1. Stanford Nanocharacterization Laboratory, Stanford University, Stanford, California 94305, United States
2. Curriculum in Applied Sciences and Engineering, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States
3. Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States
4. Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States