1. Research Institute for Biomimetics and Soft Matter Fujian Provincial Key Laboratory for Soft Functional Materials Research College of Materials Xiamen University 422 Siming Nan Road Xiamen 361005 P. R. China
2. Department of Chemistry Molecular Engineering and Sciences Institute University of Washington Seattle WA 98195 USA
3. Key Laboratory of Optoelectronic Information Materials of Hebei Province Key Laboratory of Digital Medical Engineering of Hebei Province College of Electron and Information Engineering Hebei University Baoding 071002 P. R. China
4. Department of Materials Science and Engineering University of Washington Seattle WA 98195 USA
5. College of Physical Science and Engineering Xiamen University 422 Siming Nan Road Xiamen 361005 P. R. China
6. Department of Physics National University of Singapore 2 Science Drive 3 Singapore 117542 Singapore