1. Materials Institute of Atomic and Molecular Science Shaanxi University of Science and Technology Xi'an 710021 P. R. China
2. Department of Electrical Engineering and Computer Science University of Missouri Columbia MO 65211 USA
3. Shanxi‐Zheda Institute of Advanced Materials and Chemical Engineering Taiyuan 030024 P. R. China
4. 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 101400 P. R. China