1. Guangdong Key Laboratory of Optical Fiber Sensing and Communications Guangzhou Key Laboratory of Vacuum Coating Technologies and New Energy Materials Siyuan Laboratory Guangdong Provincial Engineering Technology Research Center of Vacuum Coating Technologies and New Energy Materials Department of Physics Jinan University Guangzhou Guangdong 510632 P. R. China
2. CAS Center for Excellence in Nanoscience Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Sciences Beijing 100083 P. R. China
3. School of Materials Science and Engineering Georgia Institute of Technology Atlanta GA 30332 USA