and stretchable TFT arrays have been demonstrated, suggesting promise for next-generation printed electronics.

Photolithographically Patterned TiO2 Films for Electrolyte-Gated Transistors

Metal oxides constitute a class of materials whose properties cover the entire range from insulators to semiconductors to metals. Most metal oxides are abundant and accessible at moderate cost. Metal oxides are widely investigated as channel materials in transistors, including electrolyte-gated transistors, where the charge carrier density can be modulated by orders of magnitude upon application of relatively low electrical bias (2 V). Electrolyte gating offers the opportunity to envisage new applications in flexible and printed electronics as well as to improve our current understanding of fundamental processes in electronic materials, e.g. insulator/metal transitions. In this work, we employ photolithographically patterned TiO2 films as channels for electrolyte-gated transistors. TiO2 stands out for its biocompatibility and wide use in sensing, electrochromics, photovoltaics and photocatalysis. We fabricated TiO2 electrolyte-gated transistors using an original unconventional parylene-based patterning technique. By using a combination of electrochemical and charge carrier transport measurements we demonstrated that patterning improves the performance of electrolyte-gated TiO2 transistors with respect to their unpatterned counterparts. Patterned electrolyte-gated (EG) TiO2 transistors show threshold voltages of about 0.9 V, ON/OFF ratios as high as 1 × 10(5), and electron mobility above 1 cm(2)/(V s).

Metal oxides for optoelectronic applications

Metal oxides (MOs) are the most abundant materials in the Earth's crust and are ingredients in traditional ceramics. MO semiconductors are strikingly different from conventional inorganic semiconductors such as silicon and III-V compounds with respect to materials design concepts, electronic structure, charge transport mechanisms, defect states, thin-film processing and optoelectronic properties, thereby enabling both conventional and completely new functions. Recently, remarkable advances in MO semiconductors for electronics have been achieved, including the discovery and characterization of new transparent conducting oxides, realization of p-type along with traditional n-type MO semiconductors for transistors, p-n junctions and complementary circuits, formulations for printing MO electronics and, most importantly, commercialization of amorphous oxide semiconductors for flat panel displays. This Review surveys the uniqueness and universality of MOs versus other unconventional electronic materials in terms of materials chemistry and physics, electronic characteristics, thin-film fabrication strategies and selected applications in thin-film transistors, solar cells, diodes and memories.

Multicolored, Low-Power, Flexible Electrochromic Devices Based on Ion Gels

Ion gels composed of a copolymer and a room temperature ionic liquid are versatile solid-state electrolytes with excellent features including high ionic conductivity, nonvolatility, easily tunable mechanical properties, good flexibility and solution processability. Ion gels can be functionalized by incorporating redox-active species such as electrochemiluminescent (ECL) luminophores or electrochromic (EC) dyes. Here, we enhance the functionality of EC gels for realizing multicolored EC devices (ECDs), either by controlling the chemical equilibrium between a monomer and dimer of a colored EC species, or by modifying the molecular structures of the EC species. All devices in this work are conveniently fabricated by a "cut-and-stick" strategy, and require very low power for maintaining the colored state [i.e., 90 μW/cm(2) (113 μA/cm(2) at -0.8 V) for blue, 4 μW/cm(2) (10 μA/cm(2) at -0.4 V) for green, and 32 μW/cm(2) (79 μA/cm(2) at -0.4 V) for red ECD]. We also successfully demonstrate a patterned, multicolored, flexible ECD on plastic. Overall, these results suggest that gel-based ECDs have significant potential as low power displays in printed electronics powered by thin-film batteries.

Proton gated oxide electric-double-layer transistors for full-swing low voltage inverter applications

Low voltage full-swing inverters were built by connecting proton gated oxide electric-double-layer transistors in series with resistors.

