Engineering of TMDC-OSC Hybrid Interfaces: The Thermodynamics of Unitary and Mixed Acene Monolayers on MoS2 – Publication by A2 (Witte) and A4 (Gottfried)

In a new publication in Chemical Science, projects A2 (Witte) and A4 (Gottfried) report on the intricate desorption characteristics of pentacene (PEN) and perfluoropentacene (PFP) monolayers on the MoS2 surface. Unitary molecular monolayers are thermally stabilized by entropy due to their high mobility rather than the organic/inorganic interface bond, which hampers the formation of close-packed and well-ordered monolayers.

 

Schematic representation of characteristic TPD traces and the temperature-dependent molecular diffusivity of unitary PEN films (left hand side) and the stabilized mixed PEN:PFP monolayer (right hand side) on MoS2 (Image: P. Dombrowski). Copyright by CC-BY-NC 3.0.

Van der Waals (vdW) bound hybrid heterosystems of inorganic two-dimensional materials (2DMs) and OSCs are currently receiving great attention due to their promising characteristics for the fabrication of flexible electronics and ultra-thin devices. While prototypical devices with 2DM-OSC hybrid heterostructures have been realized, the fundamental understanding of the 2DM-OSC interface is lacking. In their new study, the authors were able to thoroughly unravel the interplay of interface and intermolecular interactions and their effect on the structure and thermal stability of molecular monolayers.

Through temperature-programmed desorption (TPD) experiments, it was shown that the first molecular layers of PEN and PFP on MoS2 are thermally more stable than subsequent molecular layers in spite of an interface bond that is weaker than the molecular interlayer bond. This is possible due to an entropic stabilization that can only occur if the molecular adlayer is highly mobile. Thus, if the first molecular layer is only stabilized by its mobility, it cannot be well-ordered and close-packed even at low temperatures as low as 100 K. The high molecular diffusivity in the gas-like unitary PEN and PFP monolayers was identified by Monte-Carlo simulations: Intermolecular repulsion of intrinsic molecular electrostatic quadrupole moments, in combination with a weak substrate interaction, prohibits the formation of a condensed molecular monolayer.

By introducing attractive intermolecular interactions, condensation of the molecular films is favored. This was achieved in mixed monolayers of PEN and PFP that adopt a well-ordered stoichiometric 1:1 intermixture. In spite of a reduced mobility, the mixed monolayer is thermally stabilized with respect to the bulk substance due to the attractive intermolecular forces and can therefore be fabricated by selective multilayer desorption. This provides a promising prospect for the fabrication and subsequent study of well-defined 2DM-OSC interfaces for future studies within SFB 1083.

 

Publication

S.R. Kachel, P.-M. Dombrowski T. Breuer, J.M. Gottfried, G. Witte
Engineering of TMDC–OSC hybrid interfaces: the thermodynamics of unitary and mixed acene monolayers on MoS2
Chem Sci. (2021) DOI:10.1039/D0SC05633B

 

Contact

Prof. Dr. Gregor Witte
Philipps-Universität Marburg
SFB 1083, project A2
Tel.: +49 6421 28-21 384
EMAIL

Prepare with Care: Low Contact Resistance of Pentacene Field-Effect Transistors with Clean and Oxidized Gold Electrodes – Publication by A2 (Witte)

In a new publication in Organic Electronics, project A2 (Witte) reports on the establishment of a complete high vacuum-based manufacturing and electronic characterization of organic field effect transistors (OFET).

Photograph and force microscopy image of the vacuum-processed OFETs, which enables their electrical characterization without exposing the devices to air (Image: Y. Radiev). Reprinted from Radiev et al, Prepare with Care: Low Contact Resistance of Pentacene Field-Effect Transistors with Clean and Oxidized Gold Electrodes, Org. Electron. 89 (2021) 106030, Copyright 2021, with permission from Elsevier.

The electronic coupling between OSC and metallic electrodes is of key importance for the efficiency of charge carrier injection in organic electronic devices, such as OFETs or photovoltaic cells, as it determines their idle power. Surface science-based model studies have mainly focused on the energy level alignment at such metal-organic interfaces without measuring real contact resistances, while device studies are typically performed without any microscopic structural and electronic interface characterization. In the present work, the authors introduced a high vacuum-based manufacturing of bottom contact OFETs that enables cleaning and controlled modification of metal contacts before the organic film deposition. This approach not only excludes any exposure to air, it also allows to examine the influence of controlled exposure to air on the device characteristics.

