Low Temperature Conductance Measurements Of Self-assembled Monolayers Of 1,4-phenylene Diisocyanide.

In the past few years, significant progress has been made in the fabrication and demonstration of molecular wires, molecular diodes and switches. Many of these advances have been made possible by using the self-assembly of molecules on nanofabricated semiconductor and/or metallic structures. The most studied molecular system for electronic transport is the AuSR system, where a self-assembled monolayer (SAM) of an oligomer (R) binds to a gold surface via a thiol group. In order to measure the electrical current through such a molecular layer, a second electrode is needed. This counterelectrode can be provided by an STM tip, by another gold wire that can be approached using a mechanical break-junction 10] or by evaporation of a gold layer on top of the SAM. Other techniques employ electromigration of Au or Au particles to achieve small interelectrode distances. In spite of a growing number of publications dealing with electronic transport through molecules, even the conductance and transport mechanisms of relatively simple molecules are not well understood. The reasons lie in the difficulty of providing stable, welldefined metallic contacts at two ends of a molecule, which can allow reproducible transport measurements. It is the purpose of this paper to present two novel sample designs for conductance measurements which were applied to SAMs of the same molecule and to discuss the possible transport mechanisms involved. One idea for implementing elementary molecular electronic computational functions requires the use of a three-terminal molecular-scale transistor. To this day, only a small number of experiments showing field-effect behavior of molecules have been published. 14, 15] One requirement for a transistor is that it exhibits signal gain which has not been achieved so far in molecular three-terminal devices. Another condition which has to be fulfilled in order to build a molecular field-effect transistors is a strong variation of the density of states (DOS) near the Fermi level of the molecules used. Ideally, the molecules should exhibit a very low conductance at off-resonance conditions and a high transmission in the case that the Fermi level is shifted electrostatically with a gate voltage to resonance. Lang and Avouris have shown theoretically that 1,4-phenylene diisocyanide (PDC), the molecule used in this study, should meet this condition and would therefore be a good candidate for a field-effect device. Temperature-dependent conductance measurements were carried out by Chen et al. with SAMs of PDC, which showed that both hopping and thermal emission of charge carriers could play a role in the conduction mechanisms involved.