The answer lies in two fundamentally different strategies known as direct electron transfer (DET) and mediated electron transfer (MET). In DET, bacteria physically contact the electrode and pass electrons across through specialized molecular machinery. In MET, microbes release or rely on soluble redox compounds, called mediators or electron shuttles, that ferry electrons from the cell surface to the anode. The review emphasizes that understanding and optimizing both pathways is essential for designing efficient MFC systems, because the effectiveness of these electron transfer processes ultimately determines the net power output and stability of the entire device. Which strategy dominates depends on the microbial species involved, the electrode material, and the chemistry of the wastewater itself.

The star performers of direct electron transfer are the metal-reducing bacteria Geobacter and Shewanella. Geobacter sulfurreducens oxidizes acetate and channels the resulting electrons toward external acceptors through conductive pili, often called microbial nanowires, and through a family of multiheme c-type cytochromes embedded in the outer membrane. These redox proteins, with potentials approaching one volt, form conductive pathways that bridge intracellular electron donors and extracellular acceptors. Research on G. sulfurreducens grown with electrodes as the sole electron acceptor revealed a remarkable 1900-fold increase in transcription of the genes encoding outer-membrane cytochromes, showing that the bacteria actively rewire their molecular machinery when an electrode is the only place to dump electrons. Shewanella oneidensis, by contrast, is a versatile dual player, capable of both direct transfer via outer-membrane cytochromes and mediated transfer using soluble flavins it secretes itself.

Those nanowires deserve special attention. These filamentous appendages extend from the bacterial cell surface and conduct electrons over micrometer distances, allowing bacteria to link electrically to distant electrodes or even to other cells. The protein OmcS is indispensable to the nanowire structure of Geobacter, while in the archaeon Methanosarcina acetivorans, the c-type cytochrome MmcA serves as an electron transport carrier. Electron microscopy has captured these structures directly: scanning electron micrographs show dense Geobacter biofilms coating electrospun lignin carbon felt anodes, and transmission electron microscopy reveals bacilli, spiral and spherical forms bristling with the filamentous pili essential for direct transfer. Biofilms are not passive passengers either. They dramatically increase the effective surface area of the anode and protect the community from environmental threats, desiccation and antibacterial agents.

Not every microbe can play this game directly. The outer layers of many bacterial species consist of nonconductive lipid membranes, peptidoglycans and lipopolysaccharides that block physical electron transfer, which is precisely where mediators come in. An effective mediator must penetrate the cell membrane easily, react quickly at the electrode, dissolve well in the anolyte, remain nontoxic and nonbiodegradable, and be inexpensive. Endogenous mediators are produced by the microbes themselves: Pseudomonas aeruginosa secretes the phenazine pyocyanin, while S. oneidensis relies on secreted flavins for roughly 80 to 90 percent of its extracellular electron transfer. Exogenous mediators such as methylene blue, neutral red, thionine and anthraquinone-2,6-disulfonate (AQDS) can be added artificially, though their instability and toxicity restrict practical use. Riboflavin and humic acid, both naturally abundant, have proven efficient shuttles; adding just 9.1 millimoles per liter of riboflavin accelerated the reduction of the azo dye Mordant Yellow 10 by 61 percent in anaerobic granular sludge.

Because electron transfer happens at the electrode surface, the material the electrode is made from matters enormously. The review documents a striking range: graphite anodes yield a maximum power density of roughly 0.7 milliwatts per square meter, while magnesium anodes coated with chitin particles reach approximately 1878 milliwatts per square meter, a thousandfold difference. Over the past five years, reported power densities have spanned from 6.12 to 6119 milliwatts per square meter depending on anode modifications. Nanotechnology has driven much of this progress. Nitrogen-doped carbon/iron oxide nanotube composite arrays on carbon cloth achieved a power density of 4.11 milliamperes per square centimeter with 89 percent coulombic efficiency, while titanium nitride nanoarrays on carbon cloth captured 97.2 percent of Geobacter from mixed sludge, ensuring long-term stability. Iron phthalocyanine modification of carbon cloth raised the Geobacter population from 6.97 to 44.83 percent and boosted power density from 560 to 2419 milliwatts per square meter, apparently by mimicking the iron porphyrin active centers of c-type cytochromes and improving their affinity for the surface.