Sub-0.5 V Highly Stable Aqueous Salt Gated Metal Oxide Electronics

Recently, growing interest in implantable bionics and biochemical sensors spurred the research for developing non-conventional electronics with excellent device characteristics at low operation voltages and prolonged device stability under physiological conditions. Herein, we report high-performance aqueous electrolyte-gated thin-film transistors using a sol-gel amorphous metal oxide semiconductor and aqueous electrolyte dielectrics based on small ionic salts. The proper selection of channel material (i.e., indium-gallium-zinc-oxide) and precautious passivation of non-channel areas enabled the development of simple but highly stable metal oxide transistors manifested by low operation voltages within 0.5 V, high transconductance of ~1.0 mS, large current on-off ratios over 10(7), and fast inverter responses up to several hundred hertz without device degradation even in physiologically-relevant ionic solutions. In conjunction with excellent transistor characteristics, investigation of the electrochemical nature of the metal oxide-electrolyte interface may contribute to the development of a viable bio-electronic platform directly interfacing with biological entities in vivo.

Printable Solid-State Lithium-Ion Batteries: A New Route toward Shape-Conformable Power Sources with Aesthetic Versatility for Flexible Electronics

Forthcoming flexible/wearable electronic devices with shape diversity and mobile usability garner a great deal of attention as an innovative technology to bring unprecedented changes in our daily lives. From the power source point of view, conventional rechargeable batteries (one representative example is a lithium-ion battery) with fixed shapes and sizes have intrinsic limitations in fulfilling design/performance requirements for the flexible/wearable electronics. Here, as a facile and efficient strategy to address this formidable challenge, we demonstrate a new class of printable solid-state batteries (referred to as "PRISS batteries"). Through simple stencil printing process (followed by ultraviolet (UV) cross-linking), solid-state composite electrolyte (SCE) layer and SCE matrix-embedded electrodes are consecutively printed on arbitrary objects of complex geometries, eventually leading to fully integrated, multilayer-structured PRISS batteries with various form factors far beyond those achievable by conventional battery technologies. Tuning rheological properties of SCE paste and electrode slurry toward thixotropic fluid characteristics, along with well-tailored core elements including UV-cured triacrylate polymer and high boiling point electrolyte, is a key-enabling technology for the realization of PRISS batteries. This process/material uniqueness allows us to remove extra processing steps (related to solvent drying and liquid-electrolyte injection) and also conventional microporous separator membranes, thereupon enabling the seamless integration of shape-conformable PRISS batteries (including letters-shaped ones) into complex-shaped objects. Electrochemical behavior of PRISS batteries is elucidated via an in-depth analysis of cell impedance, which provides a theoretical basis to enable sustainable improvement of cell performance. We envision that PRISS batteries hold great promise as a reliable and scalable platform technology to open a new concept of cell architecture and fabrication route toward flexible power sources with exceptional shape conformability and aesthetic versatility.

Ink-Jet Printed CMOS Electronics from Oxide Semiconductors

Complementary metal oxide semiconductor (CMOS) technology with high transconductance and signal gain is mandatory for practicable digital/analog logic electronics. However, high performance all-oxide CMOS logics are scarcely reported in the literature; specifically, not at all for solution-processed/printed transistors. As a major step toward solution-processed all-oxide electronics, here it is shown that using a highly efficient electrolyte-gating approach one can obtain printed and low-voltage operated oxide CMOS logics with high signal gain (≈21 at a supply voltage of only 1.5 V) and low static power dissipation.

Superconductivity Series in Transition Metal Dichalcogenides by Ionic Gating

Functionalities of two-dimensional (2D) crystals based on semiconducting transition metal dichalcogenides (TMDs) have now stemmed from simple field effect transistors (FETs) to a variety of electronic and opto-valleytronic devices, and even to superconductivity. Among them, superconductivity is the least studied property in TMDs due to methodological difficulty accessing it in different TMD species. Here, we report the systematic study of superconductivity in MoSe₂, MoTe₂ and WS₂ by ionic gating in different regimes. Electrostatic gating using ionic liquid was able to induce superconductivity in MoSe₂ but not in MoTe₂ because of inefficient electron accumulation limited by electronic band alignment. Alternative gating using KClO₄/polyethylene glycol enabled a crossover from surface doping to bulk doping, which induced superconductivities in MoTe₂ and WS₂ electrochemically. These new varieties greatly enriched the TMD superconductor families and unveiled critical methodology to expand the capability of ionic gating to other materials.

