The two clusters at the heart of this work are chemical cousins with a fascinating structural relationship. M44, formally Au24Ag20 and protected by twenty-four 4-tert-butylphenylacetylide ligands and two chlorides, consists of a hollow icosahedral Au12 kernel wrapped in a fullerene-like Ag20 inner shell and capped by twelve exterior gold atoms. M81, with the formula Au43Ag38, is essentially a dimer of M44: two Au12 units fuse through an additional gold atom to form an Au25 kernel, while the two Ag20 shells merge with the loss of two silver atoms to create an Ag38 shell, all further wrapped by eighteen gold atoms and ligands. This face-fusion architecture is distinct from the side-on biicosahedral fusion seen in related clusters such as Au20Ag32 and Au23. Kohn-Sham molecular orbital analysis reveals that M44 behaves as a superatom resembling Au25, whereas M81 shows molecular-like character reminiscent of Au38, giving the researchers a matched pair in which electronic structure alone dictates whether a cluster leans toward photoluminescence or photothermal behavior.

The synthetic route is elegant in its simplicity. A mixture of chloro(dimethylsulfide)gold(I) and silver acetate dissolved in dichloromethane and methanol is combined with the deprotonated acetylene ligand, forming a reddish-yellow Au-Ag-alkynyl polymeric intermediate within minutes. Reduction with tert-butylamine borane, followed by reaction in the dark for one to three days, yields the clusters. Rather than relying on reaction timing and direct crystallization as in earlier work, the team employed thin-layer chromatography to separate the two products, which appear as distinct bands and were unambiguously identified by ultraviolet-visible-near-infrared absorption spectroscopy. Reaction time biases the outcome: quenching at roughly twenty-four hours favors M44, while extending to thirty-six or seventy-two hours promotes M81 growth. This post-synthetic purification strategy makes both clusters accessible from a single pot, a practical advantage for scaling up.

Optically, the monomer-to-dimer transformation produces dramatic consequences. M44 shows absorption peaks at 365 nanometers, convoluted features at 450 and 505 nanometers, and a broad band at 640 nanometers with tailing into the near-infrared. M81 displays additional bands at 640 and 765 nanometers and extends its absorption tail into the NIR-II window beyond 1000 nanometers. Most strikingly, the dimerization boosts the molar extinction coefficient at the long-wavelength band by roughly sixty times, from about 850 to 50,000 per molar per centimeter, consistent with the stronger transition dipole of the rod-like M81 structure. Both clusters emit photoluminescence in the NIR-II region, peaking near 1080 nanometers for M44 and 1320 nanometers for M81, the latter being among the rarest emission depths achieved by atomically precise clusters, previously matched only by gold quantum rods.

The photoluminescence quantum yields, however, are deliberately low: 3 percent for M44 and 1.5 percent for M81 in deuterated chloroform solution. Herein lies the counterintuitive design principle behind the work. Strong electron-phonon coupling quenches radiative emission and instead channels the absorbed photon energy into lattice vibrations, releasing it as heat. Transient absorption spectroscopy confirmed the underlying excited-state dynamics, with approximately one-picosecond components attributed to internal conversion and intersystem crossing, and longer nanosecond components matching the emission lifetimes of roughly 300 nanoseconds for M81. The long-lived triplet states even enable singlet oxygen generation in M44, as confirmed by the diagnostic 1275-nanometer singlet-oxygen phosphorescence signal under oxygen purging, while M81’s emission energy of 0.89 electron volts falls just below the 0.97-electron-volt gap needed for triplet-to-singlet oxygen energy transfer, so no such signal appears.

