Probing Nanostructures on Surfaces with Scanning Tunneling Microscopy and Tip-Enhanced Raman Spectroscopy

In nanostructures and chemistry on surfaces, subtle interactions and highly localized chemical environments are paramount as the system is confined to two dimensions. Here, a hybrid technique that combines imaging and spectroscopy was used to achieve ångström scale investigations of nanostructures on surfaces. Scanning tunneling microscopy (STM) provides the ability to probe and manipulate individual atoms and molecules on a surface, revealing local structures with atomic-scale topographic imaging. When the STM tip is composed of a plasmonic metal, illumination with focused light can result in a highly localized and atomically confined electromagnetic enhanced field at the apex of the tip. This results in the ability to perform tip-enhanced Raman spectroscopy (TERS) where a TERS spectrum reveals the highly sensitive vibrational fingerprint of a molecule or material. In ultra-high vacuum (UHV) and cryogenic temperatures, the spatial resolution of TERS can be pushed to the ångström scale, providing the means to spectroscopically characterize the on-surface interactions and reactions of individual molecules. Low temperature UHV-STM-TERS was used to investigate the fundamental interactions that underlie nanostructures on surfaces. The strength of molecule–substrate interactions were found to depend on the intrinsic properties of the molecules and substrate. A surface-catalyzed dehydrocyclization reaction was studied at the single-molecule level on various noble metal substrates, including a single-atom alloy. Additionally, a common reaction-based method used to fabricate carbon nanostructures on surfaces was investigated. It was found that local chemical environments can steer the selectivity of coupling reactions and that the inclusion of functional groups in the precursor monomer results in the formation of nanostructures defined by competing interactions. Finally, a subnanoscale spectroscopic study of interfacial interactions in a newly realized organic/borophene vertical heterostructure is described. This work concludes with a discussion of the future directions and outlook for TERS, framing it within the broader context of light–matter interactions at the atomic-scale. Fundamental studies at the spatial limit provide the means to visualize the intrinsic properties and local interactions of individual molecules and materials that are essential to the realization of new atomically precise nanostructures.