Understanding the Plasmonics of Nanostructured Atomic Force Microscopy Tips

TL;DR

This study investigates how the geometry of AFM tips influences their plasmonic properties, revealing that spherical tips support localized surface plasmon resonances while sharp tips do not, impacting optical spectroscopy techniques.

Contribution

It provides experimental and numerical evidence that tip geometry critically affects plasmonic responses, guiding the design of better tips for spectroscopic applications.

Findings

Spherical AFM tips exhibit localized surface plasmon resonances.

Sharp AFM tips show minimal plasmonic response.

Tip geometry is crucial for plasmon-enhanced spectroscopy.

Abstract

Structured metallic tips are increasingly important for optical spectroscopies such as tip-enhanced Raman spectroscopy (TERS), with plasmonic resonances frequently cited as a mechanism for electric field enhancement. We probe the local optical response of sharp and spherical-tipped atomic force microscopy (AFM) tips using a scanning hyperspectral imaging technique to identify plasmonic behaviour. Localised surface plasmon resonances which radiatively couple with far-field light are found only for spherical AFM tips, with little response for sharp AFM tips, in agreement with numerical simulations of the near-field response. The precise tip geometry is thus crucial for plasmon-enhanced spectroscopies, and the typical sharp cones are not preferred.