Fabrication of transistors on flexible substrates: from mass-printing to high-resolution alternative lithography strategies

Synthesis of micro-crosslinked adamantane-containing matrix resins designed for deep-UV lithography resists and their application in nanoimprint lithography

A certain type of photoresist used for deep-UV lithography (DUVL) can also be used for other types of photolithography. Thus, to meet the requirements of two or more lithography technologies simultaneously, it is necessary to design a variety of corresponding functional groups in the molecules of materials and obtain the required properties. Herein, we designed four matrix resins based on acrylate for DUVL, employing alkyl sulfide, adamantane, methyladamantane, and hydroxyl as dangling groups and a microcrosslinking network by adding a small amount of crosslinker. These polymers were used in the thermal nanoimprint lithography (NIL) process, and distinct patterns with a resolution of 100 nm were observed. The acrylate copolymers designed for DUVL in this work can be used as thermal NIL resists and to obtain good patterns. It was found that ethylene dimethacrylate (EDMA) and adamantane endowed the matrix resins with good thermal stability and that PMMHM demonstrated the best patterning performance among the four resins. These polymers can be applied in the manufacturing of high-density integrated circuits, nano-transistors, optoelectronic devices and other components in the future.

Printable devices for neurotechnology

Printable electronics for neurotechnology is a rapidly emerging field that leverages various printing techniques to fabricate electronic devices, offering advantages in rapid prototyping, scalability, and cost-effectiveness. These devices have promising applications in neurobiology, enabling the recording of neuronal signals and controlled drug delivery. This review provides an overview of printing techniques, materials used in neural device fabrication, and their applications. The printing techniques discussed include inkjet, screen printing, flexographic printing, 3D printing, and more. Each method has its unique advantages and challenges, ranging from precise printing and high resolution to material compatibility and scalability. Selecting the right materials for printable devices is crucial, considering factors like biocompatibility, flexibility, electrical properties, and durability. Conductive materials such as metallic nanoparticles and conducting polymers are commonly used in neurotechnology. Dielectric materials, like polyimide and polycaprolactone, play a vital role in device fabrication. Applications of printable devices in neurotechnology encompass various neuroprobes, electrocorticography arrays, and microelectrode arrays. These devices offer flexibility, biocompatibility, and scalability, making them cost-effective and suitable for preclinical research. However, several challenges need to be addressed, including biocompatibility, precision, electrical performance, long-term stability, and regulatory hurdles. This review highlights the potential of printable electronics in advancing our understanding of the brain and treating neurological disorders while emphasizing the importance of overcoming these challenges.

Rapid Fabrication of High-Resolution Flexible Electronics via Nanoparticle Self-Assembly and Transfer Printing

Printed electronic technology serves as a key component in flexible electronics and wearable devices, yet achieving compatibility with both high resolution and high efficiency remains a significant challenge. Here, we propose a rapid fabrication method of high-resolution nanoparticle microelectronics via self-assembly and transfer printing. The tension gradient-electrostatic attraction composite-induced nanoparticle self-assembly strategy is constructed, which can significantly enhance the self-assembly efficiency, stability, and coverage by leveraging the meniscus Marangoni effect and the electric double-layer effect. The close-packed nanoparticle self-assembly layer can be rapidly formed on microstructure surfaces over a large area. Inspired by ink printing, a transfer printing strategy is further proposed to transform the self-assembly layer into conformal micropatterns. Large-area, high-resolution (2 μm), and ultrathin (1 μm) nanoparticle microelectronics can be stably fabricated, yielding a significant improvement over fluid printing methods. The unique deformability, recoverability, and scalability of nanoparticle microelectronics are revealed, providing promising opportunities for various academic and real applications.

The solvent effect on the morphology and molecular ordering of benzothiadiazole-based small molecule for inkjet-printed thin-film transistors

A small molecule organic semiconductor, D(D'-A-D')₂ comprising benzothiadiazole as an acceptor, 3-hexylthiophene, and thiophene as donors, was successfully synthesized. X-ray diffraction and atomic force microscopy were used to investigate the effect of a dual solvent system with varying ratios of chloroform and toluene on film crystallinity and film morphology via inkjet printing. The film prepared with a chloroform to toluene ratio of 1.5 : 1 showed better performance with improved crystallinity and morphology due to having enough time to control the arrangement of molecules. In addition, by optimizing ratios of CHCl₃ to toluene, the inkjet-printed TFT based on 3HTBTT using a CHCl₃ and toluene ratio of 1.5 : 1 was successfully fabricated and exhibited a hole mobility of 0.01 cm² V^-1 s^-1 due to the improved molecular ordering of the 3HTBTT film.

Towards Digital Twin Implementation in Roll-To-Roll Gravure Printed Electronics: Overlay Printing Registration Error Prediction Based on Printing Process Parameters

Roll-to-roll gravure (R2Rg) has become highly affiliated with printed electronics in the past few years due to its high yield of printed thin-film transistor (TFT) in active matrix devices, and to its low cost. For printing TFTs with multilayer structures, achieving a high-precision in overlay printing registration accuracy (OPRA) is a key challenge to attain the high degree of TFT integration through R2Rg. To address this challenge efficiently, a digital twin paradigm was first introduced in the R2Rg system with an aim to optimize the OPRA by developing a predictive model based on typical input variables such as web tension, nip force, and printing speed in the R2Rg system. In our introductory-level digital twin, errors in the OPRA were collected with the variable parameters of web tensions, nip forces, and printing speeds from several R2Rg printing processes. Subsequently, statistical features were extracted from the input data followed by the training of a deep learning long-short term memory (LSTM) model for predicting machine directional error (MD) in the OPRA. As a result of training the LSTM model in our digital twin, its attained accuracy of prediction was 77%. Based on this result, we studied the relationship between the nip forces and printing speeds to predict the MD error in the OPRA. The results indicated a correlation between the MD error in the OPRA and the printing speed, as the MD error amplitude in the OPRA tended to decline at the higher printing speed.

