Despite the fact that thin films of conjugated molecules are entering the market place as the active elements in various opto-electronic devices the basic understanding of their electronic structure, crucial to their function, is lacking. The electronic band structure, E(k) versus k, of the conjugated π-system defines both the electronic properties, such as charge transport, and the optical properties of the so-called organic semiconductors.
Here we report the electronic π-band structure of sexiphenyl (6P) obtained from a (20-3) oriented crystalline film and compared them to ab initio calculations. This orientation can be grown on weakly interacting anisotropic substrates, such as Cu(110)-p(2x1)O or TiO2(110), which uniaxially align the molecules parallel to the atomic corrugation of the substrate. With angle resolved UV-photoemission we have followed the photoelectron emission angle and thus the k-vector dependence of the 6P p-band orbitals in the directions parallel to the molecular axis (E(k) versus (kx)) and in two directions perpendicular to the molecular axis. Note that these are the directions important for the charge transport ability of the molecular crystal, these are however, not high symmetry directions of the crystal structure, generally used in band structure calculations. In the direction parallel to the molecular axis the observed dispersion is seen to be determined by the molecule itself. This intramolecular dispersion provides a textbook example of the band structure of a quasi one-dimensional system of finite size. In the directions perpendicular to the molecules strongly dispersing continuous bands reflecting the crystal lattice periodicity are observed.
Supported by the Austrian Science Foundation (FWF). |