Röthel Christian, Radziown Michal, Resel Roland, Zimmer Andreas, Simbrunner Clemens, Werzer Oliver
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.
Institute of Semiconductor and Solid State Physics, Johannes Kepler Universität Linz , Altenbergerstraße 69, 4040 Linz, Austria.
Cryst Growth Des. 2015 Sep 2;15(9):4563-4570. doi: 10.1021/acs.cgd.5b00833. Epub 2015 Jul 31.
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.
在技术以及制药应用中，由于形态和晶体结构与物理、电学和光学性质直接相关，因此非常需要对有机薄膜进行精确制造。在这项工作中，研究了通过热壁外延（HWE）在白云母上沉积咖啡因的定向生长。光学和原子力显微镜测量结果表明，存在咖啡因针状晶体，这些针状晶体相对于确定的云母表面取向在方位角方向上呈现出优选的排列。镜面X射线衍射和X射线衍射极图测量结果证明，咖啡因的β多晶型物在云母表面形成。所有结果均表明，咖啡因分子具有边缘取向构象，即其与表面的相互作用面积最小。此外，长咖啡因针轴的方位角排列沿着云母的[11̅0]、[100]和[110]实空间方向进行；每隔60°方位角观察到针状晶体。虽然云母具有带有镜面和降低的氧行的复杂表面结构，但略微受扰的三重对称性决定了晶体排列。这与之前关于在云母上溶液浇铸咖啡因生长的研究结果不同。对于热壁外延，针状晶体仅沿云母的主方向排列，而溶液浇铸的针状晶体由于咖啡因相对于表面的不同排列而显示出额外的针状分裂。
