title: Complex Behavior of Caffeine Crystallites on Muscovite Mica Surfaces.
abstract: Defined fabrication of organic thin films is highly desired in technological, as well as pharmaceutical, applications since morphology and crystal structure are directly linked to physical, electrical, and optical properties. Within this work, the directed growth of caffeine deposited by hot wall epitaxy (HWE) on muscovite mica is studied. Optical and atomic force microscopy measurements reveal the presence of caffeine needles exhibiting a preferable alignment in the azimuthal directions with respect to the orientation of the defined mica surface. Specular X-ray diffraction and X-ray diffraction pole figure measurements give evidence that the β-polymorphic form of caffeine forms on the mica surface. All results consent that caffeine molecules have an edge-on conformation i.e. minimizing their interaction area with the surface. Furthermore, the azimuthal alignment of the long caffeine needle axis takes place along the [11̅0], [100], and [110] real space directions of mica; needles are observed every 60° azimuthally. While mica has a complex surface structure with mirror planes and lowered oxygen rows, the slightly disturbed 3-fold symmetry dictates the crystal alignment. This is different to previous findings for solution cast caffeine growth on mica. For HWE the needles align solely along the mica main directions whereby solution cast needles show an additional needle splitting due to a different alignment of caffeine with respect to the surface.
authors.affiliation: Institute of Pharmaceutical Sciences, Department of Pharmaceutical Technology, Karl-Franzens Universität Graz , Universitätsplatz 1, 8010 Graz, Austria ; Institute of Solid State Physics, Graz University of Technology , Petersgasse 16, 8010 Graz, Austria.
authors.forename: Christian
authors.initials: C
authors.lastname: Röthel
authors.affiliation: Institute of Semiconductor and Solid State Physics, Johannes Kepler Universität Linz , Altenbergerstraße 69, 4040 Linz, Austria.
authors.forename: Michal
authors.initials: M
authors.lastname: Radziown
authors.affiliation: Institute of Solid State Physics, Graz University of Technology , Petersgasse 16, 8010 Graz, Austria.
authors.forename: Roland
authors.initials: R
authors.lastname: Resel
authors.affiliation: Institute of Pharmaceutical Sciences, Department of Pharmaceutical Technology, Karl-Franzens Universität Graz , Universitätsplatz 1, 8010 Graz, Austria.
authors.forename: Andreas
authors.initials: A
authors.lastname: Zimmer
authors.affiliation: Institute of Semiconductor and Solid State Physics, Johannes Kepler Universität Linz , Altenbergerstraße 69, 4040 Linz, Austria ; Institute of Solid State Physics, University of Bremen , Otto-Hahn-Allee 1, 28359 Bremen, Germany.
authors.forename: Clemens
authors.initials: C
authors.lastname: Simbrunner
authors.affiliation: Institute of Pharmaceutical Sciences, Department of Pharmaceutical Technology, Karl-Franzens Universität Graz , Universitätsplatz 1, 8010 Graz, Austria.
authors.forename: Oliver
authors.initials: O
authors.lastname: Werzer