Analysis of rectifying molecular thin films

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.