1. Department of Physics, University of South Florida, 4202 East Fowler Avenue, Tampa, Florida 33620, United States
2. Materials Science and Engineering Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, Stop 8500, Gaithersburg, Maryland 20899, United States
3. Department of Chemistry and Biochemistry, University of Maryland, 091 Chemistry Building, College Park, Maryland 20742, United States