1. Department
of Physics, University of California at Berkeley, Berkeley, California 94720, United States
2. Collaborative Innovation Center of Advanced Microstructures, Nanjing 210093, China
3. National Institute for Materials Science, 1-1 Namiki, Tsukuba, 305-0044, Japan
4. Materials
Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
5. Kavli
Energy NanoSciences Institute at the University of California, Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States