Negative piezoelectricity in low--dimensional materials

Abstract

In this study, we investigate the underlying mechanisms of the negative piezoelectricity in lowdimensional materials by carrying out first-principles calculations. Two-dimensional ferroelectric CuInP2S6 is analyzed in detail as a typical example, but the theory can be applied to all other lowdimensional piezoelectrics. Similar to three-dimensional piezoelectrics with negative piezoelectric responses, the anomalous negative piezoelectricity in CuInP2S6 results from its negative clampedion term, which cannot be compensated by the positive internal strain part. Here, we propose a more general rule that having a negative clamped-ion term should be universal among piezoelectric materials, which is attributed to the "lag of Wannier center" effect. The internal-strain term, which is the change in polarization due to structural relaxation in response to strain, is mostly determined by the spatial structure and chemical bonding of the material. In a low-dimensional piezoelectric material as CuInP2S6, the internal-strain term is approximately zero. This is because the internal structure of the molecular layers, which are bonded by the weak van der Waals interaction, responds little to the strain. As a result, the magnitude of the dipole, which depends strongly on the dimension and structure of the molecular layer, also has a small response with respect to strain. An equation bridging the internal strain responses in low-dimensional and three-dimensional piezoelectrics is also derived to analytically express this point. This work aims to deepen our understanding about this anomalous piezoelectric effect, especially in low-dimensional materials, and provide strategies for discovering materials with novel electromechanical properties.

Concurrence of negative in-plane piezoelectricity and photocatalytic properties in 2D ScAgP2S6 monolayers

“…In 2019, You et al [34] performed quasi-static and dynamic electro strain measurements on multilayered 2D CuInP 2 S 6 , which is held together by weak van der Waals forces and the piezoelectric coefficient was found to be d 33 ≈ −95 pC/N. In 2020, Qi et al [35] has explained theoretically the origin of negative piezoelectric coefficient in 2D materials. Similar to the three-dimensional piezoelectrics with negative piezoelectric coefficients, the same in CuInP 2 S 6 (CIPS) was also shown to originate from a negative clamped-ion and an approximately zero internal strain term.…”