A comparison of gold and silver nanocones and geometry optimisation for tip‐enhanced microscopy. (14th September 2020)

• Record Type: Journal Article
• Authors: McCourt, Luke R.; Ruppert, Michael G.; Routley, Ben S.; Indirathankam, Sathish C.; Fleming, Andrew F.
• Is Part Of: Journal of Raman spectroscopy. Volume 51:Number 11(2020)
• Page Start: 2208
• Page End: 2216
• Publication Date: 2020-09-14
• Subjects: nano‐antenna -- nanocone -- near‐field enhancement -- tip‐enhanced raman spectroscopy (TERS); Raman spectroscopy -- Periodicals; 535.846
• DOI: 10.1002/jrs.5987
• Languages: English
• ISSNs: 0377-0486

Abstract

Abstract: In this article, boundary element method simulations are used to optimise the geometry of silver and gold nanocone probes to maximise the localised electric field enhancement and tune the near‐field resonance wavelength. These objectives are expected to maximise the sensitivity of tip‐enhanced Raman microscopes. Similar studies have used limited parameter sets or used a performance metric other than localised electric field enhancement. In this article, the optical responses for a range of nanocone geometries are simulated for excitation wavelengths ranging from 400 to 1000 nm. Performance is evaluated by measuring the electric field enhancement at the sample surface with a resonant illumination wavelength. These results are then used to determine empirical models and derive optimal nanocone geometries for a particular illumination wavelength and tip material. This article concludes that gold nanocones are expected to provide similar performance to silver nanocones at red and near‐infrared wavelengths, which is consistent with other results in the literature. In this article, 633 nm is determined to be the shortest usable illumination wavelength for gold nanocones. Below this limit, silver nanocones will provide superior enhancement. The use of gold nanocone probes is expected to dramatically improve probe lifetime, which is currently measured in hours for silver coated probes. Furthermore, the elimination of passivation coatings is expected to enable smaller probe radii and improved topographical resolution. Abstract : Boundary element method simulations are used to calculate the near‐field enhancement at plasmon resonance for a range of gold and silver nanocones above a glass substrate. Empirical models are presented which allow the resonance wavelength and near‐field enhancement to be predicted for gold and silver nanocones at visible excitation wavelengths.