1. Department of Microtechnology and Nanoscience, Chalmers University of Technology, S-41296 Göteborg, Sweden
2. National Physical Laboratory, Teddington TW110LW, United Kingdom and Royal Holloway, University of London, Egham TW200EX, United Kingdom
3. Department of Physics, Chemistry and Biology (IFM), Linköping University, S-58183 Linköping, Sweden and Graphensic AB, SE-58330 Linköping, Sweden