1. International Joint Research Laboratory of New Energy Materials and Devices of Henan Province, School of Physics and Electronics, Henan University, Kaifeng 475004, P.R. China
2. Institute of Semiconductor Science and Technology, South China Normal University, Guangzhou 510631, P.R. China
3. Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Sciences Beijing 100083, P.R. China
4. School of Materials Science and Engineering Georgia Institute of Technology Atlanta, Georgia 30332-0245, United States