Flexible electronics look set to completely change our everyday lives. Indeed, a host of novel applications – such as electronic paper, touch screens, artificial skin, "smart" clothing and electronic eye-like imagers – have already been produced. Ink-jet printing is one of the best ways to make large amounts of plastic electronics, and a variety of components, like transistors, photovoltaic devices, organic light-emitting diodes and displays, can be fabricated using this technique. Ink-jet printing is also simple and only has a few processing steps.

So-called drop-on-demand ink-jet printing (originally used to print text and graphics) is now a tried and trusted method to print thin-film transistors based on organic and semiconducting inks. However, the devices made this way do not hold up when compared with standard silicon technology. Researchers have tried to remedy this by using inks based on zinc-oxide nanoparticles and carbon nanotubes, but such inks are not stable in ordinary solvents, so chemically modified solvents need to be employed. These solvents tend to degrade over time and metal nanoparticles also oxidize after printing.

Better transistors
Graphene – a 2D sheet of carbon with unique electronic and mechanical properties – could come into its own here, say Andrea Ferrari and colleagues who have succeeded in producing an ink from the material by exfoliating pieces of graphite in a liquid. What is more, preliminary thin-film transistors printed using this ink already seem to be better performing than state-of-the-art ink-jet printed devices. They have electron mobilities of up to 95 cm2V–1s–1 for example. In comparison, ink-jet printed TFTs based on organic semiconducting polymers have mobilities ranging from just 0.01 to 0.5 cm2V–1s–1, but better on/off ratios of up to 105.

The Cambridge researchers made their graphene ink as follows: they first began by treating graphite flakes in a sonic bath containing the solvent N-methylpyrrolidone for several hours. The flakes were then left to settle for a few minutes after sonication. Next, the team decanted the dispersions and centrifuged the samples for an hour to filter out any flakes bigger than 1 µm across that might clog the printer nozzle.

The ink can be used to print electronic devices like thin-film transistors on a variety of substrates, including silicon dioxide and quartz.

Compatible with existing technology
"Our technique is not new and the graphene ink produced should thus be compatible with existing standard ink-jet machines," Ferrari told nanotechweb.org. "This will hopefully allow the ink to be used in existing printed electronics."

The researchers – who report their work on the arXiv pre-print server – now plan to optimize the process parameters. "We shall also make contact with the major players in the printed electronics industry to try and implement the ink in useful devices," revealed Ferrari.