Molecular Plasmonics: Graphene Plasmons in the Picoscale Limit

Adam Lauchner Department of Electrical and Computer Engineering, Rice University, Houston, Texas, USA Laboratory for Nanophotonics, Rice University, Houston, Texas, USA Andrea Schlather Department of Chemistry, Rice University, Houston, Texas, USA Laboratory for Nanophotonics, Rice University, Houston, Texas, USA Yao Cui Department of Chemistry, Rice University, Houston, Texas, USA Laboratory for Nanophotonics, Rice University, Houston, Texas, USA Michael McClain Department of Chemistry, Rice University, Houston, Texas, USA Laboratory for Nanophotonics, Rice University, Houston, Texas, USA Alejandro Manjavacas Department of Physics and Astronomy, Rice University, Houston, Texas, USA Laboratory for Nanophotonics, Rice University, Houston, Texas, USA Javier F. García de Abajo ICFO-Institut de Ciencies Fotoniques, Mediterranean Technology Park, Castelldefels, Spain ICREA-InstitucióCatalana de Recerca i Estudis Avancats, Passeig Lluís Companys, Barcelona, Spain Gustavo Scuseria Department of Chemistry, Rice University, Houston, Texas, USA Department of Physics and Astronomy, Rice University, Houston, Texas, USA Peter Nordlander Department of Electrical and Computer Engineering, Rice University, Houston, Texas, USA Department of Physics and Astronomy, Rice University, Houston, Texas, USA Laboratory for Nanophotonics, Rice University, Houston, Texas, USA Naomi Halas Department of Electrical and Computer Engineering, Rice University, Houston, Texas, USA Department of Chemistry, Rice University, Houston, Texas, USA Department of Physics and Astronomy, Rice University, Houston, Texas, USA Laboratory for Nanophotonics, Rice University, Houston, Texas, USA

We report the experimental observation of plasmon resonances in polycyclic aromatic molecules (PAHs) with the addition of a single electron to the neutral molecule. We observe that the charged PAHs support intense absorption in the visible regime with geometrical tunability analogous to plasmonic resonances of much larger nanoscale systems. These observations confirm earlier predictions that picoscale-confinement of graphene structures, containing only a few dozen atoms, should possess a plasmon resonance fully switched on by the addition or removal of a single electron.[1] Graphene supports surface plasmons in the mid- to far-infrared that are both electrically and spatially tunable. Reduced-dimensional graphene structures including nanoribbions and nanodisks exhibit higher frequency plasmons throughout the mid- and near-infrared regimes due to additional electronic confinement of the electrons to smaller length scales. To facilitate charge transfer to and from PAH molecules, a three-electrode electrochemical cell with optical access was designed, where current is passed through a nonaqueous electrolyte solution that contains a known concentration of PAH molecules. In contrast to larger graphene nanostructures, the PAH absorption spectra possess a rich and complex fine structure that we attribute to the coupling between the molecular plasmon and the vibrational modes of the molecules. We calculate vibrationally-resolved absorption spectra with time-dependent density functional theory to support this hypothesis.

Manjavacas, A., Marchesin, F., Thongrattanasiri, S., Koval, P., Nordlander, P., Sánchez-Portal, D., García de Abajo, F.J. Tunable Molecular Plasmons in Polycyclic Aromatic Hydrocarbons. ACS Nano 7, 3635–43 (2013).

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