Tobias Breuer

Epitaxial growth of π-stacked perfluoropentacene on graphene-coated quartz.

Chemical-vapor-deposited large-area graphene is employed as the coating of transparent substrates for the growth of the prototypical organic n-type semiconductor perfluoropentacene (PFP). The graphene coating is found to cause face-on growth of PFP in a yet unknown substrate-mediated polymorph, which is solved by combining grazing-incidence X-ray diffraction with theoretical structure modeling. In contrast to the otherwise common herringbone arrangement of PFP in single crystals and “standing” films, we report a π-stacked arrangement of coplanar molecules in “flat-lying” films, which exhibit an exceedingly low π-stacking distance of only 3.07 Å, giving rise to significant electronic band dispersion along the π-stacking direction, as evidenced by ultraviolet photoelectron spectroscopy. Our study underlines the high potential of graphene for use as a transparent electrode in (opto-)electronic applications, where optimized vertical transport through flat-lying conjugated organic molecules is desired.

Efficient Syntheses of Novel Fluoro-Substituted Pentacenes and Azapentacenes: Molecular and Solid-State Properties

Non-symmetrical 6,13-disubstituted pentacenes bearing trifluoromethyl and aryl substituents have been synthesized starting from pentacenequinone. Diazapentacenes with a variety of fluorine substituents were prepared either via a Hartwig-Buchwald aryl amination route or by a SNAr strategy. As a result of a non-symmetric substitution pattern containing electron-donating substituents in combination with electron-accepting fluorine substituents, the synthesized compounds feature distinct molecular dipoles. All compounds are analyzed regarding their optoelectronic properties in solution with special focus on the frontier orbital energies as well as their molecular packing in the crystal structures. The analyses of isolated molecules are complemented by thin-film studies to examine their solid-state properties. A precise comparison between these and the molecular properties gave detailed insights into the exciton binding energies of these compounds, which are explained by means of a simple model considering the molecular packing and polarizabilities.

Lattice matching as the determining factor for molecular tilt and multilayer growth mode of the nanographene hexa-peri-hexabenzocoronene.

The microstructure, morphology, and growth dynamics of hexa-peri-hexabenzocoronene (HBC, C42H18) thin films deposited on inert substrates of similar surface energies are studied with particular emphasis on the influence of substrate symmetry and substrate-molecule lattice matching on the resulting films of this material. By combining atomic force microscopy (AFM) with X-ray diffraction (XRD), X-ray absorption spectroscopy (NEXAFS), and in situ X-ray reflectivity (XRR) measurements, it is shown that HBC forms polycrystalline films on SiO2, where molecules are oriented in an upright fashion and adopt the known bulk structure. Remarkably, HBC films deposited on highly oriented pyrolytic graphite (HOPG) exhibit a new, substrate-induced polymorph, where all molecules adopt a recumbent orientation with planar π-stacking. Formation of this new phase, however, depends critically on the coherence of the underlying graphite lattice since HBC grown on defective HOPG reveals the same orientation and phase as on SiO2. These results therefore demonstrate that the resulting film structure and morphology are not solely governed by the adsorption energy but also by the presence or absence of symmetry- and lattice-matching between the substrate and admolecules. Moreover, it highlights that weakly interacting substrates of high quality and coherence can be useful to induce new polymorphs with distinctly different molecular arrangements than the bulk structure.

Thermally activated intermixture in pentacene-perfluoropentacene heterostructures

Using thermal desorption spectroscopy (TDS) the thermal stability of binary pentacene/perfluoropentacene (PEN/PFP) thin films has been investigated for various preparation protocols. Variation of stoichiometry ratio reveals a significantly enhanced thermal stability in comparison to the single compounds only for films with equimolar stoichiometry. The stabilization also depends on the preparation method and was found for co-deposition as well as for multi-stacks and subsequently grown PEN/PFP-stacks but not for stacks grown in the reversed order. By systemically varying the substrate temperature during deposition, we prove that the resulting intermixture is caused by a thermally activated diffusion during film growth and not due to post-deposition diffusion induced upon heating during TDS measurements. The different extents of thermal stabilization are discussed in the context of the film morphology studied by means of atomic force microscopy (AFM). For complementary information, optical absorption spectra of the heterostructures are analyzed, where the arisal of new absorption bands and the extinction of excitonic bands existing in the pure compounds are identified as decisive criteria to judge the efficiency of intermixture.

Diffusion-Controlled Growth of Molecular Heterostructures: Fabrication of Two-, One-, and Zero-Dimensional C60 Nanostructures on Pentacene Substrates

A variety of low dimensional C60 structures has been grown on supporting pentacene multilayers. By choice of substrate temperature during growth the effective diffusion length of evaporated fullerenes and their nucleation at terraces or step edges can be precisely controlled. AFM and SEM measurements show that this enables the fabrication of either 2D adlayers or solely 1D chains decorating substrate steps, while at elevated growth temperature continuous wetting of step edges is prohibited and instead the formation of separated C60 clusters pinned at the pentacene step edges occurs. Remarkably, all structures remain thermally stable at room temperature once they are formed. In addition the various fullerene structures have been overgrown by an additional pentacene capping layer. Utilizing the different probe depth of XRD and NEXAFS, we found that no contiguous pentacene film is formed on the 2D C60 structure, whereas an encapsulation of the 1D and 0D structures with uniformly upright oriented pentacene is achieved, hence allowing the fabrication of low dimensional buried organic heterostructures.

