1. CAS Center for Excellence in NanoscienceBeijing Institute of Nanoenergy and NanosystemsChinese Academy of Sciences Beijing 100083 China
2. School of Nanoscience and TechnologyUniversity of Chinese Academy of Sciences Beijing 100049 China
3. Center on Nanoenergy ResearchSchool of Physical Science and TechnologyGuangxi University Nanning Guangxi 530004 P. R. China