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.