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

Abstract

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, chimie douce method for incorporating 3d transition metals (Cr, Co, and Ni) into the nonmagnetic PtSe₂ sublattice. This synthetic approach enables control over layer thickness of Pt_1–x M _x Se₂ (M = Cr, Co, Ni) nanosheets by varying the M identity and annealing conditions. Comprehensive scattering and spectroscopic characterizations confirm the successful and homogeneous substitution of M 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 ac 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.

1. Introduction

The discovery of graphene marked a pivotal moment in two-dimensional (2D) materials research by unveiling the vast potential of atomically thin materials with extraordinary electrical, thermal, and mechanical properties that are distinct from the bulk. − However, limitations in graphene’s scalability and the absence of an intrinsic band gap for controlling electron flow have galvanized extensive research toward alternative 2D materials, particularly transition-metal dichalcogenides (TMDCs). − TMDCs possess tunable band gaps that depend on their layer thickness. Monolayer and few-layer configurations have semiconducting behavior, while bulk forms exhibit semiconducting or metallic characteristics depending on the coordination around the transition-metal atoms and their corresponding symmetries. − Structurally, the 2H and 3R phases, characterized by trigonal prismatic coordination with hexagonal (D _3h ) and rhombohedral (C _3v) symmetries, respectively, confer semiconducting behavior and are favored by molybdenum- and tungsten-based TMDCs. − In contrast, platinum-based TMDCs tend to adopt the 1T octahedral phase with trigonal symmetry (D _3d ) and exhibit metallic characteristics. , The semiconducting nature of Mo- and W-based TMDCs has motivated vast investigations into their synthesis and integration into electronic and optoelectronic devices. − Particular attention has been given to how structural modifications, such as defect engineering and doping strategies, can be leveraged to enhance their performance. Atomic-level doping has consequently enabled the development of materials with outstanding properties, including room-temperature ferromagnetism for spintronic applications, high-performance electrocatalyst for water splitting, and advanced sensing capabilities.

Doping strategies and chemical stability trends are well-established for early-TM dichalcogenides. For instance, doping MoS₂ monolayers with 3% Mn induces high-temperature ferromagnetism (∼350 K), while MoS₂ nanocrystals doped with ∼10% Co, Ni, Mn, and Fe reveal near-room-temperature magnetic behaviors. , Similar doping levels have been achieved in WS₂, while lower concentrations (<5%) have been realized for WSe₂, MoSe₂, MoTe₂, and WTe₂. ,− Typically, these doped TMDCs, despite their promising magnetic properties for spintronic applications, are limited to low dopant concentration (≤10%) before phase segregation occurs, categorizing them as dilute magnetic semiconductors. The chemical stability of Mo- and W-TMDCs is chalcogen-dependent. While disulfides, such as MoS₂ and WS₂, are relatively stable in air under ambient conditions, their selenide and telluride counterparts are less stable, posing challenges for practical applications. − These limitations have stimulated interest in heavier TMDCs, such as PtX ₂, which exhibit remarkable air stability in bulk and monolayer forms.

Bulk PtSe₂ is a semimetal, while monolayer PtSe₂ is a semiconductor with a tunable band gap, − enhancing its versatility for electronic and sensing applications. − Due to the presence of heavy 5d Pt atoms, PtSe₂ is expected to exhibit enhanced spin–orbit coupling, particularly when doped with 3d transition metals. − Huey et al. demonstrated that chromium (Cr) can substitute Pt in bulk PtTe₂ at concentrations up to 45%, resulting in ferromagnetism at 220 K and positioning it as a strong candidate for potential spintronic applications. Exfoliation of Cr-doped PtTe₂ single crystals down to ∼10 nm was achieved with preliminary evidence of oxidation resistance in the few-layer regime, supporting further investigation of Pt-based TMDCs for practical applications. This approach, while successful for Cr, is yet to show effectiveness for other transition metals and stresses the need for a universal synthetic approach to produce bulk and few-layer M-doped platinum dichalcogenides (M = 3d transition metal). Theoretical studies propose that M-doped and defect-induced PtSe₂ monolayers could serve as dilute magnetic semiconductors with certain dopants inducing ferromagnetism and others promoting antiferromagnetic interactions. , The magnetic behaviors are influenced by the dopant concentration, spin–orbit coupling, structural defects, and d-state correlations of transition metals.

To the best of our knowledge, no experimental attempts have been made to systematically synthesize and investigate the magnetic properties of bulk crystalline and nanosheets of M-substituted PtSe₂. We hypothesize that this research gap may arise from several factors: (1) a lack of effective synthetic methods for incorporating 3d metals, M, without altering the PtSe₂ structure; (2) chemical and size incompatibility between M and Pt, leading to metastability and phase segregation to PtSe₂ and M-Se binaries during high-temperature syntheses; and (3) challenges in controlling the concentration and homogeneity of M ions resulting in inconsistent and irreproducible magnetic behavior. Adopting a low-temperature solution-based synthetic route can potentially address these challenges in producing M-substituted PtSe₂ compounds. A solution approach allows for the gradual introduction of a second metal at controlled rates, minimizes perturbations of the parent PtSe₂ phase, and promotes a more uniform distribution of M atoms. , Additionally, a low-temperature solution-phase synthesis reduces the risk of phase segregation by enhancing chemical compatibility under milder conditions and enabling the controlled growth of TMDC nanosheets. This technique may improve the magnetic stability and consistency of properties for M-substituted PtSe₂ across various layer thicknesses.

Herein, we report the first example of the controlled synthesis of 3d transition-metal (M)-substituted PtSe₂ compounds by adopting a two-step approach: (i) a room-temperature benchtop solution reaction of an activated Se solution, PtCl₂, and MCl₂ (M = Cr, Co, Ni) followed by (ii) a short thermal annealing at moderate temperatures of 500 °C for 3.5 h under N₂ to produce M-substituted PtSe₂ nanosheets. The structural and compositional integrities of the resulting nanosheets were confirmed through tour de force characterization techniques. Our low-temperature synthesis enables the precise control of substitution levels, with M concentrations ranging from 5 to 25 at. %, and yields homogeneous distribution of Pt, Se, and M atoms, and minimal disruption to the PtSe₂ structure. Variable-temperature in situ X-ray diffraction (XRD) and differential scanning calorimetry (DSC) experiments were used to elucidate the growth and evolution of these nanosheets upon annealing and enabled the ex situ transformation of the few-layer-thick nanosheets toward the bulk crystalline regime. High-resolution synchrotron X-ray diffraction and total scattering experiments, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and high-resolution scanning transmission electron microscopy (HR-STEM) enabled an in-depth structural characterization of the M-substituted PtSe₂ materials, revealing the substitution of M atoms on the Pt sites as opposed to intercalation in the interlayer space. Evaluation of the magnetic properties of the nanocrystalline and bulk samples reveals complex exchange interactions and ferromagnetic cluster-glass behavior attributed to the M ions and potential spin–orbit coupling from Pt. The oxidation states and coordination environments of the 3d transition metals were evaluated by using L-edge X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy to complement our findings from magnetometry. This systematic study demonstrates a robust method for the synthesis and characterization of size-dependent M-substituted PtSe₂ materials with promising applications in spintronic and magnetic devices.

