1. Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA
2. School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China
3. Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA
4. Department of Physics, Oakland University, Rochester, Michigan 48309, USA
5. Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, USA
6. Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA