The strength, flexibility, transparency and high electrical conductivity of single-layer graphene make it a potentially unique and valuable material for the next generation of electronic devices. Made of carbon atoms arranged in a honeycomb pattern – think of a chicken-wire fence – it is 97 percent transparent and 1,000 times stronger than steel.
Researchers are working on ways to tune the properties of graphene for specific electronic applications. One way to do that is by doping – introducing small amounts of other elements, such as nitrogen or phosphorus, that either add or subtract electrons from the system. Widely used in silicon technology, doping has been carried out experimentally in single-layer graphene sheets; but until now, the details of how the dopant atoms fit into the sheet and bond with their carbon neighbors remained elusive. In a study reported Aug. 9 in Science, researchers from Columbia University, Sejong University in Korea and SLAC and Brookhaven national laboratories used a combination of four techniques to make the first detailed images of nitrogen-doped graphene film. They showed that individual nitrogen atoms had taken the places of carbon atoms in the two-dimensional sheet; that about half of the extra electron contributed by each nitrogen atom was distributed throughout the graphene lattice; and that this changed the electronic structure of the graphene sheet only within a short distance – about the width of two carbon atoms – from the dopant atoms. The ability to control the electronic structure at the atomic level has important implications for tuning the unique electronic properties of graphene for particular device applications.
“We’re not trying to work on existing systems and make them better. We’re looking for new directions that can potentially enable much higher efficiencies,” said paper co-author Theanne Schiros, a surface scientist at the Department of Energy’s Energy Frontier Research Center at Columbia, who is investigating graphene as a possible electrode for novel photovoltaic devices.