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How Nitrogen Improves Graphene Films

A form of carbon just one atom thick, known as monolayer graphene, has the potential to make a big impact on the field of nanoelectronics – very tiny, very fast electronic components that could radically change computers and other devices. But scientists still need to understand better the properties of monolayer graphene so they can learn how to tailor them.

To this end, scientists working in part at NSLS have uncovered atomic-level information about monolayer graphene that was “doped” with nitrogen atoms during the growth process. Their work addresses the atomic and low-energy electronic structural changes caused by the introduction of foreign atoms into the carbon lattice.

The research was performed by scientists from Columbia University, Sejong University in Korea, Stanford Synchrotron Radiation Lightsource (SSRL), and Brookhaven National Laboratory.

The group learned that the nitrogen dopants increased the carrier density of the film – the number of electrons that contribute to its conductivity – yet disrupted the electronic structure only in the immediate area around each one. These discoveries indicate that nitrogen doping may be an excellent way to create high-quality monolayer graphene films with superior electronic behaviors.

Doping, a practice commonly used to improve a material's properties, can entail adding atoms to a material or substituting them in, as in this case, where a nitrogen atom replaced a carbon atom at random points in the graphene’s honeycomb-shaped lattice. The doping concentration was about 0.30 percent on average, meaning roughly three nitrogen atoms were present for every 1,000 carbon atoms, for a total of about 13 trillion nitrogens per square centimeter.

Using several investigation techniques, including x-ray spectroscopy at NSLS beamline U7A, the researchers probed the graphene’s atomic structure and the electronic structure of the region surrounding each nitrogen atom. They calculated that each nitrogen atom contributed about half an electron, on average, to the pool of charge carriers. This may seem like a small contribution, but it significantly increases the charge carrier density of the film.

At the same time, the nitrogen atoms had a minimal impact on the overall electronic behavior of the film, causing perturbations that disappeared within a few lattice spacings of each dopant. This is important because large perturbations can be detrimental to the flow of current in graphene.

“Doping is a key requirement necessary for all modern semiconductor electronics, and our work shows that nitrogen doping can give graphene the functionality required to make electronic devices similar to the ones that exist today,” said Columbia University physicist Abhay Pasupathy, the study’s corresponding author.

Parts of the research were also conducted at Brookhaven's Center for Functional Nanomaterials and SSRL, which is a directorate of SLAC National Accelerator Laboratory.

The work is published in the August 19, 2011, online edition of Science.

Laura Mgrdichian

2012-3012  INT/EXT  |  Media & Communications Office

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