Eco-friendly screen printing of silver nanowires for flexible and stretchable electronics

Screen printing is a promising route towards high throughput printed electronics. Currently, the preparation of nanomaterial based conductive inks involves complex formulations with often toxic surfactants in the ink's composition, making them unsuitable as an eco-friendly printing technology. This work reports the development of a silver nanowire (AgNW) ink with a relatively low conductive particle loading of 7 wt%. The AgNW ink involves simple formulation and comprises a biodegradable binder and a green solvent with no toxic surfactants in the ink formulation, making it an eco-friendly printing process. The formulated ink is suitable for printing on a diverse range of substrates such as polydimethylsiloxane (PDMS), polyethylene terephthalate (PET), polyimide (PI) tape, glass, and textiles. By tailoring the rheological behaviour of the ink and developing a one-step post-printing process, a minimum feature size of 50 μm and conductivity as high as 6.70 × 106 S m-1 was achieved. Use of a lower annealing temperature of 150 °C makes the process suitable for plastic substrates. A flexible textile heater and a wearable hydration sensor were fabricated using the reported AgNW ink to demonstrate its potential for wearable electronic applications.

High-Frequency Rheological and Piezo-Voltage Waveform Characterization of Inkjet-Printed Polymer-Based Dopant-Source Inks

This work focuses on developing an understanding of the rheological properties of polymer-based dopant-source inks at the timescales relevant to inkjet printing and their corresponding roles in determining the production of defect-free droplets. Ink-specific optimization of printing processes for phosphorus and boron dopant-source inks with different compositions is demonstrated. Rheological flow curves measured by a piezo axial vibrator (PAV) were used to study the changes in complex viscosity (η*) and in the elastic (G') and viscous (G″) components of the shear modulus (G*) with respect to changes in frequency (from f_min = 1 kHz to f_max = 10 kHz) to obtain an insight into the high-frequency behaviour of inks, as well as the effects of temperature (25 °C and 45 °C) and the natural aging time of the inks. Inks demonstrating complex viscosity η*_min ≥ 2 mPas to η*_max ≤ 20 mPas and an elastic modulus G' ≤ 20 Pa, produced droplets with negligible defects. Of the three rheological parameters (η*, G' and G″), the elastic component (G') of the shear modulus was observed to have the greatest significance in determining the stability and homogeneity of ink droplets, thus dictating the quality of the printed structures. The reliability and stability of droplet formation were further investigated through voltage waveform simulation using an oscilloscope.

Ultraviolet Photodetectors Based on Polymer Microwire Arrays toward Wearable Medical Devices

Polymer micro/nanoarchitectures have attracted intense interest for wearable medical applications due to their excellent mechanical flexibility, solution processability, and tunable optoelectronic properties. Based on polymer micro/nanostructures, high-performance ultraviolet (UV) photodetectors can not only functionalize the accurate image sensing but also sustain the biocomfortable flexible devices for real-time health monitoring. The main challenges are focused on the integration of medical wearable devices, which requires large-scale assembly of polymer micro/nanostructures with controlled morphology and strict alignment. Herein, we utilized a confined assembly system through the cautious regulation for the growth of high-quality polymer 1D arrays. UV photodetectors based on these polymer microwire arrays perform a high on/off ratio of 137 and responsivity of 19.1 mA W^-1. Polymer microarray photodetectors facilitate the scale-up fabrication of 14 × 18 multiplexed image sensors for highly accurate capturing the signals of Arabic numerals "1," "2," and "3." Flexible UV photodetectors based on these arrays present excellent flexibility and bending durability, maintaining 97% of their original on/off ratio after 4000 cycles with a 10 mm bending radius. UV photodetection signals were also collected from the attached flexible devices on the back skin of the mouse, demonstrating the great potential in wearable medical photodetection.

Bubble-pen lithography: Fundamentals and applications: Nanoscience: Special Issue Dedicated to Professor Paul S. Weiss

Developing on-chip functional devices requires reliable fabrication methods with high resolution for miniaturization, desired components for enhanced performance, and high throughput for fast prototyping and mass production. Recently, laser-based bubble-pen lithography (BPL) has been developed to enable sub-micron linewidths, in situ synthesis of custom materials, and on-demand patterning for various functional components and devices. BPL exploits Marangoni convection induced by a laser-controlled microbubble to attract, accumulate, and immobilize particles, ions, and molecules onto different substrates. Recent years have witnessed tremendous progress in theory, engineering, and application of BPL, which motivated us to write this review. First, an overview of experimental demonstrations and theoretical understandings of BPL is presented. Next, we discuss the advantages of BPL and its diverse applications in quantum dot displays, biological and chemical sensing, clinical diagnosis, nanoalloy synthesis, and microrobotics. We conclude this review with our perspective on the challenges and future directions of BPL.

