1. Center for
Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico
87185, United States
2. Department
of Materials Science
and Engineering and the Laboratory for Research on the Structure of
Matter, University of Pennsylvania, Philadelphia,
Pennsylvania 19104, United States, and
3. Penn Regional Nanotechnology Facility, University of Pennsylvania, Philadelphia, Pennsylvania,
19104, United States