1. Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA
2. Department of Mechanical Engineering, Stanford University, Stanford, California 94305, USA
3. Department of Mechanical Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, South Korea
4. Naval Research Laboratories, Washington, DC 20375, USA
5. Department of Materials Science and Engineering, Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA