1. School of Stomatology, Lanzhou University, Lanzhou, P. R. China
2. National and Local Joint Engineering Laboratory for Optical Conversion Materials and Technology of National Development and Reform Commission, Department of Materials Science, School of Materials and Energy, Lanzhou University, Lanzhou, P. R. China
3. College of Materials Science and Engineering, Qingdao University, Ningxia Road, No. 308, Qingdao 266071, P. R. China