Solution-Processed Titanium Dioxide Ion-Gated Transistors and Their Application for pH Sensing

Titanium dioxide (TiO2) is an abundant metal oxide, widely used in food industry, cosmetics, medicine, water treatment and electronic devices. TiO2 is of interest for next-generation indium-free thin-film transistors and ion-gated transistors due to its tunable optoelectronic properties, ambient stability, and solution processability. In this work, we fabricated TiO2 films using a wet chemical approach and demonstrated their transistor behavior with room temperature ionic liquids and aqueous electrolytes. In addition, we demonstrated the pH sensing behavior of the TiO2 IGTs with a sensitivity of ∼48 mV/pH. Furthermore, we demonstrated a low temperature (120°C), solution processed TiO2-based IGTs on flexible polyethylene terephthalate (PET) substrates, which were stable under moderate tensile bending.

Monitoring Symptoms of Infectious Diseases: Perspectives for Printed Wearable Sensors

Infectious diseases possess a serious threat to the world's population, economies, and healthcare systems. In this review, we cover the infectious diseases that are most likely to cause a pandemic according to the WHO (World Health Organization). The list includes COVID-19, Crimean-Congo Hemorrhagic Fever (CCHF), Ebola Virus Disease (EBOV), Marburg Virus Disease (MARV), Lassa Hemorrhagic Fever (LHF), Middle East Respiratory Syndrome (MERS), Severe Acute Respiratory Syndrome (SARS), Nipah Virus diseases (NiV), and Rift Valley fever (RVF). This review also investigates research trends in infectious diseases by analyzing published research history on each disease from 2000-2020 in PubMed. A comprehensive review of sensor printing methods including flexographic printing, gravure printing, inkjet printing, and screen printing is conducted to provide guidelines for the best method depending on the printing scale, resolution, design modification ability, and other requirements. Printed sensors for respiratory rate, heart rate, oxygen saturation, body temperature, and blood pressure are reviewed for the possibility of being used for disease symptom monitoring. Printed wearable sensors are of great potential for continuous monitoring of vital signs in patients and the quarantined as tools for epidemiological screening.

Analyses of pattern quality in roll-to-roll digital maskless lithography with positional errors

In roll-to-roll digital maskless lithography (R2R DML) equipment, it is difficult to achieve high quality, owing to surface deformation that affects the pattern position. To address this issue, we simulated the patterning results of R2R DML to analyze the relationship between positional errors and pattern quality. Errors perpendicular to the pattern direction exhibited a 1.3-2 times greater effect on the linewidth and line edge roughness compared to those parallel to this direction. We confirmed that positioning errors could lead to defects in which the photoresists were not fully exposed. Finally, through simulations, we found that the effect of positional errors could be reduced by controlling the array spot separation length.

Printability of the Screen-Printed Strain Sensor with Carbon Black/Silver Paste for Sensitive Wearable Electronics

Printing technology enables not only high-volume, multipurpose, low-impact, low-cost manufacturing, but also the introduction of flexible electronic devices, such as displays, actuators, and sensors, to a wide range of consumer markets. Consequently, in the past few decades, printed electronic products have attracted considerable interest. Although flexible printed electronic products are attracting increasing attention from the scientific and industrial communities, a systematic study on their sensing performance based on printability has not been reported so far. In this study, carbon black/Ag nanocomposites were utilized as pastes for a flexible wearable strain sensor. The effects of the rheological property of the pastes and the pattern dimensions of the printed electrodes on the sensor’s performance were investigated. Consequently, the printed sensor demonstrated a high gauge factor of 444.5 for an applied strain of 0.6% to 1.4% with a durability of 1000 cycles and a linearity of R2 = 0.9974. The sensor was also stable under tough environmental conditions.

Azopolymer-Based Nanoimprint Lithography: Recent Developments in Methodology and Applications

Nanofabrication on soft polymeric surfaces is an essential process in many fields, for example, chip manufacturing, microfluidics, high efficiency solar cells, and anticounterfeiting. In order to achieve these applications, various nanofabrication methods have been explored. Among them, nanoimprint lithography (NIL) has drawn worldwide attention because of its cheap and fast processability. In this minireview, an overview of azopolymer-based NIL is provided. Since their discovery, azopolymers have demonstrated versatile photoresponsive characteristics due to their unique physical and chemical properties that originate from the photoisomerization of azobenzene chromophores. As such, two aspects are reported in this minireview. On the one hand, various azopolymers showing photofluidization and photoswitchable glass transition temperatures have been developed, thus facilitating methodological advancements in NIL. On the other hand, these on-demand NIL methods provide greater opportunities for azopolymer-based applications, such as templating of optics, directional photo-manipulation of nanopatterns, and micro photo-actuators. Also the challenges are discussed that remain in this field.

