Electron Microscopy Characterization of Pentacene and Perfluoropentacene Grown on Different Substrates

This thesis deals with the study of the morphology, arrangement and orientation of organic semiconductor films by (scanning) transmission electron microscopy ((S)TEM) techniques. The organic semiconductor perfluoropentacene (PFP) as well as the organic heterostructures of pentacene (PEN) and PFP have been investigated. PFP has been grown on graphene substrate, while the organic mixtures formed by PEN and PFP have been deposited with different mixing ratios on two different substrates, i.e. SiO2 and KCl. PFP deposited on graphene exhibits an epitaxial growth in island shapes where the molecules lie flat and parallel to the substrate adopting the so called ‘π-stacked polymorph’. Within this work, the lateral alignment of the PFP molecules with respect to the graphene substrate has been determined. It was found that the long molecular axis of PFP is aligned along the zig-zag direction of the graphene. However, this alignment is not exactly parallel, but exhibits a small offset. Furthermore, the morphology of the PFP islands has been investigated. A characteristic angle around 68° was found between confining edges of PFP islands. The combination of TEM micrographs and electron diffraction patterns has enabled the determination of the planes that run parallel to the confining edges of the islands ‘as seen’ by the electron beam in the two-dimensional projection. From that the possible side facets associated with each confining edge have been suggested. Finally, electron tomography experiments were used to gain insight into the shape of the PFP islands, allowing the 3D reconstruction of them. PEN:PFP blends have been prepared with mixing ratios of [2:1], [1:1] and [1:2] on an inert substrate such as SiO2. Although different phases and morphologies have been observed for each mixture, a mixed phase made out of PEN and PFP which exhibits similar lattice parameters in all cases has been found independently of the mixing ratio. The monocrystalline SAED pattern of the mixed phase has been shown for the first time on this substrate. The diffraction pattern is rather similar to the one of the pure PEN in �0 0 1� direction, suggesting that the crystal structure of the mixed phase is similar to the one of pure PEN. For non-equimolecular blends, the respective pure phase in excess is present apart from the mixed phase. A different morphology was observed for the different PEN:PFP mixing ratios. The equimolecular mixture of PEN and PFP exhibits fiber-like structures consisting of the mixed phase. For the mixture with PFP in excess, some fibers are formed on a background layer. The PFP is contained in the fibers, while the background layer is made out of the mixed phase. For the mixture with PEN in excess, a grainy structure (grain size of 10 nm-60 nm) with contributions of pure PEN and of the mixed phase is detected. PEN:PFP blends with mixing ratios of [2:1] and [1:2] grown on KCl substrates have been investigated too. The mixed phase formed by PEN and PFP is also present and both blends reveal a quite different morphology. The composition, orientation and crystalline details of each phase have been inspected. In the blend with PEN in excess, the mixed phase together with the pure PEN phase are found in a uniform layer formed with domains that are rotated in-plane by 90° towards each other. In contrast, the blend with excess of PFP presents two different arrangements. The majority of the sample exhibits some spicular fibers made out of PFP on a background layer composed by the mixed phase. The other arrangement present to a lesser extent consists of a film of pure PFP lying in direct contact with the KCl substrate. The importance of PFP grown on graphene lies in the relevance of the graphene substrate together with the π-stacked arrangement exhibited by PFP on this substrate. This motif enhances charge carrier mobility along the stacking direction. The knowledge of the relative alignment as well as the faceting are a key information since the physical properties depend on these parameters. Furthermore, considering the role of the organic heterostructures in the development of organic electronic devices, a detailed understanding of the basic arrangement of the organic molecules in the organic blend is a requirement for the development of new organic devices