1. Department of Physics, University of Basel, 4056 Basel, Switzerland
2. Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Campus UAB, 08193 Bellaterra, Spain
3. NEST, Istituto Nanoscienze-CNR and Scuola Normale Superiore, 56127 Pisa, Italy
4. Swiss Nanoscience Institute, University of Basel, 4056 Basel, Switzerland