1. Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
2. Department of Physics and Astronomy and California NanoSystems Institute, University of California, Los Angeles, Los Angeles, California 90095, United States
3. Department of Chemical and Biological Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong 999077, China
4. Physics Department, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
5. Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States