1. Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6272, United States
2. DuPont Nanocomposite Technologies, Central Research & Development, E.I. DuPont de Nemours & Co., Inc., P.O. Box 400, Wilmington, Delaware, 19880-0400, United States
3. Department of Physics and Astronomy, The University of Sheffield, Sheffield, S3 7RH, United Kingdom