1. Department of Physics, University of California, Berkeley, California 94720, United States
2. Material Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
3. Electrical
Engineering and Computer Sciences, University of California, Berkeley, California 94720, United States
4. Kavli Energy Nanosciences Institute at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States