All-Printed, Foldable Organic Thin-Film Transistors on Glassine Paper

Facile filter cloth brush-coating of large-area uniform silver nanowire conductive films for paper-based heater

Filter cloth brush-coating (FCBC), using soft filter cloth as a brush-coating medium, in conjunction with viscous silver nanowire (AgNW) conductive solution, is used to prepare AgNW conductive films. The density and uniformity of AgNWs deposited on the substrate are controlled by the interplay between the filter cloth aperture, the conductive solution viscosity, and the brush-coating speed. Further, with appropriate AgNW concentration and flow rate, uniform AgNW transparent conductive film with sheet resistance of 18 Ω sq-1and transmittance of 94% at 550 nm is acquired by FCBC. Due to the precise control of the coating process in FCBC, large-area uniform AgNW conductive film fabricated on printing paper has a low non-uniformity factor of 1.2% at a sheet resistance of 19.0 Ω sq-1. The resultant paper-based AgNW film heater shows sensitive and stable heating performance. FCBC shows great potential in producing large-area uniform AgNW films on various substrates.

Graphene-interfaced flexible and stretchable micro-nano electrodes: from fabrication to sweat glucose detection

Flexible and stretchable wearable electronic devices have received tremendous attention for their non-invasive and personal health monitoring applications. These devices have been fabricated by integrating flexible substrates and graphene nanostructures for non-invasive detection of physiological risk biomarkers from human bodily fluids, such as sweat, and monitoring of human physical motion tracking parameters. The extraordinary properties of graphene nanostructures in fully integrated wearable devices have enabled improved sensitivity, electronic readouts, signal conditioning and communication, energy harvesting from power sources through electrode design and patterning, and graphene surface modification or treatment. This review explores advances made toward the fabrication of graphene-interfaced wearable sensors, flexible and stretchable conductive graphene electrodes, as well as their potential applications in electrochemical sensors and field-effect-transistors (FETs) with special emphasis on monitoring sweat biomarkers, mainly in glucose-sensing applications. The review emphasizes flexible wearable sweat sensors and provides various approaches thus far employed for the fabrication of graphene-enabled conductive and stretchable micro-nano electrodes, such as photolithography, electron-beam evaporation, laser-induced graphene designing, ink printing, chemical-synthesis and graphene surface modification. It further explores existing graphene-interfaced flexible wearable electronic devices utilized for sweat glucose sensing, and their technological potential for non-invasive health monitoring applications.

Papertronics: Marriage between Paper and Electronics Becoming a Real Scenario in Resource-Limited Settings

Integrating electronic applications with paper, placed next to or below printed images or graphics, can further expand the possible uses of paper substrates. Consuming paper as a substrate in the field of electronics can lead to significant innovations toward papertronics applications as paper comprises various advantages like being disposable, inexpensive, biodegradable, easy to handle, simple to use, and easily available. All of these advantages will definitely spur the advancement of the electronics field, but unfortunately, putting electronics on paper is not an easy task because, compared to plastics, the paper surface is not just rough but also porous. For example, in the case of lateral flow assay testing the sensor response is delayed if the pore size of the paper is enormous. This might be a disadvantage for most electrical devices printed directly on paper. Still, some methods make it compatible when fit with a rough, absorbent surface of the paper. Building electronic devices on a standard paper substrate have sparked much interest because of its lightweight, environmental friendliness, minimal cost, and simple fabrication. A slew of improvements have been achieved in recent years to make paper electronics perform better in various applications, including transistors, batteries, and displays. In addition, flexible electronics have gained much interest in human-machine interaction and wireless sensing. This review briefly examines the origins and fabrication of paper electronics and then moves on to applications and exciting possible paths for paper-based electronics.

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.

Flexible organic optoelectronic devices on paper

Paper substrate has many advantages, such as low cost, bendable, foldable, printable, and environmentally friendly recycling. Nowadays, paper has been further extended as a flexible platform to deliver electronic information with the integration of organic optoelectronic devices, such as organic thin-film transistor, organic solar cell, organic electrochromic device, and organic light-emitting device. It has great potential to become the new generation of flexible substrate. Given rough surface and porous of paper, many efforts have been underway in recent years to enable the compatibility between optoelectronics and paper substrate. In this review, we present the development history of paper and its physicochemical properties, and summarize the current development of paper-based organic optoelectronic devices. We also discuss the challenges that need to be addressed before practical uses of paper-based organic optoelectronic devices.

Paper-based field-effect transistor sensors

The present scenario in the world largely demands affordable, fast, recyclable, and flexible electronic devices for bio sensing. Varieties of paper-based devices such as microfluidics paper electrodes, paper diodes, and paper-based transistors etc. have been developed and validated. Most of the fabrication techniques published so far have focused on economic, environment-friendly straightforward methods to develop paper-based field-effect transistors (PFET) sensors, additionally, explored their applications. The synthetic-free, mechanically flexible, biocompatible, and signal amplification capability render PFET based sensors for wearable device makers. Modified organic/inorganic PFETs identify target analytes based on the electrical signal and endow them as perfect transducers. In the field of PFET bio sensing technology, numerous challenges are needed to be discussed to proceed forward in biomedical and other analytical applications. Realizing biologically or chemically modified PFET having an exceptional signal to noise ratio, specificity, with rapid detection ability is challenging. This review recapitulates the fabrication techniques, performances of various PFET sensors, and summarizes the report by concluding remarks including the limitations of the existing PFET based sensors and the future holds in regards to the sustainable nature of PFET.

