Two-Dimensional Ferroelasticity and Domain-Wall Flexoelectricity in HgX₂ (X = Br or I) Monolayers

Abstract

Electromechanical phenomena in two-dimensional (2D) materials can be related to sizable electric polarizations and switchable spontaneous ferroelasticity, allowing them to be used as miniaturized electronic and memory devices. Even in a parent centrosymmetric (nonpolar) ferroelastic (FE) material, non-zero polarization can be produced around the FE domain wall, owing to the strain-gradient-induced flexoelectricity. Compared with the negligibly weak flexoelectric effect in bulk compounds, significant electric polarizations can be expected in 2D FE materials that sustain a large elastic strain and a strain gradient. Using first-principles calculations, we predict that spontaneous 2D ferroelasticity and domain-wall flexoelectricity can be simultaneously realized in synthetic HgX2 (X = Br or I) monolayers. The FE phase renders three oriented variants, which form FE domain walls with a large strain gradient and the associated domain-wall flexoelectric polarizations. Our thermodynamic stability analysis and kinetic barrier simulations allow us to manipulate the domain-wall flexoelectricity via applied mechanical stress, thereby enabling future electromechanical applications in nanoelectronics.

Strong Sliding Ferroelectricity and Interlayer Sliding Controllable Spintronic Effect in Two-Dimensional HgI2 Layers

“…In experiment, owing to the noncentrosymmetric structural character, HgI 2 based nonlinear optical crystals have been applied for the second-order photonic processes. 51 From the atomic structural point of view, structural polarity of bulk HgX 2 along the out-of-plane c axis manifests as nonzero Hg polar displacement relative to octahedral center (d Hg tabulated in Table 1) and interlayer dipoles arise from locating Hg cations of one monolayer right above/below halogen anions from the adjacent monolayer (indicated by red arrows in Figure 1a), as a result of net sliding between two adjacent 52 vertically polarized HgX 2 bulk can be obtained after combining the centrosymmetric HgX 2 monolayers, a signature of interlayer sliding induced ferroelectricity. 26 In order to explore the potential sliding ferroelectricity within bulk HgX 2 , we first construct a high-symmetry hypothetical reference phase via periodic stacking of two centrosymmetric HgX 2 monolayers in the AA stacking sequence, where the net interlayer sliding along the b axis is absent.…”

Flexoelectricity‐Enhanced Self‐Powered Photodetection in 2D van der Waals Heterojunctions With Large Curvatures

“…However, studies on the flexoelectric effect of 2D materials remain limited. [ 33 – 35 ] Deng et al. obtained the longitudinal flexoelectric coefficient of suspended 2D‐layered materials using piezoresponse force microscopy and theoretical calculations for the first time.…”

“…However, studies on the flexoelectric effect of 2D materials remain limited. [33][34][35] Deng et al obtained the longitudinal flexoelectric coefficient of suspended 2D-layered materials using piezoresponse force microscopy and theoretical calculations for the first time. [36] Wang et al reported an equivalent piezoelectric coefficient of 21.9 pm/V in a few-layer InSe endowed by out-ofplane flexoelectric polarization.…”