News

On the Role of Collective Electrostatic Effects in Electronic Level Pinning and Work Function Changes by Molecular Adlayers: The Case of Partially Fluorinated DNTTs Adsorbed Flat-Lying on Various Metals and Hetero-Structures – Publication by A2 (Witte) and A8 (Koert)

In a new publication in Advanced Materials Interfaces, the groups of Gregor Witte (A2), Ulrich Koert (A8) as well as Jérôme Cornil from the University of Mons report on the formation of an internal interface dipole at a metal/2D metal/organic hetero-interface, which can be modified by the outer organic monolayer.

Schematic representation of the outer and internal interface dipoles at the FxDNTT/cesium/copper hetero-interface (Image: Maximilian Dreher, CC BY-NC-ND 4.0).

The use of organic contact layers is a versatile tool to control the work function of metal electrodes. While partial fluorination of robust organic molecules leads to a significant shift of their frontier energy levels in the isolated molecules, this effect can be wiped out for organic films adsorbed on high work function metal substrates leading to an equalization of the corresponding HOMO levels. Consequently, also the work function shift is equalized in the condensed phase, which is often referred to as HOMO (resp. LUMO) level pinning. Especially the LUMO level pinning has been reported only on a theoretical level in literature yet.

By using partially fluorinated DNTTs, which were synthesized by project A8 and exhibit such a HOMO level pinning on high work function noble metals, the group of Jérôme Cornil (Mons, Belgium) demonstrated that a LUMO level pinning also exists on low work function Cs(110) surfaces on a theoretical level. To face low work function surfaces experimentally, Maximilian Dreher and coworkers used atomically thin cesium layers that grow epitaxial on Cu(100) single crystals and provide more inert, low work function surfaces. In contrast to the expectation, this copper/cesium/organic hetero-stack reveals no LUMO pinning effect. Complementary DFT calculations demonstrate, that the contributions prevailing on the work function shift can be separated into (i) an outer interface dipole between the organic layer and the 2D cesium layer and (ii) an inner, buried interface dipole at the metal/cesium interface. While the outer interface dipole is again equalized for the different FxDNTT species, the buried interface dipole is modulated dependent on the degree of fluorination of the molecules.

Such a sandwich hetero-interface provides new possibilities to effectively tailor contact layers between metal electrodes and active organic layers improving their energy level alignment and emphasizes the importance of internal interfaces.

Publication

M. Dreher, D. Cornil, M. W. Tripp, U. Koert, J. Cornil, G. Witte
On the Role of Collective Electrostatic Effects in Electronic Level Pinning and Work Function Changes by Molecular Adlayers: The Case of Partially Fluorinated DNTTs Adsorbed Flat-Lying on Various Metals and Hetero-Structures
Adv. Mater. Interfaces (2022) DOI:10.1002/admi.202200361

Contact

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

German Science and Humanities Council recommends research building for materials sciences

A research building together with a modern transmission electron microscope will be established on the Lahnberge campus of Philipps-University Marburg

Dr. Andreas Beyer, a researcher in SFB 1083, operates a Transmission Electron Microscope, which provides important insights in the development of new materials.

On the Lahnberge Campus, a new research building for a transmission electron microscope for the investigation of novel materials will be established. The German Science and Humanities Council gave its recommendation for the project, which is called ATEMMA (Advanced Transmission Electron Microscopy, Marburg). ATEMMA comprises a volume of 10 Mio €. This is divided into 4 Mio € for the building itself as well as 6 Mio € for the new (S)TEM.

ATEMMA strengthens the focus on material sciences and especially on interfaces at the Philipps-University Marburg and paves the way for high-quality research, e.g., on new materials used for communication and energy technologies, as these represent extremely important topics in our today’s society. The new research lab combines structural characterization with the development of new methods. This combination will boost the research on novel materials also with respect to device applications.

ATEMMA will be used jointly by different groups from physics, chemistry and material sciences distributed over the Philipps-University Marburg as well as Justus-Liebig-University Giessen and the Forschungscampus Mittelhessen. Several of the groups are also part of the SFB 1083, highlighting the importance of interface-related research for ATEMMA.