Highly Sensitive, Selective, Flexible and Scalable Room-Temperature NO2 Gas Sensor Based on Hollow SnO2/ZnO Nanofibers

Semiconducting metal oxides can detect low concentrations of NO₂ and other toxic gases, which have been widely investigated in the field of gas sensors. However, most studies on the gas sensing properties of these materials are carried out at high temperatures. In this work, Hollow SnO₂ nanofibers were successfully synthesized by electrospinning and calcination, followed by surface modification using ZnO to improve the sensitivity of the SnO₂ nanofibers sensor to NO₂ gas. The gas sensing behavior of SnO₂/ZnO sensors was then investigated at room temperature (~20 °C). The results showed that SnO₂/ZnO nanocomposites exhibited high sensitivity and selectivity to 0.5 ppm of NO₂ gas with a response value of 336%, which was much higher than that of pure SnO₂ (13%). In addition to the increase in the specific surface area of SnO₂/ZnO-3 compared with pure SnO₂, it also had a positive impact on the detection sensitivity. This increase was attributed to the heterojunction effect and the selective NO₂ physisorption sensing mechanism of SnO₂/ZnO nanocomposites. In addition, patterned electrodes of silver paste were printed on different flexible substrates, such as paper, polyethylene terephthalate and polydimethylsiloxane using a facile screen-printing process. Silver electrodes were integrated with SnO₂/ZnO into a flexible wearable sensor array, which could detect 0.1 ppm NO₂ gas after 10,000 bending cycles. The findings of this study therefore open a general approach for the fabrication of flexible devices for gas detection applications.

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.

3D-Printable Carbon Nanotubes-Based Composite for Flexible Piezoresistive Sensors

The intersection between nanoscience and additive manufacturing technology has resulted in a new field of printable and flexible electronics. This interesting area of research tackles the challenges in the development of novel materials and fabrication techniques towards a wider range and improved design of flexible electronic devices. This work presents the fabrication of a cost-effective and facile flexible piezoresistive pressure sensor using a 3D-printable carbon nanotube-based nanocomposite. The carbon nanotubes used for the development of the material are multi-walled carbon nanotubes (MWCNT) dispersed in polydimethylsiloxane (PDMS) prepolymer. The sensor was fabricated using the direct ink writing (DIW) technique (also referred to as robocasting). The MWCNT-PDMS composite was directly printed onto the polydimethylsiloxane substrate. The sensor response was then examined based on the resistance change to the applied load. The sensor exhibited high sensitivity (6.3 Ω/kPa) over a wide range of applied pressure (up to 1132 kPa); the highest observed measurement range for MWCNT-PDMS composite in previous work was 40 kPa. The formulated MWCNT-PDMS composite was also printed into high-resolution 3-dimensional shapes which maintained their form even after heat treatment process. The possibility to use 3D printing in the fabrication of flexible sensors allows design freedom and flexibility, and structural complexity with wide applications in wearable or implantable electronics for sport, automotive and biomedical fields.

Tunable Electrical Properties of Embossed, Cellulose-Based Paper for Skin-like Sensing

This article describes a process of fabricating highly porous paper from cellulosic fibers and carbon black (CB) with tunable conductivity. By embossing such paper, its porosity decreases while its conductivity increases. Tuning the porosity of composite paper alters the magnitude and trend of conductivity over a spectrum of concentrations of conductive particles. The largest increase in conductivity from 8.38 × 10^-6 to 2.5 × 10^-3 S/m by a factor of ∼300 occurred at a percolation threshold of 3.8 wt % (or 0.36 vol %) with the composite paper plastically compressed by 410 MPa, which caused a decrease of porosity from 88% to 42% on average. Our composite paper showed stable piezoresistive responses within a broad pressure range from 1 kPa up to 5.5 MPa for 800 cycles. The piezoresistive sensitivities of the composite paper were dependent on concentration and decreased with pressure. Composite paper with 7.5 wt % CB had sensitivities of -0.514 kPa^-1 over applied pressures ranging from 1 to 50 kPa and -0.215 kPa^-1 from 1 to 250 kPa. This piezoresistive paper with embossed patterns enabled touch sensing and detection of damage from darts and punches. Understanding the percolation behavior of three-phase composites (cellulosic fibers/conductive particles/air) and their response to damage, pressure, and processing conditions has the potential to enable scalable applications in prosthetics and robotics, haptic feedback, or structural health monitoring on expansive surfaces of buildings and vehicles.

Microfluidic Behavior of Alumina Nanotube-Based Pathways within Hydrophobic CNT Barriers

The use of porous micro-and nanostructured materials within microfluidic devices results in unique fluid transport characteristics. In this paper, we investigate the microfluidic behavior of hybrid alumina nanotube-based pathways within the hydrophobic carbon nanotube (CNT) barriers. These hybrid systems provide unique benefits for potential liquid transport control in porous structures with real-time sensing of fluids. In particular, we examine how the alignment of the alumina nanostructures with high internal porosity enables increased capillary action and sensitivity of detection. Based on the Lucas and Washburn model (LW) and the modified LW models, the microfluidic behavior of these systems is detailed. The time exponent prediction from the models for capillary transport in porous media is determined to be ≤0.5. The experimental results demonstrate that the average capillary rise in the nanostructured media driven by a capillary force follows t^0.7. The hydrophilic/electrically insulating and hydrophobic/electrically conductive patterned structures of the device are used for electronic measurements within the microfluidic channels. The device structure enables the detection of fluid samples of very low analyte concentrations (1 μM) that can be achieved due to the very high surface area of the hybrid structure combined with the electrical conductivity of the CNT support structure.

Low-voltage 2D materials-based printed field-effect transistors for integrated digital and analog electronics on paper

Paper is the ideal substrate for the development of flexible and environmentally sustainable ubiquitous electronic systems, which, combined with two-dimensional materials, could be exploited in many Internet-of-Things applications, ranging from wearable electronics to smart packaging. Here we report high-performance MoS2 field-effect transistors on paper fabricated with a "channel array" approach, combining the advantages of two large-area techniques: chemical vapor deposition and inkjet-printing. The first allows the pre-deposition of a pattern of MoS2; the second, the printing of dielectric layers, contacts, and connections to complete transistors and circuits fabrication. Average ION/IOFF of 8 × 103 (up to 5 × 104) and mobility of 5.5 cm2 V-1 s-1 (up to 26 cm2 V-1 s-1) are obtained. Fully functional integrated circuits of digital and analog building blocks, such as logic gates and current mirrors, are demonstrated, highlighting the potential of this approach for ubiquitous electronics on paper.

Thermostable Ion Gels for High-Temperature Operation of Electrolyte-Gated Transistors

High-temperature durability is critical for application of organic materials in electronic devices that operate in harsh environments. In this work, thermostable physically cross-linked polymer electrolytes, or thermostable physical ion gels, were successfully developed using crystallization-induced phase separation of semicrystalline polyamides (PAs) in an ionic liquid (IL). In these ion gels, phase-separated PA crystals act as network junctions and enable the ion gels to maintain their mechanical integrity up to 180 °C. ILs and ion gels are suitable electrolyte candidates for thin-film devices operating at high temperatures because they outperform other electrolytes that use aqueous and organic solvents, owing to their superior thermal stability and nonvolatility. In addition to thermal stability, the PA gels exhibited high ionic conductivity (∼1 mS/cm) and specific capacitance (∼10 μF/cm²) at room temperature; these values increased significantly with increasing temperature, while the gel retained its solid-state mechanical integrity. These thermostable ion gels were successfully used as an electrolyte gate dielectric in organic thin-film transistors that operate at high temperatures (ca. 150 °C) and low voltages (<1 V). The transistors gated with the dielectrics had a high on/off current ratio of (3.04 ± 0.24) × 10⁵ and a hole mobility of 2.83 ± 0.20 cm²/V·s. By contrast, conventional physical ion gels based on semicrystalline polymers of poly(vinylidene fluoride-co-hexafluoropropylene) and polyvinylidene fluoride lost their mechanical integrity and dewetted from a semiconductor channel at lower temperatures. Therefore, these results demonstrate an effective method of generating thermally stable, mechanically robust, and highly conductive solid polymer electrolytes for electronic and electrochemical devices operating in a wide temperature range.

Aerosol-Jet-Printed Graphene Immunosensor for Label-Free Cytokine Monitoring in Serum

Graphene-based inks are becoming increasingly attractive for printing low-cost and flexible electrical circuits due to their high electrical conductivity, biocompatibility, and manufacturing scalability. Conventional graphene printing techniques, such as screen and inkjet printing, are limited by stringent ink viscosity requirements properties and large as-printed line width that impedes the performance of printed biosensors. Here, we report an aerosol-jet-printed (AJP) graphene-based immunosensor capable of monitoring two distinct cytokines: interferon gamma (IFN-γ) and interleukin 10 (IL-10). Interdigitated electrodes (IDEs) with 40 μm finger widths were printed from graphene-nitrocellulose ink on a polyimide substrate. The IDEs were annealed in CO₂ to introduce reactive oxygen species on the graphene surface that act as chemical handles to covalently link IFN-γ and IL-10 antibodies to the graphene surfaces. The resultant AJP electrochemical immunosensors are capable of monitoring cytokines in serum with wide sensing range (IFN-γ: 0.1-5 ng/mL; IL-10: 0.1-2 ng/mL), low detection limit (IFN-γ: 25 pg/ml and IL-10: 46 pg/ml) and high selectivity (antibodies exhibited minimal cross-reactivity with each other and IL-6) without the need for sample prelabeling or preconcentration. Moreover, these biosensors are mechanically flexible with minimal change in signal output after 250 bending cycles over a high curvature (Φ = 5 mm). Hence, this technology could be applied to numerous electrochemical applications that require low-cost electroactive circuits that are disposable and/or flexible.

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.

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.

Solvent-free electrically conductive Ag/ethylene vinyl acetate (EVA) composites for paper-based printable electronics

Solvent-free electrically conductive composites have been applied to flexible electronics to obtain high electrical conductivity. However, some of the proposed composites have low electrical conductivities and are unable to meet the requirements of commercial printable electronics. In this study, solvent-free electrically conductive Ag/EVA (ethylene vinyl acetate) composites for paper-based printable electronics were prepared by a thermal melting method. The properties of these electrically conductive Ag/EVA composites, including particle sizes, morphologies and phase purities of the flake silver flake powders, were investigated using a particle size analyzer, scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The results showed that nanometer-thick flake silver flake powders with smooth and flat surfaces were made by the nanofilm transition technique. These obtained powders were able to form smooth face-to-face contacts, which facilitated the formation of an excellent conductive network in the conductive system. Dynamic mechanical analysis (DMA) was conducted to investigate the mechanical properties of EVA and Ag/EVA composites. A Fourier transformation infra-red (FTIR) spectrometer, laser micro-Raman spectrometer and thermogravimetric analyzer were used to analyze the organic functional groups, glass transition temperatures and thermal weight losses of the EVA resin and solvent-free electrically conductive composites. The solvent-free electrically conductive Ag/EVA composite, which contained 55 wt% of the as-prepared flake silver flake powders, was found to have an extremely low volume resistivity of 1.23 × 10^-4 Ω cm as well as excellent bending performance and adhesion. These features indicate the great potential of these composites for application in printed electronics.

Recent Progress in Inkjet-Printed Thin-Film Transistors

Drop-on-demand inkjet printing is one of the most attractive techniques from a manufacturing perspective due to the possibility of fabrication from a digital layout at ambient conditions, thus leading to great opportunities for the realization of low-cost and flexible thin-film devices. Over the past decades, a variety of inkjet-printed applications including thin-film transistors (TFTs), radio-frequency identification devices, sensors, and displays have been explored. In particular, many research groups have made great efforts to realize high-performance TFTs, for application as potential driving components of ubiquitous wearable electronics. Although there are still challenges to enable the commercialization of printed TFTs beyond laboratory-scale applications, the field of printed TFTs still attracts significant attention, with remarkable developments in soluble materials and printing methodology. Here, recent progress in printing-based TFTs is presented from materials to applications. Significant efforts to improve the electrical performance and device-yield of printed TFTs to match those of counterparts fabricated using conventional deposition or photolithography methods are highlighted. Moreover, emerging low-dimension printable semiconductors, including carbon nanotubes and transition metal dichalcogenides as well as mature semiconductors, and new-concept printed switching devices, are also discussed.

Flexible Electronics Based on Micro/Nanostructured Paper

Over the past several years, a new surge of interest in paper electronics has arisen due to the numerous merits of simple micro/nanostructured substrates. Herein, the latest advances and principal issues in the design and fabrication of paper-based flexible electronics are highlighted. Following an introduction of the fascinating properties of paper matrixes, the construction of paper substrates from diverse functional materials for flexible electronics and their underlying principles are described. Then, notable progress related to the development of versatile electronic devices is discussed. Finally, future opportunities and the remaining challenges are examined. It is envisioned that more design concepts, working principles, and advanced papermaking techniques will be developed in the near future for the advanced functionalization of paper, paving the way for the mass production and commercial applications of flexible paper-based electronic devices.

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.

A general gelation strategy for 1D nanowires: dynamically stable functional gels for 3D printing flexible electronics

3D printing of functional inorganic nanowires has been accomplished using extruded nanowire-based inks obtained by incorporating nanofillers into polymeric matrices or thickeners. However, the presence of inactive additives poses a critical challenge for fully realizing the functionality of the nanowires in the printed structures, which remains a fundamental hurdle to overcome. Here, to construct 3D-printed electronics with high performance, we developed a versatile gelation strategy to obtain thixotropic nanowire gels through formation of dynamically stable 3D networks using small amounts of flexible, water-soluble and single-layer 2D nanosheets, such as graphene oxide and MXene, as physical cross-linkers. The nanosheets can knot-tie and stabilize the nanowire junctions in the aqueous suspension, leading to the formation of stable and thixotropic gels with viscosities up to ∼80 000 Pa s at 0.01 s-1 in the absence of polymer thickener. Gels of varioius metallic and semiconductive nanowires have been successfully prepared and printed into 3D and self-supported architectures via extrusion-based 3D-printing. The synergism of nanowires and nanosheets not only conquers the restraints of harsh post-treatments to remove additives after printing, but also maximizes the functionality of the nanowires in the printed architectures. The printed 3D structures solidified by ambient drying, coagulation, or freeze-drying exhibit remarkable functionalities. For example, the electrical conductivity of the 3D-printed silver nanowire-based architectures can reach 40 000 S cm-1. The feasibility of these functional nanowire gels was demonstrated by fabricating a series of printed flexible electronics via extrusion-based 3D-printing.

