1. Department of Physics and School of Engineering, Brown University, Providence, Rhode Island 02912, USA
2. Semiconductor and Dimensional Metrology Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
3. Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, USA
4. Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
5. Department of Electrical and Computer Engineering, University of California, San Diego, California 92093, USA