The material at the center of the work, MoS2, belongs to a family of compounds that can be peeled into sheets just three atoms thick while retaining useful semiconducting behavior. Unlike graphene, which conducts electricity superbly but lacks an intrinsic bandgap, MoS2 offers a genuine gap between its valence and conduction bands, making it a serious candidate for transistors that could push past the scaling limits of silicon. But a transistor is only as good as its contacts, and in two-dimensional devices the metal–semiconductor junction often dominates the total resistance. When a metal such as antimony is deposited on MoS2, charge carriers must cross an energy barrier known as the Schottky barrier, whose height depends delicately on the band structure of the semiconductor at the interface. Any factor that reshapes that band structure, including how many layers of the material are stacked together, can therefore move the performance needle dramatically.

To quantify this layer dependence systematically, the researchers fabricated field-effect transistors built on MoS2 channels of varying thickness, from a single atomic layer up to many layers, each fitted with antimony contacts. Rather than relying on a single device measurement, which can be skewed by fabrication quirks, they extracted contact resistance using the transmission line method, a well-established technique in which identical devices are patterned with channels of several different lengths. By measuring the total resistance of each device and plotting it against channel length, the extrinsic contribution from the contacts separates cleanly from the intrinsic resistance of the MoS2 channel itself, allowing the junction resistance to be isolated with confidence. The team repeated this analysis for devices built on two different gate dielectrics, silicon dioxide and hafnium dioxide, to check that the observed trends were robust properties of the MoS2–antimony interface rather than artifacts of one particular substrate chemistry.

The resulting dataset revealed the striking non-monotonic behavior. Moving from a monolayer to a trilayer, contact resistance steadily decreased, bottoming out at 308 ohm-micrometers for the three-layer devices, a figure that represents a substantial improvement over the monolayer case. But the improvement did not continue. As the researchers stacked on additional layers beyond three, the contact resistance reversed course and began climbing again, erasing the gains made in the few-layer regime. The fact that the same pattern appeared on both silicon dioxide and hafnium dioxide dielectrics strengthens the conclusion that the minimum at three layers reflects intrinsic contact physics rather than some accident of device processing. For device engineers, the message is unusually concrete: when optimizing a MoS2 transistor with antimony contacts, the thickness of the channel is not a free parameter to be tuned casually, because the sweet spot sits at a specific, countable number of atomic layers.

Explaining why three layers occupy the optimum required the authors to dig into the electronic band structure of few-layer MoS2 and the way charge carriers actually move through a stacked crystal. In a monolayer, the bandgap is at its largest, roughly in the range of 1.8 to 1.9 electronvolts in direct-gap form, and as layers are added the gap narrows and the band character shifts. That narrowing matters enormously at a metal contact, because the height of the Schottky barrier that electrons must surmount is tied directly to the position of the conduction band edge relative to the metal’s Fermi level. A smaller bandgap pulls the band edge downward, lowering the barrier and letting carriers flow more easily across the junction. In the few-layer regime, this bandgap reduction effect works in favor of the contact, and it explains the steady drop in resistance observed between one and three layers.

But the same stacking that shrinks the bandgap also introduces a penalty. In a multilayer crystal, carriers injected at the top surface, where the metal contact sits, must hop or tunnel between adjacent layers to traverse the material and reach the conduction pathway. Each interlayer hop adds its own resistance, and the more layers there are, the more hops a carrier must make and the more the transport pathway resembles a series of resistive steps rather than a smooth highway. In thin stacks this interlayer transport resistance is negligible compared with the barrier-lowering benefit, but as thickness grows the penalty accumulates. Eventually, according to the study’s analysis, the growing interlayer resistance overtakes the diminishing returns of bandgap narrowing, and the total contact resistance begins to rise. The crossover between these two trends lands at three layers, producing the minimum the measurements captured so cleanly.

This competition between bandgap reduction and interlayer transport is more than a curiosity of one material pair; it illustrates a general design principle for two-dimensional electronics. Researchers have long known that layer number tunes the electronic properties of layered semiconductors, but the new results show that the optimal thickness for a device is set not by the channel alone but by the contact physics at its edges. A channel that is ideal for switching characteristics may be far from ideal for injecting current, and vice versa. The authors argue that their findings offer practical guidance for contact engineering and channel thickness selection in next-generation devices, suggesting that designers of MoS2-based transistors should treat trilayer channels with antimony contacts as a particularly favorable combination rather than defaulting to the monolayer devices that dominate much of the literature.

The choice of antimony as the contact metal is itself noteworthy. Metals that form low-barrier contacts to MoS2 without damaging the fragile atomic surface are scarce, and semimetallic electrodes have attracted attention precisely because their modest density of states and weak Fermi-level pinching can keep injection barriers low. The fact that a clear, reproducible layer-dependent signature emerges with antimony electrodes on two different dielectric substrates suggests the effect is a robust feature of the interface, one that other laboratories replicating the fabrication recipe should be able to observe. The transmission line methodology, meanwhile, offers a template for how the field can move beyond anecdotal single-device reports toward systematic, statistically grounded characterizations of contact behavior across thickness series.

For the broader effort to bring two-dimensional semiconductors out of the laboratory and into manufactured electronics, the study lands at an opportune moment. As silicon transistors approach their physical limits, the semiconductor industry has been evaluating atomically thin channels as candidates for future technology nodes, and contact resistance remains one of the most cited roadblocks in those assessments. Results like these sharpen the engineering map: they identify not only which material combinations lower the barrier but also how the internal transport architecture of the channel itself feeds back into contact performance. The three-layer optimum found by Liu and colleagues is unlikely to be the final word, since different contact metals, dielectrics and doping strategies will shift the crossover point, but the underlying principle, that bandgap narrowing and interlayer resistance trade off against each other as thickness grows, gives researchers a quantitative framework for finding the new optimum in each variant. In a field where single atomic layers make the difference between a record-setting device and a mediocre one, knowing exactly where the sweet spot lies is a valuable piece of the puzzle.

Subject of Research: Layer-dependent contact resistance at MoS2–antimony interfaces in two-dimensional field-effect transistors

Article Title: Non-monotonic thickness dependence of MoS 2 –Sb contact resistance driven by bandgap and interlayer transport

Article References: Liu, L., He, Z., Li, Y., Lin, S., & Li, S. (2026). Non-monotonic thickness dependence of MoS2–Sb contact resistance driven by bandgap and interlayer transport. PLOS One, 21(10), e0360960. https://doi.org/10.1371/journal.pone.0360960

Image Credits: AI Generated

DOI: 10.1371/journal.pone.0360960

Keywords: MoS2, contact resistance, two-dimensional semiconductors, Schottky barrier, field-effect transistors, transmission line method, antimony contacts, bandgap engineering, interlayer transport, PLOS One, Non-monotonic, thickness

The study, conducted entirely with the SCAPS-1D simulation software developed at Ghent University, systematically explored how every major design parameter of a solar cell affects its output. The researchers modeled an n-i-p architecture of the form FTO/TiO2/Cs2BiAgI6/Cu2O/Ag, in which a titanium dioxide electron transport layer extracts electrons from the Cs2BiAgI6 absorber, while a copper oxide hole transport layer ferries holes to a silver back contact. Under the standard AM 1.5G solar spectrum at 1000 W/m2 and a temperature of 300 K, the optimized structure produced an open-circuit voltage of 1.2998 V, a short-circuit current density of 26.68 mA/cm2, a fill factor of 83.80 percent, and the headline efficiency of 29.06 percent.

One of the most striking findings concerns the choice of back-contact metal. The team tested silver, gold, and platinum electrodes and found remarkably similar performance across all three. The platinum variant actually edged ahead with a simulated efficiency of 30.03 percent, an open-circuit voltage of 1.2816 V, a current density of 26.69 mA/cm2, and a fill factor of 84.94 percent, while the gold contact yielded 29.00 percent. Because the differences were marginal, the researchers selected silver for their reference design, a pragmatic choice given silver’s cost advantage over platinum and its favorable work function for hole collection.

Layer thickness proved to be a delicate balancing act. Varying the electron transport layer revealed that 120 nm of TiO2 delivered the optimal 29.06 percent efficiency, with efficiency rising only slowly as the layer thickened before the fill factor and power conversion efficiency began to decline. The hole transport layer told a similar story: at 140 nm of Cu2O, the device reached its peak, and the relationship between thickness and efficiency was found to be directly proportional up to that point. These results echo earlier work on organic semiconductor devices, where transport layers must be thick enough to resist field-induced degradation, typically between 10 and 20 nm at minimum, yet thin enough to avoid parasitic light absorption and excessive series resistance.

The absorber layer itself, the heart of the device, was swept from 650 nm to 1050 nm. Efficiency rose steadily with thickness, confirming a positive correlation, although the fill factor moved in the opposite direction. The optimum landed at 750 nm, where the simulated device achieved its maximum performance. This thickness optimization matters because the absorber must be thick enough to capture incoming photons and generate electron-hole pairs, but not so thick that photogenerated carriers recombine before reaching the transport layers.

