Unveiling the Enigma: Exploring the Intricacies of 2D Material Interfaces

In a recent breakthrough, scientists have accentuated the importance of vertically stacking two-dimensional materials, especially when a subtle twist angle is introduced. This seemingly small twist leads to remarkable physical phenomena, heralding a new era of possibilities in the domain of twisted electronics.

Researchers have devised a novel methodology for scrutinizing the internal structures within vertically stacked two-dimensional materials. Through their endeavors, they have unveiled atomic reconstructions that exert a profound influence on the materials’ physical properties. This groundbreaking research holds the promise of deepening our comprehension and application of twisted electronics.

The act of vertically stacking two-dimensional (2D) materials to create van der Waals homo- or hetero-structures has emerged as a potent technique for modulating their physical and mechanical characteristics. It is particularly intriguing that when a minor twist angle is introduced at the interface of these stacked 2D structures, they often exhibit a plethora of captivating physical phenomena, owing to the distinctive interlayer coupling that occurs.

Take, for instance, the case of bilayer graphene with a slight twist angle. At the twisted interface, a spontaneous atomic reconstruction takes place. This phenomenon arises from the interplay between interlayer stacking energy and intralayer elastic strain energy, as depicted schematically in Figure 1.

Figure 1. Schematic representations of the atomic structures before and after the reconstruction of twisted bilayer graphene. Credit: ©Science China Press

This peculiar stacked configuration leads to a host of unexpected phenomena, including the emergence of a Mott insulating state, unconventional superconductivity, and spontaneous ferromagnetism. Recent revelations indicate that twisted interfaces are not limited to surface layers; they can also be embedded within the van der Waals structures, potentially giving rise to even richer physical behaviors.

These intriguing 2D architectures exhibit a heightened sensitivity to the stacking state of their internal layers and interfaces. Unfortunately, the precise characterization of embedded stacking structures remains a formidable challenge. Moreover, whether these embedded twisted interfaces undergo atomic reconstruction and the ramifications of such reconstruction on neighboring atomic layers and the overall stacked units are questions that continue to pique scientific curiosity.

Enter the Breakthrough Research

To address these enigmatic questions, Professor Qunyang Li’s group at Tsinghua University and Professor Ouyang Wengen’s group at Wuhan University have pioneered a novel approach based on conductive atomic force microscopy (c-AFM). This method enables the characterization and reconstruction of the internal stacking state of twisted layered materials through straightforward surface conductivity measurements. The results of their rigorous experimentation indicate that even when embedded 10 atomic layers below the surface, twisted interfaces can still undergo atomic reconstruction, significantly influencing surface conductivity, as depicted in Figure 2.

For a more comprehensive understanding of the atomic structure of these twisted multilayer systems, a molecular dynamics (MD) simulation model was constructed, closely mirroring the experimental samples. This simulation unveiled that small-angle twisted interfaces, when nestled within the material’s interior, indeed experience atomic reconstruction. This reconstruction, in turn, promotes in-plane rotational deformation of adjacent graphene layers. However, this rotational deformation diminishes as one moves away from the twisted interface, as illustrated in Figure 2.

The Proposed Model and Its Implications

Drawing upon the atomic structures elucidated through MD simulations, the research group introduced a groundbreaking Series Spreading Resistance model (SSR model). This model quantifies the impact of the stacking state of twisted multilayer systems on their surface conductivity. Remarkably, it allows for a direct correlation between surface conductivity and the internal stacking structure, even in the presence of complex crystal defects such as dislocations.

This pioneering research presents a straightforward, convenient, and high-resolution means to scrutinize the internal stacking structures of twisted layered materials. Such insights are pivotal for fundamental inquiries into 2D stacked structures and the advancement of emerging technologies in the realm of twisted electronics.

Reference: “Deducing the internal interfaces of twisted multilayer graphene via moiré-regulated surface conductivity” by Huan Wang, Sen Wang, Shuai Zhang, Mengzhen Zhu, Wengen Ouyang, and Qunyang Li, published on June 19, 2023, in National Science Review. DOI: 10.1093/nsr/nwad175

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• National Science Review: The original research paper titled “Deducing the internal interfaces of twisted multilayer graphene via moiré-regulated surface conductivity” by Huan Wang, Sen Wang, Shuai Zhang, Mengzhen Zhu, Wengen Ouyang, and Qunyang Li, published in National Science Review.