1. School of Materials Science and Engineering Beihang University Beijing 100191 P. R. China
2. College of Physics and Optoelectronic Engineering Shenzhen University Shenzhen 518060 P. R. China
3. Shenzhen Institute for Technology Innovation NIM Shenzhen 518060 P. R. China
4. School of Physics and Electronic Science East China Normal University Shanghai 200241 P. R. China