1. Department of Mechanical Engineering, City University of Hong Kong, Hong Kong 999077, P. R. China
2. Department of Physics, City University of Hong Kong, Hong Kong 999077, P. R. China
3. Argonne National Laboratory, X-Ray Science Division, Argonne, Illinois 60439, USA
4. Shenzhen Research Institute, City University of Hong Kong, Shenzhen 518057, P. R. China