Conductive Green Graphene inks for Printed Electronics

Conductive graphene inks have attracted extensive research interests for printed electronics due to the excellent electrical and mechanical properties of graphene. While the traditional organic solvents used for graphene inks will result in high environmental cost, environmentally friendly solvents are required for conductive graphene inks. In this work, we demonstrate a highly conductive aqueous graphene ink printed on a paper substrate using screen printing technique. The well-tuned rheological properties of the graphene ink enable the printed lines to achieve a high resolution as fine as 45 µm in width. After being dried at 70 °C and roller compressed, the printed graphene lines present a high conductivity, up to 4.23 × 104 S m−1 at 3 µm thickness. It also shows an excellent bending stability during the roller compression process, which is compatible for large-scale manufacturing. Combining with inkjet-printed graphene oxide as the sensing layer, the graphene lines are exploited as the electrodes for a fully printed humidity sensor on the paper substrate. Such a low drying temperature, high printing resolution and low-cost printing method offers an example that two-dimensional materials are cooperatively utilized for fully printed flexible electronic devices.

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

Internet of Things (IoT) has become one of the trending technologies in recent years. With the wide applications and rapid development of wireless communication and IoT, there are growing number of requirements on novel device design, such as flexible antennas and tunable antennas. The demand of transparent and flexible wireless communicational devices is ever-increasing for a wide range of scenarios, such as noninvasive healthcare, real-time wearable electronics, etc. Besides, a highly integrated antenna with tunable bandwidth and low energy consumption is in pressing demands in order to reduce the number of antennas and achieve better signal strength and higher speed signal transmission. Graphene materials have drawn much attention due to its extraordinary optical properties, high electrical and thermal conductivity, and excellent mechanical properties. All of these properties underline that graphene can be considered as a highly competitive material for wearable communication devices. Moreover, the electrical resistance of graphene and graphene oxide could be simply changed by external electric fields, applied strain and environment, which alters the electrical characteristics of the graphene-based microwave devices. Accordingly, graphene materials have a great potential in various reconfigurable or tunable devices. Our group fabricated the first graphene-based flexible and transparent wireless antenna with 2D large single-crystalline graphene and 1D AgNW hybrid material based on the features of graphene, such as high carrier mobility, transparency and flexibility, etc. The antenna exhibited a 75% light transmittance and an ultrawide bandwidth ranging from 5.6 to 12.8 GHz, combining with outstanding durability and stability with an almost negligible variation of reflection coefficients over 1000 bending cycles. Further research confirmed that the existence of graphene harmonized the compatibility between antenna and integrated chips, hence improved the total performance of the radio frequency identification (RFID) tags. Graphene/AgNW-based RFID tag was fabricated with an operating UHF band of 850–960 MHz and a reading distance of 130 cm, showing a great commercial potential. The excellent performance and unique properties of graphene/AgNW-based antenna and RFID tag came from the synergistic effect between 1D and 2D nanomaterials, providing a new vision for the design of next generation wireless electronics. Meanwhile, a slot antenna with tunable bandwidth was proposed. The antenna consisted of a planar slot antenna with graphene ink printed at the gap between the ground and a small patch. The graphene ink was made from the mixture of graphene powder and graphene oxide aqueous dispersion. Graphene oxide sheets functioned as effective surfactants rendering assistance to form uniform printable inks, displayed alterable electrical resistance by optimization of mixing proportions between graphene and graphene oxide. Dried graphene inks displayed tunable electrical resistance under applied DC voltages. The variation in the resistance of graphene ink affected radiation of the slot antenna, resulting in a shift of operating bandwidth. The proposed antenna achieved a shifting bandwidth of 0.54 from 6 to 5.46 GHz after applying DC voltage less than 15 V. Such cost-effective, easily acquired, and printable graphene inks integrated the dual advantages of graphene and graphene oxide. It could be further applied in many other fields of printable electronic devices.

