Strain Engineering for 2D Ferroelectricity in Lead Chalcogenides

Abstract

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 Tc 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.

Review on Recent Developments in 2D Ferroelectrics: Theories and Applications

“…[82] For example, strain has been predicted to soften the polar phonon mode for inducing ferroelectric polarization, which can reach ≈1.9 × 10 −10 C m −1 at a biaxial strain of 4% in centrosymmetric lead chalcogenides PbXs (X = S, Se, and Te) monolayer. [20] The NbN monolayer has been predicted to have two stable states, one is the metallic s-NbN state and the other is the non-centrosymmetric semiconductor h-NbN state with an intrinsic irreversible out-of-plane polarization due to the huge energy barrier. Considering the piezoelectricity in h-NbN, a tensile strain could be used to reduce the polar buckling and energy barrier, possibly making the strained h-NbN ferroelectric.…”

“…[ 82 ] For example, strain has been predicted to soften the polar phonon mode for inducing ferroelectric polarization, which can reach ≈1.9 × 10 −10 C m −1 at a biaxial strain of 4% in centrosymmetric lead chalcogenides PbXs (X = S, Se, and Te) monolayer. [ 20 ]…”

“…Meanwhile, various origins of ferroelectric polarization in 2D vdW ferroelectrics have been discovered. In addition, stable ferroelectric polarization can be induced even in non-ferroelectric 2D materials through various external factors such as electron/hole doping, [19] strain engineering, [20] surface functionalization, [21] and defect engineering. [22] After several years of research, many 2D ferroelectrics have been reported to exhibit special properties, such as locking between in-plane and out-of-plane ferroelectric polarization in 2D In 2 Se 3 [23] and the odd-even layer effect in few-layer SnS.…”

“…Meanwhile, various origins of ferroelectric polarization in 2D vdW ferroelectrics have been discovered. In addition, stable ferroelectric polarization can be induced even in non‐ferroelectric 2D materials through various external factors such as electron/hole doping, [ 19 ] strain engineering, [ 20 ] surface functionalization, [ 21 ] and defect engineering. [ 22 ]…”

Recent Progress in Optical Control of Ferroelectric Polarization

“…In recent years, the unexpected discovery of ferroelectricity in van der Waals layered structures has attracted much attention. [65] Van der Waals ferroelectrics are regarded as nextgeneration optoelectronic building blocks since they promise ferroelectricity persisting down to monolayer thickness, as well as flexible layer-by-layer assembly strategies regardless of lattice matching. [66] Among these emerging low-dimensional ferroelectric materials, hexagonal α-In 2 Se 3 crystal possesses robust in-plane and out-of-plane ferroelectricity at room temperature, and is more conductive than conventional ferroelectrics (such as BTO).…”