1. Center
for Nanochemistry, Beijing National Laboratory for Molecular Sciences
(BNLMS), College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
2. Department
of Physics, University of Texas at Austin, Austin, Texas 78712, United States
3. Physical
Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia
4. Department
of Physics, Durham University, Durham DH1 3LE, United Kingdom