Microstructured SiOx/COP Stamps for Patterning TiO2 on Polymer Substrates via Microcontact Printing

Microcontact printing (μCP) techniques have sparked a surge of interests in microfabrication since they help produce arrays on a wide range of target substrates in a facile and efficient manner. Polydimethylsiloxane (PDMS), as a well-established material for stamps, has constraints resulting from its hydrophobicity and softness, and the replication of PDMS stamps usually requires rigid masters or processes using a photoresist. Herein, a novel μCP stamp based on cyclo-olefin polymer (COP) is produced through vacuum ultraviolet (VUV) lithography. 2,4,6,8-Tetramethylcyclotetrasiloxane is selectively deposited at the affinity-patterns on the COP surface, and these patterned siloxane films are converted into SiO_x meanwhile protecting the COP beneath them from the VUV photoetching. By this means, a patterned relief is fabricated on the COP plates, resulting in a hydrophilic SiO_x/COP μCP stamp with punch heights of ∼180 nm. The novelty arises from the simplicity of the master- and photoresist-free microstructuring, and the higher stiffness of SiO_x/COP stamps prevents the deformation during pressing. Finally, an example μCP is given to transfer titania precursor gel and produce TiO₂ micropatterns on flexible polymer substrates. The SiO_x/COP stamps and the μCP of TiO₂ provide simple and cost-effective patterning techniques, which should contribute to the future design and creation of flexible devices.

Advanced Nanomaterials, Printing Processes, and Applications for Flexible Hybrid Electronics

Recent advances in nanomaterial preparation and printing technologies provide unique opportunities to develop flexible hybrid electronics (FHE) for various healthcare applications. Unlike the costly, multi-step, and error-prone cleanroom-based nano-microfabrication, the printing of nanomaterials offers advantages, including cost-effectiveness, high-throughput, reliability, and scalability. Here, this review summarizes the most up-to-date nanomaterials, methods of nanomaterial printing, and system integrations to fabricate advanced FHE in wearable and implantable applications. Detailed strategies to enhance the resolution, uniformity, flexibility, and durability of nanomaterial printing are summarized. We discuss the sensitivity, functionality, and performance of recently reported printed electronics with application areas in wearable sensors, prosthetics, and health monitoring implantable systems. Collectively, the main contribution of this paper is in the summary of the essential requirements of material properties, mechanisms for printed sensors, and electronics.

Printing ZnO Inks: From Principles to Devices

Solution-based printing approaches permit digital designs to be converted into physical objects by depositing materials in a layer-by-layer additive fashion from microscale to nanoscale resolution. The extraordinary adaptability of this technology to different inks and substrates has received substantial interest in the recent literature. In such a context, this review specifically focuses on the realization of inks for the deposition of ZnO, a well-known wide bandgap semiconductor inorganic material showing an impressive number of applications in electronic, optoelectronic, and piezoelectric devices. Herein, we present an updated review of the latest advancements on the ink formulations and printing techniques for ZnO-based nanocrystalline inks, as well as of the major applications which have been demonstrated. The most relevant ink-processing conditions so far explored will be correlated with the resulting film morphologies, showing the possibility to tune the ZnO ink composition to achieve facile, versatile, and scalable fabrication of devices of different natures.

Effect of Core Oligomer Length on the Phase Behavior and Assembly of π-Conjugated Peptides

Biohybrid molecules are a versatile class of materials for controlling the assembly behavior and functional properties of electronically active organics. In this work, we study the effect of the size of the π-conjugated core on the assembly and phase behavior for a series of π-conjugated peptides consisting of oligothiophene cores of defined lengths flanked by sequence-defined peptides (OTX, where X = 4, 5, 6 is the number of thiophene core units). Interestingly, we find that π-conjugated peptides with relatively short OT4 cores assemble into ordered, high aspect ratio, one-dimensional (1D) structures, whereas π-conjugated peptides with longer OT5 and OT6 cores assemble into disordered structures or lower aspect ratio 1D structures depending on assembly conditions. Phase diagrams for assembled materials are experimentally determined as a function of ionic strength, pH, temperature, and peptide concentration, revealing the impact of molecular sequence and π-conjugated core length on assembled morphologies. Molecular dynamics (MD) simulations are further used to probe the origins of microscale differences in assembly that arise from subtle changes in molecular identity. Broadly, our work elucidates the mechanisms governing the assembly of π-conjugated peptides, which will aid in efficient materials processing for soft electronic applications. Overall, these results highlight the complex phase behavior of biohybrid materials, including the impact of molecular sequence on assembly behavior and morphology.

Ink synthesis and inkjet printing of electrostatically stabilized multilayer graphene nanoshells

HYPOTHESIS

Most functional inkjet inks are sterically stabilized nanoparticle dispersions that require a post-printing-process to remove stabilizing materials and gain functionality. This post-process limits material selection and increases fabrication time and complexity for printed devices. By optimizing the electrostatic stability of a carbon nanomaterial dispersed in water or ethylene glycol via pH adjustment, a stable and printable ink should be attainable without a steric stabilizing material and hence the post-process may be avoided.

EXPERIMENTS

The electrostatic stability of multilayer graphene nanoshells (MGNS)-an inexpensive and net carbon-negative nanomaterial-dispersed in water and ethylene glycol was studied by measuring zeta potential as a function of pH and modeling energetic potentials between particles. Requirements for electrical percolation of printed MGNS were analyzed and corroborated with electrical measurements.

FINDINGS

Electrostatic stability improved with increased zeta potential caused by an increased pH. Ionic strength also increased with pH, causing strong destabilization. By increasing zeta potential while minimizing ionic strength, the maximum solid-loading of MGNS in DI water and ethylene glycol was increased up to 20%. For the MGNS solid-loading achieved here, electrical percolation occurs with 20-30 consecutively printed layers producing a resistivity of 30 Ω-cm. The inexpensive, environmentally-friendly MGNS are a promising material for printed, flexible electronics.

Thermal control of SZ2080 photopolymerization in four-beam interference lithography

Photopolymerization by four-beam interference lithography on a preheated SZ2080 sample was explored at different initial temperatures of the sample: 20 °C, 50 °C, 75 °C, 100 °C, 125 °C, and 150 °C, and at exposure times ranging from 0.5 s to 5 s. The average laser power selected was ∼100 mW for the 300 ps duration pulses at a 1 kHz repetition rate. The experimental results demonstrate that the higher initial temperature of the sample positively influences the crosslinking of the patterns. These findings will improve polymerization protocols for multi-beam interference lithography.

Inkjet-printed aligned quantum rod enhancement films for their application in liquid crystal displays

Semiconductor quantum rods (QRs) show a polarized emission, which opens up the possibility of the enhancement of both brightness and color for liquid crystal displays (LCD) in the form of quantum rod enhancement films (QREFs) for LCD backlights. However, the QR alignment over a large area, suitable for displays, is a challenge. Inkjet printing of QREFs, introduced here, allows fabrication of well-aligned, uniform QREFs on photoaligned substrates using optimized QR inks. We observed that the ink composition and printing conditions affect the QR alignment quality significantly. A relative humidity of 50% with an exposure energy of 1 J cm^-2 for the photoalignment process provided optimal conditions for QREFs. We have successfully shown a good QR alignment for 2.5-inch films and were able to align QRs in multiple layers. Thus, fabricated QREFs show a polarization ratio of 7.2 : 1 for the emitted light. These QREFs were combined with a blue LED and deployed as a backlight unit for an LCD which shows a brightness of ∼250 nits with an optical efficiency of ∼8%, reaching an NTSC of 109% in a CIE1976 color space. Thus, these printed QREFs, over a large area, provide an unprecedented increase of 77% in the optical efficiency of the LCDs and simultaneously offer better color performance.

Technologies and Fabrication of Intelligent Packaging for Perishable Products

The preservation of perishable products to maintain their quality is of paramount importance for food safety and security, and is attracting more attention due to increasing concerns regarding food quality, healthcare, and quality of life. Advances in technology and materials in recent years have led to the development and implementation of intelligent packaging for perishable products that can monitor or even control their quality in a supply chain. In this paper, the techniques used in intelligent packaging (i.e., indicators, sensors, and identification technology) and the major printing methods for fabricating electronics (i.e., inkjet printing, screen printing, and gravure printing) are reviewed with a focus on the packaging of perishable products. Although the high manufacturing costs pose a major challenge the commercialization and large-scale deployment of perishable products, it is expected that the technological progresses in printing electronics will significantly reduce the manufacturing cost of intelligent packaging to a threshold of acceptance by markets. In addition, the broad applications of intelligent packaging can facilitate the traction and monitoring of perishable products for better control of the product quality and improved management of the supply chain.

Polymer-Assisted Metal Deposition (PAMD) for Flexible and Wearable Electronics: Principle, Materials, Printing, and Devices

The rapid development of flexible and wearable electronics favors low-cost, solution-processing, and high-throughput techniques for fabricating metal contacts, interconnects, and electrodes on flexible substrates of different natures. Conventional top-down printing strategies with metal-nanoparticle-formulated inks based on the thermal sintering mechanism often suffer from overheating, rough film surface, low adhesion, and poor metal quality, which are not desirable for most flexible electronic applications. In recent years, a bottom-up strategy termed as polymer-assisted metal deposition (PAMD) shows great promise in addressing the abovementioned challenges. Here, a detailed review of the development of PAMD in the past decade is provided, covering the fundamental chemical mechanism, the preparation of various soft and conductive metallic materials, the compatibility to different printing technologies, and the applications for a wide variety of flexible and wearable electronic devices. Finally, the attributes of PAMD in comparison with conventional nanoparticle strategies are summarized and future technological and application potentials are elaborated.

Facile and scalable fabrication of self-assembled Cu architecture with superior antioxidative properties and improved sinterability as a conductive ink for flexible electronics

The inherent susceptibility to oxidation and poor sinterability significantly limit the practical application of Cu-based conductive inks. Most methodologies employed for the inks like organic polymer coatings and inorganic metal deposition are generally ineffective. Herein, we report the design of a novel hierarchical Cu architecture to simultaneously improve the antioxidative and sinterability via a self-passivation mechanism and loose interior structures. The hierarchical Cu architecture was prepared using copper hydroxide, L-ascorbic acid, and polyvinylpyrrolidone in aqueous solution; 40 g Cu were prepared in a scale-up experiment. A possible growth mechanism is proposed, involving the Cu₂O-templated and mediated nucleation and growth of Cu nanocrystals, followed by the PVP-directed electrostatic self-assembly of Cu nanocrystals. The synthesized Cu shows high oxidation resistance after stored in ambient environment for 90 d by self-passivation, wherein the dense oxidized external layer prevented further oxidation of Cu, unlike other antioxidative strategies. In addition, the structure became 2D flake after a simple ball-milling for 10 min of 2000r, thus forming a good conductive network at the temperature of 180 °C. Importantly, no obvious decline in the electrical performance after severe surface oxidation. Although the structure cannot offer excellent conductive performance, but it proposes a new solution for the balance of antioxidative capabilities and good sinterability in Cu nanomaterials, thus facilitating greater utilization of Cu-based conductive inks for emerging flexible electronic applications.

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.

