Molecular Plasmonics: Graphene Plasmons in the Picoscale Limit
Master of Science
Doped graphene supports surface plasmons in the mid- to far-infrared that are both electrically and spatially tunable. Graphene has been shown to enable greater spatial confinement of the plasmon and fewer losses than typical noble metals. Reduced-dimensional graphene structures, including nanoribbons, nanodisks, and other allotropes including carbon nanotubes exhibit higher frequency plasmons throughout the mid- and near-infrared regimes due to additional electronic confinement of the electrons to smaller length scales. Recent theoretical predictions have suggested that further spatial confinement to dimensions of only a few nanometers (containing only a few hundred atoms) would result in a near-infrared plasmon resonance remarkably sensitive to the addition of single charge carriers. At the extreme limit of quantum confinement, picoscale graphene structures known as Polycyclic Aromatic Hydrocarbons (PAHs) containing only a few dozen atoms should possess a plasmon resonance fully switched on by the addition or removal of a single electron. This thesis reports the experimental realization of plasmon resonances in PAHs with the addition of a single electron to the neutral molecule. Charged PAHs are observed to support intense absorption in the visible regime with geometrical tunability analogous to plasmonic resonances of much larger nanoscale systems. 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. The natural abundance, low cost, and extremely large variety of PAH molecules available could make extremely large-area active color-switching applications, such as walls, windows or other architectural elements, even vehicles, a practical technology.
plasmon; graphene; polycyclic aromatic hydrocarbon; molecular plasmon; plasmonics