title: Interplay of Bias-Driven Charging and the Vibrational Stark Effect in Molecular Junctions.
abstract: We observe large, reversible, bias driven changes in the vibrational energies of PCBM based on simultaneous transport and surface-enhanced Raman spectroscopy (SERS) measurements on PCBM-gold junctions. A combination of linear and quadratic shifts in vibrational energies with voltage is analyzed and compared with similar measurements involving C60-gold junctions. A theoretical model based on density functional theory (DFT) calculations suggests that both a vibrational Stark effect and bias-induced charging of the junction contribute to the shifts in vibrational energies. In the PCBM case, a linear vibrational Stark effect is observed due to the permanent electric dipole moment of PCBM. The vibrational Stark shifts shown here for PCBM junctions are comparable to or larger than the charging effects that dominate in C60 junctions.
authors.affiliation: Department of Physics and Astronomy, MS 61, Rice University , 6100 Main Street, Houston, Texas 77005, United States.
authors.forename: Yajing
authors.initials: Y
authors.lastname: Li
authors.affiliation: Department of Physics and Astronomy, MS 61, Rice University , 6100 Main Street, Houston, Texas 77005, United States.
authors.forename: Pavlo
authors.initials: P
authors.lastname: Zolotavin
authors.affiliation: Center for Nanophase Materials Sciences, Oak Ridge National Laboratory , Oak Ridge, Tennessee 37831-6493, United States.
authors.forename: Peter
authors.initials: P
authors.lastname: Doak
authors.affiliation: Department of Materials and Interfaces, Weizmann Institute of Science , Rehovoth 76100, Israel.
authors.forename: Leeor
authors.initials: L
authors.lastname: Kronik
authors.affiliation: Molecular Foundry, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.|Department of Physics, University of California at Berkeley , Berkeley, California 94720, United States.|Kavli Energy Nanosciences Institute at Berkeley , Berkeley, California 94720, United States.
authors.forename: Jeffrey B
authors.initials: JB
authors.lastname: Neaton
authors.affiliation: Department of Physics and Astronomy, MS 61, Rice University , 6100 Main Street, Houston, Texas 77005, United States.|Department of Electrical and Computer Engineering, MS 366, Rice University , Houston, Texas 77005, United States.|Department of Materials Science and Nanoengineering, MS 325, Rice University , Houston, Texas 77005, United States.
authors.forename: Douglas
authors.initials: D
authors.lastname: Natelson
keywords: Molecular junction; charge transfer; surface-enhanced Raman spectroscopy; vibrational Stark effect