Thickness‐Dependent Semiconductor‐Metal Transition in Two‐Dimensional Nonlayered Magnetic CuCo₂S₄

ABSTRACT

Two‐dimensional (2D) magnetic semiconductors that simultaneously host tunable charge transport and robust magnetic order remain scarce because of the intrinsically competing requirements of semiconductivity and magnetism. Here, we report the chemical vapor deposition synthesis of high‐quality 2D CuCo₂S₄ nanoflakes and systematically investigate the evolution of their electronic properties as a function of thickness. Owing to dimensional reduction in the nonlayered spinel structure, CuCo₂S₄ undergoes a pronounced metal to semiconductor transition at a critical thickness of ∼20 nm, while long‐range ferromagnetic order persists above room temperature. Field‐effect transport reveals n‐type semiconducting behavior in the thin nanoflakes, in contrast to metallic conduction in thicker counterparts, highlighting thickness‐engineered electronic reconstruction. The semiconducting CuCo₂S₄ nanoflakes further exhibit broadband photodetection from visible to near‐infrared region with high photoresponse metrics, underscoring the multifunctionality enabled by reduced dimensionality. This work demonstrates dimensional reduction of spinel chalcogenides as an effective strategy for coupling semiconducting transport with robust ferromagnetism.

Molecular sieve‐assisted chemical vapor deposition enables the growth of two‐dimensional nonlayered CuCo₂S₄ nanoflakes with thicknesses down to sub‐10 nm. Dimensional confinement induces a pronounced metal to semiconductor transition at a critical thickness of ∼20 nm, while robust room‐temperature ferromagnetism is preserved. These findings establish ultrathin CuCo₂S₄ as an intrinsic two‐dimensional magnetic semiconductor enabled by thickness engineering.

1. Introduction

Two‐dimensional (2D) materials offer an exceptional platform for accessing electronic and magnetic phenomena that are fundamentally distinct from those of bulk crystals, owing to reduced dimensionality, enhanced surface‐to‐volume ratios, and highly tunable electronic structures. Over the past decades, van der Waals (vdW) layered materials have enabled the discovery of layer‐dependent electronic, optical, and magnetic properties, leading to rapid advances in nanoelectronics and spintronics [1, 2, 3, 4]. 2D magnetic semiconductors, which integrate charge and spin degrees of freedom within a single crystalline phase, represent a long‐standing challenge yet a highly desirable material class for spin logic, magneto‐optoelectronics, and low‐power information technologies [5, 6, 7]. Despite the rapid progress, the realization of materials that simultaneously host semiconducting transport and long‐range magnetic order in the 2D limit remains rare, and the underlying physical mechanisms need to be further understood.

The scarcity of 2D magnetic semiconductors originates from the intrinsically conflicting requirements of magnetism and semiconducting behavior. Long‐range magnetic order typically relies on strong exchange interactions, which are often mediated or enhanced by itinerant charge carriers and are therefore favored in metallic or semimetallic systems [8, 9]. In contrast, semiconducting transport requires a finite bandgap and suppressed carrier density, conditions that tend to weaken exchange coupling and destabilize magnetic order, particularly in reduced dimensions where thermal fluctuations are enhanced [3, 4, 10]. As a result, the introduction of a sizable bandgap frequently leads to the suppression of magnetic ordering, and strong spin splitting commonly observed in metals diminishes once semiconducting behavior emerges. Several strategies have been explored to overcome this fundamental incompatibility. These include thinning intrinsically magnetic layered semiconductors such as CrI₃ and Cr₂Ge₂Te₆ to the monolayer limit [1, 2], introducing magnetic dopants into 2D semiconductors [11, 12], engineering defect‐induced magnetism [13, 14], and exploiting proximity effects in vdW heterostructures such as MoS₂/CrI₃ [15]. While these approaches have enabled important proof‐of‐concept demonstrations, they are often accompanied by practical and physical limitations, including low magnetic ordering temperatures, chemical instability, inhomogeneous magnetic phases, and limited scalability [3, 4, 5, 10, 16, 17, 18]. More recently, dimensionality reduction in rhombohedrally stacked Cr₂Se₃ has been shown to induce a metallic‐to‐semiconducting transition while preserving antiferromagnetic order [19], highlighting thickness engineering as a promising route toward magnetic semiconductors. Nevertheless, such layered systems generally suffer from weak interlayer exchange and reduced Curie or Néel temperatures [20], which constrain their applicability at or above room temperature. Beyond vdW layered materials, non‐layered 2D materials have recently emerged as an alternative and largely unexplored materials platform [21, 22, 23, 24]. In contrast to vdW crystals, nonlayered 2D materials are held together by strong covalent or ionic bonding, leading to higher structural rigidity, distinct surface coordination environments, and fundamentally different responses to dimensional confinement. In these systems, reducing thickness does not simply truncate a bulk band structure but can profoundly reconstruct carrier density, defect energetics, exchange pathways, and symmetry‐driven interactions. Such characteristics provide new opportunities to engineer coupled electronic and magnetic functionalities that are difficult to achieve in conventional layered materials.

Spinel chalcogenides with the general formula AB₂X₄ (A and B are metal cations, and X is a chalcogenide anion) represent a particularly compelling yet underexplored family within this class. Their mixed‐valence electronic configurations, geometrically frustrated lattices, and strong coupling between charge, spin, and lattice degrees of freedom give rise to a rich spectrum of correlated phenomena, including high conductivity [25], magnetism [26], and broadband optical absorption [27]. Among them, CuCo₂S₄ has attracted attention as a chemically stable ferromagnetic sulfide with a high magnetic ordering temperature [28, 29]. In its bulk and thick‐film forms, CuCo₂S₄ typically exhibits metallic transport, in which itinerant carriers mediate magnetic exchange between mixed‐valence Co ions [30]. However, how this intertwined electronic‐magnetic ground state evolves under extreme thickness reduction and whether semiconducting transport can emerge without quenching ferromagnetism remains largely unexplored.

Here, we report the chemical vapor deposition (CVD) synthesis of 2D nonlayered spinel CuCo₂S₄ nanoflakes with varied thicknesses, and present a systematic investigation of their thickness‐dependent electronic and magnetic properties. While robust ferromagnetic ordering is retained at room temperature, a pronounced transition from metallic to semiconducting behavior is uncovered as the nanoflake thickness decreases. This transition establishes these CuCo₂S₄ nanoflakes with a thickness below 20 nm as a new kind of 2D ferromagnetic semiconductor. The suppressed carrier concentration in these thin CuCo₂S₄ semiconductors facilitates the photoresponse over a broad spectrum range from 450 to 1310 nm, providing a platform for resolving the complex interaction between charge, spin, and photons. These findings establish 2D thickness‐engineered CuCo₂S₄ as a promising candidate for next‐generation spintronic and magneto‐optoelectronic devices.

2. Results and Discussion

CuCo₂S₄ crystallizes in a normal spinel structure with a cubic space group Fd‐3m. As shown in Figure 1a, sulfur atoms form a close‐packed face‐centered cubic framework. Within this lattice, Cu atoms occupy one‐eighth of the tetrahedral sites, while Co atoms reside in half of the octahedral sites, giving rise to a three‐dimensionally connected network of corner‐ and edge‐sharing polyhedra. This robust nonlayered framework distinguishes CuCo₂S₄ fundamentally from vdW layered materials and is expected to support strong electronic coupling and magnetic exchange interactions along all crystallographic directions. Importantly, when thinned down to the 2D limit, such a framework is highly susceptible to surface coordination breaking and carrier redistribution, providing a fertile ground for emergent thickness‐dependent properties. 2D CuCo₂S₄ nanoflakes were synthesized via CVD using CuCl and CoCl₂ as metal precursors, with a molecular sieve layer introduced to suppress premature precursor sublimation and enable controlled vapor delivery (see Experimental Section). Owing to its highly porous structure and large specific surface area, the molecular sieve can temporarily adsorb a portion of the evaporated precursor species and subsequently release them in a more gradual manner as the temperature increases. As a result, the instantaneous precursor concentration in the vapor phase is significantly reduced and stabilized during the growth process. This buffering effect suppresses the rapid precursor burst that would otherwise occur at elevated temperatures and provides a more controllable precursor flux to the substrate. A freshly cleaved fluorophlogopite mica substrate was employed to promote anisotropic growth. Optical microscopy (OM) reveals the coexistence of in‐plane (IP) and out‐of‐plane (OOP) nanoflake growth modes on the same substrate (Figure 1b), indicating competing kinetic and thermodynamic pathways during nucleation and growth. The OOP‐oriented nanoflakes appear as dark quadrilateral domains, while the IP‐grown counterparts exhibit lighter contrast and truncated triangular morphologies. Tilted scanning electron microscopy (SEM) image (Figure 1c) also confirms the inclined orientations of the OOP nanoflakes, highlighting the nontrivial growth habit of this nonlayered material.

