Self assembled monolayers beyond thiols dithiocarbamates from pure layers to ternary assembly systems
Since microelectronics started to play its dominant role in the innovation of technologic and electronic instruments the reduction of the size of transistors and other functional devices was in the main focus of research and development. Therefore molecular nanotechnology and molecular electronics attracted a great deal of attention in the last years. The understanding of the fundamentals of electron transport through molecules is essential for the development and exploration of possible electronic components. Many different approaches have been introduced so far to characterise metal molecule metal junctions. The most reliable measurements on single molecular junctions were done on break junctions, metal gaps, which have been produced by the combination of metal microfabrication and metal electromigration as well as on the Scanning Tunneling Microscope. The molecules under investigation were almost exclusively thiol functionalised. A detailed understanding of a molecule and how it affects the transport properties of a metallic nano gap must include the exact gap structure, the molecular backbone and the interfacial structure of the chemical/physical linker of the molecule to the metal contacts. The aim of this work was to investigate the electron transport abilities of molecules containing dithiocarbamate (DTC) as the linker group on the single molecular level. Therefore the chemisorption process was investigated for different aromatic and non aromatic thiol and DTC compounds by Scanning Tunneling Microscopy, X Ray Photoelectron Spectroscopy (XPS) and Cyclic Voltammetry (CV) The obtained results were published under the title: Dithiocarbamates: Functional and Versatile Linkers for the Formation of Self Assembled Monolayers. The most important findings presented in that publication were that the dithiocarbamates as sodium salts and also the oxidized form of dithiocarbamates the thiurams are chemisorbing in a bidentate structure on Au(111) The thiurams are decomposing at the disulfide bridge as the sodium dithiocarbamate releases the cation. Reductive desorption experiments were accomplished in a home built electrochemical set up, which allowed for the investigation of the same samples, investigated by XPS and STM before. Comparing all these data quantitatively it was shown that the reductive desorption is a one electron transfer process. A second publication was written on the novel and very interesting surface structure of diethyldithiocarbamate: Supramolecular surface assembly of dithiocarbamates on Au(111) Chiral trimeric domains and amorphous structures. Here we report on a first time observation that chemisorbing molecules are forming trimeric complexes, which are hexagonally packed. This is the first experimental evidence of a chemisorbing supramolecular surface assembly. The next experiments were designed in order to produce STM accessible molecular surface structures i.e. islands of one molecular species placed within a matrix of another thiol compound. Therefore we investigated the formation of mixed monolayers at the solid liquid interface of a non polar and a polar thiol species. These results will be published as: Co assembly concentration and mixing dependent structures: Mechanisms of molecular 2D ordering investigated by NEXAFS and STM. The most important result from this work is a detailed model, which can explain all the observed effects as there are ostwald ripening of vacancy islands, the existence of a minimal stable island size of the low dosed species and the diffusion limits of thiol adsorption. The formation of ternary assemblies comprising dithiocarbamate and thiol linked molecules within an alkanethiols self assembled monolayer matrix made the first comparative dithiocarbamate/thiol STM study possible. A gel assisted assembly technique enabled us to produce molecular gradient assemblies. The combination of NEXAFS and STM lead to the identification of the molecules bound to the surface. Finally the STM data analysis lead to the first experimental evidence on the single molecular level, that dithiocarbamate is a more conductive linker than a thiolate linker. The results of these investigations were submitted under the title: Gelassisted host/guest assembly of dithiocarbamates and thiols: Analysis of local orientation and conductivity within ternary assemblies.