Substrate-Induced Order and Multilayer Epitaxial Growth of Substituted Phthalocyanine Thin Films

Thin films of hexadecafluorophthalocyaninatooxovanadium (F 16 PcVO) are vapor-deposited under organic molecular beam epitaxy (OMBE) conditions on the (100) surfaces of NaCl, KCl, and KBr and on quartz glass. Beginning at submonolayer coverages and extending to a film thickness of several tens of nanometers the film structure is determined in situ by reflection high energy electron diffraction (RHEED). Following deposition the film morphology is characterized by tapping mode atomic force microscopy (AFM) and chromophore coupling of the molecules within the films is studied by optical absorption spectroscopy. Highly ordered growth of crystalline domains of F 16 PcVO with dimensions in the micrometer range leading to films of quite uniform thickness is observed. On KBr a commensurate √10 × √10, R = 18.4° square lattice is seen in RHEED of the first monolayers with the molecules parallel to the substrate surface which is also preserved at higher film thickness. On KCl a surface lattice of the same size is formed which is, however, understood as a result of point-on-line coincidence. Diffraction of transmitted electrons yield a constant three-dimensional crystal structure of the films on KBr and KCl with a tetragonal unit cell of a = b = 1.47 nm and c = 0.62 nm. On NaCl with its smaller lattice constant no ordered relative orientation is possible and hence an increased part of the film appears amorphous. On quartz glass on the other hand, ordered films are formed with the molecular plane predominantly oriented cofacially parallel to each other and vertical to the surface. Calculations of molecular mechanics as well as of periodic surface potentials are performed to support the proposed structures and to discuss the crystallization in thin films of phthalocyanines and related materials. Implications of the present results for the electrical properties to be expected from such layers and their potential use in molecular semiconductor thin films are discussed.