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

Three-Dimensional High-Resolution Laser Lithography of CsPbBr3 Quantum Dots in Photoresist with Sub-100 nm Feature Size

Perovskite quantum dots (PQDs), with their excellent optical properties, have become a leading semiconductor material in the field of optoelectronics. However, to date, it has been a challenge to achieve the three-dimensional high-resolution patterning of perovskite quantum dots. In this paper, an in situ femtosecond laser-direct-writing technology was demonstrated for three-dimensional high-resolution patterned CsPbBr3 PQDs using a two-photon photoresist nanocomposite doped with the CsPbBr3 perovskite precursor. By adjusting the laser processing parameters, the minimum line width of the PQDs material was confirmed to be 98.6 nm, achieving a sub-100 nm PQDs nanowire for the first time. In addition, the fluorescence intensity of the laser-processed PQDs can be regulated by the laser power. Our findings provide a new technology for fabricating high-resolution display devices based on laser-direct-writing CsPbBr3 PQDs materials.

From Printed Devices to Vertically Stacked, 3D Flexible Hybrid Systems

The pursuit of miniaturized Si electronics has revolutionized computing and communication. During recent years, the value addition in electronics has also been achieved through printing, flexible and stretchable electronics form factors, and integration over areas larger than wafer size. Unlike Si semiconductor manufacturing which takes months from tape-out to wafer production, printed electronics offers greater flexibility and fast-prototyping capabilities with lesser resources and waste generation. While significant advances have been made with various types of printed sensors and other passive devices, printed circuits still lag behind Si-based electronics in terms of performance, integration density, and functionality. In this regard, recent advances using high-resolution printing coupled with the use of high mobility materials and device engineering, for both in-plane and out-of-plane integration, raise hopes. This paper focuses on the progress in printed electronics, highlighting emerging printing technologies and related aspects such as resource efficiency, environmental impact, integration scale, and the novel functionalities enabled by vertical integration of printed electronics. By highlighting these advances, this paper intends to reveal the future promise of printed electronics as a sustainable and resource-efficient route for realizing high-performance integrated circuits and systems.

Ultra-high precision nano additive manufacturing of metal oxide semiconductors via multi-photon lithography

As a basic component of the versatile semiconductor devices, metal oxides play a critical role in modern electronic information industry. However, ultra-high precision nanopatterning of metal oxides often involves multi-step lithography and transfer process, which is time-consuming and costly. Here, we report a strategy, using metal-organic compounds as solid precursor photoresist for multi-photon lithography and post-sintering, to realize ultra-high precision additive manufacturing of metal oxides. As a result, we gain metal oxides including ZnO, CuO and ZrO2 with a critical dimension of 35 nm, which sets a benchmark for additive manufacturing of metal oxides. Besides, atomic doping can be easily accomplished by including the target element in precursor photoresist, and heterogeneous structures can also be created by multiple multi-photon lithography, allowing this strategy to accommodate the requirements of various semiconductor devices. For instance, we fabricate an ZnO photodetector by the proposed strategy.

Multiobjective Optimization Strategy for Enhancing the Efficiency and Quality of Organic Thin-Film Manufacturing with Electrohydrodynamic Atomization Coating

Electrohydrodynamic atomization coating technology is well-suited for micro-/nanoscale thin-film additive manufacturing. However, there are still some challenges in quality control and parameter adjustment during the coating process. Especially when coating on nonconductive and nonhydrophilic substrates, film quality and thickness uniformity are difficult to control. This paper proposes an optimization strategy for enhancing the efficiency and quality of thin-film manufacturing on nonconductive, nonhydrophilic glass substrates. In this paper, a visual inspection system was developed for in situ inspection and identification of droplet deposition states in the substrate surface. Then, the statistical relationship between the operating parameters and the quality of the deposition state was analyzed by response surface methodology. On this basis, machine learning models and intelligent recommendation frameworks for small data sets were developed to rapidly optimize operating parameters and improve the quality of thin-film coating. Optimization strategy developed by applying the principles of statistical modeling, analysis of variance, and global optimization are more efficient and less costly than traditional parameter screening methods. The experimental results show that optimum deposition quality can be obtained with the recommended operating parameters. And, validation results show a 12.8% improvement in film thickness uniformity. At the same time, no mura defects appeared on the thin-film surface. The proposed optimization strategy can improve the efficiency and quality of additive manufacturing of micro and nano thin films and is beneficial for advancing industrial applications of the electrohydrodynamic atomization coating.

