1. Department of Materials Science, University of Cambridge, Pembroke Street, Cambridge
CB2 3QZ, United Kingdom
2. CEA, Laboratory for Electronics and Information Technology (LETI), Minatec
Campus, 17 rue des Martyrs, Grenoble Cedex 38054, France
3. Department of Electrical and Computer Engineering, Texas A&M University, College Station, Texas 77843-3128, United States
4. Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico
87545, United States