Doping can add or remove electrons from a material. Chemical doping is widely used in the silicon industry but it was still not clear before this new study if it could be applied to graphene. "In our work we clearly show that individual nitrogen atoms displace carbon atoms from the graphene lattice," team leader Abhay Pasupathy of Columbia University told "And, what's more, we have actually seen how the nitrogen atoms 'sit' in the lattice for the first time."

The technique could be used to dope different patches of a single graphene sheet in different ways, by varying the amount of chemical doping across the sheet, he added. Graphene might also be doped with other elements, such as boron, that take away electrons from graphene instead of adding them. "This so-called complementary doping is a key feature behind all silicon transistor technology," said Pasupathy. "Our experiments show that such processes could now be applied to graphene too."

The researchers also found that the extra nitrogen atoms do not significantly modify the basic structure of graphene sheets. It is the same for silicon: a small amount of phosphorus is routinely used to dope the material but it does not fundamentally change its basic structure either.

Extra valence electrons
Pasupathy and colleagues used graphene films that they had grown by passing carbon-containing gases and small amounts of ammonia over a copper foil at high temperatures. Careful imaging using a scanning tunnelling microscope revealed that nitrogen atoms substitute for carbon atoms within the regular honeycombed lattice of graphene. "In direct analogy to phosphorus dopants in silicon, the nitrogen atom has one extra valence electron compared with carbon," explained Pasupathy. "We found that approximately half of this extra electronic charge is distributed throughout the graphene lattice while the remainder is localized around the nitrogen atoms."

Yunqi Liu of the ICCAS in China, whose group has also n-doped graphene using nitrogen in the past, comments: "It is well-known that graphene is a typical semi-metallic material. However, to make graphene-based electronics, both p- and n-type graphene is needed. This new study involving detailed scanning tunnelling microscopy on nitrogen-doped graphene samples shows that nitrogen dopants are inserted into the monolayer graphene lattices, which strongly modifies the electronic structure of graphene. The research will be valuable for both fundamental scientific studies and the practical application of substitutional doped graphene."

Pasupathy's team, which includes scientists from Sejong University in Seoul, SLAC National Accelerator Laboratory and the Brookhaven National Laboratory, also hypothesizes that nitrogen-doped graphene may be more chemically reactive then pristine graphene. It could potentially be used in applications like chemical sensors as well as for making conventional transistors and other electronics devices.

The researchers say that their priority is now to study the chemical reactivity of the material. "We are also looking at other forms of doping and chemical functionalization of graphene," revealed Pasupathy.

The work was reported in Science.