1. College of Electrical Engineering, Sichuan University, Chengdu 610065, P. R. China
2. School of Materials Science and Engineering, Liaocheng University, Liaocheng 252000, P. R. China
3. Laboratory of Theoretical and Computational Nanoscience, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Chinese Academy of Sciences, Beijing 100190, P. R. China
4. State Key Laboratory of the Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an 710049, P. R. China