Metal chalcogenide clusters on doped TMDC-layers – Publication by A9 (Dehnen) & A6 (Tonner)

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In their study published in the Journal of the American Chemical Society, Eike Dornsiepen, Fabian Pieck, Ralf Tonner, and Stefanie Dehnen report on the synthesis of molecular model systems for the up to now unknown adsorption of organotin chalcogenide cluster molecules on TMDC surfaces. Computational studies reveal similar covalent bonding interactions for the model system as well as for the adsorption on a TMDC surface.

Reprinted with permission from Journal of the American Chemical, 2019, 141,41, 16494-16500.

Transition metal dichalcogenides (TMDCs) like MoS2 or WS2 have gained large interest for their potential in electronic applications. Combinations with other 2D materials in heterostructures have already been demonstrated as useful in various devices, such as tunneling transistors or solar cells. Hybrid systems combining 2D materials with layers of adsorbed molecules have proven to be interesting for optoelectronic applications, as they allow for tailoring of electronic properties and high photoabsorption of molecular materials. In most of these studies, the adsorbed molecules were organic molecules like pentacene or coordination compounds like phthalocyanines, which interacted with the surface by means of dispersion interactions. The interaction of larger organometallic systems with TMDCs, however, has not yet been studied. This is in part due to the fact that the chemisorption of molecules on TMDC surfaces is relatively weak, hence adsorbents tend to move randomly around.

With the aim to mimic the yet unknown covalent deposition of metal chalcogenide clusters on transition metal dichalcogenide MoS2 or WS2 layers, and thereby explore the interaction between the two systems and potential consequences for the physical properties of the TMDC material, the authors synthesized heterobimetallic model systems. The heterocubane-type cluster [(SnCl3) WCp)3S4], the organotin-sulfidomolybdate cluster [{(PhSn)3SnS6}{(MoCp)3S4}], and the corresponding tungstate [(PhSn)3SnS6{(WCp)3S4}] were obtained in ligand exchange reactions from [(PhSn)4S6] and [M(CO)3CpCl] (with M = Mo, W). Indeed, the {M3S4} cages in the three compounds resemble a section of the respective TMDC monolayers, altogether representing minimal molecular model systems for the adsorption of organotin sulfide clusters on MoS2 or WS2. The interaction between the {(MCp)3S4} and {(PhSn)3SnS6} subunits is characterized by multicenter bonding, rendering the respective Sn atom as Sn(II), hence driving the clusters into a mixed-valence Sn(IV)/Sn(II) situation, and the M atoms as M(IV) upon an in situ redox process. The attachment is thus weaker than via regular covalent M-S bonds, but definitely stronger than via van der Waals interactions that have been characteristic for all known interactions of clusters on TMDC surfaces so far. Calculations of a periodic model system that simulates the attachment of the {(PhSn3S6} fragment to MS2 surfaces reveal striking similarities in structure and bonding situation, given the MS2 surfaces are doped with titanium or other electron-poor metal atoms. This renders the new compounds as relevant molecular models for covalent attachment of larger organometallic systems on TMDCs.

Publication

E. Dornsiepen, F. Pieck, R. Tonner, S. Dehnen, [{(PhSn)3SnS6}{(MCp)3S4}] (M = W, Mo): minimal molecular models of the covalent attachment of metal chalcogenide clusters on doped transition metal dichalcogenide layers, J. Am. Chem Soc. (2019) DOI: 10.1021/jacs.9b09209

Contact

Prof. Dr. Stefanie Dehnen
Philipps-Universität Marburg
SFB 1083 project A9
Tel.: 06421 28 25751
EMAIL

Growth of extended DNTT fibers on metal substrates by suppression of step-induced nucleation – Publication by A2 (Witte)

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In their study published in Nanoscale Horizons, Maximilan Dreher, Dayeon Kang, Tobias Breuer and Gregor Witte introduce and validate a new concept to suppress the defect-driven fiber nucleation at surface steps by selective blocking of the active step sites using small molecules, so that the formation of crystalline, organic fibers is only governed by the intrinsic epitaxial growth on ideal, defect-free surface regions.

DNTT fiber structures grown on Ag(111) substrates without (left) and with (right) pre-exposition of oxygen to the surface. The oxygen suppresses the DNTT molecules to adsorb at the step edges, which leads to straight, elongated and epitaxially aligned fibers. (Image: M. Dreher). Reproduced with permission from the Royal Society of Chemistry.

