1. Electrical Engineering and Computer Sciences, University of California, Berkeley, California 94720, United State
2. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
3. The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
4. Center for Integrated Nanotechnologies, Material Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States