Characterization of Rotational Stacking Layers in Large-Area MoSe2 Film Grown by Molecular Beam Epitaxy and Interaction with Photon

Stacking Order Engineering of Two-Dimensional Materials and Device Applications

Stacking orders in 2D van der Waals (vdW) materials dictate the relative sliding (lateral displacement) and twisting (rotation) between atomically thin layers. By altering the stacking order, many new ferroic, strongly correlated and topological orderings emerge with exotic electrical, optical and magnetic properties. Thanks to the weak vdW interlayer bonding, such highly flexible and energy-efficient stacking order engineering has transformed the design of quantum properties in 2D vdW materials, unleashing the potential for miniaturized high-performance device applications in electronics, spintronics, photonics, and surface chemistry. This Review provides a comprehensive overview of stacking order engineering in 2D vdW materials and their device applications, ranging from the typical fabrication and characterization methods to the novel physical properties and the emergent slidetronics and twistronics device prototyping. The main emphasis is on the critical role of stacking orders affecting the interlayer charge transfer, orbital coupling and flat band formation for the design of innovative materials with on-demand quantum properties and surface potentials. By demonstrating a correlation between the stacking configurations and device functionality, we highlight their implications for next-generation electronic, photonic and chemical energy conversion devices. We conclude with our perspective of this exciting field including challenges and opportunities for future stacking order engineering research.

Bipolar Resistive Switching in 2D MoSe2 Grown by Atmospheric Pressure Chemical Vapor Deposition

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) are highly promising nanomaterials for various electronic devices such as field-effect transistors, junction diodes, tunneling devices, and, more recently, memristors. 2D MoSe2 stands out for having high electrical conductivity, charge carrier mobility, and melting point. While these features make it particularly appropriate as a switching layer in memristive devices, reliable and scalable production of large-area 2D MoSe2 still represents a challenge. In this study, we manufacture 2D MoSe2 films by atmospheric-pressure chemical vapor deposition and investigate them on the atomic scale. We selected and transferred MoSe2 bilayer to serve as a switching layer between asymmetric Au-Cu electrodes in miniaturized crossbar vertical memristors. The electrochemical metallization devices showed forming-free, bipolar resistive switching at low voltages, with clearly identifiable nonvolatile states. Other than low-power neuromorphic computing, low switching voltages approaching the range of biological action potentials could unlock hybrid biological interfaces.

Thermal Conductivity of Large-Area Polycrystalline MoSe2 Films Grown by Chemical Vapor Deposition

It is of great importance to understand the thermal properties of MoSe₂ films for electronic and optoelectronic applications. In this work, large-area polycrystalline MoSe₂ films are prepared using a low-cost, controllable, large-scale, and repeatable chemical vapor deposition method, which facilitates direct device fabrication. Raman spectra and X-ray diffraction patterns indicate a hexagonal (2H) crystal structure of the MoSe₂ film. Ellipsometric spectra analysis indicates that the optical band gap of the MoSe₂ film is estimated to be ∼1.23 eV. From the analysis of the temperature-dependent and laser-power-dependent Raman spectra, the thermal conductivity of the suspended MoSe₂ films is found to be ∼28.48 W/(m·K) at room temperature. The results can provide useful guidance for an effective thermal management of large-area polycrystalline MoSe₂-based electronic and optoelectronic devices.

TMDs beyond MoS2 for Electrochemical Energy Storage

Atomically thin sheets of two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted interest as high capacity electrode materials for electrochemical energy storage devices owing to their unique properties (high surface area, high strength and modulus, faster ion diffusion, and so on), which arise from their layered morphology and diversified chemistry. Nevertheless, low electronic conductivity, poor cycling stability, large structural changes during metal-ion insertion/extraction along with high cost of manufacture are challenges that require further research in order for TMDs to find use in commercial batteries and supercapacitors. Here, a systematic review of cutting-edge research focused on TMD materials beyond the widely studied molybdenum disulfide or MoS₂ electrode is reported. Accordingly, a critical overview of the recent progress concerning synthesis methods, physicochemical and electrochemical properties is given. Trends and opportunities that may contribute to state-of-the-art research are also discussed.

Thermal transport properties of monolayer MoSe2 with defects

The defects in monolayer MoSe₂ have a significant effect on its lattice thermal conductivity.

Molecular Beam Epitaxy of Highly Crystalline MoSe2 on Hexagonal Boron Nitride

Molybdenum diselenide (MoSe₂) is a promising two-dimensional material for next-generation electronics and optoelectronics. However, its application has been hindered by a lack of large-scale synthesis. Although chemical vapor deposition (CVD) using laboratory furnaces has been applied to grow two-dimensional (2D) MoSe₂ cystals, no continuous film over macroscopically large area has been produced due to the lack of uniform control in these systems. Here, we investigate the molecular beam epitaxy (MBE) of 2D MoSe₂ on hexagonal boron nitride (hBN) substrate, where highly crystalline MoSe₂ film can be grown with electron mobility ∼15 cm²/(V s). Scanning transmission electron microscopy (STEM) shows that MoSe₂ grains grown at an optimum temperature of 500 °C are highly oriented and coalesced to form continuous film with predominantly mirror twin boundaries. Our work suggests that van der Waals epitaxy of 2D materials is tolerant of lattice mismatch but is facilitated by substrates with similar symmetry.