1. Institute of Microelectronics, Tsinghua University, Beijing 100084, China.
2. Department of Electrical and Computer Engineering, Northeastern University, Boston, MA 02115, USA.
3. Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, CA 94720, USA.
4. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
5. Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA.
6. Department of Chemistry, Northeastern University, Boston, MA 02115, USA.
7. Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, Berkeley, CA 94720, USA.
8. Department of Physics, University of California, Berkeley, Berkeley, CA 94720, USA.