1. Shenzhen Research Institute School of Physics and Electronics Central South University Changsha 410083 Hunan P. R. China
2. College of Materials Science and Engineering Hunan Province Key Laboratory for Advanced Carbon Materials and Applied Technology Hunan University Changsha 410082 Hunan P. R. China
3. School of Materials Science and Engineering Zhengzhou University Zhengzhou 450001 Hunan P. R. China
4. Department of Materials Science and Engineering School of Materials and Chemical Technology Tokyo Institute of Technology Tokyo 152‐8503 Japan