1. Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, PA 16802, USA.
2. Instituto de Física, Universidade de São Paulo, São Paulo, SP 05508-090, Brazil.
3. Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais 30123-970, Brazil.
4. Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P. R. China.
5. Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou 215123, P. R. China.
6. Department of Material Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA.
7. Institute of Carbon Science and Technology, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan.
8. School of Physics, Huazhong University of Science and Technology, Wuhan, Hubei 430074, P. R. China.
9. Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA.