1. Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, Georgia 30332-0826, USA
2. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, USA
3. School of Chemical Sciences and Fredrick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
4. LNM, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China
5. George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0405, USA