Nanoscale Visualization of Crystal Habit Modification by Atomic Force Microscopy

Atomic force microscopy (AFM) studies of N-[4-[4-cyano-2-(2-furanylmethylene)-2,5-dihydro-5-oxo-3-furanyl]phenyl]-1- butanesulfonamide (1), a crystalline dye used in photographic film, are described. AFM of the largest crystal face of freshly cleaved blade-shaped crystals of 1 revealed molecular scale contrast with periodicity identical with the low-energy oleophilic (100) plane, and ledges oriented along [001] with step heights equal to the a lattice parameter, which corresponds to the height of single molecules of 1. These features are consistent with the solid-state structure of the dye, which reveals a low-energy oleophilic (100) plane and strong intermolecular π-π interactions along [001]. Real-time in situ AFM reveals that growth of 1 near equilibrium conditions proceeds by flow of the [001] ledges along the [010] direction, consistent with the relative surface energies of the crystal planes and a terrace-ledge-kink mechanism. Addition of an ionic octyl oligoether sulfate surfactant to the growth medium resulted in significant curvature of the ledge topography in a manner consistent with the formation of [010] ledges, suggesting attachment of surfactant molecules to higher energy {001} or {011} step planes. This is manifested in the suppression of the nucleation of 1, and changes in the crystal habit of submicron crystals from needles to blocks due to inhibited growth along the otherwise fast growing [001] direction. These studies provide direct and rapid observation of the origins of crystal habit modification by molecular additives at the nanoscale level.