1. Key Laboratory of Advanced Materials (MOE)School of Materials Science and EngineeringTsinghua University Beijing 100084 P. R. China
2. Center for Brain Inspired Computing Research (CBICR)Tsinghua University Beijing 100084 P. R. China
3. Key Laboratory of Microelectronic Devices and Integrated TechnologyInstitute of MicroelectronicsChinese Academy of Sciences Beijing 100029 P. R. China