The transformation mechanisms among cuboctahedra, Ino's decahedra and icosahedra structures of magic-size gold nanoclusters

Abstract

Gold nanoclusters possess multiple competing structural motifs with small energy differences, enabling structural coexistence and interconversion. Using a high-accuracy machine learned potential trained on some 20'000 density functional theory reference data points, we investigate transformation pathways connecting both high-symmetry and amorphous cuboctahedra, Ino's decahedra and icosahedra for Au55, Au147, Au309 and Au561 nanoclusters. Our saddle point searches reveal that high-symmetry transformations from cuboctahedra and Ino's decahedra to icosahedra proceed through a single barrier and represent soft-mode-driven jitterbug-type and slip-dislocation motions. In addition, we identify lower-barrier asymmetric transformation pathways that drive the system into disordered, Jahn-Teller-stabilized amorphous icosahedra. Minima Hopping sampling further uncovers, in this context, many such low-symmetry minima. Some of the newly identified global minima for Au309 and Au561 have energies that are up to 2.8 eV lower than the previously reported global minima. Hence, both the shapes and the transformation pathways studied in previous investigations are not the physically relevant ones. In contrast to the previously studied pathways, our transformation pathways give reasonable transformation times that are in rough agreement with experiments.

Figures (20)

• Figure 1: Correlation of the energies calculated by three different force fields for the structures in the training dataset of the MLIP.
• Figure 2: New GM structures for Au$_{309}$ (top row) and Au$_{561}$ (bottom row). (a,d) Atoms whose positions coincide with those of an I$_h$ are shown in gold. (b,e) Complete nanocluster structures, where ordered (I$_h$-matching) atoms are shown in golden and disordered or amorphous atoms are shown in red. (c,f) Isolated views of the amorphous (red) atoms extracted from the corresponding structures.
• Figure 3: Energy levels near the Fermi level for the Au$_{55}$-I$_h$ nanocluster as obtained from scaled LJ, MLIP-relaxed, and VASP-relaxed. Occupied levels are shown as solid lines, unoccupied levels as dashed lines, and the Fermi level is indicated by black dotted lines.
• Figure 4: Histogram comparing the bond length distributions of the Au$_{55}$-I$_h$ nanocluster as obtained from scaled LJ, MLIP-relaxed, and VASP-relaxed structures.
• Figure 5: Examples of two partially amorphous O$_{h}$-Au$_{147}$ (top row) and two I-D$_{5h}$-Au$_{147}$ (bottom row). Red spheres indicate atoms that deviate from their ideal positions in the corresponding high-symmetry nanocluster.