Electrocrystallization of the organic superconductor bis(ethylenedithio)tetrathiafulvalene triiodide, (ET)2I3, on a highly ordered pyrolytic graphite (HOPG) substrate has been visualized using in situ atomic force microscopy (AFM). Previous studies have revealed the formation of a coincident epitaxial monolayer with a structure identical to that of a (001) layer of the superconducting beta phase of this material prior to bulk crystal growth. However, the symmetry of the HOPG substrate leads to domain boundary defects during self assembly of the separately growing domains. The number of defects is significantly reduced after an electrochemical annealing process in which the potential is cycled about the monolayer deposition potential. Annealing of these films is important if they are to be used in electronic devices as the defects may serve as barriers to electron transport in the two-dimensional layers.
In addition to (ET)2I3, monolayer growth also has been visualized during electrocrystallization of (ET)2ReO4 on HOPG. The role of coincident epitaxy with HOPG in the monolayer formation and molecular orientation will be discussed.
Structurally modified substrates have also been investigated. Studies of the electrocrystallization of (ET)2I3 on HOPG, thermally treated to produce well-defined monolayer depth pit structures, have demonstrated that (ET)2I3 monolayer growth can occur inside the pit structures. The presence of monolayer domain boundaries within large pits indicates that multiple, independent nucleation events can occur in the pits, providing an opportunity to determine critical nucleation sizes by varying the pit dimensions. Recently we have discovered that MoS2 substrates can be electrochemically etched, giving rise to monolayer deep triangular pits.