Electron structure and charge transport properties of thiols and dithiocarbamates in self-assembled monolayers

It is accepted that the potential of molecular electronics for future device applications critically depends on the formation of a stable and defined link between the organic layer and the metallic contacts. Here we present investigations on dithiocarbamates used as an alternative connecting group for metals, from which their promising properties with respect to thermal stability and electrical conductance emerge. The structure and the electronic properties of dithiocarbamate- and comparable thiolate-based self-assembled monolayers is elucidated using photoelectron spectroscopy, scanning tunneling microscopy (STM), contact angle measurements and interlinked nanoparticle arrays. The experimental results are interpreted in the light of density functional theory calculations, that show the principal difference between the two anchor groups. In the first chapter of the thesis, we show that dithiocarbamates improve the electrical coupling to the metal due to the presence of delocalized electronic states at 0.5±0.1 eV below the Fermi level of Au. The significantly increased density of states at the interface, as revealed by photoelectron spectroscopy and density functional theory calculations, proves that these states are related to the hybridization of the metal d band with delocalized orbitals on the dithiocarbamate anchor group. A low charge injection barrier between the monolayer and the metal is the consequence. Finally, the improved stability of dithiocarbamates on gold is shown by thermal desorption experiments. In the second chapter, the overlayer structure of alkanethiol, benzyl-mercaptan and highly conjugated methyl-phenyl-dithiocarbamate self-assembled monolayers on Au(111) is studied by STM and the conductance of those monolayers measured by current-distance spectroscopy. Whereas alkanethiol monolayers exhibit the known c(4 x 2) overlayer structure, benzyl-mercaptan monolayers show a novel reconstruction, consisting of extended, striped phase domains having a commensurate, p(4½√3 x 2) overlayer structure with an oblique unit cell. In contrast, methyl-phenyl-dithiocarbamate monolayers are found to be disordered. The tunnelling decay constant β for the molecular medium, as well as the molecular conductance at the STM tip-monolayer contact point, are determined. A decay constant of β = 1/Å for alkanethiols and β = 0.5/Å for the phenyl ring is found, in line with reported values, whereas the methyl-phenyl-dithiocarbamate is roughly one order of magnitude more conductive than benzyl-mercaptan. In the last chapter, the structure and the electrical properties of self-assembled monolayers of cyclic aromatic and aliphatic dithioacetamides and of mixed dithioacetamide/alkanethiol monolayers are characterized. The co-assembly and the insertion method are compared for the formation of mixed dithioacetamide/alkanethiol monolayers, and it is found that small and well defined dithioacetamide domains are realized by insertion of dithioacetamides into defect sites of closely packed octanethiol monolayers. These domains are used to determine the molecular conductance by means of STM height profiles, using molecular lengths resulting from density functional theory calculations. The difference in the tunneling decay constant β measured for aromatic dithioacetamides (β = 0.74-0.76/Å) and for aliphatic dithioacetamides (β = 0.84-0.91/Å) highlights the influence of the conjugation within the cyclic core on molecular conductance. In conclusion, different alternative approaches have been used to determine the conductance of molecular junctions, and it is shown that molecules coupled to metals via the dithiocarbamate anchor group could overcome some of the fundamental limitations currently encountered in molecular electronics.