1. Department
of Chemistry, University of California Berkeley, Berkeley, California 94720, United States
2. Department
of Physics, University of California Berkeley, 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