Reactive Metal-Organic Interfaces Studied with Adsorption Calorimetry and Photoelectron Spectroscopy
This doctoral thesis presents investigations on the reactive interactions between Ca and oligomeric organic thin films with X-ray photoelectron spectroscopy (XPS), adsorption microcalorimetry, and scanning transmission electron microscopy (STEM). By combining X-ray photoelectron spectroscopy (XPS...
Wechselwirkung und Reaktion an der Grenzfläche zwischen Metallschichten und oligomerbasierten organischen Dünnfilmen
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|This doctoral thesis presents investigations on the reactive interactions between Ca and
oligomeric organic thin films with X-ray photoelectron spectroscopy (XPS), adsorption
microcalorimetry, and scanning transmission electron microscopy (STEM).
By combining X-ray photoelectron spectroscopy (XPS) and adsorption microcalorimetry,
it is concluded that Ca exhibits a rather different growth behavior on oligomeric organic thin
films compared with their polymeric homologs: the diffusion of Ca atoms into the oligomeric
organic thin films is generally stronger than into the comparable polymeric thin films,
resulting in thicker reaction zones. These effects can be observed in the Ca/α-sexithiophene
(Ca/6T) system at room temperature, where a thick reaction zone (ca. 30 nm) with a constant
composition of the reacted and unreacted sulfur species is present. This finding is possibly
due to steric hindrance effects in the reaction zone. Such hindrance effects are presumably
the result of massive backbone structural modifications, which are introduced by ring
opening reactions at the thiophene units of 6T, subsequently followed by CaS cluster
formation. The initially measured adsorption enthalpy of Ca on 6T (275 kJ/mol) is discussed
in the context of previous calculations and measurements on similar systems. In contrast to
expectation, the initial value for the measured adsorption enthalpy remains independent of
coverage up to 4 monolayers of Ca (1 monolayers corresponding to 7.4×1018 atoms/m2).
This indicates a continuous exposure of yet unreacted sulfur to Ca from the gas phase up to
this point, resulting in the formation of an extended reaction zone. Based on literature data,
one would expect that a closed metallic Ca layer would form on top of the 6T film
significantly below the observed threshold of 4 monolayers, quenching the reaction between
Ca and 6T at an earlier stage during the Ca deposition.
In order to improve the existing adsorption microcalorimeter setup, a reliable and
multifunctional data acquisition system is built with the LabVIEW programming
environment. To accommodate the pendulum style movement of the calorimeter's chopper
for Ca pulse generation, synchronized timing of the desktop data acquisition program and
step motor’s internal control program is realized through the calibrated compensations and
the employment of pulse pair as the unit experimental step. Possible solutions for the balance
between the ease of programming and runtime reliability are proposed together with the
ideas for code efficiency improvement. As for the hardware development of the calorimeter,
a possible transition from the PVDF film detector to the LiTaO3 single crystalline detector
is also proposed, with a prototype detector being designed and manufactured.