In Situ Discrimination between Axially Complexed and Ligand-Free Co Porphyrin on Au(111) with Tip-Enhanced Raman Spectroscopy

Simultaneous chemical and topographic information about cobalt tetraphenyl-porphyrin (CoTPP) adlayers formed on a Au(111) single crystal is obtained with tip-enhanced Raman (TER) spectroscopy. We distinguish in situ between sample areas covered with an ordered adlayer of CoTPP and areas covered with a spontaneously formed disordered phase. The Raman vibrational fingerprints collected from the nanometer-sized near-field region just below a scanning tunnelling microscope (STM) tip are correlated with the adsorbate structures seen in the STM images. We assign the TER spectral features of the disordered phase to CoTPP complexes with CO and/or NO axial ligands, whereas the TER spectrum obtained from the ordered phase does not show any indication of additional axial complexation of CoTPP.