Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopy technique to extend the inherent specificity of Raman spectroscopy by sensitivities down to the single molecule level. The ultra-high sensitivities observed in SERS are reliant on the plasmonic properties of silver (Ag) and gold (Au) nanoparticles. Plasmons, the oscillation of the metal particle electron cloud, give rise to the electric fields responsible for SERS enhancement. Plasmonic properties depend crucially on features of the metallic nanostructure. Optimal plasmonic response for SERS applications is achieved on tailored nanostructures with Ag and Au nanoparticle sizes ranging from 10 to 100 nm at the lowest possible particle edge distances. SERS benefits further from the unique properties of semiconductor/metal composites. A promising methodology to produce such nanostructured composites is photocatalytic metal deposition on semiconductors. Herein, optimisation of the pulsed laser-assisted photocatalytic deposition process for SERS applications is demonstrated. The utilised semiconductor, a transparent titanium dioxide thin film, was tailored for the laser-assisted process to capitalise on laser writing and direct laser interference patterning (DLIP) capabilities. Effects of key process parameters on Ag and Au nanostructure were systematically investigated by Design of Experiment. Several significant parameter effects were identified, including the effect of nanosecond compared to picosecond pulse lengths. The Au deposition process showed distinct differences from the Ag deposition. Informed by Design of Experiment, optimised Ag and Au nanostructures on semiconductor thin film for use as SERS substrates were prepared. SERS performance was studied at excitation wavelengths of 532, 633 and 785 nm using Thiophenol as the probe molecule. Utilising the laser writing approach, broadband SERS performance with enhancement factors (EF) of 10^5 to 10^6 was achieved for both metals. Utilising the DLIP approach multiscale patterned nanostructures at sub-micron periods were fabricated. Diffractive far-field coupling as well as the coupled plasmonic modes in periodic resonator chains increased SERS EFs of Ag and Au nanostructures further, to 10^6 to 10^7.
Pulsed Laser-Assisted Photocatalytic Metal Deposition for SERS Applications
Hoffmann, M. (Author). 2023
Student thesis: Doctoral Thesis › Doctor of Philosophy