1. Department of Physics, University of California at Berkeley, Berkeley, California 94720, United States
2. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
3. Department of Physics, Yonsei University, Seoul 03722, Korea
4. Center for Nanomedicine, Institute for Basic Science, Seoul 03722, Korea
5. School of Physical Sciences and CAS Key Laboratory of Vacuum Physics, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China
6. Kavli Energy NanoSciences Institute at the University of California at Berkeley, Berkeley, California 94720, United States