1. State Key Laboratory of Optoelectronic Materials and Technologies
2. Nanotechnology Research Center
3. School of Physics & Engineering
4. Sun Yat-sen University
5. Guangzhou 510275, P. R. China
6. Key Laboratory of Particle & Radiation Imaging (Tsinghua University) of Ministry of Education
7. Department of Engineering Physics
8. Tsinghua University
9. Beijing 100084, P. R. China