The team grew their films by metal organic chemical vapor deposition, or MOCVD, an industrial-standard technique prized for producing high-quality, uniform coatings over large areas. In a hot-wall, low-pressure horizontal tube reactor held at 0.5 Torr, molybdenum hexacarbonyl powder vaporized at 30 degrees Celsius reacted with hydrogen sulfide gas delivered at a precisely controlled flow rate, with argon carrying the molybdenum precursor into the reaction zone. Fused quartz substrates, cleaned sequentially in acetone, alcohol and deionized water, were coated at deposition temperatures spanning 250 to 750 degrees Celsius. What emerged was a forest of nanosheets standing on edge, perpendicular to the substrate, at every temperature tested.

Temperature, however, dramatically changed what that forest looked like. At 250 degrees Celsius the films consisted of vertical sheets roughly 100 to 150 nanometers long and only a few nanometers thick, with a total film height near 180 nanometers. At 550 degrees Celsius the sheets thickened to 40 to 50 nanometers and the film grew to about 300 nanometers, while at 750 degrees Celsius sheet lengths stretched to roughly 350 nanometers and film thickness reached 600 nanometers. Crucially, sheet density dropped as temperature rose. Because the exposed edges of these vertical sheets terminate crystal planes and host defects, plasmonic behavior and dangling bonds, controlling their dimensions is effectively controlling the material’s active optical surface.

Structural analysis told a consistent story. X-ray diffraction showed the 250-degree film to be essentially amorphous or too thin to diffract, while films grown at 550 and 750 degrees matched the hexagonal 2H phase of MoS2, with reflections characteristic of semi-epitaxial growth parallel to the substrate and of the vertical sheets themselves. Rietveld refinement revealed grain sizes increasing from 31 to 47 nanometers, microstrain falling from 2 percent to 0.3 percent, and dislocation density dropping from 850 to 450 per square micrometer as temperature climbed. Raman spectroscopy reinforced the picture: the characteristic in-plane and out-of-plane vibrational modes sharpened dramatically, with the width of the out-of-plane mode narrowing from 16 to 4 inverse centimeters, signaling steadily improving crystallinity.

The linear optical properties shifted in parallel. All films showed bandgaps between 2.05 and 2.25 electronvolts, well above the 1.3 electronvolts of bulk MoS2, and the gap decreased as sheet size increased with deposition temperature. The smallest sheets, grown at 250 degrees, displayed a strong absorption peak near 475 nanometers but weak or absent A and B exciton features. The researchers attribute this to the enormous surface-to-volume ratio of tiny nanostructures, where edge defects, lattice distortions and dangling bonds create localized states that trap photo-excited carriers before coherent delocalized excitons can form. Larger sheets, with fewer such traps, showed the expected excitonic absorption in the 600 to 750 nanometer range, slightly shifted by residual strain.

The real surprises came when the team probed nonlinear behavior with a Z-scan setup, sweeping samples through the focus of a 532-nanometer continuous-wave laser and recording transmission with microwatt precision over 12,000 points per scan. Films grown at 250 degrees showed positive nonlinear absorption, with an effective photo-thermal coefficient reaching about 900 centimeters per watt at 25 milliwatts. Films grown at 550 degrees flipped behavior depending on power: positive at low power, but strongly negative at 50 and 75 milliwatts, with coefficients as large as roughly 10,000 centimeters per watt and temperature rises inferred to reach 70 units. Films grown at 750 degrees showed negative, comparatively stable values at all powers. The team links the stronger response of the 550-degree film to its higher sheet density, since sheet thicknesses at 550 and 750 degrees are nearly identical.

Here the authors exercise unusual and commendable caution. Continuous-wave illumination, unlike ultrafast pulses, pumps heat into the nanostructures continuously, far longer than the microsecond-to-millisecond thermal diffusion time of MoS2. The result is a thermal lens and temperature-dependent absorption changes that can masquerade as electronic saturable or reverse saturable absorption. The team therefore interprets their continuous-wave coefficients strictly as effective photo-thermal figures of merit tied to their exact experimental geometry, substrate thermal conductivity and exposure time, not as intrinsic electronic properties. Their analysis of transmission versus incident power showed no saturation behavior consistent with a standard two-level saturable absorption model; instead, transmission rose exponentially with power, pointing to thermally induced bleaching amplified by the films’ remarkable light-trapping absorption of roughly 97 percent.

To isolate genuine electronic effects, the researchers turned to a twin-detector configuration with 20-nanosecond laser pulses at 532 nanometers, energies from 10 to 80 millijoules. Under pulsed excitation the picture inverted cleanly: all three films, regardless of growth temperature, showed transmission decreasing as pulse intensity increased, the hallmark of reverse saturable absorption. The extracted nonlinear absorption coefficients, between 10 and 4.5 times 10 to the minus 4 centimeters per watt, were orders of magnitude smaller than the continuous-wave values and of opposite sign for the hotter films, confirming that thermal accumulation dominated the continuous-wave regime. The pulsed response likely arises from excited-state absorption or defect-assisted transitions through edge states, though the authors note that confirming carrier lifetimes and specific decay pathways will require ultrafast pump-probe spectroscopy beyond this study.

