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. Department of Physics, Chemistry and Biology, Linköping University, 58183 Linköping, Sweden
3. Physics Department E20, Technical University of Munich, D-85748 Garching, Germany