1. SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of Ministry of Education, School of Electronic Science & Engineering, Southeast University, Nanjing, Jiangsu 210096, P. R. China
2. National Graphene Products Quality Inspection and Testing Center (Jiangsu), Special Equipment Safety Supervision Inspection Institute of Jiangsu Province, Yanxin Road 330, Wuxi, Jiangsu 214174, P. R. China