To Substitute Rather Than Intercalate: <i>Chimie douce</i> Approach to Induce Ferromagnetism in Metastable Pt_0.8<i>M</i>_0.2Se₂ (<i>M</i> = Cr, Co, Ni)

Two-dimensional (2D) magnetic materials with exotic magnetic properties have garnered significant interest due to their potential applications in spintronics and data storage technologies. However, the limited availability of intrinsic 2D magnetic materials has driven efforts to induce and manipulate magnetism in otherwise nonmagnetic 2D systems through approaches such as chemical intercalation, defect engineering, and substitutional doping. Herein, we present a facile, <i>chimie douce</i> method for incorporating 3<i>d</i> transition metals (Cr, Co, and Ni) into the nonmagnetic PtSe₂ sublattice. This synthetic approach enables control over layer thickness of Pt_1-<i>x</i><i>M</i>_<i>x</i>Se₂ (<i>M</i> = Cr, Co, Ni) nanosheets by varying the <i>M</i> identity and annealing conditions. Comprehensive scattering and spectroscopic characterizations confirm the successful and homogeneous substitution of <i>M</i> atoms at the Pt site, rather than intercalation, and reveal a strong correlation between nanosheet thickness and the identity of the substituting metal. High-temperature annealing of the nanosheets promotes an irreversible transformation toward the bulk phase, allowing for detailed characterization of structural and magnetic properties. A case study of Pt_0.8Cr_0.2Se₂ reveals that nanosheet thickness plays a critical role in modulating local magnetic interactions. While Cr atoms in the as-synthesized few-layers-thick nanosheets exhibit predominantly short-range antiferromagnetic interactions, the emergence of short-range ferromagnetic exchange is revealed in the bulk material. Detailed <i>ac</i> susceptibility and remanent magnetization measurements further demonstrate that bulk Pt_0.8Cr_0.2Se₂ adopts a frustrated magnetic ground state with clear signatures of ferromagnetic cluster-glass behavior. The systematic investigation presented herein establishes a clear and robust protocol for the synthesis and in-depth characterization of 2D transition-metal-substituted PtSe₂ materials with varying layer thickness and paves a path toward their realization in spintronic and magnetic device applications.