Table of Contents:
The growth and structure of molecular thin films is of high interest for the improvement
of organic electronic devices (e.g. organic feld effect transistors (OFETs))
because their optoelectronic properties are strongly dependent on their microscopic
structure. A close understanding of the growth behaviour is needed to tune structural
parameters like crystalline phase and orientation, molecular orientation, degree
of crystallinity, topography etc.. As organic semiconductors the molecular class of
phthalocyanines, of which some have already been successfully implemented for industrial
production, is frequently used. Nevertheless a complete understanding of the
growth modi, allowing to predict the structure and optoelectronic properties of thin
films with newly designed phthalocyanine-derivates, has not been achieved so far. To
gain deeper knowlege on the determining growth prozesses, further detailed studies
on thin film growth of phthalocyanines are needed. Within the class of phthalocyanines,
titanyl-phthalocyanine (TiOPc) and copper(II)-phthalocyanine (CuPc) are
frequently used molecules, which have already been applied in organic electronics,
making them ideal candidates for a detailed growth analysis. While several aspects
of their growth behaviour have already been addressed by different studies, some aspects
of their growth are still controversially discussed. Additionally, despite the fact
that multilayer growth can signifcantly differ from chemisorbed monolayer growth,
the multilayer growth behaviour has not been addressed in a systematic way so far.
Therefore this work characterizes the growth behaviour from monolayer to multilayer
regime of CuPc as well as TiOPc on gold and silver, which are frequently used as
electrode material in organic electronic devices.
To achieve detailed informations on different aspects of the film structures a multitechnique
approach is used. As imaging methods scanning tunneling microscopy
(STM) and atomic force microscopy (AFM) are employed, while x-ray diffraction
(XRD) is used to determine the degree of crystallinity als well as crystalline and
molecular orientation. These methods are complemented by x-ray photoelectron spectroscopy
(XPS) to determine thin film covering on larger scale and near edge x-ray
absorption fine structure (NEXAFS) spectroscopy for the determination of the molecular
orientation in the monolayer regime. By the combination of these techniques a
comprehensive picture of the molecular thin film growth behaviour can be obtained.
CuPc, in contrast to other aromatic hydrocarbon molecules like pentacene, adopts
a recumbent molecular orientation not only on smooth but also on microscopically
roughened gold substrates. While the roughness only causes a decreased crystallinity,
a contamination of the gold surfaces by air contact leads to a molecular reorientation.
Additionally a molecular reorientation may occur after a certain film thickness
is reached and when the crystallinty is low. This effect can be explained by the limited
height of small crystallites, leading to the formation of new crystalline domains
with a standing molecular orientation on top of other crystallites. Additionally an
increased substrate temperature will increase the crystallinity but at the same time
cause distinct dewetting, attributed to strain and surface energy minimization.
The analysis of the TiOPc thin film growth on Ag(111) reveals a complete wetting
with molecularly flat domains, only seperated by trenches between differently oriented
domains. These domains grow in an energetically favourable bilayerwise growth mode.
Additionally the TiOPc thin films show heteroepitactic growth. Switching to Au(111)
substrates the formation of tilted domains in the initial film growth, which was reported
by Zhu et al, is ruled out. Exclusively the phase I polymorph of TiOPc is
adopted with a recumbent molecular confguration on Au(111), resembling the case
of TiOPc on Ag(111).
Using the well characterized model system of TiOPc multilayers on Ag(111), their
experimental NEXAFS signature and dichroism was additionally studied in detail.
The signatures at the C and N K-edges are comparable to those of CuPc. Nevertheless
the dichroic behavior of TiOPc at the C and N K-edge is much more complex due
to tilted transition dipole moments of the non-planar molecule. A model to estimate
the systematic error when neglecting this effect is presented, helping in the evaluation
of NEXAFS dichroism data of non-planar molecules. Additionally the O K-edge of
TiOPc is analyzed in detail for the first time revealing an inverted dichroism due to
different target states.
In summary a detailed unterstanding of the growth mechanism of phthalocyanines
on gold and silver surfaces is achieved, which may be complemented by future studies,
e.g. on the effect of fluorination of TiOPc. Eventhough there is still a long way to go,
the gained knowlege will help to improve the models for the complex molecular thin
film growth. For now procedures to grow desired structures of TiOPc and CuPc and
analogous Pcs can be created on basis of the achieved knowlege on phthalocyanine
thin film growth.