A Combined XRD, Solvatochromic, and Cyclic Voltammetric Study of Poly (3,4-Ethylenedioxythiophene) Doped with Sulfonated Polyarylethersulfones: Towards New Conducting Polymers

Despite the poor solubility in organic solvents, poly (3,4-ethylenedioxythiophene) (PEDOT) is one of the most successful conducting polymers. To improve PEDOT conductivity, the dopants commonly used are molecules/polymers carrying sulfonic functionalities. In addition to these species, sulfonated polyarylethersulfone (SPAES), obtained via homogeneous synthesis with different degrees of sulfonation (DS), can be used thanks to both the tight control over the DS and the charge separation present in SPAES structure. Here, PEDOTs having enhanced solubility in the chosen reaction solvents (N,N-dimethylformamide, dimethylacetamide, dimethyl sulfoxide, and N-methyl-2-pyrrolidone) were synthesized via a high-concentration solvent-based emulsion polymerization with very low amounts of SPAES as dopant (1% w/w with respect to EDOT monomer), characterized by different DS. The influence of solvents and of the adopted doping agent was studied on PEDOT_SPAESs analyzing (i) the chemical structure, comparing via X-ray diffraction (XRD) the crystalline structures of undoped and commercial PEDOTs with PEDOT_SPAES' amorphous structure; (ii) solvatochromic behavior, observing UV absorption wavelength variation as solvents and SPAES' DS change; and (iii) electrochemical properties: voltammetric peak heights of PEDOT_SPAES cast onto glassy carbon electrodes differ for each solvent and in general are better than the ones obtained for neat SPAES, PEDOTs, and glassy carbon.

High-Performance, Ultrathin, Ultraflexible Organic Thin-Film Transistor Array Via Solution Process

Ultrathin organic thin-film transistors (OTFTs) have received extensive attention due to their outstanding advantages, such as extreme flexibility, good conformability, ultralight weight, and compatibility with low-cost and large-area solution-processed techniques. However, compared with the rigid substrates, it still remains a challenge to fabricate high-performance ultrathin OTFTs. In this study, a high-performance ultrathin 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) OTFT array is demonstrated via a simple spin-coating method, with mobility as high as 11 cm2 V-1 s-1 (average mobility: 7.22 cm2 V-1 s-1 ), on/off current ratio of over 106 , switching current of >1 mA, and a good yield ratio as high as 100%. The ultrathin thickness at ≈380 nm and the ultralight weight at ≈0.89 g m-2 enable the free-standing OTFTs to imperceptibly adhere onto human skin, and even a damselfly wing without affecting its flying. More importantly, the OTFTs show good electrical characteristics and mechanical stability when conformed onto the curved surfaces and even folded in a book after 100 folding cycles. These results illustrate the broad application potential of this simply fabricated ultrathin OTFT in next-generation electronics such as foldable displays and wearable devices.

Self-Sensing Paper Actuators Based on Graphite-Carbon Nanotube Hybrid Films

Soft actuators have demonstrated potential in a range of applications, including soft robotics, artificial muscles, and biomimetic devices. However, the majority of current soft actuators suffer from the lack of real-time sensory feedback, prohibiting their effective sensing and multitask function. Here, a promising strategy is reported to design bilayer electrothermal actuators capable of simultaneous actuation and sensation (i.e., self-sensing actuators), merely through two input electric terminals. Decoupled electrothermal stimulation and strain sensation is achieved by the optimal combination of graphite microparticles and carbon nanotubes (CNTs) in the form of hybrid films. By finely tuning the charge transport properties of hybrid films, the signal-to-noise ratio (SNR) of self-sensing actuators is remarkably enhanced to over 66. As a result, self-sensing actuators can actively track their displacement and distinguish the touch of soft and hard objects.

Functional inks and printing of two-dimensional materials

Graphene and related two-dimensional materials provide an ideal platform for next generation disruptive technologies and applications. Exploiting these solution-processed two-dimensional materials in printing can accelerate this development by allowing additive patterning on both rigid and conformable substrates for flexible device design and large-scale, high-speed, cost-effective manufacturing. In this review, we summarise the current progress on ink formulation of two-dimensional materials and the printable applications enabled by them. We also present our perspectives on their research and technological future prospects.

Direct Printing of Graphene Electrodes for High-Performance Organic Inverters

Scalable fabrication of high-resolution electrodes and interconnects is necessary to enable advanced, high-performance, printed, and flexible electronics. Here, we demonstrate the direct printing of graphene patterns with feature widths from 300 μm to ∼310 nm by liquid-bridge-mediated nanotransfer molding. This solution-based technique enables residue-free printing of graphene patterns on a variety of substrates with surface energies between ∼43 and 73 mN m^-1. Using printed graphene source and drain electrodes, high-performance organic field-effect transistors (OFETs) are fabricated with single-crystal rubrene (p-type) and fluorocarbon-substituted dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDIF-CN₂) (n-type) semiconductors. Measured mobilities range from 2.1 to 0.2 cm² V^-1 s^-1 for rubrene and from 0.6 to 0.1 cm² V^-1 s^-1 for PDIF-CN₂. Complementary inverter circuits are fabricated from these single-crystal OFETs with gains as high as ∼50. Finally, these high-resolution graphene patterns are compatible with scalable processing, offering compelling opportunities for inexpensive printed electronics with increased performance and integration density.

