NANOSCALE SURFACE-TOPOGRAPHY AND GROWTH OF MOLECULAR-CRYSTALS - THE ROLE OF ANISOTROPIC INTERMOLECULAR BONDING

Atomic force microscopy (AFM) and phase measurement interferometric mi croscopy (PMIM) of the molecular crystals alpha-glycine and (TMTSF)(2) ClO4 (TMTSF = tetramethyltetraselenafulvalene) reveal that crystal top ography, growth, and etching reflect the relative strengths of solid s tate intermolecular bonding. The (010), (110), and (011) faces of alph a-glycine exhibit terraces, ledges, and kinks that can be interpreted on the basis of intermolecular hydrogen bonding in these planes. A str ong preference for [100] ledges on the (001) face of (TMTSF)(2)ClO4 is a consequence of strong intermolecular charge transfer interactions b etween TMTSF molecules stacked along this direction. Dynamic in situ m easurements of growth and etching indicate that the topographic struct ure is formed and preserved during these active processes. AFM studies of crystal growth and etching of (TMTSF)(2)ClO4 are particularly conv enient, as these processes can be controlled through adjustment of the electrochemical potential applied to single crystals. In this case, l ayer-by-layer growth or etching, in which the layers correspond to sin gle unit cell heights, occurs by a terrace-ledge-kink mechanism with t he direction of fastest growth or etching oriented along the TMTSF sta cking axis. In both alpha-glycine and (TMTSF)(2)ClO4, the nanoscale to pographic structure resembles the macroscopic morphology, suggesting s elf-similarity across the length scales examined. The role of excess i nterfacial energy during crystal growth is evident from the distributi ons of terraces, ledges, and kinks, which differ from those observed u nder equilibrium conditions.