1. College of Chemistry; Chemical Engineering and Materials Science; Soochow University; Jiangsu 215123 China
2. Department of Materials Science and Engineering and Department of Energy and Resources Engineering; College of Engineering; Peking University; Beijing 100871 P. R. China
3. Department of Physics and Astronomy; California State University; Northridge CA 91330 USA
4. Center for Functional Nanomaterials; Brookhaven National Laboratory; Upton NY 11973 USA
5. Testing and Analysis Center; Soochow University; Jiangsu 215123 China