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

Writing in Advanced Composites and Hybrid Materials, the group led by Tata Sanjay Kanna Sharma, Jin Suk Chung, and Won Mook Choi describes a bifunctional heterostructure in which phase-pure bismuth telluride nanosheets are woven into a matrix of titanium carbide MXene, a conductive ceramic-like material derived by etching aluminum from layered titanium carbide precursors. The strategy is deceptively simple in concept: let the topological insulator supply the catalytic chemistry while the MXene supplies the electrical plumbing and the mechanical armor. In practice, achieving that partnership required precise control over synthesis, because bismuth telluride is notoriously easy to form with defects and secondary phases that ruin its electronic character.

The researchers produced their bismuth telluride nanosheets through a surfactant-assisted hydrothermal method, a wet-chemical route in which crystals grow from a hot aqueous solution under the guidance of organic molecules that steer nucleation along particular crystal faces. The surfactant acts like a molecular traffic controller, encouraging the growth of thin, well-defined platelets rather than irregular clumps. Once the nanosheets were formed, they were integrated into the MXene framework, whose negatively charged, functionalized surfaces, denoted Ti3C2Tx in the literature, help anchor the chalcogenide sheets and create intimate interfacial contact between the two layers.

What makes the interface so important is the physics of charge transfer. In electrocatalysis, every molecule of hydrogen gas produced or oxygen bond broken requires electrons to move from the electrode to the reacting species, or vice versa. Any bottleneck along that path, whether from poor conductivity, a sluggish interface, or a corroding surface, shows up as extra voltage that must be supplied to drive the reaction. That extra voltage, called overpotential, is the enemy of efficient energy conversion, because it represents energy wasted as heat. By fusing the two materials at the atomic scale, the team created a highway for electrons that bypasses the conductivity limitations of bulk bismuth telluride entirely.

The evidence for this synergy comes from an unusually thorough battery of structural probes. Synchrotron-based X-ray analysis allowed the researchers to examine the crystal structure of the composite with high precision, confirming that the bismuth telluride retained its phase purity even after integration with the MXene. Temperature-dependent Raman spectroscopy, which tracks how the vibrational fingerprints of a material shift as it is heated and cooled, revealed that the MXene incorporation modifies the structural and vibrational characteristics of the bismuth telluride, a sign that the two materials interact strongly at their junction rather than merely sitting side by side.

Perhaps the most consequential finding concerns durability. Bismuth telluride, like many chalcogenides, is vulnerable to oxidation, and a corroded surface loses the very electronic properties that make it interesting. Time-of-flight secondary ion mass spectrometry, a technique capable of mapping chemical species layer by layer with exquisite sensitivity, indicated that the MXene effectively passivates the chalcogenide surface, shielding it against oxidation. In other words, the MXene does double duty: it is both a conductive scaffold that ferries electrons and a structural buffer that protects the delicate topological insulator from the chemical assault of its own working environment.

The electrochemical performance numbers are striking. For the hydrogen evolution reaction, the cathodic half of water splitting, the composite required an overpotential of just 260 millivolts to sustain a current density of 100 milliamperes per square centimeter, a current level relevant to industrial electrolyzers rather than laboratory curiosities. For the oxygen evolution reaction, the anodic counterpart and historically the more difficult of the two reactions, the overpotential was 490 millivolts at the same demanding current density. The material also catalyzed the oxygen reduction reaction with an onset potential of 0.81 volts, the reverse chemistry that powers fuel cells and rechargeable metal-air batteries. In every case, the heterostructure significantly outperformed pristine bismuth telluride, confirming that the improvement stems from the engineered interface rather than the topological insulator alone.

Mechanistic probing added a layer of understanding that goes beyond performance metrics. The experiments indicate that the hydrogen evolution reaction proceeds predominantly at bismuth sites, enabled by partial bismuth-oxygen surface termination, a subtle surface chemistry that appears to tune the binding energy of hydrogen intermediates into the optimal range. The MXene, meanwhile, plays a supporting but indispensable role: it supplies rapid electron conduction to the active sites and acts as a sink for defects, stabilizing the catalytic centers that would otherwise degrade. This division of labor, with one material hosting the chemistry and the other managing the electrons and the damage, offers a design template that could be applied well beyond this particular pairing.

The broader significance lies in the emerging field of van der Waals heterostructures, in which atomically thin layers of different materials are stacked and coupled to produce properties that neither constituent possesses alone. The keywords that accompany the paper, including active-site engineering, charge-transfer kinetics, and energy storage electrodes, signal the authors’ ambition to treat the interface itself as the active component. Because both bismuth telluride and MXenes are two-dimensional, their junctions can in principle be designed with atomic precision, opening a path to rational optimization of charge transport and ion kinetics rather than the trial-and-error mixing that has historically dominated composite electrode research.

Challenges remain before such materials reach commercial electrolyzers or fuel cell stacks. MXenes are themselves susceptible to oxidation in water, scaling up surfactant-assisted hydrothermal synthesis demands careful process control, and long-term stability testing under industrial conditions will be essential. Still, the demonstration that a single interface-engineered heterostructure can drive hydrogen evolution, oxygen evolution, and oxygen reduction with high efficiency is a compelling proof of concept. If the approach generalizes to other topological insulators and MXene chemistries, the exotic surface states first predicted in quantum physics laboratories may one day help split water on an industrial scale, turning a celebrated curiosity of condensed matter into a workhorse of the clean energy economy.

Subject of Research: Interface engineering of Bi2Te3-MXene two-dimensional heterostructures for electrocatalytic energy conversion and storage

Article Title: Interface-engineered Bi2Te3-MXene heterostructure for multifunctional energy conversion and storage

Article References: Sharma, T. S. K., Babu, B. M., Jana, J., Solaiappan, L., Kumar, R., Kuriam, M. M., Lee, Y., Hur, S. H., Shin, T. J., Chung, J. S., & Choi, W. M. (2026). Interface-engineered Bi2Te3-MXene heterostructure for multifunctional energy conversion and storage. Advanced Composites and Hybrid Materials. https://doi.org/10.1007/s42114-026-02072-3

Image Credits: AI Generated

DOI: 10.1007/s42114-026-02072-3

Keywords: bismuth telluride, MXene, topological insulator, heterostructure, hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, electrocatalysis, two-dimensional materials, charge transfer, energy storage, interface engineering