1. Key Laboratory of Luminescence and Optical Information Ministry of Education Beijing Jiaotong University Beijing 100044 P. R. China
2. Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization The University of Toledo Toledo OH 43606 USA
3. State Key Laboratory for Superlattices and Microstructures Institute of Semiconductors Chinese Academy of Sciences Beijing 100083 P. R. China
4. National Center for Nanoscience and Technology Beijing 100190 P. R. China