Structure of perylene-tetracarboxylic-dianhydride thin films on alkali halide crystal substrates

The growth and crystal structure of epitaxial films of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA), vacuum-deposited onto freshly cleaved (001) surfaces of NaCI and KCI, have been investigated with electron diffraction, electron microscopy (TEM) and X-ray diffraction. Besides the known crystal structure, a new polymorph of PTCDA was found in the vapour-deposited films (which is also present in the commercially obtained powder). A structural model of this second polymorph could be derived from the results of the diffraction measurements. The new polymorph, β-PTCDA, is very similar to the previously known modification, except for the inclination of the a-axis (stacking axis) with respect to the molecular plane. Films deposited on alkali halide substrates kept at a temperature of about 200°C grow in the form of oriented islands, with the molecules lying flat on the surface. The azimuthal orientation of the crystallites appears to be determined by a good overlap between the functional groups of a first PTCDA molecule and the ions of the substrate surface; the final orientation is then determined by a certain degree of lattice fit between the PTCDA and the respective substrate crystal lattice.