Dynamics of Liquid Transfer from Nanoporous Stamps in High-Resolution Flexographic Printing

Printing of ultrathin layers of polymeric and colloidal inks is critical for the manufacturing of electronics on nonconventional substrates such as paper and polymer films. Recently, we found that nanoporous stamps overcome key limitations of traditional polymer stamps in flexographic printing, namely, enabling the printing of ultrathin nanoparticle films with micron-scale lateral precision. Here, we study the dynamics of liquid transfer between nanoporous stamps and solid substrates. The stamps comprise forests of polymer-coated carbon nanotubes, and the surface mechanics and wettability of the stamps are engineered to imbibe colloidal inks and transfer the ink upon contact with the target substrate. By high-speed imaging during printing, we observe the dynamics of liquid spreading, which is mediated by progressing contact between the nanostructured stamp surface and by the substrate and imbibition within the stamp-substrate gap. From the final contact area, the volume of ink transfer is mediated by rupture of a capillary bridge; and, after rupture, liquid spreads to fill the area defined by a precursor film matching the stamp geometry with high precision. Via modeling of the liquid dynamics, and comparison with data, we elucidate the scale- and rate-limiting aspects of the process. Specifically, we find that the printed ink volume and resulting layer thickness are independent of contact pressure; and that printed layer thickness decreases with retraction speed. Under these conditions, nanoparticle films with controlled thickness in the <100 nm regime can be printed using nanoporous stamp flexography, at speeds commensurate with industrial printing equipment.

A Rapid Thermal Nanoimprint Apparatus through Induction Heating of Nickel Mold

Thermal nanoimprint lithography is playing a vital role in fabricating micro/nanostructures on polymer materials by the advantages of low cost, high throughput, and high resolution. However, a typical thermal nanoimprint process usually takes tens of minutes due to the relatively low heating and cooling rate in the thermal imprint cycle. In this study, we developed an induction heating apparatus for the thermal imprint with a mold made of ferromagnetic material, nickel. By applying an external high-frequency alternating magnetic field, heat was generated by the eddy currents and magnetic hysteresis losses of the ferromagnetic nickel mold at high speed. Once the external alternating magnetic field was cut off, the system would cool down fast owe to the small thermal capacity of the nickel mold; thus, providing a high heating and cooling rate for the thermal nanoimprint process. In this paper, nanostructures were successfully replicated onto polymer sheets with the scale of 4-inch diameter within 5 min.

Inkjet Printing in Liquid Media: Intra-Volumetric Drop Coalescence in Polymers

The tendency to develop flexible and transparent materials has been growing in the last decade. As inkjet printing technology has become a widespread method for the fabrication of functional materials, the investigation of the inkjet printing process seems to be essential with regard to polymers, which are a viscous and flexible media. In this study, we evaluated the dependence of ink drop coalescence on process parameters such as polymer viscosity (controlled by polymerization time), drop spacing and drop speed. The results showed that drop coalescence was mostly influenced by drop speed, while drop spacing and substrate polymer viscosity did not significantly affect the printing results. The presented data are crucial for understanding the processes involved in the fabrication of flexible materials by inkjet printing.

Ambipolar Conjugated Polymers with Ultrahigh Balanced Hole and Electron Mobility for Printed Organic Complementary Logic via a Two-Step CH Activation Strategy

High mobility ambipolar conjugated polymers are seriously absent regardless their great potential for flexible and printed plastic devices and circuits. Here, ambipolar polymers with ultrahigh balanced hole and electron mobility are developed via a two-step CH activation strategy. Diketopyrrolopyrrole-benzothiadiazole-diketopyrrolopyrrole (DBD) and its copolymers with thiophene/selenophene units (short as PDBD-T and PDBD-Se) are used as examples. PDBD-Se exhibits highly efficient ambipolar transport with hole and electron mobility up to 8.90 and 7.71 cm² V^-1 s^-1 in flexible organic field-effect transistors, presenting a milestone for ambipolar copolymer screening. Based on this performance metrics and good solubility, PDBD-Se is investigated as inkjet-printable semiconductor ink for organic complementary logic circuits. Under ambient processing, maximum hole and electron mobilities reach 6.70 and 4.30 cm² V^-1 s^-1 , respectively. Printed complementary inverter and NAND gates with transition voltages near V_DD /2 are fabricated, providing an easy-handling, general material for printed electronics and logic.

A Sequential Process of Graphene Exfoliation and Site-Selective Copper/Graphene Metallization Enabled by Multifunctional 1-Pyrenebutyric Acid Tetrabutylammonium Salt

This paper reports a procedure leading to shear exfoliation of pristine few-layer graphene flakes in water and subsequent site-selective formation of Cu/graphene films on polymer substrates, both of which are enabled by employing the water soluble 1-pyrenebutyric acid tetrabutylammonium salt (PyB-TBA). The exfoliation with PyB-TBA as an enhancer leads to as-deposited graphene films dried at 90 °C that are characterized by electrical conductivity of ∼110 S/m. Owing to the good affinity of the tetrabutylammonium cations to the catalyst PdCl₄^2-, electroless copper deposition selectively in the graphene films is initiated, resulting in a self-aligned formation of highly conductive Cu/graphene films at room temperature. The excellent solution-phase and low-temperature processability, self-aligned copper growth, and high electrical conductivity of the Cu/graphene films have permitted fabrication of several electronic circuits on plastic foils, thereby indicating their great potential in compliant, flexible, and printed electronics.

