Protruding hydrogen atoms as markers for the molecular orientation of a metallocene

A distinct dumbbell shape is observed as the dominant contrast feature in the experimental data when imaging 1,1'-ferrocene dicarboxylic acid (FDCA) molecules on bulk and thin film CaF₂(111) surfaces with non-contact atomic force microscopy (NC-AFM). We use NC-AFM image calculations with the probe particle model to interpret this distinct shape by repulsive interactions between the NC-AFM tip and the top hydrogen atoms of the cyclopentadienyl (Cp) rings. Simulated NC-AFM images show an excellent agreement with experimental constant-height NC-AFM data of FDCA molecules at several tip-sample distances. By measuring this distinct dumbbell shape together with the molecular orientation, a strategy is proposed to determine the conformation of the ferrocene moiety, herein on CaF₂(111) surfaces, by using the protruding hydrogen atoms as markers.