Carbon nanomaterials have emerged as perhaps the most versatile electrode platform. Graphene, reduced graphene oxide and carbon nanotubes combine high conductivity with enormous surface areas, supporting stable biofilm formation. A three-dimensional graphene aerogel anode achieved an output voltage of 0.488 volts and a peak power density of 2381 milliwatts per cubic meter, far exceeding the 708.89 milliwatts per cubic meter of the control. A graphene aerogel scaffold decorated with platinum nanoparticles reached 1460 milliwatts per square meter with Shewanella oneidensis MR-1, 5.3 times better than carbon fabric. Nitrogen-doped multiwalled carbon nanotube/graphene aerogels pushed power density to 2977.8 milliwatts per square meter, and porous iron oxide nanofibers integrated with carbon nanotubes achieved 1959 milliwatts per square meter thanks to their highly porous, three-dimensional interpenetrating network. Even biomass waste is being recruited: tea polyphenol-functionalized reduced graphene oxide on carbon cloth delivered 773.9 milliwatts per square meter while cutting charge transfer resistance to 14.6 ohms.

The microbial side of the equation is equally dynamic. Community analyses reveal no single dominant ‘winner’ on the anode; instead, diverse consortia including Comamonadaceae, Geobacter, Clostridium, Rhodopseudomonas, Bacillus and Shewanella coexist, and even gram-positive species such as Lysinibacillus sphaericus D-8 can perform extracellular electron transfer despite their thick, nonconductive cell walls. Cocultures frequently outperform pure strains: pairing Escherichia coli with P. aeruginosa yielded 190.44 milliwatts per square meter versus 139.24 and 158.76 for the pure cultures alone. Engineered consortia go further. A ternary culture of Cellulomonas Lsc-8, Bacillus subtilis C9 and G. sulfurreducens PCA converted cellulose into electricity at a peak current density of 796 microamperes per square centimeter, with the first two species breaking down cellulose and releasing riboflavin while Geobacter consumed the resulting acetate. Genetic engineering adds another lever: integrating the flavin biosynthesis pathway into S. oneidensis raised power density to 238.7 milliwatts per square meter, and multiplying riboflavin operon copies in Bacillus subtilis RH33 similarly enhanced output.

Photosynthetic microalgae are adding a solar dimension to the field. In photosynthetic microalgal-assisted MFCs, algae at the cathode release oxygen through photosynthesis, providing a free, cost-effective electron acceptor while yielding valuable biomass, lipids, pigments and proteins as byproducts. A system using live Chlorella pyrenoidosa reached a striking 6030 milliwatts per square meter under optimized conditions, and one with Haematococcus lacustris achieved 33.77 milliwatts per square meter alongside astaxanthin production and 46 percent chemical oxygen demand removal. Diatom-based fuel cells reached 12.62 milliwatts per square meter with a 22.95 percent coulombic efficiency while boosting lipid production by 64.28 percent, and diatomite nanocomposites at the anode degraded the dye rhodamine with 88.23 percent efficiency. Wastewater applications are scaling impressively: constructed wetland-MFC hybrids treating real leather tannery wastewater achieved nearly 99.8 percent COD removal, and heat-pretreated sludge eliminated methanogens within four hours while maintaining a power density of 134 milliwatts per square meter.

Commercialization, however, remains the stubborn frontier. The review is candid that large-scale power generation with MFCs is currently impractical: energy output remains insufficient to compete with alternative power technologies, installation and maintenance costs can reach hundreds of thousands of euros per kilowatt, and corrosive wastewater degrades infrastructure over time. There are no certification standards, renewable-energy credits or feed-in tariffs specifically for MFC modules, and reproducibility at commercial scale is still unproven. Yet the authors see a clear roadmap: cheaper biomass-derived electrodes, 3D-printed porous structures, alternative catalysts to replace expensive platinum, better separators such as clayware plates and bipolar membranes, and bioinformatics-driven identification of electroactive species. If those pieces come together, the humble bacteria that have been quietly passing electrons for billions of years may finally power the water infrastructure of the future, turning every treatment plant into a modest power station.

