1. Wuhan National Laboratory for Optoelectronics School of Optical and Electronic Information Huazhong University of Science and Technology Wuhan Hubei 430074 P. R. China
2. Department of Materials Science and Engineering University of California Los Angeles CA 90095 USA
3. Department of Materials Science and Engineering Beijing Institute of Technology Beijing 100081 P. R. China
4. Department of Materials Science and Engineering College of Engineering Peking University Beijing 100871 P. R. China
5. Department of Materials Engineering Monash University Victoria 3800 Australia