1. Collaborative Innovation Center for Nanomaterials & DevicesCollege of PhysicsQingdao University Qingdao 266071 P. R. China
2. Medical CollegeQingdao University Qingdao 266071 P. R. China
3. Department of Mechanical EngineeringColumbia University New York NY 10027 USA
4. Center for Nanofibers & NanotechnologyDepartment of Mechanical EngineeringNational University of Singapore Singapore 117574 Singapore