1. SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of Ministry of Education, Southeast University, Nanjing 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 214174, Jiangsu, P. R. China