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.
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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