1. Department of Physics Chongqing University Chongqing 400044 P. R. China
2. Department of Physics College of Basic Medical Sciences Army Medical University Chongqing 400038 P. R. China
3. College of Physics and Electronic Engineering Chongqing Normal University Chongqing 401331 P. R. China
4. Key Lab for Optoelectronic Technology and Systems Ministry of Education College of Optoelectronic Engineering Chongqing University Chongqing 400044 P. R. China
5. Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Sciences Beijing 100083 P. R. China
6. School of Materials Science and Engineering Georgia Institute of Technology Atlanta GA 30332 USA