Stacking and Twisting of Layered Materials Enabled by Screw Dislocations and Non-Euclidean Surfaces

Yuzhou Zhao,Song Jin

doi:10.1021/accountsmr.1c00245

Abstract

ConspectusThe rise of van der Waals layered materials such as graphene, hexagonal boron nitride, and transition-metal dichalcogenides opens up enormous opportunities for exploring novel quantum phenomena at the two-dimensional (2D) atomic limit. The physical properties of van der Waals materials are often affected by the symmetry of the crystal lattices. For example, breaking the inversion symmetry can lead to a variety of phenomena, such as second-harmonic generation, valley polarization, and ferroelectricity. The symmetry and symmetry breaking of layered materials are determined by the atomic arrangements within the layer and the layer stackings. The screw-dislocation-driven growth mechanism is general to 2D materials and can provide effective kinetic pathways to influence the layer stacking and generate diverse and complex layer stackings with different symmetry.Furthermore, different van der Waals layered materials can also be stacked vertically to create artificial new structures. In such stacked structures, the interlayer twist angle between stacked layers results in the formation of large-scale moiré superlattices that manipulate the electronic structures of van der Waals materials and provide an additional degree of freedom for tuning their physical properties. The stacking and twisting of layered materials lead to observations of new quantum phenomena, such as unconventional superconductivity, tunable Mott insulators, moiré excitons, and various magnetic and ferroelectric orderings. Previously, such twisted 2D structures were often fabricated by mechanically stacking exfoliated layers, but it was recently demonstrated that twisted van der Waals structures can form via direct growth. Moreover, continuously twisted structures of 2D materials can be realized through two distinct mechanisms that involve screw dislocations. The Eshelby twist mechanism induced by the strain of screw dislocation gives rise to twisted van der Waals nanowires with small interlayer twists. Beyond the Eshelby twist mechanism, supertwisted spirals have recently been enabled by a non-Euclidean twist mechanism which arises from the mismatched geometry between van der Waals crystals and non-Euclidean (curved) surfaces.In this Account, we start with reviewing the stacking configurations in the diverse polytypic structures of layered materials using transition-metal dichalcogenides as examples. We further discuss the twisted structures of layered materials from a structural point of view. After introducing screw-dislocation-driven growth and showing its generality in the crystal growth of layered materials, we further discuss how the unique structures of screw dislocations influence the stacking and symmetry of layered materials. Moreover, we highlight how screw dislocations enable interlayer twisting through the Eshelby twist mechanism and the non-Euclidean twist mechanism. In the end, the challenges and future perspectives are discussed for the study of screw-dislocated layered materials to control the stacking and twist for exotic physical properties.

Highlights

Layered materials such as graphene, hexagonal boron nitride, and transition-metal dichalcogenides are characterized by dangling-bond-free covalently bonded 2D layers held together by weak out-of-plane van der Waals forces and are given the name van der Waals materials.[1]
The breaking of inversion symmetry can lead to a variety of phenomena, such as even-order nonlinear optical effects, valley polarization, ferroelectricity, and so on.[2]
We have shown that screw-dislocation-driven growth is a general growth mechanism for layered materials

Summary

■ INTRODUCTION

Layered materials such as graphene, hexagonal boron nitride, and transition-metal dichalcogenides are characterized by dangling-bond-free covalently bonded 2D layers held together by weak out-of-plane van der Waals forces and are given the name van der Waals materials.[1]. The selected nanobeam electron diffraction patterns at positions along the nanowire show progressive twisting of the crystal lattice along the nanowire axis, which in theory forms twisted moiré patterns along the nanowire (Figure 5c) that progressively change the optoelectronic properties detected by locally excited optical spectroscopy.[12] In addition, a portion of the nanowire with the Eshelby twist is pinned by the substrate on both ends, which constrains its freedom to untwist when the nanowire radius becomes larger, as predicted by Eshelby theory.[11] Further radial growth on those nanowires results in mesoscale GeS with discretized twisting sections and the formation of twist boundaries (Figure 5d), in which the increasing dislocation strain energy is relieved through the rotational slip of van der Waals layers. Article wide range and in theory is exclusively determined by the substrate geometry, regardless of the size of the 2D crystals, which allows the twisting structures to scale up to any lateral dimension

■ CONCLUSIONS AND OUTLOOK

■ REFERENCES