1. Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China
2. National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, P. R. China
3. School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, P. R. China
4. University Research Facility in Materials Characterization and Device Fabrication, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China
5. Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China
6. Institute of Advanced Energy Materials, Fuzhou University, Fuzhou 350002, P. R. China