1. Department of Physics, University of California, Berkeley, California 94720, United States
2. Department of Chemistry, University of California, Berkeley, California 94720, United States
3. Department of Physics, Pohang University of Science and Technology, Pohang, Kyungbuk 37673, Korea
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