Subject of Research: Electron transfer mechanisms between electroactive bacteria and electrodes in microbial fuel cells

Article Title: Electron transfer mechanism in microbial fuel cells

Article References: Mishra, K., Islam, M. M., Soni, U., & Vinayak, V. (2026). Electron transfer mechanism in microbial fuel cells. Discover Electrochemistry, 3(1), Article 33. https://doi.org/10.1007/s44373-026-00118-1

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00118-1

Keywords: microbial fuel cells, electron transfer, Geobacter, Shewanella, bioelectricity, nanowires, cytochromes, redox mediators, electrode materials, wastewater treatment, microalgae, biofilms

The choice of host material is no accident. Graphitic carbon nitride is a metal-free, two-dimensional polymer-like semiconductor that has become one of the most studied photocatalysts of the past decade thanks to its chemical robustness and earth-abundant composition. Yet it suffers from a well-known weakness: it absorbs only the blue portion of visible light, and ultraviolet light, which many wider-bandgap semiconductors rely on, accounts for less than five percent of the energy reaching Earth from the sun. Coupling g-C3N4 with a partner semiconductor that harvests longer wavelengths is therefore essential if solar energy is to be used efficiently. CsPbI3 is an ideal candidate because, among the all-inorganic cesium lead halide perovskites, it possesses the narrowest band gap, allowing absorption to stretch from the visible deep into the near-infrared region.

Working with CsPbI3 is not without difficulty. The compound is notoriously unstable in polar and humid environments because the cesium ion is slightly too small to fully stabilize the lead-iodide octahedral lattice, which triggers structural distortion and phase degradation. Moreover, perovskite nanostructures are typically synthesized in hydrophobic solvents that coat their surfaces with long-chain organic ligands such as oleic acid and oleylamine. These ligands are useful for controlling growth but act as insulating barriers that block intimate contact and charge transfer at a heterojunction. The research team, led by Xiao Zhang and San Ping Jiang, overcame both obstacles with a two-step fabrication strategy: a mechano-chemical pre-treatment that partially strips the ligands and promotes hydrogen-bonding between the perovskite and the nitrogen-rich nanosheet surface, followed by a solvothermal treatment at 150 degrees Celsius that locks the two components into a robust, face-to-face architecture.

The structural characterization reveals just how intimate that contact is. Transmission electron microscopy shows cubic CsPbI3 nanocrystals, averaging roughly 18 nanometers, anchored uniformly across the g-C3N4 sheets without agglomeration, with lattice fringes at 0.61 nanometers confirming the cubic (100) plane. The nanowire composites tell a different story: wires of approximately 3.8 nanometers in diameter become partially embedded within the nanosheets rather than sitting loosely on top, forming the kind of continuous, ordered interface that engineers of solar materials dream about. X-ray diffraction confirms the cubic perovskite phase in both composites alongside the characteristic (002) reflection of g-C3N4 at 27.5 degrees, while elemental mapping verifies the even distribution of cesium, lead, and iodine across the hybrid structures.

Spectroscopic measurements then demonstrate why these interfaces matter. Photoluminescence, the light a semiconductor emits when excited electrons recombine with holes, is dramatically quenched in both composites, a signature of efficient charge separation. The average fluorescence lifetime of the nanocrystal composite lengthens from about 15 to nearly 27 nanoseconds, indicating that an interfacial built-in electric field is suppressing wasteful radiative recombination. Transient photocurrent measurements reinforce the point: the nanowire composite generates the highest photocurrent density of all samples, reflecting the ability of one-dimensional structures to shuttle carriers directionally along their long axis. X-ray photoelectron spectroscopy adds further evidence, with the nitrogen, cesium, lead, and iodine core-level peaks all shifting upon coupling, a fingerprint of electronic interaction and interfacial electron redistribution between the two semiconductors.

The band alignment explains the underlying physics. Tauc analysis of diffuse reflectance data yields band gaps of 2.78 electronvolts for g-C3N4 and 1.95 electronvolts for the CsPbI3 nanowires, while Mott-Schottky measurements place the flat-band potential of the nanowires at a more negative value than that of the nanosheets. When the two semiconductors touch, electrons spontaneously flow from the higher-Fermi-level perovskite into the carbon nitride until equilibrium is reached, creating a depletion region and an internal electric field across the junction. Electrochemical impedance spectroscopy shows that both composites have smaller semicircle radii than their parent materials, confirming lower charge-transfer resistance. Together, these measurements establish an S-scheme charge transport mechanism, in which excited electrons in the conduction band of g-C3N4 recombine with holes in the valence band of CsPbI3, preserving the most strongly reducing electrons and the most strongly oxidizing holes for chemistry.

That mechanism pays off spectacularly in carbon dioxide reduction. Under visible light, the nanowire composite produces carbon monoxide at a rate of 4.6 micromoles per gram per hour, outperforming the nanocrystal version at 3.6 and far exceeding bare g-C3N4. Remarkably, carbon monoxide is the only detectable product, corresponding to 100 percent selectivity, a feat attributed to the well-matched redox potentials of the preserved carriers. Because the two-electron pathway from carbon dioxide to carbon monoxide is both thermodynamically and kinetically favorable, and because separating mixed gas products is a costly downstream burden, this level of selectivity is highly valuable for both mechanistic studies and practical deployment. Radical scavenging experiments with ascorbic acid and isopropanol show that superoxide radicals play the decisive role, indicating that photogenerated electrons activate dissolved oxygen to form reactive intermediates that assist the reduction chemistry.

Stability and safety, the perennial Achilles heels of lead-halide perovskites, also fare well. After ten consecutive carbon dioxide reduction cycles, the nanowire and nanocrystal composites retain 91 and 94 percent of their initial activity respectively, and the X-ray diffraction pattern of the cycled nanowire sample remains unchanged. Lead ion leakage into solution after testing measures a mere 0.31 nanomolar, kept low by the small perovskite loading and by the residual hydrophobic ligands that suppress dissolution. For a class of materials often dismissed as too fragile and too toxic for real-world catalysis, these numbers represent a meaningful step toward credibility.

The air-purification results are equally compelling, and here the morphology story flips. The nanocrystal composite removes 69.5 percent of nitric oxide at a realistic concentration of 600 parts per billion, compared with 59.2 percent for the nanowire version and just 23.9 percent for pristine g-C3N4. More importantly, the fraction of nitric oxide converted into nitrogen dioxide, a toxic byproduct that contributes to acid rain, drops from 14.2 percent for bare carbon nitride to only 5.0 percent for the nanocrystal system. The S-scheme pathway generates an abundance of superoxide radicals, which oxidize nitric oxide first to nitrogen dioxide and then further to nitrate, effectively preventing the dangerous intermediate from accumulating. Hydroxyl radicals, generated through a chain involving hydrogen peroxide formed on the water-adsorbing carbon nitride surface, drive the final conversion to stable nitrous and nitric acids. The nanocrystal catalyst also retained its full activity over five successive reaction cycles.

The deeper lesson of this work lies in quantum confinement and defect engineering. The 3.8-nanometer-diameter nanowires possess a wider band gap of about 1.95 electronvolts, while the larger 18-nanometer nanocrystals sit at roughly 1.68 electronvolts, absorbing more effectively in the red. The nanocrystals also exhibit well-defined facets and far fewer surface defects than the defect-rich nanowires, whose vacancies and dangling bonds can trap carriers and diminish oxidative power. By simply choosing which morphology to grow, the researchers can dial the same chemical system toward reduction chemistry or oxidation chemistry, a strategy they describe as morphology-governed redox tuning. As the world searches for technologies that simultaneously address the carbon problem and the air-quality crisis, this study suggests that some of the most powerful answers may come not from new compounds, but from reshaping the ones we already have.

