Despite the many success and promises of graphene, it is yet to be widely used in mainstream devices. One of the big hurdles in this area is producing graphene. There are many methods to produce graphene, but they each have their problems: those that produce the highest-quality graphene cannot produce enough, and those that produce lots often give graphene that is too poor for most applications.
The original isolation method was the now-famous sticky tape method. Here, chunks of graphite are peeled away using sticky tape, and these are then placed onto a flat surface. More sticky tape is then pressed onto the chunks and peeled away again, giving thinner chunks. If this process is repeated, eventually there are flakes that are only a single atom thick. However, by this time the flakes are very small (only a few microns across) and they are buried within a crowd of thick flakes. This makes finding and investigating the flakes difficult. They are, however, of a very high quality and so this graphene is useful for early stage research. But it cannot make enough for any applications.
The question then becomes: is there a way to separate graphene sheets on a much bigger scale? The first steps in this direction came from chemical exfoliation. In this method, graphite is oxidised, which then allows water to move easily between sheets. With some stirring, the sheets then separate in water quite easily. Now there are many single layer sheets floating in solution. However, these sheets are not really graphene, they are graphene oxide. Removing the oxygen from graphene oxide is still providing many challenges and pristine graphene is yet to be recovered from graphene oxide.
Separating sheets without oxidising would be the obvious next step. Recent efforts have been made to do this using high shear mixing. Here, a mixer creates forces on the graphite layers that is strong enough to separate the sheets, and a surfactant (like soap) can coat the sheets to stop them from restacking. Again, this method often produces thin sheets, but the sizes are still too small (tens of microns) for many applications.
A route to large area graphene that has lots of promise is chemical vapour deposition. Here, metals are heated to 1000°C, and carbon-containing gases like methane are introduced. The metals break the gases down into carbon atoms, which then arrange onto the metal surface to form graphene. This method produces high quality graphene and has been scaled up to metre sizes. The downside here is that the graphene is attached to a metal surface, and efforts to transfer the graphene off have yet to be perfected. Further, growing graphene in this way on a non-metallic substrate are still in their infancy.
In summary, the current research efforts in graphene production are along these lines:
- Can the oxygen on graphene oxide be removed completely, and yield perfect, high-quality graphene?
- Can liquid exfoliation give bigger sheets, and more routinely give only single layer graphene?
- Is there a way to transfer graphene perfectly, leaving no contaminants, wrinkles, or defects?
- Can we find a way to grow perfect graphene on any surface that we want?