Perhaps the most consequential parameter was the bulk defect density of the Cs2BiAgI6 absorber. The researchers varied defect concentrations from 10^11 to 10^15 cm−3 and found that efficiency peaked at the lowest density, 10^11 cm−3, remaining nearly constant below that threshold. Simulations at even lower densities suffered from convergence failures in the iterative solver, a common numerical limitation when defect populations become vanishingly small. The physics here is governed by Shockley-Read-Hall recombination, the dominant loss pathway in perovskite devices: defects act as traps that capture charge carriers and dissipate their energy non-radiatively, shortening carrier lifetimes and eroding both quantum efficiency and open-circuit voltage. The recombination rate depends on the carrier capture cross-section, thermal velocity, and trap density, so every order-of-magnitude reduction in defects translates directly into longer diffusion lengths and better charge collection.

Temperature emerged as a persistent adversary. Sweeping the operating temperature from 300 K to 380 K, the team observed that open-circuit voltage, fill factor, and efficiency all declined as temperature rose, while short-circuit current density increased. The mechanism is well understood: additional thermal energy generates more charge carriers and slightly narrows the bandgap, boosting current, but it simultaneously raises series resistance, lowers shunt resistance, and accelerates recombination, dragging down voltage and fill factor. The open-circuit voltage follows a logarithmic dependence on the ratio of short-circuit current to the dark saturation current, so any increase in recombination-driven dark current exacts a heavy toll on voltage. The best performance was therefore obtained at the coolest simulated condition, 300 K, a reminder that thermal management will be essential in real-world deployments.

Bandgap engineering also proved decisive. The Cs2BiAgI6 absorber’s native bandgap of 1.5 eV sits close to the ideal value for single-junction solar energy conversion, and the simulations confirmed that this is where the device shines, delivering roughly 29 percent efficiency. As the bandgap was widened, the material absorbed fewer photons from the solar spectrum, reducing current density, fill factor, and overall efficiency, with the lowest recorded efficiency of 26.08 percent at a wider gap. Beyond about 1.6 eV the simulation became numerically infeasible. The authors note that alloying strategies, such as substituting small amounts of tin into related double perovskites like Cs2AgBiBr6, can push bandgaps as low as 0.5 eV, underscoring how compositional tuning could open further design space without introducing toxicity.

Electron affinity, the energy difference between the vacuum level and the conduction band, was varied from 3.8 to 4.3 eV. Efficiency fell as the affinity increased, with the optimum at 4.00 eV yielding the familiar 29.06 percent benchmark and the worst case at 4.3 eV producing 28.65 percent. The band alignment between the absorber and its transport layers, characterized by conduction band offsets, directly shapes how easily electrons flow out of the absorber and how much voltage is lost at the interfaces. The study highlights that conduction band offset and absorber thickness are among the most critical levers for maximizing performance in double perovskite devices.

Beyond the headline efficiency, the team examined quantum efficiency across the solar spectrum. The simulated devices achieved approximately 98.68 percent quantum efficiency at a wavelength of 360 nm, holding strong through much of the visible range before declining toward 820 nm, while the photon energy of the absorbed light fell from 4.13 eV at 300 nm as wavelength increased. Defects and interface imperfections can devastate quantum efficiency, with prior studies reporting drops from over 90 percent to as little as 20 percent when carrier capture cross-sections grow, which is why precise external quantum efficiency measurements, accurate to within 5 percent for estimating short-circuit current, remain essential for validating any future experimental device. The authors caution that translating these simulated results into the laboratory will be challenging: environmental factors such as moisture and temperature sensitivity, long-term stability, and the economics of large-scale manufacturing all stand between a 29 percent numerical model and a commercial product. Still, the message of the study is unambiguous. With careful control of absorber thickness, defect density, doping, band alignment, and contact materials, lead-free Cs2BiAgI6 double perovskites can theoretically rival the efficiency of lead-based champions while eliminating one of the technology’s most serious environmental liabilities, offering a credible path toward sustainable, eco-friendly photovoltaics.

Subject of Research: Numerical optimization of lead-free Cs2BiAgI6 double perovskite solar cells using SCAPS-1D simulation

Article Title: A performance analysis of inorganic Cs 2 BiAgI 6 perovskite solar cell: A comprehensive study using SCAPS -1D

Article References: Shawon, M., Fatema, K., Akter, R., Hossain, S., & Shalauddin (2026). A performance analysis of inorganic Cs2BiAgI6 perovskite solar cell: A comprehensive study using SCAPS -1D. Results in Optics, Article 101187. https://doi.org/10.1016/j.rio.2026.101187

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101187

Keywords: perovskite solar cells, Cs2BiAgI6, lead-free photovoltaics, SCAPS-1D, double perovskite, power conversion efficiency, Shockley-Read-Hall recombination, defect density, bandgap engineering, electron transport layer, hole transport layer, quantum efficiency

The computational campaign began with a rigorous geometry optimization performed in the ORCA 6.1 quantum chemistry package. The total electronic energy dropped sharply from -753.9 to -756.2 electron volts within the first sixteen optimization steps, then settled at -756.92 electron volts after 44 iterations, a stabilization of roughly 3.0 electron volts, or nearly 1890 kilocalories per mole. Convergence was tracked through root-mean-square and maximum gradients, which fell by two orders of magnitude to 0.0020 and 0.0058 electron volts per angstrom respectively, alongside shrinking step sizes. Crucially, a vibrational frequency calculation on the relaxed structure returned no imaginary frequencies, confirming that the doped dot sits at a true local minimum on the potential energy surface and is intrinsically thermally stable rather than a computational artifact.

The heart of the study lies in its frontier molecular orbital analysis. The highest occupied molecular orbital, with an energy of -3.68 electron volts, is delocalized across the carbon skeleton with contributions from the oxygen-bearing groups, while the lowest unoccupied orbital at -2.21 electron volts concentrates around the nitrogen dopant sites and adjacent carbons. That spatial separation matters enormously: when a photon promotes an electron from HOMO to LUMO, the excited electron and the hole it leaves behind end up on different regions of the molecule, which naturally suppresses recombination and promotes charge separation. The 1.47 electron volt gap between them is dramatically narrower than the 2.5 to 3.5 electron volt range typical of undoped carbon dots, and the authors attribute the narrowing to mid-gap states created cooperatively by the two dopant species.

Time-dependent DFT calculations at the same level of theory confirmed that the electronic gap translates into genuine optical behavior. The lowest singlet excitation was computed at 1.62 electron volts, corresponding to a wavelength of 765 nanometers in the near-infrared, with an oscillator strength of 0.21 and a transition that is 92 percent HOMO-to-LUMO in character. Higher-energy excitations appeared at 2.18 electron volts, or 569 nanometers, and 2.93 electron volts, or 423 nanometers, each with distinct orbital origins. The presence of multiple accessible excited states spanning the visible spectrum, combined with the spatially separated frontier orbitals, sketches a material that can absorb broadly, separate charge efficiently, and do so without a single atom of cadmium or lead.

Charge polarization emerged as the second pillar of the story. Electrostatic potential maps draped over the electron density surface revealed dense equipotential contours around the heteroatoms, with strongly negative regions hugging the oxygen sites and electron-deficient zones around nitrogen and its neighboring carbons. Mulliken population analysis quantified the effect: the carbonyl oxygen carries -0.63 elementary charges, the hydroxyl oxygen and its hydrogen carry +0.58 and +0.61 respectively, the two nitrogen dopants carry +0.88 and +0.17, and the carbon atom sandwiched between nitrogen dopants is pushed to a striking +1.43, while a neighboring carbon swings to -1.09. Because Mulliken charges are notoriously basis-set dependent, the team repeated the analysis with the Hirshfeld stockholder scheme, which reproduced the same qualitative polarization pattern and yielded a total dipole moment of 2.096 atomic units, independently confirming the molecule’s built-in polarity.

Spectroscopy provided the experimental bridge. Simulated infrared spectra, computed without any empirical scaling factor, showed intense peaks at 1440.84, 1353.93, and 908.42 wavenumbers, assigned to carbon-hydrogen bending with ring stretching, carbon-hydrogen bending with carbon-nitrogen stretching, and ring deformation respectively, alongside strong carbon-oxygen and carbon-nitrogen stretches between 1000 and 1300 wavenumbers. A broadened band near 2500 wavenumbers betrayed the hydrogen-bonded hydroxyl stretch of the carboxylic acid group, and skeletal carbon-oxygen and carbon-carbon double-bond modes appeared near 1630 wavenumbers. The calculated frequencies deviated by less than five percent from experimental carbon-dot spectra reported in the literature, lending credibility to the assignments and confirming that the modeled functional groups are the same ones chemists actually observe on doped dots.

Nuclear magnetic resonance calculations using the gauge-including atomic orbital method then mapped the local electronic environment atom by atom. The simulated spectrum, broadened with a 5 parts-per-million Lorentzian, separated into an upfield aliphatic zone, a mid-field aromatic and heteroatom zone, and a strongly downfield zone where one carbon resonates near 365 parts per million and one nitrogen near 390. Isotropic shieldings ranged from moderately shielded nitrogen at 12.6 parts per million to deeply deshielded nitrogen at -125.2, while the two oxygens, at 105.6 and 339.1 parts per million, clearly distinguished carbonyl-like from hydroxyl-like environments. The deshielded carbon correlates directly with the large positive Mulliken charge found nearby, tying the spectroscopic fingerprint to the charge analysis and demonstrating that the doping effects are localized at specific atomic sites rather than smeared across the dot.