The practical implications are considerable. Reverse saturable absorption under nanosecond pulses is exactly the property sought in optical limiters, devices that become more opaque as incoming light grows more intense, protecting detectors and eyes from laser damage. Meanwhile, the giant photo-thermal coefficients observed under continuous-wave illumination, among the largest reported for MoS2 on quartz, suggest potential in thermally managed photonic components. The work also delivers a clear design rule: deposition temperature is a single, accessible knob that tunes sheet size from 100 to 350 nanometers, sheet thickness from a few to about 50 nanometers, bandgap from 2.25 down to 2.05 electronvolts, and the sign and magnitude of nonlinear absorption. For a material already central to two-dimensional electronics and optoelectronics, that kind of process-structure-property control brings practical nonlinear optical devices a meaningful step closer.

Subject of Research: Temperature-controlled MOCVD growth of vertical MoS2 nanosheet thin films and the dependence of their nonlinear optical responses on morphology

Article Title: Morphological and deposition temperature dependence of the nonlinear optical responses in nanostructures MoS2 thin films synthesized by the metal organic chemical vapor deposition

Article References: Khattab, Y., jazmati, A., & Allaham, A. (2026). Morphological and deposition temperature dependence of the nonlinear optical responses in nanostructures MoS2 thin films synthesized by the metal organic chemical vapor deposition. Results in Optics, 25, Article 101168. https://doi.org/10.1016/j.rio.2026.101168

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101168

Keywords: MoS2, two-dimensional materials, MOCVD, nanosheets, nonlinear optics, Z-scan, saturable absorption, reverse saturable absorption, optical limiting, bandgap tuning, photothermal effects, thin films

Zinc oxide is one of the most attractive semiconductors in nanotechnology. It combines a wide direct bandgap of about 3.37 electron volts with a high exciton binding energy of 60 milli-electron volts, excellent electron mobility, and strong transparency in the visible spectrum. It is also non-toxic, chemically stable, mechanically robust, and environmentally benign—qualities that have made it a staple candidate for ultraviolet photodetectors, transparent conductive coatings, gas sensors, photocatalysts, and solar energy converters. Yet pristine ZnO suffers from well-known shortcomings, including surface-related charge trapping, electron–hole recombination losses, and difficulty achieving uniform dopant incorporation. Conventional doping strategies rely on trivalent metals such as aluminum, gallium, and indium, which are expensive and raise environmental and health concerns.

The research team, led by Ezekiel Clinton Oroke of the Federal Polytechnic Ado-Ekiti with colleagues from Alex Ekwueme Federal University and Ebonyi State University, sidestepped metal dopants altogether. They prepared biochar by slow pyrolysis of fresh oil palm fingers collected from a plantation reserve in Ebonyi State, a material authenticated at the Ekiti State University herbarium. Half a gram of this biochar was added directly into each chemical bath during ZnO deposition, an approach that differs from the impregnation, co-precipitation, hydrothermal, and ball-milling routes that dominate the biochar–ZnO literature, which have mostly targeted adsorption, photocatalysis, and biomedical uses rather than semiconductor tailoring.

Structural analysis confirmed that the biochar does not destroy the crystal backbone of ZnO. X-ray diffraction showed the characteristic hexagonal wurtzite phase, indexed against the standard JCPDS card 36-1451, with an average crystallite size of just 22.1 nanometers. That small size highlights the biochar’s role as a growth-regulating matrix that suppresses excessive particle agglomeration during low-temperature synthesis. Refined lattice constants of 3.253 and 5.211 angstroms, a c/a ratio near 1.602, and a unit cell volume of roughly 47.8 cubic angstroms sat close to bulk ZnO values, with slight deviations pointing to lattice distortion and strain induced by interfacial interactions between ZnO nanocrystals and oxygen-containing functional groups on the carbonaceous scaffold. The team calculated an average microstrain of 3.41 × 10⁻³ and a dislocation density of 3.64 × 10⁻³ per square nanometer, alongside an estimated crystallinity of about 78.4 percent—a slight reduction attributed to amorphous carbon domains threading through the crystal lattice.

Raman spectroscopy told a complementary story about defects and interfaces. The spectra displayed well-defined ZnO vibrational modes, including the E1 (transverse optical) band at 412 wavenumbers and the defect-sensitive E1 (longitudinal optical) band at 634 wavenumbers, the latter signaling increased concentrations of oxygen vacancies and zinc interstitials created by the biochar interaction. The carbon D and G bands near 1347 and 1583 wavenumbers showed an intensity ratio of roughly one, indicating a balanced mix of ordered graphitic domains and defect or edge sites—a balance the authors describe as ideal for functional carbon–semiconductor hybrid systems, because it can enhance charge transfer, facilitate bandgap modulation, and reduce electron–hole recombination. Additional bands associated with carbonyl, aliphatic C–H, and surface hydroxyl groups revealed a high density of surface-active sites capable of inducing surface band bending and stronger light–matter interaction.