For further information, please see the press release by the Philipps-Universität Marburg (in German).

Contact

Prof. Dr. Kerstin Volz
Department of Physics and Materials Science Center
Philipps-Universität Marburg
Tel.: 06421 28-22297
EMAIL

Dr. Gerson Mette (B5) completed his habilitation at the Philipps-University Marburg

We congratulate Dr. Gerson Mette, former PI of SFB project B5, on completing his habilitation in experimental physics at the Philipps-University Marburg.

Dr. Gerson Mette studied physics at the Philipps-University Marburg and finished his PhD in the group of Prof. Höfer in 2012. After working as a postdoc at the University of Zurich for two years, he went back to Marburg and became a research associate in 2015 while simultaneously joining the SFB 1083 as a young researcher and co-PI of project B5.

With his broad background in surface science and laser spectroscopy, he has set up new SHG imaging microscopy for pump-probe experiments of van der Waals heterostructures and explored the dynamics of charge-transfer processes across interfaces of 2D materials in well-defined environments. Furthermore, he explored the influence of electronic interface states on the ultrafast charge-transfer at buried GaP/Si interfaces.

In February 2022 he gave his habilitation talk on “How big is the proton? The proton radius puzzle” and completed his habilitation in experimental physics. The members of the SFB thank Dr. Mette for his work and commitment for the SFB 1083 and wish him all the best on his future career path.

Terahertz Fingerprint of Monolayer Wigner Crystals – Publication by B9 (Malic) in Nano Letters

The Ultrafast Quantum Dynamics group of Ermin Malic (Project B9) together with Rudolf Bratschitsch from the University of Münster revealed unexpected transport behavior of excitons in ultrathin semiconductors

Sketch of the 2D Wigner crystal with a honeycomb lattice and alternating spin polarization. The colored curves underneath the particles illustrate their wave functions. Reprinted with permission from Brem et al. Copyright 2022 American Chemical Society.

Wigner crystals are solid, crystalline phases of electrons, formed at low temperatures in order to minimize their repulsive energy. This formation is one of the most intriguing quantum phase transitions and their experimental realization remains challenging since their theoretical prediction. However, the strong Coulomb interaction in monolayer semiconductors represents a unique opportunity for the realization of Wigner crystals without external magnetic fields.

In this work, the group of Ermin Malic predicts that the formation of monolayer Wigner crystals can be detected by their terahertz response spectrum, which exhibits a characteristic sequence of internal optical transitions. The density matrix formalism was used to derive the internal quantum structure and the optical conductivity of the Wigner crystal and to microscopically analyze the multipeak shape of the obtained terahertz spectrum. Moreover, a characteristic shift of the peak position as a function of charge density for different atomically thin materials was predicted and showed how the results can be generalized to an arbitrary two-dimensional system.

The results will guide future experiments toward the detection of Wigner crystallization and help to study the interaction dynamics in pure and generalized Wigner crystals in twisted bilayers.

Publication

S. Brem, E. Malic
Terahertz Fingerprint of Monolayer Wigner Crystals
Nano Lett. (2022) DOI:10.1021/acs.nanolett.1c04620

Contact

Prof. Dr. Ermin Malic
Philipps-Universität Marburg
SFB 1083 project B9
Tel.: 06421 28-22640
EMAIL

Dark exciton anti-funneling in atomically thin semiconductors – Publication by B9 (Malic) in Nature Communication

The Ultrafast Quantum Dynamics group of Ermin Malic (Project B9) together with Rudolf Bratschitsch from the University of Münster revealed unexpected transport behavior of excitons in ultrathin semiconductors

Adapted from Rosati et al. (full citation see below) licensed by CC BY 4.0.

