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.

This project is funded by the Austrian Science Fund under contract number P24528-N20.