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Local density of states effects at the metal-molecule interfaces in a molecular device

Abstract

Clarifying the nature of interactions between metal electrodes and organic molecules still represent one of the challenging problems in molecular electronics that needs to be solved in order to optimize electron transport through a molecular device. For this purpose, electronic properties at metal–molecule interfaces were studied by combining experimental and theoretical methods. Applying a novel electrochemical approach, strictly two-dimensional Pd islands were prepared on top of 4-mercaptopyridine self-assembled monolayers (4MP-SAMs) which, in turn, were deposited on (111)-oriented Au single crystals. Electron spectroscopy together with density functional theory calculations revealed strong interactions between the molecules and the islands due to Pd–N bonds, resulting in a drastically reduced density of states (DOS) at the Fermi level EF for a nearly closed Pd monolayer, and even non-metallic properties for nanometre-sized islands. Similarly, a significantly reduced DOS at EF was observed for the topmost Au layer at the Au–SAM interface due to Au–S interactions, suggesting that these effects are rather general.

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Figure 1: Morphology of the metal–molecule–metal junction after different steps of preparation.
Figure 2: UPS spectra acquired from Pd-covered samples after subtracting the contribution due to the support, thus only representing the properties of the Pd contacts.
Figure 3: Pd-3d 5/2 core-level spectra measured with monochromatized X-rays.
Figure 4: Electronic DOS as calculated within DFT for a Pd monolayer together with experimental results for a 0.7 ML film deposited on top of the SAM.
Figure 5: Photoemission results (h ν=40.8 eV) related to the Au–SAM interface.

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Acknowledgements

We thank J. A. Rodriguez (Chemistry Department, Brookhaven National Laboratory) for helpful discussions and M. Manolova (Abteilung Elektrochemie, Universität Ulm) for technical assistance. This work was supported by the Deutsche Forschungsgemeinschaft (DFG) within SFB 569, the Fonds der Chemischen Industrie, the Swiss National Science Foundation (NF) and the NCCR ‘Nanoscale Science’.

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Correspondence to Hans-Gerd Boyen.

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Boyen, HG., Ziemann, P., Wiedwald, U. et al. Local density of states effects at the metal-molecule interfaces in a molecular device. Nature Mater 5, 394–399 (2006). https://doi.org/10.1038/nmat1607

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