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 (Δt_CT) 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

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

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

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

Andreas Beyer and coworkers achieved the determination and spatial resolution of electric fields at interfaces with the transmission electron microscope.

Adapted from Beyer et al. (full citation see below) licensed by CC BY-NC-ND 4.0.

Nanometer-scale built-in electric field are the basis of many modern (opto)electronic devices, such as solar cells, lasers or batteries. Optimization of these devices requires precise characterization of such fields at small length scales. With a fast pixelated-detector, A. Beyer and coworkers in SFB project A5 (Volz) acquire a 2D diffraction pattern for every real-space position of the impinging electron beam. In doing so, the momentum transfer of an electric field (or a charge) on the electron beam can be measured, and the electric field, which is invisible in “normal high angle annular dark field images”, can be calculated from the 4D data-set.

In this work, key characteristics, like doping concentration or polarity, of GaAs-based p-n junctions were quantitatively obtained by 4D scanning transmission electron microscopy (4DSTEM). The values are in excellent quantitative agreement with results from other techniques, which – of course – lack lateral resolution.

Publication

A. Beyer, M.S. Munde, S. Firoozabadi, D. Heimes, T. Grieb, A. Rosenauer, K. Müller-Caspary, K. Volz
Quantitative Characterization of Nanometer-Scale Electric Fields via Momentum-Resolved STEM
Nano Lett. (2021) DOI:10.1021/acs.nanolett.0c04544

Johanna Heine and Sangam Chatterjee break boundaries in two-dimensional materials’ design towards enhanced light-harvesting and emitting capabilities of hybrid perovskites

Adapted from Klement et al. (full citation see below) licensed by CC BY-NC-ND 4.0.

Low-dimensional organic−inorganic perovskites synergize the virtues of organic perovskites and inorganic two-dimensional (2D) materials featuring intriguing possibilities for next-generation optoelectronics: they offer tailorable building blocks for atomically thin, layered materials while providing the enhanced light-harvesting and emitting capabilities. However, the quest for new materials is limited by the generally-accepted paradigm that atomically thin materials require covalent in-plane bonding.

The groups of Dr. Heine (A11) and Prof. Chatterjee (B2) within the SFB 1083 lift this apparent paradigm and report single layers of the 1D organic–inorganic perovskite [C₇H₁₀N]₃[BiCl₅]Cl. Its unique 1D–2D interface structure enables single layers and the formation of self-trapped excitons, which show white-light emission. The thickness dependence of the emission energy may enable facile color tuning for next-generation lighting and display technologies.

This class of materials enables interface-controlled device integration of brightly luminescent 1D and 0D hybrid perovskites and offers a promising pathway for the non-covalent functionalization of classical 2D materials through heterostructures.

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