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.

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

SFB 1083 extended by four more years

The German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) has granted the Collaborative Research Center SFB 1083 „Structure and Dynamics of Internal Interfaces“ 12.3 Million Euros for a third funding period from July 2021 to June 2025.

SFB 1083 was established at Philipps-Universität Marburg in 2013. It included a guest project from the Donostia-International Physics Center in San Sebastián, Spain. Meanwhile groups from the universities of Gießen, Leipzig and Münster as well as the Forschungszentrum Jülich participate in the center. From October 2013 to June 2021, Marburg and the participating institutions received DFG funding that amounts to 20.4 Million Euros. Together the researchers have published more than 330 scientific papers. For a report on the scientific activities from 2013 to 2019 see the activity report. A video clip, also available in German, explains the relevance of research on internal interfaces to the general public and highlights selected contributions of SFB 1083. With the new grant, SFB 1083 will be supported for altogether 12 years, the maximum funding period for a DFG Collaborative Research Center.

The 3rd SFB funding period will bring a number of changes. Kerstin Volz will become the new spokesperson and follow Ulrich Höfer, who initiated the SFB more than ten years ago and successfully guided it in the first and second funding period. Seven projects of the 2nd funding period will end, either because their principle investigators (PIs) reached retirement age, or because of a shift of scientific focus. Instead, eight new projects will become part of the center. Three of these new projects will be led by new PIs, namely Marina Gerhard, Jens Güdde and Ermin Malic. Altogether, SFB 1083 will consist of 19 scientific and three service projects in its last funding period. The projects will be led by 21 professors, senior scientists or junior group leaders. They will involve a total of about 80 scientists working in physics, chemistry and materials sciences.

Scientifically, the SFB will focus on a couple of new aspects in the coming years, such as the influence of the interface on lateral charge-carrier transport and the tailored synthesis at interfaces to design desired structures bottom-up. Research on interfaces of 2D materials, which started with the 2nd funding period in 2017, will be further extended. Last but not least, applications and devices will become more in to focus. The research on novel interface-dominated lasers will be intensified, including new material systems and emission wavelengths. Moreover, strong THz-emitters based on charge-carrier recombination across interfaces are included in the program due to promising results from the 2nd funding period. More applications and devices are envisioned, particularly as a result of research on hybrid organic/inorganic materials. Another important focus of the SFB will be on development and usage of sophisticated experimental methods, which allow unprecedented insights into processes at the nanoscale across interfaces.

Present spokesman Ulrich Höfer (left) and future spokeswoman Kerstin Volz in front of a poster introducing SFB 1083, Foto: Stefan Kachel.

See also Press Release of Philipps-Universität Marburg (in German) and the German Research Foundation (DFG) for more detail.

 

Contact

Prof. Dr. Kerstin Volz
Department of Physics and Materials Science Center
Philipps-Universität Marburg
Tel: + 49 6421 28-22297
Email: kerstin.volz@physik.uni-marburg.de

 

Biphenylene Network: A Nonbenzenoid Carbon Allotrope – Publication by A4 (Gottfried) and A8 (Koert/Dürr) in Science

Not graphene: Dr. Qitang Fan and coworkers of SFB 1083 discover new type of atomically thin carbon material

Carbon exists in various forms, of which graphene is one of the most astonishing. In this atomically thin material, each carbon atom is linked to three neighbors, forming hexagons arranged in a honeycomb network. Researchers in the SFB 1083 projects A4 (Gottfried) and A8 (Koert/Duerr) have now discovered a new carbon network, which is planar like graphene, but is made up of squares, hexagons, and octagons forming an ordered lattice. In collaboration with physicists from Aalto University in Finland, the unique structure was confirmed using high-resolution scanning probe microscopy methods. In addition, it was found that the electronic properties of the new material are very different from those of graphene.

Structure of the new carbon network. The upper part shows schematically the linking of the carbon atoms, forming squares, hexagons, and octagon. The lower part is an image of the network, obtained with atomic force microscopy.

Biphenylene network, as the new material is named, is made from organic molecules on an atomically smooth gold surface. These molecules first form polymer chains, which consist of linked hexagons. A subsequent reaction connects these chains and forms the squares and octagons. An important feature of the chains is that they are chiral. Chains of the same type aggregate on the gold surface forming well-ordered assemblies, before they connect. This is critical for the formation of the new carbon material, because reaction between two different types of chains leads to the well-known graphene.

In contrast to graphene and other forms of carbon, the new material has metallic properties. Therefore, it can be used as conducting wires in future carbon-based electronic devices. The authors of the study are confident that their synthesis method will contribute to the discovery of further novel carbon networks. For now, their goal is to prepare larger sheets of the material and to study its interface-related properties.

For further information, please see the press release of the university of Marburg (available in German).

Publication

Q.T. Fan, L.H. Yan, M.W. Tripp, O. Krejči, S. Dimosthenous, S.R. Kachel, M.Y. Chen, A.S. Foster, U. Koert, P. Liljeroth, J.M. Gottfried
Biphenylene Network: A Nonbenzenoid Carbon Allotrope
Science 372 (2021) 852 DOI:10.1126/science.abg4509

Contact

Prof. Dr. Michael Gottfried
Philipps-Universität Marburg
SFB 1083 project A4
Tel.: 06421 28 22541
EMAIL