1. Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, 215123 Suzhou, Jiangsu People’s Republic of China
2. Institute of Physics, University of Siegen, Walter-Flex-Strasse 3, D-57068 Siegen, Germany
3. Jiangsu Key Laboratory of Advanced Negative Carbon Technologies, Soochow University, 215123 Suzhou, Jiangsu, People’s Republic of China