1. Department of Physics and Princeton Materials Institute, Princeton University, Princeton, NJ 08544
2. Department of Chemical Engineering and Princeton Materials Institute, Princeton University, Princeton, NJ 08544
3. National Institute of Standards and Technology, Gaithersburg, MD 20899
4. Princeton Materials Institute, Princeton University, Princeton, NJ 08544, and Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195