Perhaps the most novel contribution is the study’s reduced density gradient analysis, which the authors describe as the first combined application of this technique to a nitrogen and oxygen co-doped carbon quantum dot. The non-covalent interaction analysis revealed a stabilizing web of secondary forces: attractive hydrogen-bond regions around the carboxylic acid and nitrogen-hydrogen groups accounted for 28 percent of the non-covalent electron density, weak van der Waals contacts contributed 65 percent, and repulsive steric zones a mere 7 percent. Hydrogen bond strengths estimated from electron densities at bond critical points fell in the moderate range typical of organic systems. These secondary interactions, the authors argue, work hand in hand with covalent doping, polarizing orbital energies and constraining the geometry in ways that favor electron delocalization and stabilize charge-separated states.

The density-of-states analysis completed the mechanistic picture and delivered the study’s most striking quantitative result. The projected density of states showed the HOMO region dominated by carbon 2p orbitals plus oxygen lone pairs, while the LUMO region drew mainly from nitrogen 2p states hybridized with adjacent carbons, with strong carbon-nitrogen mixing between -5 and 0 electron volts and carbon-oxygen mixing from 0 to +4. Benchmark calculations quantified the synergy: the pristine dot’s gap of 2.83 electron volts fell to 1.92 with nitrogen doping alone and 2.15 with oxygen alone, but co-doping achieved 1.47, a reduction of 1.36 electron volts that exceeds the additive expectation by 0.23 electron volts. That cooperative bonus, the authors conclude, is the signature of genuinely synergistic doping rather than two independent effects. With a narrow gap, strong polarization, efficient exciton dissociation, and a non-toxic composition, the modeled material now offers experimentalists a concrete design recipe for next-generation light-emitting diodes, photodetectors, photocatalysts, and sensors built entirely from carbon, nitrogen, and oxygen.

Subject of Research: First-principles DFT investigation of the electronic and optoelectronic properties of nitrogen and oxygen co-doped carbon quantum dots

Article Title: First principles investigation of electronic and optoelectronic properties of N and O codoped carbon quantum dots

Article References: Katariya, G., Katariya, D., Jadav, G., Dodia, S., Kataria, B., & Markna, J. H. (2026). First principles investigation of electronic and optoelectronic properties of N and O codoped carbon quantum dots. Discover Chemistry, 3(1), Article 468. https://doi.org/10.1007/s44371-026-00927-8

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00927-8

Keywords: carbon quantum dots, nitrogen doping, oxygen doping, density functional theory, HOMO-LUMO gap, optoelectronics, charge transfer, B97-3c, NMR spectroscopy, non-covalent interactions, photocatalysis, bandgap engineering

The material at the center of the study is bismuth oxychloride, or BiOCl, a layered semiconductor that has long attracted attention in photocatalysis circles. Its crystal structure consists of alternating sheets of bismuth and oxygen interleaved with chlorine layers, a geometry that creates internal electric fields capable of separating charge when light is absorbed. BiOCl is chemically stable, inexpensive relative to many catalytic materials, and easy to synthesize through solvothermal routes in which precursors react in a sealed vessel at elevated temperature and pressure. Yet for all its virtues, BiOCl suffers from two crippling drawbacks when it comes to artificial photosynthesis. Its bandgap, the energy threshold that photons must exceed to excite electrons, is large, which means the material responds only weakly to visible light and effectively ignores most of the solar spectrum. Worse, the excited electrons and the holes they leave behind tend to recombine rapidly, annihilating each other before they can drive any useful chemistry.

The Harbin team’s solution was to decorate BiOCl with carbon quantum dots derived from nitrogen-doped lignin, abbreviated NLCQDs in the study. Carbon quantum dots are nanoscale carbon-based particles, typically just a few nanometers across, that exhibit unusual optical and electronic properties, including fluorescence and the ability to accept, store and shuttle electrons. Unlike the heavy-metal quantum dots familiar from imaging research, carbon dots are generally considered benign and can be made from renewable feedstocks. Lignin, the aromatic polymer that gives wood its rigidity and is generated in enormous quantities as a byproduct of the paper and pulp industry, is an ideal carbon source. It is cheap, widely available and rich in the aromatic rings that translate well into the conjugated carbon frameworks of functional nanocarbons. The researchers synthesized their dots hydrothermally, combining alkaline lignin with ethylenediamine, a molecule that donates nitrogen atoms to the carbon structure. Nitrogen doping is a well-known strategy for tuning the electronic properties of carbon materials, introducing additional active sites and modifying the density of electronic states near the Fermi level.

With the dots in hand, the team coupled them to solvothermally prepared BiOCl to create a series of composite photocatalysts labeled BiOCl/NLCQDs-X, where X denotes the volume of the quantum dot solution used during assembly: 20, 30 or 40 milliliters. This systematic variation of loading is critical, because the amount of modifier on a photocatalyst surface often determines whether it helps or hinders. Too little decoration leaves the underlying deficiencies of the semiconductor untouched; too much can block active sites, scatter light away from the absorber or even act as a recombination center in its own right. The optimal balance, as it turned out, lay in the middle of the series.

A battery of structural characterizations confirmed that the composites had been successfully fabricated, and revealed something rather elegant about how the modification works. The introduction of the lignin-derived dots barely changed the crystal phase or the morphology of the BiOCl, meaning the underlying semiconductor retained the structure that makes it attractive in the first place. What did change was the specific surface area: the best-performing composite, BiOCl/NLCQDs-30, gained 8.7523 square meters per gram relative to pristine BiOCl. In catalysis, surface area is real estate. Reactions happen at interfaces, and more accessible surface means more places where carbon dioxide molecules can adsorb, be activated and be reduced. A nearly nine-square-meter-per-gram increase, achieved without disrupting the crystal structure, represents a meaningful gain in the number of available reaction sites.

The photoelectrochemical measurements told an even more compelling story about the electronic consequences of the modification. According to the study, the nitrogen-doped lignin carbon quantum dots narrow the effective bandgap of BiOCl, which allows the composite to harvest a larger fraction of visible light than the pristine material could manage on its own. Just as importantly, the dots serve as electron trapping sites and charge transport channels. When photons strike the composite, electrons excited within the BiOCl can migrate onto the carbon dots, where they are held away from the holes left behind. This spatial separation of positive and negative charges is the holy grail of photocatalysis, because it prevents the recombination that otherwise wastes the absorbed energy as heat or light. With electrons parked on the dots and holes remaining in the semiconductor, both charge carriers survive long enough to participate in chemical reactions at the surface.

The payoff came when the materials were tested under simulated visible-light illumination for the reduction of carbon dioxide. The optimal composite, BiOCl/NLCQDs-30, delivered a carbon monoxide evolution rate of 14.13 micromoles per gram per hour, four times the rate achieved by pure BiOCl. Carbon monoxide may sound like an odd prize, but in the context of artificial photosynthesis it is a valuable one: CO is a key feedstock for syngas, the mixture of carbon monoxide and hydrogen that underpins the industrial synthesis of liquid fuels and countless chemicals. Converting a waste molecule into a chemical building block using sunlight is precisely the kind of closed-carbon-loop chemistry that a decarbonizing economy will need. The product selectivity matters as much as the rate, and the fact that the composite channels electrons into CO2 reduction rather than side reactions is a sign that the engineered charge dynamics are doing their job.

Durability is where many promising photocatalysts stumble, and here too the composite performed well. The study reports that the material displays favorable cycling stability, meaning it can be used repeatedly without catastrophic loss of activity. For any technology hoping to move from the laboratory bench to a solar refinery, stability over many hours and many cycles is non-negotiable. Photocatalysts can deactivate through photocorrosion, aggregation, poisoning of active sites or the gradual degradation of surface modifiers. The fact that the lignin-derived dots remained functional partners to the BiOCl across repeated runs suggests that the coupling between the two components is robust, and that the carbon dots are not simply consumed as a sacrificial electron sink.

What makes this work resonate beyond its headline numbers is the sustainability of the strategy itself. The authors describe their approach as facile, and the ingredients could hardly be more earth-abundant: bismuth salts, chlorine precursors, industrial lignin waste and ethylenediamine, assembled through standard hydrothermal and solvothermal chemistry. There are no precious metals, no rare-earth elements and no exotic synthesis infrastructure required. The work was financially supported by the National Natural Science Foundation of China and the Heilongjiang Provincial Natural Science Foundation, reflecting the strategic priority that China and its international partners place on carbon conversion technologies. As the researchers note, the study optimizes the charge separation behavior of BiOCl and offers a sustainable route to efficient photocatalytic CO2 conversion.

The broader lesson is one that keeps reappearing in materials chemistry: the most powerful upgrades often come not from inventing entirely new compounds but from pairing familiar ones in clever ways. BiOCl has been studied for years, and carbon dots have been celebrated for their optical tricks since their discovery, but the marriage of a wood-waste-derived, nitrogen-doped carbon dot with a layered bismuth oxyhalide turns out to be more than the sum of its parts. By widening the light-harvesting window, multiplying the available surface area and giving excited electrons somewhere to go, the composite attacks the fundamental bottlenecks of artificial photosynthesis from several directions at once. If similar strategies can be extended to other bandgap-limited semiconductors, the vision of solar reactors that sip carbon dioxide from flue gases or even from air and exhale chemical fuels moves a little closer to reality. For now, a humble byproduct of the paper industry has shown that the building blocks of a carbon-neutral fuel cycle may be lying in the waste streams we already produce.