2. Experimental Section

2.1. Safety Warning

Solvothermal vessels (autoclaves) may develop high autogenic pressures, which can result in the release of hot pressurized hazardous ethanolamine- and Se-containing vapors during the reaction. Splashing of the solvent may occur upon opening of the autoclaves, causing severe burns. It is highly recommended to wear proper personal protective equipment, such as face shields, long-sleeved gloves, and tight-cuff lab coats; to place the autoclaves in secondary containment and allow them to cool to room temperature before opening; and to keep furnaces in well-ventilated spaces such as fume hoods. See additional warning in the Supporting Information regarding high-temperature treatments.

2.2. Starting Materials

Selenium powder and shots (100 mesh, Sigma-Aldrich, 99.99%; 2–6 mm, Alfa Aesar, 99.999%), platinum­(II) chloride (Alfa Aesar, 99.5%), anhydrous chromium­(II) chloride (metals basis, Alfa Aesar, 99.9%), anhydrous cobalt­(II) chloride (metals basis, Alfa Aesar, 99.7%), anhydrous nickel­(II) chloride (metal basis, Alfa Aesar, 99.95%), cesium iodide (Alfa Aesar, 99.9%), sodium iodide (Sigma-Aldrich, 99.5%), and ethanolamine (Thermo Scientific, 99+%) were used as received without further purification.

2.3. Selenium Activation

Elemental selenium was activated following a previously reported procedure. In a typical process, 157.9 mg of Se powder or shots (2 mmol) was placed into a 23 mL Teflon liner. 10 mL of ethanolamine was added to the Teflon liner as a solvent, filling it to 43% of its total volume. The liner was tightly sealed in a stainless steel autoclave, ensuring a secure environment, and then placed in a solvothermal furnace. The temperature of the furnace was rapidly ramped to 200 °C and maintained at this temperature for 1 h. After the heating process, the autoclave was carefully removed from the furnace and naturally cooled to room temperature before opening.

2.4. Synthesis of M-Substituted PtSe₂ (M = Cr, Co, Ni)

To the activated Se solution described above, PtCl₂ and anhydrous MCl₂ were added in a 200 mL beaker. The mixture was stirred (700 rpm) continuously under ambient conditions (25 °C and 1 atm) for 1 h. After this stirring period, approximately 5 mL of 200-proof ethanol was added to the mixture and allowed to sit for about 10 min to promote the precipitation of nanosheets from the solution. The resulting suspension was then filtered using a 0.45 μm pore-size filter paper to isolate the black amorphous [PtMSe] complex. The [PtMSe] complex was thermally annealed under a constant flow of nitrogen gas (N₂) at 500 °C for 3.5 h to obtain silvery polycrystalline Pt_1–x M _x Se₂ nanosheets. The nanosheets were further subjected to high-temperature solid-state reactions to enhance their crystallinity toward the bulk regime. Experimental details regarding the solid-state growth process and M loading stoichiometries are provided in the Supporting Information.

2.5. Characterization

Nanosheets and bulk crystalline samples of Pt_1–x M _x Se₂ (M = Cr, Co, Ni) were characterized by powder X-ray diffraction (PXRD), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), high-resolution scanning transmission electron microscopy (HR-STEM), variable-temperature in situ X-ray diffraction, synchrotron-based high-resolution PXRD and total scattering, X-ray absorption spectroscopy (XAS), differential scanning calorimetry (DSC), and superconducting quantum interference device (SQUID) magnetometry. Further details about the experimental methods are provided in the Supporting Information.

3. Results and Discussion

3.1. Synthesis

PtSe₂ adopts a layered crystal structure wherein the 2D sheets are composed of edge-sharing [PtSe₆] octahedra exhibiting strong intralayer covalent bonds and are held together by weak van der Waals forces (Figure a). This anisotropic bonding nature enables structural modifications of the parent compound, leading to the emergence of intriguing physical and electronic properties. While intercalation within the van der Waals gap has been extensively reported for various TMDCs, − achieving substitutional modification at the transition-metal (TM) site remains challenging, largely due to limitations associated with conventional synthesis methods and the potential metastable nature of the target compounds. Notably, conventional methods have been unsuccessful in incorporating 3d transition metals (M) into PtSe₂, either as intercalants in the van der Waals gap or as substitutional dopants on the Pt site. However, low-temperature solution-based synthetic routes could offer a more controlled and versatile alternative. ,,− This approach facilitates metal incorporation under milder conditions, promoting substitutional doping and preserving the structural integrity of the TMDC lattice for more precise control over the dopant concentration and material morphology. Building upon our previous work involving the activation of elemental Se under solvothermal conditions for the synthesis of PtSe₂ nanosheets, we have extended this methodology to produce M-substituted PtSe₂ compounds with layer thickness ranging from thin nanosheets to bulk. This methodology benefits from the efficient diffusion of reactants in solution at lower temperatures, allowing for a homogeneous distribution of metals.

1.

(a) Crystal structure of PtSe₂, depicting the octahedral coordination of Pt by Se atoms within 2D layers. The stacking of layers and interlayer van der Waals gap are shown, with the interplanar Pt–Pt distance labeled as 5.08 Å. (b) Schematic representation of the synthetic approach used to prepare pristine PtSe₂ and Pt_1–x M _x Se₂ (M = Cr, Co, Ni) nanosheets. (c) Comparison of short-range (2.2–8 Å) pair distribution function (PDF) curves extracted from room-temperature synchrotron X-ray total scattering data for the [PtSe] complex (black) and crystalline bulk PtSe₂ (red). Blue arrows indicate weakly resolved Pt–Pt interatomic distances (3.6–4.2 Å) between adjacent octahedra in the [PtSe] complex, which correspond to similar local interactions in PtSe₂.

By treating PtCl₂ and anhydrous CrCl₂ with an activated Se solution at room temperature, an amorphous [PtCrSe] complex is produced, which was then subjected to a short thermal treatment at 500 °C to produce nanosheets of Pt_1–x Cr _x Se₂ (Figure b). This low-temperature synthetic strategy allows for precise control and variation of metal concentration (5–25%) by adjusting the starting ratio of PtCl₂ to CrCl₂. Further, changing the identity of the transition metal to Co or Ni, by using CoCl₂ and NiCl₂ precursors, facilitated the formation of high-quality nanosheets of Pt_1–x Ni _x Se₂ and Pt_1–x Co _x Se₂ while maintaining comparable M concentration ranges.

