1. College of Mechanical and Electrical Engineering; North China University of Technology; Beijing 100144 P. R. China
2. Changchun Institute of Optics, Fine Mechanics and Physics; Chinese Academy of Sciences; Changchun 130033 P. R. China
3. School of Materials Science and Engineering; Beijing Institute of Technology; Beijing 100081 P. R. China
4. Materials Research Institute, Pennsylvania State University; University Park; PA 16802 USA