Optical evidence for substrate-induced growth of ultrathin hexa-peri-hexabenzocoronene films on highly oriented pyrolytic graphite

We describe the influence of the substrate on the optical properties of ultrathin hexa-peri-hexabenzocoronene (HBC) layers deposited by organic molecular beam epitaxy. For that purpose, in situ differential reflectance spectroscopy (DRS) was employed, providing unsurpassed sensitivity for the thickness-dependent optical analysis. From the DR spectra, the optical functions were extracted using a numerical algorithm. The obtained spectra vary significantly for different substrates. We interpret these variances as being due to the different growth modes of HBC films altered by the influence of the respective substrate. HBC shows polycrystalline island structures on fused quartz in atomic force microscopy. On the other hand, layer-by-layer growth along with the formation of quasi-one-dimensional stacks on highly oriented pyrolytic graphite is known from previously published structural examinations. In this contribution, the optically observed monomer -> dimer -> oligomer transition, indicated by the occurrence of isosbestic points, is related to the findings for perylenetetracarboxylic dianhydride layers on mica.