1. Chinese Academy of Sciences, Institute of Plasma Physics, P. O. Box 1126 Hefei 230031, China
2. Tsinghua University, School of Materials Science and Engineering, Laboratory of Advanced Materials, Beijing 100084, China
3. Advanced Technology & Materials Co., Ltd., Beijing 100081, China
4. Hefei Center for Physical Science and Technology, Hefei 230031, China
5. Chinese Academy of Sciences, Hefei Science Center, Hefei 230031, China