Adsorption of organic molecules on single crystal surfaces

The physics and chemistry of conducting organic material has attracted considerable interest in the recent past. Conjugated organic molecules are a particularly interesting class of materials, due to their wide range of possible applications like light emitting devices, photodiodes, solar cells and also fully organic transistors and integrated circuits. One of the most promising approach for device fabrication is the production of crystalline organic thin films. However, the present know-ledge concerning the growth kinetics and the structure of these thin films is still in its infancy. The objective of this proposal therefore is the systematic investigation of the kinetics and energetics of adsorption of organic molecules on various surfaces. For this purpose we will apply surface science techniques like Thermal Desorption Spectroscopy (TDS), Low Energy Electron Diffraction (LEED) and X-Ray Photoelectron Spectroscopy (XPS). All the investigations will be performed under well defined Ultrahigh Vacuum (UHV) conditions. With respect to the adsor-bates we will focus on the oligo-phenylenes (p-terphenyl to p-sexiphenyl). These molecules will be adsorbed on more or less reactive single crystal metal surfaces (Au, Cu, Ni, Pd), but also on nonmetallic surfaces like mica or semiconducting surfaces. In this proposal we will tackle the following questions, by using a surface science approach: How does the first adsorbed layer influence the epitaxial growth of the thin organic film. What is the influence of the substrate on the layer growth. To which extent can preadsorbed particles effect the adsorption probability of the organic molecules and how stable is the grown layer against post- adsorbed particles. How large is the sticking coefficient and what is the saturation coverage of the first layer. The know-ledge of these parameters is of interest from a fundamental point of view, but is also important for the tailoring of the growth conditions. In addition to these investigations concerning the kinetics of adsorption we will focus on the energetics and the structure of the submonolayer, monolayer and few multilayer regime of the organic molecules. These studies will be performed within a cluster project in which several research groups from Austrian universities are participating.

There is an enormous worldwide effort both in basic scientific research as well as in industrial development in the area of organic electronics. It is expected that electronic devices based on organic material will be of significant relevance in the next future. In this context a thorough understanding of the interface formation, film growth and functionality of organic materials is of utmost importance. A control of these aspects will allow the realisation of totally new device concepts exploiting the enormous flexibility inherent in organic chemistry. The investigations of organic thin films can be partitioned into three areas of focus: (1) the development and production of devices, (2) thin film characterisation, and more recently (3) the molecular level control using surface and interface science. The latter two aspects were the topic of this project. We have studied the adsorption and the thin film layer growth of organic model molecules (quaterphenyl, sexiphenyl) on gold surfaces. These molecules are of great technological relevance because they show efficient luminescence in the blue and ultraviolet spectral range. Gold is very often used as contact electrodes for devices and is therefore a relevant substrate. Prototypes of blue light emitting diodes have indeed already been fabricated. The main emphasis of this work, however, was to understand the structure and morphology of the ultra thin oligo- phenyl films from a fundamental point of view. Therefore we have performed experiments under well defined conditions in ultrahigh vacuum and have applied surface analytical tools to fully control the adsorption and growth processes. The main results of these investigations can be summarized as follows: The rod-like molecules form well ordered organic crystals if they are properly evaporated onto the gold surface (organic molecular beam epitaxy). This is a pre-requisite for the proper operation of the electronic devices. We were able to demonstrate that the particular layer growth, i.e. the orientation of the molecules in the film, the crystallite size and the morphology of the crystallites can be influenced significantly by the substrate temperature, the substrate crystallographic orientation and the chemical composition (impurities) of the substrate. This allows a purposeful tailoring of the organic thin film to optimally fulfil the demands. In particular, we were able to show how important the structure and energetics of the first monolayer is for the subsequent growth of the thin film. The experiments which were performed for these investigations ranged from sub-monolayer studies under ultra-high vacuum conditions using various surface analytical techniques and electron spectroscopy, via bulk structure characterisation with X-rays and synchrotron radiation, to morphological investigations with scanning electron microscopy and atomic force microscopy. For these purpose various national and international co-operations have been established which were very successful.