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

Abstract

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 (A_1g) of the defective MoS₂ sheets’ Raman spectra. This red-shifting in the A_1g 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.