Geometry-controlled competition between axis centering and detwinning in fivefold-twinned gold nanoparticles

Abstract

Fivefold-twinned metal nanoparticles host a central wedge disclination that strongly influences their mechanical and catalytic properties. Yet the atomistic mechanisms governing the stability, migration, and annihilation of this topological defect remain incompletely understood. Here we present a systematic molecular dynamics study of gold Marks decahedra in which the fivefold axis is artificially brought close to the surface by controlled geometric modifications. By generating concave and convex morphologies with varying axis depth, we uncover a geometry-controlled competition between axis centering and detwinning. Concave geometries promote surface diffusion that restores fivefold symmetry, either by recentering the original disclination or by nucleating a new subsurface axis through collective atomic rearrangements. In contrast, convex structures with a shallow axis undergo rapid detwinning within the first nanoseconds via surface glide, leading to single-twin or fully FCC configurations. Remarkably, positioning the axis just two atomic layers beneath the surface suppresses detwinning and restores stability. Our results demonstrate that surface curvature and defect depth critically regulate disclination mobility and twin stability, providing a mechanistic framework to understand the structural evolution of multi-twinned nanoparticles and to guide the controlled design of defect-engineered nanomaterials.

Figures (9)

• Figure 1: Marks decahedra selected for geometry modification. The fivefold axis is parallel to the plane of observation, and the re-entrances that characterize the Marks geometry are clearly visible. The size ranges from $N=348$ to 766 atoms, with axis heights varying between 8 and 11 atoms.
• Figure 3: Time-sequence of the recentering process in a 277-atom $\check{1}$ cluster at 600 K. The disclination axis (red) and its first-neighbor columns (orange) are stationary while surface atoms migrate toward the re-entrant facets. A) Initial peripheral axis (0 ns); B) intermediate filling of the concavity (8 ns); C) final restored fivefold symmetry (1 µ s).
• Figure 4: fivefold axis migration during the evolution at 550K of the $\check{1}$ structure obtained from Dh434 captured between 388.78 and 388.80 ns. A) The axis is mainly composed of the same atoms as the original one (colored in pink); in B) we observe a collective movement of groups of columns which leads to the nucleation of a new fivefold axis (red atoms) in C), distinct from the original one and located in a more central layer.
• Figure 5: Evolution of a 234-atom $\check{0}$ cluster at 500 K. In the first sequence, atom belonging to the twin-planes and to the fivefold axis are colored in blue and red, respectively, to highlight the developing decahedral arrangement. In the second sequence, atoms belonging to the original axis are colored in pink, and atoms belonging to the parallel subsurface column that will become the fivefold axis are colored in red. A) Initial unstable exposed axis. B) Subsurface nucleation (9 ns): a new fivefold axis forms in a subsurface column through collective shearing. C) Covering of the new axis (13 ns). D) Final centered decahedron (1 µ s).
• Figure 6: Picosecond-scale zoom of the disclination migration in 234-atom $\check{0}$ structure. A) (5.00 ns) Pre-transition state with unstable HCP configurations; B) (5.38 ns) lattice distortion and curving of surface columns; C) (5.40 ns) collective glide of the surface layer leading to the emergence of the new disclination core.