Self-assembled films for molecular rectification
The original model for molecular rectification was proposed by Aviram and Ratner (Chemical Physics Letters, 1974, 29, 277-284.), who suggested that donor-electron bridge-acceptor molecules could be organic counterparts of the inorganic p-n junction. Experimental verification was difficult and the first unambiguous study was reported only three years ago. However, other researchers were still sceptical of the data, but results in this thesis now confirm that molecular rectification is a real effect. Thirteen compounds have been studied of which seven have shown Aviram-Ratner type rectifying behaviour. Self-assembled monolayer films were studied by scanning tunnelling spectroscopy (STS) which provided their current-voltage characteristics. The principal group of materials studied that exhibited electrical asymmetry, involved hemicyanine dyes where the length of the group that connected the chromophore to the gold-coated substrate was varied from S-C3H6 to S-C10H20. Interestingly, the different alkyl analogues showed almost indistinguishable current-voltage characteristics. This suggested that recent theoretical models were incorrect. Furthermore, the chromophore is readily protonated resulting in disruption of the donor-acceptor combination, and it was found that exposure to acid switched the rectification off, which was reversible by subsequent exposure to base. This provided proof that the electrical asymmetry resulted from the donor-bridge-acceptor chromophore and not from any extrinsic effect. It is believed that this is the first conclusive study of molecular rectification.