Molecular interactions in thin films of hexadecafluorophthalocyaninatozinc(F16PcZn) as compared to islands of N,N '-dimethylperylene-3,4,9,10-bicarboximide (MePTCDI)

Thin films (1-200 nm) of the title compounds were prepared by vapor deposit ion on glass at a controlled temperature; Film growth was studied in situ b y optical absorption spectroscopy and measurement of the electrical conduct ivity. Independent of substrate temperature, a layered growth was found for ultrathin films of F16PcZn. During further deposition, this was followed b y island formation (Stranski-Krastanov). Island growth from the beginning o f deposition (Volmer-Weber) was found for MePTCDI. Both growth modes were c onfirmed by atomic force microscopy (AFM). Substrate temperature had a clea r influence on the crystal structure of F16PcZn. A structure consisting of parallel stacks of molecules is formed first under all conditions. At lower temperature,this growth continues including, however, an increasing portio n of amorphous material, whereas' a square lattice of molecules is formed a t higher temperature. This was found to be the stable modification of F16Pc Zn since films deposited at lower temperature could be irreversibly transfo rmed into this structure by annealing of the films. A reversible dependence of optical absorption spectra on temperature was found for the stable modi fication of both materials in the range from 78 to 450 K. Bands were found to narrow considerably at lower temperature, and shifts were observed that were characteristic for stronger intermolecular interaction which was very well- defined at lower temperature. A considerable mobility of molecules on the lattice site as well as between sites is indicated by the results of t his study. The optical data are discussed in-terms of an established model of transition dipole coupling.