1. Department of Physics and Astronomy, University of California, Los Angeles, California 90095, United States
2. California NanoSystems Institute, University of California, Los Angeles, California 90095, United States
3. Department of Materials Science and Engineering, University of Virginia, Charlottesville, Virginia 22904, United States
4. NanoElectronic Imaging, Inc., Los Angeles, California 90095, United States
5. Charles L. Brown Department of Electrical and Computer Engineering, University of Virginia, Charlottesville, Virginia 22904, United States