Transport of charge carriers is at the heart of current nanoelectronics. In conventional materials, electronic transport can be conveniently controlled by applying external electric fields. However, the optoelectronic properties of the emerging material class of atomically thin semiconductors are governed by tightly bound excitons. These are neutral Coulomb-bound electron-hole pairs and as such their propagation cannot be controlled by electrical fields. Recently, strain engineering has been introduced to manipulate the propagation of excitons in these technologically promising materials. Strain-induced energy gradients give rise to exciton funneling up to a micrometer range. Excitons have been observed to propagate towards spatial regions with the strongest strain gradient, where the energy is minimal. However, the transport of dark excitons, which govern the optoelectronic response of these materials, has remained literally in the dark up till now.

In this joint theory-experiment work, the research groups of Ermin Malic and Rudolf Bratschitsch combined spatiotemporal photoluminescence measurements with microscopic many-particle theory to track the way of excitons in time, space and energy. They found that excitons surprisingly move away from high-strain regions. This anti-funneling behavior can be traced back to the dominating role of propagating dark excitons, which possess an opposite strain-induced energy variation compared to bright excitons. The findings open new possibilities to control the transport in materials dominated by excitons.

See also the press release by Philipps-University Marburg (in German).

Publication

R. Rosati, R. Schmidt, S. Brem, R. Perea-Causín, I. Niehues, J. Kern, J.A. Preuß, R. Schneider, S.M. de Vasconcellos, R. Bratschitsch, E. Malic
Dark exciton anti-funneling in atomically thin semiconductors
Nat. Commun. 12 (2021) 7221 DOI:10.1038/s41467-021-27425-y

Contact

Prof. Dr. Ermin Malic
Philipps-Universität Marburg
SFB 1083 project B9
Tel.: 06421 28-22640
EMAIL

Publication of new SFB 1083 Image Brochure

SFB 1083 published a new image brochure introducing the projects and the principle investigators in the third funding period.

Cover of the image brochure of the third funding period. Design by Bosse&Meinhard.

In October 2021, the SFB 1083 updated its image brochure to feature the goals and the focus of the research center in the third funding period. The image brochure gives a general introduction to the research on internal interfaces and portraits the participating researchers mainly for interested students and for the general public.  The numbers on the SFB for the past two as well as the current funding period can also be found in the booklet.

The image brochure (German) can be downloaded here.

A printed version of the image brochure is available upon request.

Contact

Sonderforschungsbereich 1083
Philipps-Universität Marburg
Hans-Meerwein-Str. 6
35043 Marburg
Tel.: 06421 28-24223
EMAIL

 

Polarization Resolved Optical Excitation of Charge-Transfer Excitons in PEN:PFP Cocrystalline Films: Limits of Nonperiodic Modeling– Publication by A2 (Witte)

In their combined experimental and theoretical study published in The Journal of Physical Chemistry Letters, the groups of Caterina Cocchi and Gregor Witte investigated the nature of charge transfer excitons in crystalline PEN:PFP heterostructures.

Absorption and schematic representation of CTX that are only formed in crystalline solids and not in dimers (Image: D. Günder, Reprinted with permission from J. Phys. Chem. Lett. 2021, 12, 40, 9899–9905. Copyright 2021 American Chemical Society.)

Charge-transfer excitons (CTX) at organic donor/acceptor interfaces are considered important intermediates for charge separation in photovoltaic devices. While typically blends are used in real solar cells, their mostly amorphous arrangement prevents microscopic insights into the nature of such CTX states. In contrast, crystalline model systems allow to derive structure-property interrelations and also enable detailed theoretical modeling based on the known molecular arrangement.

In this study Prof. Witte and coworkers characterized the CTX of the prototypical molecular donor/acceptor system pentacene:perfluoropentacene (PEN:PFP). Using template controlled co-crystalline films of different orientation, allowed to precisely determine the polarization of the CTX state from angular-resolved UV/Vis absorption spectroscopy. Complementary, this co-crystalline system was analyzed theoretically in the group of Prof. Cocchi (Oldenburg) by first-principles many-body calculations and solving the Bethe-Salpeter equation, which confirms that the lowest-energy excitation is a true CTX state with a polarization along the molecular stacking direction. In addition, it was shown that analogous simulations performed on bimolecular clusters are unable to reproduce this state, which is ascribed to the lack of long-range interactions and wave-function periodicity in these calculations and represents an important finding for the description of molecular donor/acceptor systems.

