1. School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA
2. Laboratoire de Physique et Chimie des Nano-objets, Institut National des Sciences Appliquees, 31077 Toulouse, Cedex 4, France
3. Nuclear and Radiological Engineering and Medical Physics Program, School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA