Robust probes for high resolution chemical detection and imaging

Abstract

There are needs for both high resolution imaging and high sensitivity detection/analysis of surface chemistry on a nanometer scale. These needs can be addressed with Raman spectroscopy coupled with schemes that provide extraordinary enhancement of the Raman signal, namely surface enhanced (SERS) and tip enhanced Raman spectroscopy (TERS). Advances in applications of high resolution imaging and high sensitivity detection will be enabled by two specific improvements: increased signal enhancement and increased robustness of the plasmonic structures needed to achieve enhancement. Robustness and stability are especially important for those plasmonic structures made of silver that usually provide the best enhancements. Here we focus particularly on TERS, in which a plasmonic structure is placed on a scanning probe microscope tip in order to achieve high lateral resolution imaging. We have demonstrated that aluminum oxide protected silver plasmonic structures show significantly increased robustness against chemical and mechanical degradation when compared to unprotected analogues without loss of enhancement. A 2-3 nm thick coating of aluminum oxide prevents chemical attack of the underlying silver film for three months in a desiccator, significantly increasing the storage life of current probes. The same protective coating also extends the scanning life of the probe when the probe is used to image a hard patterned silicon substrate.