Manuscript received March 5, 2019; revised April 17, 2019; accepted April 29, 2019. Date of publication May 17, 2019; date of current version June 19, 2019. This work was supported in part by the National Natural Science Foundation of China under Grant 61404011, in part by the Natural Science Foundation of Hunan Province under Grant 2015JJ3001, and in part by the Scientific Research Fund of Hunan Provincial Education Department under Grant 17B007. The review of this paper was arranged by Editor C. Surya. (Corresponding author: Hai–Qing Xie.) H.–Q. Xie is with the School of Physics and Electronic Science, Chang-sha University of Science and Technology, Changsha 410114, China, and also with the Hunan Provincial Key Laboratory of Flexible Electronic Materials Genome Engineering, Changsha University of Science and Technology, Changsha 410114, China (e–mail: xiehq@csust.edu.cn).
