Growing graphene layers on metals like cobalt or nickel is interesting for two reasons, say team leaders George Flynn and Tony Heinz of the Nanoscale Science and Engineering Center at Columbia and Mark Hybertsen at Brookhaven. First, the lattice constants of Co(0001) and Ni(111) 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. Second, nickel and cobalt are ferromagnetic materials that are used in spintronics applications – devices that exploit the spin of the electron as well as its charge.

Using a vacuum evaporation technique, the researchers placed solid-phase carbon-containing precursor molecules (contorted hexabenzacoronene) inside a UHV chamber and raised the temperature to 605 K to deposit the molecules onto a clean Co(0001) surface. Next, they 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.

The team 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). It also observed strong coupling between graphene and the cobalt surface (compared with the surface of other metals, like silver, copper or platinum). This means that the electronic structure of graphene that is in contact with the Co(0001) is very different from that of isolated graphene.

Opening up a gap
"The hybridization between graphene p- and cobalt d-states opens up a gap feature around the Fermi level in the electronic structure of graphene," Flynn told "This is mainly due to the asymmetric environment of two carbon atoms of the graphene unit cell that arises when they sit on Co(0001)."

Graphene might be used as a spin-carrying material in the future and cobalt is one of the main ferromagnetic materials exploited in spintronics applications. The structural and electronic coupling between graphene and cobalt in the contact region revealed by this work will thus be very important when developing potential graphene-based spintronics devices.

"For example, a gap feature opens in the electronic structure of graphene on cobalt thanks to the strong coupling between the two materials. This indicates that one of the key parameters for efficient spin injection into graphene is the adjustment of the coupling strength between graphene and the ferromagnetic material," explained Flynn. Such an adjustment could be achieved by inserting a monolayer-thick surface oxide or non-magnetic metal between the two materials.

The work was published in Nano Letters.