X-rays Illuminate a Microscopic Picture of the Correlation between Nitrogen-dopant Bond Type and Electronic Effects in Single-layer Graphene

February 2013 SSRL Science Summary by Lori Ann White, SLAC Office of Communications

The cone represents the electronic structure of graphene, the cone center is the "Dirac point"; which is equivalent to the "Fermi level" in graphene. Grey indicates electrons. If there are electrons (shaded grey) above the Fermi level, the graphene is n-type doped, if there is a shortage of electrons (grey below the Fermi level), the graphene is p-type doped. In neutral, pristine graphene (undoped) the Fermi level indicates the top of the electronic levels occupied with electrons.

Doping graphene with small amounts of another element such as nitrogen or boron enables scientists to "tune" its properties to make it more suitable for a variety of applications, such as contact material in solar cells. Determining the chemically distinct species and different bond types that result from of doping monolayer graphene - even sub-percent-level doping -can be done using the high energy resolution and tunable polarization and energy of synchrotron light such as provided by SSRL.

As reported in the June 29, 2012, online edition of Nano Letters, a group led by Theanne Schiros, a research fellow at the Energy Frontier Research Center at Columbia University, working in close collaboration with SSRL scientist Dennis Nordlund, used SSRL Beam Lines 10-1 and 13-2 to study doped graphene samples. The researchers discovered that several bond types could exist between carbon and nitrogen atoms, even within the same graphene sheet, and that the different N-bond types have profoundly different macroscopic effects on the electronic structure of the graphene. This understanding, coupled with increasing control over N2/N3 ratios in N-doped graphene, opens new avenues for tailoring the carrier density and electronic properties of graphene at the atomic level.

This work demonstrates that synchrotron radiation-based spectroscopies provide invaluable, atom-specific tools to determine the electronic properties of different dopant and defect structures in graphene, even at sub-percent dopant levels, which is necessary for controlling the bonding type of dopant substitutions. Combined with continuing advances in the growth and controlled doping of graphene and cleaner transfer procedures, the detailed correlation of bond type and electronic structure demonstrated here promises to facilitate the atomic-level control of electronic properties for next-generation graphene-based devices.

 

Primary Citation

T. Schiros, D. Nordlund, L. Pálová, D. Prezzi, L. Zhao, K. S. Kim, U. Wurstbauer, C. Gutierrez, D. Delongchamp, C. Jaye, D. Fischer, H. Ogasawara, L. G. M. Pettersson, D. R. Reichman, P. Kim, M. S. Hybertsen and A. N. Pasupathy, "Connecting Dopant Bond Type with Electronic Structure in N-Doped Graphene", Nano Lett. 12, 4025 (2012) [DOI: 10.1021/nl301409h]

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Theanne Schiros, Energy Frontier Research Center, Columbia University