Doctor of Philosophy
When the electromagnetic field of light is incident on metallic nanostructures of dimensions smaller than the incident wavelength of the light, there is a strong interaction, resulting in an enhanced, highly confined electromagnetic field in the vicinity of the nanostructure. This effect is referred to as a localized surface plasmon resonance, most commonly exploited for plasmon-enhanced spectroscopies, such as surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS). The location, number and intensity of these regions of enhancement, or “hotspots”, can be tuned by changing the nature of the metal, the size, shape and arrangement of the nanoparticles, its surroundings, or the wavelength of the incident light. When a molecule is located within these nanoscale hotspots, it is possible to obtain detailed spectroscopic information about the molecule with high sensitivity.
The decay process of these plasmon resonances can result in the ejection of high energy “hot” carriers, either hot electrons or hot holes, and the subsequent heating of the nanoparticle lattice. When a molecule is adsorbed to the surface of the nanoparticle, the presence of hot electrons and holes or the elevation of temperature can favour a chemical reaction. This effect is most prevalent in metallic nanostructures that exhibit hotspots at their surface, that can in turn be used to photocontrol surface reactions through plasmon excitation.
In this thesis, plasmon-mediated reactions are investigated using a variety of spectroscopic and microscopic techniques, along with the modelling of the light-matter interaction. The reduction of aryl diazonium salts on a gold nanostructured surface is plasmon-catalyzed. For a tip-enhanced Raman spectroscopy system involving a gold tip and a silver nanoplate as a substrate, plasmonic heating is discussed and modelled using finite element methods. New fractal metallic nanostructures are developed and studied for future applications in plasmon-mediated chemistry.
Summary for Lay Audience
Nanoparticles are particles of dimensions less than 100 nanometres, or about a thousand times smaller than the diameter of a human hair. Through a variety of synthesis approaches and techniques, nanoparticles can be made of many different materials and in many different shapes. Of note are metallic nanoparticles, most often made in gold or silver, which can exhibit a very strong interaction when illuminated with visible light. Interestingly, this interaction can be used for enhancing the signal of optical sensing techniques, improving the detection limit so that a signal can be obtained from a few, or even one, molecule. Some side effects of this process are the production of high energy electrons and local heating. These side effects can cause chemical reactions to occur under illumination by laser light when certain molecules are attached to these nanoparticles.
As these reactions are happening on such small scales, techniques with high spatial resolution are needed to observe their progress. These involve the interaction of light with the nanoparticles, with the signal enhanced by the local electric field of the nanoparticles, and the interaction of a sharp nanoscopic tip running along the surface of the nanoparticles, like a record player on the nanoscale, to obtain the topography of the sample. Combination of these two techniques, running a nanoparticle-decorated tip along a surface, gives both the topography and an enhanced signal with a spatial resolution of several nanometres.
Here, these reactions are studied using these techniques, along with the response as predicted by simulations. One such reaction studied here is the grafting of a diazonium salt on a gold surface. The rate of chemical reactions can be augmented by the production of high energy electrons, or through local heating, as heat acts as an energy source for the reaction. To study the latter for a more complex system, involving a sharp gold-coated tip and a flat silver nanoplate, the heating process is modelled by simulations and compared to experimental observations. Finally, new nanoparticles are made based on different self-similar, fractal patterns, and these are characterized for future applications in chemistry.
McRae, Danielle, "Plasmon-Enabled Physical and Chemical Transformations of Nanomaterials" (2020). Electronic Thesis and Dissertation Repository. 7068.