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In situ atomistic insight into the growth mechanisms of single layer 2D transition metal carbides

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Summary

In situ aberration-corrected scanning transmission electron microscopy (STEM) with heating reveals homoepitaxial growth of hexagonal TiC adlayers on defunctionalized Ti\(_3\)C\(_2\) MXene above roughly 500 °C, producing Ti\(_4\)C\(_3\)- and Ti\(_5\)C\(_4\)-like local compositions. Density functional theory and ReaxFF-based hybrid Monte Carlo and molecular dynamics (fbMC/MD) simulations interpret adatom diffusion, binding strengths, and step energies, supporting a Frank–van der Merwe growth mode for layered TiC on the MXene template. Electron energy-loss spectroscopy distinguishes carbon-rich adlayers from bare titanium metal, complementing the atomistic picture of carbon addition during growth.

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Methods

Microscopy. Experiments use a Nion UltraSTEM 100 with Protochips Fusion heating holders near \(10^{-9}\) Torr base pressure; Ti\(_3\)C\(_2\)T\(_x\) flakes are annealed while imaging. Beam–thermal coupling is discussed in the article because the electron beam can assist local chemistry.

Spectroscopy. EELS provides elemental contrast for C versus Ti in growing adlayers.

Theory. DFT supercells evaluate adsorption and step energetics for h-TiC on Ti\(_3\)C\(_2\). ReaxFF fbMC/MD captures bond-making and bond-breaking events during growth at elevated temperature.

Atomistic modeling (DFT + ReaxFF fbMC/MD). The article couples DFT supercells on Ti\(_3\)C\(_2\) with h-TiC adlayer models for adsorption, step-edge, and binding energetics, and uses ReaxFF-based hybrid force-biased Monte Carlo / molecular dynamics (fbMC/MD) to sample bond-making and bond-breaking during homoepitaxial growth. fbMC/MD alternates Monte Carlo bond events with MD propagation to access rare reactive steps at temperatures relevant to the experiments; reproducing published kinetics requires matching the ReaxFF parameter file, timestep, temperature schedule, and fbMC attempt frequencies in Nature Communications Methods / SI. Engine / code: reactive molecular dynamics with ReaxFF in the publication’s fbMC/MD workflow (N/A — LAMMPS vs other engine not restated in the indexed excerpt used here). System / composition: periodic slab/supercell models of the MXene template and adlayers (N/A — atom counts and lateral cell vectors in article/SI). Boundaries: three-dimensional periodic boundary conditions (PBC). Ensemble / timestep / duration: N/A — explicit NVE/NVT/NPT labels, integration timestep (fs), and total sampled time not recovered from the p1–2 extract; use Methods/SI. Thermostat / barostat: N/A — not in the indexed excerpt for the fbMC/MD segment. Temperature: targets thermal conditions for high-temperature growth discussed alongside experiment. Hydrostatic pressure control: N/A — not used in the atomistic growth models summarized here. Electric field in MD: N/A — not used (STEM beam treated experimentally, not as a classical MD bias). Enhanced sampling: fbMC/MD (MC bond moves + MD); N/A — umbrella / metadynamics / replica exchange not indicated in the indexed text.

Findings

Frank–van der Merwe growth of TiC monolayers proceeds with Ti and C supplied from the flake; h-TiC adlayers appear above 500 °C with beam assistance and at 1000 °C under thermal driving alone in the conditions reported. DFT highlights low adatom diffusion barriers, high surface energies for certain h-Ti facets, step-edge penalties, and strong h-TiC binding that favor layered addition rather than isolated islanding. EELS supports carbon-rich adlayers relative to metallic titanium patches.

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Cross-scale consistency. The paper’s argument combines in situ microscopy evidence for layer addition, EELS contrast for carbon enrichment, DFT step and adsorption energetics, and fbMC/MD kinetics. Readers should treat each layer as mutually supporting rather than independently sufficient: microscopy shows morphology, DFT supplies relative energies for candidate pathways, and reactive MD addresses bond events at finite temperature.

Beam vs thermal driving. Because STEM can assist local chemistry, the article distinguishes conditions where adlayers appear with beam help near 500 °C from more thermally dominated regimes at 1000 °C; reproducing the narrative requires citing the exact experimental protocols on the canonical page.

Limitations

Electron-beam effects overlap with thermal driving forces; duplicate PDFs should be consolidated in the manifest. Full numerical settings appear in the main article and SI on the canonical slug.

Relevance to group

van Duin group coauthors (Yilmaz, Lotfi, Ostadhossein, van Duin); this page records alternate PDF bytes for the same DOI.

Citations and evidence anchors

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