1. School of Instrumentation Science and Engineering, Harbin Institute of Technology, Harbin 150080, P. R. China
2. School of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150040, P. R. China
3. School of Physics, Shandong University, Jinan 250100, P. R. China
4. Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, USA