1. Department of Physics, University of California at Berkeley, Berkeley, California 94720, United States
2. Graduate Group in Applied Science and Technology, University of California at Berkeley, Berkeley, California 94720, United States
3. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
4. Department of Physics, Chinese University of Hong Kong, Hong Kong 999077, China
5. Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
6. International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
7. Kavli Energy NanoSciences, University of California Berkeley and Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States