1. Institute of Microelectronics, Tsinghua University, Beijing 100084, China.
2. Beijing National Research Center for Information Science and Technology (BNRist), Tsinghua University, Beijing 100084, China.
3. Nano-Materials Group (FIRST Nano Co-op Center), National Nanofab Center (NNFC) at KAIST, Daejeon 34141, Republic of Korea.