CRC TRR 61 B07 - Characterization of epitaxially assembled multi-layers of functional organic molecules by atomic force microscopy

Basic data for this project

Description

In this project we investigate self-assembly processes of functional organic molecules using high resolution atomic force microscopy (AFM) in ultrahigh vacuum. While the scanning tunnelling microscope (STM) is an ideal tool to investigate molecular interactions with a solid substrate up to the monolayer regime, it poses severe limitations to the intended application of these nanoscale functional elements requiring multilayer systems in many cases. In particular, epitaxial multilayer structures with poor conductivity and structures on non-conducting substrates must be analyzed by atomic force microscopy methods. We furthermore plan to exploit the recently developed method of 3D force field spectroscopy for studying the intramolecular contrast observed on organic molecules. Ordered vacuum-deposited organic molecular mono- and multilayers represent ideal model systems for the investigation of the physical properties of organic/inorganic interfaces. They open perspectives to investigate the electronic properties of individual molecules fixed within a molecular layer. The substrate/molecule interaction may induce significant shifts in the molecular electronic properties depending on its chemical interaction with the substrate. As a consequence both electronic as well as optical properties may be controlled by the precise epitaxial growth conditions and the number of molecular layers. Organic mono- and double layers on conducting substrates can be directly inspected by scanning tunnelling microscopy (STM) (see project B3). However, thicker layers or layers on insulating substrates, which are of special interest for optical and electrical applications, can only be inspected by highresolution atomic force microscopy (AFM) techniques. While molecular resolution imaging of organic molecules with STM has been successfully demonstrated for many years, only very recently similar resolution was achieved with non-contact AFM in vacuum environments. A further related question concerns the intramolecular contrast that we have observed on organic molecules, which was found to depend on the local chemical environment of the individual molecules. Theoretical ab-initio simulations have indicated that, for example, PTCDA adsorbed on metal surfaces suffers from severe changes of the internal charge distribution, which in turn will modify the electronic properties. We want to investigate the contrast mechanisms giving rise to the observed intramolecular features in non-contact AFM using high resolution 3D force field microscopy.