The increasing scientific and industry interest in 2D MX 2 materials within the field of nanotechnology has made the single crystalline integration of large area van der Waals (vdW) layers on commercial substrates an important topic. The c-plane oriented (3D crystal) sapphire surface is believed to be an interesting substrate candidate for this challenging 2D/3D integration. Despite the many attempts that have been made, the yet incomplete understanding of vdW epitaxy still results in synthetic material that shows a crystallinity far too low compared to natural crystals that can be exfoliated onto commercial substrates. Thanks to its atomic control and in situ analysis possibilities, molecular beam epitaxy (MBE) offers a potential solution and an appropriate method to enable a more in-depth understanding of this peculiar 2D/3D hetero-epitaxy. Here, we report on how various sapphire surface reconstructions, that are obtained by thermal annealing of the as-received substrates, influence the vdW epitaxy of the MBE-grown WSe 2 monolayers (MLs). The surface chemistry and the interatomic arrangement of the reconstructed sapphire surfaces are shown to control the preferential in-plane epitaxial alignment of the stoichiometric WSe 2 crystals. In addition, it is demonstrated that the reconstructions also affect the in-plane lattice parameter and thus the inplane strain of the 2D vdW-bonded MLs. Hence, the results obtained in this work shine more light on the peculiar concept of vdW epitaxy, especially relevant for 2D materials integration on large-scale 3D crystal commercial substrates.

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As interest in layered van der Waals (vdW) materials keeps increasing, fundamental knowledge about their synthesis is gaining more and more value. The defect-free heteroepitaxial integration of vdW materials on large-area substrates is currently thoroughly being researched since it might encompass a successful transition of these materials to industrial applications.To date, Transition Metal Dichalcogenides (TMDs) are considered as one of the most promising vdW materials within the field of nanoelectronics. Nevertheless, the electrical characterization of heteroepitaxially grown TMDs still shows inferior performance as compared to exfoliated TMD flakes. This is mainly attributed to the high density of defects resulting from their challenging vdW heteroepitaxial synthesis. In this work, we have investigated in depth the vdW homoepitaxial synthesis of the WSe2 TMD compound. We have demonstrated that even for homoepitaxy, the simplest type of crystal growth, challenges such as the formation of 60 o twins need to be addressed. We evidenced the presence of 60 o twins during vdW homoepitaxy which is assigned to stacking faults. The formation of these stacking faults is associated with their very similar binding energy as revealed by Density Functional Theory (DFT) calculations. Therefore, stacking faults are identified in this work to be the fundamental limitation of lowly-defective TMD vdW epitaxy. Furthermore, a generalized model is developed that determines the lower limit of the defect density based on the degree of control on the bilayer stacking phase and the nucleation density of the TMD compound. This model therefore assesses and quantifies for the first time the ultimate defect density level that can be achieved with vdW epitaxially grown 2D materials.

Layered materials held together by weak van der Waals (vdW) interactions are a promising class of materials in the field of nanotechnology. Besides the potential for single layers, stacking of various vdW layers becomes even more promising since unique properties can hence be precisely engineered. The synthesis of stacked vdW layers, however, remains to date, hardly understood. Therefore, in this work, the vdW epitaxy of transition metal dichalcogenides (TMDs) on single-crystalline TMD templates is investigated in depth. It is demonstrated that the role of lattice mismatch is insignificant. More importantly is the role of surface energy, calculated using density functional theory, which plays an essential role in the activation energy for adatom diffusion, hence nucleation density. This in turn correlates with defect density since the stacking sequence in vdW epitaxy is generally poorly controlled. Moreover, the vapor pressure of the transition metal is also found to correlate with adatom diffusion. Consequently, the proposed study enables important and new insight in the vdW epitaxy of multilayer 2D homo-/heterostructures.

We present in this paper the use of Gas Source Molecular Beam Epitaxy for the large-scale growth of transition metal dichalcogenides. Fiber-textured MoS2 co-deposited thin films (down to 1 MLs) are grown on commercially 200 mm wafer size templates where MX2 crystalline layers are achieved at temperatures ranging from RT to 550 °C. Raman Spectroscopy and photoluminescence measurements along with X-Ray Photoelectron Spectroscopy show that a low growth rate is essential for complete Mo sulfurization during MoS2 co-deposition. Finally, cross-section Transmission Electron Microscopy investigations are discussed to highlight the influence of SiO2 and Al2O3 used surfaces on MoS2 deposition.

Two-dimensional (2D) material research is rapidly evolving to broaden the spectrum of emergent 2D systems. Here, we review recent advances in the theory, synthesis, characterization, device, and quantum physics of 2D materials and their heterostructures. First, we shed insight into modeling of defects and intercalants, focusing on their formation pathways and strategic functionalities. We also review machine learning for synthesis and sensing applications of 2D materials. In addition, we highlight important development in the synthesis, processing, and characterization of various 2D materials (e.g., MXnenes, magnetic compounds, epitaxial layers, low-symmetry crystals, etc.) and discuss oxidation and strain gradient engineering in 2D materials. Next, we discuss the optical and phonon properties of 2D materials controlled by material inhomogeneity and give examples of multidimensional imaging and biosensing equipped with machine learning analysis based on 2D platforms. We then provide updates on mix-dimensional heterostructures using 2D building blocks for next-generation logic/memory devices and the quantum anomalous Hall devices of high-quality magnetic topological insulators, followed by advances in small twist-angle homojunctions and their exciting quantum transport. Finally, we provide the perspectives and future work on several topics mentioned in this review.

In this paper, we explore the impact of changing the growth conditions on the substrate surface during the metal-organic vapor phase epitaxy of 2D-transition metal dichalcogenides. We particularly study the growth of molybdenum disulfide (MoS 2 ) on sapphire substrates at different temperatures. We show that a high temperature leads to a perfect epitaxial alignment of the MoS 2 layer with respect to the sapphire substrate underneath, whereas a low temperature growth induces a 30°epitaxial alignment. This behavior is found to be related to the different sapphire top surface re-arrangement under H 2 S environment at different growth temperatures. Structural analyses conducted on the different samples confirm an improved layer quality at high temperatures. MoS 2 channel-based metal-oxide-semiconductor field-effect transistors are fabricated showing improved device performance with channel layers grown at high temperature.

Two-dimensional transition metal dichalcogenide (TMD) semiconductors have risen as an important material class for novel nanoelectronic applications. Molybdenum disulfide (MoS2) is the most representative TMD compound due to its superior stability and attractive properties for (opto-) electronic devices. However, the synthesis of single-crystalline and functional MoS2 across large-area substrates remains crucial for its successful integration in semiconductor industry platforms. Therefore, this work focuses on the study of MoS2 epitaxy via two well-established industry-compatible synthesis methods, promising for the large-area and single-crystalline integration of van der Waals (vdW) materials. These methods are molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE) and have studied MoS2 quasi-vdW heteroepitaxy on reconstructed sapphire substrates and MoS2 vdW homoepitaxy on exfoliated MoS2 flakes. By examining the MoS2 structural properties using diffraction and spectroscopy techniques, the epitaxial relation and crystal quality are assessed, which reveals insights into the prevalence of inter- and intragrain defects such as grain boundaries and sulfur vacancies. The MBE method yields superior epitaxial MoS2 registry on both sapphire and MoS2 surfaces as compared to MOVPE, although inferior defectivity arises from the typical lower MBE growth temperature and chalcogen partial pressure. Moreover, both synthesis methods generate high densities of twinned MoS2 grain boundaries, which hamper defect-free integration. As a result, this challenging integration might become an important bottleneck for industrial TMD-based applications with a low tolerance for material defects.