Area-Controllable Stamping of Semicrystalline Copolymer Ionogels for Solid-State Electrolyte-Gated Transistors and Light-Emitting Devices

Two types of thin-film electrochemical devices (electrolyte-gated transistors and electrochemical light-emitting cells) are demonstrated using area-controllable ionogel patches generated by transfer-stamping. For the successful transfer of ionogel patches on various target substrates, thermoreversible gelation by phase-separated polymer crystals within the ionogel is essential because it allows the gel to form a conformal contact with the acceptor substrate, thereby lowering the overall Gibbs energy of the system upon transfer of the ionogel. This crystallization-mediated stamping provides a much more efficient deposition route for producing thin films of ionically conductive high-capacitance solid ionogel electrolytes. The lateral dimensions of the transferred ionogels range from 1 mm × 1 mm to 40 mm × 40 mm. These ionogel patches are incorporated in organic p-type and inorganic n-type thin-film transistors and electrochemical light-emitting devices. The resulting transistors show sub-1 V device operation with high transconductance currents, and the optoelectronic devices emit orange light through a series of electrochemical redox reactions. These results demonstrate a simple yet versatile route to employ physical ionogels for various solid-state electrochemical device applications.

Combustion-Assisted Photonic Annealing of Printable Graphene Inks via Exothermic Binders

High-throughput and low-temperature processing of high-performance nanomaterial inks is an important technical challenge for large-area, flexible printed electronics. In this report, we demonstrate nitrocellulose as an exothermic binder for photonic annealing of conductive graphene inks, leveraging the rapid decomposition kinetics and built-in energy of nitrocellulose to enable versatile process integration. This strategy results in superlative electrical properties that are comparable to extended thermal annealing at 350 °C, using a pulsed light process that is compatible with thermally sensitive substrates. The resulting porous microstructure and broad liquid-phase patterning compatibility are exploited for printed graphene microsupercapacitors on paper-based substrates.

High-Resolution Transfer Printing of Graphene Lines for Fully Printed, Flexible Electronics

Pristine graphene inks show great promise for flexible printed electronics due to their high electrical conductivity and robust mechanical, chemical, and environmental stability. While traditional liquid-phase printing methods can produce graphene patterns with a resolution of ∼30 μm, more precise techniques are required for improved device performance and integration density. A high-resolution transfer printing method is developed here capable of printing conductive graphene patterns on plastic with line width and spacing as small as 3.2 and 1 μm, respectively. The core of this method lies in the design of a graphene ink and its integration with a thermally robust mold that enables annealing at up to ∼250 °C for precise, high-performance graphene patterns. These patterns exhibit excellent electrical and mechanical properties, enabling favorable operation as electrodes in fully printed electrolyte-gated transistors and inverters with stable performance even following cyclic bending to a strain of 1%. The high resolution coupled with excellent control over the line edge roughness to below 25 nm enables aggressive scaling of transistor dimensions, offering a compelling route for the scalable manufacturing of flexible nanoelectronic devices.

Flexible and Foldable Fully-Printed Carbon Black Conductive Nanostructures on Paper for High-Performance Electronic, Electrochemical, and Wearable Devices

In this work, we demonstrate the first example of fully printed carbon nanomaterials on paper with unique features, aiming the fabrication of functional electronic and electrochemical devices. Bare and modified inks were prepared by combining carbon black and cellulose acetate to achieve high-performance conductive tracks with low sheet resistance. The carbon black tracks withstand extremely high folding cycles (>20 000 cycles), a new record-high with a response loss of less than 10%. The conductive tracks can also be used as 3D paper-based electrochemical cells with high heterogeneous rate constants, a feature that opens a myriad of electrochemical applications. As a relevant demonstrator, the conductive ink modified with Prussian-blue was electrochemically characterized proving to be very promising toward the detection of hydrogen peroxide at very low potentials. Moreover, carbon black circuits can be fully crumpled with negligible change in their electrical response. Fully printed motion and wearable sensors are additional examples where bioinspired microcracks are created on the conductive track. The wearable devices are capable of efficiently monitoring extremely low bending angles including human motions, fingers, and forearm. Here, to the best of our knowledge, the mechanical, electronic, and electrochemical performance of the proposed devices surpasses the most recent advances in paper-based devices.

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.

Repurposing compact discs as master molds to fabricate high-performance organic nanowire field-effect transistors

Organic field-effect transistors (OFETs) have been developed over the past few decades due to their potential applications in future electronics such as wearable and foldable electronics. As the electrical performance of OFETs has improved, patterning organic semiconducting crystals has become a key issue for their commercialization. However, conventional soft lithographic techniques have required the use of expensive processes to fabricate high-resolution master molds. In this study, we demonstrated a cost-effective method to prepare nanopatterned master molds for the fabrication of high-performance nanowire OFETs. We repurposed commercially available compact discs (CDs) as master molds because they already have linear nanopatterns on their surface. Flexible nanopatterned templates were replicated from the CDs using UV-imprint lithography. Subsequently, 6,13-bis-(triisopropylsilylethynyl) pentacene nanowires (NWs) were grown from the templates using a capillary force-assisted lithographic technique. The NW-based OFETs showed a high average field-effect mobility of 2.04 cm² V^-1 s^-1. This result was attributed to the high crystallinity of the NWs and to their crystal orientation favorable for charge transport.

