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 Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
4. Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, California 94720, United States
5. Department of Physics, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China
6. National Institute for Materials Science, Tsukuba 305-0044, Japan
7. Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
8. Kavli Energy Nano Sciences Institute at the University of California Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States