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. Center on Nanoenergy Research, School of Physical Science and Technology, Guangxi University, Nanning, 530004, P. R. China
4. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA