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Over the last few decades, topological insulators have emerged as an important class of materials for quantum technologies, offering robust conducting surface states with potential applications in low-energy field-effect transistors and heterogeneous catalysis.$^{1-3}$ Recent studies of two-dimensional (2D) materials known as transition metal dichalcogenides (TMDs) have identified topological insulating states in 1T’ MX$_2$ monolayers (M = Mo, W; X = S, Se, Te),$^{4,5}$ which have been proposed for application in low-energy transistors allowing switching via a topological phase transition.
However, the difficulty in synthesising these high-energy phases has prompted the development of a new TMD allotrope based on the experimental observation that square-octagon grain boundaries significantly influence the transport and optical properties of MoS$_2$ monolayers.$^6$ By introducing these boundaries as line defects into the hexagonal ground state of a TMD, a stable phase is obtained, which undergoes a semiconductor-topological insulator phase transition under roughly 5% equi-biaxial tensile strain.$^7$
In this work, we investigate the effect of these line defects in Janus-type TMD monolayers in which the two chalcogen faces of the layer are elementally distinct, such as MoSSe. Using density functional theory, we evaluate the relaxed atomic structures, relative thermodynamic stability, and strain-dependent electronic properties of defective Janus monolayers. Localized Wannier-function models are subsequently constructed to investigate the possible topological characteristics of strain-induced insulating phases, determine the $\mathbb{Z}_2$ topological invariant through Wilson-loop analysis, and identify edge-state signatures.
This study aims to establish whether square–octagon line defects offer a viable strategy for engineering topological phases in Janus dichalcogenide monolayers and to clarify how broken out-of-plane symmetry influences the electronic response of defective 2D materials.
1 Nadeem 2021 https://doi.org/10.1021/acs.nanolett.1c00378
2 Li 2019 https://doi.org/10.1016/j.nanoen.2019.01.007
3 Chen 2011 https://doi.org/10.1103/PhysRevLett.107.056804
4 Tang 2017 https://doi.org/10.1038/nphys4174
5 Qian 2014 https://doi.org/10.1126/science.1256815
6 van der Zande 2013 https://doi.org/10.1038/nmat3633
7 Li 2017 https://doi.org/10.1039/C6NR07851F
| I am the presenting author | Yes |
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