Orbital mapping

The aim of this project is the development of a method to obtain orbital information from element specific signals in the electron microscope. The theory will be developed as a combination of inelastic electron interaction with elastic diffraction on the lattice with the aim of simulating and predicting energy filtered images in electron microscopy and spectrum imaging in scanning transmission microscopy. An innovative aspect is the propagator approach for the density matrix of the probe electron. Density functional theory and many-body perturbation theory for the excited states will be used for the inelastic scattering kernel. The expected results will allow to obtain spatial information on final state orbitals in electronic transitions. As a proof of principle predictions will be compared with the results of a key experiment on graphene and monolayered BN using the best available instruments. In the second part of the project, refined models will be used for the simulation of bonding at interfaces and in defects. The possibility to interpret the respective images in the electron microscope will increase our understanding of many interface-related problems in materials science. Cases of physical and technological interest will be analyzed theoretically, such as foreign atoms (nitrogen) in the graphene lattice or single layer boron nitride. The combination of the theory to be developed with the experimental proof of principle is a prerequisite for future innovative applications of this method in nanotechnology, such as direct mapping of orbitals and bonds.

The aim of this project was the development of a method to obtain orbital information from element-specific signals in the electron microscope. Obtaining such signals with atomic resolution has become possible with the realization of aberration corrected transmission electron microscopes in the last decade, providing true maps of the different atomic species in the lattice. However, no attempt has been made to see transitions between individual orbitals that are involved in an inelastic interaction with the probe beam, although theory based on work dating back to the 1930ies and 1980ies suggests that with modern electron microscopes, this should be possible.The practical difficulties that prevented such results are rooted a) in the intricate combination of elastic electron scattering on the lattice largely caused by the atomic nuclei with the inelastic scattering on the electronic subsystem largely caused by medium or outer shell bonding electrons; and b) in the limited stability of the specimen under electron bombardment. We succeeded in predicting maps of orbital electronic transitions with atomic resolution, i.e. in a real image of the specimen, including the multiple scattering processes, by combining and extending available simulation software. An ideal model system graphene shows clear anisotropy in the form of lobes of the involved -sp2 or (anti)-bonds depending on the viewing angle. An experimental verification is still pending because the bonding structure of the system is destroyed too rapidly under electron irradiation. Rutile (TiO2) turned out to be a more robust model system. Here, we could experimentally confirm the simulations, showing oriented electronic (anti)-bonds between neighbouring atomic columns.The results open the road for future applications in nanotechnology where bonding information on the sub-nanometric scale is essential.