Turning Up the Heat Reshapes MoS2 Nanosheets and Their Optical Behavior

Molybdenum disulfide, the layered semiconductor second only to graphene in popularity among two-dimensional materials, has just revealed a new degree of control over its optical personality. A research team led by Y. Khattab, A.K. Jazmati and A. Allaham reports in Results in Optics that simply changing the temperature at which MoS2 thin films are grown can tune the size, thickness and density of vertically oriented nanosheets, and with them the films’ nonlinear optical responses. The finding matters because those responses underpin technologies ranging from optical limiters that protect sensors and eyes from intense laser light to mode-locked pulsed lasers and next-generation photodetectors.

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