micro-focused ARPES study of electronic structure in Janus monolayer transition metal dichalcogenides

Janus monolayer transition metal dichalcogenides (TMDs) represent a novel class of 2D materials [1,2]. In Janus structures, one side (top or bottom side) of the chalcogen atomic layer within the TMD is substituted with a different chalcogen element. This distinctive configuration induces an out-of-plane electronic polarization, leading to several unique physical properties not observed in usual monolayer TMDs [1]. In our study, we utilized micro-focused angle-resolved photoemission spectroscopy (micro-ARPES)[3] on a Janus monolayer WSeS to examine the modification of the electronic band structure resulting from the substitution of the top selenium atomic layer with sulfur. The Janus monolayer WSeS was synthesized using the plasma treatment [4] on a mechanically exfoliated monolayer WSe₂ sample which was specially prepared for micro-ARPES measurement [5], while being continuously monitored with photoluminescence (PL). We successfully obtained the clear ARPES images on both monolayer WSe₂ and WSeS. Our results reveal that the substitution of the selenium atomic layer with a sulfur atomic layer results in a momentum shift of the K-valley due to in-plane lattice shrinkage, as well as a notable variation of the band structure around the Γ-point. These observations are basically in good agreement with the first-principles band calculations, indicating the emergence of electronic bands peculiar to the Janus compound.

[1] A.-Y. Lu, et al., Nat. Nanotechnol. 12, 744 (2017).

[2] J. Zhang, et al., ACS Nano 11, 8192 (2017).

[3] M. Kitamura et al., Rev. Sci. Instrum. 93, 033906 (2022).

[4] D. B. Trivedi et al., Adv. Mater. 32, 2006320 (2020).

[5] S. Masubuchi et al., Sci. Rep. 12, 10936 (2022).