Publication

D. Günder, A.M. Valencia, M. Guerrini, T. Breuer, C. Cocchi, G. Witte
Polarization Resolved Optical Excitation of Charge-Transfer Excitons in PEN:PFP Cocrystalline Films: Limits of Nonperiodic Modeling
J. Phys. Chem. Lett. 12 (2021) 9899 DOI:10.1021/acs.jpclett.1c02761

Contact

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

Ultrafast charge transfer in twisted TMDC heterostructures – Publication by B5 (Höfer/Mette)

In a new publication in ACS Nano, Zimmermann and coworkers investigate ultrafast charge-transfer processes in twisted heterostructures of transition metal dichalcogenides by means of time-resolved SHG imaging microscopy.

Two-dimensional heterostructures of transition metal dichalcogenides (TMDC) represent very well-defined and at the same time highly versatile model systems of van-der-Waals interfaces. Many material combinations feature a type-II band alignment, which can separate photoexcited electrons and holes into different layers through ultrafast charge transfer leading to the formation of so-called interlayer excitons. Since the coupling within these structures depends considerably on the layer stacking, a strong influence of the interlayer twist on the ultrafast charge-transfer, recombination and other properties of the interlayer excitons has been expected.

In their study, Zimmermann and coworkers have employed time- and polarization-resolved second-harmonic imaging microscopy to investigate the ultrafast charge-carrier dynamics across the MoS2/WSe2 heterostructure interface for different stacking configurations. The excellent time resolution made it possible to identify stacking-dependent differences in the ultrafast charge transfer that were not accessible in previous approaches. For lower excitation energies of 1.70 eV, ultrafast electron transfer from WSe2 to MoS2 is found to depend considerably on the stacking angle and the transfer time is reduced by a factor of seven when going from a larger rotational mismatch towards 2H-stacking. At higher excitation energies, hole transfer processes from MoS2 to hybridized states at the Γ-point and to the K-points of WSe2 have to be considered in addition. The respective decay dynamics, however, does not show a significant dependence on the stacking angle indicating that radiative recombination of indirect Γ-K excitons becomes the dominant decay route for all samples.

The pump-probe SHG measurements upon 1.70-eV photoexcitation reveal a strong stacking-dependence of the ultrafast electron transfer (ΔtCT) from WSe2 to MoS2. At higher excitation energy of 1.85 eV, the observed decay dynamics indicate radiative recombination (τ) of indirect Γ-K excitons independent of the stacking configuration. Reprinted with permission from ACS Nano 2021, 15, 9, 14725–14731. Copyright 2021 American Chemical Society.

Publication

J.E. Zimmermann, M. Axt, F. Mooshammer, P. Nagler, C. Schüller, T. Korn, U. Höfer, G. Mette
Ultrafast Charge-Transfer Dynamics in Twisted MoS2/WSe2 Heterostructures
ACS Nano (2021) DOI:10.1021/acsnano.1c04549

Contact

Dr. Gerson Mette
Philipps-Universität Marburg
SFB 1083 subproject B5
Tel.: 06421 28-24123
EMAIL

Engineering of Printable and Air-Stable Silver Electrodes with High Work Function using Contact Primer Layer: From Organometallic Interphases to Sharp Interfaces – Publication by A2 (Witte)

Felix Widdascheck, Daniel Bischof and Gregor Witte developed a robust method to prepare air-stable molecular contact primer layers allowing to reduce hole injection barriers of printable silver electrodes  into organic semiconductors.

Contact engineering is an important issue for organic electronics as it allows to reduce charge carrier injection barriers. While the use of molecular contact primer layers was demonstrated in many concept studies for single crystalline model substrates, the processability of electrodes and their robustness in real devices must also be considered. Although silver electrodes can be printed using silver ink, their low work function and sensitivity to oxidation severely limits their use for printable organic electronics.

