Laboratoire Léon Brillouin
UMR12 CEA-CNRS, Bât. 563 CEA Saclay
91191 Gif sur Yvette Cedex, France
+33-169085241 llb-sec@cea.fr
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.
References:
(1) Lenfant, S.; Guérin, D.; TranVan, F.; Chevrot, C.; Palacin, S.; Bourgoin, J. P.; Bouloussa, O.; Rondelez, F.; Vuillaume, D.; Electron transport through rectifying self-assembled monolayer diodes on silicon: Fermi level pinning at the molecule-metal interface; J. Phys. Chem. B 2006, 110, 13947-13958.
(2) Versatility of Aqueous Micellar Solutions for Self-Assembled Monolayers Engineering,
L. Patrone, S. Palacin, J.P. Bourgoin, M.H.V. Werts, Langmuir 20, 11577-11582 (2004)
(3) Direct comparison of the electronic coupling efficiency of sulfur and selenium alligator clips for molecules adsorbed onto gold electrodes
L. Patrone, S. Palacin and J. P. Bourgoin, Applied Surface Science, Volumes 212-213 (2003), 446.
(4) Evidence of the Key Role of Metal-Molecule Bonding in Metal-Molecule-Metal Transport Experiments
L. Patrone, S. Palacin, J. Charlier, F. Armand, J. P. Bourgoin, H. Tang, and S. Gauthier, Phys. Rev. Lett. 91, 96802 (2003)
(5) J. P. Bourgoin et al., Revue de l'Electricité et de l'Electronique, (2003)
• Électronique et optique du futur › Matériaux pour l'électronique et l'optique - Electronique organique et moléculaire
• Service de Physique et Chimie des Surfaces et des Interfaces • Laboratory of Physics and Chemistry of Surfaces and Interfaces
• Laboratoire Innovation, Chimie des Surfaces Et Nanosciences- LCSI (LICSEN-LCSI) • Laboratory of Chemistry of Surfaces and Interfaces (LCSI)