1. Key Laboratory for the Physics and Chemistry of Nanodevices, School of Electronics, Peking University, Beijing 100871, China
2. Beijing Key Laboratory of Microstructure and Property of Solids, Faculty of Materials and Manufacturing, Beijing University of Technology, Beijing 100124, China
3. National Key Laboratory of Nano/Micro Fabrication Technology, School of Integrated Circuits, Peking University, Beijing 100871, China