Tracing the film structure of an organic semiconductor with photoemission orbital tomography
Photoemission orbital tomography (POT) is a powerful tool for investigating the orbitals and electronic band structure of oriented layers of organic molecules. In many cases, POT allows conclusions to be drawn regarding the geometric structure, but so far it has been mainly applied to (sub)monolayers and rarely to bilayers, raising the question of whether POT can also provide structure information for thicker films. Here, we use POT to analyze the band dispersion in up to eight layers of $\alpha$-sexithiophene (6T) adsorbed on Cu(110)-p($2\times1$)O. This linear oligomer turns out to be a textbook example that exemplifies the concepts of intra- and intermolecular band dispersion in molecules. Moreover, the rich band and orbital structure information available from POT for this system enables us to trace subtle changes in the crystal structure as a function of layer thickness. Specifically, we find that the periodicity of an intermolecular band changes with film thickness, revealing an increase of the intralayer distance between the molecules with the number of layers. At the same time, the momentum distribution of photoemission from the highest occupied molecular orbital of 6T discloses a decrease of the molecular tilt angle. Following the evolution of tilt angle and lattice constant with layer thickness, we observe -- purely based on electronic structure data -- that the surface-templated monolayer structure relaxes into the structure of bulk 6T crystals. The experimental findings agree well with the results of density functional theory calculations.