Using the example of the prototypical OSC pentacene it is demonstrated that FET structures with thoroughly cleaned gold electrodes reveal a remarkably low contact resistance. This can be further improved if the electrodes are O2 plasma treated before the pentacene film growth, which results in a thin gold oxide layer and yields one of the lowest contact resistances ever reported for this system. It is shown that this not only causes an improved energy level alignment at the metal-organic interface but also suppresses a pronounced dewetting. In addition, it was demonstrated that controlled exposure to air – even for a short time – significantly affects the device performance.

The present study is an important milestone as it enables detailed electronic transport measurements through metal-OSC interfaces with poly- and single crystal organic semiconductors. This work paves the way for a knowledge transfer about the properties of idealized model interfaces to real electronic devices applications.

Publication                                     

Yurii Radiev, Felix Widdascheck, Michael Göbel, Alrun Aline Hauke and Gregor Witte,
Prepare with Care: Low Contact Resistance of Pentacene Field-Effect Transistors with Clean and Oxidized Gold Electrodes
Org. Electron. 89 (2021) 106030 DOI:10.1016/j.orgel.2020.106030

Contact

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

Prof. Dr. F.S. Tautz won a price at „Falling Walls – The World Science Summit“

At „Falling Walls – The World Science Summit“ during the Berlin Science Week, Prof. Dr. F.S. Tautz, PI of SFB project A12 (Tautz/Bocquet/Kumpf), was declared the winner in the category „Engineering and Technology“ with his contribution „Breakting the Wall of Building with Molecules“.

Tautz Portait

„Falling Walls“ is an event that brings together some of the best researchers of the world, discussing and celebrating the latest breakthroughs in science and society since over 10 years. The breakthroughs represent significant advances, groundbreaking developments and innovative ideas stretching over ten different categories.

 

Falling Walls 2020 during the Berlin Science Week. Copyright by Falling Walls Foundation.

Prof. Tautz (SFB project A12) submitted his 5-min long nomination film (link see below) in the category “Engineering and Technology” titled “Breaking the Wall of Building with Molecules”. In this video, Prof. Tautz gives a short insight into his research. Although manipulating atoms on surfaces is already performed for several years, researchers are struggling with moving and arranging molecules. He explains how an artificial intelligence was trained by reinforcement learning in reality and in model systems at the same time. Consequently, the agent got highly adaptive and become able to successfully manipulate molecules.

With this technique, targeted fabrication of molecular machines can be achieved giving rise to new promising technologies with high-level functionalities. Winning this price is not only a great honor for Prof. Tautz, but also represents the high importance and appreciation of the SFB’s research.

For further details, see the nomination film.

 

Contact

Prof. Dr. Stefan Tautz
FZ Jülich, SFB project A12
PGI, Experimental Physics
Tel.: +49 (0)2461 61-4561
EMAIL

Super-resolution lightwave tomography of electronic bands in quantum materials – Publication by B4

The researchers developed a method, which was published in Science, to reconstruct the band structure of quantum materials with very high precision

A light flash (yellow) induces the movement of electrons in the band structure (red curve) resulting in the formation of localized electronic interference combs (peaks). The emitted radiation (red) enables the analysis of the electronic band structure. © Markus Borsch, University of Michigan, USA. Reprinted with permission from AAAS.

Stacking of two-dimensional (2D) materials yield promising properties for the development of new devices with outstanding functionality. However, detailed knowledge of the electronic structure is necessary to tailor their properties. Within the cooperation of Prof. Huber (Uni Regensburg), Prof. Kira (Uni Michigan) and Prof. Koch (Uni Marburg, project B4), a new method was developed enabling the reconstruction of the band structure of 2D materials.

The quantum material WSe2 was simultaneously excited by two laser pulses in the visible range and in the THz spectral range. The resonant excitation of the weak optical pulse and simultaneous irradiation with a strong THz pulse result in harmonic sideband generation (HSG) in the transmitted spectrum. Due to the wave-particle dualism of electrons, electronic interference combs evolve in momentum space (cf. Figure on the left). By analyzing multiple sideband spectra at different THz frequencies and intensities, it is possible to reconstruct the band structure with super-resolution.

