Sulfurization of MoO3 in the Chemical Vapor Deposition Synthesis of MoS2 Enhanced by an H2S/H2 Mixture.

The typical layered transition metal dichalcogenide (TMDC) material, MoS2, is considered a promising candidate for the next-generation electronic device due to its exceptional physical and chemical properties. In chemical vapor deposition synthesis, the sulfurization of MoO3 powders is an essential reaction step in which the MoO3 reactants are converted into MoS2 products. Recent studies have suggested using an H2S/H2 mixture to reduce MoO3 powders in an effective way. However, reaction mechanisms associated with the sulfurization of MoO3 by the H2S/H2 mixture are yet to be fully understood. Here, we perform quantum molecular dynamics (QMD) simulations to investigate the sulfurization of MoO3 flakes using two different gaseous environments: pure H2S precursors and a H2S/H2 mixture. Our QMD results reveal that the H2S/H2 mixture could effectively reduce and sulfurize the MoO3 reactants through additional reactions of H2 and MoO3, thereby providing valuable input for experimental synthesis of higher-quality TMDC materials.

Defect Healing in Layered Materials: A Machine Learning-Assisted Characterization of MoS2 Crystal Phases.

Monolayer MoS2 is an outstanding candidate for a next-generation semiconducting material because of its exceptional physical, chemical, and mechanical properties. To make this promising layered material applicable to nanostructured electronic applications, synthesis of a highly crystalline MoS2 monolayer is vitally important. Among different types of synthesis methods, chemical vapor deposition (CVD) is the most practical way to synthesize few- or mono-layer MoS2 on the target substrate owing to its simplicity and scalability. However, synthesis of a highly crystalline MoS2 layer remains elusive. This is because of the number of grains and defects unavoidably generated during CVD synthesis. Here, we perform multimillion-atom reactive molecular dynamics (RMD) simulations to identify an origin of the grain growth, migration, and defect healing process on a CVD-grown MoS2 monolayer. RMD results reveal that grain boundaries could be successfully repaired by multiple heat treatments. Our work proposes a new way of controlling the grain growth and migration on a CVD-grown MoS2 monolayer.

Role of H Transfer in the Gas-Phase Sulfidation Process of MoO3: A Quantum Molecular Dynamics Study.

Layered transition metal dichalcogenide (TMDC) materials have received great attention because of their remarkable electronic, optical, and chemical properties. Among typical TMDC family members, monolayer MoS2 has been considered a next-generation semiconducting material, primarily due to a higher carrier mobility and larger band gap. The key enabler to bring such a promising MoS2 layer into mass production is chemical vapor deposition (CVD). During CVD synthesis, gas-phase sulfidation of MoO3 is a key elementary reaction, forming MoS2 layers on a target substrate. Recent studies have proposed the use of gas-phase H2S precursors instead of condensed-phase sulfur for the synthesis of higher-quality MoS2 crystals. However, reaction mechanisms, including atomic-level reaction pathways, are unknown for MoO3 sulfidation by H2S. Here, we report first-principles quantum molecular dynamics (QMD) simulations to investigate gas-phase sulfidation of MoO3 flake using a H2S precursor. Our QMD results reveal that gas-phase H2S molecules efficiently reduce and sulfidize MoO3 through the following reaction steps: Initially, H transfer occurs from the H2S molecule to low molecular weight Mo xO y clusters, sublimated from the MoO3 flake, leading to the formation of molybdenum oxyhydride clusters as reaction intermediates. Next, two neighboring hydroxyl groups on the oxyhydride cluster preferentially react with each other, forming water molecules. The oxygen vacancy formed on the Mo-O-H cluster as a result of this dehydration reaction becomes the reaction site for subsequent sulfidation by H2S that results in the formation of stable Mo-S bonds. The identification of this reaction pathway and Mo-O and Mo-O-H reaction intermediates from unbiased QMD simulations may be utilized to construct reactive force fields (ReaxFF) for multimillion-atom reactive MD simulations.

Nanocarbon synthesis by high-temperature oxidation of nanoparticles.

High-temperature oxidation of silicon-carbide nanoparticles (nSiC) underlies a wide range of technologies from high-power electronic switches for efficient electrical grid and thermal protection of space vehicles to self-healing ceramic nanocomposites. Here, multimillion-atom reactive molecular dynamics simulations validated by ab initio quantum molecular dynamics simulations predict unexpected condensation of large graphene flakes during high-temperature oxidation of nSiC. Initial oxidation produces a molten silica shell that acts as an autocatalytic 'nanoreactor' by actively transporting oxygen reactants while protecting the nanocarbon product from harsh oxidizing environment. Percolation transition produces porous nanocarbon with fractal geometry, which consists of mostly sp(2) carbons with pentagonal and heptagonal defects. This work suggests a simple synthetic pathway to high surface-area, low-density nanocarbon with numerous energy, biomedical and mechanical-metamaterial applications, including the reinforcement of self-healing composites.

Facile Five-Step Heteroepitaxial Growth of GaAs Nanowires on Silicon Substrates and the Twin Formation Mechanism.

Monolithic integration of III-V semiconductors with Si has been pursued for some time in the semiconductor industry. However, the mismatch of lattice constants and thermal expansion coefficients represents a large technological challenge for the heteroepitaxial growth. Nanowires, due to their small lateral dimension, can relieve strain and mitigate dislocation formation to allow single-crystal III-V materials to be grown on Si. Here, we report a facile five-step heteroepitaxial growth of GaAs nanowires on Si using selective area growth (SAG) in metalorganic chemical vapor deposition, and we further report an in-depth study on the twin formation mechanism. Rotational twin defects were observed in the nanowire structures and showed strong dependence on the growth condition and nanowire size. We adopt a model of faceted growth to demonstrate the formation of twins during growth, which is well supported by both a transmission electron microscopy study and simulation based on nucleation energetics. Our study has led to twin-free segments in the length up to 80 nm, a significant improvement compared to previous work using SAG. The achievements may open up opportunities for future functional III-V-on-Si heterostructure devices.

Prediction intervals for a noisy nonlinear time series based on a bootstrapping reservoir computing network ensemble.

Prediction intervals that provide estimated values as well as the corresponding reliability are applied to nonlinear time series forecast. However, constructing reliable prediction intervals for noisy time series is still a challenge. In this paper, a bootstrapping reservoir computing network ensemble (BRCNE) is proposed and a simultaneous training method based on Bayesian linear regression is developed. In addition, the structural parameters of the BRCNE, that is, the number of reservoir computing networks and the reservoir dimension, are determined off-line by the 0.632 bootstrap cross-validation. To verify the effectiveness of the proposed method, two kinds of time series data, including the multisuperimposed oscillator problem with additive noises and a practical gas flow in steel industry are employed here. The experimental results indicate that the proposed approach has a satisfactory performance on prediction intervals for practical applications.