Electron microscopy reinforced the picture of a well-engineered hybrid. Scanning electron micrographs revealed a highly porous, interconnected surface morphology of densely packed quasi-spherical nanograins, with an average particle size of 59 ± 3 nanometers distributed across the biochar matrix. The raw biochar itself, by contrast, appeared as a collapsed fibrous network with a partially preserved plant framework, layered structures, micro-cavities, and an average particle size of 147 ± 27 nanometers. Those pores and defects, the researchers note, provide abundant nucleation sites for ZnO deposition and help disperse the nanoparticles, curb aggregation, and boost electron mobility. X-ray fluorescence further documented a rich elemental profile dominated by silicon, calcium, potassium, and aluminum oxides—multiple oxide species that the authors link to the improved optical conductivity of the modified films.

The optical results are where the practical promise sharpens. Across deposition temperatures from 30 to 100 degrees Celsius, the biochar-modified films showed blue-shifted absorption edges between 300 and 350 nanometers—shorter than the 365.5 nanometers at which ZnO normally absorbs—along with a general cutoff in the ultraviolet region suggesting potential as efficient UV detectors. Bandgap energies swung non-monotonically from 3.54 through 3.00, 3.10, and up to 3.70 electron volts depending on bath temperature, with the minimum at 50 degrees Celsius marking what the team interprets as optimal carbon–ZnO coupling, where π-conjugated pathways and defect states narrow the gap and extend absorption toward the visible. Films deposited at that temperature reached the highest transmittance, 65.3 percent, and the thinnest dimension, about 12.1 nanometers—close to quantum-dot scale. Refractive index values climbed from the published bulk figure of roughly 2.0 to as high as 2.64, a significant boost for optical switches, filters, and modulators.

Residence time in the deposition bath proved equally influential. As deposition stretched from 10 to 60 minutes, the modified nanoparticles showed non-monotonic bandgap evolution—3.7, 3.4, 2.51, 3.70, and 2.70 electron volts—while pristine ZnO followed a simpler, steadily narrowing path from 3.5 to 2.5 electron volts as crystallinity improved. The contrast, the authors argue, shows that biochar alters the electronic structure through interfacial interactions and defect-state modulation rather than plain crystal growth. Modified films absorbed progressively into the visible region at longer residence times and outperformed pristine ZnO in light harvesting, with the 60-minute sample delivering the strongest and broadest UV–visible absorption. Optical conductivity peaked in films prepared at 60 minutes, and the dielectric response—real part falling and imaginary part rising with photon energy—matched the inter-band transition behavior expected of semiconductors, with the real part proportionally higher than the imaginary throughout.

Not every metric moved upward, and the researchers are candid about the trade-off. Pristine ZnO, as crystallization progressed with residence time, achieved the highest optical conductivity overall, around 4.2 × 10⁶ siemens across the visible region, whereas the biochar-modified samples plateaued near or below 1.1 × 10⁶ siemens. The team attributes this suppression of carrier concentration to the biochar matrix, which promotes more controlled charge transfer through interfacial interactions and carrier trapping—an effect they argue is advantageous for curbing electron–hole recombination and improving photocatalytic performance, even at the cost of absolute conductivity. The modified films, they suggest, are strong candidates for UV and infrared photodetectors, photocatalytic devices, transparent optoelectronics requiring spectral selectivity, and solar energy conversion, where defect-rich, strained ZnO structures are known to offer enhanced charge-carrier mobility, more surface-active sites, and better adsorption behavior.

Beyond the numbers, the study carries a sustainability argument that aligns with several United Nations Sustainable Development Goals, including affordable clean energy, responsible consumption and production, and climate action. Oil palm bunch fingers are an agricultural residue with no known economic value, and valorizing them as a multi-element, eco-friendly dopant addresses two problems at once: it diverts waste from the environment while replacing toxic, costly metal dopants in semiconductor fabrication. Chemical bath deposition itself operates below 100 degrees Celsius, keeping the energy footprint low and making bath temperature a surprisingly effective, low-energy dial for band-structure engineering. The authors conclude that oil palm fingers biochar effectively tailors the optoelectronic properties of ZnO without compromising its wurtzite crystal integrity, and they frame the resulting nanoparticles as sustainable candidates for the energy-efficient devices the industry will need as demand for transparent, flexible, and low-power electronics continues to climb.

Subject of Research: Biochar-modified zinc oxide nanoparticles synthesized from oil palm agro-waste for enhanced optoelectronic properties

Article Title: Eco-friendly biochar-modified ZnO nanoparticles for enhanced optoelectronic properties

Article References: Oroke, E. C., Nwabue, F. I., Nworie, F. S., & Afuwape, O. M. (2026). Eco-friendly biochar-modified ZnO nanoparticles for enhanced optoelectronic properties. Discover Green Chemistry, 1(1), Article 34. https://doi.org/10.1007/s44509-026-00037-9

Image Credits: AI Generated

DOI: 10.1007/s44509-026-00037-9

Keywords: biochar, zinc oxide nanoparticles, optoelectronics, Elaeis guineensis, chemical bath deposition, bandgap tuning, green chemistry, agro-waste valorization, semiconductor doping, Raman spectroscopy, X-ray diffraction, wurtzite ZnO