1. College of Materials Xiamen University 422 Siming Nan Road Xiamen 361005 P. R. China
2. 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
3. Pacific Northwest National Laboratory Richland WA 99352 USA
4. Research Institute for Biomimetics and Soft Matter Fujian Provincial Key Laboratory for Soft Functional Materials Research College of Physical Science and Engineering Jiujiang Research Institute Xiamen University 422 Siming Nan Road Xiamen 361005 P. R. China
5. Department of Physics National University of Singapore 2 Science Drive 3 Singapore 117542 Singapore