1. CAS Center for Excellence in NanoscienceBeijing Key Laboratory of Micro‐Nano Energy and SensorBeijing Institute of Nanoenergy and NanosystemsChinese Academy of Sciences Beijing 100083 P. R. China
2. School of Nanoscience and TechnologyUniversity of Chinese Academy of Sciences Beijing 100049 P. R. China
3. Department of Photonics EngineeringTechnical University of Denmark DK‐2800 Kongens Lyngby Denmark
4. Department of PhysicsUniversity of West Georgia Carrollton GA 30118 USA
5. School of Materials Science and EngineeringGeorgia Institute of Technology Atlanta GA 30332‐0245 USA