1. Shaanxi Key Laboratory of Nano Materials and Technology, College of Materials Science and Engineering, Xi'an University of Architecture and Technology, Xi'an 710055, P. R. China
2. School of Physics and Opto-Electronic Technology, Baoji University of Arts and Sciences, Baoji, Shanxi 721016, P. R. China
3. Institute of Guizhou Aerospace Measuring and Testing Technology, Guiyang 550009, P. R. China