Site-dependent TERS study of a porphyrin molecule on Ag(100) at 7 K

Single-molecule tip-enhanced Raman spectroscopy (TERS) has emerged as an important technique for structural analysis at sub-molecular scale. Here in this work, we report a TERS study of an isolated free-base porphyrin molecule adsorbed on the Ag(100) surface at cryogenic temperature (similar to 7 K). Site-dependent TERS spectra reveal distinct local vibrational information for the chemical constituents within a single molecule. Moreover, distinct spatial features among different Raman peaks can be resolved from the TERS mapping images. These images are found to associate with related vibrational modes, enabling to resolve the mode associated with N-H bonds at the sub-nanometer level. This study will provide deep insights into the symmetry of adsorption configurations and local vibrational information within a single molecule.