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. Material Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
4. Research Center for Electronic and Optical Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
5. Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
6. Kavli Energy NanoSciences Institute at University of California Berkeley and Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States