Abstract:
The
mercury-drop electrode technique, when used to measure the current/voltage
characteristics of thin films of molecular rectifiers, is a useful, if awed method of
analysis. Current/voltage plots taken from a number of samples confirm previous
studies taken using the scanning tunnelling microscope (STM), but indicate significant
levels of
penetration of the mercury into the sample or stabilising monolayer. Quartz
crystal microbalance (QCM) studies of the adsorption of 7,7,8,8°-
tetracyanoquinodimethane (TCNQ) onto clean gold indicate small but significant
adsorption, with possible formation of a salt, via conversion of TCNQ into the radical
anion. This has
implications for the results of previous experiments, which indicate
alternative sources for the rectification observed from donor-bridge-acceptor systems,
but also
suggest use of the TCNQ radical anion as a electron donor when used to
replace the counterion present in a number of self-assembled systems. Previous STM-
based measurements confirming this latter suggestion are supported by mercury-drop
studies. The results
support conclusions previously drawn by a number of authors,
removing much of the ambiguity surrounding their interpretation, whilst clarifying the
adsorption characteristics of thiol-based molecules that contain a terminal TCNQ group,
and to some extent, TCNQ itself. It is suggested that the mercury-drop technique should
never be used in isolation to
quantify rectification in monolayers of molecular rectifiers,
but that it is suitable for use only i confirming and checking results from more
established procedures.