Subject of Research: Morphology-dependent CsPbI3/g-C3N4 S-scheme heterostructures for photocatalytic CO2 reduction and NO removal

Article Title: Perovskite CsPbI3 Nanocrystal and Nanowire Incorporated G‐C3N4 Nanosheets for CO2 Reduction and NO Removal

Article References: Perovskite CsPbI3 Nanocrystal and Nanowire Incorporated G‐C3N4 Nanosheets for CO2 Reduction and NO Removal. (n.d.). https://doi.org/10.1002/advs.78119

Image Credits: AI Generated

DOI: 10.1002/advs.78119

Keywords: photocatalysis, CsPbI3 perovskite, graphitic carbon nitride, CO2 reduction, NO removal, S-scheme heterojunction, nanowires, nanocrystals, quantum confinement, charge separation, solar fuels, air purification

The appeal of lithium-sulfur chemistry lies in its theoretical energy density, which far exceeds that of today’s lithium-ion cells because sulfur can in principle exchange two electrons per atom at a lightweight atomic mass. That promise has made Li-S batteries a favorite candidate for structural energy storage, an emerging field in which the battery itself doubles as a load-bearing component of an aircraft fuselage, a car chassis, or a robot’s limb. The problem is that sulfur cathodes are mechanically and chemically fragile. When a sulfur-loaded electrode is bent around a curved surface, the active material cracks, the interface between the cathode and the current collector delaminates, and electrical contact is progressively lost. Under repeated curvature changes and external loading, these failure modes compound, and the cell’s performance collapses.

The KAIST-led team attacked this problem at the level of materials architecture rather than cell packaging. Their starting point is ordinary woven carbon cloth, a flexible, conductive textile made of carbon fibers. Onto this fabric they grew dense arrays of nickel-cobalt precursor nanowires directly, without any polymer binder, and then converted the nanowires into nickel cobalt selenide, NiCo2Se4, through a selenization process. The result is a single monolithic scaffold in which polar, polysulfide-grabbing NiCo2Se4 surfaces are seamlessly fused to a continuous carbon-fiber network that shuttles electrons throughout the electrode. Because the nanowires are grown rather than glued, there is no weak interface between the active material and the conductive substrate, which is precisely where conventional composite cathodes tend to fail under mechanical stress.

The chemistry of the selenide coating matters as much as its mechanical integration. During discharge in a lithium-sulfur cell, sulfur is converted through a series of intermediate polysulfide species that dissolve into the electrolyte and migrate between the electrodes, a phenomenon known as the polysulfide shuttle that erodes capacity and fouls the lithium anode. Polar transition-metal compounds such as NiCo2Se4 chemically adsorb these polysulfides, anchoring them near the conductive network where they can be reconverted instead of wandering off. In comparative tests, the sulfur-loaded nanowire scaffold, designated S/NCSe@CC, showed markedly stronger polysulfide adsorption than sulfur deposited on plain carbon cloth or on a precursor-coated cloth lacking the selenide conversion, confirming that the selenide surface is the active ingredient in taming the shuttle problem.

Electrochemically, the scaffold delivered impressive numbers. At a modest rate of 0.1 C, the S/NCSe@CC cathode achieved a discharge capacity of 1,544 milliampere-hours per gram of sulfur, close to the theoretical limit of the chemistry, with a voltage polarization of only 0.13 volts between charge and discharge plateaus. Reduced charge-transfer resistance and accelerated sulfur-redox kinetics, measured against the control electrodes, indicate that the interconnected nanowire-fiber network provides both fast electron transport and efficient ionic access deep inside the electrode. In practical terms, the scaffold functions simultaneously as a current collector, a polysulfide trap, and a mechanical backbone, collapsing three functions that normally require separate layers into one integrated textile.

