Probing the Spectral Signatures of Energy Transfer in Metal-Decorated Plasmonic Nanoprisms

This dissertation focuses on imaging localized surface plasmon resonances (LSPRs) by utilizing electron energy-loss spectroscopy (EELS) and resonance-Rayleigh scattering to understand energy transfer and changes in LSPR morphology dependent upon the location of a catalytic metal in contact with a plasmonic metal.

Imaging and understanding the unique properties supported by plasmonic metals has become increasingly more intriguing in the plasmonics community since EELS is able to spatially map LSPRs. We are able to utilize EELS to spatially image the LSPR mode structure of gold nanoprisms and gold nanoprisms of the same size decorated with a catalytic metal, particularly, platinum. Importantly, we can use the plasmon mode maps to understand LSPR behavior of gold when in the presence of a catalytic metal. We demonstrate the location-dependence of the catalytic metal on the LSPR mode structure changes. We further elucidate that conservation of three-fold symmetry in the nanoprisms inherently causes no LSPR mode differences. Finally, we use simulations only to show that, although gold and platinum couple, better energy transfer may be facilitated in a system whose LSPRs spectrally overlap, such as aluminum and platinum.

Although EELS is capable of spatially mapping LSPRs, a cheaper and more universally available spectroscopy can spectrally image low-energy LSPR modes (dipole and quadrupole modes) with high resolution. Using depolarized and polarized plane-wave excitation in the dark-field, we excite the LSPR of a single platinum-decorated gold nanoprism to demonstrate a splitting of the low-energy plasmon mode. This splitting relies on the location of the platinum decoration on the nanoprism. Like EELS, three-fold symmetry conservation results in a single dipolar LSPR mode; however, when the decoration breaks the three-fold symmetry of the nanoprism system, a distinct splitting of the dipole LSPR mode is observed. We show that resonance Rayleigh scattering coupled with high-resolution transmission electron microscope (HR-TEM) images is capable of probing the location differences of the platinum decoration on a single gold nanoprism.

Finally, we show through experiment only how electron beam lithography (EBL) affords great control over nanodimer fabrication including junction distances of less than 10 nm. Of interest in the molecular and refractive index sensing communities, the Fano effect is observable under plane-wave excitation. Unfortunately, it remains unclear if a Fano interference can be probed by an electron beam. In order to elucidate the Fano effect under a near-field probe, nanodimers with very small junction distances (< 10 nm) must be fabricated. EBL allows for systematic fabrication of nanodimers with small junctions.