1. Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, CA 94720, USA
2. Department of Applied Physics, University of the Basque Country (UPV/EHU), 48013 Bilbao, Spain
3. Department of Electrical and Computer Engineering, University of California, Los Angeles, CA 90095, USA
4. Advanced Light Source, Lawrence Berkeley National Lab, Berkeley, CA 94720, USA
5. Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, USA
6. Mechanical and Aerospace Engineering Department, University of California, Los Angeles, CA 90095, USA
7. Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, NY 14853, USA
8. California NanoSystems Institute, Los Angeles, CA 90095, USA
9. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA