Voltammetric and in situ frequency modulation atomic force microscopic investigation of phenalenyl derivatives adsorbed on graphite surfaces

A combined electrochemical and in situ frequency modulation atomic force microscopy (FM-AFM) study of phenalenyl derivatives, tetra tert-butyl derivatives of s-indacenodiphenalene (TTB-IDPL), adsorbed on a highly oriented pyrolytic graphite (HOPG) electrode was investigated. The cyclic voltammetry (CV) showed stable and adsorption-type redox peaks from TTB-IDPL at the double-layer region of the HOPG basal plane, but the peak became much more intense by adding the anodic potential. In situ FM-AFM images of the molecular film revealed the casted molecules formed aggregated islands on the HOPG surface at cathodic potentials. When anodic potentials were applied, the height of aggregates increased and finally became mobile on the surface. By stepping back to the cathodic potentials, the aggregates dispersed as isolated molecules that were invisible by AFM measurements. This morphology change resulted in the increase of the CV peak originated from TTB-IDPL. Reversible dispersion-aggregation processes depending on the electrode potential were imaged by FM-AFM. The size of aggregates at each potential indicates that the origin of the dispersion of aggregates could be the change of the interfacial free energy, which is dependent on the electrode potential. These AFM results aid in the correlation of the structure of the adsorbate layers with their electrochemical response.