The most striking result, however, concerns mechanical durability. The researchers subjected the cathode scaffold to 1,000 curvature-conditioning cycles at a bending radius of just 2.5 millimeters, an aggressive deformation that would destroy most conventional electrodes. Microscopic examination afterward showed that the nanowire-fiber integration remained intact, with no catastrophic cracking or delamination of the active layer. Even more importantly, pouch cells assembled with electrodes that had already endured this conditioning displayed discharge behavior comparable to pristine, never-bent cells. This suggests that the scaffold does not merely survive bending in a laboratory test; it can be pre-shaped into a curved geometry and still deliver full electrochemical performance once installed in a real device.

To test that idea under realistic conditions, the team assembled pouch cells and bent them around cylindrical mandrels. Cells held in the curved state retained 94.0 percent of their flat-state discharge capacity, a loss of only six percent attributable to the curvature itself. The researchers then went further, integrating the pouch cells into cylindrical, oval, and square geometries, and finally wrapping them around a drone-leg structure to mimic the kind of non-planar component a structural battery would actually inhabit. In every case the cells remained electrically operational after geometric insertion and fixation, demonstrating that the materials-to-architecture strategy holds up not just on a bending jig but in the awkward, compound-curved shapes that real engineering demands.

Why does this matter beyond the laboratory? Structural batteries promise to eliminate the dead weight of dedicated battery packs by making the airframe or chassis itself an energy store, and for lightweight platforms such as drones, eVTOL aircraft, and small satellites, every gram saved translates directly into range or payload. Lithium-sulfur chemistry is particularly attractive here because of its low areal density, but until now nobody had shown a Li-S cathode that could be reliably integrated into a non-planar structural element. The carbon-cloth substrate is itself a textile, which means it can in principle be draped, stitched, or laminated into composite structures using processes familiar to the aerospace industry, and the binder-free growth process avoids the polymer additives that add weight and creep under load.

The work also offers a template that extends beyond nickel cobalt selenide. The core insight, that a polar conversion compound grown monolithically on a flexible conductive textile can simultaneously solve the chemical and mechanical failure modes of a sulfur cathode, could be adapted to other transition-metal chalcogenides or phosphides as polysulfide hosts. The KAIST and Chalmers collaboration, which bridges mechanical engineering, materials science, and structural-battery expertise, is well positioned to push the concept toward full structural composites in which the cathode textile is embedded within a load-bearing laminate. The research was supported by the National Research Foundation of Korea and the Korean Institute for Advancement of Technology, reflecting sustained national investment in next-generation energy-storage materials.

Challenges remain before curved lithium-sulfur structural cells fly on real aircraft. The reported results focus on cathode performance and curvature tolerance; full structural-battery systems must also address lithium-metal anode stability, electrolyte containment under load, and the trade-off between mechanical stiffness and ionic transport that defines the entire structural-power field. Cycle-life data at the pouch-cell level under sustained mechanical stress will be essential, and scaling the nanowire growth process to meter-scale textiles is an open engineering question. Still, the demonstration that a Li-S pouch cell can be wrapped around a drone leg and keep working, retaining 94 percent of its capacity under cylindrical curvature, is a vivid proof of concept. It suggests a future in which the battery is not a box bolted onto a machine but a material woven into its very shape, flexible enough to follow the designer’s geometry and tough enough to carry the load.

Subject of Research: Monolithic NiCo2Se4 nanowire-carbon cloth cathode scaffolds for non-planar lithium-sulfur structural battery integration

Article Title: Monolithically integrated NiCo2Se4 nanowire–carbon cloth cathode scaffolds for non-planar lithium-sulfur structural-battery integration

Article References: Ha, J., Garai, M., Ullah, Z., Ardhi, R. E. A., Kim, C., Kim, A., Mahato, M., Kim, H. Y., Asp, L. E., & Oh, I.-K. (2026). Monolithically integrated NiCo2Se4 nanowire–carbon cloth cathode scaffolds for non-planar lithium-sulfur structural-battery integration. Advanced Composites and Hybrid Materials. https://doi.org/10.1007/s42114-026-02093-y

Image Credits: AI Generated

DOI: 10.1007/s42114-026-02093-y

Keywords: lithium-sulfur batteries, structural batteries, nickel cobalt selenide, nanowires, carbon cloth, polysulfide shuttle, flexible electrodes, energy storage, curvature tolerance, pouch cells, drone integration, KAIST

P3HT is one of the most studied semiconducting polymers in the world, prized for its strong absorption in the visible spectrum, its processability from common solvents and its ability to self-organise into ordered structures. Third-order nonlinear optics, the regime in which a material’s optical response scales with the square of the light intensity, is where such polymers can outshine conventional glass. Conjugated small molecules such as cyanoethynylethenes exhibit third-order susceptibilities roughly a thousand times greater than silica, and P3HT monolayers with improved molecular ordering have shown exceptionally high values of the complex third-order susceptibility, χ3. Yet most research has focused on molecular design rather than on a more fundamental question: what happens when the same polymer chains shift from a tangled, amorphous state into tightly packed crystalline aggregates?

To answer that question, the team exploited a remarkably simple preparation route. Regioregular P3HT was dissolved in anhydrous toluene at a 1:200 weight-to-volume ratio, stirred at 75 degrees Celsius for four hours and then left in the dark for 72 hours. During this quiet incubation the polymer chains slowly aggregated into nanowires, forming crystalline H-aggregates in which neighbouring backbones stack face to face. The dark conditions also prevented photodegradation. The resulting suspension took on a bluish hue, while amorphous P3HT dispersed in tetrahydrofuran appeared burgundy, a visible sign of the electronic reorganisation within. Electron microscopy confirmed the transformation, revealing interconnected fibrillar nanowires with a mean diameter of about 16 nanometres, in stark contrast to the diffuse, irregular nanostructures of the amorphous phase.

Optical characterisation reinforced the picture. Spectroscopic ellipsometry, fitted with a Tauc-Lorentz oscillator model, showed that the amorphous films had refractive indices of roughly 1.2 to 1.3, while the nanowire films ranged from 1.85 to 1.95, both consistent with literature values. The nanowires displayed a steep anomalous dispersion slope near 2.33 electronvolts, driven by a red-shifted central oscillator and narrower broadening coefficient, direct optical evidence of highly ordered pi-pi stacking through Kramers-Kronig relations. High-resolution Raman spectroscopy added a decisive signature: the dominant carbon ring stretching mode narrowed from a full width at half maximum of 49 wavenumbers in the amorphous phase to just 26 wavenumbers in the nanowires, indicating straighter backbones, longer effective conjugation lengths and well-formed crystalline domains.

The heart of the study was the Z-scan technique, a sensitive single-beam method for extracting both the nonlinear refractive index and the nonlinear absorption coefficient. A continuous-wave diode-pumped solid-state laser at 532 nanometres, delivering 50 milliwatts and a focal intensity of 2.63 kilowatts per square centimetre, probed ultra-dilute P3HT suspensions at five concentrations between 0.0006 and 0.0010 percent. Open-aperture detection captured nonlinear absorption, while closed-aperture detection, with a 0.42 aperture, captured nonlinear refraction. The 532-nanometre line was chosen deliberately: P3HT absorbs strongly near 520 nanometres with vibronic features extending to about 600 nanometres, so the nanowires sat close to resonant excitation while the amorphous phase sat on its absorption blue edge.

The results overturned a straightforward expectation. One might assume that the crystalline nanowires, with their dense conjugation and dielectric confinement, would deliver the stronger nonlinear response. Instead, the amorphous phase consistently showed larger effective nonlinear absorption coefficients, peaking at 1.31 times ten to the minus five centimetres per watt at the lowest concentration, compared with 1.05 times ten to the minus five for the nanowires. The explanation lies in defects. Disordered chains are riddled with torsional defects and localised electronic states that promote sequential excited-state absorption, a hallmark of reverse saturable absorption. In the crystalline nanowires, delocalised electronic states and enhanced interchain stacking suppress these defect-mediated pathways, yielding lower but more stable absorption nonlinearity.

