Optical Spectroscopic Probing and Atomic Visualization of the Motion of N-Heterocyclic Carbenes on Ag(111)

The application of N-heterocyclic carbenes (NHCs) as versatile anchors for planar surface modifications has been well documented over the past decade. Despite its fundamental importance to the formation of self-assembled NHC monolayers on surfaces, the microscopic mechanism behind the mobility of NHCs has primarily been explored through theoretical studies; an atomic-level experimental understanding of NHC motion on surfaces remains elusive. Here, we combine tip-enhanced Raman spectroscopy (TERS) and scanning tunneling microscopy (STM) to investigate the mobility of a model NHC on Ag(111). Two distinct molecular behaviors are observed, depending on substrate preparation. Room-temperature deposition leads to diffusing NHC-Ag adatom complexes exhibiting a ballbot-like motion, chemically identified by TERS through their spectroscopic fingerprint. By contrast, NHCs deposited at low temperature are stabilized on Ag(111) as isolated single molecules directly bound to the substrate. Significantly, a desorption/readsorption scenario is suggested for the displacement of NHCs by moving otherwise immobile single NHCs deposited at low temperature via STM manipulation, with their trajectory traced to atomic precision. This study provides chemical and atomic-level insights into the mobility of NHCs, which will advance the understanding of the fundamental properties of NHC-based surface modifications at the spatial limit.