“Two-dimensional” materials have the potential to make materials lighter and stronger and to make batteries more efficient.
But to unlock that potential for industrial application we need to be able to make such nano-sheets in large amounts, according to an Irish researcher who has just written a major review paper on the subject for the journal Science.
It all kicked off back in 2004 when Andre Geim and Konstantin Novoselov described how they produced graphene, a form of carbon that is just one atom thick.
The duo subsequently won a Nobel Prize for making the ultra-thin and extremely strong nanosheets, and their starting point was graphite, the stuff you find in your pencil, explains Jonathan Coleman, who is a professor of physics at Trinity College Dublin and a principal investigator at the Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN).
Beyond graphene
Graphite is a layered compound and you get graphene by separating out the layers – it's like taking a deck of cards and separating them into individual cards," he says.
But peeling the graphene flakes away individually meant the process was expensive and time-consuming, so Coleman worked out a relatively simple way to make graphene flakes quickly: he put the cheap graphite into a soapy solution and applied energy, and the nano-flakes exfoliated off into the liquid in their billions. Coleman and CRANN researcher Prof Valeria Nicolosi subsequently found that the approach could work for inorganic layered materials too.
Graphene had quickly earned a reputation as one of the strongest materials known, and people also started looking at other layered materials that had the potential to make ultra-thin nano-flakes – in part because of the usefulness of their shape, notes Coleman.
“A lot of applications require materials to sit on surfaces, and if you want something to sit on a surface a flat object is ideal,” he explains.
“And people realised that graphite was just one of many layered compounds, there are probably as many as 500 of them, and that graphene is not the only game in town. While graphene has one set of properties that are very interesting and exciting it can’t do everything, but if you have got 500 different two-dimensional materials all with different properties it opens up the potential for finding other applications in areas like electronics and even drug delivery in medicine – there may be a two-dimensional material for it.”
And as the field has developed, interest has also grown in techniques to exfoliate the flakes, adds Coleman, and he and Nicolosi are co-authors of a review paper out this week in Science on the various methods to exfoliate ultra-thin sheets in liquid.
“The realisation has come over the last few years that the key to being able to unlock the potential of these two-dimensional materials is to be able to exfoliate them, to make them in liquids,” says Coleman.
“You take the layered material, put it in a suitable liquid, you add energy and it naturally breaks up into the two-dimensional sheets.”
Scaling up
Research at CRANN has been developing the liquid exfoliation technique with a view to scaling it up for industrial use, according to Coleman.
“The method we started formed the basis, but we have moved far beyond that, we are a good few rungs up the ladder now and we are making the [nano-sheets] much faster than we were previously.”
With funding from the European Research Council and through Science Foundation Ireland's planned AMBER centre, Coleman and Nicolosi are now working on two-dimensional materials for a range of potential applications, including lighter materials and efficient energy storage.
“SFI and CRANN has made a rather big investment in this area,” says Coleman. “And while a year ago it might have been seen as a bit of a gamble, now people are realising this is a shrewd gamble.”