FIGURE 1.

CVD growth and characterizations of 2D CuCo₂S₄ nanoflakes. (a) Atomic model of CuCo₂S₄ with the spinel structure. (b) OM image of as‐grown CuCo₂S₄ nanoflakes on the mica substrate showing both IP and OOP growth directions. (c) A tilted SEM image of CuCo₂S₄ nanoflakes on the mica substrate. (d) OM image of CuCo₂S₄ nanoflakes transferred onto a SiO₂/Si substrate by mechanical pressing. Inset: AFM image of a CuCo₂S₄ nanoflake showing a thickness of ∼9 nm. (e) Raman spectra of both IP and OOP CuCo₂S₄ nanoflakes. (f–h) High‐resolution XPS spectra of Cu 2p, Co 2p, and S 2p for 2D CuCo₂S₄, respectively.

Compared with IP‐grown nanoflakes, the OOP‐oriented CuCo₂S₄ nanoflakes are weakly coupled to the substrate through their edges rather than their basal planes. This geometry enables facile and polymer‐free mechanical transfer to arbitrary substrates (Figure S1), which is particularly advantageous for electronic and magnetic measurements where polymer residues can obscure intrinsic properties. After transfer onto SiO₂/Si substrates, the nanoflakes retain their structural integrity without noticeable cracking, wrinkling, or contamination (Figure 1d). Atomic force microscopy (AFM) reveals a representative thickness of ∼9 nm and an ultrasmooth surface with a root‐mean‐square roughness of only 0.14 nm, underscoring the mechanical robustness and high crystalline quality of the transferred nanoflakes.

The relative prevalence of IP vs. OOP growth is strongly correlated with growth temperature (Figure S2). At lower temperatures (∼620°C), IP growth dominates, whereas increasing the temperature progressively favors OOP growth, with nearly exclusive OOP orientation observed at ∼700°C. This transition can be understood in terms of precursor supply and surface energetics: higher temperatures enhance metal precursor flux and adatom mobility, reducing the relative energetic penalty of exposing high‐energy facets and thereby favoring OOP growth. Similar growth‐mode transitions have been reported in other nonlayered and layered 2D systems, such as Bi₂O₂Se [31] and Bi₂SiO₅ [32], where kinetic control plays a decisive role. Consistent with this picture, both the lateral sizes and thicknesses of OOP‐grown nanoflakes increase with growth temperature (Figure S3), reflecting accelerated vertical growth in the absence of facet‐selective passivation. Raman spectroscopy was employed to probe lattice vibrations and bonding configurations in both IP‐grown and transferred OOP‐grown nanoflakes. As shown in Figure 1e, four characteristic Raman modes at ∼465.1, 508.8, 601.7, and 670.9 cm⁻¹ are observed for both growth orientations, corresponding to Co‐S stretching, two distinct Cu‐S vibrational modes, and S‐S stretching vibrations, respectively [33]. The absence of Raman features associated with binary CuS_x or CoS_x phases confirms the phase purity of the CuCo₂S₄ nanoflakes. Furthermore, comparative Raman measurements were performed on OOP‐grown CuCo₂S₄ nanoflakes before and after the mechanical pressing transfer. As shown in Figure S4, the Raman spectra remain nearly unchanged after transfer, with negligible peak‐position shifts and no additional Raman features observed, suggesting that this transfer process does not introduce detectable localized strain into the CuCo₂S₄ nanoflakes. Raman mapping integrated from the most prominent peak position exhibits a highly uniform spatial distribution, further confirming the structural uniformity and the absence of strain fluctuations after transfer. X‐ray photoelectron spectroscopy (XPS) further corroborates the stoichiometric composition and appropriate chemical states. The Cu 2p, Co 2p, and S 2p spectra (Figure 1f–h) are consistent with mixed‐valence Cu and Co ions coordinated by sulfide anions, in agreement with the expected valence states of CuCo₂S₄. These results collectively confirm the successful synthesis of phase‐pure, stoichiometric, and orientation‐tunable 2D CuCo₂S₄ nanoflakes.