Subject of Research: Photocatalytic CO2 reduction using BiOCl composite photocatalysts modified with nitrogen-doped lignin carbon quantum dots

Article Title: Preparation and CO2 reduction performance of BiOCl composite photocatalysts modified with lignin carbon quantum dots

Article References: Li, L., Wu, Y., Taran, O. P., Sun, F., Yan, H., & Hu, Y. (2026). Preparation and CO2 reduction performance of BiOCl composite photocatalysts modified with lignin carbon quantum dots. Journal of Materials Science, 61(43), 33532-33552. https://doi.org/10.1007/s10853-026-13581-3

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13581-3

Keywords: photocatalysis, carbon dioxide reduction, BiOCl, carbon quantum dots, lignin, nitrogen doping, charge separation, visible light, carbon monoxide, sustainable chemistry, bandgap engineering, renewable feedstocks

Two-dimensional materials have captivated physicists and engineers ever since graphene was first isolated, but the field has long faced a fundamental dilemma. Graphene itself is a superb conductor, yet it lacks a bandgap, the energy barrier that separates occupied from unoccupied electronic states and makes a material useful as a semiconductor. Hexagonal boron nitride, at the other extreme, is a wide-gap insulator. Sandwiching carbon between boron and nitrogen in a single atomic plane offers a tantalizing middle ground: a lattice built entirely from light elements that could, in principle, combine the robustness of boron nitride with semiconducting behavior suitable for transistors, photodetectors, and solar cells. The new study adds a fresh entry to this growing family of borocarbonitride compounds and subjects it to one of the most demanding tests a two-dimensional material can face: sustained mechanical stretching.

The researchers began by evaluating whether the proposed BCN lattice could exist at all. Stability is the perennial Achilles heel of hypothetical two-dimensional crystals, many of which look beautiful on paper but fall apart the moment their atomic vibrations are examined. To probe this, the team performed phonon analysis, a technique that calculates the full spectrum of vibrational modes in the crystal. If any of these modes have imaginary frequencies, the lattice is dynamically unstable and will spontaneously distort or disintegrate. The BCN monolayer passed this test convincingly, remaining dynamically stable not only in its pristine form but across a wide range of applied biaxial tensile strain. That resilience matters enormously for practical applications, because any real device built on a flexible substrate will subject its active layer to mechanical stress, and a material that cannot survive deformation is of little use in flexible electronics.

In its unstrained state, the pristine BCN monolayer exhibits a direct bandgap of approximately two electronvolts. A direct bandgap means that an electron dropping from the conduction band to the valence band can emit a photon without needing a change in momentum, which is precisely the property that makes materials like gallium arsenide valuable for LEDs and lasers. A gap of roughly two electronvolts places the material in the visible-light range, meaning it can absorb and potentially emit photons in the portion of the spectrum most relevant to human vision and to solar energy conversion. This combination of a direct gap and an all-light-element composition is rare, and it immediately positions the monolayer as a candidate for ultrathin, lightweight optoelectronic components.

The most striking results, however, emerged when the researchers applied biaxial tensile strain, uniformly stretching the lattice in both in-plane directions. The bandgap proved highly sensitive to this deformation, decreasing substantially as the strain increased. This behavior follows a well-established principle in semiconductor physics: stretching a lattice weakens and lengthens the bonds between atoms, which alters the overlap of their electronic orbitals and shifts the energies of the band edges. In many two-dimensional semiconductors, including the widely studied molybdenum disulfide, strain has been shown to modify the gap by hundreds of millielectronvolts. What makes the new BCN monolayer notable is the magnitude of the effect combined with the material’s demonstrated stability under the same conditions, meaning the tuning window can actually be exploited rather than merely observed before the crystal fails.

The optical consequences of this strain sensitivity are equally significant. The team calculated the material’s absorption spectrum and found it to be highly anisotropic, meaning it absorbs light differently depending on the polarization direction relative to the crystal axes. Anisotropic absorption is a valuable property for polarization-sensitive photodetectors, which can distinguish the orientation of incoming light and are used in imaging, sensing, and communications technologies. The prominent absorption peaks in the spectrum shifted to longer wavelengths, a redshift, as tensile strain increased, tracking the narrowing of the electronic bandgap. Because the gap defines the minimum photon energy the material can absorb, stretching the lattice effectively broadens the range of low-energy photons it can capture, pulling previously invisible parts of the spectrum into play.

Perhaps the most application-relevant finding concerns the near-infrared and visible regions of the spectrum. The calculations showed that light absorption in these bands, which are the workhorse ranges for photovoltaics, imaging, and telecommunications, can be effectively enhanced by applying tensile strain. In practical terms, a thin film of this material could be made a better solar absorber or a more sensitive infrared detector simply by stretching it, without any chemical doping or compositional change. Because strain can be applied dynamically, through flexible substrates, piezoelectric actuators, or patterned substrate topography, this opens the possibility of devices whose spectral response is actively adjustable in operation, a degree of freedom that conventional bulk semiconductors cannot offer.

The study also examined excitons, the bound pairs of photoexcited electrons and holes that form when light is absorbed in a semiconductor. In two-dimensional materials, reduced dielectric screening makes excitons exceptionally tightly bound, which is scientifically fascinating but technologically problematic: an exciton that will not separate cannot contribute its charges to a photocurrent. The researchers found that the exciton binding energy in the BCN monolayer is largely reduced as tensile strain increases. Weaker exciton binding facilitates the effective spatial separation of photoexcited electrons and holes, allowing the charge carriers to escape their mutual attraction and be collected as current. For photovoltaic and photodetector applications, this strain-induced reduction in binding energy could translate directly into improved conversion efficiency and faster response times.

Methodologically, the work stands out for its rigor. Rather than relying on standard density functional theory alone, which notoriously underestimates bandgaps and neglects excitonic effects, the team employed many-body perturbation theory in the GW and Bethe-Salpeter framework, the gold standard for predicting the excited-state properties of low-dimensional materials. They combined the QUANTUM ESPRESSO simulation suite with the Yambo code for excited-state calculations, using PBE exchange-correlation functionals and optimized norm-conserving pseudopotentials from the PseudoDojo database. This level of theoretical treatment is essential for obtaining trustworthy optical spectra and exciton energies, and it lends considerable weight to the quantitative predictions about strain tuning.

The broader significance of the study lies in its demonstration that light-element two-dimensional materials deserve the same strain-engineering attention that has been lavished on transition metal dichalcogenides. The authors frame their findings as both the discovery of a new member of the light-elemental two-dimensional family and an emphasis on its potential in strain-engineered optoelectronic devices. The work was supported by the National Natural Science Foundation of China and Jiangsu University of Science and Technology. As experimentalists continue to synthesize increasingly exotic borocarbonitride phases, from graphitic BCx compounds to hexagonal BC2N films grown by chemical vapor deposition, computational studies like this one serve as a map, pointing synthetic chemists toward the structures most likely to survive the journey from theory to working devices, and toward the mechanical tricks that could unlock their full optical potential.

Subject of Research: Strain-dependent stability and electro-optical properties of a two-dimensional boron-carbon-nitride monolayer

Article Title: Impact of biaxial tensile strain on stability and electro-optical properties of new B–C–N hybrid monolayer

Article References: Dong, Y., Shu, H., & Xu, H. (2026). Impact of biaxial tensile strain on stability and electro-optical properties of new B–C–N hybrid monolayer. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13900-8

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13900-8

Keywords: BCN monolayer, two-dimensional materials, biaxial tensile strain, bandgap engineering, first-principles calculations, many-body effects, exciton binding energy, optical absorption, strain engineering, optoelectronics, phonon stability, near-infrared

The physics behind the excitement lies in a deceptively simple scaling law. In rectifying power electronics, the Baliga figure of merit, which captures how well a semiconductor performs as a switch, is proportional to the bandgap raised to the power of 5.5. That steep exponent means that moving from silicon’s 1.1 electronvolts to gallium nitride’s 3.4 electronvolts already transforms what is possible in converters, chargers and grid infrastructure. Moving to a colossal bandgap above 6 electronvolts, the regime the Cornell-led team now enters, promises another leap in critical electric field and therefore in the voltage a device can block per unit thickness. Thinner devices switch faster, dissipate less heat and shrink the passive components around them, which is why the pursuit of ever-wider bandgaps has become one of the most competitive frontiers in materials science.

Yet widening the bandgap has historically come at a punishing cost: the loss of dopability. The defining trick of semiconductors is that their conductivity can be tuned over many orders of magnitude by deliberately introducing impurity atoms, a capability that underpins every transistor and diode ever made. As bandgaps grow, dopants tend to become so deeply bound that they fail to release their carriers at room temperature, and materials that were once dismissed as insulators, including gallium nitride, silicon carbide, aluminium nitride, gallium oxide and germanium oxide, only became useful semiconductors after years of work reducing defect densities and finding the right dopants. Diamond and cubic boron nitride, despite their superb intrinsic properties, remain locked behind enormous synthesis barriers: high pressures, tiny crystals, prohibitive costs and difficulty scaling. The new result breaks that trend by achieving record bandgap and record conductivity on one of the cheapest substrates in the industry.

