Organic molecular beam epitaxial growth of substituted phthalocyanine thin films – tetrapyridotetraazaporhyrins on alkali halide (100) surfaces

Abstract

Thin films of tetrapyridotetraazaporphyrinatozinc (TPyTAPZn) and of tetrapyridotetraazaporphyrinatooxovanadium (TPyTAPVO) which both can be looked at as substituted phthalocyanines Pc were prepared on the (100) surfaces of NaCl, KCl, KBr and on quartz glass under ultra high vacuum (UHV) conditions. The films were studied by reflection high energy electron diffraction (RHEED), UV-vis absorption spectroscopy, tapping mode atomic force microscopy (AFM) and modeled by molecular mechanics calculations. Highly ordered crystalline films were obtained for the first monolayers with the molecules oriented in square lattices in close relationship to the substrate crystal axes as determined by RHEED. 2√2×2√2, R=45° surface lattices were obtained for TPyTAPZn on KBr and KCl relative to the respective surface unit mesh. Somewhat distorted 2√2×2√2, R=15° and 1.25√5×1.25√5, R=18.4° surface lattices were found for TPyTAPVO on KBr and KCl, respectively, instead of high crystallinity whereas a clear √10×√10, R=18° surface lattice was obtained on NaCl. The pyrido groups in the ligand instead of the benzo groups of Pc led to these new molecular orientations by providing different interaction with the substrate surface, within the molecular plane and between layers as proven by molecular mechanics geometry optimization. Very sharp absorption bands shifted to lower transition energy also indicate crystalline growth in square lattices for thicker films. The band position and hence the size of the exciton splitting is discussed based on a point dipole model and used to estimate the vertical interaction range in the thin films. AFM allowed a detailed analysis of crystal morphology and film texture. The island growth of the organic crystals at step edges of the alkali halide substrates indicate a high mobility of molecules on terraces as well as across step edges of the substrate under deposition conditions.