1. National Laboratory of Solid States Microstructures and Key Laboratory of Modern Acoustics MOE Institute of Acoustics Nanjing University Nanjing 210093 P. R. China
2. National Laboratory of Solid States Microstructures College of Engineering and Applied Sciences and Collaborative Innovation Center of Advanced Microstructures Nanjing University Nanjing 210023 P. R. China
3. Department of Materials Science and Engineering University of California Irvine Irvine CA 92697 USA
4. College of Electronic Engineering Nanjing Xiaozhuang University Nanjing 211171 P. R. China