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 Guangxi 530004 P. R. China
4. College of Physics and Optoelectronic Engineering Shenzhen University Shenzhen 518060 P. R. China
5. School of Materials Science and Engineering Georgia Institute of Technology Atlanta Georgia 30332-0245 USA