From fano to nano:

: Mechanisms and applications of surface enhanced raman spectroscopy

Abstract

Surface enhanced Raman scattering/spectroscopy (SERS) is a trace sensing technique that uses collective surface electron oscillation-light excitations (plasmon-polaritons) to enhance the inelastically scattered light (Raman scattering) from a material of choice. However, SERS studies suffer from a trade-off between sensitivity and signal uniformity, which may be hindering analytical applications. Plasmon mixing can increase local electric fields and when this involves a spectrally broad resonance and another narrower resonance, very sharp asymmetric peak shapes can result, known as Fano resonances. This may be useful in SERS: more energy confined to the region near the nanostructures and proximal analyte molecules, provides a larger SERS enhancement. In the literature, there appears to be an evasion of an experimental SERS investigation with a ‘real world’ sensing substrate. In this thesis, we examine gold-metallised, polyurethane nanodome arrays for evidences of Fano resonance benefit to experimental SERS (in a multi-wavelength study). This is complemented by experimental far-field and numerical data. It is concluded that plasmon mixing is involved in an unexpectedly high SERS enhancement in samples with relatively low nanostructure packing density and that the positions of the respective ‘isolated’ and ‘system’ resonances, along with coupling degree and relative linewidth, may be projected via a classical analogue of coupled oscillators. In a subsequent study, leaning gold-covered silicon nanopillars are analysed where plasmonic interference and ‘substrate-solution hydrophobic effects’ are provided as explanations for the magnitudes and patterns of SERS enhancements observed via numerical studies. This is alongside a more conventional numerical investigation into dimer systems with nanometric-sized gaps. Very small inter-dimer separations (< 2 nm) or even a degree of coalescence are favoured. Finally, detecting explosive precursors is explored as part of a large European consortium: ‘Bomb Factory Detection by Networks of Advanced Sensors.’ Via a novel chemical derivatisation approach, nitric acid is detected via SERS down to 100 ppb with ppt range projected.

Date of Award	Dec 2019
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Robert Pollard (Supervisor) & Miryam Arredondo-Arechavala (Supervisor)