1. School of Material Science and EngineeringGeorgia Institute of Technology Atlanta GA 30332 USA
2. School of Light Industry and Food EngineeringGuangxi University Nanning 530004 P. R. China
3. Beijing Institute of Nanoenergy and NanosystemsChinese Academy of Sciences National Center for Nanoscience and Technology (NCNST) Beijing 100083 P. R. China