Graphene is a sheet of carbon atoms arranged in a honeycomb-like lattice just one atom thick. Since its discovery in 2004, this "wonder material" has continued to amaze scientists with its growing list of unique electronic and mechanical properties. Some believe that graphene could find uses in a number of technological applications – even rivalling silicon as the electronic industry's material of choice. This is because electrons whizz through graphene at extremely high speeds, behaving like "Dirac" particles with no rest mass, conferring graphene with an ultrahigh conductivity that could be used to make transistors and other devices that are faster than any that exist today.

Making devices using graphene will inevitably involve contacting the material to metal electrodes and the material will need to be grown on metal substrates. Growing graphene layers on metals like cobalt is interesting because the lattice constants of Co(0001) surfaces match the in-plane lattice constants of graphene. This means that stable layers can be grown without having to make complex superstructures, which would be the case if using metals with a large lattice mismatch.

To do this, Mark Hybertsen of the Brookhaven National Laboratory and colleagues used a vacuum evaporation technique to place solid-phase carbon-containing precursor molecules (contorted hexabenzacoronene) inside a UHV chamber and raised the temperature to more than 600 K to deposit the molecules onto a clean Co(0001) surface. Next, the researchers thermally annealed the cobalt substrate with the precursor molecules on it at 600 K for 20 minutes in the vacuum chamber to produce well defined isolated graphene nanoislands. Finally the sample was cooled down to 4.9 K for the STM measurements.

In previous work, Hybertsen and co-workers found that one of the two carbon atoms of the graphene unit cell sits on top of an underlying cobalt atom while the other carbon atom is located in a three-fold hollow site of Co(0001). They also observed strong coupling between graphene and the cobalt surface.

The same team has now used STM again to look at the structure and measure the electronic properties of the graphene nanoislands grown on the cobalt surface. “Often, the bright features that appear in the images we obtained correspond directly to carbon atoms on the surface,” explains Hybertsen. “This allows us to see that our islands have straight and relatively well ordered edges.”

Extra complication

However, there are some situations in which the electronic properties of the surface change the apparent image obtained, he adds. “Sometimes, some atoms appear dark, and inside the graphene islands in our images, every other atom is bright. These atoms look different because they sit over a hollow in the cobalt surface. In other instances, the bright spots shift – for example to a position between atoms. We see this effect in the images for one type of disordered graphene edge – known as a zigzag edge – in our experiments.”

This extra “complication” means that calculations for model atomic structures are very important when interpreting the data obtained. “We use density functional theory based quantum mechanical calculations to determine both the physical and atomic structure of competing models for the graphene island edges and to simulate the way these models should appear under the conditions of the scanning tunnelling microscope,” says Hybertsen.

The calculations do confirm that zigzag edges form easily on the cobalt surface, but what about other types of edges?

Another type of edge structure

“We realized that there was another type of edge structure, parallel to the zigzag one, but where the end carbon atom is almost by itself and bonded to only one other carbon atom,” explained Hybertsen. “This structure, called a Klein edge, usually has a very high energy when graphene is suspended (because of the number of neighbouring carbon bonds) but on the cobalt surface (when it is over a hollow site), the graphene edge bends down (as what happens when a zigzag edge forms) and the extra carbon atom stabilizes by bonding to a cobalt atom.”

The calculations by the team also show that the Klein edge is easy to form in terms of energy cost, again backing up the observations made in the STM.

Transition metal surfaces, like cobalt (but more often copper) are routinely used as both catalysts and support substrates for growing large areas of nearly perfect graphene from various molecular feedstocks. However, the atomic-scale mechanisms behind graphene growth are still not very well understood so the new results may help here. “They may also help us reproducibly grow specific graphene nanostructures, like nanoribbons, with better control over how wide they are and by keeping perfect edge structures. This would really boost the prospects for graphene in real-world electronic devices,” added Hybertsen.

The team includes Deborah Prezzi in Hybertsen’s group, who performed the theoretical calculations, in part, using facilities of the Center for Functional Nanomaterials at Brookhaven National Laboratory; Daejin Eom, Kwang Rim and Hui Zhou in Tony Heinz’s and George Flynn’s teams prepared the samples and performed the STM measurements. Shengxiong Xiao in Colin Nuckolls’ team prepared the contorted HBC molecules.

The researchers detail their work in ACS Nano DOI: 10.1021/nn500583a.