1. Key Laboratory of Green Chemistry and Technology of Ministry of EducationCollege of Chemistry, and State Key Laboratory of Polymer Materials EngineeringSichuan University Chengdu 610064 P. R. China
2. State Key Laboratory for Mechanical Behavior of MaterialsXi'an Jiaotong University Xi'an 710049 P. R. China
3. Department of Electrical and Electronic EngineeringThe University of Hong Kong Hong Kong 999077 P. R. China