1. Key Laboratory of Luminescence and Optical Information Ministry of Education Beijing JiaoTong University Beijing 100044 China
2. Department of Physics and Astronomy University of Kansas Lawrence KS 66045 USA
3. Department of Materials Science and Engineering Shanghai Jiao Tong University Shanghai 200240 China
4. School of Microelectronics and State Key Laboratory for Mechanical Behavior of Materials Xi'an Jiaotong University Xi'an 710049 China