1. Low-dimensional
Materials and Devices Laboratory, Tsinghua-Berkeley Shenzhen Institute, Tsinghua University, Shenzhen, Guangdong 518055, People’s Republic of China
2. Department
of Materials Science and NanoEngineering, Rice University, Houston, Texas 77005, United States
3. Institute
of Advanced Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen, Guangdong 518055, People’s Republic of China