Decoding Defect-Driven Ultrafast and Nonlinear Optical Properties of MoS2 Nanocracks

Abstract Ultrafast spectroscopic techniques have emerged as powerful tools for unravelling the fundamental carrier dynamics, charge transfer, and excitonic interactions in low-dimensional materials. Among these, two-dimensional transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2) exhibit remarkable light–matter coupling and nonlinear optical (NLO) behavior. However, most previous investigations have primarily focused on intrinsic excitonic processes, often overlooking the critical influence of structural imperfections such as defect-, grain boundary-, and nanocrack-driven inhomogeneities that naturally arise during synthesis and afterward. These imperfections drastically modify charge transfer pathways and exciton relaxation, thereby altering the material’s nonlinear optical response. In the present work, MoS2 thin films were synthesized via atmospheric pressure chemical vapor deposition (APCVD) and were subjected to ultrafast transient absorption (UTA) spectroscopy, revealing a distinct subpicosecond relaxation component, attributed to either self-trapped exciton (STE) formation or highly efficient energy funneling at defect-rich regions. This process occurs on a time scale faster than the typical exciton-trapping time (τ1) reported for MoS2, indicating nearly instantaneous exciton localization and transfer within the crystal lattice. A strain-dependent analysis was also carried out to correlate the lattice deformation with the observed charge-carrier dynamics with an estimated strain potential of D ≈ 0.295 eV. Apart from this, open- and closed-aperture Z-scan measurements demonstrated strong third-order nonlinearity, dominated by two-photon absorption (TPA) and self-defocusing effects. The calculated nonlinear parameter absorption coefficient (β), 2.22 × 10–15 m/W, refractive index (η2), −1.17 × 10–12 m2/W, and third-order susceptibility (χ3), 1.91 × 10–12 esu, confirm a robust defect-assisted nonlinearity in the system. These findings establish a direct correlation between structural integrity, defect-mediated exciton dynamics, and nonlinear optical response in APCVD-grown MoS2, offering new insights into the design of ultrafast photonic and optoelectronic devices based on two-dimensional materials.