1. Department of Physics, University of York, Heslington, York YO10 5DD, United Kingdom
2. York JEOL Nanocentre, University of York, Heslington, York YO10 5BR, United Kingdom
3. Department of Electronics, University of York, Heslington, York YO10 5DD, United Kingdom
4. PRESTO, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan