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