X-ray diffraction and infrared multiple-angle incidence resolution spectroscopic studies on the crystal structure and molecular orientation of zinc-porphyrin thin films on a SiO₂/Si substrate

The crystal structure and molecular orientation of zinc-octaethylporphyrin [Zn(OEP)] thin films deposited on a SiO2 /Si (111) substrate have been investigated using x-ray diffraction (XRD) and infrared multiple-angle incidence resolution spectroscopy (MAIRS), respectively. XRD results show that the Zn(OEP) thin films have two kinds of crystallites with a diffraction plane (2θ=7.9°) parallel to the substrate and with that (2θ=7.1°) inclined mainly by 60°-70° to the substrate. Comparison with simulated XRD results of the Zn(OEP) single crystal indicates that the diffraction angle of 2θ=7.9° is consistent with that of the (01 1-) plane of the single crystal, while the angle of 2θ=7.1° is slightly different from that of 7.3° corresponding to the (010) plane of the single crystal. On the other hand, MAIRS results demonstrate that Zn(OEP) molecules in the crystallite corresponding to 2θ=7.9° are inclined by 52.2±0.9° to the substrate, which is in a good agreement with that of ca. 50° estimated from the Zn(OEP) single crystal structure. In addition, these crystalline sizes are estimated to be approximately 20 nm and independent of film thickness. Accordingly, it can be concluded from the XRD and MAIRS results that the Zn(OEP) films have two kinds of grains. One has the single crystal structure, in which Zn(OEP) molecules are inclined by 52.2°±0.9° to the substrate. On the other hand, the other has a crystal structure close to the single crystal, in which Zn(OEP) molecules would be inclined by 10°-20° to the substrate.