1. Department of Physics, Harbin Institute of Technology, Harbin 150001, P. R. China
2. School of Life Science and Technology, Harbin Institute of Technology, Harbin 150080, P. R. China
3. Department of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, P. R. China
4. Center for Advanced Optoelectronic Functional Materials Research, Key Laboratory of UV-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, P. R. China
5. Department of Materials Science, Fudan University, Shanghai 200433, P. R. China