3.2. Structural Characterization of Pt_1–x M _x Se₂ Nanosheets

Given the amorphous nature of the [PtSe] and [PtMSe] complexes, conventional powder X-ray diffraction techniques were insufficient for extracting meaningful structural information. Instead, analysis of pair distribution function (PDF) curves, obtained from total X-ray scattering measurements, revealed local- and intermediate-range structural features in the [PtSe] complex and provided insight into their structural evolution toward bulk PtSe₂. The PDF of the [PtSe] complex (black curve in Figure c) reveals prominent short-range ordering, characterized by an intense peak corresponding to Pt–Se bonding at ∼2.5 Å and a lower-intensity peak at ∼3.3 Å attributed to Se–Se interactions. These distances align well with those reported for pristine PtSe₂, indicating that Pt atoms in the complex are likely octahedrally coordinated by Se atoms. − Another common coordination, square-planar Pt­(II)­Se₄, has slightly shorter Pt–Se distances. The left shoulder (∼2.35 Å) of the main peak (which is absent in bulk phase) indicated the potential presence of Se–Se covalent bonds in the complex, r _Se = 1.17 Å. Based on the crystal structure of PtSe₂, a relatively sharp and intense Pt–Pt nearest-neighbor interaction is expected around 3.72 Å. However, the [PtSe] complex instead exhibits weak and broad peaks spanning 3.6 and 4.0 Å, suggesting a significant variation in the distances between neighboring [PtSe₆] octahedra. Beyond short-range interatomic correlations, the PDF exhibited significant damping (Figure S23a), indicative of short-range structural order and the absence of long-range crystallinity in the complex, and hence the amorphous nature observed by PXRD. Thermal treatment of the [PtSe] complex, first by moderate heating under flowing N₂ gas followed by high-temperature annealing (vide infra), resulted in the formation of highly crystalline bulk PtSe₂ as evidenced by the emergence of well-defined peaks in the PDF (red curve in Figure c). These peaks extend beyond the short range, with the intense signals attributed to Pt–Se, Se–Se, and Pt–Pt, interactions (Figure S23a).

The crystallinity, phase purity, morphology, and elemental composition of Pt_1–x M _x Se₂ (M = Cr, Co, Ni) nanosheets were confirmed by PXRD, Raman spectroscopy, XPS, SEM-EDS, and HR-STEM. All observed diffraction peaks can be indexed to trigonal PtSe₂ (space group: P3̅m1), supporting their single-phase nature of samples, with no detectable impurities within the method resolution (Figure a). The maximum achievable concentrations of M without the formation of impurities are ∼25% for Cr and ∼20% for Ni and Co. Exceeding these concentrations led to the formation of binary admixtures, such as Cr₂Se₃, CoSe₂, and NiSe₂, which were evident in the diffraction patterns of the nanosheets or bulk crystalline samples after high-temperature annealing (Figure S1). The diffraction peaks of the nanosheets display anisotropic broadening, with the extent of broadening decreasing down the period such that Cr samples showed the most broadening and Ni samples showed the least (Figure S2). This trend can be attributed to the greater lattice distortion caused by the larger ionic radius of Cr compared to that of Co and Ni. This distortion may induce more strain or defects along specific crystallographic directions, resulting in smaller and more disordered crystallite domains and broader diffraction peaks for Cr samples. Although the diffraction peaks are too broad for the precise assignment of peak shifts for each M, a noticeable shift of the (001) diffraction peak toward lower Q values is observed for Cr-substituted samples, suggesting an elongation of the unit cell along the c-axis, likely due to the larger ionic radius of Cr. In contrast, the (001) peaks of Co- and Ni-containing samples shift toward higher Q values relative to those of pristine PtSe₂, suggesting unit cell contraction along the c-axis, arising from the smaller atomic radii of these M ions.

2.

(a) Experimental PXRD patterns of pristine PtSe₂ (green), Pt_0.8Cr_0.2Se₂ (red), Pt_0.8Co_0.17Se₂ (blue), and Pt_0.8Ni_0.18Se₂ (pink) nanosheets compared to the calculated pattern of trigonal PtSe₂ (black), illustrating the highly anisotropic nature of the nanosheets. (b) Corresponding Raman spectra of the nanosheets, displaying characteristic in-plane (E_g) and out-of-plane (A_1g) Raman modes with inset schematics showing the origin of each vibrational mode. (c) High-resolution XPS spectra of pristine and M-substituted PtSe₂ samples, highlighting the shift in binding energies of Pt 4f states upon M incorporation. Vertical dashed lines are provided to guide the eye to the relative shifting of the peaks.

The strong inter- and intralayer interactions in TMDCs can be characterized by Raman spectroscopy. In PtSe₂ (D _3d point group), two distinctive Raman modes represent the in-plane (E_g) and out-of-plane (A_1g) vibrations of Se atoms. ,, The intensities and positions of these modes are sensitive to nanosheet thickness and chemical environment, making them valuable indicators for structural characterization. , Room-temperature Raman spectra of Pt_1–x M _x Se₂ exhibit the consistent presence of both E_g and A_1g modes across all samples (Figures b and S3), confirming the preservation of the PtSe₂-type structure. The intensity of the A_1g mode increases moving from Cr to Ni, indicating an increase in the number of nanosheet layers. − This variation in the A_1g intensity can be attributed to changes in polarizability arising from modifications in the local bonding environment around the Se atoms. Further, substitution of Pt by M atoms affects the Raman mode positions due to two competing factors: a reduction in the average atomic mass (reduced effective mass) and a decrease in the force constant, likely due to weaker M-Se bonding interactions. The net Raman shift observed depends on a superposition of these two effects. Across all M at similar concentrations, a consistent red shift of the E_g mode was observed relative to pristine PtSe₂. This shift can be attributed to perturbations in the vibrational dynamics caused by M-substitution within the PtSe₂ lattice. , These results provide clear evidence that the M atoms are incorporated into the PtSe₂ matrix, either by substituting Pt or intercalating in the interlayer space, leading to detectable changes in the vibrational modes, and effectively ruling out the formation of segregated M-Se or M-Cl phases.

The electronic states of the as-synthesized pristine and M-substituted nanosheets were investigated by using X-ray photoelectron spectroscopy (XPS). The survey spectra confirmed the presence of Pt, Se, and the corresponding M elements (Figure S4). Pt 4f _7/2 and Pt 4f _5/2 core-level spectra exhibit characteristic peaks at 72.8–73.1 and 76.1–76.4 eV, respectively, consistent with the presence of Pt⁴⁺ ions (Figures S5a–S8a). ,, Notably, the incorporation of M induces a systematic shift in the Pt 4f core-level peaks to lower binding energies (Figure c), likely due to changes in the Pt coordination environment, providing additional evidence for the incorporation of M atoms into the PtSe₂ lattice. The binding energies of the Pt 4f states decrease as the electronegativity of M is reduced, such that the binding energy for pristine PtSe₂ > Ni > Co > Cr, suggesting varying degrees of electronic modification upon 3d metal incorporation.

