1. Department of Physics, Blackett Laboratory, Imperial College London, London SW7 2AZ, U.K.
2. Department of Materials, Imperial College London, London SW7 2BP, U.K.
3. Faculty of Electrical Engineering, Czech Technical University in Prague, Technická 2, Prague 166 27, Czech Republic
4. Material Science and Technology Department, Technical University of Cluj-Napoca, 103-105 Muncii, Cluj-Napoca 400020, Romania
5. Instituto de Fisica, Universidade Federal do Rio de Janeiro, 21941-972 Rio de Janeiro, Rio de Janeiro, Brazil
6. Center for Electron Microscopy and Analysis, 1305 Kinnear Road, Columbus, Ohio 43212, United States