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 tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) single crystals in aqueous solution have been studied by in situ electrochemical scanning tunneling microscopy (ECSTM) both at equilibrium and under electrochemically driven dissolution conditions. The TTF and TCNQ molecules present in segregated molecular stacks at the crystal surface have been resolved at atomic resolution in different solutions and at various applied electrochemical potentials. The Images display the same atomic features seen by STM in air. Surface features and defects on the molecular scale, such as flat terraces, ledges of monomolecular height, kinks due to molecules of reduced coordination at the ends of molecular stacks, and vacancies within stacks due to missing molecules, have been seen to play a crucial par? in the dissolution processes at these electrodes. Observations of interfacial dissolution and electrochemical reactions under controlled potential by dynamic ECSTM imaging at a molecular level suggest that the kinetics of these processes are dependent on the orientations of surface ledges and kink density in relation to the crystallographic axes of the crystal. The mechanism of dissolution is found to involve preferential removal of molecules along the molecular stacks in a molecule-by-molecule sequence occurring at the kink sites. These phenomena can be rationalized in terms of their relation to the anisotropic properties of this material which arise from strong intermolecular 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 discussed.