1. Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA.
2. Shenzhen Geim Graphene Center, Tsinghua-Berkeley Shenzhen Institute and Tsinghua Shenzhen International Graduate School, Tsinghua University, Shenzhen 518055, China.
3. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
4. Stanford Institute for Materials and Energy Science, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA.