Self-assembled monolayers (SAMs) are versatile systems to tune surface properties. The general concept of SAM formation involves the selective binding of molecules to a flat surface followed by periodic rearrangement of the molecular backbone. Here, an alternative binding group, the dithiocarbamate (DTC) group is introduced and new structural and electronic properties are demonstrated.

Chemisorption of dithocarbamates probed by XPS

Thiram a thiuramdisulfide was used to exemplify the reactive chemisorption of dithiocarbamate compounds. X-ray Photoelectron Spectroscopy (XPS) is a capable method to selectively identify atomic species e.g. sulfur in the S-S single bond compared to C=S double bond. Thus in the thiram powder sample two chemical sulfur species are detected as shown in Figure 1. The surface assembled thiram molecules, in contrast, exhibit only one species, which proves the chemisorption in the symmetric bi-dentate state (Fig.1 top-right). The N1s XPS peaks of diethyldithiocarbamate (DEDT) (Figure 1 top-right) and phenylbisdithiocarbamate (PBDT) (Figure 1 bottom-right) show a full width at half maximum (FWHM) peak of 1.0 eV and 1.8 eV respectively, which can be assigned to one and two chemical nitrogen species. Such a splitting occurs because of the charge transfer between the gold substrate and the nitrogen. The observation of charge transfer within the delocalized DTC-Au structure makes DTC an interesting chemical linker group with promising applications in organic devices and molecular electronics.   

Figure 1 XPS data of different self assemblies on Au(111): On the left the sulfur 2p signals of Thiram as powder and SAM compared to the standard decanethiol SAM is presented. On the right side the Nitrogen signals of two different SAM forming dithiocarbamates are shown.

A novel electrochemistry cell for reductive desorption experiments 

The quantitative analysis of Cyclic Voltammetry experiments – in a home built cell as shown in Figure 2 – provides a measure for the number of charges involved in the reductive desorption process of the studied SAM. In combination, XPS and CV reveal the desorption stoichiometry of 1±0.2 electron per molecule for DEDT [1]. This result confirms the bi-dentate state structure of DTC on gold, as proposed already on the basis of the XPS results, above. Further properties of the resonant DTC-Au structure are currently under investigation. 

Figure 2 Cyclic voltammetry on flat gold substrates. In order to obtain electrochemical information i.e. reductive desorption currents of the same SAMs on which we performed XPS before. We used a home built electrochemical cell (d) where we measured the round samples (c).

Reference

[1] Morf, P.; Raimondi, F.; Nothofer, H.-G.; Schnyder, B.; Yasuda, A.; Wessels, J.M. and Jung, T.A. Langmuir 22 (2006) 658.

Contact
Dr. P. Morf: peter.morfipsi.ch
Dr. T.A. Jung: thomas.jungpsi.ch

Academic partners
Dr. J.M. Wessels, Sony Deutschland GmbH.
Prof. Dr. Christian Schönenberger, University of Basel.

Financal support
Sony Deutschland GmbH – Materials Science Laboratory.