1. Key Laboratory of Atomic and Molecular Physics & Functional Materials of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, P. R. China
2. Institute of Nanomaterials Application Technology, Gansu Academy of Science, Lanzhou 730000, P. R. China
3. School of Electrical and Electronics Engineering, Nanyang Technological University, 639798 Singapore
4. Laboratory of Clean Energy Chemistry and Materials, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China