1. Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory L-211, 7000 East Avenue, Livermore, California, Stanford Nanofabrication Facility, Stanford University, Palo Alto, California, Laboratory for Surface Science and Technology (LSST), ETH Hönggerberg, HCI F536, CH-8093 Zurich, Switzerland, and Institute of Bioengineering and Institute of Chemical Sciences and Engineering, Station 15, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland