Growth of Monolayer and Multilayer MoS2 Films by Selection of Growth Mode: Two Pathways via Chemisorption and Physisorption of an Inorganic Molecular Precursor

Temperature-Dependent Structural and Electrical Properties of Metal-Organic CVD MoS2 Films

Metal-Organic CVD method (MOCVD) allows for deposition of ultrathin 2D transition metal dichalcogenides (TMD) films of electronic quality onto wafer-scale substrates. In this work, the effect of temperature on structure, chemical states, and electronic qualities of the MOCVD MoS2 films were investigated. The results demonstrate that the temperature increase in the range of 650 °C to 950 °C results in non-monotonic average crystallite size variation. Atomic force microscopy (AFM), transmission electron microscopy (TEM), and Raman spectroscopy investigation has established the film crystal structure improvement with temperature increase in this range. At the same time, X-Ray photoelectron spectroscopy (XPS) method allowed to reveal non-stoichiometric phase fraction increase, corresponding to increased sulfur vacancies (VS) concentration from approximately 0.9 at.% to 3.6 at.%. Established dependency between the crystallite domains size and VS concentration suggests that these vacancies are form predominantly at the grain boundaries. The results suggest that an increased Vs concentration and enhanced charge carriers scattering at the grains' boundaries should be the primary reasons of films' resistivity increase from 4 kΩ·cm to 39 kΩ·cm.

Effect of Twist Angle on Interfacial Thermal Transport in Two-Dimensional Bilayers

Advances in two-dimensional (2D) devices require innovative approaches for manipulating transport properties. Analogous to the electrical and optical responses, it has been predicted that thermal transport across 2D materials can have a similar strong twist-angle dependence. Here, we report experimental evidence deviating from this understanding. In contrast to the large tunability in electrical transport, we measured an unexpected weak twist-angle dependence of interfacial thermal transport in MoS2 bilayers, which is consistent with theoretical calculations. More notably, we confirmed the existence of distinct regimes with weak and strong twist-angle dependencies for thermal transport, where, for example, a much stronger change with twist angles is expected for graphene bilayers. With atomic simulations, the distinct twist-angle effects on different 2D materials are explained by the suppression of long-wavelength phonons via the moiré superlattice. These findings elucidate the unique feature of 2D thermal transport and enable a new design space for engineering thermal devices.

Charge Mobility and Strain Engineering in Two-Step MS-Grown MoS2/Seed Layer Heterointerface and Photo-Excitation Mechanism

Two-dimensional (2D) materials have potential application and wide development prospects in photoelectron and spintronic devices. However, the properties of different growth conditions are challenging to study in the future. This, in turn, hinders further research into 2D materials and the manufacture of high-quality devices. A comprehensive understanding of the ultrafast laser spectroscopy and dynamics that take into account the substrate-transition metal dichalcogenide (TMD) interaction is lacking. Here, the strain effect is elucidated by systematically investigating the interfacial interaction between different substrates and MoS₂. The strain and interface engineering of MoS₂/seeds layer heterointerface and light-matter coupling are discussed in the Raman and photoluminescence spectra. The dramatic enhanced PL originates from the phase transition of MoS₂ on different substrates and electron-hole pairs dissociated by exciton screening effect. Finite-difference time-domain simulation confirmed that the electric field, magnetic field, and polarization field of the heterojunction system changed after the strain was applied. In addition, based on the dependence of physical parameters of MoS₂, the relative numerical changes of physical parameters of MoS₂ films on different substrates as well as the photoelectric transfer, strain, and charge doping levels on the surface or interface will provide a direction for optimizing the selection of various devices.

Continuous Large Area Monolayered Molybdenum Disulfide Growth Using Atmospheric Pressure Chemical Vapor Deposition

The growth of large crystallite continuous monolayer materials like molybdenum disulfide (MoS₂) with the desired morphology via chemical vapor deposition (CVD) remains a challenge. In CVD, the complex interplay of various factors like growth temperatures, precursors, and nature of the substrate decides the crystallinity, crystallite size, and coverage area of the grown MoS₂ monolayer. In the present work, we report about the role of weight fraction of molybdenum trioxide (MoO₃), sulfur, and carrier gas flow rate toward nucleation and monolayer growth. The concentration of MoO₃ weight fraction has been found to govern the self-seeding process and decides the density of nucleation sites affecting the morphology and coverage area. A carrier gas flow of 100 sccm argon results in large crystallite continuous films with a lower coverage area (70%), while a flow rate of 150 sccm results in 92% coverage area with a reduced crystallite size. Through a systematic variation of experimental parameters, we have established the recipe for the growth of large crystallite atomically thin MoS₂ suitable for optoelectronic devices.

