[1] Henan Univ, Key Lab Special Funct Mat, Minist Educ, Kaifeng 475004, Peoples R China
[2] Georgia Inst Technol, Sch Mat Sci & Engn, Atlanta, GA 30318 USA
[3] Henan Univ, Sch Phys & Elect, Kaifeng 475004, Peoples R China
[4] Chinese Acad Sci, Beijing Inst Nanoenergy & Nanosyst, Beijing 100083, Peoples R China