1. Technical Institute of Physics and Chemistry; Chinese Academy of Sciences; 100190 Beijing P. R. China
2. Fritz Haber Institute of Max Planck Society; Faradayweg 4-6 14195 Berlin Germany
3. School of Electronic Science and Applied Physics; Hefei University of Technology; Hefei 230009 P. R. China
4. Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power; Shanghai University of Electric Power; 200090 Shanghai P. R. China