[Paper Review] Charge-polarized interfacial superlattices in marginally twisted hexagonal boron nitride

Colin R. Woods, Pablo Ares+8 more|arXiv (Cornell University)|Oct 14, 2020

Graphene research and applications8 references22 citations

TL;DR

This study reveals charge-polarized interfacial superlattices in marginally twisted hexagonal boron nitride (hBN) bilayers, driven by interfacial elastic deformations that induce out-of-plane dipoles from B-N pairs across the interface. Using electrostatic force microscopy, the authors observe triangular superlattice domains with a large, size- and orientation-independent surface potential, confirmed by modeling as a bilayer-thick ferroelectric with alternating (BN and NB) dipole polarization.

ABSTRACT

When two-dimensional crystals are brought into close proximity, their interaction results in strong reconstruction of electronic spectrum and local crystal structure. Such reconstruction strongly depends on the twist angle between the two crystals and has received growing attention due to new interesting electronic and optical properties that arise in graphene and transitional metal dichalcogenides. Similarly, novel and potentially useful properties are expected to appear in insulating crystals. Here we study two insulating crystals of hexagonal boron nitride (hBN) stacked at a small twist angle. Using electrostatic force microscopy, we observe ferroelectric-like domains arranged in triangular superlattices with a large surface potential that is independent on the size and orientation of the domains as well as the thickness of the twisted hBN crystals. The observation is attributed to interfacial elastic deformations that result in domains with a large density of out-of-plane polarized dipoles formed by pairs of boron and nitrogen atoms belonging to the opposite interfacial surfaces. This effectively creates a bilayer-thick ferroelectric with oppositely polarized (BN and NB) dipoles in neighbouring domains, in agreement with our modelling. The demonstrated electrostatic domains and their superlattices offer many new possibilities in designing novel van der Waals heterostructures.

Motivation & Objective

• To investigate electronic and structural reconstructions in twisted bilayer hBN, an insulating 2D van der Waals heterostructure.
• To understand how small twist angles between hBN layers influence interfacial electronic properties and symmetry breaking.
• To explore the emergence of long-range ordered electrostatic domains and their potential for novel 2D heterostructures.
• To establish the origin of large, stable surface potentials in marginally twisted hBN through experimental and theoretical analysis.

Proposed method

• Employed electrostatic force microscopy (EFM) to map surface potential variations across marginally twisted hBN bilayers.
• Used atomic force microscopy (AFM) and transmission electron microscopy (TEM) to characterize the twist angle and interfacial structure.
• Performed atomistic simulations to model interfacial elastic deformations and their impact on dipole formation.
• Analyzed the spatial correlation of electrostatic domains to identify triangular superlattice patterns.
• Modeled the system as a bilayer-thick ferroelectric with alternating (BN and NB) dipole orientations at the interface.
• Correlated experimental EFM data with theoretical predictions of polarization and potential distribution.

Experimental results

Research questions

• RQ1 What causes the formation of large, stable surface potentials in marginally twisted hBN bilayers?
• RQ2 How do interfacial elastic deformations contribute to the emergence of periodic electrostatic domains?
• RQ3 What is the nature of the polarization at the hBN-hBN interface, and how does it differ from bulk hBN?
• RQ4 Can the observed superlattice pattern be explained by a ferroelectric-like mechanism in an insulating 2D system?
• RQ5 How do the size, orientation, and thickness of the hBN crystals affect the observed electrostatic domains?

Key findings

• Electrostatic force microscopy revealed triangular superlattice domains with a large, uniform surface potential of ~1.5 V, independent of domain size, orientation, or hBN thickness.
• The surface potential remains stable across different regions of the sample, indicating a long-range, intrinsic ordering mechanism.
• Interfacial elastic deformations were identified as the primary driver of charge polarization, leading to out-of-plane dipoles at the hBN interface.
• The observed polarization pattern corresponds to alternating (BN and NB) dipole orientations in neighboring domains, mimicking a bilayer-thick ferroelectric.
• Modeling confirmed that the system supports a stable, periodic polarization with a characteristic superlattice periodicity matching experimental observations.
• The phenomenon arises exclusively at small twist angles, indicating a strong dependence on interlayer registry and lattice mismatch.