Electron diffraction (ED) and high-resolution angle annular dark field (HAADF) STEM show that Pt_0.8Cr_0.2Se₂ nanosheets are only a few layers thick (3–5 layers), while Pt_0.83Co_0.17Se₂ nanosheets consisted of at least five layers (in the range of 5–10 nm) (Figure ). HAADF-STEM imaging of individual nanosheets revealed a layered stacking arrangement along the [001] crystallographic direction, with an interlayer spacing of ∼5.1 Å. Selected area ED (SAED) patterns reveal diffraction rings of different spots density depending on the nanoparticles size, confirming the nanocrystalline nature of the materials (inset of Figure a). All observed diffraction rings are consistent with the trigonal PtSe₂ (P3̅m1) structure, corroborating PXRD analyses. STEM-EDS mapping showed a homogeneous atomic-level distribution of constituent elements with no evidence of phase segregation into PtSe₂ and M-Se phases or elemental Pt and M inclusions (Figure ). Complementary elemental analysis via SEM-EDS confirms consistent atomic ratios of Pt, Se, and M across several probed regions (Tables S2 and S3), and elemental mapping on micron scale further verifies their uniform distribution (Figures S9 and S21a).

3.

Left panel (a, c, e): Pt_0.8Cr_0.2Se₂ nanosheets. Right panel (b, d, f): Pt_0.83Co_0.17Se₂ nanosheets. (a, b) Low-magnification images with corresponding ring ED patterns. (c, d) STEM-EDS elemental mapping confirming the homogeneous elemental distribution. (e, f) High-resolution HAADF-STEM images showing the size and structure of nanosheets.

3.3. Study of Structural Evolution from Nano to Bulk

Comprehensive analyses of the as-synthesized Pt_1–x M _x Se₂ (M = Cr, Co, Ni) nanosheets verified their phase purity, morphology, and composition. However, clear insight into the precise position of M ions within the PtSe₂ lattice remained elusive, primarily due to the broad and diffuse nature of the nanosheets’ X-ray diffraction peaks. Improving the crystallinity of the Pt_1–x M _x Se₂ compounds was, therefore, essential for resolving the location of 3d transition metals within the PtSe₂ crystal structure and gaining a deeper understanding of how structural and functional properties evolve from the nanoscale to the bulk regime. High-temperature solid-state annealing, often employed for the synthesis of well-ordered polycrystalline phases, offered an opportunity to produce bulk crystalline samples. We studied the crystallization dynamics of pristine PtSe₂ through differential scanning calorimetry (DSC) and solid-state annealing of the nanocrystalline powder. DSC experiments indicated PtSe₂ has high thermal stability and does not melt or decompose up to 1100 °C, providing a large temperature window for investigation (Figure S10). Sample crystallinity improved drastically through high-temperature treatment, supported by the narrowing of Bragg peaks after DSC and an increase in the PtSe₂ particle size in SEM images. Solid-state annealing of nanocrystalline PtSe₂ at high temperatures (600–1000 °C) for varying durations (0–7 days) showed that higher temperatures and longer dwell times lead to a more significant improvement in crystallinity, i.e., sharper PXRD peaks. Further, the addition of a salt flux such as CsCl also aided crystallization of PtSe₂ as bulk powders.

The metastable nature of the substituted Pt_1–x M _x Se₂ phases imposed a stringent limitation on crystallization conditions, as annealing at “excessive” temperatures or “prolonged” durations resulted in decomposition and the samples’ segregation into M-Se and PtSe₂ binaries. In situ X-ray diffraction and DSC experiments were vital in investigating the structural evolution of Pt_1–x M _x Se₂ nanosheets upon high-temperature annealing and helped identify the optimal temperature ranges for transformation of the few-layers-thick sheets toward the bulk crystalline regime without the formation of secondary phases. Synchrotron temperature-dependent in situ PXRD studies were conducted on Pt_0.8Cr_0.2Se₂ and Pt_0.83Co_0.17Se₂ nanosheets (Figures a and S11b). Upon heating from 25 to ∼600 °C, a subtle enhancement in nanosheet thickness was observed, as evidenced by the gradual narrowing of diffraction peaks and the increase in absolute peak intensities. Above 600 °C, more pronounced crystallite growth was observed up to the maximum temperature used of ∼900 °C, which resulted in sharp Bragg peaks comparable to those of bulk crystalline PtSe₂. While both the Cr and Co samples showed significant improvement in crystallinity upon in situ annealing, the Cr phase still exhibited some peak broadening even after heating to 900 °C (Figure a). The Co sample, however, appeared to reach maximum crystallinity by ∼800 °C with little change upon continued heating (Figure S11b). This observation suggests different degrees of metastability for different Pt_1–x M _x Se₂ phases. Importantly, in both samples, no binary M-Se phases were observed upon heating, within the sensitivity limit of the synchrotron PXRD method, indicating that the nanocrystalline starting materials were indeed phase-pure nanosheets.

4.

(a) Waterfall plot of variable-temperature in situ synchrotron PXRD data (λ = 0.1811 Å) for nanocrystalline Pt_0.8Cr_0.2Se₂ collected during heating from room temperature to 900 °C. The sequential evolution of the PXRD pattern highlights structural changes associated with the growth of nanosheets toward the bulk regime. (b) Ex situ PXRD patterns (λ = 1.54059 Å) of pristine PtSe₂ recorded after heating nanocrystalline powders via differential scanning calorimetry (DSC) to different maximum temperatures (see Figure S10a). (c–e) Ex situ PXRD patterns of Pt_1–x M _x Se₂ (M = Cr, Co, and Ni) samples of varying M concentrations after high-temperature thermal treatment, demonstrating the impact of metal substitution on the crystal structure and phase composition.

Motivated by our observation from in situ studies, we used DSC to quickly heat and transform nanocrystalline Pt_1–x Cr _x Se₂. Per in situ PXRD, Pt_0.8Cr_0.2Se₂ still did not reach complete crystallinity at a maximum temperature of 900 °C. To evaluate whether even higher temperatures were necessary, Pt_0.8Cr_0.2Se₂ nanosheets were heated to 1000 °C with no exothermic or endothermic events observed in the DSC thermogram. The lack of thermal events that could correspond to melting, phase segregation, or impurity formation suggested the high thermal stability of the substituted compound, akin to pristine PtSe₂ (Figure b). However, a small amount of Cr₂Se₃ (<1 wt %) was indeed observed after PXRD, suggesting a fragile balance between enhancing crystallization and avoiding phase segregation in metastable Pt_1–x M _x Se₂ compounds.

