Werzer, O.; Kunert, B.; Roblegg, E.; Zimmer, A.; Oehzelt, M.; Resel, R.: Surface Induced Order of Solution Processed Caffeine Needles on Silica and Muscovite Mica. Crystal Growth & Design 13 (2013), p. 1322-1328
10.1021/cg301812m

Abstract:
In this work the orientational order of needle-shaped caffeine crystallites at silica and muscovite mica surfaces is investigated by X-ray diffraction experiment and optical microscopy. The investigation of thin films reveals the formation of caffeine in the anhydrous monoclinic beta-polymorphic structure independent of the surface with the disclike caffeine molecules assemble in an edge-on conformation. The silica surface provides isotropic properties for crystal growth which leads to a fiber texture arrangement of the needles; a majority of needles have a 010 texture with all other directions being randomly distributed. The mica surface with its highly regular surface structure induces defined textures and azimuthal order of the caffeine needles resulting in a fully defined epitaxial order. The edge-on and disclike caffeine molecules align along the [100](m), [110](m), and [1 (1) over bar0](m) real space directions of the mica substrate and show a mirror symmetry around the [110](m) The experiments on these two surfaces show that the type of surface is able to induce order during caffeine needle growth.