1. Department of Micro and Nanofabrication, International Iberian Nanotechnology Laboratory, Braga 4715-330, Portugal
2. College of Engineering, Swansea University, Bay Campus, Fabian Way, Swansea SA1 8EN, United Kingdom
3. Computational Physics Laboratory, Tampere University, P.O. Box 692, Tampere 33014, Finland
4. Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain
5. Department of Chemistry, Iowa State University, Ames, Iowa 50011, United States
6. Ames Laboratory, U.S. Department of Energy, Ames, Iowa 50011, United States
7. ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Catalonia, Spain
8. Department of Physics, Norwegian University of Science and Technology, Trondheim 7491, Norway