Picking up the pace
Ellis Davies reports on a new method to produce single-crystal graphene in larger quantities.
Single-crystal graphene can currently only be produced in samples of a few centimetres at best. Now, a team at the Institute for Basic Science (IBS), South Korea, has developed a method that it claims increases the amount of graphene produced to 5 x 50cm2, in a process that takes 20 minutes. The team has reported the synthesis of a large sheet of monolayer single-crystal graphene that could stimulate further work on large-scale folding of graphene sheets to create origami or kirigami-like shapes, which could be applied to future flexible circuits.
In the study, graphene was grown on a 5 x 50cm2 piece of copper (Cu) foil by heating it to 1,030oC in an oven and allowing a temperature slope from hot-to-cold to move the grain boundary onwards to create a ‘perfect’ single-crystal. This crystal forms the base of the method, as large single-crystal Cu foil is not commercially available.
The heating and cooling of the foil causes copper atoms to migrate inside the material and arrange themselves into an ordered structure that causes fewer defects. Once the foil is created, the team uses chemical vapour disposition – a process of synthesising materials on to the surface of a substrate – to form millions of parallel graphene islands. These islands grow as more atoms are deposited on the foil until they coalesce to form a near-perfect single-crystal graphene layer across the whole surface.
This method is an improvement over previous iterations because of the size of the resulting sample and the short time taken to make it. Professor Feng Ding, Group Leader at IBS, told Materials World, ‘Both the growth rate and the size were greatly improved compared with previous reports. Based on this technique, we can easily synthesise single-crystal graphene at the cost close to that of producing polycrystalline graphene, which can be considered as a revolutionary change in graphene synthesis.’
To optimise the technique, the researchers considered four technical challenges, as Ding explained. ‘The rapid synthesis of single-crystal Cu foil in a large area is achieved by using a continuous source to assist the graphene growth on the Cu surface. Oxygen has the function of greatly enhancing the feedstock decomposition and assisting the attachments of carbon atoms to the graphene edge. The alignment of graphene islands on the Cu surface is guided by the (111) texture of the single-crystal Cu surface. It is suspected that the oxygen also plays an important role here. The seamless stitching of the graphene islands is assisted by their alignment and might also be helped by the existence of oxygen during growth,’ he explained.
Copper also helps to keep the cost low – the technique for annealing polycrystalline Cu into single-crystal graphene is fast and therefore cheap, and the condition of graphene growth can be easily controlled. The team claims that the only restriction on this method is the size of the copper surface, meaning that scaling up the single-crystal graphene production to industrial scale should be easily achievable.
‘In principle, both the size of the Cu foil and the single-crystal graphene can be unlimited. If we increase the size of the oven to be metre sized, using 100 metre-long Cu foil, then single-crystal graphene of 1 x 100m2 could be produced,’ explained Ding.
A big claim
The IBS team’s study boasts some bold claims, so Materials World spoke to Professor Andrea Ferrari, University of Cambridge, UK, about the results. ‘This paper seems to report impressive results as it shows the growth of a very large piece of graphene on copper foil. The key challenge these days is to grow larger areas of graphene with a high quality, so it should be, as much as possible, a single crystal. The results reported are quite reasonable, and overall it is very impressive work,’ he said.
Ferrari was keen to stress that the study does not yet present definitive proof that a single large monocrystal of graphene was produced. ‘The result is a large flake with very good alignment of the grains. This means that the flake is made up of many different grains with the same orientation. Even if this is not a single grain, the paper represent another milestone towards industrial applications of graphene in (opto)electronics,’ he explained.