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Dirac Point Resonances, Transport Gaps and Conductance Quantization in Graphene Nanoribbons with Adsorbed Atoms and Molecules

We present calculations of electronic quantum transport in graphene nanoribbons with adsorbed H, F, OH and O, based on a tight binding model derived from extended Huckel theory. The relaxed atomic geometries of the adsorbates and graphene are calculated using density functional theory. Our model includes the effects of the local rehybridization of the graphene from the sp2 to sp3 electronic structure that occurs when H, F, OH or O bonds covalently to the graphene. It captures the physics of the scattering resonances that are induced in the graphene near the Dirac point by the presence of these adsorbates. We find these Dirac point resonances to play a dominant role in quantum transport in ribbons with these adsorbates: Even at low adsorbate concentrations the conductance of the ribbon is strongly suppressed and a transport gap develops for electron Fermi energies near the resonance. The transport gap is centered very near the Dirac point energy of for H, below it for F and OH and above it for O. We predict ribbons with these adsorbed species under appropriate conditions to exhibit quantized conductance steps of equal height similar to those that have been observed by Lin et al. [Phys. Rev. B 78, 161409 (2008)] at moderately low temperatures, even for ribbons with conductances a few orders of magnitude smaller than 2e2/h.