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Stable anchoring chemistry for room temperature charge transport through graphite-molecule contacts

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Science Advances  09 Jun 2017:
Vol. 3, no. 6, e1602297
DOI: 10.1126/sciadv.1602297
  • Fig. 1 Single-molecule conductance measurements.

    Left panels illustrate the schematics of Au/HOPG junctions in (A) blank experiment without adding molecules and (B to D) with trapped single molecules: (B) molecular junction with a probable dendrimeric structure, which can be obtained upon AB grafting; (C) molecular junction with grafted DMAB; and (D) noncovalently anchored molecular junction with PPD. The corresponding 1D and 2D conductance histograms, which are constructed from more than 1000 individual withdrawing traces, are shown in the middle and right panels, respectively. The STM-BJ results in (A) to (C) are obtained in argon atmosphere without solvent. The measurements shown in (D) are carried out with PPD dissolved in 1,2,4-trichlorobenzene up to a concentration of 0.1 mM. In all cases, a positive bias voltage of 100 mV is applied. In 2D histograms, Δz represented the relative vertical displacement of the STM tip with respect to the substrate.

  • Fig. 2 Junction geometries and transmission coefficients.

    (A) Representative structure used for the evaluation of the dependence of (E) the binding energy to graphene on the distance between two adsorbed DMAB molecules. Transmission coefficient, T, as a function of energy, E, for (F) DMAB and the longer dendrimer, and for (G) PPD at different angles. The used structure for the transmission calculation for DMAB is shown in (B), the one for AB with an aminobenzene unit attached to model a longer dendrimer is shown in (C), and the structure for PPD at an angle of 21° from the HOPG plane is shown in (D) (the PPD structures at different angles are shown in fig. S12). For PPD, the lowest energy is found at an angle of 3°, and the energy increases with increasing pulling angle. Note that the full scattering region used in the calculations includes six layers of graphene for the HOPG electrode and five layers of Au, whereas here only two layers are shown for each electrode for clarity. A detailed explanation of the structural relaxations performed to obtain the atomic structures shown in (B) to (D) is given in Materials and Methods.

  • Fig. 3 Bias dependence.

    (A) Bias-dependent conductance of the HOPG/molecule/Au junctions for DMAB and PPD molecules obtained from both experiment (symbols) and theory. The calculations are performed at an electronic temperature of 300 K (solid lines) as well as 30 K (dashed lines). For DMAB, we use the structure shown in Fig. 2B, and for the PPD molecule, we used the configuration at 3°, where the molecule is nearly flat between Au and the graphite substrate, which exhibits the highest transmission. Because of the temperature-induced broadening of the Fermi distribution, the drop in conductance at very low bias is washed out at 300 K, whereas it is visible at 30 K. (B) Illustration of rectification behavior in opposite direction for covalent and noncovalent molecule-graphite attachment.

  • Fig. 4 Finite bias transmission and conductance.

    (A) Scattering region for the DMAB molecule between HOPG and Au and (B) independent non–self-consistent potential profiles for 0.2 V considered in the calculation of the (C) conductance at finite bias for this structure. We compare the conductance for a linear drop of the potential (ΔVH,1) to the one for a drop very close to the HOPG (ΔVH,2) or Au (ΔVH,3) surfaces, respectively. Transmission at ±0.2 eV for (D) DMAB and (E) PPD, for the geometries used to calculate the bias-dependent conductance in Fig. 3A. The yellow shaded area corresponds to the bias window (EF ± eV/2) for an applied bias voltage with an absolute value of 0.2 V so that the total current at ±0.2 V of bias is approximately proportional to the area underneath the corresponding transmission curves in that energy range (see Eq. 1).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/6/e1602297/DC1

    section S1. Synthesis

    section S2. Grafting and characterization of modified HOPG

    section S3. Conductance measurements—Additional data

    section S4. Conductance calculations—Additional data and discussion

    scheme S1. Synthetic route of 3,5-dimethyl-4-nitrobenzenediazonium tetrafluoroborate salt.

    fig. S1. 1H NMR spectrum of 1-amino-3,5-dimethyl-4-nitrobenzene (500 MHz, CDCl3).

    fig. S2. 1H NMR spectrum of 3,5-dimethyl-4-nitrobenzenediazonium tetrafluoroborate salt (500 MHz, DMSO-d6).

    fig. S3. Grafting on HOPG.

    fig. S4. Characterization of modified HOPG surface.

    fig. S5. Dendrimeric structures.

    fig. S6. Blank experiment in argon.

    fig. S7. Blank experiment in 1,2,4-trichlorobenzene.

    fig. S8. Individual conductance traces.

    fig. S9. Geometries of grafted DMAB molecules.

    fig. S10. Transmission of different DMAB structures.

    fig. S11. Transmission of AB, DMAB, and PPD.

    fig. S12. Structures of PPD junctions.

    References (4855)

  • Supplementary Materials

    This PDF file includes:

    • section S1. Synthesis
    • section S2. Grafting and characterization of modified HOPG
    • section S3. Conductance measurements—Additional data
    • section S4. Conductance calculations—Additional data and discussion
    • scheme S1. Synthetic route of 3,5-dimethyl-4-nitrobenzenediazonium
      tetrafluoroborate salt.
    • fig. S1. 1H NMR spectrum of 1-amino-3,5-dimethyl-4-nitrobenzene (500 MHz,
      CDCl3).
    • fig. S2. 1H NMR spectrum of 3,5-dimethyl-4-nitrobenzenediazonium
      tetrafluoroborate salt (500 MHz, DMSO-d6).
    • fig. S3. Grafting on HOPG.
    • fig. S4. Characterization of modified HOPG surface.
    • fig. S5. Dendrimeric structures.
    • fig. S6. Blank experiment in argon.
    • fig. S7. Blank experiment in 1,2,4-trichlorobenzene.
    • fig. S8. Individual conductance traces.
    • fig. S9. Geometries of grafted DMAB molecules.
    • fig. S10. Transmission of different DMAB structures.
    • fig. S11. Transmission of AB, DMAB, and PPD.
    • fig. S12. Structures of PPD junctions.
    • References (48–55)

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