Schmitt trigger using a self-healing ionic liquid gated transistor

Electrical double layer transistors using ionic liquids as the gate and ZnO as the semiconductor exhibit stable operation in the presence of redox active additives. The characteristics of the device enable single components with the response of a Schmitt trigger.

A general route toward complete room temperature processing of printed and high performance oxide electronics

Critical prerequisites for solution-processed/printed field-effect transistors (FETs) and logics are excellent electrical performance including high charge carrier mobility, reliability, high environmental stability and low/preferably room temperature processing. Oxide semiconductors can often fulfill all the above criteria, sometimes even with better promise than their organic counterparts, except for their high process temperature requirement. The need for high annealing/curing temperatures renders oxide FETs rather incompatible to inexpensive, flexible substrates, which are commonly used for high-throughput and roll-to-roll additive manufacturing techniques, such as printing. To overcome this serious limitation, here we demonstrate an alternative approach that enables completely room-temperature processing of printed oxide FETs with device mobility as large as 12.5 cm(2)/(V s). The key aspect of the present concept is a chemically controlled curing process of the printed nanoparticle ink that provides surprisingly dense thin films and excellent interparticle electrical contacts. In order to demonstrate the versatility of this approach, both n-type (In2O3) and p-type (Cu2O) oxide semiconductor nanoparticle dispersions are prepared to fabricate, inkjet printed and completely room temperature processed, all-oxide complementary metal oxide semiconductor (CMOS) invertors that can display significant signal gain (∼18) at a supply voltage of only 1.5 V.

Solvothermal synthesis of gallium-indium-zinc-oxide nanoparticles for electrolyte-gated transistors

Solution-processed field-effect transistors are strategic building blocks when considering low-cost sustainable flexible electronics. Nevertheless, some challenges (e.g., processing temperature, reliability, reproducibility in large areas, and cost effectiveness) are requirements that must be surpassed in order to achieve high-performance transistors. The present work reports electrolyte-gated transistors using as channel layer gallium-indium-zinc-oxide nanoparticles produced by solvothermal synthesis combined with a solid-state electrolyte based on aqueous dispersions of vinyl acetate stabilized with cellulose derivatives, acrylic acid ester in styrene and lithium perchlorate. The devices fabricated using this approach display a ION/IOFF up to 1 × 10(6), threshold voltage (VTh) of 0.3-1.9 V, and mobility up to 1 cm(2)/(V s), as a function of gallium-indium-zinc-oxide ink formulation and two different annealing temperatures. These results validates the usage of electrolyte-gated transistors as a viable and promising alternative for nanoparticle based semiconductor devices as the electrolyte improves the interface and promotes a more efficient step coverage of the channel layer, reducing the operating voltage when compared with conventional dielectrics gating. Moreover, it is shown that by controlling the applied gate potential, the operation mechanism of the electrolyte-gated transistors can be modified from electric double layer to electrochemical doping.

Electrochemical processes and mechanistic aspects of field-effect sensors for biomolecules

Electronic biosensing is a leading technology for determining concentrations of biomolecules. In some cases, the presence of an analyte molecule induces a measured change in current flow, while in other cases, a new potential difference is established. In the particular case of a field effect biosensor, the potential difference is monitored as a change in conductance elsewhere in the device, such as across a film of an underlying semiconductor. Often, the mechanisms that lead to these responses are not specifically determined. Because improved understanding of these mechanisms will lead to improved performance, it is important to highlight those studies where various mechanistic possibilities are investigated. This review explores a range of possible mechanistic contributions to field-effect biosensor signals. First, we define the field-effect biosensor and the chemical interactions that lead to the field effect, followed by a section on theoretical and mechanistic background. We then discuss materials used in field-effect biosensors and approaches to improving signals from field-effect biosensors. We specifically cover the biomolecule interactions that produce local electric fields, structures and processes at interfaces between bioanalyte solutions and electronic materials, semiconductors used in biochemical sensors, dielectric layers used in top-gated sensors, and mechanisms for converting the surface voltage change to higher signal/noise outputs in circuits.

