1. Department of Physics and Key Laboratory for Low-Dimensional Structures and Quantum Manipulation (Ministry of Education), Hunan Normal University 1 , Changsha 410081, China
2. Key Laboratory for Matter Microstructure and Function of Hunan Province, Hunan Normal University 2 , Changsha 410081, China
3. Department of Chemical and Materials Engineering, University of Alberta 3 , Edmonton, Alberta T6G 2H5, Canada