Systematic photothermal measurements across varying concentrations and optical power densities revealed a crucial and often overlooked trade-off: photothermal conversion efficiency and maximum temperature rise are counteracting parameters. At a modest optical power density of 0.5 watts per square centimeter and low absorbance, M81 achieved its record 91 percent efficiency with a temperature rise of 11.2 degrees Celsius. Pushing the power density to 2.5 watts per square centimeter dropped the efficiency to 38 percent but drove the temperature rise to 35.4 degrees Celsius, reaching a solution temperature of nearly 60 degrees. At higher absorbance, M81 heated toluene to 96.8 degrees Celsius. M44 followed similar trends with a peak efficiency of 69 percent and temperature rises up to 58 degrees. Under 1064-nanometer excitation deep in the NIR-II window, M81 still produced a measurable temperature rise despite weak extinction there, one of the first demonstrations of direct NIR-II photothermy from atomically precise clusters.

The solid-state results are where the numbers become genuinely startling. Thin films of the clusters embedded in a polymethyl methacrylate matrix, irradiated at 2.5 watts per square centimeter, reached temperature increases of 226 degrees Celsius for M44 and 155 degrees for M81. Such extreme heating in a solid film is precisely what is needed for energy-harvesting devices, since converting light into electricity requires a large, localized temperature gradient rather than the rapid dissipation that occurs in dilute solutions. Crucially, ultraviolet-visible-near-infrared spectra recorded before and after irradiation in all three media, organic solution, aqueous suspension, and solid film, showed no discernible change, confirming that both clusters survive months of ambient storage and repeated intense heating without degradation. The team also encapsulated both clusters in the FDA-approved poloxamer Pluronic F-127, producing water-soluble supraparticles around 150 nanometers in diameter with photothermal efficiencies near 50 percent in water, a regime well suited to photothermal tumor therapy.

The culmination of the study is the first thermoelectric generator application of atomically precise nanoclusters. The researchers deposited a layer of clusters onto the hot side of a commercial bismuth telluride thermoelectric generator and irradiated it with the same 808-nanometer laser, deliberately omitting any external cooling on the cold side to demonstrate electricity generation under minimal, device-integrated conditions. The Seebeck effect, in which a temperature gradient across a thermoelectric material drives directional charge-carrier diffusion, converted the cluster-generated heat into stable open-circuit voltages of 146 millivolts for M44 and 127 millivolts for M81, sustained over 900 seconds of continuous irradiation. Cycling experiments showed the devices could be switched on and off repeatedly without measurable degradation, and control experiments on bare thermoelectric generators produced no detectable voltage, confirming that the clusters themselves are responsible for the effect. Thermal paste at the interface yielded comparable voltages, indicating that direct cluster deposition already provides efficient thermal coupling.

The broader implications extend well beyond the laboratory laser experiments. More than half of the total solar radiation arrives in the near-infrared, yet conventional solar energy harvesting captures predominantly the ultraviolet and visible portions. The absorption profiles of both clusters overlap substantially with the solar spectrum, suggesting that direct sunlight could in principle drive the same photo-thermo-electric cascade. Encapsulating the clusters within heat-resistant carbon or silica shells could allow operation at even higher temperatures under focused solar illumination. Perhaps most conceptually significant is the reframing of electron-phonon coupling, typically dismissed as an unwanted quenching mechanism, into a deliberate design principle for near-infrared photonic energy harvesting. Key challenges remain, including broadening the absorption further into the shortwave infrared, improving thermal coupling, and demonstrating long-term operational stability, but this work establishes atomically precise nanoclusters as functional device components rather than mere solution-phase chromophores, and points toward a future in which quantum-sized metal clusters help reclaim the invisible half of sunlight.

Subject of Research: Near-infrared photothermal conversion and photo-thermo-electric energy generation using atomically precise bimetallic gold-silver nanoclusters

Article Title: Near‐Unity Photothermal Conversion in Bimetallic M44 and M81 Nanoclusters Enables NIR‐Driven Photo‐Thermo‐Electricity Generation

Article References: Sardar, A., Mazumder, A., Ji, W., He, G., Wang, Y., Luo, L., Chen, S., & Jin, R. (2026). Near‐Unity Photothermal Conversion in Bimetallic M 44 and M 81 Nanoclusters Enables NIR‐Driven Photo‐Thermo‐Electricity Generation. Advanced Science, Article e78061. https://doi.org/10.1002/advs.78061

