Molecular Electronics:

From Simple Alkyls to Proteins

David Cahen*

Weizmann Institute of Science

Is there a basic reason that electrical currents in nature are carried by ions, rather than by electrons and that electron transport is limited to step-wise electron transfer processes? If we force them, how do electrons cross organic molecules, especially those that we view as "insulating" ones? Can such molecules, be electronic (solid state) components at all?

To be able to answer such curiosity-driven questions and see if there is any new science to learn there, we need on the one hand models for electronic charge transport through molecules with predictive power, and on the other hand sufficiently robust data sets so that any model will have to be tested against them, rather than only those that the experimenter/ theoretician chooses . . .

We found semiconductor/alkyl chain monolayers/metal structures, with the molecules directly chemically bound to the semiconductor (Si, GaAs) and/or the metal (Hg, Au, Pd) to give the required reproducible and reliable data sets. This is underscored by our finding that they form nearly ideally insulating layers (which make them an interesting model systems for certain solar cells) and that we can actually controllably dope those layers, something that is possible only if residual defects do not anymore dominate the electrical transport.

Combining our transport results with those from photoemission and electronic structure calculations led us to question some "conventional wisdoms", which I will discuss and which illustrate our "ignorance".

In spite of these uncertainties, our work with these simple systems showed a way to work with much more complicated biological ones. I'll illustrate the latter briefly with results on bacteriorhodopsin. While much more work is needed, results already hold some surprises.

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* with Y. Cohen, F. Thieblemont, A. Salomon, O. Seitz, H. Shpaisman, G. Nesher, A. Vilan, I. Ron, Y.-D. Jin (Weizmann Inst.), T. Boecking, J. Gooding_, UNSW, Sydney, and with the A. Kahn, Princeton Un., E. Umbach, Würzburg Un., N. Ueno, Chiba Un.,, L. Kronik, R. Naaman, M. Sheves (Weizmann Inst.) groups.