Design, fabrication and characterisation of improved tip-enhanced Raman Spectroscopy probes

Probes for tip-enhanced Raman spectroscopy (TERS) are currently constructed from silicon atomic force microscopy (AFM) cantilevers with a grainy silver coating. The grainy coating creates randomly dispersed metal nanoparticles that can concentrate incident light into a volume with a radius on the order of 10,nm. This locally enhanced light enables sub-diffraction limited optical microscopy and Raman spectroscopy; however, a number of issues arise from the random grain formation at the tip apex, including unreliable enhancement, an offset between TERS and AFM maps, suboptimal AFM and TERS resolution, and artefacts in the TERS images. Furthermore, silver probes corrode rapidly in ambient conditions, which limits the effective lifetime to one day or less. These issues significantly limit the performance and practicality of tip-enhanced microscopy and spectroscopy applications. This thesis investigates the use of gold nanocone probes as an alternative to grainy silver-coated AFM probes. Rather than relying on random grain formation, gold nanocones are fabricated with dimensions that result in surface plasmon resonance at the intended illumination wavelength. Compared to silver coated probes, gold nanocones are chemically stable and result in only one centre of enhancement, which is located at the mechanical apex of the probe. These properties address many of the issues experienced with silver coated probes; however, gold nanocones must be designed to optimise the resolution and enhancement, and the probe fabrication becomes significantly more challenging. This thesis describes the optical modelling, design, and fabrication of gold nanocone probes for improved tip-enhanced Raman spectroscopy in the visible spectrum. The gold nanocone probes are compared to commercially available TERS probes using a new method for performance measurement, which utilises single-walled carbon nanotubes as 1D scattering objects. Experimental results demonstrate that gold nanocone probes provide higher spatial resolution, artefact-free imaging, longer lifetime, and improved image contrast comparable to grainy silver probes. The final contribution of this thesis is a new imaging mode for tip-enhanced Raman spectroscopy named modulated-illumination intermittent-contact TERS (MIIC-TERS). This mode combines the high enhancement of contact TERS with the lower shear forces of intermittent contact mode. MIIC-TERS is demonstrated using commercial probes and a gold nanocone probe.