Atomic reconstruction and moiré patterns in transition metal dichalcogenide van der Waals heterostructures (1911.12282v1)

Abstract: Van der Waals layered materials, such as transition metal dichalcogenides (TMDs), are an exciting class of materials with weak interlayer bonding which enables one to create van der Waals heterostructures (vdWH). Recent work has shown that control of the twist angle between layers can have a dramatic effect on vdWH properties. For TMD vdWH, twist angle has been treated solely through the use of rigid-lattice moir\'e patterns. No atomic reconstruction, i.e. any rearrangement of atoms within the individual layers, has been reported experimentally to date. Here we demonstrate that vdWH of MoSe2/WSe2 and MoS2/WS2 at twist angles less than 1{\deg} undergo significant atomic level reconstruction leading to discrete commensurate domains divided by narrow domain walls, rather than a smoothly varying rigid-lattice moir\'e pattern as has been assumed in prior work. Using conductive atomic force microscopy (CAFM), we show that the stacking orientation of the two TMD crystals has a profound impact on the atomic reconstruction, consistent with recent theoretical work on graphene/graphene and MoS2/MoS2 structures and experimental work on graphene bilayers and hBN/graphene vdWH. Transmission electron microscopy (TEM) provides additional evidence of atomic reconstruction in MoSe2/WSe2 structures and demonstrates the transition between a rigid-lattice moir\'e pattern for large angles and atomic reconstruction for small angles. We use density functional theory to calculate the band structures of the commensurate reconstructed domains and find that the modulation of the relative electronic band edges is consistent with the CAFM results and photoluminescence spectra from reconstructed vdWH. The presence of atomic reconstruction in TMD heterostructures and the observed impact on nanometer-scale electronic properties provides fundamental insight into the behavior of this important class of heterostructures.

Overview of Atomic Reconstruction and Moiré Patterns in TMD van der Waals Heterostructures

The paper focuses on the detailed paper of the atomic reconstruction and moiré patterns in transition metal dichalcogenide (TMD) van der Waals heterostructures (vdWH), with primary attention given to MoSe<sub\>2</sub>/WSe<sub\>2</sub> and MoS<sub\>2</sub>/WS<sub\>2</sub> systems. This research builds on the understanding that the control of twist angles in TMDs has significant implications for the electronic and optoelectronic properties of the materials. Traditionally, the effects of these twist angles have been understood within the framework of rigid-lattice moiré patterns without considering atomic-level reconstructions. The paper challenges this conventional model by revealing the significant reconstruction that occurs at twist angles ≤ 1°, leading to the formation of distinct commensurate domains.

The paper employs multiple techniques to investigate these phenomena, including conductive atomic force microscopy (CAFM), transmission electron microscopy (TEM), and density functional theory (DFT) calculations. These methodologies collectively highlight the presence of atomic reconstruction and provide a comprehensive understanding of its effects on the properties of vdWH.

Key Findings and Methodologies

1. Atomic Reconstruction Evidence: The authors present both CAFM and TEM data to demonstrate the presence of atomic reconstruction at small twist angles (≤ 1°). While CAFM showcases conductive differences between triangular domains (0° alignment) and hexagonal domains (60° alignment) indicative of different atomic structures, TEM corroborates these findings by showing clear transitions between regions exhibiting rigid-lattice moiré patterns and those showing reconstructed lattice domains.
2. Stacking Energy and Domain Formation: The DFT calculations reveal that for twist angles ≤ 1°, stacking orientations such as AB, BA, and ABBA represent global minima in interlayer stacking energy, promoting the formation of large, stable domains. The CAFM data validates this finding by showcasing different conductivity levels attributed to different stacking orders, with DFT indicating varying band edge positions for each stacking configuration.
3. Optical Properties and Band Structure: Photoluminescence (PL) measurements offer additional insights into the impact of atomic reconstruction on optical properties. A significant shift in emission wavelengths between 0° and 60° structures supports the alteration in interlayer exciton behavior due to varying bandgaps, as predicted by DFT.

Implications and Future Directions

The discovery of atomic reconstruction at specific twist angles advances our understanding of vdWH beyond the traditional rigid-lattice moiré model. This phenomenon has profound implications for the band structure, conductivity, and optical properties of these materials, impacting potential applications in electronics and optoelectronics. Recognizing the importance of atomic-level rearrangements also prompts reconsiderations of theoretical models and calls for more sophisticated descriptions in simulations and predictions of vdWH systems.

Future research directions may encompass exploring a broader spectrum of TMD materials and other heterostructures beyond TMD-TMD combinations. Additionally, in-depth theoretical investigations could further elucidate the conditions for atomic reconstruction and its impact across various domains and materials. A more intricate understanding of interface quality could drive the development of methods to enhance TMD vdWH fabrication for practical applications, potentially unlocking new functionalities beyond current electronic and optoelectronic limits.