1. Roy & Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, United States
2. Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6396, United States
3. Department of Materials Science and Engineering, University of Sheffield, Sheffield, S1 3JD, United Kingdom
4. Department of Physics, Zhejiang Sci-Tech University, 310018 Hangzhou, China