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Atomically Precise Nanoclusters are Nanoparticles that can be synthesized with exact purity
Atomically-precise nanoclusters are valued as nanoparticles that can be isolated with exact compositional purity and often crystallized to determine their exact structures. They are tunable by size, metal doping, or exchange of some or all ligands, giving them great flexibility to be tuned for various applications in catalysis, sensing, or photonics.
Several hundred of these nanoclusters have been reported in the last 18 years. While gold is synthetically facile, clusters of silver, copper, and other metals are increasingly being reported as synthetic techniques mature. Ligands such as halides, phosphines, thiolates, N-heterocyclic carbenes, and more, play a key role in stabilizing clusters and determinging their geometric and electronic structure.
Despite the excitement over their potential as "designer nanoparticles," understanding the properties of atomically-precise nanoclusters remains a challenge, and we lack quantitative intuitive frameworks to guide synthetic efforts to leverage their tunability to optimize their performance. Our goal is to use precision measurements to create predictive models describing the electronic effects of doping, ligand exchange, and structural transformations.
A depiction of the scale of gold nanoclusters as a function of their composition. Clusters of ten or fewer atoms lie below 1 nm and have discrete HOMO-LUMO gaps, while clusters of several hundreds of atoms reach the size at which metallic behavior dominates.
Understanding the effects of doping on electronic properties
Doping of nanoclusters is a key mechanism to manipulate their electronic structure without significantly altering their geometric structure. However, quantifying the effects of dopants is a challenge, even for the most well-studied clusters and dopant species. This is largely due to two challenges: 1.) isolating clusters with specific numbers of dopants is challenging, and thus atomic precision is lost, and 2.) experimental probes of electronic structure do not provide sufficient detail to extract the changes induced by doping and why they occur.
We are able to eliminate both of these challenges: we can purify any mixture arbitrarily using mass spectrometry, which filters all but the exact composition that we want to study. Then, at low temperature, we record spectra of the purified clusters, giving us resolution near to quantum limit for a sample that we have identified unambiguously. This allows us to track the changes induced by dopants atom by atom!
The HOMO-LUMO gap of Au25-nAgn(SR)18 anionic clusters shows a complex even/odd alternating pattern that is not resolvable using other experimental techniques.
Controlling electronic properties using ligand chemistry
Ligand chemistry is less explored than dopant chemistry for nanoclusters, but it is well developed for inorganic complexes in general. We are showing that ligands can be used to systematically fine tune the electronic properties of nanoclusters in similar ways to dopants. Many familiar concepts can be adapted from coordination complexes to explain ligand effects on nanocluster electronic structure, though the effects are sometimes the opposite of those in complexes!
The HOMO-LUMO gap of Au8 and Au9 nanoclusters can be controlled by manipulating the electron donating character of substituents on ligand phenyl rings.
This project has been supported by by the Air Force Office of Scientific Research under grant numbers FA9550-17-1-0373 and FA9550-19-1-0105 and the Department of Energy under grant number DE-SC0021991.
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