1. CAS Center for Excellence in Nanoscience Beijing Key Laboratory of Micro‐nano Energy and Sensor Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Sciences Beijing 100083 P. R. China
2. School of Nanoscience and Technology University of Chinese Academy of Sciences Beijing 100049 P. R. China
3. Department of Photonics Engineering Technical University of Denmark Kongens Lyngby DK‐2800 Denmark
4. Department of Physics Universidad Carlos III de Madrid Leganés Madrid ES‐28916 Spain
5. School of Materials Science and Engineering Georgia Institute of Technology Atlanta GA 30332‐0245 USA