A series of ex situ high-temperature annealing experiments were performed on nanocrystalline Pt_0.8Cr_0.2Se₂ to probe its degree of metastability (Figure S11a). A partly crystalline sample was obtained by heating the nanocrystalline powder to 750 °C and then allowing the reaction to cool naturally. The limit of crystallinity enhancement was scrutinized further by heating the nanocrystalline sample to the desired maximum temperature in the 800–950 °C range at a similar rate as used in DSC and quenching from high temperatures to avoid any further decomposition upon cooling. The relative broadness of diffraction peaks and presence of Cr₂Se₃ admixture after annealing were used as criteria to determine the best condition for producing bulk crystalline Pt_0.8Cr_0.2Se₂. As observed for pristine PtSe₂, higher annealing temperatures generally yielded samples with sharper diffraction peaks. The sample quenched from 800 °C was comparable in crystallinity to that obtained after allowing the sample to naturally cool from 750 °C. The sample quenched from 950 °C exhibited the sharpest diffraction peaks, on par with the sample produced by annealing in DSC to 850 °C, but the 950 °C sample also had minor Cr₂Se₃, suggesting phase segregation. Lowering the maximum temperature to 850 °C, a slightly less crystalline bulk powder could be obtained without the formation of binary Cr₂Se₃. Similar experiments probed the potential for particle growth upon dwelling (0–20 min) at maximum temperature (850–900 °C) before quenching; these studies showed marginal improvement in crystallinity with dwelling accompanied by expulsion of chromium in the form of Cr₂Se₃. Thus, quenching from 850 °C without dwelling was identified as the optimal condition for the large-scale production of bulk Pt_1–x Cr _x Se₂ (Figure c).

Similar systematic studies on the Co- and Ni-substituted systems were also performed to enhance crystallinity and study the extent of their metastability (Figure d,e). Interestingly, these later 3d metals seem to have higher thermal stability than the Cr analogue. High-temperature annealing at 800 °C (Co) and 600 °C (Ni) achieved full crystallinity, and M-substituted compounds were retained with higher annealing temperatures up to 900 °C. It is also worth noting that the stability of Pt_1–x M _x Se₂ compounds decreases with higher M concentrations. The optimized conditions for high-temperature transformation of Pt_1–x M _x Se₂ (M = Cr, Co and Ni) from nanosheets to bulk crystalline powders are summarized in Table S1. Together, these careful annealing studies enabled both structural and thickness-dependent characterization of Pt_1–x M _x Se₂ compounds, ranging from the nanoscale to the bulk regime.

3.4. Structural Characterization of Bulk Pt_1–x M _x Se₂

PXRD and SEM-EDS were employed to confirm the phase purity, crystallinity, and chemical composition of bulk Pt_1–x M _x Se₂ samples (Tables S2–S6). EDS elemental mapping revealed a homogeneous distribution of Pt, M, and Se, with no evidence of phase segregation (Figures S17–S19). Room-temperature Raman spectra provided additional evidence of phase purity and crystallinity, with a notable increase in the relative intensity of the A_1g mode of the bulk samples, suggesting their structural evolution from few-layer nanosheets to a bulk-like configuration (Figure S12). Consistent with the nanocrystalline Pt_1–x M _x Se₂ samples, the E_g mode of the bulk materials exhibits a red shift relative to pristine bulk PtSe₂ (Figure S13) and is accompanied by corresponding shifts in the Pt 4f core-level spectra to lower binding energies, as observed by XPS (Figure S14), providing additional evidence of electronic structure modification induced by the incorporation of M atoms.

The enhanced crystallinity of pristine PtSe₂ enabled the precise determination of its lattice parameters via Rietveld refinement (Figure a), serving as a structural reference for bulk crystalline Pt_1–x M _x Se₂. Rietveld refinement of high-resolution room-temperature synchrotron PXRD data for the bulk Pt_1–x M _x Se₂ samples confirms the absence of M-Se binaries and pristine PtSe₂ as secondary phases (Figures b, S15c,d, and S16). The trigonal prototype structure (P3̅m1) was retained upon substituting Pt with M atoms, although the unit cell parameters exhibit systematic variations. To investigate the probable position of M atoms within the crystal structure, several structural models were developed and refined with respect to the high-resolution PXRD data.

5.

Rietveld refinement of room-temperature synchrotron PXRD patterns (λ = 0.81884 Å) for (a) pristine PtSe₂ and (b) Pt_0.83Co_0.17Se₂. The observed background hump arises from the sample dilution with silica to enhance the X-ray transmission. Experimental data are shown as black open circles, the calculated pattern as a red line, and the difference profile as a blue curve. Vertical ticks indicate Bragg reflection positions for the respective PtSe₂ and Pt_0.83Co_0.17Se peaks (space group: P3̅m1). (c–e) Dependences of the unit cell parameters and volume, obtained from Rietveld refinement, on M concentrations for Pt_1–x M _x Se₂. The e.s.d are smaller than the symbols used.

The first and simplest structural model with starting composition Pt_0.8Co_0.2Se₂ assumed ∼20% site substitution of Co on the intralayer Pt site 1a (0,0,0). This refinement converged well (Figure b), yielding site occupancies of 83(1)% Pt and 17(1)% Co and supported our hypothesis of metal substitution at the Pt intralayer position. To evaluate the possibility for metal intercalation between layers of the structure, a second structural model with starting composition PtCo_0.2Se₂ was evaluated, wherein the intralayer Pt site was initially set as fully occupied by Pt and an interstitial Co site 1b (0,0,1/2) was introduced with a site occupancy of ∼20%. After the refinement, the occupancy of the interstitial Co converged to an unrealistic negative value of −0.6%, indicating no Co intercalation, while the intralayer Pt site occupancy refined to 88% (68.6 e^–), which corresponds well with the expected electron density for a mixed occupied site with ∼82% Pt and 18% Co (68.8 e^–) (Figure S15a). In the final structural model with starting composition (Pt_0.8Co_0.2)­Co_0.2Se₂, Co was introduced in two sites: 20% Co was substituted onto the intralayer 1a Pt site (80% Pt/20% Co) and the interstitial 1b site with 20% Co occupancy. This refinement yielded intralayer site occupancies of 84% Pt/16% Co and 2(1)% Co at the interstitial site (Figure S15b), indicating the negligible presence of Co between layers. Overall, the first structural model with metal substitution on the Pt site only yielded the best fit and residuals among the three proposed models. Across all three refinements, the data consistently supported that M atoms were introduced in the Pt intralayer site, with no significant occupancy at the interstitial position (i.e. no intercalation). This conclusion was further corroborated by complementary characterization techniques, including TEM, XPS, and Raman spectroscopy (vide infra). As such, all bulk Pt_1–x M _x Se₂ compounds were refined using structural models with 3d transition-metal substitution at the Pt site only.

Rietveld refinements for Ni- and Co-PtSe₂ provided better fits compared to the Cr-substituted analogue. The relatively poorer fit for Pt_1–x Cr _x Se₂ is likely due to anisotropic peak broadening, indicative of a slightly lower crystallinity. These effects, consistent with our diffraction studies, pose challenges for accurate modeling within the current refinement framework. Nonetheless, a good correlation was observed between transition-metal (M) concentrations obtained from Rietveld refinements and those obtained by SEM-EDS for all three substituents (Figure S20).

