Influence of functional groups on charge transport in molecular junctions

It is well known that side group functionalization can be used to modify and control a molecules chemical and electronic properties.[1] In this work we have investigated the possibility of using side group functionalization to control the electrical conductivity of a metal-molecule-metal nano-junction for applications in molecular electronics.[2] To this end we have used density functional theory (DFT) in combination with a Green's function transport scheme [3] to obtain the conductance of functionalized benzenedithiolate (BDT) and benzenediamine (BDA) molecules connected to Au electrodes. We found that the qualitative effect of a given side group on the conductance can be inferred from its known electronic property (whether it is s/p donating/withdrawing) thus providing simple guidelines for the synthesis of molecules with specific electrical properties.  The change in the calculated conductance upon attachment of different side groups is in qualitative agreement with experiments [4], however, the actual conductance values were predicted to be five times larger than in experiment. The quantitative disagreement was traced to the incorrect description of electron ionization and affinities by Kohn-Sham theory. After correcting for this using the scissors operator introduced in [5], the calculated values all lie within 50% of  the experiments. 

Figure  Isosurfaces and eigenenergies (in eV) for the highest occupied molecular orbital of BDT species (squares) and BDA species (circles) for different side groups. Dashed and dotted lines indicate the level position when adsorbed on a (111) gold surface (relative to the metal’s Fermi level) and in the gas phase (relative to vacuum), respectively. The conductance of the junction follows the variation in the HOMO position (the closer to Fermi level, the higher the conductance), which in turn can be inferred from the side-groups known electronic property. The red lines indicate the position of the energy levels when self-interaction and image-charge effects, not included in the DFT calculation, are taken into account.

[1] J. Clayden, N. Greeves, S. Warren, and P. Wothers, Organic Chemistry (Oxford University Press, Oxford 2001)

[2] G. Cuniberti, G. Fagas, and K. Richter, Introducing Molecular Electronics, Lecture Notes in Physics, Springer Verlag Vol. 680, (2005)

[3] Kristian S. Thygesen and K. W. Jacobsen, Chem. Phys. 319 111 (2005)

[4] Latha Venkataraman and Young S. Park and Adam C. Whalley and Colin

Nuckolls and Mark S. Hybertsen and Michael L. Steigerwald, Nano Lett. 7 502 (2007)

[5] S. Y. Quek, Y. Su, L. Venkataraman, H. J. Choi, S. G. Louie, M. S. Hybertsen, J. B. Neaton, Nano Lett. 7 3477 (2007).