Substrate-induced order and multilayer epitaxial growth of substituted phthalocyanine thin films

Thin films of hexadecafluorophthalocyaninatooxovanadium (F16PcVO) are vapor -deposited under organic molecular beam epitaxy (OMBE) conditions on the (1 00) surfaces of NaCl, KCl, and KBr and on quartz glass. Beginning at submon olayer coverages and extending to a film thickness of several tens of nanom eters the film structure is determined in situ by reflection high energy el ectron diffraction (RHEED). Following deposition the film morphology is cha racterized by tapping mode atomic force microscopy (AFM) and chromophore co upling of the molecules within the films is studied by optical absorption s pectroscopy. Highly ordered growth of crystalline domains of F16PcVO with d imensions in the micrometer range leading to films of quite uniform thickne ss is observed. On KBr a commensurate root 10 x root 10, R = 18.4 degrees s quare lattice is seen in RHEED of the first monolayers with the molecules p arallel to the substrate surface which is also preserved at higher film thi ckness. On KCI a surface lattice of the same size is formed which is, howev er, understood as a result of point-on-line coincidence. Diffraction of tra nsmitted 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 nn 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 am orphous. On quartz glass on the other hand, ordered films are formed with t he molecular plane predominantly oriented cofacially parallel to each other and vertical to the surface. Calculations of molecular mechanics as well a s of periodic surface potentials are performed to support the proposed stru ctures and to discuss the crystallization in thin films of phthalocyanines and related materials. Implications of the present results for the electric al properties to be expected from such layers and their potential use in mo lecular semiconductor thin films are discussed.