NANOSCALE VISUALIZATION OF CRYSTAL HABIT MODIFICATION BY ATOMIC-FORCEMICROSCOPY

Atomic force microscopy (AFM) studies of N-[4-[4-cyano-2-(2-furanylmet hylene)-2, ihydro-5-oxo-3-furanyl]phenyl]-l-butanesulfonamide (1), a c rystalline dye used in photographic film, are described. AFM of the la rgest crystal face of freshly cleaved blade-shaped crystals of 1 revea led molecular scale contrast with periodicity identical with the low-e nergy oleophilic (100) plane, and ledges oriented along [001] with ste p heights equal to the a lattice parameter, which corresponds to the h eight of single molecules of 1. These features are consistent with the solid-state structure of the dye, which reveals a low-energy oleophil ic (100) plane and strong intermolecular pi<-pi> interactions along [0 01]. Real-time in situ AFM reveals that growth of 1 near equilibrium c onditions proceeds by flow of the [001] ledges along the [010] directi on, consistent with the relative surface energies of the crystal plane s and a terrace-ledge-kink mechanism. Addition of an ionic octyl oligo ether sulfate surfactant to the growth medium resulted in significant curvature of the ledge topography in a manner consistent with the form ation of [010] ledges, suggesting attachment of surfactant molecules t o higher energy {001} or {011} step planes. This is manifested in the suppression of the nucleation of 1, and changes in the crystal habit o f submicron crystals from needles to blocks due to inhibited growth al ong the otherwise fast growing [001] direction. These studies provide direct and rapid observation of the origins of crystal habit modificat ion by molecular additives at the nanoscale level.