1. School of Physics and Electronics Central South University Changsha 410083 P. R. China
2. Department of Materials Science and Engineering Southern University of Science and Technology Shenzhen 518055 P. R. China
3. Nanoinstitut München Fakultät für Physik Ludwig-Maximilians-Universität München 80539 München Germany
4. School of Materials Science and Engineering Zhengzhou University Zhengzhou 450001 P. R. China
5. National Synchrotron Radiation Research Center Hsinchu 300 Taiwan