Large tunable image-charge effects in single-molecule junctions

Journal name:
Nature Nanotechnology
Volume:
8,
Pages:
282–287
Year published:
DOI:
doi:10.1038/nnano.2013.26
Received
Accepted
Published online

Abstract

Metal/organic interfaces critically determine the characteristics of molecular electronic devices, because they influence the arrangement of the orbital levels that participate in charge transport. Studies on self-assembled monolayers show molecule-dependent energy-level shifts as well as transport-gap renormalization, two effects that suggest that electric-field polarization in the metal substrate induced by the formation of image charges plays a key role in the alignment of the molecular energy levels with respect to the metal's Fermi energy. Here, we provide direct experimental evidence for an electrode-induced gap renormalization in single-molecule junctions. We study charge transport through single porphyrin-type molecules using electrically gateable break junctions. In this set-up, the position of the occupied and unoccupied molecular energy levels can be followed in situ under simultaneous mechanical control. When increasing the electrode separation by just a few ångströms, we observe a substantial increase in the transport gap and level shifts as high as several hundreds of meV. Analysis of this large and tunable gap renormalization based on atomic charges obtained from density functional theory confirms and clarifies the dominant role of image-charge effects in single-molecule junctions.

At a glance

Figures

  1. Illustration of the experiments.
    Figure 1: Illustration of the experiments.

    a, Structural formula of ZnTPPdT. b, Layout of the MCBJ set-up. c, False colour scanning electron microscope (SEM) image of a three-terminal MCBJ device. The gate is made of aluminium and covered with a plasma-enhanced native aluminium oxide layer. The gold electrodes are deposited on top of the gate dielectric. d, False colour SEM image of a two-terminal MCBJ.

  2. Mechanical gating of charge transport in ZnTPPdT junctions.
    Figure 2: Mechanical gating of charge transport in ZnTPPdT junctions.

    a,b, Current–voltage characteristics (a) and differential conductance (b) for MCBJ devices that have been exposed to a solution of ZnTPPdT. The estimated electrode displacement is relative to d0, the initial electrode separation. c,d, The same quantities are plotted for junctions exposed to the pure solvent. e,f, Two-dimensional visualization of dI/dV for ZnTPPdT as a function of bias voltage and electrode displacement while fusing sample A (e) and for three making/breaking cycles of a different device (sample B) (f). A clear dependence of the Coulomb gap on electrode spacing is apparent. The differential conductance has been normalized.

  3. Level shifts by electrostatic gating.
    Figure 3: Level shifts by electrostatic gating.

    a,b, Gate diagrams recorded on sample C for different junction configurations and during different breaking events. Colour-coded dI/dV plotted versus gate and bias voltage. The dependence of the resonance shift on gate voltage allows us to attribute resonances in a to an occupied level (HOMO-like, located at ~0.3 eV for zero gate voltage) and those in b to an unoccupied level (LUMO-like, located at ~0.75 eV for zero gate voltage). The corresponding electrostatic gate coupling value (EGC) is given in the figure. c, Effect of a rigid shift of the levels under electrostatic gating by a potential Vg applied to a gate electrode below the junction for an occupied and unoccupied level. Here, β is the electrostatic gate coupling, φm the metal workfunction, Δ is the shift of the potential VS outside the surface due to the presence of the molecule, and εF is the Fermi energy of the metal. εocc, εunocc and εocc, εunocc are the occupied and unoccupied levels for Vg = 0 and Vg ≠ 0, respectively.

  4. Level shifts by mechanical gating.
    Figure 4: Level shifts by mechanical gating.

    a,b, Systematic IV series for sample C, recorded immediately after Fig. 3a and b, respectively. HOMO-like (located at ~0.3 eV for zero displacement), a) and LUMO-like (located at ~0.75 eV for zero displacement, b) levels both move away from the Fermi energy with increasing electrode spacing. The corresponding mechanical gate coupling value (MGC) is displayed. c, Shift of occupied and unoccupied molecular orbital levels with distance to the metal. The effects contained in Δ shift all levels in the same direction, while image-charge effects are responsible for occupied and unoccupied levels moving closer to the Fermi energy of the metal (gap renormalization). φm represents the metal workfunction, Δ the interfacial dipole, V the potential at infinity, VS the potential at the surface and εF the Fermi energy. εocc, εunocc and εocc, εunocc are now the occupied and unoccupied levels of the molecule in the gas phase and at the interface, respectively.

  5. Transport calculation and image-charge model.
    Figure 5: Transport calculation and image-charge model.

    a, Zero-bias transmission and molecular orbital levels of ZnTPPdT coupled to gold, from DFT and DFT + NEGF calculations, respectively. The Fermi energies are reported with respect to the Fermi energy of the metal electrodes, which is marked by the vertical black line. ZnTPPdT is located 2.59 Å from each lead, with hollow-site binding. b, Image-charge model geometry, with the image plane located 1 Å outside the first atomic layer (uncertainty bands derived from a 0.25 Å deviation), as indicated by the two arrows. c, Level shifts (relative to the relaxed junction with an electrode separation of 2.23 nm) predicted by the image-charge model (with uncertainties) showing the occupied and unoccupied levels both shifting towards εF for increasing electrode separation. The values in the plot represent the expected resonance shift in the experiments, assuming a symmetrically applied bias, and yield mechanical gate couplings in the range 0.4–2.8 V nm−1. These values may be significantly reduced for realistic electrode geometries.

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Affiliations

  1. Kavli Institute of Nanoscience, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands

    • Mickael L. Perrin,
    • Christopher J. O. Verzijl,
    • Christian A. Martin,
    • Joseph M. Thijssen,
    • Herre S. J. van der Zant &
    • Diana Dulić
  2. Department of Chemical Engineering, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands

    • Ahson J. Shaikh,
    • Rienk Eelkema &
    • Jan H. van Esch
  3. Kamerlingh Onnes Laboratory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands

    • Jan M. van Ruitenbeek

Contributions

D.D. and H.v.d.Z. designed the project. C.M., H.v.d.Z. and J.v.R. designed the set-up and the devices. M.P. and C.M. fabricated the devices. A.S., R.E. and J.v.E provided the molecules. M.P and D.D. performed the experiments. C.V., M.P. and J.T. performed the calculations. M.P., C.V., D.D., J.T. and H.v.d.Z. wrote the manuscript. All authors discussed the results and commented on the manuscript.

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The authors declare no competing financial interests.

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