Novel probes for multimodal imaging at the nanoscale

Atkins, David (2026) Novel probes for multimodal imaging at the nanoscale. PhD thesis, University of Glasgow.

Abstract

Tip enhanced Raman spectroscopy is a technique which allows the user to retrieve topographic, morphological and chemical information of a sample with single molecule resolution, far beyond the Raman spatial resolution of conventional techniques. Its usefulness extends into many scientific fields, such as medicine, electronics and material science. Conventional probes used to conduct TERS , where metal is deposited onto commercial AFM tips, tend to be made of silver. This is due to silver having strong plasmon resonances in the visible and near-infrared regime, alongside low optical losses and there are well established processes for making them. The issue with these probes is that they are commonly deposited one at a time and silver is known to grow a sulphide layer. This sulphide film inhibits the electric field, shifting the plasmon resonance and limits the lifetime of these probes to 24-48hrs. The use of a conventional AFM tip as a substrate for the plasmonic film is also problematic since the coupling of the far field radiation to the tip plasmon occurs close to the tip and is facilitated by local roughness, leading to critical and irreproducible alignment requirements for the illuminating beam. The issue of tip contamination and lifetime may be eliminated by the use of gold instead of silver, at the expense of plasmonic performance. The use of a long-lived metal allows the use of more elaborate coupling methods based on plasmonic gratings or slot couplers fabricated by focused ion beam milling or self-aligned plasma processing. Both techniques of silver growth and gold coupler tip fabrication are irreproducible however, they do not have nm resolution placement or techniques to ensure the same tip is made over again using plasma or chemical etches. The work-around for this is to use a focused ion beam with the benefit of very high resolution (nm), but each probe has to be milled one at a time. A lot of effort goes into making these tips, where if one fails due to over/under etch or other reasons, another metal deposition or plasma processing has to occur as the tip is scrapped.

A method for the wafer scale fabrication of novel TERS-AFM probes is presented. The probes are based on the use of a grating to couple light from free space into the dielectric of the tip at some distance from its apex, the grating allowing for the use of fixed – angle illumination at a known position far from the tip. Waveguide to surface plasmon coupling occurs near the tip and plasmons are then concentrated to the tip by a triangular metal structure. The fabrication was successfully completed, although operation at useful wavelengths was precluded by poor reproducibility of the probe dielectric thickness. Development work on the fabrication of scanning electro-chemical microscopy probes, where insulator adhesion to the metal electrode is a key issue, has also been been conducted.

To test the TERS probes, novel calibration samples were made. These samples employed statistical and correlation alignment strategies with an e-beam lithography tool to produce 70nm thick gold dots and dimers with separation distances of 1nm and above (1nm increments) in the x and y direction. Once coated with Raman Active molecules this sample will be useful for testing of TERS probes, allowing the study of separation distance vs. enhancement for different probe archetypes. A topography free sample with interdigitated electrode active areas has also been developed for the test of Scanning Electrochemical Microscope probes. This sample removes the introduction of topographic artefacts from a SECM scan, whilst retrieving useful information on the electro-chemical nature of the probes.

After extensive and rigorous testing, probes failed to exhibit local plasmonic enhancement at the available wavelength of the TERS systems. The lack of enhancement was confirmed by extensive processing of the acquired data using baseline correction and adaptive smoothness penalised least squares (asPLS) methods. For comparison of the SERS and TERS spectra the data was also intensity normalised and cosmic rays removed. This is attributed to poor control of dielectric deposition thickness preventing phase matching of the guided light to the surface plasmon polariton: Possible solutions are presented and their practicality discussed.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: T Technology > T Technology (General)
• Colleges/Schools: College of Science and Engineering > School of Engineering
• Supervisor's Name: Weaver, Professor Jonathan and Dobson, Dr. Phil
• Date of Award: 2026
• Depositing User: Theses Team
• Unique ID: glathesis:2026-85683
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 15 Jan 2026 11:56
• Last Modified: 15 Jan 2026 12:02
• URI: https://theses.gla.ac.uk/id/eprint/85683