1. Department of Physics, University of California Berkeley, Berkeley, California 94720, USA
2. Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
3. Kavli Energy NanoScience Institute of Berkeley, Berkeley, California 94720, USA
4. Department of Chemistry and the PULSE Institute, Stanford University, Stanford, California 94305, USA
5. SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA