DOI: 10.1002/admt.201800546 manufacturing processes and relatively high production cost.[12,13] PE has been explored for the manufacturing of flexible and stretchable electronic devices by printing functional inks containing soluble or dispersed materials,[14–16] which has enabled a wide variety of applications, such as transparent conductive films (TCFs), flexible energy harvesting and storage, thin film transistors (TFTs), electroluminescent devices, and wearable sensors.[17–24] The global PE market should reach $26.6 billion by 2022 from $14.0 billion in 2017 at a compound annual growth rate of 13.6%.[25] PE devices are manufactured by a variety of printing technologies. Typical printing technologies can be divided into two broad categories: noncontact patterning (or nozzle-based patterning) and contact-based patterning. The noncontact techniques include inkjet printing, electrohydrodynamic (EHD) printing, aerosol jet printing, and slot die coating, while screen printing, gravure printing, and flexographic printing are examples of the contact techniques. Each of these techniques has its own advantages and disadvantages, but they all rely on the principle of transferring inks to a substrate. Understanding the characteristics and recent advances of each printing technique is important to further the progress in PE. Moreover, to promote the lab-scale printing technologies to large-scale production process, roll-toroll (R2R) printing, which is one of the manufacturing methods to obtain large-area films with low cost and excellent durability, has drawn much attention from both industry and the research community. Nearly all of devices based on PE require conductive structures, interconnects, and contacts; therefore, highly conductive patterns, usually with high transparency and/or high resolution, fabricated by means of printing conductive materials are one of the most critical components in PE devices. Various printable conductive nanomaterials, such as metal nanomaterials (e.g., metal nanoparticles and metal nanowires) and carbon nanomaterials (e.g., graphene and carbon nanotubes (CNTs)), have been explored and used as major materials for PE. Applying printing technology to deposition of the conductive nanomaterials requires formulation of suitable inks. After depositing inks on different substrates, post-printing treatment, Printed electronics is attracting a great deal of attention in both research and commercialization as it enables fabrication of large-scale, low-cost electronic devices on a variety of substrates. Printed electronics plays a critical role in facilitating widespread flexible electronics and more recently stretchable electronics. Conductive nanomaterials, such as metal nanoparticles and nanowires, carbon nanotubes, and graphene, are promising building blocks for printed electronics. Nanomaterial-based printing technologies, formulation of printable inks, post-printing treatment, and integration of functional devices have progressed substantially in the recent years. This review summarizes basic principles and recent development of common printing technologies, formulations of printable inks based on conductive nanomaterials, deposition of conductive inks via different printing techniques, and performance enhancement by using various sintering methods. While this review places emphasis on conductive nanomaterials, the printing techniques and ink formulations can be applied to other materials such as semiconducting and insulating nanomaterials. Moreover, some applications of printed flexible and stretchable electronic devices are reviewed to illustrate their potential. Finally, the future challenges and prospects for printing conductive nanomaterials are discussed.

Printing Conductive Nanomaterials for Flexible and Stretchable Electronics: A Review of Materials, Processes, and Applications

An accurate extraction of physiological and physical signals from human skin is crucial for health monitoring, disease prevention, and treatment. Recent advances in wearable bioelectronics directly embedded to the epidermal surface are a promising solution for future epidermal sensing. However, the existing wearable bioelectronics are susceptible to motion artifacts as they lack proper adhesion and conformal interfacing with the skin during motion. Here, we present ultra-conformal, customizable, and deformable drawn-on-skin electronics, which is robust to motion due to strong adhesion and ultra-conformality of the electronic inks drawn directly on skin. Electronic inks, including conductors, semiconductors, and dielectrics, are drawn on-demand in a freeform manner to develop devices, such as transistors, strain sensors, temperature sensors, heaters, skin hydration sensors, and electrophysiological sensors. Electrophysiological signal monitoring during motion shows drawn-on-skin electronics’ immunity to motion artifacts. Additionally, electrical stimulation based on drawn-on-skin electronics demonstrates accelerated healing of skin wounds. Designing efficient wearable bioelectronics for health monitoring, disease prevention, and treatment, remains a challenge. Here, the authors demonstrate an ultra-conformal, customizable and deformable drawn-on-skin electronics which is robust to motion artifacts and resistant to physical damage.

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Ultra-conformal drawn-on-skin electronics for multifunctional motion artifact-free sensing and point-of-care treatment.

Integrating materials with distinct lattice symmetries and dimensions is an effective design strategy toward realizing novel devices with unprecedented functionalities, but many challenges remain in synthesis and device design Here, a heterojunction memory made of wurtzite ZnO nanorods grown on perovskite Nb-doped SrTiO3 (NSTO) is reported, the electronic properties of which can be drastically reconfigured by applying a voltage and light Despite of the distinct lattice structures of ZnO and NSTO, a consistent nature of single crystallinity is achieved in the heterojunctions via the low-temperature solution-based hydrothermal growth In addition to a high and persistent photoconductivity, the ZnO/NSTO heterojunction diode can be turned into a versatile light-switchable resistive switching memory with highly tunable ON and OFF states The reversible modification of the effective interfacial energy barrier in the concurrent electronic and ionic processes most likely gives rise to the high susceptibility of the ZnO/NSTO heterojunction to external electric and optical stimuli Furthermore, this facile synthesis route is promising to be generalized to other novel functional nanodevices integrating materials with diverse structures and properties

A Versatile Light‐Switchable Nanorod Memory: Wurtzite ZnO on Perovskite SrTiO3

Rod–coil and all-conjugated block copolymers for photovoltaic applications

Low-resistivity metal-semiconductor (M-S) contact is one of the urgent challenges in the research of 2D transition metal dichalcogenides (TMDs). Here, we report a chloride molecular doping technique which greatly reduces the contact resistance (Rc) in the few-layer WS2 and MoS2. After doping, the Rc of WS2 and MoS2 have been decreased to 0.7 kohm*um and 0.5 kohm*um, respectively. The significant reduction of the Rc is attributed to the achieved high electron doping density thus significant reduction of Schottky barrier width. As a proof-ofconcept, high-performance few-layer WS2 field-effect transistors (FETs) are demonstrated, exhibiting a high drain current of 380 uA/um, an on/off ratio of 4*106, and a peak field-effect mobility of 60 cm2/V*s. This doping technique provides a highly viable route to diminish the Rc in TMDs, paving the way for high-performance 2D nano-electronic devices.

Chloride Molecular Doping Technique on 2D Materials: WS2 and MoS2

Semiconducting carbon nanotubes (CNTs) printed into thin films offer high electrical performance, significant mechanical stability, and compatibility with low-temperature processing. Yet, the imple...

Flexible, Print-in-Place 1D-2D Thin-Film Transistors Using Aerosol Jet Printing.

Electronic waste can lead to the accumulation of environmentally and biologically toxic materials and is a growing global concern. Developments in transient electronics—in which devices are designed to disintegrate after use—have focused on increasing the biocompatibility, whereas efforts to develop methods to recapture and reuse materials have focused on conducting materials, while neglecting other electronic materials. Here, we report all-carbon thin-film transistors made using crystalline nanocellulose as a dielectric, carbon nanotubes as a semiconductor, graphene as a conductor and paper as a substrate. A crystalline nanocellulose ink is developed that is compatible with nanotube and graphene inks and can be written onto a paper substrate using room-temperature aerosol jet printing. The addition of mobile sodium ions to the dielectric improves the thin-film transistor on-current (87 μA mm−1) and subthreshold swing (132 mV dec−1), and leads to a faster voltage sweep rate (by around 20 times) than without ions. The devices also exhibit stable performance over six months in ambient conditions and can be controllably decomposed, with the graphene and carbon nanotube inks recaptured for recycling (>95% recapture efficiency) and reprinting of new transistors. We demonstrate the utility of the thin-film transistors by creating a fully printed, paper-based biosensor for lactate sensing. All-carbon thin-film transistors—made using crystalline nanocellulose as a dielectric, carbon nanotubes as a semiconductor and graphene as a conductor—can be printed onto paper substrates and the constituent materials subsequently recycled.

Printable and recyclable carbon electronics using crystalline nanocellulose dielectrics

Room-temperature printing of conductive traces has the potential to facilitate the direct writing of electronic tattoos and other medical devices onto biological tissue, such as human skin. However, in order to achieve sufficient electrical performance, the vast majority of conductive inks require biologically harmful post-processing techniques. In addition, most printed conductive traces will degrade with bending stresses that occur from everyday movement. In this work, water-based inks consisting of high aspect ratio silver nanowires are shown to enable the printing of conductive traces at low temperatures and without harmful post-processing. Moreover, the traces produced from these inks retain high electrical performance, even while undergoing up to 50% bending strain and cyclic bending strain over a thousand bending cycles. This ink has a rapid dry time of less than 2 minutes, which is imperative for applications requiring the direct writing of electronics on sensitive surfaces. Demonstrations of conductive traces printed onto soft, nonplanar materials, including an apple and a human finger, highlight the utility of these new silver nanowire inks. These mechanically robust films are ideally suited for printing directly on biological substrates and may find potential applications in the direct-write printing of electronic tattoos and other biomedical devices.

Silver nanowire inks for direct-write electronic tattoo applications

Scaling Printable Zn–Ag2O Batteries for Integrated Electronics

Semiconducting carbon nanotube (CNT) networks exhibit electrical, mechanical, and chemical properties attractive for thin-film applications, and printing allows for scalable and economically favorable fabrication of CNT thin-film transistors (TFTs). However, device-to-device variation of printed CNT-TFTs remains a concern, which largely stems from variations in printed CNT thin-film morphology and resulting properties. In this work, we overcome the challenges associated with printing uniformity and demonstrate an aerosol jet printing process that yields devices exhibiting a hole mobility of μh = 12.5 cm2/V·s with a relative standard deviation as small as 4% (from over 38 devices). The enabling factors of such high uniformity include control of the CNT ink bath temperature during printing, ink formulation with nonvolatile and viscosifying additives, and a thermal treatment for polymer removal. It is discovered that a low CNT ink temperature benefits aerosol jet printing uniformity and stability in both short-term (∼1 min) and long-term (∼1 h) printing settings. These findings shed light on the effect of a commonly overlooked dimension of CNT aerosol jet printing and provide a practical strategy for large-scale, high-consistency realization of CNT-TFTs.

Nicholas X. Williams

Papers

Flexible, Print-in-Place 1D-2D Thin-Film Transistors Using Aerosol Jet Printing.

Printable and recyclable carbon electronics using crystalline nanocellulose dielectrics

Silver nanowire inks for direct-write electronic tattoo applications

Scaling Printable Zn–Ag2O Batteries for Integrated Electronics

Uniform and Stable Aerosol Jet Printing of Carbon Nanotube Thin-Film Transistors by Ink Temperature Control