Enhanced Magnetization of MoS ₂ Nanosheets through the MoS _<i>x</i> O _<i>y</i> Interphase via Controlled Oxidation: Implications for Spintronics Applications

The magnetic properties of MoS2 can be tuned by tailoring defects, doping magnetic cations, or controlling the phase and composition. Here, we present an enhancement in magnetization of pristine MoS2 nanosheets upon introducing structural and compositional changes through a controlled oxidation process. Conversion of MoS2 to the MoO3 phase is enabled either by longer annealing with natural cooling at (i) 350 °C for 45 min (MoS2-350-45 min) and (ii) 500 °C for 30 min (MoS2-500-30 min) or by rapid annealing for 5 min with immediate cooling at (iii) 400 °C (MoS2-400-5 min), (iv) 450 °C (MoS2-450-5 min), and (v) 500 °C (MoS2-500-5 min) in an air atmosphere to investigate magnetic properties. The presence of the MoS2:MoO3 hybrid phase is confirmed in MoS2-350-45 min, MoS2-400-5 min, and MoS2-450-5 min samples through various spectroscopic and transmission electron microscopy techniques. However, a complete phase transition is observed in MoS2-500-5 min and MoS2-500-30 min samples with ∼40–50 and ∼400–500 nm MoO3 crystallites, respectively. Pristine MoS2 nanosheets exhibit weak ferromagnetism with a saturation magnetization (MS) of ∼6 × 10–3 emu/g (at T = 10 K). MS 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 Mo5+ 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 MoO3 reduces defect states, thus resulting in a decrease in MS (∼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 MoS2:MoO3 hybrid phase, suggesting a significant role played by the MoSxOy 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.