Plasmonic Nanostructures for Molecular Spectroscopy
Abstract: This thesis focuses on the fabrication, characterization and utilization of nanostructures for chemical sensing and optical enhancement processes. Noble metal nanostructures can support localized surface plasmon resonances (LSPR’s), which amplify the strength of the electromagnetic field close to the structures. The LSPR is very sensitive to the surrounding refractive index, making metal nanostructures an interesting alternative to the commercially available flat-film surface plasmon chemo- and biosensors of today. Furthermore, the increased local field can enhance the efficiency of light-scattering and luminescence processes from molecules, which can prove useful in a wide range of applications, for example, the life sciences. Nanoplasmonic sensor surfaces in the form of thin gold films perforated by nanoholes were fabricated on different supporting substrates using colloidal lithography. It was shown that the bulk refractive index sensitivity could be increased by factor of ca 40% by fabricating the sensing structure on Teflon instead of on conventional glass substrates. The reason for this amplification is a redistribution of the evanescent field into the aqueous sensing region due to the similar refractive indices of Teflon and water. Preliminary studies of surface-enhanced fluorescence (SEF) were performed using gold nanodisks of varying radii but fixed height, fabricated using electronbeam lithography (EBL). Enhancement as a function of the spectral overlap between the fluorophore and the nanoparticle resonance was studied. Two fluorophores, 5-carboxy-x-rhodamine (ROX) and 6-carboxy-fluorescein (FAM) were conjugated with thiolated DNA, which was chemisorbed to nanodisks of varying LSPR wavelength. The results indicate that the relative enhancement is largest for resonances overlapping with the fluorophore absorption and emission spectra. Further, enhancement processes were studied for molecules adsorbed to colloidal surface-supported nanoparticle aggregates acting as optical antennas. In particular, the directionality of surface-enhanced Raman scattering (SERS) was measured by recording Fourier images in an optical microscope. Rhodamine 6G (R6G) physisorbed to colloidal dimers and trimers was found to scatter light at angles above the critical angle of the air-glass interface, stressing the use of high-NA objectives for SERS studies. In addition, the emission from molecules adsorbed to nanoparticle dimers was found to produce antenna lobes perpendicular to the dimer axis, demonstrating that the "hot site" in the junction between the particles dominates the SERS response. In contrast, for nanoparticle trimers, the scattering was circularly symmetric in the aggregate plane.
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