1. Key Laboratory of Materials Physics CAS Center for Excellence in Nanoscience Anhui Key Laboratory of Nanomaterials and Nanotechnology Institute of Solid State Physics Chinese Academy of Sciences Hefei 230031 P. R. China
2. University of Chinese Academy of Sciences Beijing 100049 P. R. China
3. China Star Optoelectronics Technology Co., Ltd Shenzhen 518800 P. R. China
4. Department of Materials Science & Engineering University of Science and Technology of China Hefei 230026 P. R. China
5. Institut für Physik & IMN MacroNano@ (ZIK) Technische Universität Imenau Ilmenau 98693 Germany