1. School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, P. R. China
2. Key Laboratory of Functional Materials for Electronic Information (B), MoE, Huazhong University of Science and Technology, Wuhan, 430074, P. R. China
3. Department of Microelectronic Science and Engineering, Ningbo University, Ningbo 315211, P. R. China