From building blocks to 2D networks

The aim of this work is to further the understanding of the
important parameters in the formation process of 2D nanostructures
and therewith pioneer for novel applications. Such 2D
nanostructures can be composed of specially designed organic
molecules, which are adsorbed on various surfaces. In order to
study true 2D structures, monolayers were deposited. Their
properties have been investigated by scanning tunneling microscopy
(STM) under ultra-high vacuum (UHV) conditions as well as under
ambient conditions. The latter is a highly dynamic environment,
where several parameters come into play. Complementary surface
analysis techniques such as low-energy electron diffraction (LEED),
X-Ray photo-emission spectroscopy (XPS), and Raman spectroscopy
were used when necessary to characterize these novel molecular
networks. In order to conduct this type of experiments, high
technical requirements have to be fulfilled, in particular for UHV
experiments. Thus, the focus is on a drift-stable STM, which lays
the foundation for high resolution STM topographs. Under ambient
conditions, the liquid-solid STM can be easily upgraded by an
injection add-on due to the highly flexible design. This special
extension allows for adding extra solvent without impairing the
high resolution of the STM data. Besides the device, also the
quality of the tip is of pivotal importance. In order to meet the
high requirements for STM tips, an in vacuo ion-sputtering and
electron-beam annealing device was realized for the
post-preparation of scanning probes within one device. This
two-step cleaning process consists of an ion-sputtering step and
subsequent thermal annealing of the probe. One study using this STM
setup concerned the incorporation dynamics of coronene (COR) guest
molecules into pre-existent pores of a rigid 2D supramolecular host
networks of trimesic acid (TMA) as well as the larger analogous
benzenetribenzoic acid (BTB) at the liquid-solid interface. By
means of the injection add-on the additional solution containing
the guest molecules was applied to the surface. At the same time
the incorporation process was monitored by the STM. The
incorporation dynamics into geometrically perfectly matched pores
of trimesic acid as well as into the substantially larger pores of
benzentribenzoic acid exhibit a clearly different behavior. For the
BTB network instantaneous incorporation within the temporal
resolution of the experiment was observed; for the TMA network,
however, intermediate adsorption states of COR could be visualized
before the final adsorption state was reached. A further issue
addressed in this work is the generation of metal-organic
frameworks (MOFs) under ultra-high vacuum conditions. A suitable
building block therefore is an aromatic trithiol, i.e.
1,3,5-tris(4-mercaptophenyl)benzene (TMB). To understand the
specific role of the substrate, the surface-mediated reaction has
been studied on Cu(111) as well as on Ag(111). Room temperature
deposition on both substrates results in densely packed trigonal
structures. Yet, heating the Cu(111) with the TMB molecules to
moderate temperature (150 °C) yields two different porous metal
coordinated networks, depending on the initial surface coverage.
For Ag(111) the first structural change occurs after annealing the
sample at 300 °C. Here, several disordered structures with
partially covalent disulfur bridges were identified. Proceeding
further in the scope of increasing interaction strength between the
building blocks, covalent organic frameworks (COFs) were studied
under ultra-high vacuum conditions as well as under ambient
conditions. For this purpose, a promising strategy is covalent
coupling through radical addition reactions of appropriate
monomers, i.e. halogenated aromatic molecules such as
1,3,5-tris(4-bromophenyl)benzene (TBPB) and 1,3,5-tris(4-
iodophenyl)benzene (TIPB). Besides the correct choice of a
catalytic surface, the activation energy for the scission of the
carbon-halogen bonds is an essential parameter. In the case of
ultra-high vacuum experiments, the influence of substrate
temperature, material, and crystallographic orientation on the
coupling reaction was studied. For reactive Cu(111) and Ag(110)
surfaces room temperature deposition of TBPB already leads to a
homolysis of the C-Br bond and subsequent formation of
proto-polymers. Applying additional heat facilitates the
transformation of proto-polymers into 2D covalent networks. In
contrast, for Ag(111) just a variety of self-assembled and rather
poorly ordered structures composed of intact molecules has emerged.
The deposition onto substrates held at 80 K has never resulted in
proto-polymers. For ambient conditions, the polymerization reaction
of 1,3,5-tri(4-iodophenyl)benzene (TIPB) on Au(111) was studied by
STM after drop-casting the monomer onto the substrate held either
at room temperature or at 100 °C. For room temperature deposition
only poorly ordered non-covalent arrangements were observed. In
accordance with the established UHV protocol for halogenated
coupling reaction, a covalent aryl-aryl coupling was accomplished
for high temperature deposition. Interestingly, these covalent
aggregates were not directly adsorbed on the Au(111) surface, but
attached on top of a chemisorbed monolayer comprised of iodine and
partially dehalogenated TIPB molecules. For a detailed analysis of
the processes, the temperature dependent dehalogenation reaction
was monitored by X-ray photoelectron spectroscopy under ultra-high
vacuum conditions.