1. Nanotechnology Center and Applied Science Department, University of Arkansas at Little Rock, Little Rock, Arkansas 72204, Department of Physics and Astronomy, University of Arkansas at Little Rock, Little Rock, Arkansas 72204, National Institute for Research and Development of Isotopic and Molecular Technologies, P.O. Box 700, R-400293 Cluj-Napoca, Romania, and Physics Department, University of Arkansas, Fayetteville, Arkansas 72701