a
School of Materials Science and Engineering, Seoul National University, Daehak-dong, Kwanak-gu, Seoul 151-744, S. Korea
E-mail: kshongss@plaza.snu.ac.kr
b
Department of Materials Science and Engineering, Ajou University, Suwon 443-749, Korea
E-mail: dwkim@ajou.ac.kr
c
Department of Mechanical Engineering, Stanford University, California 94305, USA