1. School of Materials, University of Manchester, Manchester, M13 9PL, U.K.
2. SuperSTEM Laboratory, STFC Daresbury Campus, Daresbury WA4 4AD, U.K.
3. Jeol Nanocentre and Department of Physics, University of York, Heslington, York YO10 5DD, U.K.
4. School of Chemical and Process Engineering and School of Physics, University of Leeds, Leeds LS2 9JT, U.K.