Dokument

Summary:
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.

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