1. Department of Electrical and Computer Engineering; University of California San Diego; 9500 Gilman Dr MC 0407 La Jolla California 92093 USA
2. Division of Physics and Applied Physics; Nanyang Technological University; 21 Nanyang Link Singapore 637371
3. Center for Integrated Nanotechnologies; Los Alamos National Laboratory; P.O.Box 1663 MC K771 Los Alamos New Mexico 87545 USA