Determination of the Ground-State Atomic Structures of Size-Selected Au Nanoclusters by Electron-Beam-Induced Transformation

The equilibrium ground state atomic structures of nanoparticles are critical to understanding the relationship between their structure and functionality, e.g., in catalysis, and are the standard output of first principles and semiempirical theoretical treatments. We demonstrate a method of obtaining a stable population of the structural isomers of supported Au clusters from a metastable initial array via electron beam irradiation. Statistical investigation of size-selected Au clusters containing $923\pm 23$ atoms via aberration-corrected scanning transmission electron microscopy shows that virtually all of the icosahedral (Ih) clusters undergo structural transformations into decahedral (Dh) (primarily) or fcc isomers while Dh and FCC clusters generally retain their atomic structures after electron irradiation of each cluster individually for up to 400 s at a dose of $2.4\times {10}^4{e}^{-}/{\mathrm{angstrom}}^2/\mathrm{frame}$. Intermediate phases are often observed in the image series (videos) before the appearance of the new stable isomers, the relative structural populations of which can be controlled via the electron beam dose. The comprehensive results reported here should provide a valuable experimental reference for testing or refining potential models and for kinetic or dynamical treatments of the atomic configurations.

Figure 1

Typical HAADF-STEM images of ${\mathrm{Au}}_{923}$ clusters and corresponding simulations. (a)–(d) the HAADF images for (a) a decahedron along a fivefold axis; (b) fcc polyhedron along the $⟨110⟩$ axis; (c) icosahedron along a fivefold axis; and (d) icosahedron along a twofold axis, respectively. (e)–(h) the simulated images obtained from the standard atomic models of the Ino-decahedron, cuboctahedron, and icosahedron, respectively. The arrow in (a) marks the edge where no reentrant structure is observed.

Figure 2

Statistical results of the structural transformations obtained for 79 ${\mathrm{Au}}_{923}$ clusters. Left panel shows the structural populations of ${\mathrm{Au}}_{923}$ clusters initially and after electron beam irradiation. Right panel shows a few typical examples of structural transformation (for Ih clusters) or structural persistence (for Dh and fcc clusters).

Figure 3

Systematic investigations of the cluster structural transformation times and structural populations as a function of irradiation time. (a) Statistical distribution of transformation times from Ih to Dh or fcc. (b) The structural population of 79 ${\mathrm{Au}}_{923}$ clusters as a function of electron beam dose.

Figure 4

Individual frames from two series of HAADF images ($6\times 6\mathrm{nm}$ for each). (a)–(d) frames taken of an icosahedral cluster which undergoes a structural transition to Dh. (e)–(h) frames from a $⟨110⟩$ fcc polyhedral cluster exhibiting significant surface reconstruction under electron beam irradiation. The arrow in (g) marks an incomplete surface layer nucleated as a result of surface reconstruction.