Recent Advances in Patterning Strategies for Full-Color Perovskite Light-Emitting Diodes

Metal halide perovskites have emerged as promising light-emitting materials for next-generation displays owing to their remarkable material characteristics including broad color tunability, pure color emission with remarkably narrow bandwidths, high quantum yield, and solution processability. Despite recent advances have pushed the luminance efficiency of monochromic perovskite light-emitting diodes (PeLEDs) to their theoretical limits, their current fabrication using the spin-coating process poses limitations for fabrication of full-color displays. To integrate PeLEDs into full-color display panels, it is crucial to pattern red-green-blue (RGB) perovskite pixels, while mitigating issues such as cross-contamination and reductions in luminous efficiency. Herein, we present state-of-the-art patterning technologies for the development of full-color PeLEDs. First, we highlight recent advances in the development of efficient PeLEDs. Second, we discuss various patterning techniques of MPHs (i.e., photolithography, inkjet printing, electron beam lithography and laser-assisted lithography, electrohydrodynamic jet printing, thermal evaporation, and transfer printing) for fabrication of RGB pixelated displays. These patterning techniques can be classified into two distinct approaches: in situ crystallization patterning using perovskite precursors and patterning of colloidal perovskite nanocrystals. This review highlights advancements and limitations in patterning techniques for PeLEDs, paving the way for integrating PeLEDs into full-color panels.

Stretchable conductors for stretchable field-effect transistors and functional circuits

Stretchable electronics have received intense attention due to their broad application prospects in many areas, and can withstand large deformations and form close contact with curved surfaces. Stretchable conductors are vital components of stretchable electronic devices used in wearables, soft robots, and human-machine interactions. Recent advances in stretchable conductors have motivated basic scientific and technological research efforts. Here, we outline and analyse the development of stretchable conductors in transistors and circuits, and examine advances in materials, device engineering, and preparation technologies. We divide the existing approaches to constructing stretchable transistors with stretchable conductors into the following two types: geometric engineering and intrinsic stretchability engineering. Finally, we consider the challenges and outlook in this field for delivering stretchable electronics.

Additive manufacturing of Zn with submicron resolution and its conversion into Zn/ZnO core-shell structures

Electrohydrodynamic redox 3D printing (EHD-RP) is an additive manufacturing (AM) technique with submicron resolution and multi-metal capabilities, offering the possibility to switch chemistry during deposition "on-the-fly". Despite the potential for synthesizing a large range of metals by electrochemical small-scale AM techniques, to date, only Cu and Ag have been reproducibly deposited by EHD-RP. Here, we extend the materials palette available to EHD-RP by using aqueous solvents instead of organic solvents, as used previously. We demonstrate deposition of Cu and Zn from sacrificial anodes immersed in acidic aqueous solvents. Mass spectrometry indicates that the choice of the solvent is important to the deposition of pure Zn. Additionally, we show that the deposited Zn structures, 250 nm in width, can be partially converted into semiconducting ZnO structures by oxidation at 325 °C in air.

Copper inks for printed electronics: a review

Conductive inks have attracted tremendous attention owing to their adaptability and the convenient large-scale fabrication. As a new type of conductive ink, copper-based ink is considered to be one of the best candidate materials for the conductive layer in flexible printed electronics owing to its high conductivity and low price, and suitability for large-scale manufacturing processes. Recently, tremendous progress has been made in the preparation of cooper-based inks for electronic applications, but the antioxidation ability of copper-based nanomaterials within inks or films, that is, long-term reliability upon exposure to water and oxygen, still needs more exploration. In this review, we present a comprehensive overview of copper inks for printed electronics from ink preparation, printing methods and sintering, to antioxidation strategies and electronic applications. The review begins with an overview of the development of copper inks, followed by a demonstration of various preparation methods for copper inks. Then, the diverse printing techniques and post-annealing strategies used to fabricate conductive copper patterns are discussed. In addition, antioxidation strategies utilized to stabilize the mechanical and electrical properties of copper nanomaterials are summarized. Then the diverse applications of copper inks for electronic devices, such as transparent conductive electrodes, sensors, optoelectronic devices, and thin-film transistors, are discussed. Finally, the future development of copper-based inks and the challenges of their application in printed electronics are discussed.

