Electronic artwork

A common conductive ink contains metal nanoparticles, such as silver or copper, encapsulated by a thin dispersive material (usually a polymer) and diluted in one or several solvents. Such ‘nano-inks’ require sintering as part of the printing process onto a substrate – heating the nanoparticles to 50–80% of their melting point – so that the nanoparticles fuse together and form a continuous conductive structure. If nanoparticle powders are used, the sintering temperature can be even lower, for example, 20–30% of the melting temperature of the metal; without sintering, nanoparticle inks have poor conductivity. In the process of sintering, solvent and dispersants are removed.

Nanoparticle technology offers a number of advantages for conducting inks. For example, nano-inks can produce electronic circuits with much finer detail than is possible with typical paste-based inks and require lower sintering temperatures, explains Brett Walker, ceo of Electroninks.

The ink in Electroninks’ rollerball pen is based on silver nanoparticles developed initially by Jennifer Lewis’ team at the University of Illinois, US. Silver has many advantages over carbon and copper based inks, explains Walker. For example, it is more conductive than carbon, and will not oxidise at room temperature like copper. But the stand-out feature of this ink, he says, is that the ink dries within seconds, flows freely from the pen and is stable for about a year. Carbon-based inks on the market, he adds, take several hours to dry and must also be shaken and squeezed out of the tube.

Electroninks already sells a variation of its rollerball ink to industry customers who make flexible integrated circuits for uses ranging from smart packaging to biomedical devices. Developed in 2012, this nano-ink comprises a transparent solution of silver acetate and ammonia (JACS, 2012, 134(3), 1419). The silver remains dissolved in the solution until it is printed, when the liquid evaporates, yielding a conductive pattern.

‘For printed electronics applications, you need to be able to store the ink for several months because silver is expensive,’ says Walker. ‘Since silver nanoparticles don’t actually form until the ink exits the nozzle and the ammonia evaporates, our ink remains stable for very long periods. For fine-scale nozzle printing, that’s a rarity.’

Walker says this ink is much faster to make than the rollerball ink and can print through 100nm nozzles, an order of magnitude smaller than particle-based inks, an important feature for printed microelectronics. Moreover, the ink’s low viscosity makes it suitable for ink-jet printing, direct ink writing or airbrush spraying over large areas. Another advantage is its low processing temperature, he adds. The sintering temperatures for many particle-based inks are too high for many inexpensive plastics or paper. By contrast, the silver nano-ink has an electrical conductivity approaching that of pure silver upon sintering at 90°C.

Silver nano-inks already have an established market; they are used in solar cells, keyboards and mobile phones, for example; and the market is predicted to reach $249.4m in 2024, up from $6.2m in 2014, according to report published in March by IDtechEx. This sits within a total global market for conductive inks forecast to be $1875m in 2014 and $2777m in 2024, says report author Khasha Ghaffarzadeh. Emerging products, such as inks based on copper nanoparticles or graphene, are slowly penetrating the market, too. They are either displacing existing products or creating new markets based on their attributes of higher conductivity, inkjet printability, flexibility or enhanced surface smoothness.

Copper-based nano-inks are attracting interest as copper is a far cheaper raw material than silver. Researchers are working on improving their stability in air and reducing the required sintering temperatures, which limits their use and application on heat-sensitive surfaces.

One team led by Shlomo Magdassi, from the Institute of Chemistry and the Center for Nanoscience and Nanotechnology at the Hebrew University of Jerusalem, Israel, claims to have developed oxidation-resistant copper nano-inks that require a low sintering temperature, enabling the printing of low-cost conductive patterns on heat sensitive plastic substrates.

The team is working on two different types of inks: one composed of copper nanoparticles, the other comprising a precursor that deposits sintered copper nanoparticles on printing. In the former, the particles are protected with a polymeric stabiliser; they can be printed by several methods, including inkjet printing; and require sintering at ca 250°C to form the conductive pattern. In the precursor inks, the copper nanoparticles are deposited after printing as a result of decomposition/reduction of the precursor on heating.

The researchers are also working on two different types of precursor inks. Magdassi explains: ‘One is a copper complex-based ink that after printing is converted into a conductive pattern on heating at 140–150°C (under nitrogen); the other comprises copper sub-micron particles, which on printing and heating to ca 180°C (under nitrogen) converts to a conductive pattern.’

‘The copper-based nano-ink invented by Professor Magdassi solves some of the major limitations that are preventing widespread use of conductive inks, and we are certain that this novel ink will become an important aspect of the growing industry of printed electronics,’ says Yaacov Michlin, ceo of Yissum, the R&D spinout of the Hebrew University of Jerusalem.