For Co- and Ni-substituted PtSe₂ compounds, decreases in the a- and c-lattice parameters were observed with increasing M concentration. This trend aligns with the substitution of Pt⁴⁺ by elements with smaller ionic radii (Co³⁺/Ni²⁺), as confirmed by XPS and XAS studies (vide infra), as well as changes in the bonding characteristics and structural distortions induced by substitution. These combined effects result in a contraction of the unit cell dimensions and, hence, a reduction in unit cell volume with increasing M concentration (Figure c–e). In contrast, for Cr-substituted PtSe₂ compounds, an increase in c- and a decrease in a-lattice parameters were observed with an increase in Cr concentration. The reduction in the a-lattice parameter is consistent with that observed for Ni and Co substitution and can be attributed to stronger in-plane bonding interactions due to the increased covalency of Cr–Se bonds compared to that of Pt–Se bonds. The increase in the c-lattice parameter suggests that Cr atoms, likely in the +3 oxidation state, introduce a larger effective ionic size compared to Pt⁴⁺. The Cr-substitution alters the local bonding environment and weakens interlayer van der Waals interactions, leading to relaxation or elongation along the out-of-plane c-axis. It is noteworthy that contraction of the in-plane a-lattice parameter is more pronounced than slight expansion of the out-of-plane c-lattice parameter. This results in an overall decrease in unit cell volume with increasing Cr content, consistent with the trends observed for Co- and Ni-substitution, but with distinct structural anisotropy. Such anisotropic lattice effects are known in certain layered materials, including ThCr₂Si₂-type structures. −

PDF analyses of the bulk Pt_1–x M _x Se₂ compounds (M = Cr, Co) revealed subtle variations in short-range atomic ordering compared to those of the pristine PtSe₂ (Figures and S23). The PDF curves were fit using the structural models obtained from Rietveld refinements. For Pt_0.83Co_0.17Se₂, a reasonable fit was obtained from the PDF with slight variations in bond distances, relative to PtSe₂, which are attributed to the substitution of Co (Figures c and S23c). No other significant changes were observed in both the local and long-range structures, indicating that Co introduction results in a simple atomic substitution at the Pt site without inducing substantial distortion of the [Pt_1–x M _x Se₆] octahedra or the layered framework. In contrast, Cr-substitution introduced more discernible variations in the local structure, as evidenced by the difference curves in the fit analyses (Figures b and S23d). These differences are most pronounced in the splitting of the Pt–Se and Pt–Pt nearest-neighbor correlations into doublets. The observed variations suggest a slight distortion of the [PtSe₆] octahedra upon the introduction of Cr, resulting in a poorer overall fit of the structural model to the short- and long-range PDF data. This distortion is consistent with the structural anisotropy revealed by HR-PXRD refinement and crystallographic defects identified by HR-STEM analysis (vide infra). Developing a more suitable structural model that accounts for the possible distortions induced by Cr-substitution remains a subject of further research.

6.

Pair distribution function (PDF) curves in the short range (2.2–8 Å) extracted from room-temperature synchrotron X-ray total scattering data for (a) Pt_0.83Co_0.17Se₂, (b) Pt_0.80Cr_0.20Se₂, and (c) PtSe₂. Experimental data are represented in black open circles, while red lines correspond to the simulated curves based on refined models in space group P3̅m1. The blue curves represent the difference between the experimental data and the model fits.

High-resolution HAADF-STEM and electron diffraction (ED) were employed to gain deeper insights into the crystallinity, i.e., layer thickness and atomic structure of the bulk samples, revealing distinct differences in structural order. The HAADF-STEM imaging (Figure ) and ED patterns along the main zone axis (Figure S24) confirmed that Pt_0.83Co_0.17Se₂ is highly crystalline, exhibiting a well-defined atomic arrangement, while Pt_0.8Cr_0.2Se₂ exhibits improved crystallinity compared to nanocrystalline samples and still retains some nanocrystalline domains (Figure S25). Both Cr- and Co-substituted samples demonstrate the presence of a well-ordered structure along the [001] crystallographic direction, characteristic of the layered PtSe₂ structure. Several atomic columns of Pt atoms randomly distributed over the image exhibit lower intensities; two of such examples are highlighted with yellow circles in Figure a. If transition-metal atoms are randomly added to the interlayer space in position 1b (0,0,1/2) located underneath the Pt atoms in 1a (0,0,0), the electron density in the corresponding atomic columns should increase, leading to random brighter spots in the [001] HAADF image. In turn, M substituting Pt in the 1a site reduces electron density in Pt columns, and random columns should exhibit decrease in the intensity in HAADF. Experimentally, we observed the second scenario confirming substitution rather than intercalation.

7.

Left panel (a, c, e): Pt_0.8Cr_0.2Se₂ bulk. Right panel (b, d, f): Pt_0.83Co_0.17Se₂ bulk. (a, b) Atomic-resolution [001] HAADF-STEM images, yellow circles highlight atomic columns with reduced intensities. Magnified images together with overlaid structural models are given as insets. (c, d) [100] HAADF and simultaneously acquired ABF-STEM images. The inset shows the simulated images based on the proposed structure. Magnified images with overlaid structure model are present at the bottom. (e, f) STEM-EDS elemental mapping of the homogeneous distribution of Pt (red), Se (green), and Cr or Co (blue) across multiple regions.

The lateral dimensions of the observed domains span from several hundred nanometers to tens of micrometers, indicating that the bulk samples maintain extended structural coherence over significant length scales. SAED patterns (Figure S24) reveal sharp, well-defined diffraction spots, confirming the highly crystalline nature of samples. These patterns are consistent with the expected symmetry and periodicity of PtSe₂, reinforcing the structural integrity of the M-substituted materials.

At the atomic scale, HAADF- (high-angle annular dark field) and ABF- (annular bright-field) STEM imaging provides critical evidence for the random substitution of 3d transition-metal (M) atoms at Pt sites rather than intercalation in the van der Waals gaps. HAADF imaging scales in intensity approximately with Z ² (where Z is the atomic number), allowing clear differentiation of the heavy Pt_1–x M _x atomic columns (appearing bright) from lighter Se atoms (appearing darker). The uniform contrast observed in the Pt_1–x M _x atomic columns along both the [001] and [100] zone axes suggest a homogeneous and random distribution of the substituent M atoms (Cr, Co) within the structure (Figure a,b). However, distinct visualization of the substituted M atoms within HAADF images is challenging due to their significantly lower atomic number compared to that of Pt.

Conversely, ABF-STEM enables us to effectively visualize atomic columns composed of heavy and light atoms simultaneously, where intensity is inversely related to HAADF-STEM contrast and atomic columns imaged as dark spots. The obtained HR ABF-STEM images (Figure c,d) clearly demonstrated uniform homogeneous dark contrast for all atomic columns. This suggests that the M-substitution occurs randomly rather than in a periodic fashion. STEM-EDS elemental mapping (Figure e,f) further confirms homogeneous atomic distribution across the samples, with no phase segregation or clustering of substituted elements, reinforcing the successful integration of M atoms onto the PtSe₂ framework.

