1. Materials and Interfaces Department, Weizmann Institute of Science, Rehovot 76100, Israel, Institute of Solid State Research and Ernst-Ruska Centre for Microscopy and Spectroscopy with Electrons, Research Centre Jülich GmbH, 52425 Jülich, Germany, Electron Microscopy Unit, Weizmann Institute of Science, Rehovot 76100, Israel, and Physikalische Chemie, Technische Universität Dresden, 01062 Dresden, Germany