1. CAS Center for Excellence in NanoscienceBeijing Key Laboratory of Micro–Nano Energy and Sensor Beijing Institute of Nanoenergy and NanosystemsChinese Academy of Sciences Beijing 100083 P. R. China
2. School of Nanoscience and TechnologyUniversity of Chinese Academy of Sciences Beijing 100049 P. R. China
3. Center on Nanoenergy ResearchSchool of Physical Science and TechnologyGuangxi University Nanning 530004 P. R. China
4. School of Material Science and EngineeringGeorgia Institute of Technology Atlanta GA 30332 USA