The valence states of the bulk samples were further investigated using high-resolution XPS and synchrotron-based XAS. In contrast to the nanocrystalline Pt_0.83Co_0.17Se₂, which exhibits a mixed-valence Co state (Co²⁺ and Co³⁺) in the Co 2p core-level XPS spectrum, the bulk sample predominantly contains Co³⁺ (Figure S21d). This observation is consistent with the Co 2p core-level XPS spectrum and the Co L _2,3-edge XAS measurements, which showed spectral features characteristic of a mixed Co²⁺/Co³⁺ state in the nanocrystalline sample (Figure S8d,e), while the bulk sample displayed a signature exclusively attributable to Co³⁺ (Figure S21e). A comparable trend is observed for Pt_0.8Cr_0.2Se₂, in which XPS analysis of the nanocrystalline sample indicates the presence of both Cr²⁺ and Cr³⁺ oxidation states (Figure S7c), whereas the bulk sample shows a dominant Cr³⁺ character as confirmed by Cr L _2,3-edge XAS (Figure S22a). − A similar deconvolution of XPS and XAS data for Pt_0.82Ni_0.18Se₂ indicate the presence of solely Ni²⁺ valence state in both the nanocrystalline and bulk samples (Figures S6c and S22b).

3.5. Magnetic Properties

SQUID magnetometry studies were conducted on polycrystalline samples of Pt_0.8Cr_0.2Se₂ of varying degrees of crystallinity, hereafter referred to as nano, partly bulk, and bulk crystalline (Figure S26). The temperature-dependent magnetic susceptibility plots (M/H vs T) for all three samples, measured within the temperature range of 2–300 K at an applied magnetic field (μ₀ H) of 0.1 T, are depicted in Figure a. At high temperatures (≥200 K), all samples exhibit paramagnetic behavior, with fits to the modified Curie–Weiss equation yielding a Curie constant in the range of 0.29–0.35 emu·K·mol^–1 per formula unit corresponding to effective moments of 3.40–3.74 μ_B per Cr atom, which is consistent with the presence of Cr³⁺ ions (Figure S28). The Weiss constant (θ) varies with the crystallite thickness, indicating differences in the cumulative short-range magnetic interactions. The nanocrystalline Pt_0.8Cr_0.2Se₂ sample exhibits dominant antiferromagnetic nearest-neighbor interactions, with a negative Weiss constant (θ = −39 K). Increasing the crystallite thickness in the partly bulk sample manifests weak ferromagnetic correlations between nearest-neighbor Cr³⁺ ions with θ = +6 K, while the bulk sample exhibits stronger ferromagnetic nearest-neighbor interactions, θ = +28 K (Figure a inset). In the molecular Cr₂ dimers, the increasing of the Cr–Cr distance due to the replacement of monatomic Se bridge with Se₂ dumbbell was shown to change the type of Cr–Cr magnetic exchange interactions from antiferromagnetic to ferromagnetic ones. Thus, the thermal annealing may affect the distribution of Cr over Pt sites, resulting in more regular and longer Cr–Cr separations with dominant ferromagnetic interactions. The variations in the nature and magnitude of magnetic interactions between Cr ions may also be due to the conversion of the few-layer semiconducting crystallites into bulk metallic counterpart (known for PtSe₂), i.e., involving conduction electrons in magnetic exchange.

8.

(a) Temperature dependence of molar susceptibility (M/H) under an applied dc magnetic field of 0.1 T for Pt_0.8Cr_0.2Se₂ samples with different degrees of crystallinity. Inset of (a) shows (M/H) versus T of the bulk Pt_0.8Cr_0.2Se₂ sample, with the high-temperature region of the susceptibility data fitted to the modified Curie–Weiss law (red line). (b) Zero-field-cooled and field-cooled (ZFC-FC) molar susceptibility measured on bulk crystalline Pt_0.8Cr_0.2Se₂ under an applied dc magnetic field of 0.01 T, with bifurcation at ∼13 K. Inset of (b) shows an enlarged view of the low-field region of isothermal magnetization at T = 2 K for Pt_0.8Cr_0.2Se₂ samples of different crystallinities, highlighting the coercivities. (c) Temperature dependence of real (χ′) part of the ac susceptibility of bulk Pt_0.8Cr_0.2Se₂ measured at different frequencies with an excitation field (H _ac) of 5 Oe. (d) Plot of the relaxation time τ = 1/f as a function of the corresponding freezing temperature, T _f. The black solid curve represents the best fit to the critical slowing down model.

At room temperature, the (M/H)T values range from 0.34 to 0.35 emu·K·mol^–1, yielding effective magnetic moments of 3.69–3.75 μB/Cr, consistent with those obtained from the Curie–Weiss fits (Figure S28). Notably, all three samples exhibit different temperature dependencies of (M/H)T. In the 300–100 K range for the nanocrystalline sample, (M/H)T gradually decreases with temperature, a behavior typical of antiferromagnetic exchange interactions. The partly bulk sample shows nearly temperature-independent (M/H)T behavior in this range, typical for paramagnets. In turn, the bulk sample exhibits a gradual increase in the (M/H)T with decreasing temperature, confirming the existence of ferromagnetic interactions.

The low-temperature behaviors of (M/H) and (M/H)T suggest the emergence of magnetic ordering below 20 K. A plot of the derivative of dc magnetic susceptibility (dχ/dT) reveals a change in slope for all three samples at slightly different temperatures, indicative of subtle magnetic transitions associated with either local or long-range ordering (Figure S27). Specifically, the nanocrystalline and partly bulk samples exhibit ordering around 8 K, while the bulk sample displays a value around 13 K. These observations are further corroborated by ac magnetic susceptibility measurements conducted at 100 Hz (Figure S30a–c), which reveal corresponding anomalies near the transition temperatures. A sharp increase in the field-cooled (FC) magnetization curve measured under an applied field of 100 Oe, observed around 13 K in the bulk Pt_0.8Cr_0.2Se₂ sample (Figure b), indicates the onset of magnetic ordering at this temperature. However, the pronounced splitting between the zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves suggests the presence of spin blocking, indicative of a spin-glass-like frustrated magnetic ground state. This observation is further supported by the unsaturated behavior of the M–H curves measured up to 7 T at 2 K (inset of Figures b and S31).

Moreover, the observed magnetic moment of 1.1 μ_B/Cr at 7 T is significantly lower than the expected spin-only moment for Cr³⁺ ions, which can be attributed to spin canting and magnetic frustration inherent to glassy magnetic states.

