1. Department of Materials Science and Engineering, University of California 1 , Los Angeles, California 90095, USA
2. California NanoSystems Institute, University of California 2 , Los Angeles, California 90095, USA
3. Department of Physics and Astronomy “Ettore Majorana,” University of Catania 3 , Catania 95125, Italy
4. CNR-IMM, University Unit Catania, Consiglio Nazionale delle Ricerche 4 , Catania 95123, Italy
5. Istituto Nazionale di Fisica Nucleare, Sezione di Catania 5 , Catania 95123, Italy
6. InstituteQ, Department of Applied Physics, Aalto University 6 , Espoo 00076, Finland