Superatoms offer new dimension to materials chemistry palette

Across the periodic table’s broad palette of chemical colour, each element is unique. Chemists artfully blend their distinct hues to create dazzling arrays of functional material masterpieces.

Held up to the light, however, many elements have negative as well as positive aspects to their nature; characteristics that limit the real-world use of the materials they have been woven into. Next generation thin-film solar materials, for example, have been hampered by a reliance on toxic elements such as arsenic or lead. Palladium, platinum and rhodium often make exceptional catalysts, but their relative rarity and high cost prohibit many potential applications.

From this perspective, the periodic table’s elemental colour palette can seem very limited. Replacing the problematic element in a functional material with an alternative, even a near neighbour, often fundamentally changes the blend, altering the material’s structure and compromising its function.

What if we started making materials from building blocks whose properties were not so rigidly fixed? Could superatoms – small, tuneable clusters of atoms that can mimic aspects of the chemistry of particular elements, without containing a single atom of that element – fly to the rescue?

Superatoms’ origin story

The concept that atomically precise nanoclusters could act as super-surrogates for specific chemical elements began as a theoretical pursuit, says Puru Jena, who came up with superatom science alongside Shiv Khanna at Virginia Commonwealth University in the US.

‘Two experiments led us to the superatom concept,’ Jena says. One involved an apparent magic trick, and the second centred on an aluminium cluster that thought it was a halogen.

In the early 1990s, when Jena and Khanna proposed the superatom idea, nanocluster chemistry was in its infancy. Experimentalists worked mainly in the gas phase, producing nanoclusters in trace quantities from hot atomic plumes of one or more element in near-vacuum conditions.

Walter Knight at the University of California, Berkeley, in the US had been using mass spectrometry to study sodium clusters created this way. ‘They looked at the abundance of sodium clusters as a function of size, and found that clusters containing “magic” numbers of atoms – including two, eight, 20 and 40 – were unusually stable, forming pronounced peaks in the mass spectra,’ Jena says.

This observation could be rationalised in the same way as noble gas stability, Knight showed. According to his ‘jellium’ model of spherical nanocluster electronic structure, each sodium atom contributes its single valance electron to a delocalised electron system. These electrons orbit the sodium ion cluster in shells akin to atomic orbitals. In magic number sodium clusters, these shells were filled. The eight-atom sodium cluster has its electrons arranged in a noble-gas-like 1s² 1p⁶ configuration, for example, while sodium-40 also has a filled shell set.

The size and composition of those superatoms could have almost infinite variation

The second experiment was led a few years later by Will Castleman at Pennsylvania State University, US. ‘They found that if you take a cluster of 13 aluminium atoms – which has 39 delocalised electrons – and add one extra electron, then it becomes a stable 40 electron cluster just like sodium-40,’ Jena says. With its stable closed shell electron arrangement, the Al₁₃^– cluster was inert to attack by oxygen, for example. Without the extra electron and one short of a closed shell, however, aluminium-13 was highly reactive – just like a halogen atom.

By controlling the size and composition of a cluster, Jena realised, it should be possible to mimic the properties of atoms across the periodic table. He began to imagine a periodic table that was not flat like Mendeleev’s, but featured a third dimension populated by superatoms. ‘Because the size and composition of those superatoms could have almost infinite variation, that third dimension of the periodic table could be virtually unlimited,’ he says.

Superatoms also potentially offered enhancements over the elements they mimic. Toxic elements might be replaced by superatoms with the same chemistry but made only of atoms without environmental or health concerns. Rare and expensive elements could also be sidestepped. ‘If I can mimic them with a superatom made of earth-abundant elements, I have found a way to circumvent the chemistry of the periodic table to eliminate some of the properties that are undesirable,’ Jena says. As atomically precise cluster synthesis has developed, that vision is beginning to be realised.

Moving from gas phase to solid state

Cluster chemist Tatsuya Tsukuda from the University of Tokyo, Japan, makes solid-state superatom nanoclusters with real world functional promise. Tsukuda started his independent research group with the aim of testing whether superatom behaviours observed in clusters formed in near-vacuum gas phase conditions could be translated into solid phase materials. ‘I wanted to bridge the gap between metal cluster research in vacuum and the metal clusters in the real world as materials,’ Tsukuda says.