An Atlas for the Inkjet Printing of Large-Area Tactile Sensors

This review aims to discuss the inkjet printing technique as a fabrication method for the development of large-area tactile sensors. The paper focuses on the manufacturing techniques and various system-level sensor design aspects related to the inkjet manufacturing processes. The goal is to assess how printed electronics simplify the fabrication process of tactile sensors with respect to conventional fabrication methods and how these contribute to overcoming the difficulties arising in the development of tactile sensors for real robot applications. To this aim, a comparative analysis among different inkjet printing technologies and processes is performed, including a quantitative analysis of the design parameters, such as the costs, processing times, sensor layout, and general system-level constraints. The goal of the survey is to provide a complete map of the state of the art of inkjet printing, focusing on the most effective topics for the implementation of large-area tactile sensors and a view of the most relevant open problems that should be addressed to improve the effectiveness of these processes.

High-Resolution 3D Printing for Electronics

The ability to form arbitrary 3D structures provides the next level of complexity and a greater degree of freedom in the design of electronic devices. Since recent progress in electronics has expanded their applicability in various fields in which structural conformability and dynamic configuration are required, high-resolution 3D printing technologies can offer significant potential for freeform electronics. Here, the recent progress in novel 3D printing methods for freeform electronics is reviewed, with providing a comprehensive study on 3D-printable functional materials and processes for various device components. The latest advances in 3D-printed electronics are also reviewed to explain representative device components, including interconnects, batteries, antennas, and sensors. Furthermore, the key challenges and prospects for next-generation printed electronics are considered, and the future directions are explored based on research that has emerged recently.

Recent Advances in Metal−Oxide Thin−Film Transistors: Flexible/Stretchable Devices, Integrated Circuits, Biosensors and Neuromorphic Applications

Thin−film transistors using metal oxides have been investigated extensively because of their high transparency, large area, and mass production of metal oxide semiconductors. Compatibility with conventional semiconductor processes, such as photolithography of the metal oxide offers the possibility to develop integrated circuits on a larger scale. In addition, combinations with other materials have enabled the development of sensor applications or neuromorphic devices in recent years. Here, this paper provides a timely overview of metal−oxide−based thin−film transistors focusing on emerging applications, including flexible/stretchable devices, integrated circuits, biosensors, and neuromorphic devices. This overview also revisits recent efforts on metal oxide−based thin−film transistors developed with high compatibility for integration to newly reported applications.

One-step preparation of Cr2O3-based inks with long-term dispersion stability for inkjet applications

Inkjet printing of functional materials has shown a wide range of applications in advertising, OLED display, printed electronics and other specialized utilities that require high-precision, mask-free, direct-writing deposition techniques. Nevertheless, the sedimentation risk of the refractory functional materials dispensed in inks hinders their further implementation. Herein, we present a bottom-up ink preparation strategy based on Cr₂O₃ by a one-step solvothermal method. The obtained ink remained stable under an equivalent natural sediment test for 2.5 years. The chemical composition of the solvothermal product was characterized, and the mechanism of the superior dispersion stability of Cr₂O₃ particles was analysed. These amorphous Cr₂O₃ particles were capped by ligands generated via low-temperature solvothermal reactions. Ethanol and acetylacetone covering the particle surfaces play an essential role in enhancing the solubility of Cr₂O₃ particles in the solvent forming the ultrastable colloidal ink. Moreover, this ink was successfully printed using a direct-write inkjet system JetLab®II on nylon fabrics, and the printed area of the fabrics shows a spectral correlation coefficient of 0.9043 to green leaves. Finally, we believe that the one-step bottom-up fabrication method of Cr₂O₃-based pigment inks may provide a general approach for preparing metal oxide-based pigment inks with long-term dispersion stability.

Recent advances in printable thermoelectric devices: materials, printing techniques, and applications

Thermoelectric devices have great potential as a sustainable energy conversion technology to harvest waste heat and perform spot cooling with high reliability. However, most of the thermoelectric devices use toxic and expensive materials, which limits their application. These materials also require high-temperature fabrication processes, limiting their compatibility with flexible, bio-compatible substrate. Printing electronics is an exciting new technique for fabrication that has enabled a wide array of biocompatible and conformable systems. Being able to print thermoelectric devices allows them to be custom made with much lower cost for their specific application. Significant effort has been directed toward utilizing polymers and other bio-friendly materials for low-cost, lightweight, and flexible thermoelectric devices. Fortunately, many of these materials can be printed using low-temperature printing processes, enabling their fabrication on biocompatible substrates. This review aims to report the recent progress in developing high performance thermoelectric inks for various printing techniques. In addition to the usual thermoelectric performance measures, we also consider the attributes of flexibility and the processing temperatures. Finally, recent advancement of printed device structures is discussed which aims to maximize the temperature difference across the junctions.

