Efficient fabrication of gold tips by electrochemical etching for tip-enhanced Raman spectroscopy

Tip-enhanced Raman Spectroscopy (TERS) is a powerful tool for analyzing physical and chemical properties of material at the nanoscale. The most widely used probes for TERS have metal(Au or Ag)-coated AFM tips. However, the durability of metal-coated AFM tips are limited because the coating layers can be easily worn out during the measurement. Furthermore, the Ag-coated tip is oxidized when it is exposed to air. To overcome these limitations, we propose a method of fabricating a Au tip for TERS with a Au wire by electrochemical etching. Pulse waves generated by pulse width modulation was applied to an electrochemical etching system, and it was optimized by controlling etching parameters (voltage, duty cycle, frequency and immersion depth) to achieve a smoothly etched surface with a tip radius of around 20 nm. With optimized etching parameters, a Au probe tip having a desired tip radius was successfully fabricated with a yield rate of approximately 85 %. Finally, a tuning fork-based Au probe for TERS was fabricated by attaching the Au tip to a tuning fork, and its operation was confirmed by obtaining a topographical image and Raman maps of carbon-nanotubes (CNTs).