Effects of 2 MeV proton irradiation on two-dimensional monolayer and multilayer MoS 2 : critical impact of dimensionality on the optical, electronic, and structural properties

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Molybdenum disulfide (MoS 2 ) is a prototypical transition metal dichalcogenide semiconductor with excellent optical and electrical properties. In this study, the irradiation behavior of monolayer and multilayer MoS 2 was compared under irradiation with 2 MeV protons at 300 K, at fluences of 1 × 10 14, 1 × 10 15 , and 5 × 10 15 protons cm −2 . Raman spectroscopy revealed that the intensity of the longitudinal acoustic mode, indicative of structural defects, increased with irradiation fluence only in monolayer MoS 2 , and both the in-plane ( E 2g 1 ) and out-of-plane ( A 1g ) vibrational modes showed minimal sensitivity to irradiation in both samples. Photoluminescence (PL) measurements revealed a strong suppression of direct-transition emission in monolayer MoS 2 after irradiation, whereas enhanced PL near the A- and B-exciton energies was observed in multilayer MoS 2 . X-ray photoelectron spectroscopy revealed no notable oxidation in MoS 2 , but the Mo 3d and S 2p binding energies were slightly increased in monolayer MoS 2 . Kelvin probe force microscopy showed opposite trends in the MoS 2 –SiO 2 work-function difference for monolayer and multilayer samples, indicating competing effects of surface adsorption, charge transfer, and electronic property modifications induced by irradiation defects. Molecular dynamics simulations further elucidate irradiation-induced defect formation, including point defects, sputtering, and defect clustering. These results provide insights into the irradiation behavior of MoS 2 and its layer-dependent property modifications.