1. School of Physics, School of Microelectronics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R. China
2. State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China
3. State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering and Shaanxi Joint Laboratory of Graphene (NPU), Northwestern Polytechnical University, Xi’an 710072, P. R. China