Facile Electrochemical Two-Step Etching to Improve the Performance of Au Tips in Tip-Enhanced Raman Spectroscopy

Tip-enhanced Raman spectroscopy (TERS) has emerged as a pivotal tool for nanoscale chemical imaging. The tip plays the dominate role in TERS, where the geometry of the tip apex dramatically influences the efficiency of surface plasmon resonance and thus determines the TERS enhancement. However, tuning the cone angle without compromising the sharpness of the tip apex is quite challenging, often requiring time-consuming nanoprocessing techniques. Here, we developed a facile electrochemical two-step etching method to decouple cone angle tuning from tip sharpness in Au TERS tips. After the first-step conventional electrochemical etching, the second etching step leverages potential-dependent electrochemical oscillation-based anodic dissolution of Au to reshape the sharp Au tip to have a larger cone angle (15-40°) but sharp apex (< 30 nm radius). Under the optimal etching potential, this method yields 5-10 folds improvement in TERS enhancement and enables high-speed TERS imaging (0.1 s/pixel) with 3.5 nm spatial resolution, comparable to state-of-the-art conventional Au tips. This facile electrochemical two-step etching method offers a practical solution in normal TERS laboratories to significantly improve the performance of Au TERS tips by tuning the cone angle, which will accelerate the applications of TERS.