Molecular architecture of ordered thin films of crystalline organic dyes

Abstract

The factors which determine the growth mode and molecular architecture of vacuum deposited organic thin films on single crystalline substrates were investigated. Specifically, the relative importance of layer planes in the bulk structure, lattice matching between the overlayer and substrate, topographic direction by the substrate, and specific molecule-substrate interactions, in determining the growth mode were examined. The majority of the molecules studied here (ClAlPc, F₁₆ZnPc, PTCDA, C4-PTCDI, and C5-PTCDI) exhibited layer planes in their bulk structures, however, the molecular plane is coincident with the layer plane only for PTCDA and ClAlPc. ClAlPc and F₁₆ZnPc were found to adopt different flat-lying commensurate square lattices on the Cu(100) surface. In both cases, the flat-lying orientation of the molecules was dictated by specific molecule-substrate interactions, while the orientation of the lattice was dictated by lattice matching with the substrate. ClAlPc was also able to adopt an incommensurate centered rectangular lattice whose orientation was directed by alignment along step edges. Fluorescence investigation of submonolayer PTCDA and PTCDI films on alkali halide substrates demonstrated the great potential of fluorescence spectroscopy as a means of monitoring film growth. PTCDA was found to adopt a flat-lying orientation on NaCl, KCl, and KBr, while a flat-lying orientation of the PTCDI molecules was determined by the strength of the molecule-substrate interactions. From these measurements, the relative interaction strengths of the substrates were determined to be KCl > KBr > NaCl. IR dichroism showed that the expected growth along the layer planes was found only to occur for PTCDA, due to the coincidence of the layer and molecular planes. IR spectroscopy also revealed that a new polymorph of C5-PTCDI had been formed on these surfaces. These studies showed that the relative importance of the factors in determining the molecular architecture adopted within the first 1-2 MLE of a film are: (1) molecule-substrate interaction, (2) lattice matching, (3) topographic direction, (4) layer planes in the bulk structure. In addition the use of fluorescence spectroscopy to probe the evolution of vacuum deposited films was significantly advanced.