1. Department of Chemistry, Energy Institute, and Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction Hong Kong University of Science and Technology Clear Water Bay Kowloon Hong Kong
2. State Key Laboratory for Mechanical Behavior of Materials Xi'an Jiaotong University Xi'an 710049 P. R. China
3. Department of Physics Hong Kong University of Science and Technology Clear Water Bay Kowloon Hong Kong
4. Key Laboratory of Organic Solids CAS Research/Education Center for Excellence in Molecular Sciences Institute of Chemistry Chinese Academy of Sciences Beijing 100190 China
5. Department of Physics and Organic and Carbon Electronics Laboratory North Carolina State University Raleigh NC 27695 USA