This concept offers an all-optical and practical approach for a three-dimensional tomography of the electronic structure of small quantum materials as shown for WSe2. This work is a great extend to the research in the SFB as it provides the possibility to examine the electronic band structure of 2D materials, such as TMDCs, even under ambient conditions.

Publication

M. Borsch, C.P. Schmid, L. Weigl, S. Schlauderer, N. Hofmann, C. Lange, J.T. Steiner, S.W. Koch, R. Huber, M. Kira
Super-resolution lightwave tomography of electronic bands in quantum materials
Science 370 (2020) 1204 DOI:10.1126/science.abe2112

Contact

Prof. Dr. Stephan W. Koch
Philipps-Universität Marburg
Department of Physics, Theoretical Semiconductor Physics
Renthof 5, 35032 Marburg
Tel.: 06421 28-21336
EMAIL

Lars Bannow and Benedikt P. Klein awarded the dissertation prize of Philipps-Universität Marburg

Congratulation to Dr. Lars Bannow and Dr. Benedikt P. Klein, PhD-students of the GRK 1782 (SW Koch) and the SFB project A4 (Gottfried), respectively, for being awarded a prize by Philipps-Universität Marburg for their excellent dissertations presented in 2019.

Lars Bannow, GRK 1782 (SW Koch) Foto: Paul Ndimande

In the thesis of Dr. Bannow with the title “Optical and Electronic Properties of Semiconductor Materials”, he investigates optical and electronic properties of novel semiconductor materials such as Ga(AsBi), In(AsBi) and the methylammonium (MA) perovskite MAPbI3. Using a combination of density functional theory calculation and semiconductor Bloch equations, a precise prediction of opto-electronic properties was possible at a minimum of experimental data. The examined materials are promising options for the fabrication of more efficient laser diodes and more economic solar cells.

Benedikt Klein, A4 (Gottfried)

In his thesis “The Surface Chemical Bond of Non-alternant Aromatic Molecules on Metal Surfaces”, Dr. Klein explores interfaces between model organic semiconductors and metals. He compares π-electron systems with alternant and non-alternant topologies and finds that the non-alternant topology leads to much stronger interfacial interactions. These studies pave the way to novel organic semiconductors with tailored properties and provide important insight into the bonding of non-alternant defect structures in graphene with metals.

See the news release of the Philipps-Universität Marburg for details of the event.

Directional ultrafast charge transfer in a TMDC heterostructure – Publication by B5 (Höfer/Mette)

In a new publication in Nanoscale Horizons, Zimmermann and coworkers introduce time-resolved SHG imaging microscopy as a new experimental method for investigating ultrafast charge-transfer processes in heterostructures of transition metal dichalcogenides.

Time- and polarization-resolved SHG microscopy in combination with pump-photon energy dependent measurements reveals ultrafast interlayer hole transfer from WSe2 to MoSe2 and vice versa. Copyright by CC BY 3.0.

Heterostructures of transition metal dichalcogenides (TMD) feature a type-II band alignment which can separate photoexcited electrons and holes into different layers through ultrafast charge transfer. While this charge transfer is essential for potential applications, the underlying mechanisms still remain elusive. Main drawbacks of previous experiments are insufficient time-resolution of the employed microscopy setups and deficiencies of linear optical spectroscopies to address individual layers of the heterostructure selectively.In their new approach, Zimmermann and coworkers have combined the advantages of time-resolved optical second-harmonic generation (SHG) with an optical microscopy setup. On the one hand, their method allows for pump-probe experiments in µm small structures with a superior time-resolution. On the other hand, the tensorial nature of the second-order nonlinear susceptibility allows them to distinguish the response from differently oriented layers to elucidate directional interlayer charge transfer as demonstrated for a rotationally mismatched WSe2/MoSe2 heterostructure. As their results show, the new approach is particularly suited to perform systematic investigations of the charge transfer in dependence of the rotational layer mismatch in TMD heterostructures.

Publication

J. E. Zimmermann, Y. D. Kim, J. C. Hone, U. Höfer, G. Mette
Directional ultrafast charge transfer in a WSe2/MoSe2 heterostructure selectively probed by time-resolved SHG imaging microscopy
Nanoscale Horizons 5 (2020) 1603 DOI: 10.1039/d0nh00396d

Contact

Dr. Gerson Mette
Philipps-Universität Marburg
SFB 1083 subproject B5
https://internal-interfaces.de/projects/B5
Tel.: +49 6421 28-24123
EMAIL

Van der Waals bound Organic Semiconductor/2D-Material Hybrid Heterosystems: Intrinsic Epitaxial Alignment of Perfluoropentacene Films on Transition Metal Dichalcogenides – Publication by A2 (Witte)

In a new publication in Chemistry of Materials project A2 (Witte) reports on the epitaxial alignment of crystalline perfluoropentacene (PFP) films on various transition metal dichalcogenides (TMDCs). This van der Waals epitaxy results in characteristic twist angles between substrate and film lattices, which are of particular interest for the optoelectronic coupling at the interface.

Epitaxial alignment of crystalline PFP films on the basal plane of MoS2 and WSe2 (Image: M. Dreher). Reprinted with permission from 2020, 32, 20, 9034-9043. Copyright 2020 American Chemical Society.

Two-dimensional (2D) materials are a subject of current research, because their different electronic properties as well as the ability to prepare films as thin as one mono­layer opens up the prospect of producing new nanoscale heterostructures and devices. Of particular interest is the stacking of such films with controlled twist angle as it critically affects the electronic interface properties. A promising extension is the combination of TMDCs with organic semi­conductors (OSC), as it allows to combine the high charge carrier mobility of the TMDCs with the OSC’s large photo-absorption cross section, which is beneficial for photovoltaic applications.

Using the example of the prototypical OSC PFP, Maximilian Dreher and coworkers analyzed in the present study the epitaxial alignment of the crystalline molecular adlayers on the basal plane of different TMDCs (MoSe2, WSe2, MoS2 MoTe2). By utilizing the optical anisotropy of PFP films, their azimuthal alignment was analyzed by means of polarization resolved reflection anisotropy. This sensitive and non-invasive method allows to characterize the epitaxial alignment even for thin films of few nanometers. The analysis yielded specific twist angles of the crystalline adlayer domains with respect to the substrate lattice, which are characteristic for the individual material combinations. Notably, the observed epitaxial order is not caused by any higher-order commensurability between substrate and adlayer, where individual molecules are bound to locally favorable adsorption sites. Instead, it results from an energetically favored alignment of the entire crystalline adlayer on the substrate surface and can be rationalized as an on-line coincidence. This peculiar epitaxy could also be theoretically modelled using a modified scheme of projection of real-space adlayer lattice points onto the substrate unit cell. In addition, the extreme sensitivity of this van der Waals epitaxy on small lattice distortions was demonstrated by films grown at slightly higher substrate temperature. Although raising the growth temperature by about 30 K yields only a small increase of the lattice constants of the PFP film due to thermal expansion in the order of a few hundredths of Angstrom, while the more rigid TMDC surface lattice is hardly affected, it causes a distinct change of the twist angle of more than 20°. The achieved epitaxial alignment and control of twist angles is an important mile stone and will be used in future studies on the optoelectronic adlayer-substrate coupling in OSC/TMDC hybrid systems within the SFB 1083.

Publication

Maximilian Dreher, Darius Günder, Steffen Zörb, and Gregor Witte
Van der Waals Bound Organic Semiconductor/2D-Material Hybrid Heterosystems: Intrinsic Epitaxial Alignment of Perfluoropentacene Films on Transition Metal Dichalcogenides
Chem. Mater. (2020) DOI:10.1021/acs.chemmater.0c03482

Contact

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

David Peter Krug (Project A5) receives poster award at M&M 2020

David Peter Krug, PhD-student in SFB-project A5 of Prof. Dr. Kerstin Volz, was awarded with the poster prize at the virtual Microscopy & Microanalysis Meeting 2020.

