DISSOLUTION PROCESSES AT TTF-TCNQ SINGLE-CRYSTAL ELECTRODES - A DYNAMIC IN-SITU ELECTROCHEMICAL SCANNING-TUNNELING-MICROSCOPY STUDY

Surface transformations occurring at the (001) face of tetrathiafulval ene-tetracyanoquinodimethane (TTF-TCNQ) single crystals in aqueous sol ution have been studied by in situ electrochemical scanning tunneling microscopy (ECSTM) both at equilibrium and under electrochemically dri ven dissolution conditions. The TTF and TCNQ molecules present in segr egated molecular stacks at the crystal surface have been resolved at a tomic resolution in different solutions and at various applied electro chemical potentials. The Images display the same atomic features seen by STM in air. Surface features and defects on the molecular scale, su ch as flat terraces, ledges of monomolecular height, kinks due to mole cules of reduced coordination at the ends of molecular stacks, and vac ancies within stacks due to missing molecules, have been seen to play a crucial par? in the dissolution processes at these electrodes. Obser vations of interfacial dissolution and electrochemical reactions under controlled potential by dynamic ECSTM imaging at a molecular level su ggest that the kinetics of these processes are dependent on the orient ations of surface ledges and kink density in relation to the crystallo graphic axes of the crystal. The mechanism of dissolution is found to involve preferential removal of molecules along the molecular stacks i n a molecule-by-molecule sequence occurring at the kink sites. These p henomena can be rationalized in terms of their relation to the anisotr opic properties of this material which arise from strong intermolecula r bonding and partial charge transfer between the molecules within the molecular stacks but with weaker interactions between stacks. Effects caused by the proximity of the STM tip leading to local modification of the interfacial electrochemistry have also been observed and are di scussed.