Vibrational properties of graphene nanostructures: Raman spectroscopy and density-functional theory

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Graphene nanostructures like nanoribbons are expected to have fascinating properties which are determined by their size and crystallographic orientation, but also by the edge configuration. Despite new approaches for fabrication of nanoribbons, there is still a large gap between theoretical predictions (mostly narrow ribbons with long-range ordered edges) and experimental data obtained from wider, less well-defined ribbons. Here our aim is to diminish this gap by combining high-resolution vibrational spectroscopy and simulations of appropriate less idealized graphene nanostructures. If successful, local vibrational spectroscopy can give the necessary information about structural properties. Moreover, since some vibrational modes are very sensitive to doping and other changes in the electronic structure, we expect to obtain local information on the electronic properties as well. In particular, we plan to focus now on tip-enhanced Raman spectroscopy (TERS), which provides spatial resolution of ~50nm, compared with ~500nm in conventional optical spectroscopy. It therefore seems promising for obtaining the desired local information in nanoribbons like homogeneity, edges, disorder etc. Comprehensive simulations of the disorder-induced Raman modes in nanoribbons are planned in order to discriminate effects from confinement, disorder at the edge (edge roughness), crystallographic orientation, and bulk defects. We will further investigate graphene structures fabricated by novel methods like molecular beam epitaxy (MBE) on different substrates and graphene defects and folds produced by ion irradiation.