[1] Nanyang Technol Univ, Sch Elect & Elect Engn, Singapore 639798, Singapore
[2] Northwest Normal Univ, Key Lab Atom & Mol Phys & Funct Mat Gansu Prov, Coll Phys & Elect Engn, Lanzhou 730070, Peoples R China
[3] Chinese Acad Sci, Lanzhou Inst Chem Phys, Lab Clean Energy Chem & Mat, Lanzhou 730000, Peoples R China
[4] Gansu Acad Sci, Inst Nanomat Applicat Technol, Lanzhou 730000, Peoples R China