1. State Key Laboratory of Solidification Processing, Center of Nano Energy Materials; School of Materials Science and Engineering, Northwestern Polytechnical University; Xi'an 710072 P. R. China
2. Department of Electrical and Electronic Engineering; The University of Hong Kong; Hong Kong P. R. China
3. College of Physics and Electronic Information; Huaibei Normal University; Huaibei 235000 P. R. China
4. Department of Mechanical Engineering; University of Delaware; Newark DE 19716 USA