1. Key Laboratory for the Physics and Chemistry of Nanodevices and Academy for Advanced Interdisciplinary Studies; Peking University; Beijing 100871 China
2. Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics; Peking University; Beijing 100871 China
3. Suzhou Institute of Nano-Tech and Nano-Bionics; Chinese Academy of Sciences; 398 Ruoshui Road, Suzhou Industrial Park Suzhou 215123 China