1. Department of Materials Science and Engineering, University of California, Los Angeles, 410 Westwood Plaza, Los Angeles, CA 90095, USA.
2. Department of Mechanical and Aerospace Engineering, and Andlinger Center for Energy and the Environment, Princeton University, Princeton, NJ 08544, USA.
3. IBM Thomas J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, NY 10598, USA.