Flexible logic circuits based on top-gate thin film transistors with printed semiconductor carbon nanotubes and top electrodes

In this report printed thin film transistors and logic circuits on flexible substrates are reported. The top-gate thin film transistors were made of the sorted semiconducting single-walled carbon nanotubes (sc-SWCNTs) ink as channel material and printed silver lines as top electrodes and interconnect. 5 nm HfOx thin films pre-deposited on PET substrates by atomic layer deposition (ALD) act as the adhesion layers to significantly improve the immobilization efficiency of sc-SWCNTs and environmental stability. The immobilization mechanism was investigated in detail. The flexible partially-printed top-gate SWCNT TFTs display ambipolar characteristics with slightly strong p-type when using 50 nm HfO(x) thin films as dielectric layer, as well as the encapsulation layer by atomic layer deposition (ALD) at 120 °C. The hole mobility, on/off ratio and subthreshold swing (SS) are ∼ 46.2 cm(2) V(-1) s(-1), 10(5) and 109 mV per decade, respectively. Furthermore, partially-printed TFTs show small hysteresis, low operating voltage (2 V) and high stability in air. Flexible partially-printed inverters show good performance with voltage gain up to 33 with 1.25 V supply voltage, and can work at 10 kHz. The frequency of flexible partially-printed five-stage ring oscillators can reach 1.7 kHz at supply voltages of 2 V with per stage delay times of 58.8 μs. This work paves a way to achieve printed SWCNT advanced logic circuits and systems on flexible substrates.

Aerosol jet printed p- and n-type electrolyte-gated transistors with a variety of electrode materials: exploring practical routes to printed electronics

Printing electrically functional liquid inks is a promising approach for achieving low-cost, large-area, additive manufacturing of flexible electronic circuits. To print thin-film transistors, a basic building block of thin-film electronics, it is important to have several options for printable electrode materials that exhibit high conductivity, high stability, and low-cost. Here we report completely aerosol jet printed (AJP) p- and n-type electrolyte-gated transistors (EGTs) using a variety of different electrode materials including highly conductive metal nanoparticles (Ag), conducting polymers (polystyrenesulfonate doped poly(3,4-ethylendedioxythiophene, PEDOT:PSS), transparent conducting oxides (indium tin oxide), and carbon-based materials (reduced graphene oxide). Using these source-drain electrode materials and a PEDOT:PSS/ion gel gate stack, we demonstrated all-printed p- and n-type EGTs in combination with poly(3-hexythiophene) and ZnO semiconductors. All transistor components (including electrodes, semiconductors, and gate insulators) were printed by AJP. Both kinds of devices showed typical p- and n-type transistor characteristics, and exhibited both low-threshold voltages (<2 V) and high hole and electron mobilities. Our assessment suggests Ag electrodes may be the best option in terms of overall performance for both types of EGTs.

High capacitance, photo-patternable ion gel gate insulators compatible with vapor deposition of metal gate electrodes

A facile fabrication route to pattern high-capacitance electrolyte thin films in electrolyte-gated transistors (EGTs) was demonstrated using a photoinitiated cross-linkable ABA-triblock copolymer ion gel. The azide groups of poly(styrene-r-vinylbenzylazide) (PS-N3) end-blocks can be chemically cross-linked via UV irradiation (λ = 254 nm) in the self-assembly of poly[(styrene-r-vinylbenzylazide)-b-ethylene oxide-b-(styrene-r-vinylbenzylazide)] (SOS-N3) triblock copolymer in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]). Impedance spectroscopy and small-angle X-ray scattering revealed that ion transport and microstructure of the ion gel are not affected by UV cross-linking. Using a photoinduced cross-linking strategy, photopatterning of ion gels through a patterned mask was achieved. Employing a photopatterned ion gel as the high-capacitance gate insulator in thin film transistors (TFTs), arrays of TFTs exhibited uniform and high device performance. Specifically, both p-type (poly(3-hexylthiophene)) (P3HT) and n-type (ZnO) transistors displayed high carrier mobility (hole mobility of ∼ 1.4 cm(2)/ (V s) and electron mobility of ∼ 0.7 cm(2)/ (V s) and ON/OFF current ratio (∼ 10(5)) at supply voltages below 2 V. This study suggests that photopatterning is a promising candidate to conveniently incorporate high-capacitance ion gels into TFTs in the fabrication of printed electronics.