Fabrication of Au tips for tip-enhanced Raman spectroscopy

Abstract

Tip-enhanced Raman spectroscopy is a newly developed technique that can provide a very high sensitivity and spatial resolution. It requires tips with a very sharp apex and smooth surface free of any contamination to provide the necessary enhancement effect. Au tips with a smooth surface and a radius ranging from 10-30 nm were fabricated by electrochemical etching of Au wires at a constant potential in a mixture of fuming HCl and absolute ethanol with a volume ratio of 1:1. The ending point was monitored by a feeback control with a preset etching current smaller than the base current. The influence of the etching voltage on the sharpness and surface structure of the tips were systematically investigated and it was found that Au tips with a high quality could be obtained at an etching voltage of 2.2 V versus Au wire. By comparing the current-time curves of the etching process and the SEM images of each individual tip, we found that the current-time curves recorded during the etching process could be effectively used to evaluate the possible quality of tips. A curve with a good periodicity at the ending point of etching process results in a tip with a good shape and sharp apex. As a consequence, it is not necessary to use SEM to characterize the tips in the future preparation of tips, which will not only improve the efficiency during the experiments but also avoid to a large extent any possible contamination. The quality of the tip was demonstrated by getting STM image of the terrace on a HOPG surface. The obtained Au tip can also be used as micro-(nano-) electrodes and the electromagnetic field enhancer in tip-enhanced spectroscopy.