1. Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA.
2. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
3. Maryland NanoCenter, University of Maryland, College Park, MD 20742, USA.
4. School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China.
5. Physics Department and Solid State Institute, Technion-Israel Institute of Technology, Haifa 32000, Israel.