Showing papers by "Torsten Fritz published in 2006"

Advanced Modelling of Epitaxial Ordering of Organic Layers on Crystalline Surfaces

Abstract: A fine balance between weak molecule-molecule interactions and usually only weakly laterally varying molecule-substrate interactions governs the physical structure of organic-inorganic and organic-organic heteroepitaxial thin films Therefore, it is important to investigate the energetics of realistically large overlayer domains So far, Potential calculations for large domains of organic overlayers on crystalline substrates were computationally demanding due to the vast number of atoms to be considered Here, we present a technique which for the first time enables the calculation of both the molecule-substrate interaction potential and the molecule-molecule interaction potential for large molecular domains (up to several thousands of rather large molecules) by utilizing potential energy grid files This technique does not only allow the investigation of the substrate influence but also full Monte-Carlo based structural optimizations, if applied to 2D crystalline overlayers Furthermore, it provides an estimate for the energetic gain combined with the differently aligned overlayer domains In several examples we will discuss the usefulness of this method As a general result, we will provide evidence that energetically favorable lattice structures in OMBE systems are not restricted to commensurate unit cells or coincident super cells

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

Abstract: 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 $\text{monomer}\ensuremath{\rightarrow}\text{dimer}\ensuremath{\rightarrow}\text{oligomer}$ transition, indicated by the occurrence of isosbestic points, is related to the findings for perylenetetracarboxylic dianhydride layers on mica.

Epitaxial nanolayers of quaterrylene: Influence of the substrate on the growth of the first and second monolayers

Abstract: We present a combined low energy electron diffraction and scanning tunneling microscopy study on the growth of the first and second monolayers (MLs) of quaterrylene on single crystalline Au(111). For the first ML, we observe a point-on-line coincident growth of flat lying molecules which agrees well with our potential energy calculations. In contrast, we observe alternating upstanding and flat lying molecules in the second layer. This behavior nicely demonstrates that the substrate has a very strong influence on the structure of the first layer, whereas the second layer is much weakly affected and exhibits properties comparable to the bulk.