Preparation and Characterization of Optically-active Metal Probes for Scanning Chemical Microscopy

Tip-enhanced Raman spectroscopy (TERS) has enabled spatially correlated topographic and chemical imaging of biomolecules, catalysts, photovoltaics, and materials on the nanoscale. As the name suggests, the tip is an important part of the TERS experiment; tip size sets the spatial resolution, whereas tip material (typically gold or silver) and shape determine how well far-field laser light couples to electron oscillations (plasmons) in the tip.  These plasmons create a tightly confined field that enhances Raman scattering of molecules in the tip-surface gap, enabling sub-diffraction limited chemical identification. Tips are typically prepared via electrochemical etching of metal wires in strong acid or base solutions. The etching process is not well understood, and the production of sharp, plasmonically active tips remains a key challenge in the TERS field.

To address this issue, we use a highly sensitive tuning fork oscillator to study the electrochemical tip etching process. Specifically, we monitor the oscillator frequency and amplitude during electrochemical tip etching under varying wire immersion depth, etching solution concentration, and different etching parameters (e.g. voltage, current, time, etc.). In this presentation, we compare tuning fork dynamics during tip etching to overall tip morphology and the corresponding optical activity from TERS experiments in an effort to improve control of tip preparation.