In this study Prof. Witte and his coworkers demonstrate that F6TCNNQ monolayers provide a reliable approach to engineer high work function silver electrodes, which is examined for Ag(111) as well as polycrystalline and silver ink substrates. Notably, upon multilayer growth, a pronounced intercalation of silver into the molecular adlayer occurs, yielding thermally stabilized organometallic interphases extending over the entire adlayer. It is shown that heating allows their controlled desorption leaving behind a well-defined monolayer that is further stabilized by additional charge transfer. Such primer layers enhance the work function to 5.5-5.6 eV and can even withstand air exposure but show no interdiffusion into subsequently deposited p-type organic semiconductor, hence validating their use for organic electronic devices.

Preparation scheme of F6TCNNQ layers on silver electrodes yielding well-defined, charge-transfer stabilized contact primer monolayers. Adapted from Widdascheck et al. (full citation see below) licensed by CC BY-NC-ND 4.0 

For further information, please see the press release by the Philipps-Universität Marburg (in German).

Publication

F. Widdascheck, D. Bischof, G. Witte
Engineering of Printable and Air-Stable Silver Electrodes with High Work Function using Contact Primer Layer: From Organometallic Interphases to Sharp Interfaces
Adv. Funct. Mater. (2021) DOI:10.1002/adfm.202106687

Contact

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

EMAIL

Momentum-forbidden dark excitons in WS2 – Publication by B6 (Höfer/Wallauer) and B9 (Malic)

In a publication in Nano Letters, Robert Wallauer and co-workers trace the early-stage exciton dynamics in a two-dimensional semiconductor and report first results on the ultrafast formation of momentum-forbidden dark excitons.

Excitons that form out of electrons and holes at different locations of the Brillouin zone, so-called dark excitons, play a key role for the optical properties of TMDC monolayers in general and for the formation of interlayer excitons in TMDC heterostructures, in particular. Whereas dark excitations are difficult to access by purely optical experiments they can be imaged directly in momentum space by time- and angle-resolved photoelectron spectroscopy [Madéo et al., Science 370, 1199 (2020)]. With the superior time-resolution of the momentum microscope operated by B6 (Höfer/Wallauer), the dynamics of formation of a dark KΣ exciton could now be resolved for the first time.

Reprinted with permission from Nano Lett. 2021, 21, 13, 5867–5873 Copyright 2021 American Chemical Society.

The formation process occurs on timescales where coherence between valence and conduction band play a major role.  The short pump pulses of the experiment induce an optical polarisation in the K valley. This polarisation was found to decay and form bright and dark excitons in a few tens of femtoseconds after optical excitation.  A fully microscopic theory by B9 (Malic) revealed the influence of the coherence on the formation process of the excitons, in excellent agreement with the experiment that could tune the excitation energy.  The high quality WS2-samples for the measurements were provided by the Huber group in Regensburg, who earlier succeeded in probing momentum-indirect excitons via the intraexcitonic 1s-2p transition [Poellmann et al., Nat. Mater. 14, 889 (2015)].

Future experiments of this kind will address the role of dark excitons in the formation process of interlayer excitons. The excellent agreement between experiment and theory in this work holds great promise to investigate such charge transfer processes on a microscopic level.

 

Publication

R. Wallauer, R. Perea-Causin, L. Münster, S. Zajusch, S. Brem, J. Güdde, K. Tanimura, K.-Q. Lin, R. Huber, E. Malic, U. Höfer
Momentum-resolved observation of exciton formation dynamics in monolayer WS2
Nano Lett. (2021) DOI:10.1021/acs.nanolett.1c01839

 

Contact

Dr. Robert Wallauer
Philipps-Universität Marburg
SFB 1083 project B6
Tel.: 06421 28 21406
EMAIL

Prof. Dr. Ermin Malic
Philipps-Universität Marburg
SFB 1083 project B9
Tel.: 06421 28 22640
EMAIL

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