Critical Role of Surface Termination of Sapphire Substrates in Crystallographic Epitaxial Growth of MoS2 Using Inorganic Molecular Precursors

A highly reproducible route for the epitaxial growth of single-crystalline monolayer MoS2 on a C-plane sapphire substrate was developed using vapor-pressure-controllable inorganic molecular precursors MoOCl4 and H2S. Microscopic, crystallographic, and spectroscopic analyses indicated that the epitaxial MoS2 film possessed outstanding electrical and optical properties, excellent homogeneity, and orientation selectivity. The systematic investigation of the effect of growth temperature on the crystallographic orientations of MoS2 revealed that the surface termination of the sapphire substrate with respect to the growth temperature determines the crystallographic orientation selectivity of MoS2. Our results suggest that controlling the surface to form a half-Al-terminated surface is a prerequisite for the epitaxial growth of MoS2 on a C-plane sapphire substrate. The insights on the growth mechanism, especially the significance of substrate surface termination, obtained through this study will aid in designing efficient epitaxial growth routes for developing single-crystalline monolayer transition metal dichalcogenides.

Gate-controlled electron quantum interference logic

Inspired by using the wave nature of electrons for electron quantum optics, we propose a new type of electron quantum interference structure, where single-electron waves are coherently injected into a gate-controlled, two-dimensional waveguide and exit through one or more output channels. The gate-controlled interference effects lead to specific current levels in the output channels, which can be used to realize logic gate operations, e.g., NAND or NOR gates. The operating principle is shown by coherent, dynamic Wigner quantum electron transport simulations. A discussion of classical simulations (Boltzmann) allows to outline the underlying process of interference. Contrary to other electron control approaches used for advanced information processing, no magnetic or photonic mechanisms are involved.

NaCl-Assisted Temperature-Dependent Controllable Growth of Large-Area MoS2 Crystals Using Confined-Space CVD

Due to its semiconducting nature, controlled growth of large-area chemical vapor deposition (CVD)-grown two-dimensional (2D) molybdenum disulfide (MoS₂) has a lot of potential applications in photodetectors, sensors, and optoelectronics. Yet the controllable, large-area, and cost-effective growth of highly crystalline MoS₂ remains a challenge. Confined-space CVD is a very promising method for the growth of highly crystalline MoS₂ in a controlled manner. Herein, we report the large-scale growth of MoS₂ with different morphologies using NaCl as a seeding promoter for confined-space CVD. Changes in the morphologies of MoS₂ are reported by variation in the amount of seeding promoter, precursor ratio, and the growth temperature. Furthermore, the properties of the grown MoS₂ are analyzed using optical microscopy, scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX), and atomic force microscopy (AFM). The electrical properties of the CVD-grown MoS₂ show promising performance from fabricated field-effect transistors. This work provides new insight into the growth of large-area MoS₂ and opens the way for its various optoelectronic and electronic applications.

Chemical vapor deposition of 2D materials: A review of modeling, simulation, and machine learning studies

Chemical vapor deposition (CVD) is extensively used to produce large-area two-dimensional (2D) materials. Current research is aimed at understanding mechanisms underlying the nucleation and growth of various 2D materials, such as graphene, hexagonal boron nitride (hBN), and transition metal dichalcogenides (e.g., MoS₂/WSe₂). Herein, we survey the vast literature regarding modeling and simulation of the CVD growth of 2D materials and their heterostructures. We also focus on newer materials, such as silicene, phosphorene, and borophene. We discuss how density functional theory, kinetic Monte Carlo, and reactive molecular dynamics simulations can shed light on the thermodynamics and kinetics of vapor-phase synthesis. We explain how machine learning can be used to develop insights into growth mechanisms and outcomes, as well as outline the open knowledge gaps in the literature. Our work provides consolidated theoretical insights into the CVD growth of 2D materials and presents opportunities for further understanding and improving such processes

A Novel Carbon-Assisted Chemical Vapor Deposition Growth of Large-Area Uniform Monolayer MoS2 and WS2

Monolayer MoS₂ can be used for various applications such as flexible optoelectronics and electronics due to its exceptional optical and electronic properties. For these applications, large-area synthesis of high-quality monolayer MoS₂ is highly desirable. However, the conventional chemical vapor deposition (CVD) method using MoO₃ and S powder has shown limitations in synthesizing high-quality monolayer MoS₂ over a large area on a substrate. In this study, we present a novel carbon cloth-assisted CVD method for large-area uniform synthesis of high-quality monolayer MoS₂. While the conventional CVD method produces thick MoS₂ films in the center of the substrate and forms MoS₂ monolayers at the edge of the thick MoS₂ films, our carbon cloth-assisted CVD method uniformly grows high-quality monolayer MoS₂ in the center of the substrate. The as-synthesized monolayer MoS₂ was characterized in detail by Raman/photoluminescence spectroscopy, atomic force microscopy, and transmission electron microscopy. We reveal the growth process of monolayer MoS₂ initiated from MoS₂ seeds by synthesizing monolayer MoS₂ with varying reaction times. In addition, we show that the CVD method employing carbon powder also produces uniform monolayer MoS₂ without forming thick MoS₂ films in the center of the substrate. This confirms that the large-area growth of monolayer MoS₂ using the carbon cloth-assisted CVD method is mainly due to reducing properties of the carbon material, rather than the effect of covering the carbon cloth. Furthermore, we demonstrate that our carbon cloth-assisted CVD method is generally applicable to large-area uniform synthesis of other monolayer transition metal dichalcogenides, including monolayer WS₂.