1. Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, Bellaterra 08193, Barcelona, Spain
2. Dep. de Fisica, Universitat Autònoma de Barcelona, Campus UAB, Bellaterra 08193, Barcelona, Spain
3. Institut des Nanotechnologies de Lyon (INL), UMR CNRS 5270, Ecole Centrale de Lyon, 36 avenue Guy de Collongues, 69134 Ecully Cedex, France
4. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA