1. National & Local Joint Engineering Laboratory for Optical Conversion Materials and Technology School of Physical Science and Technology Lanzhou University Lanzhou 730000 P.R. China
2. Department of Applied Physics Eindhoven University of Technology Eindhoven 5600 MB the Netherlands
3. Hubei Key Laboratory of Low Dimensional Optoelectronic Materials and Devices Hubei University of Arts and Science Xiangyang Hubei 441053 China