Charge-polarized interfacial superlattices in marginally twisted hexagonal boron nitride

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.

Charge-Polarized Interfacial Superlattices in Marginally Twisted Hexagonal Boron Nitride

The paper "Charge-Polarized Interfacial Superlattices in Marginally Twisted Hexagonal Boron Nitride" provides a compelling study of the unique electrostatic properties induced by marginally twisted hexagonal boron nitride (hBN) crystals. This research emphasizes the importance of twist angle in controlling the interfacial charge polarization in van der Waals (vdW) heterostructures, which has significant implications for the engineering of novel electronic and optical devices.

In this study, the authors systematically explore the ferroelectric-like behavior emerging in hBN when two monolayers are stacked at a marginal (small) twist angle. The authors employ electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM) to examine the surface potential of these structures. Remarkably, they observe the formation of triangular domains characterized by a large density of out-of-plane polarized dipoles. These domains manifest as a charge-polarized superlattice and are independent of domain size, shape, and orientation as well as the thickness of the hBN layers, confirming the robustness of this interfacial phenomenon.

Experimental data reveal that the relative alignment between boron and nitrogen atoms at the interface plays a crucial role in the polarized series of dipoles that contribute to the ferroelectric-like behavior observed. Particularly, the parallel alignment of hBN layers at small angles results in a significant charge density modulation. In contrast, the antiparallel alignment yields negligible interfacial charge polarization, thereby emphasizing the significance of parallel alignment in inducing polarized states.

The research also contributes a theoretical framework by employing tight-binding calculations to simulate charge distributions and validate experimental findings. The calculations reveal a substantial charge polarization occurring at the interface even without explicit lattice relaxation, enhancing the potential role of twisted hBN as an emergent ferroelectric system.

Implications of this research are profound across both theoretical and applied domains. The discovery of interfacial charge polarization between two insulating hBN layers offers a novel platform for superlattice phenomenon exploration. It further suggests potential directions for leveraging charge-polarized twisted superlattices in conceiving advanced heterostructures with tailored electronic and optical properties. The ability to induce substantial interfacial charge density modulations via marginal twisting heralds exciting possibilities for creating programmable surface potentials, particularly useful in the manipulation of adjacent 2D materials like graphene. This could lead to significant advancements in fields such as quantum electronics, superconductivity, and spintronics.

Future developments may extend this line of inquiry, exploring larger arrays of materials and varying stacking configurations to tune the interfacial properties further. Additionally, advanced computational methods could help in understanding the precise mechanics of interlayer interaction strength, further enhancing our grasp of the spontaneous charge polarization observed.

Overall, this paper delivers crucial insights into the exploitation of twist-induced periodic charge polarization in vdW heterostructures, presenting new vistas for future research and technology in two-dimensional materials.