This project was supported by the Office of Naval Re- search under Award N00014-07-1-0690-DOD35CAP and by the Air Force Office of Scientific Research under Award FA9550-09-1-0669-DOD35CAP. The work was performed in part at the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Infrastructure Network (NNIN), which is sup- ported by the National Science Foundation under Award ECS-0335765. L.M. thanks the US Department of Homeland Security (DHS) for the fellow- ship. The DHS Scholarship and Fellowship Program is administered by the Oak Ridge Institute for Science and Education (ORISE) through an interagency agreement between the US Department of Energy (DOE) and DHS. ORISE is managed by Oak Ridge Associated Universities (ORAU) under DOE Contract DE-AC05-06OR23100.