1. School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, 100083, Center for Advanced Micro Structures and Devices, Louisiana State University, Baton Rouge, Louisiana 70806, College of Materials Science and Engineering, Beijing University of Chemical Technology, Department of Electrical and Computer Engineering, Old Dominion University, Norfolk, Virginia 23529, and Department of Physics, Southern University and A & M College, Baton Rouge, Louisiana 70813