title: Charge transport and rectification in molecular junctions formed with carbon-based electrodes.
abstract: Molecular junctions formed using the scanning-tunneling-microscope-based break-junction technique (STM-BJ) have provided unique insight into charge transport at the nanoscale. In most prior work, the same metal, typically Au, Pt, or Ag, is used for both tip and substrate. For such noble metal electrodes, the density of electronic states is approximately constant within a narrow energy window relevant to charge transport. Here, we form molecular junctions using the STM-BJ technique, with an Au metal tip and a microfabricated graphite substrate, and measure the conductance of a series of graphite/amine-terminated oligophenyl/Au molecular junctions. The remarkable mechanical strength of graphite and the single-crystal properties of our substrates allow measurements over few thousand junctions without any change in the surface properties. We show that conductance decays exponentially with molecular backbone length with a decay constant that is essentially the same as that for measurements with two Au electrodes. More importantly, despite the inherent symmetry of the oligophenylamines, we observe rectification in these junctions. State-of-art ab initio conductance calculations are in good agreement with experiment, and explain the rectification. We show that the highly energy-dependent graphite density of states contributes variations in transmission that, when coupled with an asymmetric voltage drop across the junction, leads to the observed rectification. Together, our measurements and calculations show how functionality may emerge from hybrid molecular-scale devices purposefully designed with different electrodes beyond the so-called "wide band limit," opening up the possibility of assembling molecular junctions with dissimilar electrodes using layered 2D materials.
authors.affiliation: Department of Applied Physics and Mathematics, Columbia University, New York, NY 10027;Department of Physics, Hankuk University of Foreign Studies, Yongin 449-791, Korea;
authors.forename: Taekyeong
authors.initials: T
authors.lastname: Kim
authors.affiliation: Molecular Foundry, Materials Sciences Division, Lawrence Berkeley National Laboratory, and Department of Physics, University of California, Berkeley, and Kavli Energy NanoSciences Institute, Berkeley, CA 94720; and.
authors.forename: Zhen-Fei
authors.initials: ZF
authors.lastname: Liu
authors.affiliation: Department of Physics, Columbia University, New York, NY 10027.
authors.forename: Chulho
authors.initials: C
authors.lastname: Lee
authors.affiliation: Molecular Foundry, Materials Sciences Division, Lawrence Berkeley National Laboratory, and Department of Physics, University of California, Berkeley, and Kavli Energy NanoSciences Institute, Berkeley, CA 94720; and lv2117@columbia.edu jbneaton@lbl.gov.
authors.forename: Jeffrey B
authors.initials: JB
authors.lastname: Neaton
authors.affiliation: Department of Applied Physics and Mathematics, Columbia University, New York, NY 10027; lv2117@columbia.edu jbneaton@lbl.gov.
authors.forename: Latha
authors.initials: L
authors.lastname: Venkataraman
keywords: density functional theory; graphite electrodes; molecular circuits