1. National Engineering Research Center for Nanotechnology, No. 28 East Jiangchuan Road, Shanghai 200241, People's Republic of China
2. School of Material Science and Engineering, Shanghai Jiao Tong University, No. 800 Dongchuan Road, Shanghai 200240, People's Republic of China
3. Department of Physics and Astronomy, Shanghai Jiao Tong University, No. 800 Dongchuan Road, Shanghai 200240, People's Republic of China