1. CAS Key Laboratory of Nanophotonic Materials and Devices CAS Key Laboratory of Standardization and Measurement for Nanotechnology CAS Center for Excellence in Nanoscience National Center for Nanoscience and Technology Beijing 100190 P. R. China
2. Center of Materials Science and Optoelectronics Engineering University of Chinese Academy of Sciences Beijing 100049 P. R. China
3. School of Life Science Peking University Beijing 100871 P. R. China
4. Department of Electronics and Nanoengineering and QTF Centre of Excellence Department of Applied Physics Aalto University Espoo 02150 Finland
5. ICFO‐Institut de Ciencies Fotoniques The Barcelona Institute of Science and Technology Castelldefels Barcelona 08860 Spain
6. ICREA‐Institució Catalana de Recerca i Estudis Avançats Passeig Lluís Companys 23 Barcelona 08010 Spain