Molecular Dynamics Simulations of the Oxidation of Aluminum Nanoparticles using the ReaxFF Reactive Force Field

We performed ReaxFF-molecular dynamics (MD) simulations of the oxidation of aluminum nanoparticles (ANPs) at three different temperatures (300, 500, and 900 K) and two different initial oxygen densities (0.13 and 0.26 g/cm³) to elucidate the mechanism of oxidation kinetics of the ANPs and to study the oxidation states in the oxide layer. Our result shows that the mechanism of the oxidation of the ANPs is as follows: hot-spots and high-temperature areas are created by adsorption and dissociation of oxygen molecules on the surface of the ANPs; void spaces are generated because of hot-spots and high-temperature areas; the void spaces significantly lower a reaction barrier for oxygen diffusion (by up to 92%) and make this process exothermic. Subsequently, an oxide layer is developed by this accelerated oxygen diffusion. Our results also indicate that the oxidation of the ANPs depends on combined effects of the temperature and the oxygen gas pressure because such conditions have effects on not only the oxide layer thickness but also the density of the oxide layer. These ReaxFF results are in good agreement with available experimental literature on aluminum oxidation kinetics

Atomistic-Scale Analysis of Carbon Coating and Its Effect on the Oxidation of Aluminum Nanoparticles by ReaxFF-Molecular Dynamics Simulations

We developed a ReaxFF reactive force field for Al/C interactions to investigate carbon coating and its effect on the oxidation of aluminum nanoparticles (ANPs). The ReaxFF parameters were optimized against quantum mechanics-based (QM-based) training sets and validated with additional QM data and data from experimental literature. ReaxFF-molecular dynamics (MD) simulations were performed to determine whether this force field description was suitable to model the surface deposition and oxidation on complex materials (i.e., carbon-coated ANPs). Our results show that the ReaxFF description correctly reproduced the Al/C interaction energies obtained from the QM calculations and qualitatively captured the processes of the hydrocarbons’ binding and their subsequent reactions on the bare ANPs. The results of the MD simulations indicate that a carbon coating layer was formed on the surface of the bare ANPs, while H atoms were transferred from the hydrocarbons to the available Al binding sites typically without breaking C–C bonds. The growth of the carbon layer depended strongly on the hydrocarbon precursors that were used. Moreover, the MD simulations of the oxidation of the carbon-coated ANPs indicate that the carbon-coated ANPs were less reactive at low temperatures, but they became very susceptible to oxidation when the coating layer was removed at elevated at elevated temperatures. These results are consistent with the experimental literature, and thus, the ReaxFF description that was developed in this study enables us to gain atomistic-scale insights into the role of the carbon coating in the oxidation of ANPs

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 MoxOy 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

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 MoxOy 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

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

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

Chemical Vapor Deposition Synthesis of MoS₂ Layers from the Direct Sulfidation of MoO₃ Surfaces Using Reactive Molecular Dynamics Simulations

Atomically thin MoS₂ layer, a promising transition metal dichalcogenide (TMDC) material, has great potential for application in next-generation electronic and optoelectronic devices. Chemical vapor deposition (CVD) is the most effective technique for the synthesis of high-quality MoS₂ layers. During CVD synthesis, monolayered MoS₂ is generally synthesized by sulfidation of MoO₃. Although qualitative reaction mechanisms for the sulfidation of MoO₃ have been investigated by previous studies, the detailed reaction processes, including atomic-scale reaction pathways and growth kinetics, have yet to be fully understood. Here, we present quantum-mechanically informed and validated reactive molecular dynamics simulations of the direct sulfidation of MoO₃ surfaces using S₂ gas precursors. Our work clarifies the reaction mechanisms and kinetics of the sulfidation of MoO₃ surfaces as follows: the reduction and sulfidation of MoO₃ surfaces occur primarily at O-termination sites, followed by unsaturated Mo sites; these local reaction processes lead to nonuniform MoO<i>_x</i>S<i>_y</i> surface structures during the CVD process. After annealing the MoO<i>_x</i>S<i>_y</i> samples, the crystallized surface structures contain voids, and three different types of local surface complexes (MoO<i>_x</i>, MoO<i>_x</i>S<i>_y</i>, and MoS₂-like surface regions), depending on the fraction of S ingredients on the MoO<i>_x</i>S<i>_y</i> surface. These results, which have been validated by our reactive quantum molecular dynamics simulations and previous experimental results, provide valuable chemical insights into the CVD synthesis of large-scale and defect-free MoS₂ layers and other layered TMDC materials