[1] Georgia Inst Technol, Sch Mat Sci & Engn, Atlanta, GA 30332 USA
[2] Xi An Jiao Tong Univ, Elect Mat Res Lab, Key Lab Minist Educ, Xian 710049, Peoples R China
[3] Xi An Jiao Tong Univ, Int Ctr Dielect Res, Xian 710049, Peoples R China
[4] Chinese Acad Sci, Beijing Inst Nanoenergy & Nanosyst, Beijing 100083, Peoples R China