Abstract:
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.
