1. Beijing Key Laboratory of Micro‐nano Energy and Sensor; CAS Center for Excellence in Nanoscience Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences Beijing 101400 P. R. China
2. Institute of Semiconductors Chinese Academy of Sciences Beijing 100083 P.R. China
3. Department of Materials Science and Engineering The Pennsylvania State University University Park PA 16802 USA
4. Department of Electrical Engineering and Materials Research Institute Pennsylvania State University University Park PA 16802 USA
5. School of Material Science and Engineering Georgia Institute of Technology Atlanta GA 30332 USA
6. School of Physical Science and Technology Guangxi University Nanning Guangxi 530004 P.R. China