1. Department of Chemistry, Virginia Commonwealth University (VCU), 1001 West Main, Richmond, Virginia 23284, USA
2. Nanotechnology and Nanometrology Lab., National institute for standards (NIS), 136 Tersa, Haram, Giza 12211, Egypt
3. Department of Physics, Virginia Commonwealth University (VCU), 701 West Grace, Richmond, Virginia 23284, USA