Electronic printing is an interdisciplinary technology of traditional printing technology and microelectronics manu- facturing. The development of electronic printing has mostly benefited from the progress of nanomaterials research. Graphene, a novel two-dimensional carbon nanomaterial, has shown excellent electrical, thermal, optical properties in flexible electrical devices. Graphene with traditional metals or polymer materials together could act as the main conductive components in conductive ink. Printing of graphene ink represents a cost-effective deposition technique to obtain patterned conductive graphene films, and further assemble them into functional electrical devices. Therefore, electronic printing based on graphene conductive ink is one of the recent research hotspots. In this paper, we review recent developments and advances in research of graphene conductive ink. This review begins with an introduction of different preparation strategies for graphene conductive inks. For printing, the three major pathways for producing graphene sheets are oxidation-reduction, solvent exfoliation and electrochemical expansion of graphite. Different preparation strategies for conductive inks are classified into three major methods: Graphene inks stabilized by surfactants or functional groups, and graphene-based composite conductive ink. The detailed review of graphene conductive ink preparation is discussed with specific examples. Preparation of graphene conductive inks of high concentrations, stability and printing adaptability is the key issue in electronic printing. Subsequently, an introduction of common printing methods and principles is given. Five printing methods are discussed in this review, including inkjet printing, gravure printing, transfer printing, direct writing and three- dimensional (3D) printing. Printing is kind of additive manufacturing, by depositing graphene onto substrates of various materials, sizes, flexibility and roughness for conductive pattern. Different printing techniques have unique requirements of ink rheological properties. The inkjet printing is becoming the most common technique employed in both academic research and industrial application. The realization of rapid, accurate, simple and controllable printing has important influence on the application of graphene conductive ink. Finally, applications of printed graphene conductive ink in flexible functional devices, including basic electrical circuits, energy storage devices and mechanical/chemical sensing devices, are envisioned. Basic electrical circuits, like flexible conductive patterns, field-effect transistors and radio-frequency circuits, play an important role in the fabrication of wearable devices. Printing also offers a cheap, scalable method of fabricating energy storage devices, including supercapacitor and lithium battery. The unique structure of graphene makes possible the fabrication of different kinds of sensors, including strain, temperature, chemical, electrochemical, photo-electricity sensors and biosensors. An outlook of potential future trends in printing graphene conductive ink research and technology is followed. In summary, printing graphene conductive ink has made many significant advances in a wide range of applications. However, the industrial-level application is still limited, and the preparation and application of graphene conductive ink still need further study. A number of key issues should be solved, including stability of graphene ink, electronic conductivity of printed circuit, limited printing resolution, etc. Overall, electronic printing technology based on graphene conductive ink is not meant as a replacement for microelectronic manufacturing engineering, but instead provides an opportunity to produce large-area flexible electronic devices at low cost. Electronic printing of graphene conductive ink will result in a diverse range of novel applications in many fields, and it calls for more research in the future.

Saturable absorbers (SA) operating at terahertz (THz) frequencies can open new frontiers in the development of passively mode-locked THz micro-sources. Here we report the fabrication of THz SAs by transfer coating and inkjet printing single and few-layer graphene films prepared by liquid phase exfoliation of graphite. Open-aperture z-scan measurements with a 3.5 THz quantum cascade laser show a transparency modulation ∼80%, almost one order of magnitude larger than that reported to date at THz frequencies. Fourier-transform infrared spectroscopy provides evidence of intraband-controlled absorption bleaching. These results pave the way to the integration of graphene-based SA with electrically pumped THz semiconductor micro-sources, with prospects for applications where excitation of specific transitions on short time scales is essential, such as time-of-flight tomography, coherent manipulation of quantum systems, time-resolved spectroscopy of gases, complex molecules and cold samples and ultra-high speed communications, providing unprecedented compactness and resolution.

