Gold And Organic Electrodes In Emerging Portable Electronics

“Portable electronics” indicate a class of tools usually available to most people for different uses, namely credit cards, mobile devices, hand-held LEDs or lasers, mini solar cells, etc. Moreover, new uses are almost ready to burst into the market such as, such as rolled electronic papers, smart windows and walls, luminescent dresses, etc. A real breakthrough of materials — organic, organometallic, or hybrid — is created by the coupling of significant performances with peculiar properties, ensuring that they are light, flexible, and stretchable.

Most of these applications require the presence of field effect transistor (FET) devices for correct operation. In this regard, organic materials, together with inorganic and organometallic molecules (single or coupled), play a decisive role as both active layers and electrodes in view of their easy availability and cost.

The crucial demand for FET devices is to facilitate the convenient current flux between the electrodes supplying the device’s proper function, especially at the interface between the insulant gate and electrically-active layers. To this end, compatibility with inorganic (electrodes) and organic (active layers) parts of the devices must be pursued. Among the possible ways to achieve this goal, the insertion of interactive layers, which can eventually react with electrodes, represents an easy method to follow, and this is the idea reported here.

Among the organic molecules suitable to for the above-mentioned role, perylene diimide derivatives (PDI) constitute ideal candidates, in view of their flat shape and the extended π-electron conjugation which eases the electrical current flux. This is especially improved if PDI contains pendant cysteine residue suitable for a strong self-assembly onto [1 1 1] Au films, due to the presence of thiol (S-H) functionality. The preparation of two different Au surfaces, terrace- or grain-like, supports the understanding of both the surface packing and morphology of the aggregated ordered films by means of a combination of X-ray photoelectron spectroscopy, atomic force microscopy, and grazing incident-wide X-ray scattering — synchrotron radiation source-techniques.

A proper correlation of the resulting data, together with conformational calculations — using a detailed molecular modeling — supplies a coherent scenario, where a clear, strong interaction between PDI molecules and Au atoms takes place.

In particular, isolated ordered pillars upstanding to the substrate plane are detected on the native gold surface, while a π-stacked aggregation normally “unregistered” to the film plane is observed when PDI derivatives are grafted onto a thermally-annealed gold surface. (see the figures below)

Fig. 1. (a) Model of PMI-Cys molecule aggregation onto gold evaporated substrate; (b) Model of PMI-Cys molecule aggregation onto the gold annealed substrate; (c) Scheme of diimide-cystein residue with sensible torsion angles indicated by arrows. Republished with permission from Elsevier from https://doi.org/10.1016/j.susc.2018.04.008.

Thanks to an established link among organic and gold electrodes, such a surface organization is expected to be useful to improve the charge mobility, thanks to the strong continuous contact between the electrode and active layers, thereby enhancing the organic field’s effect transistor performance. These reasons could lead to their use in devices suitable for both optoelectronics and photonics.

These findings are described in the article entitled Perylene diimide cysteine derivatives self-assembled onto (111) gold surface: Evidence of ordered aggregation, recently published in the journal Surface Science. This work was conducted by E. Kozma, F. Galeotti, G. Grisci, M. Catellani, G. Scavia, and W. Porzio from the Istituto per lo studio delle Macromolecole del CNR, and L. Barba and G. Arrighetti from the Istituto di Cristallografia-Sincrotrone Elettra.