1. Department of Chemistry, Howard University, Washington, D.C. 20059, United States
2. Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas 77204, United States
3. Department of Chemistry, Physics and Atmospheric Science, Jackson State University, Jackson, Mississippi 39217, United States
4. Material Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States