Direct Electrohydrodynamic Patterning of High-Performance All Metal Oxide Thin-Film Electronics

In this paper, we propose a scalable approach toward all-printed high-performance metal oxide thin-film transistors (TFTs), using a high-resolution electrohydrodynamic (EHD) printing process. Direct EHD micropatterning of metal oxide TFTs is based on diverse precursor solutions to form semiconducting materials (In₂O₃, In-Ga-ZnO (IGZO)), conductive metal oxide (Sn-doped In₂O₃ (ITO)), as well as aluminum oxide (Al₂O₃) gate dielectric at low temperatures. The fully printed TFT devices exhibit excellent electron transport characteristics (average electron mobilities of up to 117 cm² V^-1 s^-1), negligible hysteresis, excellent uniformity, and stable operation at low-operating voltage. Furthermore, integrated logic gates such as NOT and NAND have been printed and demonstrated. All-printed logic with individual gating and symmetric input/output behavior, which is crucial for large-scale integration, is also demonstrated. The devices and fabrication process described in this paper enable high-performance and high-reliability transparent electronics.

Microscale electrohydrodynamic printing of in situ reactive features for patterned ZnO nanorods

Patterning of zinc oxide (ZnO) nanorods has attracted considerable interests to enhance the performance of ZnO-based functional devices. Most of the existing techniques for patterned ZnO nanorods are based on conventional microfabrication methods that commonly require cleanroom environment, high-cost equipment and complicated processes. In this study, electrohydrodynamic (EHD) printing strategy was accommodated to fabricate microscale ZnO nanorods patterns based on in situ reactive inks. Smaller working voltage and larger nozzle-to-collector distance facilitated the formation of thinner PEO-Zn(NO₃)₂ filaments, which were decomposed into ZnO nanoparticles to serve as the seeding template for the hydrothermal growth of ZnO nanorods. The resultant ZnO nanorods can be flexibly tuned by the EHD printed patterns. The effect of growth time on the size and morphology of ZnO nanorods was investigated. Compared with the spin-coating method, the photoelectrochemical property of patterned ZnO nanorods was well controlled and showed enhanced photoelectrochemical stability. The presented method provides a flexible and rapid way to customize patterned ZnO nanorods that can be potentially used in the fields of optical detectors, biosensors or solar-driven devices.

Three-Dimensional, High-Resolution Printing of Carbon Nanotube/Liquid Metal Composites with Mechanical and Electrical Reinforcement

The formation of three-dimensional (3D) interconnections is essential in integrated circuit packaging technology. However, conventional interconnection methods, including the wire-bonding process, were developed for rigid structures of electronic devices, and they are not applicable to the integration of soft and stretchable electronic devices. Hence, there is a strong demand for 3D interconnection technology that is applicable to soft, stretchable electronic devices. Herein, we introduce the material and the processing required for stretchable 3D interconnections on the soft forms of devices and substrates with high resolutions. Liquid-metal-based composites for use as stretchable interconnection materials were developed by uniformly dispersing Pt-decorated carbon nanotubes in a liquid metal matrix. The inclusion of carbon nanotubes in the liquid metal improves the mechanical strength of the composite, thereby overcoming the limitation of the liquid metal that has a low mechanical strength. The composites can be 3D printed with various dimensions: the minimum diameters are about 5 μm and have a breakdown current density comparable to that of metal wires.

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.

Solution-processed metal-oxide thin-film transistors: a review of recent developments

Driven by the rapid development of novel active-matrix displays, thin-film transistors (TFTs) based on metal-oxide (MO) semiconductors have drawn great attention during recent years. N-type MO TFTs manufactured through vacuum-based processes have the advantages of higher mobility compared to the amorphous silicon TFTs, better uniformity and lower processing temperature compared to the polysilicon TFTs, and visible light transparency which is suitable for transparent electronic devices, etc. However, the fabrication cost is high owing to the expensive and complicated vacuum-based systems. In contrast, solution process has the advantages of low cost, high throughput, and easy chemical composition control. In the first part of this review, a brief introduction of solution-processed MO TFTs is given, and the main issues and challenges encountered in this field are discussed. The recent advances in channel layer engineering to obtain the state-of-the-art solution-processed MO TFTs are reviewed and summarized. Afterward, a detailed discussion of the direct patterning methods is presented, including the direct photopatterning and printing techniques. Next, the effect of gate dielectric materials and their interfaces on the performance of the resulting TFTs are surveyed. The last topic is the various applications of solution-processed MO TFTs, from novel displays to sensing, memory devices, etc. Finally, conclusions are drawn and future expectations for solution-processed MO TFTs and their applications are described.

