Molecular Diodes

One of the key problems in molecular electronics is to understand the supramolecular structures and the dynamics of SAMs inside tunneling junctions and how they influence charge transport across them. The dynamic nature of SAMs, especially at room temperature, is evident as the molecules readily diffuse over metal surfaces, and are in equilibrium with molecules from solution. Especially in SAMs with functional head groups, the structure of the SAMs are important to understand because subtle changes in the intermolecular interactions between the molecules can result in dramatic changes in the performance of the molecular electronic devices.

Molecular Rectifiers

The Figure on the left shows a schematic illustration of a junction with a SAM of S(CH2)10Fc (Fc is ferrocene). The Figure indicates that the Fc units may have a different tilt angle (β) than the alkyl chains (τ) and the Fc units may interact with each other differently than the alkyl chains. The arrows indicate that bottom- and top-electrodes form different interacttions with the SAMs. Thus in physical-organic studies of charge transport across organics, a good understanding of the supramolecular structure, the dynamics of the SAMs, and interfaces, are crucial.

 

We use a large range of SAM characterization techniques to study the SAM structure and we use that knowledge to interpret the data generated by these SAMs once incorporated in molecular junctions. By doing so, we have over the past years developed the best molecular diode to date with a rectification ratio of three orders of magnitude as depicted by the figure on the right (Nano Lett. 201515, 5506–5512 )

Modelling and Simulations

Our experimental data fit models of SAMs obtained by molecular dynamics simulations performed by our collaborator Damien Thompson very well, e.g., the tilt angles β of the Fc units calculated by molecular dynamics and measured by NEXAFS match surprisingly well. Besides SAMs with Fc termini, and backbones of alkyl chains, we are exploring conjugated SAMs, SAMs with end groups with interesting magnetic and luminescent properties.  The video below shows the dynamics of SAMs determined by molecular dynamics over a period of time of 10 ns. We are also studying the role of the dynamics of SAMs on their properties and, for instance, on the performance of molecular electronic devices.

We also model electronic structure. The figure below shows a computational model which became a bridge in understanding the correlation between electrochemical data and supramolecular structure (see J. Phys. Chem. C 2015, 2,1348–1354).

Fig 1: Models used to test for dependence of the near-Fermi densities on molecule packing density and gold roughness.

Controlling Leakage Current

We found that the type of anchoring group and purity of the precursors are critical to ensure the optimal performance of molecular diodes based on self-assembled monolayers (SAMs) by minimizing the leakage current. The SAMs were formed on ultra-smooth template-stripped silver (AgTS) surfaces, which served as the bottom-electrode, and a Eutectic alloy of Gallium-Indium (EGaIn) was used as the top-electrode. When these junctions incorporate SAMs of the form S(CH2)11Fc derived from HSC11Fc, they are good molecular diodes and rectify currents with rectification ratios R of ~1.0 × 10 2. Replacing the thiol by disulfide or thioacetate functionalities in the precursor resulted in molecular diodes with values of R close to unity. Cyclic voltammetry (CV) and angle resolved X-ray photoelectron spectroscopy (ARXPS) further confirm that the more defective SAMs derived from the disulfides or thioacetates result in large leakage current which, in turn, lowered the rectification ratio. We also found that purity of the thiol-precursor is crucial: 3% of disulfide present in the thiol caused a 28% decrease in R, and 10% of disulfide lowered R by 90% while the yield in non-shorting junctions remained unchanged. (see J. Am. Chem. Soc., 2014, 136 (5), pp 1982–1991)