Synthesis and low-temperature STM characterization of monolayer FeSe₂

Monolayer transition metal dichalcogenides (2d-TMDCs) of the form MX₂ ((M={Mn, Cr}, X={Se, Te}) have been identified as promising 2d magnets^1-4 with magnetic ordering up to room temperature. The related 2d-TMDC family of FeX₂, on the other hand, has only been investigated theoretically due to difficulty in material synthesis^5,6. Recently, multilayer van der Waals coupled FeSe₂ has been synthesized through chemical vapor deposition⁷.  However, the growth and characterization of FeSe₂ in the monolayer regime remains unexplored. Here we report the successful molecular beam epitaxy growth of isolated monolayer FeSe₂ islands on highly oriented pyrolytic graphite, as well as their characterization by low temperature scanned probe microscopy. We find that FeSe₂ monolayer islands are dominated by the 1T’ structural phase instead of the 1T-phase predicted by density functional theory⁵. We also observe that the structural phase of monolayer FeSe₂ can be manipulated with the STM tip. Application of different voltages to the STM tip can induce the 1T’ phase to be locally (and reversibly) switched into a hexagonal phase. Both FeSe₂ phases are stable and can be characterized using scanning tunneling spectroscopy.

¹Zhang X. et al., Nat. Commun 2021, 12, 2492

²Li B. et al., Nat. Mater. 2021, 20, 818

³Xian J.-J. et al., arxiv :2011.05620v1 2020

⁴O’Hara D. J. et al., Nano Lett. 2018, 18, 5, 3125

⁵Beiranvand R. and Mehrabi V., Phys. Scr. 2021, 96, 065803

⁶L. Liu et al., J. Phys. Chem. Lett. 2020, 11, 18, 7893

⁷H. Liu and Y. Xue, Adv. Mater. 2021, 33, 2008456