1. Department of Materials Science and Engineering Drexel University Philadelphia PA 19118 USA
2. State Key Laboratory of Low Dimensional Quantum Physics and Department of Physics Tsinghua University Beijing 100084 P. R. China
3. Department of Materials Science and Engineering Shandong University Jinan 250061 P. R. China
4. Department of Physics and Astronomy and Department of Electrical Engineering and Computer Science Vanderbilt University Nashville TN 37235 USA
5. Materials Science and Technology Division Oak Ridge National Laboratory Oak Ridge TN 37831 USA