An international team of researchers has presented strong evidence that individual clusters of twenty gold atoms (Au20) take on a pyramidal shape.
“Au20 clusters have a triangular ground plane made up of ten neatly arranged atoms, with additional triangles of six and three atoms, topped by a single atom,” said Dr. Zhe Li from the Harbin Institute of Technology and colleagues.
Using low-temperature scanning tunneling microscopy, Dr. Li and his collaborators from KU Leuven, National Tsing Hua University, and the Università di Milano-Bicocca obtained atom-resolved images of Au20 clusters.
“The scanning tunneling microscope can visualize single atoms,” they explained.
“It operates at extremely low temperatures (minus 269 degrees Celsius) and uses quantum tunneling of an electrical current from a sharp scanning metallic tip through the cluster and into the support.”
“Quantum tunneling is a process where electrical current flows between two conductors without any physical contact between them.”
The researchers used intense plasma in a complex vacuum chamber setup to sputter gold atoms from a tiny piece of gold.
“Part of the sputtered atoms grow together to small particles of a few up to a few tens of atoms, due to a process comparable with condensation of water molecules to droplets,” Dr. Li said.
“We selected a beam of clusters consisting of exactly twenty gold atoms.”
“We landed these species with one of the triangular facets onto a substrate covered with a very thin layer of NaCl, precisely three atom layers thick.”
The scientists also analyzed the peculiar electronic structure of the Au20 cluster.
“Similar to noble gas atoms or aromatic molecules, the Au20 cluster only has completely filled electron orbitals, which makes them much less reactive than clusters with one or a few atoms more or less,” they said.
“Our work demonstrates a generally applicable routine to study the intrinsic properties of well-defined clusters, as well as their sintering mechanism on surfaces,” they added.
“Detailed knowledge and understanding of morphology, size distribution, and electronic structure of supported clusters are important to evaluate their catalytic and optical performances and, hence, highly relevant to advancing the design of cluster-based catalysis and optical devices.”
The findings appear in the journal Science Advances.
Zhe Li et al. 2020. Unraveling the atomic structure, ripening behavior, and electronic structure of supported Au20 clusters. Science Advances 6 (1): eaay4289; doi: 10.1126/sciadv.aay4289
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