Image Credits: AI Generated

DOI: 10.1002/advs.78061

Keywords: atomically precise nanoclusters, gold-silver nanoclusters, photothermal conversion, near-infrared, thermoelectric generator, Seebeck effect, NIR-II photoluminescence, solar energy harvesting, electron-phonon coupling, bismuth telluride, photothermal therapy, nanomaterials

Aqueous zinc-ion batteries use a water-based electrolyte in which Zn²⁺ ions move between electrodes during charging and discharging. Water-based electrolytes are attractive because they are generally less flammable than the organic liquids used in conventional lithium-ion cells, and zinc is abundant, inexpensive and comparatively easy to handle. Yet the chemistry is challenging. Zinc-metal anodes can develop dendrites—needle-like metallic growths that may pierce separators and trigger short circuits—while repeated insertion and removal of multivalent Zn²⁺ can cause structural damage and sluggish reaction kinetics in electrode materials. Zinc-metal-free, or “rocking-chair,” architectures attempt to avoid some of these problems by storing zinc ions in both host electrodes rather than repeatedly plating and stripping zinc metal. Copper-based chalcogenides, including copper sulfides, are promising candidates because their lattices can participate in conversion reactions. In a conversion reaction, the original compound is chemically reorganized as ions and electrons are stored, potentially providing more charge than a simple intercalation process in which ions slip between existing atomic layers. The drawback is that conversion can also cause substantial structural rearrangement, making control over defects and transport pathways essential.

The Zhengzhou University team approached this problem by using copper nanoclusters as chemically defined starting materials. A nanocluster is smaller and more structurally specific than a conventional nanoparticle: it contains a precise number or narrow range of metal atoms surrounded by organic ligands. Those ligands are not merely passive stabilizers. During pyrolysis, the process in which a precursor is heated so that its organic components decompose and the inorganic framework reorganizes, the ligands can generate molecular fragments, carbon-containing residues and reactive chemical environments that influence the final solid. The researchers exploited this behavior by changing the ligand environment around the copper clusters before pyrolysis. They then produced Cu₇S₄ electrodes and examined how the choice of ligand affected the structure and electrochemical response. The central idea is comparable to programming a material before it exists: by modifying the cluster’s molecular shell, the researchers could alter the chemical events occurring during thermal conversion and thereby tune the defects and pathways that govern ion and electron movement.

One of the most important targets was the concentration of vacancies in the copper sulfide lattice. A vacancy is a missing atom at a position where one would normally be expected. In Cu₇S₄, sulfur vacancies can modify the local electronic structure, alter the charge distribution around neighboring copper atoms and create energetically favorable sites for ion transport. Defects can therefore be beneficial rather than simply imperfections. They may lower barriers for Zn²⁺ migration, expose additional electrochemically active regions and improve contact between the electrode and electrolyte. Too many defects, however, can destabilize a lattice or accelerate unwanted side reactions. Ligand engineering provides a route to balance these competing effects because different ligands decompose along different pathways and can leave behind distinct chemical fragments. According to the study, this control allows the researchers to manipulate both vacancy concentrations and the migration of ions and electrons in the pyrolysis products. The resulting material is not just Cu₇S₄ in a nominally identical chemical formula; its defect population and microscopic transport network are also engineered.

The distinction matters because electrochemical performance depends on more than the composition written on a label. When a zinc-ion battery is discharged, Zn²⁺ ions must travel through the aqueous electrolyte, cross the electrode–electrolyte interface and participate in reactions inside the active material. Electrons must simultaneously move through the electrode and external circuit. If either process is slow, the battery’s power capability suffers. A high rate performance means the electrode can retain useful charge storage when the battery is charged or discharged quickly. Specific capacity, usually expressed as charge per unit mass of active material, indicates how much charge the electrode can store, while cycling stability measures how well that capability survives repeated operation. Defects, interfaces and nanoscale dimensions can shorten transport distances and increase active surface area, but they can also make materials chemically vulnerable. The reported ligand-controlled Cu₇S₄ products outperformed materials prepared by directly pyrolyzing metal salts and ligands, showing higher specific capacity, stronger rate performance and greater overall stability. The comparison suggests that the atomically precise cluster precursor contributes more than a convenient copper source: it guides the formation of a better-organized electrochemical architecture.

