Defects-Induced Fermi Level Modulation and Exciton-to-Trion Conversion in a CVD-Grown MoS₂ Thin Film

Layered MoS2 films were synthesized through the CVD technique, and substantial sulfur vacancies were generated by using the swift heavy ion irradiation technique. The SHI-generated sulfur vacancies show an excellent opportunity for varying the optoelectronic properties of the MoS2 films. After 70 MeV Si4+ irradiation at a different fluence, the findings indicate a red shift in the out-of-plane vibration modes (A1g) of the defective MoS2 sheets’ Raman spectra. This red-shifting in the A1g phonon mode indicates the systematic tensile strain generation and n-type behavior of irradiated MoS2 sheets, which also influences the work function decrease of the MoS2 film. More precisely, a 0.23 eV decrease in the work function of the 5 × 1013 fluence-irradiated MoS2 films confirms the n-type behavior of defective MoS2. Also, the defect density that provides both radiative and nonradiative active sites for electron–hole recombination may be modulated to influence the photoluminescence (PL) intensity of MoS2; when the defect density increases, the overall PL intensity of the samples exhibits a monotonic reduction. Due to the strain-generated n-type behavior of irradiated samples, the defect-bound negative trions’ density improves with ion fluence. Also, the TRPL results show that the surface trapping and interband electron–hole recombination times continuously decrease with fluence. This study enables the systematic defects generation of MoS2, which can be employed for device or light-emitting applications.