1. Department of Chemistry, Stanford University, Stanford, California 94305, United States
2. Institute of Physics, University of Bayreuth, 95440 Bayreuth, Germany
3. Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
4. Department of Physics, University of California Berkeley, Berkeley, California 94720, United States
5. Kavli Energy NanoScience Institute at Berkeley, Berkeley, California 94720, United States