High‐resolution transmission electron microscopy (HRTEM) reveals well‐resolved lattice fringes with an interplanar spacing of ∼0.34 nm, corresponding to the (2$\overline{2}$0) crystallographic plane (Figure 2a). The inset fast Fourier transform (FFT) pattern confirms the high degree of crystallinity and hexagonal symmetry when viewed along the [111] direction. Selected‐area electron diffraction (SAED) exhibits a single set of six‐fold symmetric diffraction spots (Figure 2b), demonstrating the single‐crystalline nature of the nanoflakes. X‐ray diffraction (XRD) further confirms the cubic spinel phase, with a prominent (111) reflection at 2θ ∼ 16° (Figure 2c). The preferential orientation along [111] is consistent with the observed hexagonal atomic arrangement and may play a role in anisotropic electronic and magnetic behavior. Energy‐dispersive x‐ray spectroscopy (EDS) and elemental mapping confirm a homogeneous Cu:Co:S ratio of approximately 1:2:4 (Figure 2d,e).

FIGURE 2.

Structural and chemical characterizations of 2D CuCo₂S₄ nanoflakes. (a) HRTEM image of the transferred CuCo₂S₄ nanoflake. Inset shows the corresponding FFT pattern. (b) SAED pattern of the CuCo₂S₄ nanoflake. (c) XRD pattern of the CuCo₂S₄ nanoflakes after being transferred onto glass substrates. (d) EDS spectrum of the CuCo₂S₄ nanoflake. (e) Low‐magnification STEM image of a CuCo₂S₄ nanoflake and its corresponding EDS elemental maps of Cu, Co, and S.

The magnetic properties of CuCo₂S₄ nanoflakes were investigated using vibrating sample magnetometry (VSM). Zero‐field‐cooled (ZFC) and field‐cooled (FC) magnetization curves measured under an out‐of‐plane magnetic field of 1 kOe reveal a clear magnetic phase transition at ∼320 K (Figure 3a). The bifurcation between ZFC and FC curves below the transition temperature is a hallmark of ferromagnetic ordering, indicating the presence of magnetic anisotropy energy barriers that impede magnetization rotation in the absence of an applied field [34]. The persistence of this divergence up to room temperature suggests that the magnetic anisotropy remains significant even at elevated temperatures, which is crucial for practical spintronic applications requiring stable magnetic states. Similar behavior is observed under an in‐plane field, with a higher transition temperature above 350 K (Figure 3b). The observation of a higher transition temperature under in‐plane compared to out‐of‐plane fields implies a weak perpendicular magnetic anisotropy [35], which is consistent with the cubic spinel structure, where magnetocrystalline anisotropy is expected to be relatively small [36]. This weak anisotropy facilitates magnetization alignment along the plane of the nanoflake. Magnetic hysteresis loops measured at various temperatures (Figure 3c,d) exhibit finite coercivity and remanence, confirming long‐range ferromagnetic order. Notably, a well‐defined hysteresis loop persists at 300 K, unambiguously demonstrating room‐temperature magnetism in 2D CuCo₂S₄ nanoflakes. CuCo₂S₄ belongs to the family of mixed‐valence spinel sulfides, in which the magnetic ground state originates from the exchange interactions among transition‐metal cations occupying different crystallographic sites. Previous studies on bulk and polycrystalline CuCo₂S₄ have suggested that the magnetic properties are closely coupled to the electronic structure and carrier concentration. For example, it has been reported that substitutional doping can significantly modify the magnetic ordering behavior [28]. In particular, Ni incorporation suppresses the ferromagnetic state and eventually leads to paramagnetic behavior, indicating that the magnetic exchange interactions in CuCo₂S₄ are highly sensitive to changes in electronic structure. Such observations are commonly regarded as signatures of carrier‐dependent magnetic exchange in transition‐metal chalcogenides. The persistence of ferromagnetism in ultrathin CuCo₂S₄ is particularly noteworthy given the thickness‐dependent suppression of itinerant carriers discussed below. The magnetic domain structures of individual 2D CuCo₂S₄ nanoflakes with varied thicknesses were further explored with magnetic force microscopy (MFM) at room temperature (Figure S5). All the nanoflakes exhibit distinct magnetic contrast and single‐domain features, highlighting the robust room‐temperature ferromagnetism. To directly probe the magnetic properties of individual nanoflakes, we additionally performed room‐temperature magneto‐optical Kerr effect (MOKE) measurements on CuCo₂S₄ nanoflakes with different thicknesses. The corresponding results are presented in Figure 3e. Distinct hysteresis loops are observed for all investigated thicknesses, confirming that long‐range magnetic ordering is preserved throughout the thickness range studied. As the nanoflake thickness increases, the Kerr hysteresis loops become progressively more pronounced, accompanied by increases in both the normalized saturation Kerr rotation and remanent Kerr signal, as well as the coercive field. To gain insights into the magnetic origin, we performed first‐principles calculations on the CuCo₂S₄ slabs with one sub‐unit‐cell thickness (1‐layer, Figure S6a). The calculated spin‐polarized band structures exhibit pronounced asymmetry between the spin‐up and spin‐down channels, indicating a strongly spin‐polarized electronic ground state (Figure 3f). The spin polarization mainly originates from the Co 3d states, with additional contributions from Cu 3d orbitals through hybridization with S orbitals (Figure S6b). The calculated total magnetic moment of the 1‐layer structure remains finite, demonstrating that magnetic ordering can persist even in the thickness‐confined limit.

FIGURE 3.

Magnetic property characterizations of 2D CuCo₂S₄ nanoflakes. (a,b) Temperature‐dependent magnetic susceptibility of CuCo₂S₄ nanoflakes on SiO₂/Si substrates measured under (a) out‐of‐plane (OOP) and in‐plane (IP) magnetic fields of 1 kOe, respectively. ZFC and FC curves correspond to zero‐field‐cooled and field‐cooled processes, respectively. (c,d) Magnetic hysteresis loops of CuCo₂S₄ nanoflakes measured at different temperatures under (c) OOP and (d) IP magnetic fields, respectively. (e) Room‐temperature MOKE measurements on CuCo₂S₄ nanoflakes with different thicknesses. (f) The calculated spin‐polarized band structures of 1‐layer CuCo₂S₄.

To elucidate the electronic transport behavior, field‐effect transistor (FET) devices were fabricated using individual CuCo₂S₄ nanoflakes with varied thicknesses (Figure 4a). For the nanoflake with a thickness of 23 nm (inset in Figure 4b), the source‐drain current‐voltage (I_ds‐V_ds) curves are linear and largely insensitive to gate voltage (V_gs, Figure 4b,c), indicative of metallic transport. In contrast, a pronounced transition occurs when the thickness is reduced below ∼20 nm. A representative 19 nm‐thick device exhibits clear gate modulation and nonlinear I_ds‐V_ds characteristics (Figure 4d,e). The emergence of gate‐tunable transport marks a fundamental change in the electronic structure, where dimensional confinement has sufficiently depleted the carrier concentration to enable electrostatic control. The n‐type transport, as verified by the increase in I_ds with positive V_gs suggests that electrons dominate the transport in these thin nanoflakes. The extracted metrics for the n‐type FET device include a low off‐state current of ∼1 nA, an on/off current ratio of ∼10³, a threshold voltage of −41 V, and a field‐effect electron mobility of ∼0.01 cm² V⁻¹ s⁻¹. The relatively low on/off ratio and mobility are ascribed to the comparatively small bandgap and a high residual carrier concentration, as well as the scattering centers from the unsaturated surface dangling bonds [30]. Nevertheless, these values still represent an improvement for an intrinsic 2D magnetic semiconductor that requires no magnetic doping or proximity effects [12, 15]. The negative threshold voltage indicates the presence of electron doping, likely arising from sulfur vacancies or cation non‐stoichiometry, which is common in sulfide spinel compounds [37]. The carrier concentration was estimated from the transfer characteristics of the semiconducting devices. The sheet carrier density can be approximated by Equation (1),

