Ångström-Resolved Tip-Enhanced Raman Spectroscopy

This chapter describes the principle and application of Ångström-resolved tip-enhanced Raman spectroscopy (TERS). The theoretical background for TERS, particularly regarding the origin of ultrahigh spatial resolution, is briefly introduced from several different aspects, namely from plasmonic nanocavity to picocavity, from single molecules to single chemical bonds, and from electromagnetic to chemical enhancement. Then we describe the experimental setups for single-molecule TERS measurements, including the technical details of sample preparation and optic measurements as well as the fabrication and judgement of TERS-active tips. This is followed by a detailed account on recent advances in single-molecule TERS, mainly from three aspects: (1) imaging a single molecule with subnanometer resolution; (2) resolving single chemical bonds and their making and breaking processes, including chemical structure reconstruction through scanning Raman picoscopy, tracking bond making and breaking processes in surface reactions, and anti-Stokes Raman thermometry in a single molecule; and (3) applications in 2D materials. The conclusion and future outlook of the Ångström-resolved TERS technique are briefly described at the end of this chapter.