1. Key Laboratory of Optoelectronic Technology & Systems (Ministry of Education); College of Optoeletronic Engineering; Chongqing University; Chongqing 400044 China
2. Department of Applied Physics; College of Physics; Chongqing University; Chongqing 401331 China
3. Center For Functional Nanomaterials; Brookhaven National Laboratory; Upton NY 11973 USA