适用于针尖增强拉曼技术的Au针尖的研制

Fabrication of Au Tips for Tip-enhanced Raman Spectroscopy

采用针尖增强拉曼光谱技术(TERS)研究了刻蚀电位对Au针尖刻蚀效果的影响,初步探讨了刻蚀过程的机理.通过监控刻蚀过程中的振荡电流-时间曲线并与扫描电镜得到的结果比较,发现可以直接利用电流-时间曲线简单地判断刻蚀后针尖的可能形状,而无需再借助扫描电镜进行表征.这不但提高了实验效率,而且还可以避免针尖在转移和电镜表征过程中可能引入的污染.研究结果表明,在体积比为1∶1的发烟盐酸和无水乙醇的刻蚀液中,于2.2V的电压下,结合电化学方法控制终点可以得到形状对称尖锐的针尖.这种针尖不但适合于TERS研究,而且可用作STM针尖和微纳电极并用于其它针尖增强光学技术.

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.