Effect of the Substrate on MoS2 Monolayer Morphology: An Integrated Computational and Experimental Study

Direct Measurement of the Thermal Expansion Coefficient of Epitaxial WSe2 by Four-Dimensional Scanning Transmission Electron Microscopy

Current reports of thermal expansion coefficients (TEC) of two-dimensional (2D) materials show large discrepancies that span orders of magnitude. Determining the TEC of any 2D material remains difficult due to approaches involving indirect measurement of samples that are atomically thin and optically transparent. We demonstrate a methodology to address this discrepancy and directly measure TEC of nominally monolayer epitaxial WSe2 using four-dimensional scanning transmission electron microscopy (4D-STEM). Experimentally, WSe2 from metal-organic chemical vapor deposition (MOCVD) was heated through a temperature range of 18-564 °C using a barrel-style heating sample holder to observe temperature-induced structural changes without additional alterations or destruction of the sample. By combining 4D-STEM measurements with quantitative structural analysis, the thermal expansion coefficient of nominally monolayer polycrystalline epitaxial 2D WSe2 was determined to be (3.5 ± 0.9) × 10-6 K-1 and (5.7 ± 2) × 10-5 K-1 for the in- and out-of-plane TEC, respectively, and (3.6 ± 0.2) × 10-5 K-1 for the unit cell volume TEC, in good agreement with historically determined values for bulk crystals.

Combined Experimental and Computational Insight into the Role of Substrate in the Synthesis of Two-Dimensional WSe2

Synthesis of large-area transition-metal dichalcogenides (TMDs) with controlled orientation is a significant challenge to their industrial applications. Substrate plays a vital role in determining the final quality of monolayer materials grown via the chemical vapor deposition process by controlling their orientation, crystal structure, and grain boundary. This study determined the binding energy and equilibrium distance for tungsten diselenide (WSe2) monolayers on crystalline and amorphous silicon dioxide and aluminum dioxide substrates. Differently oriented WSe2 monolayers are considered to investigate the role of the substrate in the orientation, binding strength, and equilibrium distance. This study can pave the way to synthesizing high-quality two-dimensional (2D) materials for electronic and chemical applications.

Large-area single-crystal TMD growth modulated by sapphire substrates

Transition metal dichalcogenides (TMDs) have recently attracted extensive attention due to their unique physical and chemical properties; however, the preparation of large-area TMD single crystals is still a great challenge. Chemical vapor deposition (CVD) is an effective method to synthesize large-area and high-quality TMD films, in which sapphires as suitable substrates play a crucial role in anchoring the source material, promoting nucleation and modulating epitaxial growth. In this review, we provide an insightful overview of different epitaxial mechanisms and growth behaviors associated with the atomic structure of sapphire surfaces and the growth parameters. First, we summarize three epitaxial growth mechanisms of TMDs on sapphire substrates, namely, van der Waals epitaxy, step-guided epitaxy, and dual-coupling-guided epitaxy. Second, we introduce the effects of polishing, cutting, and annealing processing of the sapphire surface on the TMD growth. Finally, we discuss the influence of other growth parameters, such as temperature, pressure, carrier gas, and substrate position, on the growth kinetics of TMDs. This review might provide deep insights into the controllable growth of large-area single-crystal TMDs on sapphires, which will propel their practical applications in high-performance nanoelectronics and optoelectronics.

A Modified Embedded-Atom Method Potential for a Quaternary Fe-Cr-Si-Mo Solid Solution Alloy

Ferritic-martensitic steels, such as T91, are candidate materials for high-temperature applications, including superheaters, heat exchangers, and advanced nuclear reactors. Considering these alloys' wide applications, an atomistic understanding of the underlying mechanisms responsible for their excellent mechano-chemical properties is crucial. Here, we developed a modified embedded-atom method (MEAM) potential for the Fe-Cr-Si-Mo quaternary alloy system-i.e., four major elements of T91-using a multi-objective optimization approach to fit thermomechanical properties reported using density functional theory (DFT) calculations and experimental measurements. Elastic constants calculated using the proposed potential for binary interactions agreed well with ab initio calculations. Furthermore, the computed thermal expansion and self-diffusion coefficients employing this potential are in good agreement with other studies. This potential will offer insightful atomistic knowledge to design alloys for use in harsh environments.

A critical review of fabrication challenges and reliability issues in top/bottom gated MoS2field-effect transistors

The voyage of semiconductor industry to decrease the size of transistors to achieve superior device performance seems to near its physical dimensional limitations. The quest is on to explore emerging material systems that offer dimensional scaling to match the silicon- based technologies. The discovery of atomic flat two-dimensional materials has opened up a completely new avenue to fabricate transistors at sub-10 nanometer level which has the potential to compete with modern silicon-based semiconductor devices. Molybdenum disulfide (MoS₂) is a two-dimensional layered material with novel semiconducting properties at atomic level seems like a promising candidate that can possibly meet the expectation of Moore's law. This review discusses the various 'fabrication challenges' in making MoS₂based electronic devices from start to finish. The review outlines the intricate challenges of substrate selection and various synthesis methods of mono layer and few-layer MoS₂. The review focuses on the various techniques and methods to minimize interface defect density at substrate/MoS₂interface for optimum MoS₂-based device performance. The tunable band-gap of MoS₂with varying thickness presents a unique opportunity for contact engineering to mitigate the contact resistance issue using different elemental metals. In this work, we present a comprehensive overview of different types of contact materials with myriad geometries that show a profound impact on device performance. The choice of different insulating/dielectric gate oxides on MoS₂in co-planar and vertical geometry is critically reviewed and the physical feasibility of the same is discussed. The experimental constraints of different encapsulation techniques on MoS₂and its effect on structural and electronic properties are extensively discussed.

Role of tilt grain boundaries on the structural integrity of WSe2 monolayers

Grain boundaries in transition metal dichalcogenides have a profound effect on their characteristics.