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 China
2. Center of Materials Science and Optoelectronics Engineering University of Chinese Academy of Sciences Beijing 100049 China
3. Department of Electronics and Nanoengineering and QTF Centre of Excellence Department of Applied Physics Aalto University Espoo 02150 Finland