Pure, Size Tunable ZnO Nanocrystals Assembled into Large Area PMMA Layer as Efficient Catalyst

Here, we demonstrate for the first time a strategy to self-assemble ZnO nanoparticles (NP) on a large area by a facile one-step process. First, rough and random ZnO nanocrystals (NC), were produced by free-stabilizing aqueous synthesis. Therefore, a post thermal treatment at various temperatures ranging from 80 to 800 °C was necessary to obtain size-tunable and photoluminescent crystalline NP. The fabricated NP had both efficient UV photoluminescence and photocatalytic activity by photo-degradation of Methylene Blue (MB) dye. The annealed NP showed an absorption blue shift in the UV region with decreasing size. This shift was attributed to high quantum confinement effect since ZnO NP diameter reached values lower than the Bohr radius of ZnO (~2.7 nm). The photocatalytic activity displayed dependency on the particle’s size, number, and crystallinity. Subsequently, the NP were self-assembled inside poly(methyl methacrylate) (PMMA) nanoholes. Subsequently, large area substrate of homogenous properties ZnO NP was obtained. Moreover, the synthesis facility, photoemission and photocatalytic properties of ZnO NP could be a new insight into the realization of high performance and low cost UV laser devices.

Graphene inks for printed flexible electronics: Graphene dispersions, ink formulations, printing techniques and applications

Graphene inks have recently enabled the dramatic improvement of printed flexible electronics due to their low cost, ease of processability, higher conductivity and flexibility. In this review, we discuss the state-of-the-art of the fundamental formulation of graphene inks and the current printing techniques used for inks deposition, followed by recent practical applications for printed flexible electronics. The progression of science and technology for the dispersion of graphene using variety of solvents and the characteristics of the resulting conductive inks have been highlighted, with specific emphasis focused on the challenges to be resolved. The printing techniques discussed here include screen printing, gravure printing, inkjet printing and other emerging printing technologies. Each approach's pros and cons are discussed in correlation with the ink formulations and the operating principles. We also discuss the challenges and outlook of graphene ink for its future development in the world of printed flexible devices.

Fast near infrared sintering of silver nanoparticle ink and applications for flexible hybrid circuits

Near infrared(NIR) sintering technology is a photonic sintering approach for metal nanoparticle inks, which can selectively sinter metal nanoparticle inks more quickly and efficiently, and it is also compatible with high-throughput manufacturing processes. In this paper, silver nanoparticle (AgNP) ink sintered by near infrared light at a peak wavelength of 1100 nm was investigated. After only 8 seconds of exposure to NIR irradiation, resistivity of 2.78 μΩ cm was achieved for thin films printed with AgNP ink, which was only 1.7-fold higher than that of bulk silver (1.59 μΩ cm). The structure of the sintered silver film was examined by sintering printed silver nanoparticle ink samples having different thicknesses, and the results showed that AgNPs were homogeneously coalesced throughout the cross-sections of films, indicating the formation of dense silver layers. Furthermore, the morphology and electrical resistivity of the sintered AgNP film dried by NIR were compared with those of the film dried on a hot plate. It was found that drying conditions with a relatively long drying time rather than the drying temperature contributed to the reduction of voids in the film and to the improvement in its density and electrical performance. Finally, a flexible hybrid circuit integrated with a microcontroller chip on a poly(ethylene terephthalate)(PET) substrate was fabricated by screen printing with AgNP ink for interconnects, and its surface roughness and flexibility were investigated.

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.

Electrostatic Assist of Liquid Transfer between Flat Surfaces

Transfer of liquid from one surface to another plays a vital role in printing processes. During liquid transfer, a liquid bridge is formed and subjected to substantial extension but incomplete liquid transfer can produce defects that are detrimental to the operation of printed electronic devices. One strategy for minimizing these defects is to apply an electric field, a technique known as electrostatic assist (ESA). However, the physical mechanisms underlying ESA remain a mystery. To better understand these mechanisms, slender-jet models are developed for both perfect dielectric and leaky dielectric axisymmetric Newtonian liquid bridges with moving contact lines. Nonlinear partial differential equations describing the evolution of the bridge radius and interfacial charge are derived and then solved using finite element methods. For perfect dielectrics, application of an electric field enhances liquid transfer to the more wettable surface over a wide range of capillary numbers. The electric field modifies the pressure differences inside the liquid bridge and, as a consequence, drives liquid toward the more wettable surface. For leaky dielectrics, charge can accumulate at the liquid-air interface. Application of an electric field can augment or oppose the influence of wettability differences, depending on the direction of the electric field and the sign of the surface charge. Flow visualization experiments reveal that when an electric field is applied, more liquid is transferred to the more wettable surface because of a modified bridge shape that causes depinning of the contact line. The measured values of the amount of liquid transferred are in good agreement with predictions of the perfect dielectric model.