Flexible and Actuating Nanoporous Poly(Ionic Liquid)-Paper-Based Hybrid Membranes

Porous and flexible actuating materials are important for the development of smart systems. We report here a facile method to prepare scalable, flexible actuating porous membranes based on a poly(ionic liquid)-modified tissue paper. The targeted membrane property profile was based on synergy of the gradient porous structure of a poly(ionic liquid) network and flexibility of a tissue paper. The gradient porous structure was built through an ammonia-triggered electrostatic complexation of a poly(ionic liquid) with poly(acrylic acid), which were previously impregnated inside the tissue paper. As a result, these porous membranes undergo deformation by bending in response to organic solvents in the vapor or liquid phase and can recover their shape in air, which demonstrates their ability to serve as solvent sensors. Besides, they show enhanced mechanical properties due to the introduction of mechanically flexible tissue paper that allows the membranes to be designed as new responsive textiles and contractile actuators.

Direct Drawing Method of Graphite onto Paper for High-Performance Flexible Electrochemical Sensors

A simple and fast fabrication method to create high-performance pencil-drawn electrochemical sensors is reported for the first time. The sluggish electron transfer observed on bare pencil-drawn surfaces was enhanced using two electrochemical steps: first oxidizing the surface and then reducing it in a subsequent step. The heterogeneous rate constant was found to be 5.1 × 10^-3 cm s^-1, which is the highest value reported so far for pencil-drawn surfaces. We mapped the origin of such performance by atomic force microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Our results suggest that the oxidation process leads to chemical and structural transformations on the electrode surface. As a proof-of-concept, we modified the pencil-drawn surface with Meldola's blue to electrocatalytically detect nicotinamide adenine dinucleotide (NADH). The electrochemical device exhibited the highest catalytic constant (1.7 × 10⁵ L mol^-1 s^-1) and the lowest detection potential for NADH reported so far in paper-based electrodes.

Ultrafast Paper Thermometers Based on a Green Sensing Ink

With the use of an ionic liquid as the ultrathermosensitive fluid, a paper thermometer is successfully developed with intrinsic ability of ultrafast response and high stability upon temperature change. The fluidic nature allows the ionic liquid to be easily deposited on paper by pen writing or inkjet printing, affording great promise for large-scale fabrication of low-cost paper sensors. Owing to the advantages of nonvolatilization, excellent continuity and deformability, the thermosensitive ink trapped within the cellulose fibers of paper matrix has no leakage or evaporation at open states, ensuring the excellent stability and repeatability of thermal sensing against arbitrary bending and folding operation. By shortening the heat exchange distance between ionic liquid and samples, it takes only 8 s for the thermometer to reach an electrical equilibrium at a given temperature. Moreover, the paper thermometer can be applied to remotely monitor temperature change with the combination of a wireless communication technology.

Top-Contact Self-Aligned Printing for High-Performance Carbon Nanotube Thin-Film Transistors with Sub-Micron Channel Length

Semiconducting single-wall carbon nanotubes are ideal semiconductors for printed thin-film transistors due to their excellent electrical performance and intrinsic printability with solution-based deposition. However, limited by resolution and registration accuracy of current printing techniques, previously reported fully printed nanotube transistors had rather long channel lengths (>20 μm) and consequently low current-drive capabilities (<0.2 μA/μm). Here we report fully inkjet printed nanotube transistors with dramatically enhanced on-state current density of ∼4.5 μA/μm by downscaling the devices to a sub-micron channel length with top-contact self-aligned printing and employing high-capacitance ion gel as the gate dielectric. Also, the printed transistors exhibited a high on/off ratio of ∼10⁵, low-voltage operation, and good mobility of ∼15.03 cm² V^-1s^-1. These advantageous features of our printed transistors are very promising for future high-definition printed displays and sensing systems, low-power consumer electronics, and large-scale integration of printed electronics.

Rational and practical exfoliation of graphite using well-defined poly(3-hexylthiophene) for the preparation of conductive polymer/graphene composite

Processing and manipulation of highly conductive pristine graphene in large quantities are still major challenges in the practical application of graphene for electric device. In the present study, we report the liquid-phase exfoliation of graphite in toluene using well-defined poly(3-hexylthiophene) (P3HT) to produce a P3HT/graphene composite. We synthesize and use regioregular P3HT with controlled molecular weights as conductive dispersants for graphene. Simple ultrasonication of graphite flakes with the P3HT successfully produces single-layer and few-layer graphene sheets dispersed in toluene. The produced P3HT/graphene composite can be used as conductive graphene ink, indicating that the P3HT/graphene composite has high electrical conductivity owing to the high conductivity of P3HT and graphene. The P3HT/graphene composite also works as an oxidation-resistant and conductive film for a copper substrate, which is due to the high gas-barrier property of graphene.