In his online presentation, David Peter Krug gave new insights into the growth of GaP nanowires and the mechanism of the kink formation showing different predominant angles. These kinks point towards the existence of twinned interfaces in the nanowires. He studied the growth process of the nanowires by in-situ (scanning) transmission electron microscopy ((S)TEM) in gas environmental cells, in which the reaction conditions are comparable to the widely used metal organic vapor phase epitaxy (MOVPE). He brilliantly made use of the opportunities of an online presentation and implemented a live study at the (S)TEM.

The Microscopy & Microanalysis Meeting (M&M) is an annual meeting in the USA covering the research fields of microscopy, imaging, and compositional analysis. Due to the current Corona pandemic, the meeting was held online.

Poster “Formation mechanisms for the dominant kinks in GaP nanowires in an in-situ (S)TEM gas cell holder” by D. Krug, M. Widemann, F. Gruber, A. Beyer, and K. Volz (Materials Sciences Center and Faculty of Physics, Philipps-Universität Marburg) – Microscopy & Microanalysis Meeting 2020, August 04 – 07, 2020, virtual meeting.

Momentum-resolved charge transfer between two TMDC layers – Publication by B6 (Höfer/Wallauer) and A13 (Rohlfing)

How fast is the charge transfer between two layers of transition metal dichalcogenides (TMDCs) and where does it take place in momentum space? Two-photon photoemission using high-harmonic probe pulses can answer these questions as Wallauer and coworkers demonstrate for the topmost layers of MoS2.

Copyright 2020 by the American Physical Society.

The experiment of Wallauer and coworkers exploits both the high surface sensititivity of photoelectron spectroscopy and the fact, that the bandgap of the topmost layer of TMDCs is enlarged due to reduced screening. By tuning pump pulses below the top-layer gap at K, it is thus possible to excite electrons in deeper layers and probe only the topmost layer. The experiment then images the population dynamics of initially unoccupied electronic states and the charge transfer directly in momentum space with femtosecond time resolution. The results show that the electron transfer between the topmost layers of a 2H-MoS2-crystal, takes place at Σ and proceeds on a timescale of less than 20 fs.

GW-based tight binding calculations by Marauhn and Rohlfing support the experimental findings and explain why the electron transfer takes place at Σ. The GW-based tight-binding calculations not only confirm that the band gap in the surface layer is indeed considerably larger than in deeper layers. They reveal that the coupling between surface and deeper layers is strongly momentum-dependent throughout the Brillouin zone. The coupling is found to be particularly strong at at the conduction-band minimum at Σ, which explains the ultrafast interlayer charge transfer observed in the experiment at this location.

The publication is an “Editor’s Suggestion” in the September 2020 issue of Physical Preview B.

Publication
R. Wallauer, P. Marauhn, J. Reimann, S. Zoerb, F. Kraus, J. Güdde, M. Rohlfing, and U. Höfer

Momentum-resolved observation of ultrafast interlayer charge transfer between the topmost layers of MoS2
Physical Review B 102, 125417 (2020)

Contact

Dr. Robert Wallauer

Philipps-Universität Marburg
SFB 1083 subproject B6
https://internal-interfaces.de/projects/B6
Phone: +49 6421 28-21406
EMAIL

Prof. Dr. Michael Rohlfing
Westfälische Wilhelms-Universität Münster
SFB 1083 subproject A13
https://internal-interfaces.de/projects/A13
Phone: +49 251 83-36340

Professor Ralf Tonner appointed Chair for Theoretical Chemistry at the University of Leipzig

We congratulate Prof. Dr. Ralf Tonner, prinicple investigator of SFB-Project A6 “Unified density functional description of bonding and interaction at inorganic/organic interfaces” on his new postion as Chair for Theoretical Chemistry at the University of Leipzig.

Foto: Laackman Fotostudios Marburg

Professor Tonner joined SFB 1083 as a junior group leader in 2013 and has been very active and succesful in understanding inorganic/organic interfaces from electronic structure theory. In 2016, he was awarded the Hellmann-Prize for Theoretical Chemistry in recognition of his contributions to a detailed theoretical understanding of the chemical processes at surfaces and interfaces. In 2019, he received offers for a professorship (W2) for theoretical chemistry from the Universties of Chemnitz and Regensburg. He accepted the offer from Regensburg and has been working there since April 2020. In Leipzig, Professor Tonner will continue to be prinicple investigator of SFB 1083.

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