The material at the heart of the study is the corundum-structured alloy α-(Al_xGa_1−x)₂O₃, in which gallium atoms in α-gallium oxide are progressively replaced by aluminium to widen the gap. The films were grown by suboxide molecular-beam epitaxy, a technique in which the metal sources are supplied as suboxides rather than pure elements, allowing adsorption-controlled growth of stoichiometric oxide films. The team grew the layers on sapphire, which is itself crystalline aluminium oxide, meaning the substrate is chemically identical to the aluminium-rich end of the alloy system. Sapphire is abundant, inexpensive, available in exceptional crystalline quality and produced at massive scale for everything from watch faces to LED wafers, with crystals as large as hundreds of kilograms grown by the Kyropoulos method. Building colossal-bandgap electronics on such a substrate is a decisive economic advantage over diamond and cubic boron nitride, which require high-pressure synthesis and yield only small, costly samples.

The headline number is conductivity. In the colossal-bandgap regime above 6 electronvolts, the silicon-doped α-(Al_xGa_1−x)₂O₃ films exhibit room-temperature conductivities more than 100 million times higher than all previous reports for materials in this range. Previous efforts to dope the corundum-structure alloy had achieved conductive films only up to bandgaps of about 6.22 electronvolts, and the conductivity collapsed rapidly as aluminium content rose. The new work shows that silicon, acting as a donor on the cation sublattice, can still ionize and release electrons even when the bandgap exceeds 7 electronvolts, provided the growth conditions are tuned correctly. Temperature-dependent Hall measurements on the films revealed carrier freeze-out as samples were cooled, allowing the researchers to extract donor densities and donor activation energies and to confirm that the conduction observed at room temperature arises from thermally activated, deliberately introduced silicon donors rather than accidental defects.

Characterizing such films demanded an unusually broad arsenal of techniques. High-resolution scanning transmission electron microscopy revealed atomically sharp, flat interfaces between the sapphire substrate, the buffer layers and the doped channel, with misfit dislocations confined to the earliest interface. Spectroscopic ellipsometry, performed at multiple angles of incidence and azimuthal orientations to account for the anisotropy of the corundum crystal structure, was used to determine the bandgap of each layer. Secondary ion mass spectrometry quantified the silicon incorporation down to a noise floor of roughly 1.2 times 10 to the 19 per cubic centimetre, while atom probe tomography mapped the three-dimensional distribution of silicon, aluminium and gallium atoms, showing that the alloy mixing follows a random, binomial distribution with no clustering. X-ray diffraction confirmed the crystal phase and composition of the doped overlayers through sharp reflections corresponding to aluminium fractions near 0.52.

Theory played an equal role in interpreting the results. Collaborators at the University of Texas at Austin, including Viet-Anh Ha, Nick Pant and Feliciano Giustino, calculated the theoretical electron mobilities of α-(Al_xGa_1−x)₂O₃ as a function of aluminium content and carrier density using density functional theory and electron-phonon coupling methods. Their calculations, which included phonon scattering, ionized impurity scattering and alloy scattering, and which accounted for the directional dependence of transport parallel and perpendicular to the basal plane, provided benchmarks against which the measured mobilities could be judged. Because the alloy is highly anisotropic, with bandgap energies and dielectric properties that depend on crystallographic direction, such first-principles guidance is essential for designing devices that exploit the material’s best transport directions.

Crucially, the team did not stop at films. They fabricated a Schottky barrier diode, dubbed the AlphaDiode, and a field-effect transistor, the AlphaFET, both with colossal-bandgap channels. The transistor structure stacked an aluminium-rich buffer on sapphire, a silicon-doped α-(Al_0.51Ga_0.49)₂O₃ channel, and a heavily doped regrown contact layer to achieve low-resistance ohmic access to the channel. The demonstration of both a rectifying diode and a transistor proves that the material can be patterned, contacted and switched, the minimum requirements for a practical electronic platform. These first devices are proof of concept rather than production-ready components, but they establish that doping, regrowth and contact formation, the traditional stumbling blocks for ultra-wide-bandgap materials, can all be executed on this oxide system.

The broader significance is strategic. Power electronics built on silicon carbide and gallium nitride are already reshaping electric vehicles, data centres and renewable energy systems, and gallium oxide in its beta phase has attracted intense interest because it can be grown from its own melt. The corundum-structure alloy reported here extends the reachable bandgap beyond anything doped before, and it does so on sapphire wafers that are already manufactured at scale, which the authors argue facilitates rapid development and adoption of colossal-bandgap electronics. The team has filed a provisional patent covering the growth procedure for conductive silicon-doped α-(Al,Ga)₂O₃ and the device structures that use it. Challenges certainly remain, including the modest mobilities typical of oxide alloys, deep donor levels that limit carrier activation, and the need for p-type doping, which remains unsolved. But the central barrier, the belief that semiconductors above 6 electronvolts cannot be made conductive, has now been decisively broken, and with it the assumption that the future of power electronics must be built on ever more expensive and difficult crystals.

Subject of Research: Silicon-doped α-(AlxGa1−x)2O3 as a record 7-eV bandgap semiconductor for colossal-bandgap power electronics

Article Title: A 7-eV bandgap semiconductor based on silicon-doped α-(AlxGa1−x)2O3

Article References: Steele, J., Bhattacharya, D., Nomoto, K., Pieczulewski, N. A., Sorensen, P., Ha, V.-A., Pant, N., Shukla, I., Das, S., Kilic, U., Ramesh, M., Giustino, F., Mazumder, B., Senevirathna, M. K. I., Williams, M. D., Schubert, M., Muller, D. A., Xing, H. G., Jena, D., & Schlom, D. G. (2026). A 7-eV bandgap semiconductor based on silicon-doped α-(AlxGa1−x)2O3. Nature. https://doi.org/10.1038/s41586-026-11085-3

Image Credits: AI Generated

DOI: 10.1038/s41586-026-11085-3

Keywords: colossal-bandgap semiconductor, alpha-gallium oxide, aluminum gallium oxide alloy, silicon doping, suboxide molecular-beam epitaxy, sapphire substrate, power electronics, Baliga figure of merit, Schottky diode, field-effect transistor, wide-bandgap materials, molecular-beam epitaxy

Tin dioxide is a natural starting point for photocatalysis research. It is cheap, chemically robust, non-toxic, and abundant, and it has long been used in gas sensors, transparent electrodes, and catalytic supports. Yet it suffers from a fundamental limitation: its bandgap, the energy threshold that determines which photons a semiconductor can absorb, sits at roughly 3.5 to 3.6 electron volts. That means only ultraviolet light, which accounts for a small fraction of the solar spectrum, carries enough energy to excite electrons across the gap and set off the chain of oxidation and reduction reactions that break down pollutants. For a photocatalyst to make practical use of sunlight or indoor visible lighting, that gap must be narrowed, and the charges that light generates must be prevented from simply recombining and wasting the absorbed energy as heat.

The Zhengzhou team, led by Baoyan Liang together with Jingtao Wu, Cui Lyu, and Jizhou Zhang, attacked both problems at once using what they describe as an in situ urea-assisted self-reduction strategy. The chemistry hinges on their choice of precursor: tin hydroxy chloride, a compound that already carries chlorine atoms in its structure. When the precursor is heated, it decomposes toward tin dioxide, but the fate of that residual chlorine depends on the atmosphere surrounding it. In an ordinary calcination, most of the chlorine simply escapes as volatile byproducts. The researchers’ insight was to trap a localized reducing atmosphere around the sample at exactly the moment of decomposition, coaxing some of the chlorine to substitute for lattice oxygen inside the growing SnO2 crystal while simultaneously stripping oxygen atoms from the lattice to create vacancies.

The source of that reducing atmosphere is urea, an inexpensive compound better known as a fertilizer component. When heated to around 300 degrees Celsius, urea decomposes, releasing reducing gases such as ammonia and hydrogen-rich species. Rather than letting these gases dissipate, the team employed a confined dual-crucible configuration, an arrangement in which the urea and the tin precursor sit in separate crucibles within a sealed outer vessel. The decomposition products from the urea therefore build up a localized microatmosphere that bathes the precursor as it transforms. This is the self-reduction at the heart of the strategy: the reducing environment is generated in place, at the same temperature and in the same vessel as the synthesis, requiring no external hydrogen gas, no vacuum furnaces, and no post-synthesis defect-creation treatments such as plasma irradiation or high-temperature annealing under controlled atmospheres.

Comprehensive characterization of the resulting nanocrystals revealed the dual effect the researchers were aiming for. Chlorine atoms had been successfully incorporated into the SnO2 lattice in place of oxygen, and the crystals carried a high density of oxygen vacancies, sites where an oxygen atom is missing from the regular atomic arrangement. Each modification influences the electronic structure in its own way. Chlorine, being more electronegative than oxygen, perturbs the energy levels of the valence band, while oxygen vacancies tend to introduce donor-like states below the conduction band edge. Acting together, the two defects progressively narrowed the material’s bandgap from 3.50 electron volts down to 2.76 electron volts. That shift of more than 0.7 electron volts is what extends the material’s photoresponse from the ultraviolet into the visible region of the spectrum, opening the door to harvesting the blue and green wavelengths that dominate visible light.