Sessile droplets containing carbon nanotubes: a study of evaporation dynamics and CNT alignment for printed electronics

Carbon nanotubes (CNTs) are 1-dimensional (1D) and flexible nanomaterials with high electric conductivity and a high aspect ratio. These features make CNTs highly suitable materials for the fabrication of flexible electronics. CNTs can also be made into dispersions which can be used as the feedstock material for droplet-based 3D printing technologies, e.g., inkjet printing and aerosol jet printing to fabricate printed electronics. These printing techniques involve several physical processes including deposition of ink droplets on flexible polymeric substrates such as polyimides, evaporation of the solvent and formation of thin films of CNTs, all of which have not been thoroughly investigated. Besides, alignment of the CNTs in the resultant thin films dictates their electrical performance. In this work, we examine the effect of substrate temperature and CNT concentration on the evaporation dynamics and also the alignment in the deposition patterns. Evaporation-driven self-assembly of CNTs and their preferential alignment are observed. Image analysis and Raman spectroscopy are utilised to evaluate the degree of alignment of the CNT network. It is found that the contact line dynamics depends greatly on the CNT concentration. Besides, the substrate temperature plays a significant role in determining the order of the CNTs in the drying deposition pattern. Our findings show the possibility of controlling the film morphology and the degree of alignment of CNTs for printed electronics in the printing process.

Recent Advances in Transparent Electronics with Stretchable Forms

Advances in materials science and the desire for next-generation electronics have driven the development of stretchable and transparent electronics in the past decade. Novel applications, such as smart contact lenses and wearable sensors, have been introduced with stretchable and transparent form factors, requiring a deeper and wider exploration of materials and fabrication processes. In this regard, many research efforts have been dedicated to the development of mechanically stretchable, optically transparent materials and devices. Recent advances in stretchable and transparent electronics are discussed herein, with special emphasis on the development of stretchable and transparent materials, including substrates and electrodes. Several representative examples of applications enabled by stretchable and transparent electronics are presented, including sensors, smart contact lenses, heaters, and neural interfaces. The current challenges and opportunities for each type of stretchable and transparent electronics are also discussed.

Effect of Process Parameters on Organic Micro Patterns Fabricated on a Flexible Substrate Using the Near-Field Electrohydrodynamic Direct-Writing Method

A micro pattern is a key component of various functional devices. In the present study, using the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) mixed material as the direct-writing solution and photographic paper as the flexible insulating substrate, the organic micro patterns of various shapes, such as the curve of the second-order self-similar structure, the helical curve, and the wave curve, were fabricated on the flexible insulating substrate by using the near-field electrohydrodynamic direct-writing method. The effects of process parameters, such as the applied voltage, direct-writing height, flow rate of the injection system, and moving velocity of the substrate, on the width and the conductivity of the organic micro patterns were studied in the near-field electrohydrodynamic direct-writing process. The results show that the width of an organic micro pattern increases with the increase of the applied voltage of the high-voltage power supplier and the flow rate of the injection system under the condition where the three other process parameters remained constant, respectively, while the width of an organic micro pattern decreases with the increase of the direct-writing height and the moving velocity of the flexible substrate, respectively. The fabricated organic microcircuit patterns of the natural drying in air at room temperature were tested by a thin film thermoelectric tester at a detection temperature. The results show that the conductivity of a fabricated organic micro pattern decreases with the increase of the electric field intensity, while the effect of moving velocity and the flow rate on the conductivity is small under the condition where the three other process parameters remained constant.

Highly Stretchable Capacitive Sensor with Printed Carbon Black Electrodes on Barium Titanate Elastomer Composite

Wearable electronics and soft robotics are emerging fields utilizing soft and stretchable sensors for a variety of wearable applications. In this paper, the fabrication of a highly stretchable capacitive sensor with a printed carbon black/Ecoflex interdigital capacitor is presented. The highly stretchable capacitive sensor was fabricated on a substrate made from barium titanate⁻EcoflexTM 00-30 composite, and could withstand stretching up to 100%. The designed highly stretchable capacitive sensor was robust, and showed good repeatability and consistency when stretched and relaxed for over 1000 cycles.

