Funktionalisierung von selbstorganisierten Monolagen durch Bestrahlung mit niederenergetischen Elektronen

Abstract

The functionalisation of surfaces by the transformation of structurally simple molecules is one challenge of chemical research. The introduction of a specific functional group is an essential step towards this aim. Amino groups are very valuable because they can be used as linkers to connect complex molecules to the surface. One important aim of my thesis is to identify electron-induced reactions that lead to an attachment of such functional groups to simpler molecules. Previous studies had suggested that radicals produced under low-energy electron exposure of molecular films deposited on surfaces undergo further reactions that lead to activation of adjacent molecules and recombination with remaining radicals. We thus attempted to trigger a similar reaction that would introduce amino groups into a hydrocarbon by an electron-induced dissociation of NH3 yielding NH2 and H fragments in a mixture of NH3 with other molecules like ethane (C2H6). H was assumed to activate C2H6 by abstraction of another H so that, subsequently, the NH2 fragment could recombine with the resulting C2H5 fragment to produce C2H5NH2. Similarly, radicals produced by dissociation of the other molecules were also expected to recombine with NH2. In my thesis, it is shown that, instead of initiating the expected reactions, electron exposure of NH3 as pure film and in mixtures leads to production of N2 and desorption of both the starting material and the product. If, on the other hand, ethylene is used as a hydrocarbon component, exposure at energies above the ionisation threshold leads to formation of C2H5NH2. This reaction is reminiscent of a hydroamination known from organic chemistry with the difference that the electron replaces the catalyst used in the organic synthesis. The latter is required to lower the activation barrier that results from the electrostatic repulsion between neutral C2H4 and NH3. This repulsion can also be eliminated by ionisation of one of the reaction partners. Therefore, the present finding provides evidence that soft ionisation is an alternative strategy to control the outcome of electron-induced reactions. In my work it is shown that a similar reaction can be used for the functionalisation of a self-assembled monolayer.