中国物理B ›› 2025, Vol. 34 ›› Issue (11): 117305-117305.doi: 10.1088/1674-1056/ae0d57

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Emergent ferroelectricity in the two-dimensional Janus MoSSe monolayer driven by nondegenerate phonon instability

Zhi-Long Cao(曹智龙)1,2, Chen Cao(曹琛)1,2, Jia-Jun Xu(徐佳俊)1,2, Jia-Xu Yan(闫家旭)1,2,†, Lei Liu(刘雷)1,2, and De-Zhen Shen(申德振)1,2   

  1. 1 State Key Laboratory of Luminescence Science and Technology, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China;
    2 University of Chinese Academy of Sciences, Beijing 100049, China
  • 收稿日期:2025-07-16 修回日期:2025-09-19 接受日期:2025-09-30 发布日期:2025-11-13
  • 通讯作者: Jia-Xu Yan E-mail:yanjiaxu@ciomp.ac.cn
  • 基金资助:
    We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 12334014 and 11727902) and the Overseas High-level Talents Program of the Chinese Academy of Sciences.

Emergent ferroelectricity in the two-dimensional Janus MoSSe monolayer driven by nondegenerate phonon instability

Zhi-Long Cao(曹智龙)1,2, Chen Cao(曹琛)1,2, Jia-Jun Xu(徐佳俊)1,2, Jia-Xu Yan(闫家旭)1,2,†, Lei Liu(刘雷)1,2, and De-Zhen Shen(申德振)1,2   

  1. 1 State Key Laboratory of Luminescence Science and Technology, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, China;
    2 University of Chinese Academy of Sciences, Beijing 100049, China
  • Received:2025-07-16 Revised:2025-09-19 Accepted:2025-09-30 Published:2025-11-13
  • Contact: Jia-Xu Yan E-mail:yanjiaxu@ciomp.ac.cn
  • Supported by:
    We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 12334014 and 11727902) and the Overseas High-level Talents Program of the Chinese Academy of Sciences.

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摘要： We report the discovery of bistable polar states with switchable polarization in the Janus monolayer 1T-MoSSe, induced by symmetry breaking in its chalcogen atomic layers. Our results demonstrate that Janus 1T-MoSSe exhibits two out-of-plane bistable polar states with switchable polarization, rather than polarization emerging from a non-polar phase, which represents an unconventional form of ferroelectric-like behavior. First-principles calculations and phenomenological modeling reveal that the inequivalent stacking of sulfur and selenium (S/Se) atoms breaks central inversion symmetry, activating non-degenerate phonon modes at the $K$-point ($K_2/K_3$) that drive the structural transformation between metastable d1T$_{\rm S}$ and d1T$_{\rm Se}$ phases. This coupling enables bipolar control of out-of-plane polarization through atomic displacements and charge redistribution, resulting in a polarization change of ΔP ≈ ±0.3 μC/cm2. The Landau free energy analysis indicates that anharmonic terms and inter-mode coupling generate an asymmetric double-well potential, which is essential for the stabilization of bistable polar states. Molecular dynamics simulations show that the d1T$_{\rm S}$ phase remains stable at high temperatures, whereas the d1T$_{\rm Se}$ phase undergoes an irreversible phase transition near 300 K, accompanied by a Peierls-like distortion of the Mo atomic chain. This transition is driven by differences in electronegativity, atomic radius, and d-p orbital hybridization between S and Se. Our findings establish a theoretical framework for engineering nonlinear responses in two-dimensional (2D) ferroelectrics and suggest that low-energy polarization reversal at room temperature can be achieved through strain or electric-field control, offering promising opportunities for non-volatile memory and piezoelectric sensing applications.

关键词: Janus MoSSe monolayer, ferroelectric phase transitions, first-principles calculations

Abstract: We report the discovery of bistable polar states with switchable polarization in the Janus monolayer 1T-MoSSe, induced by symmetry breaking in its chalcogen atomic layers. Our results demonstrate that Janus 1T-MoSSe exhibits two out-of-plane bistable polar states with switchable polarization, rather than polarization emerging from a non-polar phase, which represents an unconventional form of ferroelectric-like behavior. First-principles calculations and phenomenological modeling reveal that the inequivalent stacking of sulfur and selenium (S/Se) atoms breaks central inversion symmetry, activating non-degenerate phonon modes at the $K$-point ($K_2/K_3$) that drive the structural transformation between metastable d1T$_{\rm S}$ and d1T$_{\rm Se}$ phases. This coupling enables bipolar control of out-of-plane polarization through atomic displacements and charge redistribution, resulting in a polarization change of ΔP ≈ ±0.3 μC/cm2. The Landau free energy analysis indicates that anharmonic terms and inter-mode coupling generate an asymmetric double-well potential, which is essential for the stabilization of bistable polar states. Molecular dynamics simulations show that the d1T$_{\rm S}$ phase remains stable at high temperatures, whereas the d1T$_{\rm Se}$ phase undergoes an irreversible phase transition near 300 K, accompanied by a Peierls-like distortion of the Mo atomic chain. This transition is driven by differences in electronegativity, atomic radius, and d-p orbital hybridization between S and Se. Our findings establish a theoretical framework for engineering nonlinear responses in two-dimensional (2D) ferroelectrics and suggest that low-energy polarization reversal at room temperature can be achieved through strain or electric-field control, offering promising opportunities for non-volatile memory and piezoelectric sensing applications.

Key words: Janus MoSSe monolayer, ferroelectric phase transitions, first-principles calculations

中图分类号:  (Nanocrystalline materials)

  • 73.63.Bd
77.80.B- (Phase transitions and Curie point) 63.20.dk (First-principles theory)