Understanding photoemission of organic molecular films

Ultra-thin films made of organic molecules form the basis of future semiconductor technologies. Because organic molecules are extremely flexible, they can be used in a whole new range of applications, making it equally possible to create pliable screens and cost-effective photo-voltaic cells. A successful application of organic semiconductors in such devices, however, asks for a better understanding of the fundamental interactions between the organic material and the inorganic carrier substances. These interactions are largely determined by the molecule`s frontier orbitals, i.e., the highest occupied and the lowest unoccupied molecular orbital. Methods to determine their spatial distribution will allow for to better understanding of how organic semiconductor components work and thus help to improve their efficiency. This proposal investigates the photoemission process from organic molecular films from an ab-initio perspective. Commonly, angle-resolved photoemission spectroscopy (ARPES) is applied to study the band structure of solids by measuring the kinetic energy versus angular distribution of the photoemitted electrons. In recent publications however, it was shown that this experimental technique can also be used to characterize discrete orbitals of large pi-conjugated molecules. In particular, a simple relation between the observed angular intensity distribution and the Fourier transform of the initial state orbital could be established enabling a reconstruction of the real space electron distribution of individual molecular orbitals. While these results demonstrated the proof of principle, several simplifying assumptions have been made. Thus, the proposed research plan will improve the theoretical description of the photoemission from organic molecular layers in several aspects, thereby allowing for a quantitative interpretation of ARPES intensity maps. The outcome of this proposal will not only be interesting from a fundamental point of view but will also serve as a valuable tool for the investigation of organic molecular films and monolayers given recent advances in ARPES instrumentation. Moreover, we anticipate that reciprocal space maps from ARPES experiments together with their theoretical interpretation could nicely complement scanning probe techniques in continued attempts to image individual molecular orbitals. This will facilitate the tailoring of organic semiconductor interfaces with desired properties.

This project has theoretically investigated the photoemission process from organic molecular films by employing a quantum-mechanical first-principles approach. It could be shown that opposite to the general belief the angular dependence of the photoemission current from oriented molecular films can be well understood by assuming a plane wave as the final state of the photoemission process. The method enabled a simple and intuitive interpretation of the measurement data in terms of the Fourier transform of the initial state orbital and has opened a number of fascinating perspectives. The reconstruction of the real space electron distribution of individual molecular orbitals could be demonstrated in two and three spatial dimensions, a procedure to recover the phase information of the quantum mechanical wave function from experimental data was developed, and stringent benchmarks for the further development of ab-initio electronic structure theories could be derived.These fundamental investigations of the interface between organic molecules and metallic surfaces are envisioned to facilitate the tailoring of organic semiconductor interfaces with desired properties in future. Ultra-thin films made of organic molecules form the basis of novel semiconductor technologies. Because organic molecules are extremely flexible, they can be used in a whole new range of applications, making it equally possible to create pliable screens and cost-effective photo-voltaic cells. A successful application of organic semiconductors in such devices, however, asks for a better understanding of the fundamental interactions between the organic material and the inorganic carrier substances. These interactions are largely determined by the molecules frontier orbitals, i.e., the highest occupied and the lowest unoccupied molecular orbital. Methods to determine their spatial distribution, such as the ones developed in this project, will allow for to better understanding of how organic semiconductor components work and thus help to improve their efficiency.