1. Department of Materials Science and Engineering, City University of Hong Kong, Hong Kong SAR 999077, China
2. Shenzhen Geim Graphene Center, Tsinghua-Berkeley Shenzhen Institute & Institute of Materials Research, Tsinghua Shenzhen International Graduate School, Tsinghua University, Shenzhen 518055, China
3. Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 816-8580, Japan
4. State Key Laboratory of Terahertz and Millimeter Waves, City University of Hong Kong, Hong Kong SAR 999077, China