1. Department of Applied Physics, Northwestern Polytechnical University, Xi’an, P. R. China
2. Center of Nanomaterials for Renewable Energy (CNRE), State Key Laboratory of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi’an Jiaotong University, Xi’an, P. R. China
3. Department of Applied Chemistry, Northwestern Polytechnical University, Xi’an, P. R. China