Narrowing the bandgap alone is not enough, however. Many doped semiconductors absorb visible light but squander the energy because their photogenerated electrons and holes recombine almost immediately. The defect engineering in this study appears to solve that problem as well. The measurements showed significantly enhanced separation of photogenerated charge carriers and faster interfacial charge transfer, meaning that electrons excited by visible light actually reach the catalyst surface and participate in chemical reactions rather than annihilating with holes. Oxygen vacancies are known to act as shallow traps that can hold one carrier population briefly while the other migrates, and the chlorine dopant modifies the local electronic landscape in a complementary fashion. The synergistic interplay between the two defect types, rather than either one alone, is what the authors identify as the key to the material’s performance.

The practical payoff was demonstrated in the degradation of methyl orange, a widely used azo dye that serves as a standard proxy for the organic pollutants found in textile and industrial wastewater. At a dye concentration of 20 milligrams per liter, the optimized chlorine-doped, vacancy-rich SnO2 destroyed 97.5 percent of the dye within 45 minutes under visible-light irradiation. To understand which chemical species were doing the destructive work, the researchers ran scavenging tests, adding compounds that selectively neutralize particular reactive intermediates. Those experiments identified superoxide radicals, the negatively charged oxygen species formed when photogenerated electrons react with dissolved oxygen, as the predominant reactive species driving the degradation. This detail matters for anyone designing treatment systems, because it points to the importance of oxygen availability at the catalyst surface and suggests that the vacancy-rich surface is particularly effective at activating molecular oxygen.

The broader context makes the result more significant. Defect engineering of tin oxide has become a crowded and competitive field, with researchers doping the material with zinc, copper, ytterbium, iron, iridium, fluorine, and cerium, or building heterojunctions with bismuth oxybromide and zinc oxide, all in pursuit of better visible-light activity. Many of these approaches require hydrothermal synthesis, solvothermal routes, deep eutectic solvents, or strong chemical reducing agents such as sodium borohydride. What distinguishes the new work is its economy of means: a one-pot synthesis, a benign and abundant reagent, a modest temperature of 300 degrees Celsius, and a simple crucible arrangement that any materials laboratory could reproduce. The authors describe the approach as facile and scalable, and the confined dual-crucible concept could in principle be adapted to other metal oxide systems where controlled defect creation is desirable.

There are, of course, the usual caveats that separate a laboratory demonstration from a deployed technology. The degradation experiments were conducted on a single model pollutant at a fixed concentration under controlled irradiation, and real wastewater presents a far messier chemical environment with competing organic matter, varying pH, and mixtures of contaminants. The long-term stability of the chlorine dopant and the oxygen vacancies under repeated photocatalytic cycles, and the question of whether the catalyst can be recovered and reused without losing its defect population, remain subjects for further study. The authors also note that the data and materials used in the research are available upon request, which should facilitate independent verification and follow-up work by other groups.

Even so, the study adds a genuinely elegant idea to the photocatalysis toolkit: the notion that the reducing atmosphere needed to create defects can be generated by the decomposition of a cheap additive in the same pot where the catalyst is born, with the precursor’s own chlorine supply doubling as the dopant. Water purification by photocatalysis has long promised a low-energy route to destroying organic pollutants using nothing more than light and a suitable semiconductor, and the obstacle has always been finding materials that respond to visible light, separate charges efficiently, and can be made cheaply at scale. A strategy that checks all three boxes using urea, a tin precursor, and a pair of crucibles is exactly the kind of unglamorous but practical innovation that could help move defect-engineered photocatalysts from the journal page toward the treatment plant. The research was supported by the International Science and Technology Cooperation Project of Henan Province.

Subject of Research: Urea-assisted synthesis of oxygen-vacancy-rich chlorine-doped SnO2 photocatalysts for visible-light degradation of organic dyes in water

Article Title: In situ urea-assisted self-reduction strategy for constructing oxygen-vacancy-rich chlorine-doped SnO2 with enhanced visible-light photocatalytic activity

Article References: Liang, B., Wu, J., Lyu, C., & Zhang, J. (2026). In situ urea-assisted self-reduction strategy for constructing oxygen-vacancy-rich chlorine-doped SnO2 with enhanced visible-light photocatalytic activity. Environmental Science and Pollution Research, 33(32), 16264-16281. https://doi.org/10.1007/s11356-026-38266-x

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38266-x

Keywords: SnO2, photocatalysis, oxygen vacancies, chlorine doping, urea, visible light, bandgap engineering, methyl orange, water purification, defect engineering, nanocrystals, superoxide radicals

The team, led by Shaghayegh Khalilzadeh, Ehsan Koushki, and Mohammad-Reza Zamani-Meymian, synthesized two types of carbon quantum dots using simple thermal decomposition methods. The pristine sample, labeled S1, was made by heating 15 grams of citric acid at 250 degrees Celsius for 20 minutes in a solvent-free solid-phase process, yielding a carbonized powder with a consistent reaction mass yield of roughly 42 percent across three replicate batches. The co-doped sample, S2, was produced by dissolving citric acid and thiourea in water and heating the mixture at 180 degrees Celsius for 40 minutes, a gentler protocol chosen specifically to prevent the degradation of thiourea and to optimize the incorporation of nitrogen and sulfur into the carbon framework. Under ultraviolet illumination at 365 nanometers, the two samples glow distinctly: S1 emits blue light, while S2 glows blue-green.

Confirming that the doping had genuinely worked required a battery of characterization techniques. Energy-dispersive X-ray spectroscopy revealed that the pristine sample contained only carbon, oxygen, and sodium, with no detectable nitrogen or sulfur, while the co-doped sample showed weight percentages of 1.28 percent nitrogen and 3.95 percent sulfur. The carbon-to-sulfur weight ratio of approximately 2 to 1 matched the ideal values reported in prior literature. Elemental mapping went further, demonstrating that the nitrogen and sulfur signals overlapped precisely with the carbon framework, proving that the heteroatoms were chemically integrated into the lattice rather than sitting in separate clusters. X-ray diffraction backed this up: both samples showed the broad (002) peak of amorphous carbon at 2 theta equals 20 degrees, but the doped sample displayed new peaks at roughly 19, 28, and 32 degrees, signatures of lattice strain from larger sulfur atoms, graphitic carbon nitride-like domains from carbon-nitrogen bonding, and localized ordering at sulfur-decorated lattice edges.

High-resolution transmission electron microscopy of the pristine sample revealed well-dispersed spherical nanoparticles with an average diameter of about 4 nanometers and clear lattice fringes with a spacing of 0.34 nanometers, corresponding to the (200) plane of graphitic structure. Fourier-transform infrared spectroscopy completed the chemical picture. The pristine dots showed the broad hydroxyl stretching band between 3000 and 3500 inverse centimeters and carboxyl peaks expected of an oxidized carbon surface, while the co-doped sample developed intense new peaks between 1030 and 1150 inverse centimeters assigned to carbon-sulfur and sulfonic group vibrations, plus an enhanced signal at 1380 inverse centimeters from covalent carbon-nitrogen stretching. Together, these results describe what the authors call bond engineering: a stable hybrid nanostructure in which nitrogen and sulfur are covalently woven into the carbon matrix, fundamentally altering its electronic charge density.

The optical consequences of this structural surgery were striking. Ultraviolet-visible absorption spectra showed a dominant band near 230 nanometers from pi-to-pi-star transitions in the aromatic carbon domains, plus a shoulder between 270 and 320 nanometers from n-to-pi-star transitions involving lone-pair electrons of the heteroatoms. In the doped sample, the absorption edge red-shifted and a pronounced tail extended into the visible range, evidence that nitrogen and sulfur had introduced new intermediate energy states between the intrinsic frontier orbitals. Yet when the team quantified the optical bandgap using Tauc analysis with an allowed direct transition exponent, they found the gap had actually widened, from 3.20 electron volts in the pristine sample to 3.45 electron volts in the doped one. This apparent paradox, a blue-shifted bandgap alongside a red-shifted absorption tail, is explained by the Burstein-Moss effect, in which nitrogen and sulfur act as electron donors that fill the lower states of the conduction band, forcing valence electrons to absorb higher-energy photons to find unoccupied states. The larger atomic radius of sulfur also induces lattice strain that fragments the extended sp2 domains into smaller clusters, strengthening the quantum confinement effect and further widening the gap.

Photoluminescence measurements told an equally compelling story. The pristine sample displayed a dual-peak emission profile, with a higher-energy peak from the intrinsic graphitic core and a lower-energy peak from oxygen-containing surface states, plus a broad near-infrared band around 760 nanometers signaling deep defect traps. After co-doping, this complicated landscape collapsed into a single, intense green emission band centered near 500 nanometers. The dopant-induced surface states act as efficient carrier trapping centers that quench the intrinsic core emission and channel all radiative recombination through one low-energy pathway. The practical payoff was dramatic: the photoluminescence intensity of the doped sample increased more than 3.5-fold, and the fluorescence quantum yield, measured against a quinine sulfate standard, jumped from 4.2 percent to 32.5 percent. The nitrogen and sulfur atoms appear to heal surface defects and dangling bonds, suppressing non-radiative recombination and preserving excitation energy for light emission.

The centerpiece of the study, however, was the Z-scan investigation of nonlinear optics. In this technique, a sample is translated through the focus of a laser beam while its transmittance is recorded, allowing researchers to extract the nonlinear absorption coefficient and the nonlinear refractive index. The team used three continuous-wave lasers at 405, 532, and 633 nanometers, violet, green, and red, at powers of 5, 10, and 15 milliwatts, with peak intensities ranging from about 1.62 to 4.84 kilowatts per square centimeter. Control measurements on the pure solvent confirmed that the nonlinear signals originated from the nanodots themselves. In the open-aperture configuration, all samples showed positive nonlinear absorption, which the authors attribute to electrostriction: polarizable nanoparticles are drawn toward the center of the beam by the electric field gradient, increasing local concentration and absorption.

The doped sample outperformed the pristine one decisively. Under green excitation at 5 milliwatts, the nonlinear absorption coefficient of the co-doped dots reached 9.74 times ten to the minus three centimeters per watt, roughly five times higher than the pristine value and orders of magnitude above many recently reported carbon-based nanomaterials, including sugarcane-derived carbon dots and yttrium-doped carbon nanodots measured at the same wavelength. The closed-aperture measurements revealed negative nonlinear refractive indices in all cases, confirming self-defocusing behavior driven by thermal lensing, in which absorbed energy heats the solvent and lowers the refractive index at the beam center. The doped sample again showed larger on-axis phase shifts, thanks to the higher polarizability imparted by the bulky, electron-rich sulfur atoms.

Perhaps the most counterintuitive finding concerned wavelength. One might expect violet photons, with energy closest to the 3.45-electron-volt bandgap, to produce the strongest electronic response. Instead, the red laser generally yielded the largest nonlinear coefficients. The authors argue that at these low powers the response is not governed by electronic transitions at all, but by the classical mechanics of electrostriction: lower-frequency red light exerts its driving force on the nanoparticles over longer time periods, making it more effective at pulling the particles toward the beam center. This frequency matching to the mechanical response of nanometer-scale particles, rather than to their electronic energy levels, is a distinctive feature of the continuous-wave regime, where the measured coefficients are effective, thermally dominated quantities rather than the ultrafast electronic Kerr responses captured by pulsed lasers.

The implications reach well beyond the laboratory bench. Materials that change their absorption and refraction in response to light intensity are the working elements of optical limiters, which protect sensitive sensors and human eyes from damaging laser exposure, as well as optical switches and modulators for photonic circuits. Because the co-doped carbon dots deliver their strongest nonlinear response under cheap, low-power continuous-wave diode lasers, and because they are biocompatible, water-soluble, and free of the toxic heavy metals found in first-generation quantum dots, they are unusually well suited to practical, cost-effective photonic devices. The study also delivers a conceptual lesson for the field: doping does not just shift energy levels, it reshapes the entire interplay between electronic structure, polarizability, and the mechanical forces that light exerts on matter. By widening the core bandgap while simultaneously seeding dense sub-bandgap surface states, nitrogen and sulfur co-doping creates a material whose linear and nonlinear optical personalities can be tuned almost independently, a degree of control that could define the next generation of carbon-based photonics.

Subject of Research: Bandgap engineering and nonlinear optics of nitrogen and sulfur co-doped carbon quantum dots

Article Title: Synthesis, characterization and low power Z -scan study of bandgap-engineered N, S-co-doped carbon quantum dots using lasers of different colors

Article References: Khalilzadeh, S., Koushki, E., & Zamani-Meymian, M.-R. (2026). Synthesis, characterization and low power Z-scan study of bandgap-engineered N, S-co-doped carbon quantum dots using lasers of different colors. Results in Optics, 25, Article 101169. https://doi.org/10.1016/j.rio.2026.101169

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101169

Keywords: carbon quantum dots, nitrogen sulfur co-doping, nonlinear optics, Z-scan, bandgap engineering, photoluminescence, electrostriction, thermal lensing, optical limiting, nanomaterials, photonics, Burstein-Moss effect

The research, led by Chien-Chih Tseng, Chang-Hsun Huang and Jui-Cheng Kao under the direction of Vincent Tung at the University of Tokyo, together with collaborators across Taiwan and Japan, reports the epitaxial growth of monolayer boron carbon nitride (BCN) with a field-effect hole mobility of 100 square centimeters per volt-second. Transistors built from the material switch with an on-off ratio of 10 to the eighth power, deliver on-currents exceeding 0.9 milliamperes per micrometer, and operate at a threshold voltage of minus 0.45 volts. Those numbers, achieved across wafer-scale arrays rather than on isolated champion devices, surpass the performance of every p-type 2D semiconductor reported to date, according to the authors. The work was published on 30 September 2026 and represents a collaboration spanning the University of Tokyo, National Taiwan University, National Yang-Ming Chiao Tung University, Academia Sinica, the National Institute for Materials Science, and Chang Gung University.

The difficulty of making a good p-type 2D semiconductor is rooted in fundamental physics. In many atomically thin materials, the valence band states that transport holes are strongly localized around particular atomic orbitals, which makes holes scatter easily off defects and phonons. Many candidate materials also exhibit a natural tendency toward electron doping, so their surfaces accumulate unwanted negative charge that degrades hole transport. Compounding these intrinsic problems is a synthetic one: to be useful in a fab, a 2D semiconductor must be grown as a continuous, single-crystal film over an entire wafer, with uniform composition and no grain boundaries that would scatter carriers or cause device-to-device variability. Achieving that uniformity for a ternary compound, in which three different elements must arrive at the growing surface in exactly the right proportions, is far harder than growing a binary crystal such as hexagonal boron nitride.

The team’s central insight concerns a problem they describe as a spatial and temporal mismatch in the delivery of boron, carbon and nitrogen atoms. The standard precursor chemistry for boron nitride growth relies on ammonia borane, a molecule that decomposes when heated. When carbon-bearing precursors are added to the mix to make BCN, the different decomposition pathways of the constituent molecules release boron, carbon and nitrogen species at different times and in different ratios. The growing lattice therefore receives an uneven supply of atoms, producing composition fluctuations, defects and discontinuous crystallization that wreck the electronic quality of the film. Rather than accepting this mismatch, the researchers engineered it away by carefully controlling the dehydrogenation and surface reaction pathways of two related precursors: monomethyl ammonia borane and ammonia borane itself.

Monomethyl ammonia borane carries a methyl group that serves as a built-in carbon source, and by tuning the balance between the two precursors the team synchronized the release of all three elements so that they arrived at the substrate together. Density functional theory calculations, performed with the VASP code, mapped the reaction landscape and identified the decomposition intermediates that would either help or hinder uniform growth. The calculations guided the choice of growth conditions, ensuring that carbon atoms and carbon dimers incorporated into the lattice at the right moment rather than segregating into graphene-like domains. The result is a film in which carbon substitutes primarily for nitrogen sites within a continuously crystallized boron nitride lattice, with local distortions around the carbon atoms but no breakdown of long-range order.

Verifying that atomic structure demanded some of the most advanced microscopy available. Aowen Li, Ryo Ishikawa and Naoya Shibata at the University of Tokyo performed high-resolution scanning transmission electron microscopy on the monolayer film, resolving individual atomic columns and confirming that carbon atoms and carbon dimers occupy nitrogen sites throughout the lattice. Complementary multi-scale characterizations, including Raman spectroscopy, X-ray photoelectron spectroscopy, Kelvin probe force microscopy and scanning tunneling spectroscopy, established the film’s composition, work function and electronic structure across the wafer. The measurements converged on a bandgap of 1.90 electronvolts, a value comfortably above the thermal energy that causes leakage in transistors, and one that places BCN squarely in the regime useful for low-power logic.

The electrical characterization is where the work becomes genuinely striking. The team fabricated arrays of p-channel field-effect transistors across the wafer, using a 20-nanometer hafnium dioxide dielectric as the gate insulator. Because the devices were measured in large numbers rather than as cherry-picked specimens, the reported figures carry statistical weight. A hole mobility of 100 square centimeters per volt-second may sound modest compared with the thousands achieved in the best silicon devices, but for a monolayer p-type 2D semiconductor it is a benchmark-setting result, and the on-current above 0.9 milliamperes per micrometer demonstrates that the material can deliver the drive current that real circuits demand. The 10-to-the-eighth on-off ratio means the transistor switches cleanly between conducting and insulating states, and the near-zero threshold voltage of minus 0.45 volts indicates that gates of practical strength can turn the devices on and off efficiently.

The broader significance lies in what BCN enables for three-dimensional monolithic integration. As silicon scaling approaches its physical limits, chipmakers are exploring stacking active layers vertically, growing one layer of 2D transistors directly on top of another to multiply transistor density without shrinking lateral dimensions. That vision requires 2D semiconductors that can be grown at temperatures compatible with the layers beneath them, with wafer-scale uniformity and both carrier polarities available. Previous work has demonstrated growth-based monolithic integration of single-crystal 2D semiconductors and complementary 2D circuits built from separate materials, but a single, scalable p-type platform grown epitaxially across a wafer has been missing. BCN, grown from simple molecular precursors on a crystalline substrate, fits the requirements in a way that exfoliated flakes or transferred films never could.

