1. Department of Materials Science & Engineering, Northwestern University, Evanston, Illinois 60208, Department of Physics, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India, Department of Radiology, Evanston Northshore Healthcare, Evanston, Illinois 60201, Department of Chemistry, Northwestern University, Evanston, Illinois 60208, and International Institute for Nanotechnology, Northwestern University, Evanston, Illinois 60208