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Plasmonic Sensing via Surface-Enhanced Spectroscopies



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Carnegie, Cloudy 


The research reported in this thesis focuses on the high field localisation formed in the nanometric gaps between gold nanoparticles. Sub-wavelength sized nanoparticles irradiated with light support electron oscillations known as plasmons, and coupling of plasmons between adjacent nanoparticles leads to hot-spots of high field enhancements in the gaps between them. The scattering processes of molecules placed in these gaps is drastically enhanced by the hot-spot, a technique utilised in surface-enhanced Raman spectroscopy (SERS). This thesis focuses on the optimisation of the SERS signals from these gaps. What is the limiting volume we can observe and what does this tell us about molecular interactions on the single-atom scale? Two types of nanostructure are utilised in this thesis for formation of nanometric gaps: long chain nano-aggregates and single junction nanoparticle-on-mirror structures. The first part of this thesis explores the use of nano-aggregates for plasmonic sensing via SERS. A key parameter to control is the gap distance between adjacent nanoparticles. Here the barrel-shaped cucubit[n]uril macromolecules are utilised. I show how nano-aggregates formed with CB[n] spacer molecules can be used for robust drug sensing of synthetic cannabi- noids down to the nanomolar regime through use of reliable gap distances in conjunction with principal component analysis (PCA) algorithms. I also show how depositing aggregates onto a substrate can effect both the inelastic and elastic scattering, through mapping the signals across a 12 × 12 μm area, demonstrating that although placing aggregates on a gold substrate can enhance the recorded signals, it is the inter-nanoparticle hotspots which dominate the signal. In the second part of this thesis I look at the subtleties of metal-molecule dynamics at single junctions formed in the nanoparticle-on-mirror (NPoM) construct. The nanoparticle- on-mirror construct comprises of a single nanoparticle spaced above a flat gold film by a molecular spacer layer. Here the spacer layers are self-assembled monolayers (SAMs) which form a robust coverage of the metal film. I show that ‘picocavities (atomic protrusions from the gold surface) can be observed and studied at room temperature. A key finding is the discovery and characterisation of new ‘flare modes’ which are millisecond increases in light intensity, shown to likely come from regions of reduced local plasma frequency, indicative of a nanoparticle grain boundary.





Baumberg, Jeremy


plasmonics, sensing, nanoparticle


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge