Energy band dispersion in well ordered $N,N′$-dimethyl-3,4,9,10-perylenetetracarboxylic diimide films

The electronic properties of well ordered $N,N′$-dimethyl-3,4,9,10-perylenetetracarboxylic dimide (DiMe-PTCDI) films prepared on sulfur passivated $GaAs(001)$ substrates were studied by means of photoemission spectroscopy. From the photon energy dependence of normal emission spectra an intermolecular energy band dispersion of about $0.2eV$ was determined for the highest occupied molecular orbital (HOMO). Simulation of the density of states reveals that the HOMO band has a single $π$ -character. The observed energy band dispersion thus originates from the intermolecular $π-π$ interaction and is modeled using the tight binding model. The analysis provides a value of $0.04eV$ for the transfer integral. The inner potential was treated as a fitting parameter such that the expected periodicity of the dispersion in the reciprocal space was obtained.