1. Departament de Física de la Matèria Condensada, Universitat de Barcelona, Barcelona 08028, Spain
2. Institut de Nanociència i Nanotecnologia (IN2UB), Universitat de Barcelona, Barcelona 08028, Spain
3. Materials Science and Engineering Department, University of Washington, Seattle, Washington 98195, United States
4. Sensors & Devices Center, FBK - Bruno Kessler Foundation, via Sommarive, 18, Povo, TN 38123, Italy
5. Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, Barcelona 08193, Spain
6. Institut de Microelectrònica de Barcelona (IMB-CNM, CSIC), Bellaterra 08193, Spain