1. Department
of Physics, South University of Science and Technology of China, Shenzhen 518055, P. R. China
2. Department
of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
3. Department
of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States
4. Department
of Physics, University of Michigan, Ann Arbor, Michigan 48109, United States
5. Department
of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel