1. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6203, United States
2. LeRoy Eyring Center for Solid State Science, Arizona State University, Tempe, Arizona 85287, United States
3. Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8371, United States
4. Department of Physics, Arizona State University, Tempe, Arizona 85287, United States