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 part 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.