Solution-Based Micro- and Nanoscale Metal Oxide Structures Formed by Direct Patterning for Electro-Optical Applications

Due to their transparency and tunable electrical, optical, and magnetic properties, metal oxide thin films and structures have many applications in electro-optical devices. In recent years, solution processing combined with direct-patterning techniques such as micro-/nanomolding, inkjet printing, e-jet printing, e-beam writing, and photopatterning has drawn much attention because of the inexpensive and simple fabrication process that avoids using capital-intensive vacuum deposition systems and chemical etching. Furthermore, practical applications of solution direct-patterning techniques with metal oxide structures are demonstrated in thin-film transistors and biochemical sensors on a wide range of substrates. Since direct-patterning techniques enable low-cost fabrication of nanoscale metal oxide structures, these methods are expected to accelerate the development of nanoscale devices and systems based on metal oxide components in important application fields such as flexible electronics, the Internet of Things (IoT), and human health monitoring. Here, a review of the fabrication procedures, advantages, limitations, and applications of the main direct-patterning methods for making metal oxide structures is presented. The goal is to highlight the examples with the most promising perspective from the recent literature.

Recent Advances of Solution-Processed Metal Oxide Thin-Film Transistors

Solution-processed metal oxide thin-film transistors (TFTs) are considered as one of the most promising transistor technologies for future large-area flexible electronics. This work surveys the recent advances in solution-processed metal oxide TFTs, including n-type oxide semiconductors, oxide dielectrics, and p-type oxide semiconductors. We first deliver a review on the history and present status of metal oxide TFTs. Then, we present the recent progress in solution-processed n-type oxide semiconductors, with a special focus on low-temperature and large-area solution-based approaches as well as emerging nondisplay applications. Next, we give a detailed analysis of the state-of-the-art solution-processed oxide dielectrics for low-power electronics. We further discuss the recent advances in solution-based p-type oxide semiconductors, which will enable the highly desirable future low-cost large-area complementary circuits. Finally, we draw conclusions and outline the perspectives over the research field.

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 performance printed oxide field-effect transistors processed using photonic curing

Oxide semiconductors are highly promising candidates for the most awaited, next-generation electronics, namely, printed electronics. As a fabrication route for the solution-processed/printed oxide semiconductors, photonic curing is becoming increasingly popular, as compared to the conventional thermal curing method; the former offers numerous advantages over the latter, such as low process temperatures and short exposure time and thereby, high throughput compatibility. Here, using dissimilar photonic curing concepts (UV-visible light and UV-laser), we demonstrate facile fabrication of high performance In₂O₃ field-effect transistors (FETs). Beside the processing related issues (temperature, time etc.), the other known limitation of oxide electronics is the lack of high performance p-type semiconductors, which can be bypassed using unipolar logics from high mobility n-type semiconductors alone. Interestingly, here we have found that our chosen distinct photonic curing methods can offer a large variation in threshold voltage, when they are fabricated from the same precursor ink. Consequently, both depletion and enhancement-mode devices have been achieved which can be used as the pull-up and pull-down transistors in unipolar inverters. The present device fabrication recipe demonstrates fast processing of low operation voltage, high performance FETs with large threshold voltage tunability.

Nanoscale coaxial focused electrohydrodynamic jet printing

Controlled patterning of nanostructures at desired positions is of great importance for high-performance M/NEMS devices. Here, we demonstrate a high-resolution, high-speed and cost-effective fabrication method, named coaxial focused electrohydrodynamic jet printing, to print functional nanostructures. A coaxial needle was designed and developed; a functional ink and high viscosity liquid are applied in the inner and outer needle, respectively. Under optimised conditions, a stable coaxial jet is formed; then, the electrical shearing force and electrical field induce viscous shearing force and internal pressure that are jointly applied on the inner functional ink, focusing the inner jet on the nanoscale. Using this stable coaxial jet with a nano-jet inside it, nanostructures with highly aligned nanowire arrays, nano-freebeams and nano-cantilever beams down to the scale of 40 nm were directly printed. The needle size was 130 μm, and the ratio of the sizes of the needle and the printed structure was as high as 3250/1. This technique realizes the controllable printing of nanoscale structures with the use of a one hundred micrometer-sized needle. The printed PZT nanostructures exhibit pure perovskite structures and distinct piezoelectric responses.

