Probing Interfacial Electronic Effects on Single‐Molecule Adsorption Geometry and Electron Transport at Atomically Flat Surfaces

Clarifying interfacial electronic effects on molecular adsorption is significant in many chemical and biochemical processes. Here, we used STM breaking junction and shell‐isolated nanoparticle‐enhanced Raman spectroscopy to probe electron transport and adsorption geometries of 4,4′‐bipyridine (4,4′‐BPY) at Au(111). Modifying the surface with 1‐butyl‐3‐methylimidazolium cation‐containing ionic liquids (ILs) decreases surface electron density and stabilizes a vertical orientation of pyridine through nitrogen atom σ‐bond interactions, resulting in uniform adsorption configurations for forming molecular junctions. Modulation from vertical, tilted, to flat, is achieved on adding water to ILs, leading to a new peak ascribed to CC stretching of adsorbed pyridyl ring and 316 % modulation of single‐molecule conductance. The dihedral angle between adsorbed pyridyl ring and surface decreases with increasing surface electronic density, enhancing electron donation from surface to pyridyl ring. STM breaking junction and shell‐isolated nanoparticle‐enhanced Raman spectroscopy techniques have been successfully applied to probe a solvent modification induced interfacial electronic effect on tuning contact geometries and electron transport of pyridyl molecules at atomically flat Au (111). Modulation of binding geometry from vertical to tilted and flat is observed and a 316 % modulation of single‐molecule conductance is achieved.