1. State Key Laboratory of Marine Resource Utilization in South China Sea, School of Materials Science and Engineering Hainan University Haikou 570228 P. R. China
2. School of Science Hainan University Haikou 570228 P. R. China
3. Ames Laboratory-U. S. Department of Energy, and Department of Physics and Astronomy Iowa State University Ames IA 50011 USA