[1] Prince Georges Community Coll, Dept Phys Sci & Engn, Largo, MD 20774 USA
[2] Johns Hopkins Univ, Dept Phys & Astron, Baltimore, MD 21218 USA
[3] Howard Univ, Howard Nanofabricat Facil, Washington, DC 20059 USA
[4] Howard Univ, Dept Chem, Washington, DC 20059 USA