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Non-chemisorbed gold–sulfur binding prevails in self-assembled monolayers

Abstract

Gold–thiol contacts are ubiquitous across the physical and biological sciences in connecting organic molecules to surfaces. When thiols bind to gold in self-assembled monolayers (SAMs) the fate of the hydrogen remains a subject of profound debate—with implications for our understanding of their physical properties, spectroscopic features and formation mechanism(s). Exploiting measurements of the transmission through a molecular junction, which is highly sensitive to the nature of the molecule–electrode contact, we demonstrate here that the nature of the gold–sulfur bond in SAMs can be probed via single-molecule conductance measurements. Critically, we find that SAM measurements of dithiol-terminated molecular junctions yield a significantly lower conductance than solution measurements of the same molecule. Through numerous control experiments, conductance noise analysis and transport calculations based on density functional theory, we show that the gold–sulfur bond in SAMs prepared from the solution deposition of dithiols does not have chemisorbed character, which strongly suggests that under these widely used preparation conditions the hydrogen is retained.

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Fig. 1: Conductance histograms for solution or SAM measurements of C12-based single-molecule junctions that comprise different sulfur terminal groups.
Fig. 2: Conductance histograms for terphenyl (Ph3)-based single-molecule junctions that comprise different sulfur terminal groups.
Fig. 3: Noise analysis for Ph3- and Cn-based single-molecule junctions.
Fig. 4: Calculated transmission and measured conductance for alkanes with terminal sulfur groups.

Code availability

The data that support the findings were acquired using a custom instrument controlled by custom software (Igor Pro, Wavemetrics). The software is available from the corresponding author upon reasonable request.

Data availability

The data that support the findings of this study not included in the Supplementary Information are available from the corresponding author upon reasonable request.

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Acknowledgements

We acknowledge discussions with M. L. Steigerwald, G. Lovat, T. Albrecht, Y. R. Leroux and P. Hapiot, and thank M. C. Buzzeo for the use of electrochemical equipment. This research was supported primarily by a Marie Skłodowska Curie Global Fellowship (M.S.I., MOLCLICK: 657247) within the Horizon 2020 Programme. This work was supported in part by the National Science Foundation grants DMR-1507440 and DMR-1807580. The computational work was supported by the US Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, under contract no. DE–AC02–05CH11231, within the Theory FWP. This work was also supported by the Molecular Foundry through the US Department of Energy, Office of Basic Energy Sciences, under the same contract number. Portions of the computational work were performed at the National Energy Research Scientific Computing Center.

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M.S.I. and L.V. conceived and led the project. M.S.I. synthesized the compounds and performed STM, XPS and electrochemical experiments. L.V. carried out the noise analyses. Z.-F.L. and J.B.N. undertook first-principles calculations. The paper was written by M.S.I. and L.V. with contributions from all the other authors.

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Correspondence to Michael S. Inkpen or Latha Venkataraman.

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Supplementary Methods, Supplementary Data, Supplementary Figs 1–28, Supplementary Tables 1–3

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Inkpen, M.S., Liu, Z., Li, H. et al. Non-chemisorbed gold–sulfur binding prevails in self-assembled monolayers. Nat. Chem. 11, 351–358 (2019). https://doi.org/10.1038/s41557-019-0216-y

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