1. School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
2. Conway Institute of Biomolecular and Biomedical Research, University College Dublin, Belfield, Dublin 4, Ireland
3. Department of Applied Physics, KTH - Royal Institute of Technology, Roslagstullbacken 21, 106 91 Stockholm, Sweden
4. Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA