1. Key Laboratory of Optoelectronics Technology Ministry of Education Beijing University of Technology Beijing 100124 China
2. Faculty of Engineering and Natural Science Tampere University Tampere 33720 Finland
3. Key Laboratory of Semiconductor Materials Science Institute of Semiconductors Chinese Academy of Sciences Beijing 100083 China
4. Nanofabrication Laboratory CAS Key Laboratory for Nanophotonic Materials and Devices CAS Key Laboratory for Nanosystems and Hierarchical Fabrication CAS Center for Excellence in Nanoscience National Center for Nanoscience and Technology Beijing 100190 China