1. School of Engineering, Brown University, Providence, Rhode Island 02912, USA
2. Institut P', Département Physique et Mécanique des Matériaux, Université de Poitiers-CNRS-ENSMA, SP2MI, Téléport 2, Bd M. et P. Curie, F-86962 Chasseneuil-Futuroscope, France
3. Nanoscale Engineering Division, Department of Physics, Chemistry and Biology, Linköping University, SE-581 83 Linköping, Sweden