1. Department of Physics and Jiangsu Key Laboratory of Advanced Metallic Materials, Southeast University, Nanjing 211189, China
2. National Laboratory of Solid State Microstructure, Nanjing University, Nanjing 210093, China
3. NanoElectronics Group, MESA+Institute for Nanotechnology, University of Twente, P.O. Box 217, Enschede 7500 AE, The Netherlands
4. Irvine Materials Research Institute, University of California, Irvine, California 92697-2800, USA