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  4. Ambient, true atomic resolution of point defects in transition metal dichalcogenide monolayers.

Ambient, true atomic resolution of point defects in transition metal dichalcogenide monolayers.

Abstract number
41
Presentation Form
Contributed Talk
DOI
10.22443/rms.mmc2023.41
Corresponding Email
[email protected]
Session
Atomic and Molecular Resolution Phenomena via AFM, STM and Scanning Probes
Authors
Edward Dunn (1), Professor Robert Young (1), Dr Samuel Jarvis (1)
Affiliations
1. Physics Department, Lancaster University
Keywords

Atomic resolution, 2D materials, cAFM, Ambient, TMDs

Abstract text

Defects have been shown to be critical to understanding the properties of transition metal dichalcogenides (TMDs) [1].  TMDs are thought to be promising candidates for a number of proposed quantum technologies, including quantum security devices [2], hydrogen evolution reaction catalysts [3] and transistors [4].  It is therefore essential to characterise the intrinsic defects of mechanically exfoliated TMDs and determine how these defects behave in the same environmental conditions as they will be applied.

Here we show that conductive atomic force microscopy (cAFM) carried out in ambient conditions can achieve `true’ atomic resolution on 2D-TMD materials. We show that it is not only possible to identify single atom defects, but also subtle changes in the density of states of the surrounding nearest neighbour atoms. Furthermore, we show that specific defect types can be identified which we attribute to metal and chalcogen substitutions, and compare their appearance and frequency across multiple 2D-TMD materials (see Figure 1). An analysis by counting determines a defect density of 0.14, 0.02 and 0.01 defects per nm2 for WSe2, MoS2 and WS2, respectively. These findings are consistent across multiple samples, suggesting that WSe2 has a much greater number of each type of defect compared to other TMD materials. The ability to characterise the atomic structure of mechanically exfoliated 2D-TMDs in ambient conditions is an important step towards understanding the influence of defects on TMD properties. In particular, we will discuss the relevance of atomic defects on optical properties, and the potential to engineer atomic defects to optimise their optical response.

Figure 1a) shows a flake before flattening where wrinkles can be seen on the surface and the flake is clearly discernable from the substrate. Next to this a cartoon depicts the sweeping away of contaminants from between the flake with an AFM tip. To the right another image of the same flake after shows how the wrinkles have been removed and the flake now lies closer to the substrate; save for some bubles where contamination has accumulated. b) Shows an atomic resolution image of MoS2 where the sulfur atoms are depicted are arranged in a triangular lattice. In several places an atom appears to be missing (there is a black void) or the lattice site is unusually bright indicating some form of point defect is present at this site. The nearest neighbour sites around these defects are significantly brighter. Next to this is an image of WSe2 where there are many more defects and the dark defects are much more common than the bright ones. c) shows cartoons depicting the 3 defect types that are suggested as common defects in TMDs: a chalcogen vacancy, where a chalcogen atom is missing; a Chalcogen substitution, where another element has replaced the chalcogen; and the Metal Substitution, where the transition metal atom has been replaced by another.

Figure 1: True atomic resolution of single atom defects in 2D-TMDs. (a) A ‘nano-squeegee’ technique [5] is used to remove contamination between the 2D flake and the graphite surface. (b) Current channel cAFM image on MoS2 (left) and WSe2 (right) revealing the distribution of chalcogen defects. Sample Bias: 0.5V. (c) Ball-and-stick cartoons depicting the possible atomic vacancies in 2D-TMDs.

References

[1] Blades, W. H.; Frady, N. J.; Litwin, P. M.; McDonnell, S. J.;  Reinke, P. J Phys. Chem. C.  2020, 124 (28), 15337–15346.
[2] Cao, Y; Robson, A. J.; Alharbi, A.; Roberts, J.; Woodhead, C. S. et al. 2D Mater. 2017, 4 (4), 045021.
[3] Zhu, J.; Yang, R.; Zhang, G. Chem. Mater. Phys. 2022, 1 (2), 102-111.
[4] Yu, Z.; Ong, Z.; Li, S.; Xu, J.; Zhang, G. et al. Adv. Funct. Mater. 2017, 27 (19), 1604093.
[5] Rosenberger M. R.; Chuang, H.; McCreary, K. M.; Hanbicki, A. T.; Sivaram, S. V. et al. ACS Appl. Mater. Interfaces 2018, 10 (12), 10379–10387.

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