Nanoscale Surface Topography and Growth of Molecular Crystals: The Role of Anisotropic Intermolecular Bonding

Atomic force microscopy (AFM) and phase measurement interferometric microscopy (PMIM) of the molecular crystals α-glycine and (TMTSF)₂ClO₄ (TMTSF = tetramethyltetraselenafulvalene) reveal that crystal topography, growth, and etching reflect the relative strengths of solid state intermolecular bonding. The (010), (110), and (011) faces of α-glycine exhibit terraces, ledges, and kinks that can be interpreted on the basis of intermolecular hydrogen bonding in these planes. A strong preference for [100] ledges on the (001) face of (TMTSF)₂C1O₄ is a consequence of strong intermolecular charge transfer interactions between TMTSF molecules stacked along this direction. Dynamic in situ measurements of growth and etching indicate that the topographic structure is formed and preserved during these active processes. AFM studies of crystal growth and etching of (TMTSF)₂ClO₄ are particularly convenient, as these processes can be controlled through adjustment of the electrochemical potential applied to single crystals. In this case, layer-by-layer growth or etching, in which the layers correspond to single unit cell heights, occurs by a terrace-ledge-kink mechanism with the direction of fastest growth or etching oriented along the TMTSF stacking axis. In both α-glycine and (TMTSF)₂ClO₄, the nanoscale topographic structure resembles the macroscopic morphology, suggesting self-similarity across the length scales examined. The role of excess interfacial energy during crystal growth is evident from the distributions of terraces, ledges, and kinks, which differ from those observed under equilibrium conditions.