Atomic reconstruction in twisted bilayers of transition metal dichalcogenides (1911.12664v2)

Abstract: Van der Waals heterostructures form a massive interdisciplinary research field, fueled by the rich material science opportunities presented by layer assembly of artificial solids with controlled composition, order and relative rotation of adjacent atomic planes. Here we use atomic resolution transmission electron microscopy and multiscale modeling to show that the lattice of MoS$_2$ and WS$_2$ bilayers twisted to a small angle, $\theta<3^{\circ}$, reconstructs into energetically favorable stacking domains separated by a network of stacking faults. For crystal alignments close to 3R stacking, a tessellated pattern of mirror reflected triangular 3R domains emerges, separated by a network of partial dislocations which persist to the smallest twist angles. Scanning tunneling measurements show that the electronic properties of those 3R domains appear qualitatively different from 2H TMDs, featuring layer-polarized conduction band states caused by lack of both inversion and mirror symmetry. In contrast, for alignments close to 2H stacking, stable 2H domains dominate, with nuclei of an earlier unnoticed metastable phase limited to $\sim$ 5nm in size. This appears as a kagome-like pattern at $\theta\sim 1^{\circ}$, transitioning at $\theta\rightarrow 0$ to a hexagonal array of screw dislocations separating large-area 2H domains.

• The paper finds that at small twist angles, TMD bilayers reconstruct into defined stacking domains with distinct electronic properties.
• It employs high-resolution STEM and vdW-DFT modeling to quantify interlayer binding energies and domain structures in MoS₂ and WS₂ bilayers.
• The reconstruction process enables tunable electronic features, guiding the design of advanced optoelectronic devices based on TMDs.

Atomic Reconstruction in Twisted Bilayers of Transition Metal Dichalcogenides

This paper presents a detailed investigation into the atomic reconstruction phenomena observed in twisted bilayer systems of transition metal dichalcogenides (TMDs), specifically focusing on MoS$_2$ and WS$_2$. The paper employs atomic resolution scanning transmission electron microscopy (STEM) and multiscale modeling to elucidate the formation of stacking domains and the emergence of distinct electronic properties at small twist angles within these bilayers.

The key aspect under investigation is the reconstruction of the atomic lattice when the TMD bilayers are twisted at a small angle ($\theta<3^\circ$). Under such conditions, the system reorganizes into energetically favorable stacking domains, delineated by a network of stacking faults. The authors observe that for alignments tending towards the 3R stacking configuration, the system exhibits a tessellated arrangement of mirror-reflected triangular domains, bordered by partial dislocation networks that persist at the smallest twist angles. Interestingly, these 3R domains reveal electronic properties distinct from those of the 2H stacked TMDs, due to the asymmetrical atomic configuration leading to layer-polarized electronic states.

In contrast, when the bilayers are aligned close to the 2H stacking configuration, the authors identify stable domains dominated by the 2H phase, punctuated by small (~5 nm) regions of a previously undetected metastable phase. At approximately $\theta \sim 1^\circ$, these metastable regions present a kagome-like pattern, which evolves into a hexagonal array of screw dislocations as $\theta$ approaches zero. This reconstructive behavior is indicative of a strong dependence of the electronic band structure on local atomic configurations within the bilayers, driven by the intricate interplay between interlayer interactions and lattice mismatch.

The experimental observations are supported by complementary theoretical modeling. Interlayer binding energy densities were computed using van der Waals density functional theory (vdW-DFT), offering insights into the energetics of the atomic domains formed. For P-oriented bilayers, the simulation predicts sizable domains of 3R stacking, supporting the experimental findings of asymmetry in electronic wavefunctions.

The implications of these phenomena are profound, given the intrinsic differences in electronic characteristics between 2H and 3R stacked structures. The paper opens avenues for tuning the electronic properties of TMD bilayers via manipulation of twist angles and stacking configurations. Practically, these insights could inform the design of novel electronic and optoelectronic devices based on TMDs, where layer polarization and induced dislocations could be harnessed for targeted functionality.

Future directions might explore further the dynamics of dislocation networks and their potential for enabling novel electronic phases. Additionally, expanding this research to explore other TMDs and heterostructures could yield a deeper understanding of moiré superlattice effects and their influence on material properties. The fundamental discoveries articulated in this paper lay the groundwork for advancing applications in flexible electronics and nano-engineered systems, where control at the atomic layer level is paramount.