1. The York Nanocentre, University of York, York YO10 5DD, UK
2. Department of Physics, University of York, York YO10 5DD, UK
3. Department of Electronic Engineering, University of York, York YO10 5DD, UK
4. International Iberian Nanotechnology Laboratory, Braga 4715-330, Portugal
5. Department of Chemistry, University of York, York YO10 5DD, UK