Sheet Size-Induced Evaporation Behaviors of Inkjet-Printed Graphene Oxide for Printed Electronics

Deep-Sintered Copper Tracks for Thermal Oxidation Resistance Using Large Pulsed Electron Beam

Thermal oxidation resistance is an important property in printed electronics for sustaining electrical conductivity for long time and/or under harsh environments such as high temperature. This study reports the fabrication of copper nanoparticles (CuNPs)-based conductive tracks using large pulsed electron beam (LPEB) by irradiation on CuNPs to be sintered. With an acceleration voltage of 11 kV, the LPEB irradiation induced deep-sintering of CuNPs so that the sintered CuNPs exhibited bulk-like electrical conductivity. Consequently, the sintered Cu tracks maintained high electrical conductivity at 220 °C without using any thermal oxidation protection additive, such as silver, carbon nanotube, and graphene. In contrast, the films irradiated with an acceleration voltage of 8 kV and irradiated by intense pulsed light (IPL) showed fast oxidation characteristics and a corresponding reduction of electrical conductivities under high temperatures owing to a thin sintered layer. The performance of highly thermal oxidation-resistant Cu films sintered by LPEB irradiations was demonstrated through the device performance of a Joule heater.

Ultraviolet Photodetecting and Plasmon-to-Electric Conversion of Controlled Inkjet-Printing Thin-Film Transistors

Direct ink-jet printing of a zinc-oxide-based thin-film transistor (ZnO-based TFT) with a three-dimensional (3-D) channel structure was demonstrated for ultraviolet light (UV) and visible light photodetection. Here, we demonstrated the channel structures by which temperature-induced Marangoni flow can be used to narrow the channel width from 318.9 ± 44.1 μm to 180.1 ± 13.9 μm via a temperature gradient. Furthermore, a simple and efficient oxygen plasma treatment was used to enhance the electrical characteristics of switching I_ON/I_OFF ratio of approximately 10⁵. Therefore, the stable and excellent gate bias-controlled photo-transistors were fabricated and characterized in detail for ultraviolet (UV) and visible light sensing. The photodetector exhibited a superior photoresponse with a significant increase of more than 2 orders of magnitude larger drain current generated upon UV illumination. The results could be useful for the development of UV photodetectors by the direct-patterning ink-jet printing technique. Additionally, we also have successfully demonstrated that a metal-semiconductor junction structure that enables plasmon energy detection by using the plasmonic effects is an efficient conversion of plasmon energy to an electrical signal. The device showed a significant variations negative shift of threshold voltage under different light power density with exposure of visible light. With the ZnO-based TFTs, only ultraviolet light detection extends to the visible light wavelength.

Experimental study and mechanism analysis on the effect of substrate wettability on graphene sheets distribution morphology within uniform printing droplets

Uniform graphene films and micro-patterns are the cornerstones of graphene-based printed electronics. However, disk-like reduced graphene oxide (RGO) sheets trend to concentrate at the edge of the drop because of the famous coffee-ring effect, resulting in non-uniform patterns. To understand the physics of coffee-ring formation for RGO droplets on hydrophilic substrates, we propose a mechanical model to elucidate the influence and its mechanism of substrate wettability on the solute migration behavior and solute distribution morphology of RGO droplets. Stronger coffee-ring morphology and a slower velocity transition on the PMMA can be observed as compared to that on the glass slides. An explanation based on the mechanical model is provided as the large contact angle on the PMMA leads to a small hindrance force and finally results in more significant coffee-ring morphology. Remarkably, we have revealed one underlying mechanism by which the hydrophilic substrate with better wettability will form more uniform patterns. This study can provide fundamental insights into the relationship between substrate wettability and RGO sheets distribution morphology. It might contribute to morphology regulation of RGO droplets in the DOD printing of graphene films and micro-patterns.

Plasma Jet Printing and in Situ Reduction of Highly Acidic Graphene Oxide

Miniaturization of electronic devices and the advancement of Internet of Things pose exciting challenges to develop technologies for patterned deposition of functional nanomaterials. Printed and flexible electronic devices and energy storage devices can be embedded onto clothing or other flexible surfaces. Graphene oxide (GO) has gained much attention in printed electronics due its solution processability, robustness, and high electrical conductivity in the reduced state. Here, we introduce an approach to print GO films from highly acidic suspensions with in situ reduction using an atmospheric pressure plasma jet. Low-temperature plasma of a He and H₂ mixture was used successfully to reduce a highly acidic GO suspension (pH < 2) in situ during deposition. This technique overcomes the multiple intermediate steps required to increase the conductivity of deposited GO. X-ray spectroscopic studies confirmed that the reaction intermediates and the concentration of oxygen functionalities bonded to GO have been reduced significantly by this approach without any additional steps. Moreover, the reduced GO films showed enhanced conductivity. Hence, this technique has a strong potential for printing conducting patterns of GO for a range of large-scale applications.

Discontinuous contact line motion of evaporating particle-laden droplet on superhydrophobic surfaces

The three-phase contact line motion on a superhydrophobic surface through particle-laden sessile droplet evaporation was investigated. Sample surfaces with micro- and nanoscale structures were generated by various durations of chemical treatment and SiO_{2} spherical particles with different sizes were used as additives of test liquid. The contact angle and contact radius profiles were studied, and the discontinuous motion of those profiles on micro- and nanostructured hierarchical surfaces was observed, while it was not observed on a nanostructured superhydrophobic surface. Suspensions with low particle concentration induced a relatively large contact radius jump compared to the high-concentrated condition; in contrast, the previous report showed the opposite trend for flat surfaces. In order to explain this result, a simple explanation was provided-that the stacked particles at the contact line region suppressed to the deformation of the liquid-vapor interface near the contact line. This is confirmed by side-view images of the deposition results because the contact line region after evaporation of the dense suspension showed a large contact angle compared to that of the diluted suspension. In addition, deposition at the contact line region was observed by scanning electron microscopy to discuss the effect of the characteristic length scale of the surface structure and particles on the contact line motion. We believe that these results will help one to understand the deposition phenomenon during particle-laden droplet evaporation on the superhydrophobic surface and its applications such as evaporation-driven materials deposition.

Controllable Bipolar Doping of Graphene with 2D Molecular Dopants

The fine control of graphene doping levels over a wide energy range remains a challenging issue for the electronic applications of graphene. Here, the controllable doping of chemical vapor deposited graphene, which provides a wide range of energy levels (shifts up to ± 0.5 eV), is demonstrated through physical contact with chemically versatile graphene oxide (GO) sheets, a 2D dopant that can be solution-processed. GO sheets are a p-type dopant due to their abundance of electron-withdrawing functional groups. To expand the energy window of GO-doped graphene, the GO surface is chemically modified with electron-donating ethylene diamine molecules. The amine-functionalized GO sheets exhibit strong n-type doping behaviors. In addition, the particular physicochemical characteristics of the GO sheets, namely their sheet sizes, number of layers, and degree of oxidation and amine functionality, are systematically varied to finely tune their energy levels. Finally, the tailor-made GO sheet dopants are applied into graphene-based electronic devices, which are found to exhibit improved device performances. These results demonstrate the potential of GO sheet dopants in many graphene-based electronics applications.

Inkjet printed highly transparent and flexible graphene micro-supercapacitors

Modern energy storage devices for portable and wearable technologies must fulfill a number of requirements, such as small size, flexibility, thinness, reliability, transparency, manufacturing simplicity and performance, in order to be competitive in an ever expanding market. To this end, a comprehensive inkjet printing process is developed for the scalable and low-cost fabrication of transparent and flexible micro-supercapacitors. These solid-state devices, with printed thin films of graphene flakes as interdigitated electrodes, exhibit excellent performance versus transparency (ranging from a single-electrode areal capacitance of 16 μF cm^-2 at transmittance of 90% to a capacitance of 99 μF cm^-2 at transmittance of 71%). Also, transparent and flexible devices are fabricated, showing negligible capacitance degradation during bending. The ease of manufacturing coupled with their great capacitive properties opens up new potential applications for energy storage devices ranging from portable solar cells to wearable sensors.

Rapid and efficient intense pulsed light reduction of graphene oxide inks for flexible printed electronics

Inkjet-printed GO patterns without additives were reduced by IPL treatment and achieved resistance as low as 760.4 Ω and acceptable flexibility.

Inkjet printed nanomaterial based flexible radio frequency identification (RFID) tag sensors for the internet of nano things

The Internet of Things (IoT) has limitless possibilities for applications in the entire spectrum of our daily lives, from healthcare to automobiles to public safety.

Polycaprolactone-templated reduced-graphene oxide liquid crystal nanofibers towards biomedical applications

Here, we report a facile method to generate electrically conductive nanofibers by coating and subsequently chemically reducing graphene oxide (GO) liquid crystals on a polycaprolactone (PCL) mat.

Improved performances of inkjet-printed poly(3-hexylthiophene) organic thin-film transistors by inserting an ionic self-assembled monolayer

Improved performances of inkjet-printed poly(3-hexylthiophene) organic thin-film transistors have been demonstrated by inserting an ionic self-assembled monolayer.