1. Department of Physics and Astronomy and California NanoSystems Institute, University of California, Los Angeles, Los Angeles, CA 90095, USA.
2. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
3. Department of Materials Science and Engineering, University of California, Irvine, Irvine, CA 92697, USA.
4. Irvine Materials Research Institute, University of California, Irvine, Irvine, CA 92697, USA.
5. Harvard John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA.
6. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
7. Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
8. Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA.
9. Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia.
10. Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong.
11. Department of Physics and Astronomy, University of California, Irvine, Irvine, CA 92697, USA.