1. College
of Physics and Energy, Shenzhen University, Shenzhen 518060, Guangdong, P.R. China
2. Department
of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
3. HKUST Shenzhen Research Institute, Shenzhen 518057, China
4. Department
of Materials Science and NanoEngineering, Department of Chemistry,
and the Smalley Institute for Nanoscale Science and Technology, Rice University, Houston, Texas 77005, United States