Plasmon-enhanced Raman scattering by graphene and carbon nanotubes

by Sebastian Heeg

Institution: Freie Universität Berlin
Department: FB Physik
Degree: PhD
Year: 2015
Record ID: 1107327
Full text PDF: http://edocs.fu-berlin.de/diss/receive/FUDISS_thesis_000000099347


The field of plasmonics studies the interaction between electromagnetic fields and free electrons in a metall, which enhance the optical near-field in the vicinity of the metal. For metallic nanostructures, the enhanced near-field is confined to a small volume called a plasmonic hotspot. Light scattering and absorption in a hotspot increases by several orders of magnitude. In this thesis I study the inelastic light scattering of graphene and carbon nanotubes subject to an enhanced near-field by Raman spectroscopy. First, I introduce and verify the concept of strained graphene as a local probe for plasmon-enhanced Raman scattering. Second, I probe the coupling of carbon nanotubes to the enhanced optical near-field in a plasmonic hotspot. To achieve the required interface, I suggest the directed dielectrophoretic deposition of nanotubes onto metallic nanostructures as a new method to couple nanotubes to plasmonic hotspots. A graphene-covered nanodimer was probed by Raman spectroscopy. The high intensity electromagnetic near-field at the plasmonic hotspot in the dimer gap enhanced the Raman signal by a factor of thousand. The enhancement occurred for strained graphene. Strain shifts the graphene phonon frequency; vibrations at the plasmonic hotspot differ in energy from vibrations originating from other areas and acts as a local probe for enhancement. We verified the Raman enhancement by the combination of spatially resolved, polarization and excitation energy dependent measurements. As these parameters do not affect the Raman signal of graphene, we proved that the experiment probed the Raman process caused by the enhanced optical near-field. For carbon nanotubes in the gap of a plasmonic dimer we observed Raman signal enhancements of the order 10^3 − 10^4. Following the approach developed using graphene, we addressed the extrinsic plasmonic and the intrinsic nanotube optical response independently by varying excitation energy and polarization. We showed that (i) the Raman enhancement scales with the projection of light polarization on the tube axis and that (ii) carbon nanotube Raman features arise from fully symmetric vibrations, even in the presence of a high intensity near-field. Raman modes that require light polarizations perpendicular to the nanotube axis were impossible to observe. This settled a long standing debate in the literature on the symmetry of the experimentally observed phonon modes. The placement of the carbon nanotubes in the gap of plasmonic dimers was achieved by directed dielectrophoretic assembly, which we suggest as a new method to achieve nanotube-nanoplasmonic interfaces. The methodologies and approaches that I developed in this thesis to couple graphene and carbon nanotubes with plasmonic structures provide a powerful and flexible tool to study the fundamentals of plasmon-enhanced Raman scattering. Das physikalische Gebiet der Plasmonik beschreibt Wechselwirkungsprozesse zwischen elektromagnetischen Feldern und freien Elektronen in Metallen. Die optischen Nahfelder in der…