The emerging field of printed electronics requires a suite of functional materials for applications including flexible and large-area displays, radio frequency identification tags, portable energy harvesting and storage, biomedical and environmental sensor arrays, and logic circuits.
To enable these technologies, functional materials must be integrated with suitable patterning technologies, such as inkjet, gravure, and flexographic printing. Because electrical conductors are a core component of electronic devices, significant effort has been devoted to conductive materials in the field of printable inks. Common conductive inks can be classified into three categories: noble metals, conductive polymers, and carbon nanomaterials.
Graphene is a 1-atom-thick layer of tightly bonded carbon atoms arranged in a hexagonal lattice. Graphene the world’s first 2D nanomaterial, is widely regarded as the “wonder material” of the 21st century due to the combination of its extraordinary properties. As a single layer of graphite, it is the thinnest material (monoatom thick), transparent, 200 times stronger than steel, yet as flexible as rubber, more conductive than copper, excellent thermal conductor and impermeable to moisture and gases. Graphene is also extraordinarily light at 0.77 mg/m2, which is roughly 1,000 times lighter than 1 m2 of paper. It is fire resistant yet retains heat.
Wearable electronic textiles (e-textiles) have become a focus of significant research interest due to their potential applications in sportswear, military uniforms, environmental monitoring and health care. There have been enormous efforts in incorporating electronic components to make e-textiles for various applications such as sensors, energy storage devices, transistors and photovoltaic devices.
Researchers are exploring the potential of graphene for the fabrication of the next generation e-textiles. Inkjet printing of graphene inks is considered to be very promising for wearable e-textile applications as benefits of both inkjet printing and extra-ordinary electronic, optical and mechanical properties of graphene can be exploited.
Vorbeck came out with their Vor-Ink series, and for the first time one could order a bucket of conductive graphene paint online. Vorbeck also patented graphene for use in conductive inks. The Cambridge Graphene center has partnered with Plastic Logic a leader in organic transistors and plastic electronics. One of the key research directions in this collaboration is printed electronics.
In September 2013, Graphene Platform and Japan’s Nissha Printing partnered to deliver products based on graphene inks developed at University of Cambridge.
In November 2013, IDTechEx recognized the potential of graphene for printed electronics and awarded Durham Graphene Science (DGS) its Best Technical Development Materials Award. DGS is a spinout company of Durham University. The company “won this award for its high specification graphene production process that can seem to scale to large-volume, utilizing a bottom-up synthesis method. DGS, has recently completed the commissioning of its first commercial scale facility, which is capable of producing one tonne of high-purity graphene nanoplatelets per year”.
The different classes of conductive inks offer unique properties suitable for particular applications. Among the noble metals, silver is the most prevalent printed conductor due to its high conductivity and oxidation resistance, and can be printed using inks based on either silver nanoparticles or silver precursors. These inks offer the highest conductivity among printed materials, but are based on expensive precursors.
Copper inks have also been introduced, but typically require core-shell nanoparticle designs or specialized photonic annealing treatments to produce conductive patterns. Conductive polymers, such as PEDOT:PSS , have also been developed for printed electronics applications. These materials offer a modest conductivity at low cost, but are limited in terms of chemical and thermal stability.
Carbon nanomaterials, including carbon nanotubes and graphene, offer a low-cost alternative with excellent environmental stability and desirable conductivity, along with unique properties suitable for a range of applications.
Carbon nanomaterials offer a number of opportunities for printed and flexible electronics. The electrical properties resulting from the sp2-bonded structure of fullerenes, carbon nanotubes, and graphene are particularly promising, and have been exploited in a number of applications from thin-film transistors (TFTs) and electrochemical sensors to supercapacitors and photovoltaics.
Graphene with high charge carrier mobility, superlative thermal and chemical stability and intrinsic flexibility, has been demonstrated for a number of applications in printed electronics including chemical and thermal sensors, micro-supercapacitors, and thin-film transistors.
A fundamental challenge to integrating carbon nanomaterials with conventional printing technologies is the production of inks suitable for various deposition processes. Here, we present recent progress in the development of graphene inks using a polymer stabilizer to enable stable, high-concentration inks of pristine graphene with tunable viscosity and solvent composition.
Scientists develop a new method to revolutionize graphene printed electronics
The development of printed conductive inks for electronic applications has grown rapidly, widening applications in transistors, sensors, antennas RFID tags and wearable electronics. Current conductive inks traditionally use metal nanoparticles for their high electrical conductivity. However, these materials can be expensive or easily oxidised, making them far from ideal for low cost IoT applications.
The team have found that using a material called dihydrolevogucosenone known as Cyrene is not only non-toxic but is environmentally- friendly and sustainable but can also provide higher concentrations and conductivity of graphene ink. Professor Zhiurn Hu said: “This work demonstrates that printed graphene technology can be low cost, sustainable, and environmentally friendly for ubiquitous wireless connectivity in IoT era as well as provide RF energy harvesting for low power electronics”.
