One of the major stumbling blocks in nanotechnology is protecting materials. When you’re making something on the scale of nanometers, just a few atoms being out of place can be a big deal. This is where molecular self-assembly can be helpful. Chemists have found many different structures that will spontaneously form interesting and useful structures just by essentially providing the right atoms. In a similar vein, several nanostructures have been found to be able to repair themselves with little prodding from researchers.
Scientists at the University of Manchester (including one of the recipients of the 2010 Nobel Prize in Physics) have now found out that graphene is one of those structures that can self-repair. This actually seems a bit unexpected, because I’ve not heard of anyone ever predicting carbon structures could self-heal. Somewhat ironically, this was observed during research on etching graphene into specific shapes. The Manchester team was trying to use metal atoms to cut-out specific shapes from a graphene sheet, as graphene’s electronic properties change depending on the the structure of its edges.
Graphene “healed” in two ways. The first way was by dumping lots of extra carbon into the system near the graphene hole. Carbon from hydrocarbon molecules was incorporated into the graphene layer by the electron beam. This didn’t result in a “pure” graphene layer though, because the hole would typically be filled by 5- and 7-atom carbon rings instead of graphene’s hexagonal, six-atom rings. An overabundance of spare carbon also tended to result in a semi-disordered, second layer of carbon growing on top of the graphene. Even without hooking the holes up with extra carbon, graphene will try to repair holes under the electron beam. The etching process would leave graphene with holes bordered by some 5- and 7-atom rings. Just putting the edges of these holes under the electron beam without a carbon source seemed to “even” them out as carbon atoms moved into graphene’s characteristic hexagonal shape, although the hole may still remain. Both processes also may incorporate the impurity atoms that caused the holes.
One could argue this doesn’t really restore the graphene, since you would still be left with awkward shapes or holes. But these can actually still be helpful in etching graphene into custom shapes, especially since the whole point of the Manchester group’s work was to cause localized defects. Years of work on chemically altering carbon nanotubes have shown that you can selectively dissolve 5-atom and 7-atom carbon rings without destroying the major 6-atom ring structure, and chemists can do weird things to the ends of 6-atom carbon rings. I do wonder if it might be a bit harder to selectively dissolve the odd rings in graphene, though, because I imagine the curvature of a nanotube helps dissolution by putting strain on the atomic bonds and presents the bond more easily to a solution. But if the selective dissolving still works on graphene, it seems like the team may already have accomplished their goal.