The device performance of organic field-effect transistors depends not only on the properties of different functional components, but it is also determined by the interfaces among them. Interface engineering has been demonstrated to be an effective approach to enhance the quality and functionality of organic semiconductor thin films, as an example optimizing the contact with electrodes in order to enable high performance of the final devices. For the past few years, two-dimensional (2D) or van der Waals materials have been suggested as substrates, on which the epitaxy of organic molecules can lead to advanced heterostructures through self-assembly. This is because their surfaces are atomically smooth without dangling bonds and exhibit no trapped charges at the interface. These key advantages enable the growth of high-quality organic crystal thin films with low amount of defects and allow exploitation of the intrinsic properties of organic semiconductors. This thesis focuses on the epitaxial growth of dihydrotetraazaheptacene (DHTA7), which is a polar and rod-like, small organic semiconductor molecule, on the 2D material graphene. The hot wall epitaxy technique was chosen for the deposition of DHTA7 molecules on micromechanically exfoliated graphene surfaces. As the main characterization tool, atomic force microscopy was used to analyze the morphology of the DHTA7 crystallites on the nanometer scale as a function of deposition temperature from 313 K to 393 K. Optical microscopy was also employed to investigate the micrometer-sized structures. It was found that the molecules adsorb in flat-lying configuration on graphene forming elongated needle-like crystallites. These crystallites follow certain growth directions that are dictated by the substrate’s three-fold symmetry. In total, six growth directions of DHTA7 on graphene were observed, with three pairs of directions split by ± 9° from three armchair directions of graphene. These growth directions can also be used to determine the crystallographic orientation of the substrate and its edges. In addition, the influence of the thickness of supporting graphene was investigated, showing that single-layer graphene is actually not favored for the growth of large and well-ordered crystalline DHTA7 needles.
|Translated title of the contribution||Wachstumsmorphologien des polaren organischen Moleküls Dihydrotetraazaheptacen auf Graphen|
|Award date||20 Oct 2017|
|Publication status||Published - 2017|
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