1. Center for Electron Microscopy and Microanalysis, University of Southern California, Los Angeles, CA 90089, USA.
2. Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA.
3. California NanoSystems Institute, University of California, Los Angeles, CA 90095, USA.
4. Department of Electrical Engineering, University of Southern California, Los Angeles, CA 90089, USA.
5. Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.