1. School of Physics and Technology; University of Jinan; Shandong 250022 P. R. China
2. School of Electrical Engineering and Automation; Hefei University of Technology; Hefei 230009 P.R. China
3. CAS Key Laboratory of Photovoltaic and Energy Conservation Materials; Institute of Plasma Physics; Chinese Academy of Sciences P.O. Box 1126 Hefei 230031 P. R. China