1. Institute of Solar Energyand Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education)School of Physics and Astronomy Shanghai Jiao Tong University Shanghai 200240 P. R. China
2. School of Environment and Chemical Engineering Yanshan University Qinhuangdao 066004 P. R. China
3. Collaborative Innovation Center of Advanced Microstructures Nanjing 210093 P. R. China