1. CAS Center for Excellence in NanoscienceBeijing Key Laboratory of Micro‐Nano Energy and SensorBeijing Institute of Nanoenergy and NanosystemsChinese Academy of Science Beijing 100083 P. R. China
2. College of Nanoscience and TechnologyUniversity of Chinese Academy of Sciences Beijing 100049 P. R. China
3. State Key Laboratory of Electronic Thin films and Integrated DevicesUniversity of Electronic Science and Technology of China Chengdu 610054 P. R. China
4. School of Material Science and EngineeringGeorgia Institute of Technology Atlanta GA 30332 USA