Developments in Tip-Enhanced Raman Scattering using silver nanowires for nanoscale characterisation of novel carbon materials

The many optical spectroscopies are known as key analytical tools throughout a wide range of science and technology, and have played a significant role in shaping our essential understanding of many fundamental phenomena. Raman spectroscopy is one example, in which the inelastic scattering of light by molecules or crystals is used to elucidate details of vibrational or electronic structure. One particular case where Raman spectroscopy is useful is with the family of nanocarbon materials. Centred on graphene, but also encompassing things such as carbon nanotubes and graphene nanoribbons, these systems have a range of outstanding properties that suggest a wide variety of potential applications in the future. They can also be easily functionalised with molecules to tailor their properties depending on circumstances. Crucially, the properties of these materials strongly depend on how they are structured at the nanoscale. However, by using light, techniques such as Raman spectroscopy and microscopy run into a fundamental resolution boundary known at the diffraction limit, beneath which they can only provide ensemble averaging. For visible light this limitation is on the order of several 100 of nanometres.

One way to overcome this using Raman spectroscopy is known as Tip-enhanced Raman spectroscopy (TERS), in which it is combined with scanning probe microscopy (SPM). By using the field-enhancing properties of silver or gold SPM tips, the Raman signal can be enhanced and localised to the area beneath the tip. This allows TERS to give correlated Raman and topographic images at a resolution below 10 nm. However, the notorious experimental difficulties of TERS make it a very challenging technique practiced successfully by only a relatively small number of research groups worldwide. The key problem with TERS is in fabricating tips with reproducible and predictable properties, which requires precise control over surface structure on the nanoscale.

To begin with, the use of Raman spectroscopy is highlighted through measurements detailing the thermal stability of molecularly functionalised graphite, the three-dimensional analogue of graphene. A simple kinetic model is combined with Raman spectroscopy at elevated temperatures to detail properties of the chemical bond formed between the molecules and graphite, and how this shows dependency on the chemical composition of the functionalising molecule.

The bulk of the thesis thereafter is focused on the development of TERS as a reliable tool for nanoscale analysis. This involves one section describing modifications to the TERS setup made over the course of the PhD. On top of this, two sections are devoted to the development of reproducible TERS tips. By using chemically synthesised silver nanowires, the traditional problems with TERS can be largely overcome due to their well-defined structure. These tips show significant reproducibility and signal enhancements over research standards. To expand the range of systems where TERS can be used, a further study involves ‘nanostructuring’ these tips through electrical cutting to further increase the enhancement. This is important because on bulk or dielectric systems, TERS signals are often very weak. The introduced nanostructure strengthens the field-enhancing properties of these probes when tested on a variety of samples.

Finally, these developments are applied back to nanocarbons through preliminary studies on two different systems. In the first case, the success of a reaction on carbon nanotubes is indicated by TERS imaging at the single nanotube level. Following this, initial studies on functionalised and bare graphene again show that TERS is positioned as a unique tool for characterising these systems. This is concluded by perspectives and directions for future studies.