The structural order of C 60 thin films is shown to be significantly improved by inserting a templating layer of diindenoperylene (DIP) between the SiO 2 substrate and C 60 . In contrast to growth on an amorphous substrate like SiO 2 , C 60 grown on DIP exhibits alignment of fcc-domains with the (111) plane parallel to the substrate and a significant increase of the coherent in-plane island size by a factor of ∼4. Modification of the structural quality of the DIP bottom layer leads to a change in structural order in the C 60 top layer. In addition, ultraviolet photoelectron spectroscopy data from templated and nontemplated C 60 films are discussed. In contrast to other anisotropic organic molecules, for C 60 the spectral broadening and density of states of the highest occupied molecular orbital region do not depend significantly on the structural order in the C 60 film, which can be rationalized by the isotropic shape of the C 60 molecule. Article pubs.acs.org/JPCC

The dynamic interaction between the traveling charges and the molecular vibrations is critical for the charge transport in organic semiconductors. However, a direct evidence of the expected impact of the charge-phonon coupling on the band dispersion of organic semiconductors is yet to be provided. Here, we report on the electronic properties of rubrene single crystal as investigated by angle resolved ultraviolet photoelectron spectroscopy. A gap opening and kink-like features in the rubrene electronic band dispersion are observed. In particular, the latter results in a large enhancement of the hole effective mass (> 1.4), well above the limit of the theoretical estimations. The results are consistent with the expected modifications of the band structures in organic semiconductors as introduced by hole-phonon coupling effects and represent an important experimental step toward the understanding of the charge localization phenomena in organic materials.

Thickness and substrate dependence of film growth, morphology, unit-cell structure, and electronic structures was thoroughly investigated for picene, the zigzag connected 5-ring molecule, by employing complementary techniques of in situ real-time X-ray reflectivity/diffraction, in situ electron spectroscopies, and atomic force microscopy. A different kind of thickness dependent structural transition was observed on SiO2 and graphite, resulting in a distinct electronic structure. On SiO2 picene films with 3D crystalline domains are formed with nearly upright molecular orientation from the initial growth stage. With increasing the film thickness the in-plane dimensions of the unit cell in the initially grown domains become smaller (in other words, more compressed), and, at the same time, crystalline domains with a more relaxed structure are nucleating on top of the compressed domains. In spite of such structural changes, the electronic structure, namely energy position of the highest occupied molecular orbital and threshold ionization potential (IPT), is not significantly altered. On graphite, on the other hand, we found a transition from a 2D (layer) to a 3D (island) growth mode with a variation of the molecular orientation from flat-lying to tilted one. The IPT changes significantly between the 2D and 3D growth regime in contrast to the SiO2 system. The origin of the different IPTs of these picene thin films is discussed. The present results are compared with other planar π-conjugated compounds, in particular pentacene which is a structural isomer of picene and shows electronic properties strongly different from picene thin films.

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Electronic devices based on organic semiconductors are a fast-growing field of technology. A detailed understanding of the interplay between optical and electronic properties of organic molecular thin films as well as the physical structure is highly beneficial for the optimization of such devices. This requires investigations by means of different yet complementary spectroscopic techniques, where the comparability between the different data sets can become confusing. Consequently, the core aspect of this work is the consistent unification of spectroscopic data obtained by a diversity of experimental techniques within the complementary pictures of molecular states and energy levels. The confusion is obvious in the example of epitaxial films of tetraphenyldibenzoperiflanthene (DBP) on graphite(0001) surfaces being discussed here. One puzzling issue is the widening of the transport gap by around 0.6 eV during film growth, whereas the optical gap remains virtually constant. Furthermore, two-photon photoemission spectroscopy (2PPE) yields several features related to the same unoccupied orbital, and it is a nontrivial task to correlate them with the results of other spectroscopies. These issues can be resolved when all spectroscopic data are consistently interpreted in the framework of a theoretical model which was recently introduced by us, taking the initial and final states of the underlying probe processes into account. Applying that model, all peak shifts are explained here consistently by means of polarization and charging energies and further physical effects in context with the microscopic structure of the DBP thin films.