Initial stages of organic film growth

Organic electronics plays an increasingly important role in modern technology and society. A variety of electronic gadgets based on organic material have already entered the market. However, there is general agreement that still much effort is necessary to improve the properties of such electronic devices. For this purpose a deeper understanding of the fundamental processes underlying organic film growth is required, because it is known that the electronic properties are mainly determined by the structure and morphology of the organic films. The proposed project will investigate the very initial steps of organic film growth, i.e. adsorption, diffusion, nucleation, coalescence and beginning multilayer formation. It is known that these initial steps are largely responsible for the properties of the final organic film. The investigations will be performed under well controlled, ultrahigh vacuum conditions. A large number of surface analytical techniques will be applied to characterize the substrates and ultra- thin organic layers. In particular, thermal desorption spectroscopy, X-ray photoelectron spectroscopy and atomic force microscopy will be used, in addition to other common surface analytical techniques. Within this project also several national and international co-operations will take place and there will also be an intensive collaboration with theory groups. The main issue of this project is to tailor the film growth by special preparation procedures and by proper structural and chemical modifications of the substrates. Nano-structuring of the substrate by grazing incidence ion sputtering and molecule deposition under grazing incidence are just two proposed ways to tackle this issue. Two different groups of organic molecules will be used for the investigations, which are either important test molecules and/or relevant for technical applications: Rod like molecules (quaterphenyl, hexaphenyl, pentacene) and plate like molecules (hexaazatriphenylene hexacarbonitrile, copper-phthalocyanine). As substrates mica, silicon oxide and single crystal surfaces of gold, silver and copper will be used. All these substrate and film materials are relevant for applications. The final goal of this project will be to develop a comprehensive model for the nucleation of organic molecules, which is a prerequisite to tailor organic thin film growth. We strongly believe that the outcome of these investigations will not only be of academic interest, but also of considerable importance for the organic electronic industry.

In the modern world, electronic equipment and gadgets are existing everywhere and influence our daily life considerably. In the future such devices will play an even more important role in the society, think just about the so-called internet of things. Wearable electronics, smart objects and biochips are some catchwords. Consequently, electronics based on organic material, possibly even biodegradable, will become increasingly important. In this project, we focused on the manufacturing and characterization of thin organic layers, which could be used in the fabrication of organic thin film transistors, light emitting diodes or solar cells. In this context, we pursued two slightly different objectives. In the one field of activity, we aimed at an in-depth understanding of the fundamental processes taking place at the initial stages of film growth (adsorption, nucleation, cluster aggregation). For this purpose, we studied the model systems pentacene and hexaphenyl on amorphous mica. Both molecules are of relevance for application. As a result of our experimental work and by collaboration with theoreticians we could gather some ground-breaking insight into the initial steps of organic film formation. One important aspect is that the nucleation and aggregation of organic molecules is limited by the attachment probability rather than by the diffusion probability, as it is the case in the formation of metal films. Another aspect is that the impinging monomers do not immediately accommodate to the surface temperature after adsorption, as assumed in the diffusion-limited aggregation model, but that they rather can move along the surface for some time as so-called hot-precursors. This characteristic also influences the emerging film morphology. Finally, we could derive a universal relationship between the island density of the ultrathin organic film, as a function of the deposition rate, and the island size distribution (or more precisely the capture zone distribution). By using this relationship, one can derive the important quantity of the critical island size, which eventually determines the film morphology, without knowing the exact physical mechanism which takes place during film formation. In the second field of activity, we studied the layer growth of a number of different organic molecules (hexaphenyl, pentacene, rubicene, indigo, quinacridone) on differently modified mica and silicon dioxide surfaces, in order to gain insight into the richness of possible behaviour patterns. Again, we could derive some quite general messages: On a reactive surface, the organic molecules form a strongly bound first layer (wetting layer) on which further molecules form needle-like islands, composed of lying molecules. By decreasing the surface energy, e.g. via surface roughening or by depositing impurities, like carbon, the molecules form islands of standing molecules. Thus, by proper surface modification one can specifically design the film growth and hence tailor the organic film for particular applications.