1. College of Mechanics and Materials, Hohai University, Nanjing 211100, PR China
2. School of Mechanical and Electrical Engineering, China University of Mining and Technology, Xuzhou 221116, Jiangsu Province, PR China
3. Department of Physics, Department of Materials Science and Engineering, and Department of Biomedical Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon 999077, Hong Kong, China