Facile Synthesis of Pt-Functionalized Meso/Macroporous SnO2 Hollow Spheres through in Situ Templating with SiO2 for H2S Sensors

Although single-nozzle electrospraying seems a versatile technique in the synthesis of spherical semiconducting metal oxide structures, the synthesized structures find limited use in gas-sensing applications because of their thick and dense morphology, which minimizes the accessibility of their inner surfaces. Herein, unprecedented spherical SiO₂@SnO₂ core-shell structures are synthesized upon calcination of single-nozzle as-electrosprayed spheres (SPs) containing tin (Sn) and silicon (Si) precursors. Subsequent etching of SiO₂ in NaOH (pH 12) affords meso/macroporous SnO₂ hollow spheres (HSPs) with short diffusion length (31.4 ± 3.1 nm), small crystallites (15.5 nm), and large Brunauer-Emmett-Teller surface area (124.8 m² g^-1). Apart from surface meso/macropores, diffusion of gases into porous SnO₂ sensing layers is realized through inner interconnection of voids of the SnO₂ HSPs into a three-dimensional network. Functionalization of the postetched SnO₂ HSPs with platinum (Pt) nanoparticles at 0.08 wt % yields gas-sensing materials with outstanding response ( R_a/ R_g = 1.6, 10.8, and 105.1-0.1, 1, and 5 ppm of H₂S, respectively) and selectivity toward H₂S against interfering gas molecules at 250 °C. The SiO₂ phase in the postcalcined SiO₂@SnO₂ SPs acts as a sacrificial template for pore creation and crystal growth inhibition, whereas the small amount of SiO₂ residues in HSPs enhances the selectivity.

Optimized inkjet-printed silver nanoparticle films: theoretical and experimental investigations

We study the influence of inkjet printing scheme and sintering parameter on the electrical resistivity of multi-layer silver nanoparticle films. A central composite Design Of Experiments (DOE) is employed to maximize experimental efficiency and improve the statistical significance of parameter estimates. The resulting mathematical correlations allow to interpret the influence of the print and sintering parameters. Detailed inspection of the correlations reveals the existence of local extrema and indicates that a structured approach such as the DOE would be significantly more effective for fabricating films with a minimum of resistivity. Furthermore, we modify the well-known Fuchs-Sondheimer Mayadas-Shatzkes model to correlate the resistivity of a multi-layer nanoparticle film with the sintering temperature and time. The modified model uses literature data but one constant inferred from two experiments. After model adjustment, the resistivities of films fabricated with different parameters can be predicted with good accuracy. This validation tremendously increases applicability and relevance of the model.

High-Resolution Inkjet-Printed Oxide Thin-Film Transistors with a Self-Aligned Fine Channel Bank Structure

A self-aligned inkjet printing process has been developed to construct small channel metal oxide (a-IGZO) thin-film transistors (TFTs) with independent bottom gates on transparent glass substrates. Poly(methylsilsesquioxane) was used to pattern hydrophobic banks on the transparent substrate instead of commonly used self-assembled octadecyltrichlorosilane. Photolithographic exposure from backside using bottom-gate electrodes as mask formed hydrophilic channel areas for the TFTs. IGZO ink was selectively deposited by an inkjet printer in the hydrophilic channel region and confined by the hydrophobic bank structure, resulting in the precise deposition of semiconductor layers just above the gate electrodes. Inkjet-printed IGZO TFTs with independent gate electrodes of 10 μm width have been demonstrated, avoiding completely printed channel beyond the broad of the gate electrodes. The TFTs showed on/off ratios of 10⁸, maximum mobility of 3.3 cm² V^-1 s^-1, negligible hysteresis, and good uniformity. This method is conductive to minimizing the area of printed TFTs so as to the development of high-resolution printing displays.

An Artificial Flexible Visual Memory System Based on an UV-Motivated Memristor

For the mimicry of human visual memory, a prominent challenge is how to detect and store the image information by electronic devices, which demands a multifunctional integration to sense light like eyes and to memorize image information like the brain by transforming optical signals to electrical signals that can be recognized by electronic devices. Although current image sensors can perceive simple images in real time, the image information fades away when the external image stimuli are removed. The deficiency between the state-of-the-art image sensors and visual memory system inspires the logical integration of image sensors and memory devices to realize the sensing and memory process toward light information for the bionic design of human visual memory. Hence, a facile architecture is designed to construct artificial flexible visual memory system by employing an UV-motivated memristor. The visual memory arrays can realize the detection and memory process of UV light distribution with a patterned image for a long-term retention and the stored image information can be reset by a negative voltage sweep and reprogrammed to the same or an other image distribution, which proves the effective reusability. These results provide new opportunities for the mimicry of human visual memory and enable the flexible visual memory device to be applied in future wearable electronics, electronic eyes, multifunctional robotics, and auxiliary equipment for visual handicapped.

