Fig.3 MoS2/WS2 heterobilayers grown by CVD method

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Ferroelectricity in Untwisted Heterobilayers of Transition Metal Dichalcogenides

Collaborators used chemical vapor deposition (CVD) to grow a untwisted MoS2/WS2 heterobilayers with a thickness of only about 1 nm. This heterobilayers has two stacking structures of 2H and 3R, both of which break the out-of-plane inversion symmetry(Fig.3). The PFM results show that the material has the out-of-plane ferroelectric property with the obvious ferroelectric hysteresis loops (Fig.4). The piezoelectric coefficient d33 is 1.95-2.09 pm/V. This value is about 6 times higher than that of monolayer α-In2Se3, which has the highest out-of-plane polarization among previously known 2D ferroelectric materials6.

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Fig.4 Ferroelectric characterization of MoS2/WS2 heterobilayers

Fig.5 The origin of the MoS2/WS2 heterobilayers’ ferroelectric behaviours

In addition, the ferroelectric thin film was constructed into a ferroelectric tunnelling junction(FTJ), shows an considerable on/off ratio(~103). By using the first-principles calculation. We had explored a reliable method to calculate the piezoelectric coefficient of 2D materials , and the calculated values of 2.28~2.40 pm/V are obtained, which are consistent with the experimental results. The calculation results show that the non-zero out-of-plane polarization comes from the interlayer charge transfer (Fig.5), and under the external electric field, the direction of interlayer charge transfer can be flipped by overcoming the 16 meV/f.u energy barrier, so that the polarization direction can be reversed through the in-plane sliding, indicating that the heterobilayers is an out-of-plane ferroelectric thin film. 

REFERENCES

1.https://www.electronics-tutorials.ws/electromagnetism/magnetic-hysteresis.html

2.Li, B., Wan, Z., Wang, C. et al. Van der Waals epitaxial growth of air-stable CrSe2 nanosheets with thickness-tunable magnetic order. Nat. Mater. 20, 818–825 (2021)
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5. Zhang, K., Wang, C., Zhang, M. et al. A Gd@C82 single-molecule electret. Nat. Nanotechnol. 15, 1019–1024 (2020). https://doi.org/10.1038/s41565-020-00778-z
6. (1) Rogée, L.; Wang, L.; Zhang, Y.; Cai, S.; Wang, P.; Chhowalla, M.; Ji, W.; Lau, S. P. Ferroelectricity in Untwisted Heterobilayers of Transition Metal Dichalcogenides. 2022, 7