1. Materials Sciences Division and Chemical Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, Institut Català de Nanotecnologia (ICN), Campus de Bellaterra-Facultat de Ciències, Edifici C-Ala Nord-Mòduls, 08193 Bellaterra, Spain, and Department of Physical Chemistry, Universitat de Barcelona, 08028 Barcelona, Spain