With the development of wearable electronics, the use of engineered functional inks with printing technologies has attracted attention owing to its potential for applications in low‐cost, high‐throughput, and high‐performance devices. However, the improvement in conductivity and stretchability in the mass production of inks is still a challenge for practical use in wearable applications. Herein, a scalable and efficient fluid dynamics process that produces highly stretchable, conductive, and printable inks containing a high concentration of graphene is reported. The resulting inks, in which the uniform incorporation of exfoliated graphene flakes into a viscoelastic thermoplastic polyurethane is employed, facilitated the screen‐printing process, resulting in high conductivity and excellent electromechanical stability. The electrochemical analysis of a stretchable sodium ion sensor based on a serpentine‐structured pattern results in excellent electrochemical sensing performance even under strong fatigue tests performed by repeated stretching (300% strain) and release cycles. To demonstrate the practical use of the proposed stretchable conductor, on‐body tests are carried out in real‐time to monitor the sweat produced by a volunteer during simultaneous physical stretching and stationary cycling. These functional graphene inks have attractive performance and offer exciting potential for a wide range of flexible and wearable electronic applications.

Laser-induced forward transfer (LIFT) is gaining significant attention as a non-contact printing technique for high-viscosity conductive inks in printed electronics. However, the high wet thickness of printed tracks is essential for achieving effective electrical pathways, a requirement that has not been thoroughly considered so far. The wet thickness is a function of ink viscosity, substrate wettability, and the laser processing parameters. In this study, the printing mechanism of conductive graphene inks with viscosities ranging from 1 to 15 Pa.s using LIFT was investigated. The effects of pulse energy (30 to 120 µJ) and gap distance (50 to 300 μm) in printing voxels with a green nanosecond laser were systematically examined, providing a phenomenological understanding of the material transfer mechanism. The findings highlight the significant role of the temporal pulse distance in enhancing the wet thickness achievable during LIFT of high-viscosity inks, attributed to capillary healing phenomena. Additionally, the acceptor substrates’ hydrophobicity was found to increase the wet thickness and improve the resolution of the printed voxels/tracks. Especially, the aspect ratio of LIFT-printed tracks was increased by more than 175% with 10 printing passes when a hydrophobic accepter was used. So, the optimal LIFT processing conditions were identified to achieve high-quality, high-aspect-ratio tracks, by considering synergistically the effects of the temporal pulse distance and the substrate wettability. Moreover, the resistivity of the LIFT-printed graphene tracks decreased by more than 84% after a 100-minute sintering step at 120 °C. This research advances understanding of LIFT printing high-viscosity conductive inks, particularly underpinning the development of high-resolution and high-aspect-ratio electrical circuits for printed electronics.

This study presents the potentials of 3D printing in the utilization of passive RFID components on a cotton substrate. New types of 3D-printable conductive materials, graphene ink and stretchable silver conductor, are used in antenna fabrication. Our measurement results confirm that both types of wireless components reach high performance: the achievable read ranges of the graphene and stretchable tags are close to 5 and 11 meters, respectively. These results are well comparable to those of previously reported silver and copper textile RFID tags.

We report on the development of terahertz (THz) saturable-absorbers exploiting printable graphene inks. The achieved 80% transparency modulation at 3.5 THz makes these devices potential candidates as passive components for THz solid-state lasers.

Here, we report a new method to prepare graphene from graphite by the liquid phase exfoliation process with sonication using graphene oxide (GO) as a dispersant. It was found that GO nanosheets act a as surfactant to the mediated exfoliation of graphite into a GO-adsorbed graphene complex in the aqueous solution, from which graphene was separated by an additional process. The preparation of isolated graphene from a single to a few layers is routinely achieved with an exfoliation yield of up to higher than 40% from the initial graphite material. The prepared graphene sheets showed a high quality (C/O ∼ 21.5), low defect (ID/IG ∼ 0.12), and high conductivity (6.2 × 10(4) S/m). Moreover, the large lateral size ranging from 5 to 10 μm of graphene, which is believed to be due to the shielding effect of GO avoiding damage under ultrasonic jets and cavitation formed by the sonication process. The thin graphene film prepared by the spray-coating technique showed a sheet resistance of 668 Ω/sq with a transmittance of 80% at 550 nm after annealing at 350 °C for 3 h. The transparent electrode was even greater with the resistance only 66.02 Ω when graphene is deposited on an interdigitated electrode (1 mm gap). Finally, a flexible sensor based on a graphene spray-coating polydimethylsiloxane (PDMS) is demonstrated showing excellent performance working under human touch pressure (<10 kPa). The graphene prepared by this method has some distinct properties showing it as a promising material for applications in electronics including thin film coatings, transparent electrodes, wearable electronics, human monitoring sensors, and RFID tags.

Electrodeposition of graphene nano-thick coating for highly enhanced performance of titanium bipolar plates in fuel cells

2D materials have emerged as new material used in electronic devices. Graphene has attract great attention due to its high charge carrier mobility and thin thickness. Different production methods have been used to produce graphene used in different electronic applications. This work highlights the validity of graphene electronic applications and drawbacks that needed to be overcome in the future. Chemical vapour deposition (CVD) grew graphene show great potential in replacing ITO in touch screen with low sheet resistance, flexibility and high transparency. Screen printed touchpad flexible keyboard can be fabricated directly from graphene inks. Graphene based touch-pad show a good conductive and flexibility while providing stable electrical performance under bending and change of humidity. Mechanical exfoliated graphene exhibits high charge carrier mobility by encapsulated with hexagonal boron nitride (h-BN).

Distinct membrane-permeabilizing interactions of an antiviral alkyl glucoside detergent: Comparison with Triton X-100 and SDS.

The declining world oil and gas reserves along with increasing gas emissions from fossil fuel consumption has received serious attention as an urgent problem worldwide. The alternative solution to the problem uses emulsified fuel which is expected to provide better combustion efficiency and will contribute to emission reductions, such as NOx and particulate matter (PM). The one of most important factor in emulsion fuel is its stability. In this work, the effect of water contents to stability of diesel-water emulsion fuels has been investigated. The stability of emulsion fuel related to coalescence and sedimentation formation. The diesel fuel-water emulsification process was carried out in a standard stirred tank consisting of a cylinder tank (93 mm in diameter) and Rushton disk turbine (40 mm in diameter). Materials used consist of diesel oil (Pertamina DEX), demineralized water, surfactants (Lecithin, Triton X-100 and Tween 80). The surfactant used is a mixture of Lecithin - Triton X-100 (76.6%: 23.4%) and Lecithin - Tween 80 (45.5%: 54.5%) at 1% total volume of emulsion. The impeller set in constant speed at 1900 rpm. To reduce emulsification energy consumption, water is added gradually into the tank at 2 mL/min of flowrate. Water content varies at 5%, 10%, 20%, and 40% by total volume of emulsion. Physical properties of emulsion fuel such as viscosity and density at room temperature were investigated periodically. The characterization of emulsion fuel stability was measured by slope value of absorbance ratio at wavelength 450 nm and 850 nm. Sediment formation was periodically by visual observation as emulsion phase percentage. Emulsion droplet size was measured by Dynamic Light Scattering (DLS) method. Physical properties of emulsions such as density and viscosity tend to constant value in order of time. Emulsion fuel with 5% water content has the largest volume of emulsion phase in the rest of time, 83 % for L-T emulsion fuel and 95% for L-Tx. The lowest slope value shown by 5% emulsion fuel water content, -8.657×10-5 for 5% L-T emulsion fuel and -2,084 × 10-4 for 5% L-Tx emulsion fuel. Droplet size measurement of emulsion shown that different amount of added water caused the different droplet size of emulsion.

Rational design and evaluation of UV curable nano-silver ink applied in highly conductive textile-based electrodes and flexible silver-zinc batteries