Phillipp Wachsmuth

Momentum-resolved electron energy-loss spectroscopy of graphene

Day of Ph.D. defence: 22.07.2014



In this work, we conduct a detailed experimental investigation and discussion of the full momentum-dependent response of graphene, that is, we directly measure the pi and pi + sigma-plasmon dispersion in free-standing single-layer graphene. The very good energy and momentum resolution as well as the wide range of probed momentum transfers enable us to use our experimental result as benchmark for a detailed comparison with ab-initio calculation. Furthermore, we study the thickness-dependent behavior of these high-energy plasmons, i.e., we investigate the transition of the collective excitations in a purely 2D system (graphene) to those in a 3D system (graphite). Here, we focus on the dispersion of the pi plasmon and discuss our results using a layered-electron gas model. For this purpose, we have improved and implemented a well-known momentum-resolved electron energy-loss technique into the newly developed Sub-Angström Low-Voltage Electron Microscopy and Spectroscopy I (SALVE-I); a transmission electron microscope optimized for the work at low accelerating voltages. Apart from the study of collective excitations, we characterize the experimental capabilities of our implemented method by also investigating high-energy core-electron excitations. In particular, we investigate the so-called magic angle condition and the importance of relativistic effects. In addition we use principal component analysis to unambiguously disentangle the two relevant contributions to our recorded core-loss spectra.

Ph.D. thesis