1. Department of Materials Science and Engineering, Yonsei University, 262 Seongsanno, Seoul 120-749, Korea
2. Department of Mechanical Engineering, University of California, Berkeley, California 94720, United States
3. Department of Mechanical and Aerospace Engineering, University of California, San Diego 92093, United States
4. National Center for Electron Microscopy, Lawrence Berkeley Laboratories, Berkeley, California 94720, United States
5. School of Mechanical Engineering, Yonsei University, 262 Seongsanno, Seoul 120-749, Korea