Assembly and Electronic Applications of Colloidal Nanomaterials

Artificial solids and thin films assembled from colloidal nanomaterials give rise to versatile properties that can be exploited in a range of technologies. In particular, solution-based processes allow for the large-scale and low-cost production of nanoelectronics on rigid or mechanically flexible substrates. To achieve this goal, several processing steps require careful consideration, including nanomaterial synthesis or exfoliation, purification, separation, assembly, hybrid integration, and device testing. Using a ubiquitous electronic device - the field-effect transistor - as a platform, colloidal nanomaterials in three electronic material categories are reviewed systematically: semiconductors, conductors, and dielectrics. The resulting comparative analysis reveals promising opportunities and remaining challenges for colloidal nanomaterials in electronic applications, thereby providing a roadmap for future research and development.

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 .

High-Performance Multiresponsive Paper Actuators

There is an increasing demand for soft actuators because of their importance in soft robotics, artificial muscles, biomimetic devices, and beyond. However, the development of soft actuators capable of low-voltage operation, powerful actuation, and programmable shape-changing is still challenging. In this work, we propose programmable bilayer actuators that operate based on the large hygroscopic contraction of the copy paper and simultaneously large thermal expansion of the polypropylene film upon increasing the temperature. The electrothermally activated bending actuators can function with low voltages (≤ 8 V), low input electric power per area (P ≤ 0.14 W cm^-2), and low temperature changes (≤ 35 °C). They exhibit reversible shape-changing behavior with curvature radii up to 1.07 cm^-1 and bending angle of 360°, accompanied by powerful actuation. Besides the electrical activation, they can be powered by humidity or light irradiation. We finally demonstrate the use of our paper actuators as a soft gripper robot and a lightweight paper wing for aerial robotics.

Direct patterning of silver electrodes with 2.4μm channel length by piezoelectric inkjet printing

The control of channel length is of great significance in the fabrication of thin film transistors (TFTs) with high-speed operation. However, achieving short channel on untreated glass by traditional piezoelectric inkjet printing is problematic due to the impacting and rebounding behaviors of droplet impinging on solid surface. Here a novel method was proposed to obtain short channel length on untreated glass by taking advantage of the difference in the retraction velocities on both sides of an ink droplet. In addition, droplets contact mechanism was first introduced in our work to explain the formation of short channel in the printing process. Through printing droplets array with optimized drop space and adjusting appropriate printing parameters, a 2.4μm of channel length for TFT, to the best of our knowledge, which is the shortest channel on substrate without pre-patterning, was achieved using piezoelectric inkjet printing. This study sheds light on the fabrication of short channel TFT for large size and high-resolution displays using inkjet printing technology.

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

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

2D-Crystal-Based Functional Inks

The possibility to produce and process graphene, related 2D crystals, and heterostructures in the liquid phase makes them promising materials for an ever-growing class of applications as composite materials, sensors, in flexible optoelectronics, and energy storage and conversion. In particular, the ability to formulate functional inks with on-demand rheological and morphological properties, i.e., lateral size and thickness of the dispersed 2D crystals, is a step forward toward the development of industrial-scale, reliable, inexpensive printing/coating processes, a boost for the full exploitation of such nanomaterials. Here, the exfoliation strategies of graphite and other layered crystals are reviewed, along with the advances in the sorting of lateral size and thickness of the exfoliated sheets together with the formulation of functional inks and the current development of printing/coating processes of interest for the realization of 2D-crystal-based devices.

Tailoring the Dielectric Layer Structure for Enhanced Performance of Organic Field-Effect Transistors: The Use of a Sandwiched Polar Dielectric Layer

To investigate the origins of hydroxyl groups in a polymeric dielectric and its applications in organic field-effect transistors (OFETs), a polar polymer layer was inserted between two polymethyl methacrylate (PMMA) dielectric layers, and its effect on the performance as an organic field-effect transistor (OFET) was studied. The OFETs with a sandwiched dielectric layer of poly(vinyl alcohol) (PVA) or poly(4-vinylphenol) (PVP) containing hydroxyl groups had shown enhanced characteristics compared to those with only PMMA layers. The field-effect mobility had been raised more than 10 times in n-type devices (three times in the p-type one), and the threshold voltage had been lowered almost eight times in p-type devices (two times in the n-type). The on-off ratio of two kinds of devices had been enhanced by almost two orders of magnitude. This was attributed to the orientation of hydroxyl groups from disordered to perpendicular to the substrate under gate-applied voltage bias, and additional charges would be induced by this polarization at the interface between the semiconductor and dielectrics, contributing to the accumulation of charge transfer.

Integrating Electronics and Microfluidics on Paper

Paper microfluidics and printed electronics have developed independently, and are incompatible in many aspects. Monolithic integration of microfluidics and electronics on paper is demonstrated. This integration makes it possible to print 2D and 3D fluidic, electrofluidic, and electrical components on paper, and to fabricate devices using them.

In situ polymerization of a novel surfactant on a graphene surface for the stable dispersion of graphene in water

A graphene surface is functionalized with a polymerizable surfactant by in situ polymerization for the stable colloidal dispersion of graphene.