Functionalised organic films

In this project the physical and chemical properties of ultra-thin films of a new class of functionalized organic molecules with photosensitive end groups will be investigated. In particular we will focus on the structure, stability and reactivity of these layers and explore the possibility to form well ordered self-assembled monolayers (SAMs). The reactivity of the self organized films and the possibility to change their reactivity by UV irradiation will be investigated for three different cases: a) Adsorption of gaseous molecules like water, small hydrocarbons and amines on the SAM, which is important for the application of such films for sensors and photolithography. b) Evaporation and film formation of metals, like gold and aluminum, on the SAM surface because of its relevance for the fabrication of contact electrodes in organo-electronic devices. c) Formation of organic heterolayers by evaporation of aromatic organic molecules, e.g. oligo-phenylenes, onto the SAM, which is important for the realization of sandwich layers in organic electronics. We will perform these investigations under well defined ultra- high vacuum conditions and a variety of surface science techniques will be applied, which allows the in-situ study of the growth kinetics, film structure and reactivity under controlled conditions. The films will be mainly prepared by physical vapour deposition (PVD), but surface science studies on films which are grown ex-situ by the traditional route from solution will also be performed. Surface analytical techniques like X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), thermal desorption spectroscopy (TDS) and infra-red absorption spectroscopy (RAIRS) will be the main tools to study the chemical composition, structure, chemical reactivity and UV sensitivity of the SAMs. In collaboration with partners we will utilize scanning electron microscopy (SEM), atomic force microcopy (AFM), scanning tunneling microscopy (STM) and X-ray diffraction (XRD) to get further information on the structure and morphology of the functionalized organic films. From these investigations we expect to obtain a deeper understanding of the underlying principles of SAM formation for this specific class of photosensitive organic molecules, which will open new perspectives in the application of such films, e.g., for selective sensing devices, patterning by photo lithography, protective coatings and for organo- electronic device fabrication in general.

In the project "Functionalized organic films" we have prepared and characterized films of large organic molecules, which can have very specific functionalities. Three different organic molecules were investigated for this purpose, which can be used for different applications. The rod-like molecules hexaphenyl, consisting or six benzene rings, show fluorescence of blue light and can therefore be used to fabricate light emitting diodes or displays. The disk- like molecules, with the short name HATCN, which consist only of carbon and nitrogen atoms, can be used to prepare organic transistors. Finally, alkanethiols with different functionalized end groups were investigated, which can be used as sensors for the detection of very specific gas molecules, e.g. in environmental engineering. These molecules have the special properties to form ordered layers on particular surfaces, so called self assembled monolayers (SAMs). In all these cases we have investigated the very initial steps of layer growth, of layers which are only a few nanometer high, i.e. their height is less then a tenthousandth of the diameter of a hair. The characteristics of all these organic films is their flexibility, which makes these materials appropriate to fabricate flexible organic electronics, e.g. flexible displays or electronic newspapers. Although this new technology has already entered the market, there are many open questions and challenges for fundamental research. In order to adequately investigate the single steps of layer growth, adsorption, desorption, diffusion and nucleation, all the experiments have been carried out under well controlled conditions, i.e. under ultra high vacuum conditions, comparable to vacuum conditions as existing in the outer space. Modern surface science has a number of surface analytical techniques to its disposal to analyze the organic thin films, which is necessary in order to specifically manipulate these films. Some of the techniques applied in our project are X-ray photoelectron spectroscopy, thermal desorption spectroscopy and atomic force microscopy. In particular the latter method can be used to obtain an image of the film morphology in the nanometer scale. We could show that many of these films do not grow in a perfect layer-by-layer fashion, but rather in form of terraced islands, similar to that of the Styrian Erzberg. In our studies we could find out the reason for this particular layer growth and the results of these investigations were published in the renowned journal "Science".