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Directed long-range molecular migration energized by surface reaction

Evidence and attribution

Authority of statements

Prose sections below (Summary, Methods, Findings, etc.) are curated summaries of the publication identified by doi, title, and pdf_path in the front matter above. They are not new primary claims by this wiki.

For definitive numerical values, reaction schemes, and interpretations, use the peer-reviewed article (and optional records under normalized/papers/ when present)—not this page alone.

Summary

This article reports scanning tunneling microscopy (STM) evidence on Si(100) for in-plane, directed, long-range migration of physisorbed ethylenic products produced by surface reactions—distinct from short-range random hopping diffusion. Thermal reactions of 1,2-dihaloethanes (F, Cl, Br) and electron-induced reactions of chemisorbed ethylene, propene, and trans-2-butene are studied. Migration distances up to on the order of 200 Å are discussed for some channels, with thermal channels showing ethylene separated from halogen pairs by tens of ångströms on average. Ab initio calculations support a picture in which asymmetric forces on the two carbons of the ethylenic unit produce torque and favor rolling/cartwheeling-like motion over the rough surface, including crossing steps and defects without losing directionality.

Methods

Experiments (STM). Scanning tunneling microscopy on Si(100)-2×1 reports six channels: three thermal reactions of 1,2-dihaloethanes (XCH\(_2\)CH\(_2\)X, X = F, Cl, Br) at room temperature (~25 °C), and three electron-induced reactions of chemisorbed ethylene, propene, and trans-2-butene. The abstract contrasts in-plane directed migration up to ~200 Å before a final attachment reaction with conventional short-range ~5 Å hopping diffusion on smooth metals.

3 — Static QM / DFT (VASP benchmarks cited in the excerpt). Program: VASP is named for the quoted heats of reaction. Functional: N/A — specific XC functional not stated on the indexed pages. Dispersion: N/A — dispersion correction not stated in the excerpt. Basis / PAW / cutoff: N/A — not transcribed from normalized/extracts/2011harikumar-nat-directed-long-range_p1-2.txt. k-sampling: N/A — k-mesh not stated in the excerpt. Structures / pathways: 1,2-dihaloethane reactions forming two chemisorbed halogens plus chemisorbed ethylene on Si(100)-2×1 as discussed in Results. Properties computed: quoted reaction energies (23.6, 22.6, 22.5 eV for DFE, DCE, DBE, respectively, as printed) as electronic-structure energy benchmarks.

1 — MD application. N/A — this paper’s primary evidence is STM plus DFT support, not classical production MD protocols.

2 — Force-field training. N/A — not applicable.

Findings

In-plane directed migration over tens to ~200 Å contrasts with the ~5 Å short-range random hopping typical of diffusion on metals in the cited literature. For dihaloethane reactions, “hot” ethylene products can appear separated from partner halogen pairs—with mean ethylene–halogen separations ~25–30 Å across the three halides (distributions biased by halogen-pair clustering for some species), and separations up to about 80 Å noted for selected DBE assignments. Directionality along the C–C axis, traversal of steps and defects without losing direction, and end-to-end inversion for propene / trans-2-butene under electron-induced conditions support rolling/cartwheeling over simple hopping. Ab initio results indicate asymmetric forces on the two carbons that generate torque, consistent with the proposed migration mechanism.

Limitations (STM interpretation). The paper notes that assigning partner halogen pairs to specific ethylene products can bias distance statistics and that clustering effects matter for some halides—caveats to keep when reading the separation histograms.

Limitations

  • Assignment of partner halogen pairs to specific ethylene products can bias distance statistics; the paper discusses clustering effects for some halides.
  • The study is specific to Si(100) chemistry and STM-accessible conditions; direct transfer to other semiconductors or metals is not claimed.

Relevance to group

Surface reaction dynamics and ab initio validation; not a ReaxFF/van Duin-group paper but relevant as surface chemistry context for silicon and hydrocarbon reactivity.

Citations and evidence anchors

  • DOI: 10.1038/nchem.1029
  • Text-aligned pointer: normalized/extracts/2011harikumar-nat-directed-long-range_p1-2.txt