1. School of Physical Science and Technology Inner Mongolia Key Lab of Nanoscience and Nanotechnology Inner Mongolia University Hohhot 010021 P. R. China
2. College of Science Nanjing Forestry University Nanjing 210037 P. R. China
3. State Key Laboratory of Luminescence and Applications Changchun Institute of Optics Fine Mechanics and Physics Chinese Academy of Sciences 3888 Eastern South Lake Road Changchun 130033 P. R. China