Application of transmission electron microscopy for microstructural characterization of perfluoropentacene thin films

The crystalline structure and orientation of perfluoropentacene (C22 F14, PFP) fibers formed upon thin-film deposition onto SiO2 substrates have been studied by means of transmission electron microscopy (TEM), atomic force microscopy (AFM), and x-ray diffraction. The synopsis of TEM micrographs and diffraction patterns enhances the understanding of local crystal orientation on small length scales. The relationship of the PFP fiber morphology with the crystalline arrangement of PFP molecules within single fibers was established using this technique. Radiation damage, which is a critical problem for TEM investigations of organic materials, is described and the sample morphology after TEM investigations is correlated with AFM measurements of samples previously examined by TEM.

Controlling Nanostructures by Templated Templates: Inheriting Molecular Orientation in Binary Heterostructures

Precise preparation strategies are required to fabricate molecular nanostructures of specific arrangement. In bottom-up approaches, where nanostructures are gradually formed by piecing together individual parts to the final structure, the self-ordering mechanisms of the involved structures are utilized. In order to achieve the desired structures regarding morphology, grain size, and orientation of the individual moieties, templates can be applied, which influence the formation process of subsequent structures. However, this strategy is of limited use for complex architectures because the templates only influence the structure formation at the interface between the template and the first compound. Here, we discuss the implementation of so-called templated templates and analyze to what extent orientations of the initial layers are inherited in the top layers of another compound to enable structural control in binary heterostructures. For that purpose, we prepared crystalline templates of the organic semiconductors pentacene and perfluoropentacene in different exclusive orientations. We observe that for templates of both individual materials the molecular orientation is inherited in the top layers of the respective counterpart. This behavior is also observed for various other molecules, indicating the robustness of this approach.

Self-assembly of partially fluorinated hexabenzocoronene derivatives in the solid state

We report on the synthesis and structural characterization of novel, partially fluorinated hexabenzocoronene (HBC) derivatives. The fluorination of polycyclic aromatic hydrocarbons (PAHs) is a well-established method to enhance the stability of organic semiconductors (OSCs) and render them n-type. For HBC it has been observed that fluorination leads to a modification of the molecular packing motif from a herringbone arrangement to a parallel-packed motif. Here, we study whether this transformation of the molecular packing is also found for the partially fluorinated HBCs 2,5-difluoro-hexa-peri-hexabenzocoronene (F2HBC) and 2,5,8,11-tetrafluoro-peri-hexabenzocoronene (F4HBC). Combining powder diffraction and NEXAFS dichroism measurements, we reveal that indeed all partially fluorinated compounds adopt a parallel molecular packing, hence maximizing their intermolecular contact area. We identify fluorine-hydrogen bonds as the mediating driving force to specifically stabilize this molecular arrangement and direct self-assembly. Furthermore, we show that the relative orientation of the HBCs on the underlying surface can be precisely controlled by varying the substrate materials. Finally, the energetic states of the compounds are analyzed using photoelectron spectroscopy, optical spectroscopy and density functional theory to identify the effects of fluorination on these fundamental electronic characteristics.

Anisotropic thermal expansion in pentacene and perfluoropentacene: Effects of molecular packing motif and fixation at the interface

Thermal expansion coefficients of molecular solids are typically significantly larger than those of inorganic materials. Since they are furthermore highly anisotropic, the molecular arrangement and consequently the intermolecular orbital overlap strongly depend on temperature, hence also affecting the energetics of optoelectronic excitations and the efficiency of charge transfer processes. Here, we report on the precise determination of the anisotropic thermal expansion coefficients of the organic semiconductor pentacene in its solid state. We compare the thermal expansion coefficients of three different pentacene polymorphs and observe distinct differences between both pentacene bulk polymorphs and the interface-stabilized thin film phase. By comparing epitaxial films with films prepared on weakly interacting, amorphous substrates, we identify a notable influence of the substrate fixation on the thermal expansion in thin pentacene films. Furthermore, the results for pentacene are compared to the thermal ...

Exceptional Dewetting of Organic Semiconductor Films: The Case of Dinaphthothienothiophene (DNTT) at Dielectric Interfaces

The novel organic semiconductor dinaphthothienothiophene (DNTT) has gained considerable interest because its large charge carrier mobility and distinct chemical robustness enable the fabrication of organic field effect transistors with remarkable long-term stability under ambient conditions. Structural aspects of DNTT films and their control, however, remain so far largely unexplored. Interestingly, the crystalline structure of DNTT is rather similar to that of the prototypical pentacene, for which the molecular orientation in crystalline thin films can be controlled by means of interface-mediated growth. Combining atomic force microscopy, near-edge X-ray absorption fine structure, photoelectron emission microscopy, and X-ray diffraction, we compare substrate-mediated control of molecular orientation, morphology, and wetting behavior of DNTT films on the prototypical substrates SiO2 and graphene as well as technologically relevant dielectric surfaces (SiO2 and metal oxides that were pretreated with self-assembled monolayers (SAMs)). We found an immediate three-dimensional growth on graphene substrates, while an interfacial wetting layer is formed on the other substrates. Rather surprisingly, we observe distinct temporal changes of DNTT thin films on SiO2 and the SAM-treated dielectric substrates, which exhibit a pronounced dewetting and island formation on time scales of minutes to hours, even under ambient conditions, leading to a breakup of the initially closed wetting layer. These findings are unexpected in view of the reported long-time stability of DNTT-based devices. Therefore, their future consideration is expected to enable the further improvement of such applications, especially since these structural modifications are equivalently observed also on the SAM-treated dielectric surfaces, which are commonly used in device processing.