Recently, Chinese researchers have reported using a copper-based ink to write circuits straight to paper. Wenjun Dong of Zhejiang Sci-Tech University in Hangzhou, and Ge Wang of University of Science and Technology Beijing start with copper nanocrystals that self-assemble into sheets, 30–100μm in diameter and a few hundred nanometres in thickness. The sheets are coated with silver nanocrystals, which prevent the oxidation of copper at high temperatures, and mixed with carboxymethyl cellulose dispersed in a solution of water and methanol to make an ink (ACS Appl. Mater. Interfaces, 2014, 6(1), 622).

The researchers have used the ink in a pen to draw patterns of lines, words and even flowers on ordinary paper. Small LED lights in the drawings lit up when the circuit was connected to a battery. They found the ink maintained 80–90% of its conductivity even when the paper was folded many times and even crumpled up. This, they say, is because the sheets are stacked on top of each other which increases the contact area under stress and maintains conductivity.

Dong and Wang envisage their ink paving the way for a wide range of new bendable gadgets, such as electronic books that look and feel more like traditional paperbacks. And because the method is straightforward and cost-effective, they say it could be used to make other kinds of aqueous metal conductive inks on flexible electronics.

Conducting inks based on graphene are also starting to gain ground in the printed electronics industry, finding their way into smart cards and radio-frequency identification tags – for example, intelligent bar codes that talk to a networked system to track a product.

Graphene, the thinnest allotrope of carbon, consists of carbon atoms arranged on a hexagonal chicken wire-like mesh. Only one atom thick, it is strong, light, nearly transparent and an excellent conductor of electricity. The challenge for graphene inks, however, is that it is difficult to harvest a sufficient amount of graphene without compromising its electronic properties. And when graphene inks are inkjet printed, they lose some of their conductivity. Inkjet printing is inexpensive, capable of printing large areas, and can create patterns on a variety of substrates, making it an attractive option for next-generation electronics.

Recently, researchers at Northwestern University, US, reported producing patterns with a graphene-based inks that are 250 times more conductive than previous attempts (J. Phys. Chem. Lett., 2013, 4(8), 1347). They developed a method to mass-produce graphene that can be carried out at room temperature. They use ethanol and ethyl cellulose to break apart graphite, which minimises residues and results in a powder with a high concentration of nm-sized graphene flakes. This is then mixed with a solvent to create the ink.

‘Graphene has a unique combination of properties that is ideal for next-generation electronics, including high electrical conductivity, mechanical flexibility, and chemical stability,’ explains Mark Hersam, professor of materials science and engineering at Northwestern. ‘By formulating an ink-jet printable ink based on graphene, we now have an inexpensive and scalable path for exploiting these properties in real-world technologies.’

The researchers demonstrated printing the ink in multiple layers, each 14nm thick, to create precise patterns. The ink’s conductivity remains virtually unchanged, they say, even when bent, suggesting that graphene inks could be used to create foldable electronic devices in the future.

Meanwhile, researchers at the University of Cambridge, UK, have designed two prototype devices to demonstrate the potential of their graphene inks made using their proprietary solution processing techniques. Their inks have three advantages, says Tawfique Hasan of the Cambridge Graphene Centre. ‘We use chemically pristine graphite flakes, which are very inexpensive. The [inks] are made without having to use any chemical functionalisation, which typically require strong acids; and the process is done at room temperature.’

Hasan, Felice Torrisi and Andrea Ferrari worked with Cambridge-based print and electronics company Novalia to design a ‘piano’. ‘The idea of the piano was to demonstrate the interfacing capability of our materials with conventional electronics, which are still based on rigid substrates,’ Hasan explains.

The keys of the transparent piano are made from graphene-based inks printed onto a plastic film. They work as electrodes and are connected to a simple electronic circuit-board, a battery and a speaker. The notes are stored in the circuitry. When a person touches a key, the change in the amount of electrical charge it holds is detected and redirected to the circuit. This commands the tiny, flat speaker positioned under the film to play the appropriate note.

The team, this time in collaboration with Printed Electronics, has also developed a flexible prototype digital display. This display uses conventional printable materials, but with a transparent, electrically conductive graphene layer on top. The team says the graphene layer is flexible, and more conductive and transparent than the conventional polymer it replaces, offering potential for use in a wide range of smart packaging applications, such as toys, labelling and board games.

‘Both of these devices show how graphene could be printed onto a whole range of surfaces, which makes it ideal for printed electronics,’ says Hasan. ‘For example, it might eventually be possible to print electronics on to clothing and to make wearable patches to monitor people with health conditions, such as a heart problem.’ Another potential application is cheap, printable sensors, which could be used to track luggage around an airport to ensure it is loaded onto the correct plane, or to follow products across a production and supply chain. The team has started a new spin-off company, Cambridge Graphene Platform, to provide printable inks derived from graphene and other 2D layered materials.

The market for conductive inks is brimming with potential products. It’s a competitive marketplace, admits Walker, but he believes there is space for products with better conductivity that require lower sintering temperatures. Also, there is a niche waiting to be filled by products that allow consumers to interact with them, he concludes.