1. School of MicroelectronicsState Key Laboratory of Crystal MaterialsShandong University Jinan 250100 P. R. China
2. School of PhysicsShandong University Jinan 250100 P. R. China
3. Technical Institute of Physics and ChemistryChinese Academy of Sciences Beijing 100190 P. R. China
4. Institute of ChemistryChinese Academy of Sciences Beijing 100190 P. R. China