There is also a pleasing historical symmetry in the result. Boron carbon nitride was first proposed as a graphite-like material in 1987, and researchers have synthesized B-C-N nanotubes and hybrid graphene-boron nitride domains since the 1990s. Carbon impurities in hexagonal boron nitride have even been identified as the source of single-photon emission, a reminder that the BCN system has long been studied at the boundaries of materials science. What distinguishes the new work is control: by solving the precursor chemistry rather than fighting it, the team converted a notoriously inhomogeneous alloy into a device-grade semiconductor. The DFT calculations, experimental growth, atomic-resolution imaging and device engineering were all executed within a single coordinated effort, a model of how modern materials discovery increasingly proceeds from first-principles prediction straight to wafer-scale demonstration.

Challenges remain before BCN transistors appear in commercial chips. The growth is epitaxial, meaning it relies on a crystalline substrate that templates the film’s orientation, and transferring large-area 2D films without introducing damage remains an unsolved industrial problem. Contact engineering, dielectric integration and long-term stability under bias and heat will all need further refinement. The authors declare no competing interests, and the data and calculation inputs are available from the corresponding authors, which should help other groups reproduce and extend the results. But the conceptual barrier has fallen: a p-type 2D semiconductor with wafer-scale uniformity, a useful bandgap and benchmark hole transport now exists. If the result replicates across laboratories, the missing half of the 2D CMOS palette may finally be in place, and the path toward atomically thin, vertically stacked logic circuits becomes considerably more concrete.

Subject of Research: Wafer-scale epitaxial growth of p-type boron carbon nitride for two-dimensional CMOS electronics

Article Title: Wafer-scale epitaxy growth of high-mobility p-type boron carbon nitride

Article References: Tseng, C.-C., Huang, C.-H., Kao, J.-C., Fu, J.-H., Li, A., Ishikawa, R., Cho, S. R., Lei, C., Qi, K., Deng, C.-Z., Yang, P.-Y., Pan, C.-T., Chen, W.-C., Wu, M.-C., Chen, F.-Z., Lin, Y.-W., Lo, Y.-C., Ho, Y.-L., Nagashio, K., … Tung, V. (2026). Wafer-scale epitaxy growth of high-mobility p-type boron carbon nitride. Nature, 658(8135), 380-387. https://doi.org/10.1038/s41586-026-11047-9

Image Credits: AI Generated

DOI: 10.1038/s41586-026-11047-9

Keywords: boron carbon nitride, two-dimensional materials, p-type semiconductor, field-effect transistors, CMOS technology, epitaxial growth, chemical vapor deposition, hole mobility, bandgap engineering, monolithic 3D integration, scanning transmission electron microscopy, density functional theory

The core insight of the paper is that knowledge gained from low-entropy systems, in which each crystal site is occupied by just one or two elements, can be transferred to high-entropy systems where the same sites host a blend of many elements. The team began by enumerating 1596 possible combinations of cesium-based chloride double perovskites, a lead-free family of semiconductors with the general formula Cs2BB’Cl6, where two different metal cations share the octahedral B-site of the crystal lattice. Rigorous stability screening narrowed this list to 275 viable semiconductors. From that curated set, the researchers extracted combinatorial rules and stability relationships, such as which pairings of cations are geometrically and thermodynamically compatible, and used them to define a rules-based element pool for constructing five-element B-site alloys.

On top of that constrained design space, the team layered an explainable machine-learning architecture that combines a Stacking ensemble with a Generalized Additive Model. Stacking blends the predictions of multiple base learners through a meta-model, capturing nonlinear interactions between elemental descriptors, while the additive component expresses the bandgap as a sum of smooth, interpretable functions of individual features. The result is a model that achieves a coefficient of determination, or R-squared, of 0.91 on its test set, meaning it accounts for roughly ninety-one percent of the variance in bandgap values across unseen compositions. Crucially, the additive framework does not behave like a black box: it yields closed-form formulas that quantitatively describe how each elemental property, most notably Pauling electronegativity, denoted by the Greek letter chi, shifts the energy gap between the material’s valence and conduction bands.

Bandgap is the single most important number in photovoltaics. It determines which slice of the solar spectrum a material can absorb, and the theoretical maximum efficiency of a single-junction solar cell peaks when the bandgap sits near 1.3 to 1.4 electron-volts. Most lead-free chloride double perovskites, however, have gaps far too wide to harvest sunlight efficiently, often exceeding two electron-volts. The new workflow revealed that by mixing five cations on the B-site, designers can either compress the gap dramatically or hold it steady, depending on how the electronegativities of the chosen elements are balanced. Because the descriptor-response functions are explicit, chemists can read off, in advance, roughly what bandgap a given five-element cocktail should produce, rather than synthesizing it and hoping for the best.

To confirm that the machine-learning predictions correspond to real physics, the authors turned to density-functional theory, the workhorse quantum-mechanical method of computational materials science. Two candidates emerged with strikingly different behaviors. The first, Cs2{ScNaInBGa}2Cl6, a compound whose B-site hosts scandium, sodium, indium, boron and gallium, showed bandgap compression from 2.96 down to 0.49 electron-volts, a reduction of nearly two and a half electron-volts that would allow the material to absorb light across almost the entire visible and near-infrared spectrum. The second, Cs2{BaZnCdSiGe}2Cl6, combining barium, zinc, cadmium, silicon and germanium, exhibited what the authors call compensatory alloying: its bandgap was retained almost perfectly, moving only from 1.56 to 1.57 electron-volts as the five elements’ opposing contributions canceled one another out. That 1.5-electron-volt regime is squarely within the sweet spot for high-efficiency single-junction photovoltaics.

Bandgap tuning alone is worthless if the resulting crystal falls apart, so the team subjected both candidates to a battery of stability tests. Tolerance-factor analysis, a geometric criterion relating the sizes of the ions to the stability of the perovskite framework, confirmed that both compositions fit the structural window in which the three-dimensional lattice can form. Calculated formation energies came out negative for both materials, indicating that they are thermodynamically favored to crystallize rather than decompose into competing phases. The researchers also evaluated the role of configurational entropy, the thermodynamic driver that stabilizes multielement alloys: the temperature-times-entropy term, T-delta-S, contributes a genuine stabilizing free-energy bonus that helps offset the strain of mixing five dissimilar cations on one lattice site.

The most demanding test was ab initio molecular dynamics, a simulation technique that tracks every atom in a crystal as it vibrates at finite temperature. The team ran 8-picosecond trajectories at 300 kelvin, roughly room temperature, for both candidate compositions. Throughout those simulations the perovskite frameworks remained intact, with no spontaneous bond breaking, cation migration or structural collapse. While 8 picoseconds is a fleeting instant compared with the years a solar panel must survive outdoors, such simulations are a standard first filter for weeding out compositions that would never survive synthesis, and both candidates passed cleanly.

What makes the O2O paradigm notable beyond its specific predictions is its philosophy. High-entropy materials research has exploded in recent years because mixing many elements confers remarkable robustness, tunability and, in some cases, entirely new functionalities, from entropy-stabilized oxides to high-entropy thermoelectrics. Yet the combinatorial explosion is brutal: with dozens of plausible cations and five slots to fill, the number of candidate compositions runs into the millions, far beyond what even the fastest high-throughput DFT pipelines can evaluate. By using interpretable rules harvested from the low-entropy regime as a prior, the new workflow effectively shrinks the search space before the expensive models ever run, a design pattern that echoes the way human chemists have always reasoned, but at machine scale and with quantified confidence.

The emphasis on explainability also matters for trust and reuse. Many state-of-the-art machine-learning models for materials, including graph neural networks, deliver impressive accuracy but offer little physical insight, making it hard for experimentalists to know when a prediction can be extrapolated safely. The Stacking-plus-GAM approach used here produces explicit descriptor-response curves, so the electronegativity dependence of the bandgap can be inspected, criticized and refined by any reader. The authors have made the entire pipeline reproducible, releasing source code, sample inputs and documentation through a public repository, which lowers the barrier for other groups to apply the same strategy to different perovskite chemistries, such as bromides, iodides or hybrid organic-inorganic frameworks.

Considerable work remains before any of these compositions light up a rooftop. The study’s candidates are prioritized hypotheses, not synthesized compounds, and the authors themselves frame the outcome as concrete composition rules, interpretable functions and a shortlist for further phase-stability assessment and experimental evaluation. Growing phase-pure five-element double perovskite crystals, verifying their gaps optically, and testing them in actual device architectures will take years. But the study offers a credible answer to one of the field’s most persistent questions: how to search an astronomically large compositional space without drowning in it. If the low-entropy-to-high-entropy knowledge transfer proves general, the approach could accelerate the discovery of lead-free, stable, spectrally matched absorbers, precisely the combination of properties that next-generation photovoltaics have been waiting for.

Subject of Research: Machine-learning-guided bandgap engineering of high-entropy lead-free chloride double perovskites for photovoltaics

Article Title: Intelligent bandgap engineering for high-efficiency perovskite photovoltaics: directing high-entropy optimization through low-entropy knowledge synergy

Article References: Hu, W., Jiang, J., Peng, L., Shi, T., Liu, X., & Lin, J. (2026). Intelligent bandgap engineering for high-efficiency perovskite photovoltaics: directing high-entropy optimization through low-entropy knowledge synergy. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13827-0

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13827-0

Keywords: high-entropy perovskites, bandgap engineering, photovoltaics, machine learning, double perovskite, density functional theory, lead-free solar materials, explainable AI, configurational entropy, ab initio molecular dynamics, chloride perovskite, materials discovery