1. Physik-Institut, University of Zürich 1 , Zürich CH-8057, Switzerland
2. Department of Physics and Astronomy, Uppsala University 2 , Uppsala SE-75120, Sweden
3. Graphene Labs, Istituto Italiano di Tecnologia 3 , Genova IT-16163, Italy
4. Leibniz-Institute for Solid State and Materials Research (IFW) 4 , Dresden D-01069, Germany
5. Swiss Light Source, Paul Scherrer Institut (PSI) 5 , Villigen CH-5232, Switzerland