1. Department of Physics, University of California at Berkeley, Berkeley, California 94720, United States
2. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
3. Kavli Energy NanoSciences Institute at the University of California at Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
4. Department of Chemistry, University of California at Berkeley, Berkeley, California 94720, United States
5. The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720 United States