Highly Concentrated, Ultrathin Nickel Hydroxide Nanosheet Ink for Wearable Energy Storage Devices

Solution-based techniques are considered as a promising strategy for scalable fabrication of flexible electronics owing to their low-cost and high processing speed. The key to the success of these techniques is dominated by the ink formulation of active nanomaterials. This work successfully prepares a highly concentrated two dimensional (2D) crystal ink comprised of ultrathin nickel hydroxide (Ni(OH)₂ ) nanosheets with an average lateral size of 34 nm. The maximum concentration of Ni(OH)₂ nanosheets in water without adding any additives reaches as high as 50 mg mL^-1 , which can be printed on arbitrary substrates to form Ni(OH)₂ thin films. As a proof-of-concept application, Ni(OH)₂ nanosheet ink is coated on commercialized carbon fiber yarns to fabricate wearable energy storage devices. The thus-fabricated hybrid supercapacitors exhibit excellent flexibility with a capacitance retention of 96% after 5000 bending-unbending cycles, and good weavability with a high volumetric capacitance of 36.3 F cm^-3 at a current density of 0.4 A cm^-3 , and an energy density of 11.3 mWh cm^-3 at a power density of 0.3 W cm^-3 . As a demonstration of practical application, a red light emitting diode can be lighted up by three hybrid devices connected in series.

An Overview of the Development of Flexible Sensors

Flexible sensors that efficiently detect various stimuli relevant to specific environmental or biological species have been extensively studied due to their great potential for the Internet of Things and wearable electronics applications. The application of flexible and stretchable electronics to device-engineering technologies has enabled the fabrication of slender, lightweight, stretchable, and foldable sensors. Here, recent studies on flexible sensors for biological analytes, ions, light, and pH are outlined. In addition, contemporary studies on device structure, materials, and fabrication methods for flexible sensors are discussed, and a market overview is provided. The conclusion presents challenges and perspectives in this field.

Laser-Material Interactions for Flexible Applications

The use of lasers for industrial, scientific, and medical applications has received an enormous amount of attention due to the advantageous ability of precise parameter control for heat transfer. Laser-beam-induced photothermal heating and reactions can modify nanomaterials such as nanoparticles, nanowires, and two-dimensional materials including graphene, in a controlled manner. There have been numerous efforts to incorporate lasers into advanced electronic processing, especially for inorganic-based flexible electronics. In order to resolve temperature issues with plastic substrates, laser-material processing has been adopted for various applications in flexible electronics including energy devices, processors, displays, and other peripheral electronic components. Here, recent advances in laser-material interactions for inorganic-based flexible applications with regard to both materials and processes are presented.

Rapid and conformal coating of polymer resins by airbrushing for continuous and high-speed roll-to-roll nanopatterning: parametric quality controls and extended applications

We present a facile and scalable coating method based on controlled airbrushing, which is suitable for conformal resin coating in continuous roll-to-roll (R2R) nanoimprint lithography (NIL) process. By controlling the concentration of UV-curable polymeric resin with mixing the volatile solvent and its airbrushing time, the coated resin film thickness can be readily tuned. After R2R NIL using a flexible nanoscale line pattern (nanograting) mold is conducted upon the airbrushed resin film, a large-area uniform nanograting pattern is fabricated with controlled residual layer thickness (RLT) based on the initial film thickness. We investigate the faithful airbrushing condition that can reliably create the uniform thin films as well as various nanopatterns with controlled morphologies. Using more diluted resin and shorter airbrushing time can reduce the RLTs favourably for many applications, yet is apt to induce the nanoscale pores and discontinued lines. We also discuss how to further improve the quality and scalability of resin airbrushing and its potential applications particularly requiring high-speed and conformal coating on highly topographic and flexible surfaces.

Tuning the Work Function of Printed Polymer Electrodes by Introducing a Fluorinated Polymer To Enhance the Operational Stability in Bottom-Contact Organic Field-Effect Transistors

Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is a promising electrode material for organic electronic devices due to its high conductivity, good mechanical flexibility, and feasibility of easy patterning with various printing methods. The work function of PEDOT:PSS needs to be increased for efficient hole injection, and the addition of a fluorine-containing material has been reported to increase the work function of PEDOT:PSS. However, it remains a challenge to print PEDOT:PSS electrodes while simultaneously tuning their work functions. Here, we report work function tunable PEDOT:PSS/Nafion source/drain electrodes formed by electrohydrodynamic printing technique with PEDOT:PSS/Nafion mixture solutions for highly stable bottom-contact organic field-effect transistors (OFETs). The surface properties and work function of the printed electrode can be controlled by varying the Nafion ratio, due to the vertical phase separation of the PEDOT:PSS/Nafion. The PEDOT:PSS/Nafion electrodes exhibit a low hole injection barrier, which leads to efficient charge carrier injection from the electrode to the semiconductor. As a result, pentacene-based OFETs with PEDOT:PSS/Nafion electrodes show increased charge carrier mobilities of 0.39 cm²/(V·s) compared to those of devices with neat PEDOT:PSS electrodes (0.021 cm²/(V·s)). Moreover, the gate-bias stress stability of the OFETs is remarkably improved by employing PEDOT:PSS/Nafion electrodes, as demonstrated by a reduction of the threshold voltage shift from -1.84 V to -0.28 V.

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.

Surface-Mediated Solidification of a Semiconducting Polymer during Time-Controlled Spin-Coating

Spin-casting a polymer semiconductor solution over a short period of only a few seconds dramatically improved the molecular ordering and charge transport properties of the resulting semiconductor thin films. In this process, it was quite important to halt spinning before the drying line propagation had begun. Here, we elucidated the effects of the substrate surface characteristics on the drying kinetics during spin-coating, systematically investigated the microstructural evolution during semiconducting polymer solidification, and evaluated the performances of the resulting polymer field-effect transistors. We demonstrated that the spin time required to enhance the molecular ordering and electrical properties of the polythiophene thin films was strongly correlated with the solidification onset time, which was altered by surface treatments introduced onto the substrate surfaces.