Space-Confined Chemical Vapor Deposition of 2D FeS With Crossover Magnetoresistance and Ultra-High Conductivity

2D iron chalcogenides (FeX, X = S, Se, Te) are emerging as an appealing class of materials, owing to their intriguing physical properties (e.g., 2D magnetism, interface superconductivity), and great potentials in next-generation applications (e.g., spintronics, quantum computing). Recently, FeS has attracted particular attentions due to the discovery of altermagnetism in bulk state. However, the intrinsic physical properties of 2D FeS are still unclear, since the syntheses of high-quality FeS with thicknesses down to 2D limit remain challenging. Herein, we report the direct syntheses of thickness-tunable, high-quality 2D FeS flakes with low defect density and thus strict stoichiometry, via a space-confined chemical vapor deposition (CVD) strategy. Significantly, we observe an unsaturated magnetoresistance with a crossover behavior (positive-negative-positive with decreasing the temperature from ≈350 to ≈2 K), and an ultra-high conductivity (≈2.8 × 10⁷ S m^-1 at 2 K) in 2D FeS. Leveraging such high conductivity and robust air stability, we demonstrate 2D FeS as ideal contact electrodes in monolayer-MoS₂-based field effect transistor, achieving a mobility more than 3 times higher than that of Ti/Au-contacted device. This work should hereby promote the fundamental property explorations of 2D nonlayered materials, and their applications in next-generation electronic/spintronic devices.