1. School of Materials Science and Engineering; Central South University of Forestry and Technology; Changsha 410004 P.R. China
2. School of Materials Science and Engineering; South China University of Technology; Guangzhou 510641 P.R. China
3. Department of Physics and Interdisciplinary Program of Biomedical, Mechanical and Electrical Engineering; Pukyong National University; Busan 608-737 Republic of Korea