Interaction of metallic contacts and organic self-assembled monolayers (SAM): A principal problem on the way towards molecular electronics

Fig. 1: Principal arrangement of a SAM on top of a metallic substrate and covered by a counter-electrode
Fig. 2: 4-mercapto-pyridine SAMs on an Au-single crystal covered by electrochemically deposited 2-dimensional Pd-islands. (a) In situ STM image demonstrating the self-assembled order within an arrangement schematically shown in (b). In (c) the coupling of the molecules to the metal electrodes via Sulfur (red) or Nitrogen (green) is sketched.
Fig. 3: Electronic density of states at EF for Pd islands of different size as determined by UPS. For comparison a Pd bulk spectrum is added.

To exploit the functionality of a given SAM on top of a metal, in many cases an additional metallic top electrode is needed. Besides the difficulty to prepare such electrodes without producing electrical shorts between the surface and the metallic substrate, the basic question arises as to how the electrodes influence and significantly change the electronic properties of the intermediate molecules. While many groups in the field are aware of this aspect, the opposite question as to the possible property changes of the electrodes induced by the molecules has practically ignored up till now.


This basic problem has been addressed recently in cooperation between electrochemists, solid state physicists and theoretical chemists by studying the effect of 4-mercapto-pyridine SAMs on the electronic properties of the Au substrate as well as strictly 2-dimensional Pd islands used as top electrodes.





Indeed, strong effects are found on the density of states at the Fermienergy EF, which are significantly reduced in both electrodes. In case of Pd-islands, this reduction even leads to a complete loss of metallicity of these nano electrodes as proven by the missing density of states at EF (cf. the following UPS spectra).



More details are found in Nature Materials, Advanced Online Publication (doi:10.1038/nmat1607)