Tsukuda focused on atomically precise gold nanocluster synthesis. The challenge with making nanoclusters outside of near-vacuum conditions is that, as soon as they come into contact, clusters want to aggregate to reduce their surface energy. ‘To protect against aggregation, we cover the clusters with organic ligands, so the metal cores cannot contact each other and combine,’ Tsukuda says.

The goal now is to make cluster-based materials with new properties 

The chemistry took a while to refine, but the effort to make these ligand-protected clusters was worth it. ‘Once I achieved atomically precise synthesis of a series of clusters, I realised there was a common concept between free clusters in vacuum and the gold clusters protected by ligands – and that was the superatom concept,’ Tsukuda says. Nanocluster electron-shell closure determined stability both in free clusters and ligand-protected clusters. ‘The goal now is to make cluster-based materials with new properties that other materials cannot achieve,’ he says.

Catalysis is one promising application. Whereas bulk state gold is chemically inert, gold nanoparticles are famously active catalysts, able to oxidise organic substrates using oxygen from the air for example. Conventional synthesis typically generates gold nanoparticles with a range of sizes, however, and the nanoparticles are again prone to aggregation, curbing their catalytic performance.

Tsukuda’s superatom synthesis methods can overcome both limitations, generating atomically precise nanoclusters with a ligand coat that prevents aggregation. Engineering the ligand environment to enable substrate access to the gold surface while still preventing superatom aggregation has been a recent focus of his work.

In one surprise finding, the team showed that gold-25 clusters coated with long chain thiolate-capped ligands can make excellent heterogeneous catalysts. Ligands on the cluster’s underside can stick the superatoms to solid carbon support through van der Waals interactions, and a controlled heat treatment then selectively strips ligands from the superatom’s upper side to expose the gold surface. These anchored superatoms catalysed alcohol oxidation to carboxylic acids in air.

‘We had initially tried removing all of the ligands, because thiolates are usually a catalyst poison,’ Tsukuba says. ‘But we found that when we intentionally left some ligands behind, their strong interaction with the support played a very important role, enhancing the catalyst’s durability by suppressing superatom migration on the surface.’

Engineered ligand environments also play a key role in the superatom catalysts recently reported by Quan-Ming Wang at Tsinghua University in Beijing, China, and his collaborators. The team made an unusual ligand-protected 40-atom gold cluster in which the catalytically active core was a gold-26 superatom and the remaining gold atoms served as stabilising structural ‘staples’. Adhered to a titania support, this superatom was a 100% selective reduction catalyst for nitro groups in arenes with multiple other reducible groups.

More recently, Wang made a highly stable superatom catalyst from a copper-45 complex. Copper has great appeal as a catalyst because it is abundant and inexpensive, but copper clusters are notoriously difficult to work with because they are inherently unstable, oxidising readily in the air. Stabilised by its 1S² 1P⁴ electron structure and strong ligand interactions, however, Wang’s copper superatom set a new standard for the efficient electrocatalytic upcycling of carbon dioxide into ethylene.

From molecules to materials

Single superatom catalysis is one aspect of these nanoclusters’ promise. Tsukuda has also shown it is possible to bond superatoms together to make supermolecules. Again, the trick is to manipulate the superatom’s ligand coating.

Inspired by Nobel-winning metal–organic framework chemistry, the team synthesised gold superatoms with selectively removable ligands at the polar position. When the team chemically removed one polar ligand, then added a bridging linker ligand, superatom dimers formed. By removing both polar ligands, multiple superatoms can start to be strung together.

The team has also manipulated gold nanocluster ligand structure so that several superatoms can fuse in controlled fashion. The resulting thin gold rods possess tuneable light absorption; the longer the rod, the more red-shifted its light capture. Wrapped in the organic light emitter rubrene, these nanorods achieved efficient photon upconversion, absorbing two near-infrared photons and combining their energy to emit a photon of visible light. Such materials could upconvert near-infrared light – a major component of the solar spectrum – into a form that solar cells or photocatalysts could use.

In Xavier Roy’s lab at Columbia University in New York, US, the team has assembled superatoms into materials with properties that are not just unusual, but unique. Roy was drawn to the field to explore the possibility that by using superatoms to assemble functional materials, rather than using regular elements, their properties could be mixed and matched for the ideal blend of behaviour.

‘We started to think of them in terms of highly tuneable building blocks, where you can design the properties of each individual block using synthetic chemistry,’ Roy recalls. ‘We can tune their luminescence, their redox behaviour, introduce magnetism, control how they bind … All before their assembly into functional materials.’

Roy initially worked on clusters synthesised from solution, where charge transfer between an electron-rich and electron-poor superatom could give rise to ionic superatomic materials. The team then transitioned to manipulating the chemistry of the superatom’s capping ligands, using synthetic chemistry to control their bonding through covalent linkages.

‘We have developed methods where we can selectively decorate our superatoms with, for example, a set of bridging ligands, and then a set of capping ligands, and use these features to control the dimensionality of the lattice,’ Roy says.

Light harvesting superatoms

Some of Roy’s most intriguing results come from a well-known material only recently recognised as superatomic in nature. ‘Rhenium selenium chloride is an old material that people have studied for decades, excising the clusters from these lattices to do chemistry on the individual superatoms,’ Roy says. ‘But we can grow these materials as large, crystalline, two-dimensional sheets of covalently bonded superatoms using high-temperature solid-state chemistry.’

Strange things happen when these crystals are illuminated with light. Like many semiconductors, light exposure generates excited electron–hole pairs known as excitons in the material. In rhenium selenium chloride, however, these excitons bind to vibrations in the lattice called acoustic phonons, forming a new quasi-particle called an acoustic polaron.

If you look at the edge of a typical crystal, there’s a lot of defect sites 

‘These acoustic polarons travel across the material without any scattering, and so we see transport phenomena that have never been observed in any other material,’ Roy says. These particles – which can only arise in superatomic materials – could be very useful for efficiently harnessing light, he says.

In traditional semiconductors, electron–hole pairs frequently recombine before anything useful can be done with them. ‘If you look at the edge of a typical crystal, there’s a lot of defect sites where excitons get trapped and recombine, and you lose that energy,’ Roy says. ‘But because the acoustic polarons in our materials seem to be completely insensitive to this problem, we’re thinking of ways that we can use these materials as antennas to funnel excitons to where we need them.’ These light-energy-harvesting antennas could be used for low light energy generation or catalysis, delivering excitons directly to catalyst particles attached at specific sites on the material’s surface.

Superatoms to the rescue in energy applications

For a concept once considered purely theoretical, superatom science has come a long way, Jena says. ‘There are now many materials enhanced by replacing an atom with a superatom of the same chemistry,’ he says.

Many early examples have involved swapping a halide for a superhalide. In hybrid perovskite solar cells such as methylammonium lead iodide, for example, lead–iodide bonds’ susceptibility to breakage contributes to the material’s notorious instability to moisture and heat. Jena’s computational modelling suggested that replacing some of the iodide with a superhalogen such as BH₄^–, SCN^– or SeCN^– could stabilise the material by strengthening bonding.

Borohydride is a polyatomic species commonly used as a reducing agent in organic synthesis, and commercially available in bulk quantities. Just like a halogen, the neutral cluster is one electron short of a closed shell, Jena notes. In lab experiments, adding borohydride not only enhanced a photovoltaic perovskite’s stability to humidity, heat and UV light, but boosted its solar power conversion efficiency from 18.43 to 21.10%. Subsequent studies have confirmed the superhalide additive effect in a range of hybrid perovskite formulations. Modelling predicting that switching to superhalides in solid-state lithium-ion battery electrolytes could improve battery performance has also been borne out experimentally.

Jena has recently focused on finding cheap superatoms that mimic the performance of precious metal catalysts. In a 2025 computational study, he calculated that earth-abundant ZrO – a superatom with the same outer electron count as palladium – outperformed the precious metal as a catalyst, more strongly binding and activating small molecules such as H₂ and CO₂.

Compared to single atom catalysts, single superatom catalysts can offer the same chemistry but with multiple sites for substrate binding and reaction. ‘Changing an atom with a superatom could lead to a new generation of catalysts,’ Jena says. Experimentalist colleagues are already testing out the idea, he adds.

As the periodic table begins to sprout a superatomic third dimension, materials chemistry’s colour palette looks increasingly bright.

James Mitchell Crow is a science writer based in Melbourne, Australia