Picocavity-based Tip-enhanced Raman Spectroscopy at the Single-molecule Level

Tip-enhanced Raman spectroscopy (TERS) is a powerful method for chemically identifying single molecules on surfaces. Although the mechanisms of the Raman signal enhancement have long been proposed, their precise evaluation and understanding remain challenging. This article introduces two key techniques to obtain single-molecule Raman signals with high reproducibility using TERS : (1) electromagnetic enhancement by a strong plasmonic field confined at atomic-scale structures of the tip apex, namely picocavities, and (2) chemical enhancement by forming an atomic point contact between the tip and the target molecule. These techniques have enabled TERS measurements on advanced and challenging samples, including molecular hydrogen physisorbed on metal surfaces and organic molecules chemisorbed on a non-plasmonic semiconductor substrate. Thus, characterizing and controlling plasmonic junctions at the atomic scale would further expand the TERS targetability.