1. Department of Chemistry, University of California, Berkeley, California 94720, United States
2. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
3. Kavli Energy NanoScience Institute, Berkeley, California 94720, United States
4. Center for NanoMedicine, Institute for Basic Science (IBS), Seoul 03772, Korea
5. Y-IBS Institute, Yonsei University, Seoul 03772, Korea
6. Department of Electronic Engineering, Tsinghua University, Beijing 100084, P. R. China
7. School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe, Arizona 85287, United States
8. Department of Materials Science and Engineering, University of California, Berkeley, California 94720, United States