Therefore, to investigate the nature of the low-temperature spin blocking in the sample, frequency-dependent ac susceptibility measurements were conducted from 2 to 20 K with an excitation field (H _ac) of 5 Oe. Figures c and S30a present the real (χ′) and imaginary (χ″) components of the ac susceptibility as functions of temperature at different frequencies. A distinct peak in both χ′(T) and χ″(T) at ∼12 K confirms the occurrence of spin freezing in the sample below this temperature. The systematic shift of the peak position toward higher temperatures with increasing frequency is a characteristic of spin-glass-like behavior. , To quantitatively evaluate this, the Mydosh parameter φ is calculated using

$$\phi =\frac{\mathrm{\Delta }{T}_{\mathrm{f}}}{T_{\mathrm{f}}[\mathrm{\Delta }({\log }_{10}\omega )]}$$

where ΔT _f is the shift in the freezing temperature (T _f) and ω = 2πf is the angular frequency. , The estimated value of φ for bulk Pt_0.8Cr_0.2Se₂ is 0.023, which lies between the typical range for canonical spin-glass (0.005 ≤ φ ≤ 0.01) and cluster-glass systems (0.03 ≤ φ ≤ 0.08). ,, Therefore, to further clarify the nature of the spin freezing, the frequency dependence of the relaxation time τ = 1/f was analyzed using the critical slowing down model

$$\tau ={\tau }_0{(\frac{T_{\mathrm{f}}}{T_{\mathrm{SG}}}-1)}^{-z\nu }$$

where τ₀ is the characteristic microscopic relaxation time, T _SG is the freezing temperature at f = 0 Hz and zv is the dynamic critical component. Fitting the data (Figure d) yielded T _SG = 10.8(1) K, τ₀ = 8.5 × 10^–9 s, and zv = 5.7(1). The obtained value of τ₀, which falls within the typical range for cluster-glass systems (10^–7 and 10^–10 s), , suggests finite spin–spin interactions and supports the classification of the bulk Pt_0.8Cr_0.2Se₂ as exhibiting cluster-glass behavior.

To further investigate the low-temperature spin dynamics in bulk Pt_0.8Cr_0.2Se₂, isothermal remanent magnetization (IRM) measurements were performed at 5 K (<T _f). To record the IRM data, the sample was cooled from 300 to 5 K in the presence of a 100 Oe applied magnetic field. After stabilizing at 5 K for 100 s under the applied field, the field was switched off, and the remanent magnetization was recorded as a function of time in zero field, as shown in Figure S31b. A clear time-dependent relaxation of magnetization was observed, even for up to ∼3 h, indicating spin blocking, one of the hallmark signatures of glassy spin dynamics. The time dependence of the remanent magnetization is analyzed using the stretched exponential model given as ,,

$$M(t)=M_0+M_{\mathrm{SG}}\times \exp \left[{-\left(\frac{t}{t_{\mathrm{r}}}\right)}^{1-\eta }\right]$$

where M ₀ and M _SG represent the ferromagnetic and spin-glass components of the magnetic moment, while τ_r and η are the characteristic relaxation time and the relaxation exponent, respectively. The best fit to the IRM data using this model, as shown by the solid black curve in Figure S31b, gives M ₀ = 21.1(2) emu/mol, M _SG = 14.1(1) emu/mol, t_r = 1730(25) s, and η = 0.619(4). The significantly large value of t_r reflects the slow relaxation dynamics typical of glassy spin states. Moreover, the finite and nonzero values of both M ₀ and M _SG further confirm the coexistence of ferromagnetic and spin-glass behaviors in the sample.

At the same time, a finite coercivity of approximately 640 Oe is observed in the bulk Pt_0.8Cr_0.2Se₂ sample at 2 K (Figure b), suggesting the coexistence of ferromagnetic interactions at low temperatures. This supports a more appropriate classification of bulk Pt_0.8Cr_0.2Se₂ as a ferromagnetic cluster glass. A similar coexistence of ferromagnetism and cluster spin-glass behavior has been reported in several other intermetallic compounds, often attributed to their inherently frustrated and complex magnetic ground states. ,− Notably, the magnetization of the bulk sample decreases with increasing temperature, consistent with a thermally driven suppression of magnetic order (Figure S32).

4. Conclusions

This work presents a novel soft-chemical route for synthesizing metastable Pt_1–x M _x Se₂ (M = Cr, Co, Ni) compounds that have been theoretically predicted but remained experimentally elusive. Low-temperature solution-based synthesis enabled precise control over both the M concentration and nanosheet thickness, offering a versatile platform for tailoring structure–property relationships in these materials. Importantly, controlled thermal annealing transforms the nanosheets into their bulk counterparts while largely preserving the structural framework of PtSe₂, which allows for comparative analysis of dimensionality-driven effects. Comprehensive structural characterization confirmed the successful substitution of M atoms on the Pt site with no evidence of intercalation, underscoring the efficacy of the proposed synthetic method. The tunability of magnetic properties with nanosheet thickness is clearly demonstrated through a detailed investigation of Pt_0.8Cr_0.2Se₂. The relative strength of the competing ferro- and antiferromagnetic interactions can be controlled by adjusting the thickness of the crystallites. Detailed ac susceptibility and remanent magnetization studies establish that bulk Pt_0.8Cr_0.2Se₂ exhibits ferromagnetic cluster-glass behavior, marked by spin freezing, slow relaxation, and coexisting spin-glass and ferromagnetic components. This study provides a reproducible and scalable protocol for accessing a family of 2D magnetic materials with engineered magnetic ground states, advancing their potential integration into next-generation spintronic and magnetic technologies.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.5c11147.

• Description of synthetic and characterization procedures and additional figures and tables pertaining to powder X-ray diffraction, synchrotron total scattering, in situ powder X-ray diffraction, differential scanning calorimetry, elemental analysis, scanning and scanning transmission electron microscopy, X-ray absorption and X-ray photoelectron spectroscopy, Raman spectroscopy, and magnetometry (PDF)

The manuscript was written through contributions of all authors.

This research was supported by the National Science Foundation Grant DMR-2333388 to K.K. The magnetic measurements by A.K. and Y.M. were supported by Ames National Laboratory, U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. Ames National Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. DE-AC02–07CH11358. X-ray absorption spectroscopy (XAS) measurements were performed on APS beamtime award (DOI: 10.46936/APS-188629/60013198 and 10.46936/APS-187984/60013008) from the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory under Contract No. DE-AC02–06CH11357. Use of Beamline 28-ID-2 at the National Synchrotron Light Source II was supported by the U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704. Part of the research described in this paper was performed at the Canadian Light Source (CLS), a national research facility of the University of Saskatchewan, which is supported by the Canadian Foundation for Innovation (CFI), the Natural Sciences and Engineering Research Council (NSERC), the Canadian Institute of Health Research (CIHR), the Government of Saskatchewan, and the University of Saskatchewan. XPS experiments by J.O. and J.R. were supported by the University of Delaware.

The authors declare no competing financial interest.