1. College of Electronic and Optical Engineering; College of Microelectronics; Nanjing University of Posts and Telecommunications; No. 9 Wenyuan Road, Nanjing 210023 China
2. National Laboratory of Solid State Microstructures; School of Electronic Science and Engineering; Nanjing University; No. 22 Hankou Road, Nanjing 210093 China
3. Physics Department and Jiangsu Key Laboratory for Chemistry of Low-Dimensional Materials; Huaiyin Normal University; No. 111 West Changjiang Road, Huaian 223300 China