1. Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
2. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
3. Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, Michigan 48824, United States
4. Department of Electronic Engineering, Chinese University of Hong Kong, Shatin, Hong Kong
5. Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
6. Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States