Face-centered cubic (fcc) metals, such as Au and Ag, usually adopt a packed crystal structure in bulk. However, the equilibrium structure could differ when only a handful of atoms compose a nanocluster. Theories have predicted that particles less than a few nanometers would favor a decahedral packing with a five-fold symmetry; when even fewer atoms are present, say less than two hundred, a 20-fold icosahedral packing would become the lowest-energy configuration. Such fluctuations of the nuclei/seeds may have played a critical role in defining the shape of colloidal nanoparticles in many wet chemical syntheses.
In recent work, a cross-institutional team led by Richard E. Palmer and Thomas J. A. Slater reported the direct observation of such fluctuations on a nearly second-by-second basis. The team synthesized Au nanoclusters containing 309±15 atoms on an amorphous carbon film through mass-selected magnetron sputtering. Subsequently, aberration-corrected scanning transmission electron microscopy (STEM) was employed to track the atomic structures of Au nanoclusters with a frame rate of 0.4–0.7 per second (Fig. 1). To identify the cluster type in each frame, the team compared them to a collection of simulated images with different cluster structures and tilt angles. The clusters exhibited highly dynamic switching between decahedral, icosahedral, and single-crystalline structures under the electron beam, which is sufficiently strong to overcome the energy barriers between such transitions.
![](https://blogs.rsc.org/nh/files/2024/02/Picture1-300x61.jpg)
Fig. 1 Au309±15 clusters fluctuating under the electron beam. High-angle annular dark field (HAADF) imaging on an aberration-corrected scanning transmission electron microscope (STEM) resolved the atomic structure of these Au nanoclusters frame by frame. Adapted from the supporting data DOI: 10.5281/zenodo.10522408, CC-BY 4.0.
Notably, the authors showed that the Au309±15 clusters favor the decahedral structure the most, followed by icosahedral and then single-crystalline structures (Fig. 2a). This result is consistent with the probabilities obtained from a snapshot of an ensemble. In theory, the lower-energy structures would have a higher probability of appearance. The ranking of isomeric preferences observed in this study indicates that the cluster size is within a range where the energy ranks in fcc > icosahedral > decahedral (Fig. 2b). Taken together, this work illustrates the possibility of atomic-resolution electron microscopy, when combined with image simulations, to track the isomeric evolution of metal nanoclusters and may shed light on how we understand and regulate nanostructures with atomic precision.
![](https://blogs.rsc.org/nh/files/2024/01/Picture1-300x185.jpg)
Fig. 1 (a) Histogram of isomer abundances from dynamic movies compared with a static image of a cluster ensemble. Reproduced from DOI: 10.1039/D3NH00291H with permission from the Royal Society of Chemistry. (b) Schematic energy landscape of cluster structures for fcc metals. A red shade indicates the cluster size range in the current study. Ih: icosahedral. Dh: decahedral. Adapted from DOI: 10.1002/anie.202015166 with permission from Wiley-VCH.
To find out more, please read:
Frame-by-frame observations of structure fluctuations in single mass-selected Au clusters using aberration-corrected electron microscopy
Malcolm Dearg, Cesare Roncaglia, Diana Nelli, El Yakout El Koraychy, Riccardo Ferrando, Thomas J. A. Slater, and Richard E. Palmer
Nanoscale Horiz., 2024, 9, 143-147
About the blogger
![]()
Jingshan S. Du is a Washington Research Foundation Postdoctoral Fellow at Pacific Northwest National Laboratory and a member of the Nanoscale Horizons Community Board. His research spans crystal formation and transformation pathways, in situ electron microscopy, and hybrid organic/inorganic nanostructures. Du received a Ph.D. in Materials Science and Engineering from Northwestern University in 2021. At Northwestern, he worked on complex nanoparticle systems, correlative electron microscopy of hybrid nanostructures, and nanoscale thermodynamics. Du received a Certificate for Management for Scientists and Engineers from Northwestern’s Kellogg School of Management in 2021 and a B.Sc. in Engineering from Zhejiang University Chu Kochen Honors College in 2015. You can follow him on Twitter @JingshanDu. The views expressed in this article do not necessarily reflect those of the author’s employer or the US government.
|