Real-time observation of growing organic nanostructures

Organic dyes are often used as active layers in opto-electronic devices like LEDs or solar cells. The devices make use of the interaction of light with the electronic states of the molecules. To study the fundamental material properties, the investigation of single crystalline samples is the most straightforward approach. However, macroscopic crystals are not always available. More often the material is deposited by vacuum sublimation so that a uniform crystalline structure without defects is only achieved on a sub-micrometer scale. The aim of the project is to correlate the optical and electronic properties of organic nanostructures (thin film phases and crystallites) at the microscopic scale. In particular photoelectron emission microscopy (PEEM) is a very powerful tool for studying the morphology (with a lateral resolution of ~50 nm) and to follow its evolution during growth and annealing in real-time. By using polarized UV-light for photoelectron excitation, the orientation of the molecules in individual crystallites can be directly determined. In addition, their local electronic structure can be probed in spectroscopic mode. Complementary to the absorption of the light is its reflectance. In contrast to PEEM in reflectivity measurements no ionization threshold has to be overcome and visible or infrared light can also be used for analysis. In a standard PEEM setup the light below the ionization threshold is totally unused, although it is technically very important. In the present project an in-situ combination of optical spectroscopy and electron microscopy will be established. The experiment is well suited to study in real-time the growth of prototype organic molecules, such as pentacene and perfluoro-pentacene on Cu(110) surfaces. In the case of mixed organic layer containing different molecular species the interaction between these molecules can be studied in detail. We plan to probe the electronic properties via PEEM and to monitor the evolution of the optical properties by differential reflectance spectroscopy (DRS). Both techniques will be applied simultaneously during the growth of organic nanostructures to obtain complementary information. Combining both techniques thus allows correlating the morphology and crystalline structure with the optical and electronic properties of the organic nanostructures. By optimizing the growth parameters it should be possible to fabricate micro-crystalline structures with designated opto-electronic properties in a controlled and reproducible fashion.

Within the project "Real-Time Observation of Growing Organic Nanostructures", methods were developed to characterize the growth of organic thin films and nanostructures during the deposition process. The aim was to combine photoelectron emission microscopy (PEEM) and optical reflectance measurements. PEEM is an imaging method, which allows to follow morphological changes of the sample during growth with sub-micron resolution. Due to the interaction with the substrate, the electronic properties of the first molecular layers on the substrate are greatly altered, which leads to a high contrast in the PEEM.In the case of differential reflectance spectroscopy (DRS), the change of the reflectance of the sample is spectrally recorded as a function of time (coverage) during an evaporation experiment. Within the project, the method was successfully integrated into the existing PEEM and further improved. Already with the first version of the DRS setup (using a photodiode as a detector), important findings concerning the polarization dependence of the photoelectron emission and the reflection of the incident UV-light from the sample surface were obtained. In the final version, the data acquisition of the PEEM and of the polarization-dependent DRS were fully synchronized. Within the optical detection system, the light is split into two mutually perpendicular linearly polarized components.The optical transitions of organic molecules are linked to their geometry. If an optical transition is mainly detected in one of the two polarization components, one can conclude on the (preferential) orientation of the molecules on the surface. Experiments, in which the organic molecule ?-sexithiophene was deposited onto different silver surfaces, confirm this hypothesis: A (111) oriented silver surface is isotropic. Correspondingly, the DRS spectra for the two polarization components show similar spectral features. Due to the large number of steps, a Ag (441) surface is strongly anisotropic and the adsorbed molecules are preferentially oriented along the step direction. Consequently, the molecular fingerprint of the molecules only shows up in one of the two polarization components.Due to the polarization dependence of the optical spectra, it could also be shown that during the growth of perfluorinated pentacene a reorientation of the molecules on the surface takes place during completion of the first molecular layer.