Scanning Tunneling-based Molecular Junctions

Since the invention of the scanning tunneling microscope (STM) in 1982 [57], it has had a tremendous impact on the developing field of nanoscale sciences. In addition to the general nanoscale imaging capabilities, the STM introduced a con- 

The fabrication of STM-based molecular junctions have relied - in most reported cases -on the deposition of thiol end-capped molecules as self-assembled monolayers on a flat gold substrate. By using an exceedingly small tip, the measurement of no more than a single or a few molecules becomes possible. A schematic illustration of an STM-based molecular junction is shown in Fig. 10.2. 

Kubiak and coworkers used STM in an ultra-high vacuum environment to study the conductance through self-assembled monolayers of simple Jt-conju-gated, thiol-terminated molecules on gold [21-23, 59] Early work, which focused on the p-xylyldithiol 82 monolayers [22, 23], was later expanded to include also short oligophenylenes (OPs) (81 and 83) that were terminated at either one or both ends by thiol or isocyanide groups [21]. Based on this work, a relatively simple model was presented to account for the observed conductance spectra of the junctions [22], 

SAMs of a molecular rectifier candidate - donor-jr-acceptor molecule 88 - were investigated with an alkanethiolate-covered STM probe by Ashwell et al. [25]. The molecular junction shown in Fig. 10.3 demonstrated highly asymmetric I(V) characteristics, which was taken as evidence for rectification of the monolayer. Depending on the experimental details, rectification ratios up to around 18 was observed. Reversible protonation and deprotonation of the donor moiety, C6H4N(CH3)2, in acidic or basic media, respectively, switched the rectification capability of the monolayer OFF and ON. This observation strongly indicates that the rectification behavior is an inherent property of the molecule. 

A surprising conductance switching between an ON and an OFF state was observed over time for single OPE molecules in the alkanethiolate matrix [27]. This switching behavior was first attributed to conformational changes in the OPE 

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