Due to their anisotropic optoelectronic properties, crystalline organic fibers constitute an interesting class of nanoscale materials with great potential for integration into future optoelectronic devices based on organic-inorganic hybrid systems. While chemical synthesis allows for flexible tailoring of electronic molecular properties, well-established structuring methods such as, e.g. lithography are hardly applicable to most molecular materials. Therefore, self-organization is an important alternative route for structuring molecular materials especially for organic/inorganic hybrid architectures. While molecular materials often form crystalline fibers, their length and orientation is, however, limited by surface defects such as steps of the supports that cannot be prevented even on very perfect, single crystalline substrates, hence drastically restricting their use in device applications.

In their study the authors analyzed the influence of surface step edges on the initial growth of fibers for the case of the high performing organic semi¬conductor dinaphthothienothiophene (DNTT) and developed a new concept to suppress the defect–driven fiber nucleation. Based on a comparison of the organic film growth on densely packed, flat noble metal surfaces and on a regularly stepped, vicinal surface, they first showed how substrate steps affect the azimuthal molecular orientation in the seed layer and also the subsequent fiber formation. In a next step they demonstrate that this parasitical step-induced fiber nucleation that occurs also on densely packed Ag(111) surfaces can be suppressed by first exposing the metal support to oxygen, or even briefly to ambient condition, which causes a selective saturation of the active step sites. They show that this not only leads to an exclusive growth of epitaxial DNTT fibers but also strongly increases the fiber size to several hundreds of microns. This novel approach is quite versatile and allows a distinct improvement of template assisted growth and thereby the quality of organic/inorganic hybrids.

Publication

M. Dreher, D. Kang, T. Breuer and G. Witte,
Growth of extended DNTT fibers on metal substrates by suppression of step-induced nucleation
Nanoscale Horizons (2019) DOI:10.1039/C9NH00422J

Poster Award
The paper’s first author Maximilian Dreher is currently a Master’s student within SFB-project A2. We congratulate him on receiving the prize for his poster on the above research which he presented at the Cecam Workshop on “Fabrication processes and molecular organization in organic thin films: Theory and simulation meet experiments” held in Lecco, Italy from July 17-20, 2019.

Contact

Prof. Dr. Gregor Witte
Philipps-Universität Marburg
SFB 1083 project A2
Tel.: 06421 28 21384
EMAIL

Lisa Pecher awarded dissertation prize of Philipps-Universität Marburg

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Congratulations to Dr. Lisa Pecher, former PhD-student of the SFB in the Tonner group (Project A6), for being awarded the Kurt-Dehnicke prize of the Department of Chemistry for her outstanding PhD thesis finalized in 2017.

Semiconductor surfaces are the basis for microtechnology and major applications like photovoltaics. Improving their efficiency and applicability toward future demands on materials requires functionalization with suitable molecules. Fundamental research can help here to understand the interaction between molecules and surfaces.

Lisa Pecher brought significant progress to the understanding of how organic molecules interact with semiconductor surfaces. In her thesis titled “Adsorption Dynamics and Bonding Analysis of Organic Molecules on Silicon(001) Surfaces” – funded and supported by SFB 1083 – she combined static and dynamic quantum-chemical methods with insightful qualitative and quantitative analysis of the electronic structure in extended systems to provide a unique view on the interaction between adsorbates and surfaces. She developed new, efficient approaches to tackle the complex interplay of atomic and electronic effects that need to be treated accurately to derive new insights. Inspired by chemical bonding concepts successfully used in molecular chemistry, she revealed surprising parallels in the realm of surface chemistry. For example, she pointed out for the first time that a well-known reaction mechanism for organic chemistry –second-order nucleophilic substitution – can be found in the reaction of ether molecules with silicon surfaces. This was the key insight explaining the product distribution and published in the chemistry flagship journal Angewandte Chemie (link).

In a further step towards actual device applications, the authors then transferred their findings and the developed preparation protocols to polycrystalline electrodes, demonstrating that the same work function changes can be observed also on “real-life” electrodes. With the end user in mind, the team also tested the air stability of their contact primers, proving that a sacrificial phthalocyanine multilayer serves well to protect the highly ordered mono- and bilayer contact primers during air transfer and can be removed by thermal desorption afterwards.

This was just one of nine publications in major scientific journals – all of them as first author, two of them highlighted on the cover pages. She summarized these impressive scientific results in a review article which was highlighted by science writers and bloggers worldwide (see the SFB news item for more details).

More coverage of the prize-giving event is found here (in German).