Molecular Electronics

Abstract

The term molecular junction (MJ) refers to a simple device where molecules (from a monolayer to a single molecule) are connected between two electrodes (metal or semiconductor), Fig. 1. The number of molecules in the MJ depends on the size of these electrodes and the MJs are usually classified as "large area MJ" and "single (or a few) MJ". Albeit the Langmuir-Blodgett method was used in the early times of ME to deposit molecules on the electrode surfaces (only large area MJs), chemisorption is nowadays widely used because it is also suitable at the nanoscale to attach few molecules between electrodes of a nanometer dimension. In that case, the molecules are equipped with anchor groups at both (or only one) ends. The chemical nature of the anchor group is chosen depending on the nature of the electrodes to permit a chemical reaction with the electrode by forming a chemical bond between the molecule and the electrode. Archetypes of anchor groups are thiols (-SH) on metals (Au, Ag,…), alkenes (-C=C-) on hydrogenated-silicon surfaces, a detailed review is given in Refs [14][15][16] . This approach is widely versatile to adapt the molecules on the electrodes of interest. However, this chemical link also has a pronounced effect on the global electronic properties of the MJs, mainly governing the electronic coupling between molecules and electrodes (see below in this chapter) and this point has been extensively studied (see a At the nanoscale, scanning tunneling microscope (STM) and conductive probe atomic force microscope (C-AFM), are the tools of choice to study a single or a few (typically ≲ 100) molecules (right part of Fig. 1). For single molecule experiments, STM break-junctions (STM-BJ) 25 and mechanically controlled break junctions (MCBJ) 26 have been developed (see a review 27,28 ). In these BJ approaches, the two electrodes are repeatedly (up to thousands or more) moved close and apart and a molecule bridges the electrode gap when its size matches the electrode gap. The signature of the molecule conductance appears as plateaus in the current vs. gap electrode distance traces. C-AFM is used to gently contact (weak loading force) the monolayer [29][30][31] on large surfaces and to measure the current voltage at few hundreds places on the monolayer. Another approach is the use of tiny nanodot electrodes (nanodot-molecule junction, NMJ). 32,33 In this latter case, hundreds to thousands of nanodots (typically 5-40 nm in diameter) are fabricated by e-beam lithography and can be measured in a "one shot" single C-AFM image. Note that all these techniques require a large number of measurements and a solid statistic analysis to get reliable conductance values of the MJs. Finally, another approach is to use a 2D network of metal