Understanding of the interplay of strain and topography in transition metal dichalcogenide monolayers

Abstract number
1357
Event
European Microscopy Congress 2020
DOI
10.22443/rms.emc2020.1357
Corresponding Email
[email protected]
Session
PSA.1 - 1D & 2D Materials
Authors
Julian Sickel (3), Marcel Asbach (3), Dr. Christoph Gammer (1), Prof. Dr. Rudolf Bratschitsch (2), Prof. Dr. Helmut Kohl (3)
Affiliations
1. Erich Schmid Institute of Materials Science
2. Physikalisches Institut, WWU
3. Interdisziplinäres Centrum für Elektronenmikroskopie und Mikroanalyse (ICEM)
Keywords

2D-materials, nano beam diffraction, NBED, transition metal dichalcogenide, TMDC, strain mapping

Abstract text

Transition metal dichalcogenide (TMDC) monolayers exhibit a non-flat topography when suspended without (underlaying) support. This gives rise to an inherent strain of the structure that varies depending on the local curvature [1]. From numerous studies of such materials we know that topography and strain have a great effect on the mechanical, electrical and optical properties [2-4], but are often hard to distinguish.
In this work, we focus on strained regions and regions with large corrugations of freely suspended TMDC monolayers, namely WSe2 and MoS2. These monolayers were exfoliated and mechanically transferred onto holey silicon nitrite membranes. This preparation method puts a lot of stress on the monolayer which leads to folds and ripples. To make the resulting strain visible we take diffraction patterns and measure the shifts of the diffraction spots. Depending on the quality of the measurement the accuracy of the measured strain can be well below 0.1% [5].
Using nano beam electron diffraction (NBED) we can achieve parallel beam diffraction conditions on areas smaller than 3nm. The spatial resolution of this method is sufficient to probe most corrugations that occur. To gain further insight we simulate NBED maps of TMDC test structures using molecular dynamics programs and multislicing routines.
In conclusion, we demonstrate the challenges of distinguishing between strain and topography in TMDC monolayers. Using only a single NBED map, it is often not possible to determine the local strain. Simulations of test structures is therefore a valuable tool to further pin down the measured structure.

References

[1] Meyer, Jannik C., et al. Solid State Commun. 143 (2007), 101-109.
[2] Zhu, Shuze, and Harley T. Johnson, Nanoscale 10 (2018), 20689-20701.
[3] Naumis, Gerardo G., et al. Rep. Prog. Phys. 80 (2017), 096501.
[4] Niehues, Iris, et al. Nano lett. 18 (2018), 1751-1757.
[5] Pekin, Thomas C., et al. Ultramicroscopy 176 (2017), 170-176.