1. Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic SystemsSchool of Materials Science and EngineeringBeijing Institute of Technology Beijing 100081 P. R. China
2. Experimental Center of Advanced MaterialsSchool of Materials Science and EngineeringBeijing Institute of Technology Beijing 100081 P. R. China
3. Department of Materials Science and EngineeringCollege of EngineeringPeking University Beijing 100871 P. R. China
4. School of Materials Science and EngineeringBeihang University Beijing 100191 P. R. China
5. School of ScienceJiangnan University Wuxi Jiangsu 214122 P. R. China