Hollow gold nanosphere optical transducers studied using femtosecond time-resolved laser spectroscopy

This dissertation presents and evaluates the unique interplay between nanoparticle structure, environment, and electronic energy relaxation. This knowledge will provide useful information for tailoring nanoparticle properties so that they can be applied to the development of more efficient transducers, such as a light-harvesting antenna. In particular, plasmonic gold nanoparticles have been synthesized, both hollow (HGNs) and solid (SGNs), and their structural properties have been characterized using transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), dynamic light scattering (DLS), and UV/Vis absorption spectrophotometry. Femtosecond pump-probe transient extinction experiments have been conducted on both isolated and aggregated HGNs and SGNs in order to elucidate their electronic energy relaxation properties. While studying how aggregated nanostructures influence optical and electronic properties, an unexpected spectral blue-shift of the surface plasmon resonance (SPR) was observed upon aggregation of HGNs using a salt solution, which led to longer electronic energy relaxation times compared to isolated HGNs. These findings were significant because previous studies have found that SGNs red-shift upon aggregation and have faster electronic energy relaxation times. In order to understand further the nature of the blue shift in HGN aggregates, alkane-thiols were used to induce the aggregation, where it was found that at a critical thickness of the HGN shell, the SPR blue-shifts due to the interaction of the electric fields within the hollow cavities of the nanoparticles. These alkane-thiol ligands provide for more controlled aggregation over the interparticle gap than other aggregating agents such as potassium chloride salt. Transient extinction experiments at high pulse energies were conducted to learn about the modulation in the SPR frequency of HGNs following excitation by a femtosecond laser pulse. The oscillation frequency and phase were determined for a wide range of HGN sizes, revealing a size-dependent excitation mechanism of the vibrational modes. In addition, transient extinction experiments were carried out at low pulse energies in order to determine the electron-phonon coupling times for a wide range of sizes of HGN and SGN samples. As the aspect ratio of the HGN increases, the electron-phonon coupling time decreases (or the electron-phonon coupling increases), whereas for SGNs, the electron-phonon coupling remains constant with increasing diameter. The electron-phonon coupling enhancement exhibited by high aspect ratio HGNs was attributed to the large surface to volume ratio of these structures, which results in non-negligible contributions from their environment. Finally, the phonon-phonon coupling properties of HGNs were investigated, which is the last step in electronic energy relaxation in metal nanoparticles. This study revealed that fluids confined to the hollow core of HGNs have different properties compared to their bulk counterparts, thereby influencing the particle-to-surroundings energy transfer rates. Hence, the cavity influences the electronic and mechanical properties of the HGNs. The structural, optical, and electronic studies on the aforementioned types of metal nanoparticles provide the basis to understand how the surface plasmons influence light absorption in a nearby molecule. Specifically, how the surface plasmons of HGNs and their aggregates interact with the discrete electric-dipole transitions of iron porphyrin molecules. Surface-enhanced Raman spectroscopy (SERS) of iron porphyrin molecules near SGN and HGN aggregate surfaces was employed to understand the interaction between the strong electric fields of HGNs and molecular electronic transitions.