1. School
of Electronic Science and Engineering, Nanjing University, Nanjing 210093, P. R. China
2. College
of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P. R. China
3. Laboratory
of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China