1. State Key Laboratory of Crystal Materials, School of Microelectronics, Shandong University, Jinan 250100, P. R. China
2. Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China
3. School of Physics and Electronics, Shandong Normal University, Jinan 250358, P. R. China
4. School of Space Science and Physics, Shandong University at Weihai, Weihai 264209, P. R. China