Both materials behaved as self-defocusing media under green continuous-wave excitation, with closed-aperture traces showing the characteristic peak-valley profile of negative nonlinear refraction. Again the amorphous phase dominated, exhibiting systematically higher negative effective refractive indices across the entire concentration range. The authors attribute this to thermal lensing: defect states accelerate non-radiative decay, heating the sample locally and driving a strong negative thermo-optic response. The ordered nanowires, lacking such trap-rich landscapes, produced a moderate and stable refractive response. Under continuous-wave illumination the extracted coefficients are therefore effective parameters that blend electronic polarisation with cumulative photothermal contributions, a caveat the team stresses throughout.

Quantitatively, the total effective third-order susceptibility averaged 2.88 plus or minus 0.84 times ten to the minus nine electrostatic units for the amorphous phase, against 2.39 plus or minus 0.47 for the nanowires, a mean suppression of 16.9 percent. The imaginary component outweighed the real part by an order of magnitude, confirming that nonlinear absorption dominates the overall profile. Intriguingly, the suppression was non-uniform: at intermediate concentrations of 0.0007 to 0.0008 percent the nanowires actually outperformed the amorphous matrix by 9.6 to 26.1 percent, because disordered chains undergo a coil-contraction dip that temporarily collapses their response. A photophysical crossover at 0.0008 percent marked the transition from isolated single-chain dynamics to collective interchain behaviour in both systems.

Perhaps the most striking comparison came against conventional organic dyes. Orange G and Methylene Blue, benchmark nonlinear chromophores under identical 532-nanometre continuous-wave excitation, exhibit effective susceptibilities confined to the ten to the minus five esu regime at far higher loadings. That P3HT architectures sustain a robust ten to the minus nine esu response at concentrations below one thousandth of a percent points to an intense intrinsic microscopic hyperpolarizability in the conjugated backbone, making the polymer highly competitive with standard optoelectronic dyes even in an ultra-dilute regime where solvent properties dominate the thermal background.

X-ray photoelectron spectroscopy added a sobering note about ageing. High-resolution carbon and sulphur core-level spectra of the nanowire films, stored for fourteen days under ambient conditions before analysis, revealed that oxidised sulphur accounted for 46.87 percent of the sulphur signal, with sulphone formation indicating cumulative chemical aging. Crystalline nanostructuring preserved the macroscopic architecture but could not shield the polymer from ambient oxidation, suggesting that encapsulation or chemical optimisation will be essential for real-world devices. The authors conclude that while the amorphous phase offers a modestly higher nonlinear response, the nanowire morphology provides the long-term stability that solid-state photonics demands, a technological advantage they judge to outweigh the 16.9 percent sensitivity compromise. Their roadmap ahead includes dopant engineering to tune the electronic landscape, femtosecond transient absorption to decouple sub-picosecond electronic responses from slower photothermal mechanisms, and Z-scan measurements under ultrafast pulsed excitation to isolate the pure electronic Kerr effect, all steps toward stable, high-performance polythiophene photonic devices.

Subject of Research: Morphology-dependent third-order nonlinear optical properties of poly(3-hexylthiophene) under continuous-wave green laser excitation

Article Title: Effect of poly(3-hexylthiophene) morphology formation on third-order nonlinear optical under green continuous wave excitation

Article References: Poad, N. A., Krishnan, G., Ros, F. C., Sa'aya, N. S. N., Halim, N. A., Manaf, N. A. A., & Demon, S. Z. N. (2026). Effect of poly(3-hexylthiophene) morphology formation on third-order nonlinear optical under green continuous wave excitation. Results in Optics, 25, Article 101172. https://doi.org/10.1016/j.rio.2026.101172

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101172

Keywords: P3HT, nonlinear optics, nanowires, Z-scan, conjugated polymers, optical limiting, self-assembly, third-order susceptibility, photothermal lensing, spectroscopic ellipsometry, XPS, organic photonics