1. Shenzhen Geim Graphene Center Tsinghua-Berkeley Shenzhen Institute and Tsinghua Shenzhen International Graduate School Tsinghua University Shenzhen 518055 P. R. China
2. International Center for Young Scientists National Institute for Materials Science Tsukuba 305-0044 Japan
3. Shenyang National Laboratory for Materials Science Institute of Metal Research Chinese Academy of Sciences Shenyang 110016 P. R. China