Mengyi Wang

We developed a ReaxFF force field for silicon carbide (SiC), specifically parametrized to capture complex, temperature-dependent vapor compositions during sublimation. ReaxFF molecular dynamics (MD) simulations of crystalline SiC at 3000–4500 K revealed a diverse range of vapor species─including SiC2, Si, Si2C, Si2C2, SiC, Si2C3 and Si3C2─closely mirroring equilibrium compositions derived from quantum mechanical (QM) benchmarks. Our simulations demonstrate that the initial stage of sublimation is Si-rich at 3000 K but transitions toward C-rich compositions at elevated temperatures, informing stoichiometry control in SiC growth. To bridge atomistic and macroscopic scales, Arrhenius analysis was employed to extract kinetic parameters for computational fluid dynamics (CFD) modeling, providing an extended kinetic description critical for optimizing SiC crystal growth. Finally, the ReaxFF force field was successfully benchmarked against a previously developed state-of-the-art machine learning potential in both accuracy and computational efficiency, establishing it as a robust tool for multiscale crystal growth simulations.

Subtle variations in local growth chemistry can fundamentally alter the morphology of layered chalcogenide thin films, yet the atomic-scale mechanisms underlying this sensitivity remain poorly understood. Here, we combine molecular beam epitaxy (MBE) experiments, reactive molecular dynamics simulations, and machine learning analysis to establish a chemistry–structure relationship for SnSe thin film growth on MgO. Experimentally, we show that modifying the Sn/Se flux ratio and deposition sequence promotes a transition between three-dimensional island formation and laterally coalesced smooth films, accompanied by pronounced changes in surface roughness. Reactive molecular dynamics simulations reveal that this transition originates from an intrinsic chemical asymmetry between Sn and Se that alters adatom surface diffusion and aggregation behavior during growth. Se-rich environments stabilize surface passivation and promote lateral growth, whereas Sn-rich conditions favor vertical aggregation through rapid Sn clustering. To assess morphology trends without imposing physical assumptions, we apply an unsupervised machine learning framework to images rendered from the simulation snapshots. The resulting low-dimensional representation of the surface morphologies captures morphology trends consistent with those observed experimentally and those in simulations, providing a complementary data-driven interpretation of the chemistry–morphology relationship. Because nanoscale roughness strongly influences carrier scattering, phonon transport, and domain stability in SnSe-based electronic and energy devices, these results provide a chemistry-guided framework for morphology control in layered chalcogenide thin films.

A detailed understanding of the composition and polymerization mechanism of elemental sulfur remains a decades long unresolved question for modern chemistry. However, the dynamic nature of molten sulfur significantly complicates its accurate characterization. To overcome this challenge, we performed the first comprehensive molecular dynamics (MD) simulations using a ReaxFF reactive force field specifically parameterized to capture the complex ring‐opening polymerization dynamics of elemental sulfur. Rigorous development of the force field parameters, trained against extensive quantum mechanical datasets, was key to enabling these large‐scale (>10 000 atoms) reactive MD simulations at polymerization‐relevant temperatures. This study provides the first detailed atomic‐level description of liquid sulfur, elucidating temperature‐dependent molecular composition and offering unprecedented insights into sulfur polymerization mechanisms. These are the first simulations to reveal the formation of remarkably large macrocyclic sulfur rings at the onset of polymerization—a discovery that challenges longstanding mechanistic misconceptions, thus reshaping our understanding of sulfur polymerization. Our findings highlight the power of molecular dynamics in exploring complex polymerization processes, with broad impact in dynamic covalent chemistry and covalent adaptable polymers.

A detailed understanding of the composition and polymerization mechanism of elemental sulfur remains a decades long unresolved question for modern chemistry. However, the dynamic nature of molten sulfur significantly complicates its accurate characterization. To overcome this challenge, we performed the first comprehensive molecular dynamics (MD) simulations using a ReaxFF reactive force field specifically parameterized to capture the complex ring‐opening polymerization dynamics of elemental sulfur. Rigorous development of the force field parameters, trained against extensive quantum mechanical datasets, was key to enabling these large‐scale (>10 000 atoms) reactive MD simulations at polymerization‐relevant temperatures. This study provides the first detailed atomic‐level description of liquid sulfur, elucidating temperature‐dependent molecular composition and offering unprecedented insights into sulfur polymerization mechanisms. These are the first simulations to reveal the formation of remarkably large macrocyclic sulfur rings at the onset of polymerization—a discovery that challenges longstanding mechanistic misconceptions, thus reshaping our understanding of sulfur polymerization. Our findings highlight the power of molecular dynamics in exploring complex polymerization processes, with broad impact in dynamic covalent chemistry and covalent adaptable polymers.

Recent advancements in the field of two-dimensional (2D) materials have led to the discovery of a wide range of 2D materials with intriguing properties. Atomistic-scale simulation methods have played a key role in these discoveries. In this review, we provide an overview of the recent progress in ReaxFF force field developments and applications in modeling the following layered and nonlayered 2D materials: graphene, transition metal dichalcogenides, MXenes, hexagonal boron nitrides, groups III-, IV- and V-elemental materials, as well as the mixed dimensional van der Waals heterostructures. We further discuss knowledge gaps and challenges associated with synthesis and characterization of 2D materials. We close this review with an outlook addressing the challenges as well as plans regarding ReaxFF development and possible large-scale simulations, which should be helpful to guide experimental studies in a discovery of new materials and devices.

Raman spectroscopy showing the initial formation of SnSe₂ followed by the stabilization of SnSe with increased growth time.

Significance: Novel membrane processes and materials are needed to combat global water scarcity. The advancement of membrane materials for water purification processes such as desalination is hindered by our limited understanding of structure–property–performance relationships. We address this knowledge gap by elucidating the mechanisms which govern the ionization behavior of nanoporous polyamide films and their influence on membrane performance. Polyamide ionization is key to enhancing the separation efficiency of charged constituents in aqueous solutions. Our results illustrate that a large portion of the ionizable moieties in polyamide films remain uncharged during typical operation due to extreme confinement effects, thus going unutilized. Complete ionization of state-of-the-art polyamide membranes offers selectivity improvements that could markedly improve desalination process efficiency.

We study the evolution of enthalpic changes across the glass transition of model sodium silicate glasses (Na₂O)_x(SiO₂)_100−x, focusing on the detection of a flexible-rigid transition and a possible reversibility window in relationship with dynamic properties. We show that the hysteresis resulting from enthalpic relaxation during a numerical cooling–heating cycle is minimized for 12% ≤ x ≤ 20% Na₂O, which echoes with the experimental observation. The key result is the identification of the physical features driving this anomalous behavior. The intermediate-flexible boundary is associated with a dynamic onset with increasing depolymerization that enhances the growing atomic motion with a reduced internal stress, whereas the intermediate-stressed rigid boundary exhibits a substantial increase in the temperature at which the relaxation is maximum. These results suggest an essentially dynamic origin for the intermediate phase observed in network glass-forming liquids.