Strain Engineering for 2D Ferroelectricity in Lead Chalcogenides

The quest for new 2D ferroelectric materials continues to arouse interest. Based on first-principles calculations, here, 2D ferroelectric properties in lead chalcogenides PbXs (X = S, Se, and Te) with a thickness of two atomic layers via strain engineering is demonstrated. Although these materials are stable in a rocksalt-type cubic structure and are intrinsically nonferroelectric materials, an appropriate mechanical strain can readily activate a paraelectric to ferroelectric phase transition and induce in-plane electric polarization in these atomic layers. The induced polarization magnitude can be as large as 1.90 × 10⁻¹⁰ C m⁻¹ at a biaxial strain of ε = 4.0%, which is comparable in magnitude to that of ultrathin ferroelectric SnTe. The Curie temperature T_c of the materials is also estimated using effective Hamiltonian simulations. The origin of the emerged ferroelectric phase is attributed to softening of the polar mode with applied strain. In addition, the band gaps of the crystals are found to be tunable with applied strain, which can be adjusted to the ideal value of 1.3 eV for photovoltaic applications. The results not only provide a new route to explore ferroelectricity in 2D materials but also suggest promising semiconducting ferroelectrics for solar applications.