1. Department of Physics, University of California at Berkeley, Berkeley, California 94720, United States
2. Department of Physics, Korea University, Seoul 02841, Republic of Korea
3. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
4. Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education), School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
5. Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California 90089, United States
6. Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, California 94720, United States
7. Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
8. International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
9. Kavli Energy NanoSciences Institute at the University of California, Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
10. Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China
11. Department of Electrical Engineering, University of Southern California, Los Angeles, California 90089, United States