1. Center of Nanomaterials for Renewable Energy (CNRE), State Key Laboratory of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi'an Jiaotong University, 710049 Xi'an, People's Republic of China
2. Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education and International Center for Dielectric Research, School of Electronic and Information Engineering, Xi'an Jiaotong University, 710049 Xi'an, People's Republic of China