$$n_{\mathrm{s}}=\frac{C_{\mathrm{ox}}\left(V_{\mathrm{gs}}-V_{\mathrm{th}}\right)}{\mathrm{q}}$$ (1)

where n _s is the sheet carrier density, C _ox is the gate capacitance per unit area, V _th is the threshold voltage, and q is the elementary charge. Using the experimentally extracted V _th of about −40 V, the sheet carrier density at V _gs = 0 V is estimated to be approximately 2.87 × 10¹² cm⁻². For a CuCo₂S₄ nanoflake with a thickness near the transition regime (∼20 nm), the corresponding volume carrier concentration is approximately 1.4 × 10¹⁸ cm⁻³, indicating a relatively high carrier density even in the semiconducting state. The thickness‐driven semiconductor‐metal transition is further confirmed by devices fabricated from 12–18 to 25–54 nm nanoflakes (Figures S7–S10). The extracted conductivity as a function of thickness (Figure 4f) reveals a critical thickness of approximately 20 nm, below which semiconducting behavior emerges. This transition is attributed to the band structure evolution and, consequently, a reduced carrier density in the ultrathin regime [19, 38], which highlights the fundamentally different nature of dimensional confinement in nonlayered 2D materials compared with vdW layered systems. Owing to the large number of atoms involved, direct calculations for experimentally relevant thicknesses approaching the critical value (∼20 nm) are computationally prohibitive. Therefore, ultrathin slab models (1 layer, 2 layer, and 4 layer in Figures S6a and S11) were employed to qualitatively investigate the trend of electronic structure evolution under dimensional confinement. The calculated polarized spin‐down band structures are shown in Figure S12. For the 1‐layer model, a narrow bandgap is observed near the Γ point. In contrast, the bandgap closes for the thicker slab models, which exhibit metallic band crossings near the Fermi level. Although the calculated critical thickness is considerably smaller than the experimentally observed transition thickness, the calculations clearly indicate that dimensional reduction can induce a transition from metallic to semiconducting electronic structures. We therefore believe that the experimentally observed semiconductor‐to‐metal transition originates from thickness‐dependent band‐structure evolution associated with quantum confinement and surface‐induced electronic reconstruction.

FIGURE 4.

Electrical characteristics of 2D CuCo₂S₄ FET devices. (a) Schematic of a 2D CuCo₂S₄ FET device. (b,c) Electrical output and transfer curves for the relatively thick 23 nm device. The inset in (b) shows the OM image of the 23 nm device. (d,e) Electrical output and transfer curves for the 19 nm device. The inset in (d) shows the OM image of the device. (f) Thickness‐dependent conductivity of CuCo₂S₄ devices (∼12–54 nm), revealing a critical semiconductor‐to‐metal transition at ∼20 nm.

