1. Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, Australian Capital Territory 2601, Australia
2. Department of Physics/Centre for Materials Science and Nanotechnology, University of Oslo, P.O. Box 1048 Blindern, N-0316 Oslo, Norway
3. Australian National Fabrication Facility ACT Node, Research School of Physics and Engineering, The Australian National University, Canberra, Australian Capital Territory 2601, Australia
4. Centre for Advanced Microscopy, The Australian National University, Canberra, Australian Capital Territory 2601, Australia