1. Department of Microtechnology and Nanoscience, Chalmers University of Technology, SE-41296 Göteborg, Sweden.
2. Catalan Institute of Nanoscience and Nanotechnology (ICN2), Consejo Superior de Investigaciones Científicas (CSIC) and Barcelona Institute of Science and Technology (BIST), Campus Universitat Autònoma de Barcelona (UAB), Bellaterra, 08193 Barcelona, Spain.
3. Universitat Autònoma de Barcelona, Campus UAB, Bellaterra, 08193 Barcelona, Spain.
4. ICREA, Institució Catalana de Recerca i Estudis Avançats, 08070 Barcelona, Spain.
5. Graphene Center, Chalmers University of Technology, SE-41296 Göteborg, Sweden.