Enhanced Magnetization of MoS ₂ Nanosheets through the MoS _x O _y Interphase via Controlled Oxidation: Implications for Spintronics Applications

The magnetic properties of MoS 2 can be tuned by tailoring defects, doping magnetic cations, or controlling the phase and composition. Here, we present an enhancement in magnetization of pristine MoS 2 nanosheets upon introducing structural and compositional changes through a controlled oxidation process. Conversion of MoS 2 to the MoO 3 phase is enabled either by longer annealing with natural cooling at (i) 350 °C for 45 min (MoS 2 -350-45 min) and (ii) 500 °C for 30 min (MoS 2 -500-30 min) or by rapid annealing for 5 min with immediate cooling at (iii) 400 °C (MoS 2 -400-5 min), (iv) 450 °C (MoS 2 -450-5 min), and (v) 500 °C (MoS 2 -500-5 min) in an air atmosphere to investigate magnetic properties. The presence of the MoS 2:MoO 3 hybrid phase is confirmed in MoS 2 -350-45 min, MoS 2 -400-5 min, and MoS 2 -450-5 min samples through various spectroscopic and transmission electron microscopy techniques. However, a complete phase transition is observed in MoS 2 -500-5 min and MoS 2 -500-30 min samples with ∼40–50 and ∼400–500 nm MoO 3 crystallites, respectively. Pristine MoS 2 nanosheets exhibit weak ferromagnetism with a saturation magnetization ( M S ) of ∼6 × 10 –3 emu/g (at T = 10 K). M S increases to ∼3 to 6 times for the samples containing the hybrid phase, primarily attributed to defects. The presence of S and/or O vacancies and Mo 5+ with a spin 1/2 state, as confirmed from electron paramagnetic resonance and X-ray photoelectron spectroscopy, may lead to a large number of overlapping bound magnetic polarons, thereby enhancing the magnetization. A complete oxidation to MoO 3 reduces defect states, thus resulting in a decrease in M S (∼10 × 10 –3 emu/g) compared to the mixed phases. Temperature-dependent magnetization studies indicate that the ferromagnetic nature sustains well above 300 K for the MoS 2:MoO 3 hybrid phase, suggesting a significant role played by the MoS x O y interphase in controlling magnetic properties. DFT studies carried out on several model configurations suggest that vacancies in the form of oxygen and sulfur are responsible for the enhancement of magnetization in the system.