Molecular scale chemical imaging of the orientation of an on-surface coordination complex by tip-enhanced Raman spectroscopy

Metal-organic coordination structures at interfaces play an essential role in many biological and chemical systems. Understanding the molecular specificity, orientation and spatial distribution of the coordination complexes at the nanometer scale is of great im-portance for effective molecular engineering of nanostructures and fabrication of functional devices with controllable properties. However, fundamental properties of such coordination systems are still rarely studied directly. In this work, we present a spectro-scopic approach on the basis of tip-enhanced Raman spectroscopy (TERS) to investigate a cobalt(II) tetraphenyl-porphyrine (CoTPP) coordination species on the scale of a single molecule under ambient conditions. Coordination species anchored on gold surfaces modified with pyridine thiol self-assembled monolayers can be spectroscopically distinguished and mapped with ca. 2 nm resolution. In addition, in combination with density functional theory simulations, the adsorption configuration and molecular ori-entation of the coordination complexes are also revealed using TERS imaging.