Multiscale analysis of large twist ferroelectricity and swirling dislocations in bilayer hexagonal boron nitride

TL;DR

The paper addresses ferroelectricity in bilayer hBN under large heterodeformations, a regime challenging for atomistic simulations due to computational cost and potential inaccuracies. It develops a density-functional-theory-informed, bicrystallography-guided multiscale (BFIM) model that predicts structural reconstruction and out-of-plane polarization for arbitrary heterodeformations by coupling GSFE and polarization landscape inputs. The authors demonstrate ferroelectric domain formation for small twists and strains and extend the approach to large-twist configurations vicinal to the $\Sigma 7$ stacking, revealing smaller Burgers vectors and field-controlled domain evolution. This BFIM framework provides a computationally efficient pathway to explore ferroelectricity in large-unit-cell 2D heterostructures and can guide design of non-volatile memory devices based on van der Waals interfaces.

Abstract

With its atomically thin structure and intrinsic ferroelectric properties, heterodeformed bilayer hexagonal boron nitride (hBN) has gained prominence in next-generation non-volatile memory applications. However, studies to date have focused almost exclusively on small heterodeformations, leaving the question of whether ferroelectricity can persist under large heterodeformation entirely unexplored. In this work, we establish the crystallographic origin of ferroelectricity in bilayer hBN configurations heterodeformed relative to high-symmetry configurations such as the AA-stacking and the 21.786789 $\circ$ twisted configuration, using Smith normal form bicrystallography. We then demonstrate out-of-plane ferroelectricity in bilayer hBN across configurations vicinal to both the AA and $Σ7$ stacking. Atomistic simulations reveal that AA-vicinal systems support ferroelectricity under both small twist and small strain, with polarization switching in the latter governed by the deformation of swirling dislocations rather than the straight interface dislocations seen in the former. For $Σ7$-vicinal systems, where reliable interatomic potentials are lacking, we develop a density-functional-theory-informed continuum framework--the bicrystallography-informed frame-invariant multiscale (BFIM) model, which captures out-of-plane ferroelectricity in heterodeformed configurations vicinal to the $Σ7$ stacking. Interface dislocations in these large heterodeformed bilayer configurations exhibit markedly smaller Burgers vectors compared to the interface dislocations in small-twist and small-strain bilayer hBN. The BFIM model reproduces atomistic simulation results and provides a powerful, computationally efficient framework for predicting ferroelectricity in large-unit-cell heterostructures where atomistic simulations are prohibitively expensive.

Figures (15)

• Figure 1: Three characteristics stacking configuration in bilayer hBN. The parallelepiped marked by dotted lines identifies the smallest unit cell.
• Figure 2: Atomic reconstruction in a $0.29°$ twisted bilayer hBN. \ref{['fig:twist_relax']} Atomic energy per atom [in $\eV$] plot showing a triangular network of interface dislocations that separate the low-energy Bernal stackings. \ref{['fig:Burgers_twist']} Displacement components $u_1$ and $u_2$ [in $\angstrom$], measured along the line ① (in \ref{['fig:twist_relax']}) and relative to the AA stacking, signify the screw characteristic of dislocations in a twisted bilayer hBN.
• Figure 3: Atomic reconstruction in a $0.42\%$ equi-biaxial heterostrained bilayer hBN. \ref{['fig:strain_relax']} Atomic energy per atom [in $\eV$] plot showing a triangular network of swirling dislocations that separate the low-energy Bernal stackings. \ref{['fig:Burgers_strain']} Displacement components $u_1$ and $u_2$ [in $\angstrom$], measured along line ①(in \ref{['fig:strain_relax']}) and relative to the AA stacking, signify the mixed characteristic of dislocations under equi-biaxial heterostrains.
• Figure 4: Generalized stacking fault energy [$meV\angstrom^{-2}$] of $0^\circ$-twist bilayer hBN computed using \ref{['fig:gsfe_dft_small']} DFT \ref{['fig:gsfe_atom']} LAMMPS, and plotted as functions of the relative displacement between the two layers. The four corners correspond to the AA stacking.
• Figure 5: The AB (blue) and BA (green) domains in a relaxed $0.29^\circ$ twisted bilayer hBN subjected to a $5V\per\angstrom$ electric field in the \ref{['fig:twist_pos']}$+X_3$ direction and \ref{['fig:twist_neg']}$-X_3$ direction.