The effect of surfactants on electrohydrodynamic jet printing and the performance of organic field-effect transistors

In this article, we report on the direct writing of multi-walled carbon nanotube (MWCNT) composite inks based on three different surfactants via the electrohydrodynamic (EHD) jet printing technique.

Review of Recent Inkjet-Printed Capacitive Tactile Sensors

Inkjet printing is an advanced printing technology that has been used to develop conducting layers, interconnects and other features on a variety of substrates. It is an additive manufacturing process that offers cost-effective, lightweight designs and simplifies the fabrication process with little effort. There is hardly sufficient research on tactile sensors and inkjet printing. Advancements in materials science and inkjet printing greatly facilitate the realization of sophisticated tactile sensors. Starting from the concept of capacitive sensing, a brief comparison of printing techniques, the essential requirements of inkjet-printing and the attractive features of state-of-the art inkjet-printed tactile sensors developed on diverse substrates (paper, polymer, glass and textile) are presented in this comprehensive review. Recent trends in inkjet-printed wearable/flexible and foldable tactile sensors are evaluated, paving the way for future research.

3D polymer objects with electronic components interconnected via conformally printed electrodes

We report the fabrication of 3D polymer objects that contain electrical components interconnected by conductive silver/carbon nanotube inks printed conformally onto their surfaces and through vertical vias. Electrical components are placed within internal cavities and recessed surfaces of polymer objects produced by stereolithography. Conformally printed electrodes that interconnect each electrical component exhibit a conductivity of ∼2 × 10⁴ S cm^-1 upon annealing at temperatures below 100 °C. Multiple 3D objects were created to demonstrate this hybrid additive manufacturing approach, including those with an embedded circuit operated by an air-suspended switch and a 3D circuit board composed of microcontroller unit, resistor, battery, light-emitting diode and sensor.

Smart Sensor Systems for Wearable Electronic Devices

Wearable human interaction devices are technologies with various applications for improving human comfort, convenience and security and for monitoring health conditions. Healthcare monitoring includes caring for the welfare of every person, which includes early diagnosis of diseases, real-time monitoring of the effects of treatment, therapy, and the general monitoring of the conditions of people's health. As a result, wearable electronic devices are receiving greater attention because of their facile interaction with the human body, such as monitoring heart rate, wrist pulse, motion, blood pressure, intraocular pressure, and other health-related conditions. In this paper, various smart sensors and wireless systems are reviewed, the current state of research related to such systems is reported, and their detection mechanisms are compared. Our focus was limited to wearable and attachable sensors. Section 1 presents the various smart sensors. In Section 2, we describe multiplexed sensors that can monitor several physiological signals simultaneously. Section 3 provides a discussion about short-range wireless systems including bluetooth, near field communication (NFC), and resonance antenna systems for wearable electronic devices.

All Inkjet-Printed Metal-Oxide Thin-Film Transistor Array with Good Stability and Uniformity Using Surface-Energy Patterns

An array of inkjet-printed metal-oxide thin-film transistors (TFTs) is demonstrated for the first time with the assistance of surface-energy patterns prepared by printing pure solvent to etch the ultrathin hydrophobic layer. The surface-energy patterns not only restrained the spreading of inks but also provided a facile way to regulate the morphology of metal oxide films without optimizing ink formulation. The fully printed InGaO TFT devices in the array exhibited excellent electron transport characteristics with a maximum mobility of 11.7 cm² V^-1 s^-1, negligible hysteresis, good uniformity, and good stability under bias stress. The new route lights a general way toward fully inkjet-printed metal-oxide TFT arrays.

3D-Printed OFETs of the 1,4-bis(3-phenylquinoxalin-2-yl)benzene-based polymer semiconductors

In this study, we polymerized a 1,4-bis(3-phenylquinoxalin-2-yl)benzene unit with DPP and isoindigo units to produce four new polymers and deeply investigated the influence of DPP and isoindigo units on the semiconductor characteristics, band gap, and orientation properties of these polymers.

Inkjet printing and rapid ebeam sintering enable formation of highly conductive patterns in roll to roll process

An illustration of an inkjet printed silver patterns on plastic substrate, sintered by electron beam irradiation in a roll-to-roll process.