The researchers also followed the pyrolysis process to connect precursor chemistry with the final electrode properties. Tracking pyrolysis is technically valuable because the transformation is often treated as a black box. A metal salt and an organic ligand may be heated together, but the resulting material can contain a mixture of phases, uncontrolled defects and carbon residues whose origins are difficult to identify. In an atomically precise cluster, by contrast, the initial arrangement of copper atoms and ligands is more clearly defined. As heating breaks chemical bonds, the evolving fragments can influence sulfur incorporation, copper coordination and the formation of vacancies. The study’s strategy therefore combines precursor engineering with pyrolysis analysis, allowing the material’s structure to be tuned through the sequence of transformations rather than only characterized after the fact. This could help explain why seemingly small changes in ligand chemistry produce measurable differences in battery behavior. It also points toward a wider design principle for energy materials: molecular fragments generated during synthesis can act as temporary templates or chemical regulators, shaping defects that later control charge storage.

The Cu₇S₄ electrode operates through conversion-type chemistry, which distinguishes it from many familiar battery hosts. In a simple intercalation electrode, guest ions enter and leave a host lattice while much of the original framework remains intact. Conversion materials instead undergo more extensive changes in bonding and phase composition as ions and electrons react with the active compound. Such reactions can unlock high capacities because more atoms participate in charge storage, but they often bring volume changes, reconstruction and loss of electrical contact. The improved stability reported for the cluster-derived electrode indicates that its defect structure and nanoscale morphology may help accommodate these transformations, although the source material does not provide a complete mechanistic account of every reaction intermediate. The researchers’ emphasis on ion and electron migration suggests that the engineered vacancies help maintain kinetic access to active sites during cycling. Their findings also fit a broader pattern in battery research, in which vacancy engineering, heterointerfaces and tailored surface chemistry are used to accelerate multivalent-ion storage. The significance here is the precursor-level control: rather than attempting to repair a finished Cu₇S₄ material, the team adjusts the chemistry that creates it.

The work does not amount to a commercial battery ready for immediate deployment, and the researchers do not present it as one. Performance in laboratory electrodes can depend on active-material loading, electrolyte composition, electrode thickness, current density and the design of the test cell. Full-cell behavior, long-term operation under practical conditions, manufacturing cost and environmental impacts would all need further evaluation. Copper and sulfur are attractive from a materials perspective, but large-scale production of atomically precise nanoclusters and ligand-controlled pyrolysis may be more complex than conventional electrode synthesis. Even so, the study offers a powerful conceptual advance for aqueous zinc-ion storage. It shows that the molecular identity of a precursor can influence the defect chemistry, transport properties and durability of a solid electrode after thermal conversion. As researchers search for safer stationary-storage technologies and alternatives to zinc-metal anodes, that connection between molecular design and electrochemical engineering could become increasingly important. The broader promise is a toolkit in which nanoclusters serve as programmable building blocks, enabling scientists to create battery materials whose vacancies, interfaces and transport channels are designed before the first ion enters the cell.

Subject of Research: Ligand-engineered copper nanocluster precursors for Cu₇S₄ anodes in aqueous zinc-ion batteries

Article Title: Regulating electrochemical performance of Cu₇S₄ electrodes via ligand engineering in copper cluster precursors

Article References: Wu, Z., Wang, L.-F., Liu, X.-F. et al. “Regulating electrochemical performance of Cu₇S₄ electrodes via ligand engineering in copper cluster precursors.” Nano Research 17, 9746–9755 (2024). Original research article

Image Credits: AI Generated

DOI: 10.1007/s12274-024-6956-z

Keywords: aqueous zinc-ion batteries, Cu₇S₄ anodes, copper nanoclusters, ligand engineering, pyrolysis, sulfur vacancies, conversion electrodes, zinc-metal-free batteries