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

The excitement around PtSe2 stems from a remarkable property: its electronic character depends dramatically on how many atomic layers are stacked together. A single layer behaves as a true semiconductor with a bandgap of about 1.2 to 1.5 electron volts. Add a second layer and the gap collapses to roughly 0.3 electron volts; a third layer pushes it below 0.1 electron volts; and by four layers the material has become a semi-metal with no gap at all. This steep transition arises from strong interlayer coupling, and it cuts both ways. On one hand, it means a single material can serve as both the semiconducting channel and the metallic electrode in an all-PtSe2 transistor. On the other, it makes thickness control absolutely unforgiving: even a stray bilayer island in an otherwise monolayer film creates a semi-metallic leak path that would ruin a transistor’s ability to switch off.

That unforgiving requirement is precisely why monolayer PtSe2 has eluded researchers for so long. Earlier attempts using thermally assisted conversion, powder-source chemical vapor deposition, or molecular beam epitaxy produced nonuniform multilayer films, tiny grains of only 10 to 20 nanometers, or multilayer flakes dotted with small monolayer patches. None of these could deliver the precise thickness control and wafer-scale uniformity that transistor manufacturing demands. The new work overcomes this barrier through two clever chemical innovations: a custom-designed platinum precursor and an unexpected growth partner, oxygen gas.

The precursor, Pt(dipivaloylmethane-sulfonate)2, was synthesized specifically for thermal stability, refusing to decompose until temperatures reach approximately 300 degrees Celsius, higher than any other platinum precursor available. That stability matters because in a typical metal-organic deposition process, precursors that break apart too easily in the gas phase release highly reactive fragments that nucleate particles in mid-air, raining uncontrolled defects onto the growing film. By keeping the precursor intact until it reaches the heated substrate, the team suppressed these gas-phase reactions entirely, allowing the film to assemble itself in an orderly, layer-by-layer fashion at a growth temperature of 470 degrees Celsius on muscovite mica substrates.

Oxygen played an equally decisive role, and here the choice of material proved fortuitous. Metal-organic precursors carry organic ligands that, if left behind on the substrate, contaminate the film with amorphous carbon and create unwanted nucleation sites that shrink grain size. Most two-dimensional semiconductors, such as molybdenum disulfide, would be damaged by oxygen during growth, but PtSe2 belongs to the noble-metal dichalcogenides, which possess a high energy barrier to oxygen dissociation and are inherently resistant to oxidation. The researchers exploited this resilience by co-injecting a small, precisely metered flow of oxygen into the growth chamber. The oxygen triggers a combustion reaction that burns the organic ligands into volatile carbon dioxide and water vapor, sweeping them away while leaving the platinum and selenium atoms untouched to form pristine PtSe2.

The spectroscopic evidence for this chemistry is striking. Films grown with oxygen show no Raman signals in the 1300 to 1600 per centimeter range, where carbon’s characteristic D and G bands would appear, while films grown with hydrogen instead of oxygen display both bands, betraying amorphous carbon contamination. X-ray photoelectron spectroscopy reinforced the picture: oxygen-grown films exhibited sharp, symmetric platinum 4f and selenium 3d peaks consistent with a clean, stoichiometric crystal, whereas hydrogen-grown films showed sub-stoichiometric phases and oxidized selenium species, signs that lingering organic residue had interfered with proper bonding. The team also demonstrated that the resulting films are exceptionally stable in air, with Raman peak positions shifting by less than 0.001 percent over seven months of ambient exposure.

Uniformity across the full substrate was verified through a battery of optical and structural measurements. Raman spectroscopy at 36 points across the film found the characteristic in-plane vibrational peak at 181.35 per centimeter, exactly where a monolayer should sit, with a standard deviation of just 0.16 per centimeter, and a narrow peak width indicating few point defects. Ultraviolet-visible absorption confirmed an optical bandgap of approximately 1.5 electron volts, matching theoretical predictions. Atomic force microscopy captured the growth in action: after four hours, triangular grains roughly 100 nanometers across had nucleated simultaneously and grown laterally, with step heights of 0.75 nanometer confirming single-layer thickness; after eight hours, the grains had merged into a fully continuous monolayer. The triangular grains even showed preferential alignment with the six-fold symmetry of the underlying mica, hinting that single-crystalline films may be within reach.

At the atomic scale, high-angle annular dark-field scanning transmission electron microscopy revealed the textbook 1T crystal structure, with each platinum atom coordinated by six selenium atoms and all three atomic sublayers clearly resolved. The measured lattice constant of 3.78 angstroms closely matched the theoretical value of 3.79 angstroms, and selected-area electron diffraction showed diffraction spots arranged in hexagonal patterns, confirming that most grains align with the mica substrate, though the film is not yet fully epitaxial.

The ultimate test came when the team fabricated arrays of bottom-gate field-effect transistors using the monolayer film as the channel, with standard photolithography, hafnium oxide gate dielectric, and gold contacts. The devices delivered a maximum on/off current ratio of 8.31 times ten to the fourth power and a minimum off-current of 3.40 times ten to the negative twelfth amperes per micrometer, the highest ratio and lowest off-current ever reported for PtSe2 film-based transistors. The large monolayer bandgap suppresses the thermally activated carriers that plague thicker, semi-metallic films in the off state. Field-effect mobility reached up to 1.37 square centimeters per volt-second, modest compared with the theoretical ceiling of nearly 1900 for monolayer PtSe2, a gap the researchers attribute to grain boundaries in the roughly 100-nanometer grains, transfer-related residues, interface traps at the dielectric, and unoptimized metal contacts, all of which they identify as targets for future optimization.

Beyond the record numbers, the significance of this work lies in its industrial compatibility. Metal-organic chemical vapor deposition is already the standard route for depositing thin films in commercial semiconductor fabs, and the growth temperature of 470 degrees Celsius is low enough for back-end integration schemes. By demonstrating that a properly designed precursor combined with oxygen-assisted ligand removal can tame a material whose layer count must be controlled to a single atomic plane, the team has established not just a new channel material candidate but a general framework for growing other emerging two-dimensional semiconductors at scale. If grain size and contact engineering can be improved in subsequent work, PtSe2 could graduate from laboratory curiosity to a serious contender in the post-silicon roadmap, with the added possibility that its own semi-metallic thicker form could one day serve as the contacts in entirely platinum-diselenide circuits.

Subject of Research: Oxygen-assisted MOCVD growth of semiconducting monolayer platinum diselenide films for field-effect transistors

Article Title: Oxygen‐Assisted MOCVD Growth of Monolayer PtSe2 Films With Bandgap Opening for Semiconducting FET Channels

Article References: Kim, Y., So, H.-S., Yoo, M., Oh, S., Kim, D., Gyeon, M., Jo, M.-K., Noh, G., Kim, T. S., Kim, M.-G., Park, J., Chai, H.-J., Kang, M., Kim, S., Ham, A., Lee, J., Lim, J., Song, S., Kwak, J. Y., … Kang, K. (2026). Oxygen‐Assisted MOCVD Growth of Monolayer PtSe 2 Films With Bandgap Opening for Semiconducting FET Channels. Advanced Science, 13(53), Article e76362. https://doi.org/10.1002/advs.76362

Image Credits: AI Generated

DOI: 10.1002/advs.76362

Keywords: PtSe2, two-dimensional materials, MOCVD, monolayer semiconductor, field-effect transistor, bandgap engineering, transition metal dichalcogenides, nanoelectronics, thin film growth, oxygen-assisted growth, crystallography, semiconductor manufacturing