A versatile experimental approach for understanding electron transport through organic materials

Abstract

This paper describes an experimentally simple method for assembling junctions with nanometer-scale, structured organic films positioned between two metal electrodes. These junctions comprise two metal electrodes that sandwich two self-assembled monolayers (SAMs) – that is, metal (mercury)–SAM//SAM–metal (mercury, gold or silver) junctions. The junctions are easy to assemble (because the mercury electrode is compliant) and they are compatible with SAMs incorporating organic groups having a range of structures. This paper describes three different variations on this type of Hg-based junction. The first junction, formed by two contacting mercury drops covered by the same type of SAM, is a prototype system that provided useful information on the structure and electrical properties of the Hg-based junctions. The second junction consists of a Hg drop covered by one SAM (Hg–SAM(1)) in contact with a second SAM supported on a silver film (Ag–SAM(2)) – that is, a Hg–SAM(1)//SAM(2)–Ag junction. This junction allowed systematic measurements of the current that flowed across SAM(2), as a function of structure (for example, using aliphatic or aromatic thiols of different length), and a common SAM(1) of hexadecane thiol. The current density follows the relation I= I 0e − βd Ag,Hg , where d Ag,Hg is the distance between the electrodes, and β is the structure-dependent attenuation factor for the molecules making up SAM(2): β was 0.87±0.1 A ̊ −1 for alkanethiols, 0.61±0.1 A ̊ −1 for oligophenylene thiols, and 0.67±0.1 A ̊ −1 for benzylic derivatives of oligophenylene thiols, in general agreement with the values calculated by other approaches. The same type of junction, but using SAM(1) and SAM(2) carrying suitable chemical groups, X and Y, was used to measure the rate of electron transfer across different types of functional groups and bonds: van der Waal interactions, H bonds, and covalent bonds. The third type of junction, Hg–SAM//R//SAM–Hg, is an electrochemical junction that can (i) trap redox-active molecules (R) in the interfacial region between the SAMs, and (ii) control the potential of the electrodes with respect to the redox potential of R using an external reference electrode. This system shows I–V curves with steps that can be interpreted in terms of redox cycling mechanism.