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

/in /by

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)

/in /by

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 /by

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