Probing interfacial electronic effects on single-molecule adsorption geometry and electron transport at atomically-flat surfaces.

Clarifying interfacial electronic effects on molecular adsorption is of great significance in many chemical and biochemical processes but remains a challenge. Here, we have successfully used STM breaking junction and shell-isolated nanoparticle-enhanced Raman spectroscopy technique to probe electron transport and adsorption geometries of pyridyl molecules at Au(111). Modifying Au(111) 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. Furthermore, fine modulation from vertical, tilted, to flat, is achieved on adding water to ILs, leading to a new peak around 1633 cm -1 ascribed to CC stretching mode of adsorbed pyridyl ring and 316% modulation of single-molecule conductance. Combining with DFT calculation reveals that the dihedral angle between adsorbed pyridyl ring and surface decreases with increasing surface electronic density, which enhances electron-donation from surface to pyridyl ring.