1. Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, USA
2. Department of Physics Education, Chonnam National University, Gwangju, 61186 Republic of Korea
3. Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853-1501, USA
4. Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA
5. Department of Electrical Engineering, University of Notre Dame, Notre Dame, Indiana 46556, USA