1. Institute of Functional Nano & Soft Materials (FUNSOM) and Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices; Soochow University; Suzhou 215123 P.R. China
2. School of Physics; Shandong University; Jinan 250100 P.R. China
3. Institut für Physik; Humboldt-Universität zu Berlin; 12489 Berlin Germany
4. Helmholtz Zentrum Berlin für Materialien und Energie; 12489 Berlin Germany
5. Graduate School of Advanced Integration Science; Chiba University; Chiba 263-8522 Japan
6. Department of Photo-Molecular Science; Institute for Molecular Science; Okazaki 444-8585 Japan