1. Institute for Advanced Study Shenzhen University Shenzhen 518060 P. R. China
2. Department of Materials Science and Engineering City University of Hong Kong Hong Kong 999077 P. R. China
3. College of Electronics and Information Engineering Shenzhen University Shenzhen 518060 P. R. China
4. Shenzhen Key Laboratory of Flexible Memory Materials and Devices Institute of Microscale Optoelectronics Shenzhen University Shenzhen 518060 P. R. China