Professor Sir Kostya Novoselov said: “Graphene is swiftly moving from research to application domain. Development of production methods relevant to the end-user in terms of their flexibility, cost and compatibility with existing technologies are extremely important. This work will ensure that implementation of graphene into day-to-day products and technologies will be even faster”. Kewen Pan, the lead author on the paper said: “This perhaps is a significant step towards commercialisation of printed graphene technology. I believe it would be an evolution in printed electronics industry because the material is such low cost, stable and environmental friendly”.
The National Physical Laboratory (NPL), who were involved in measurements for this work, have partnered with the National Graphene Institute at The University of Manchester to provide a materials characterisation service to provide the missing link for the industrialisation of graphene and 2-D materials. They have also published a joint NPL and NGI a good practice guide which aims to tackle the ambiguity surrounding how to measure graphene’s characteristics.
Professor Ling Hao said: “Materials characterisation is crucial to be able to ensure performance reproducibility and scale up for commercial applications of graphene and 2-D materials. The results of this collaboration between the University and NPL is mutually beneficial, as well as providing measurement training for Ph.D. students in a metrology institute environment.” Graphene has the potential to create the next generation of electronics currently limited to science fiction: faster transistors, semiconductors, bendable phones and flexible wearable electronics
New graphene based inks for high-speed manufacturing of printed electronics
Researchers at the University of Cambridge in collaboration with Cambridge-based technology company Novalia, have developed a low-cost, high-speed method for printing graphene inks using a conventional roll-to-roll printing process, like that used to print newspapers and crisp packets.
The method allows graphene and other electrically conducting materials to be added to conventional water-based inks and printed using typical commercial equipment, the first time that graphene has been used for printing on a large-scale commercial printing press at high speed. “This method will allow us to put electronic systems into entirely unexpected shapes,” said Chris Jones of Novalia. “It’s an incredibly flexible enabling technology.
Currently, printed conductive patterns use a combination of poorly conducting carbon with other materials, most commonly silver, which is expensive. Silver-based inks cost £1000 or more per kilogram, whereas this new graphene ink formulation would be 25 times cheaper. Additionally, silver is not recyclable, while graphene and other carbon materials can easily be recycled. The new method uses cheap, non-toxic and environmentally friendly solvents that can be dried quickly at room temperature, reducing energy costs for ink curing. Once dry, the ‘electric ink’ is also waterproof and adheres to its substrate extremely well.
Graphene-based ink may lead to printable energy storage devices
Researchers have created an ink made of graphene nanosheets, and demonstrated that the ink can be used to print 3-D structures. As the graphene-based ink can be mass-produced in an inexpensive and environmentally friendly manner, the new methods pave the way toward developing a wide variety of printable energy storage devices.
The researchers, led by Jingyu Sun and Zhongfan Liu at Soochow University and the Beijing Graphene Institute, and Ya-yun Li at Shenzhen University, have published a paper on their work in a recent issue of ACS Nano.
“Our work realizes the scalable and green synthesis of nitrogen-doped graphene nanosheets on a salt template by direct chemical vapor deposition,” Sun told Phys.org. “This allows us to further explore thus-derived inks in the field of printable energy storage.”
As the scientists explain, a key goal in graphene research is the mass production of graphene with high quality and at low cost. Energy-storage applications typically require graphene in powder form, but so far production methods have resulted in powders with a large number of structural defects and chemical impurities, as well as nonuniform layer thickness. This has made it difficult to prepare high-quality graphene inks.
In the new paper, the researchers have demonstrated a new method for preparing graphene inks that overcomes these challenges. The method involves growing nitrogen-doped graphene nanosheets over NaCl crystals using direct chemical vapor deposition, which causes molecular fragments of nitrogen and carbon to diffuse on the surface of the NaCl crystals. The researchers chose NaCl due to its natural abundance and low cost, as well as its water solubility. To remove the NaCl, the coated crystals are submerged in water, which causes the NaCl to dissolve and leave behind pure nitrogen-doped graphene cages. In the final step, treating the cages with ultrasound transforms the cages into 2-D nanosheets, each about 5-7 graphite layers thick.
The resulting nitrogen-doped graphene nanosheets have relatively few defects and an ideal size (about 5 micrometers in side length) for printing, as larger flakes can block the nozzle. To demonstrate the nanosheets’ effectiveness, the researchers printed a wide variety of 3-D structures using inks based on the graphene sheets. Among their demonstrations, the researchers used the ink as a conductive additive for an electrode material (vanadium nitride) and used the composite ink to print flexible electrodes for supercapacitors with high power density and good cyclic stability.
In a second demonstration, the researchers created a composite ink made of the graphene sheets along with binder material (polypropylene) for printing interlayers for Li−S batteries. Compared to batteries with separators made only of the conventional material, those made with the composite material exhibited enhanced conductivity, leading to an overall improvement in battery performance.
“In the future, we plan to exploit the direct chemical vapor deposition technique for the mass production of high-quality graphene powders toward emerging printable energy storage applications,” Sun said.
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