Force Spectroscopy of Iron Tetraphenylporphyrin Molecules with Cl Tips

At low sample voltages, Cl ions may be bidirectionally transferred between a Fe tetraphenylporphyrin (FeTPP) molecule on Au(111) and the tip of a low-temperature scanning tunneling microscope that also operates as an atomic force microscope. Via this effect, Cl-terminated gold tips are prepared. Chlorinated and pristine metal tips are used to probe the forces and conductances of contacts to Au(111), FeTPP, and chlorinated FeTPP. The interaction between the substrate and the Cl ion at the tip may be modeled by a point charge of ≈−0.3 e interacting with its image. The maximal attraction experienced by the tip in junctions with one Cl ion is much larger when the ion is located at the molecule. Junctions with two Cl ions exhibit two maxima of the attractive force. Simultaneous imaging of the tunneling current and the frequency shift of the force sensor with Cl tips reveals intriguing contrast changes that are tentatively interpreted in terms of tip-induced distortions of the molecules. Density functional theory calculations reproduce important aspects of the experimental data.