1. Physikalisches Institut, Goethe Universität Frankfurt, 60438Frankfurt am Main, Germany
2. Institute of Electron Microscopy and Nanoanalysis, Graz University of Technology, 8010Graz, Austria
3. Institute of Solid State Electronics, TU Wien, Gußhausstraße 25-25a, 1040Vienna, Austria
4. Christian Doppler Laboratory for Direct-Write Fabrication of 3D Nano-Probes (DEFINE), Institute of Electron Microscopy, Graz University of Technology, Steyrergasse 17, 8010Graz, Austria