Controllable Synthesis of Magnetic 2D Non‐Layered Cobalt Sulfide Nanocrystals Using Chemical Vapor Deposition

Among 2-dimensional (2D) non-layered transition-metal chalcogenides (TMCs), cobalt sulfides are highly interesting because of their diverse structural phases and unique properties. The unique magnetic properties of TMCs have generated significant interest in their potential applications in future spintronic devices. In addition, their high conductivity, large specific surface area, and abundant active sites have attracted attention in the field of catalysis. However, the synthesis of phase-controllable 2D non-layered cobalt sulfide nanocrystals remains challenging. In the present study, a method is reported in which ambient-pressure chemical vapor deposition (APCVD) is used to synthesize 2D non-layered cobalt sulfide nanocrystals on insulating substrates. By controlling the growth temperature, the transition of nanocrystal phases from pyrite-structured CoS₂ to cubic Co₃S₄ and hexagonal CoS is achieved. Magnetotransport studies revealed metallic and ferromagnetic behaviors at temperatures below the Curie temperature for CoS₂. In addition, electrical measurements of Co₃S₄- and CoS-based devices showed conventional metallic behaviors, including temperature- and magnetic field-dependent ordinary Hall effects. These findings demonstrate the potential of APCVD for synthesizing high-quality 2D non-layered cobalt sulfide nanocrystals with controllable phases, paving the way for their application in spintronics and catalysis.