Evidence of the role of the organic-metal linkage in the transport properties of metal-molecule-metal junctions

Fig. 1 : UPS spectra of T3 and Se3 SAMs on gold recorded with HeII excitation line (40.8 eV). The spectrum of gold after ion milling the organic monolayers is shown for comparison (Au). The T3 and Se3 spectra have been shifted vertically for clarity.
Inset: Zoom on the Fermi energy region.

Measurements of single molecule transport via metal-molecule-metal (MMM) junctions have been the subject of many investigations in the past five years, both from the experimental and from the theoretical points of view. Scattering models allowed for example better comprehension of conductance variations with the length of the molecule.
The exact contribution from the contact conductance has been much less investigated, mainly because the thiol-gold couple overwhelms the other interface linkages in literature, and also because comparison experiments are quite difficult.
In collaboration with the Laboratoire d'Electronique Moléculaire (IRAMIS/SPEC), we designed STM experiments that allowed us to directly compare S-Au and Se-Au bonds with respect to their effect on electron transport. We indeed designed two molecules which are identical except for the coupling group with the gold surface. More precisely, we used a STM to investigate thiol and selenol terminated terthiophene molecules – T3 and Se3, respectively – inserted in a dodecanethiol (DT) SAM on a gold surface.

First, density functional theory calculations were used to show that the two molecules isolated in vacuum have quite a similar electronic structure. In particular, their calculated ionization potentials differ by less than 1%, revealing the absence of a significant difference between the two molecules when isolated in vacuum. The electronic density representation of the orbitals confirms this similarity. This suggests to interpret our STM results as a direct consequence of a difference in adsorption of Se3 and T3 onto gold due to their different coupling groups.
Then, ultraviolet photoelectron spectroscopy (UPS) was used to study SAMs of pure T3 or Se3 were prepared onto gold substrates. T3 and Se3 spectra show a good similarity with that of thick terthiophene, which means that the electronic structure of the adsorbed molecules core remains largely undisturbed by the adsorption onto the metallic surface.
However, tiny but significant differences nevertheless exist between T3 and Se3 SAMs spectra. In particular, the two peaks at lower energy are shifted towards higher energies by about 0.2 eV in the case of T3 compared to Se3. (Fig. 1)

Fig. 2 : Typical air STM image of inserted T3 molecules into a DT SAM on gold (left); Histograms of relative height measurements for T3 and Se3 in ambient air. (right)

Thus, this analysis shows that the adsorption on gold does not disturb significantly the electronic structure of the molecules but that the difference between Au-S and Au-Se bonds shows up in the smaller EF-EHOMO value of Se3 compared to T3. That result was then used to interpret STM images obtained on T3 (resp. Se3)-DT mixed SAMs on gold.

Indeed, the elastic scattering quantum chemistry technique (ESQC) was used to model the STM images, in particular the measured apparent heights of the T3 (resp. Se3) molecules with respect to the background DT network (Fig. 2). Transmission spectra of the MMM junction could be calculated for different heights of the STM tip above the SAM and adjusted in energy with respect to each other thanks to the UPS measurements. The calculated tip heights that could be derived from those calculations were found in very good qualitative agreement with the experimental ones derived from the SPM images.

This work shows that combining experimental measurements and semi-empirical simulations allows a satisfactory description of the transport behaviour in MMM junctions. Moreover, it clearly proves that charge transport at MMM junction significantly depends on the actual nature of the chemical link between the molecule and the electrode. Our results indicate a better transmission through the Se-Au linkage when compared with the S-Au linkage, which is contrary to previous predictions from other groups.


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(3) Direct comparison of the electronic coupling efficiency of sulfur and selenium alligator clips for molecules adsorbed onto gold electrodes
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(4) Evidence of the Key Role of Metal-Molecule Bonding in Metal-Molecule-Metal Transport Experiments
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