1. School of Electrical Engineering and Automation, Hefei University of Technology, Hefei 230009, P. R. China
2. School of Microelectronics, Hefei University of Technology, Hefei 230009, P. R. China
3. School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, P. R. China
4. Academy of OptoElectric Technology, Hefei University of Technology, Hefei 230009, P. R. China
5. Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, UK