Structural studies on epitaxially grown organic films

One possible method to obtain highly aligned molecules (in combination with extended domain size) is the epitaxial growth of conjugated molecules on single crystal surfaces. A variety of thin organic films will be investigated: the organic conjugated material will be oligo-phenyles (terphenyl, quaterphenyl, quinquephenyl, sexiphenyl) and zinc- phthalocyanine; the substrates will cover a wide range of single crystal surfaces from reactive metal (Au, Cu), to semiconductors (MoSe2 , GeS) but also insulators (mica). The thin film preparation conditions will be varied in order to obtain and to optimize the epitaxial growth. The epitaxy within the thin films will be proved by X-ray diffraction pole figure technique and by transmission electron diffraction. Besides the verification of the epitaxial growth, more detailed investigations about the structural properties of the organic epitaxy will be performed by other X-ray diffraction experiments. By high-resolution X-ray diffraction and by X-ray diffraction strain-evaluation the perfection of the epitaxial grown film, the layer matching and layer homogenity as well as the strain within the epitaxial layer will be investigated. One important topic will be the connection of anisotropic strain within the epitaxial layer with the morphology of individual crystallites. These investigations should give some knowledge about the occurrence of epitaxy in organic thin films. It is expected to obtain important information about the preparation parameters of high quality epitaxially grown organic films.

Thin films of organic molecules on inorganic substrates are important for a large variety of technical applications; especially in the field of organic electronics. This project deals with films of two classes of molecules. One type are oligo-phenylenes which are wide-gap organic semiconductors and show light emission in the blue visible range in combination with high chemical stability. Oligo-phenylenes are technically applied in organic based displays. The second type of molecules are sexithiophene and pentacene which are considered as future transistor materials in organic thin film transistors. The structural properties of thin films are of essential importance for device applications, since the optical and the electrical properties are strongly anisotropic. The goal of the project is an understanding of the thin film formation of oligo-phenylenes, sexithiophene and pentacene via the investigation of the thin film structure. The crystallographic order within thin films is strongly correlated to the order in the first molecular layers (monolayers) at substrate surface. E.g. the crystal structure of epitaxial layers (layers with defined structural relation with respect to the underlying substrate) reveals important information about molecular orientation and alignment within the first molecular layers. Highly disordered monolayers result in a disordered growth of films. Within this project basic mechanisms for the formation of epitaxial thin films are obtained. Additionally to the characterisation of the crystallographic properties of the organic films also the thin film morphology is investigated and related to the crystal structure.