The semiconducting CuCo₂S₄ nanoflake (12 nm) was further evaluated for optoelectronic functionality. Under 637 nm laser illumination, the devices exhibit pronounced photocurrent enhancement compared with dark conditions (Figure 5a). The photocurrent follows a sublinear power‐law dependence on incident power (Figure 5b) with an exponent α ∼ 0.3. This value is lower than the ideal unity, indicating the presence of a high density of trap states arising from surface defects [39]. The extracted responsivity (R) and detectivity (D^*) reach maximum values of 9.91 A W⁻¹ and 7.3 × 10⁸ Jones, respectively, at low illumination power (Figure 5c), with a corresponding external quantum efficiency (EQE) of 1829%. The relatively modest D^* mainly originates from the comparatively high dark current due to the remaining high intrinsic carrier concentration. The EQE exceeding 100% indicates the presence of photoconductive gain [40], where a single photogenerated carrier induces multiple electrons to transit through the channel due to the long carrier lifetime relative to the transit time. This gain mechanism, while beneficial for responsivity, typically comes at the cost of response speed. Time‐resolved measurements reveal fast and repeatable photoresponse with rise and decay times of 27 and 38 ms, respectively (Figure 5d,e and Figure S13). The asymmetric rise and decay characteristics are consistent with the filling and emptying of trap states. Broadband photodetection is demonstrated over a spectral range spanning from visible to near‐infrared (450–1310 nm) (Figure 5f and Figures S14–S18), underscoring the strong optical absorption and efficient carrier generation in CuCo₂S₄. The gradual reduction in performance at longer wavelengths is attributed to decreased absorption and reduced photocarrier generation efficiency. The coexistence of thickness‐tunable electronic transport, broadband photoresponse capacity, and intrinsic room‐temperature ferromagnetism establishes 2D CuCo₂S₄ nanoflakes as a rare example of a nonlayered 2D magnetic semiconductor. Unlike conventional approaches relying on magnetic doping or proximity effects, dimensional confinement in CuCo₂S₄ enables an intrinsic coupling between semiconducting transport and magnetic order, while the introduction of broadband incident light provides another degree of freedom for modulating the interactions between charge, spin, and lattice.

FIGURE 5.

Optoelectronic characteristics of the thin CuCo₂S₄ nanoflakes. (a) The I_ds‐V_ds curves of the device in dark and under 637 nm laser illumination with increasing power densities. (b) The incident power density‐dependent photocurrent and the fitting curve under 637 nm illumination. (c) The calculated R, D^*, and EQE as a function of incident power densities. (d) Time‐resolved photocurrent of the device at V_ds = 1 V and 785 nm light power density of 36.29 mW·cm⁻². (e) One cycle of the time‐resolved photocurrent curve showing a rise time of 27 ms and a decay time of 38 ms. (f) Comparison of the R, D^*, and EQE under broadband illumination (450–1310 nm).

Lastly, the environmental stability of the ultrathin CuCo₂S₄ nanoflakes and devices was evaluated. As shown in Figure S19, the Raman spectra collected from the as‐grown sample, after 3 days and 7 days of ambient exposure, exhibit nearly identical characteristic Raman modes without noticeable peak shifts, peak broadening, or intensity degradation. In the XPS spectra, the Cu 2p, Co 2p, and S 2p spectra after 7 days of exposure remain nearly unchanged compared with those of the as‐synthesized samples (Figure S20), suggesting that no significant chemical oxidation or decomposition occurs during the exposure period. In addition to the material stability, the stability of the CuCo₂S₄ photodetector devices was also evaluated after prolonged air exposure. The temporal photoresponse characteristics of the same device were re‐measured after storage in ambient conditions for 7 days. As presented in Figure S21, the device exhibits nearly identical dark current, photocurrent magnitude, and photo‐switching cycling behavior compared with the freshly fabricated device. The extracted rise and decay times also remain comparable, indicating that the carrier transport and photoresponse dynamics are not noticeably degraded after air exposure. The preserved switching stability further confirms the robustness of the metal contacts and channel interface under ambient conditions. Therefore, although ultrathin nonlayered sulfides are generally considered vulnerable to ambient degradation, the present CuCo₂S₄ nanoflakes exhibit sufficiently stable structural, chemical, and device characteristics for practical electronic and optoelectronic applications.

3. Conclusion

In summary, we have demonstrated the CVD synthesis of high‐quality 2D CuCo₂S₄ nanoflakes and revealed a pronounced thickness‐driven evolution of their electronic properties. Owing to the nonlayered spinel structure, dimensional reduction induces a metal to semiconductor transition at a critical thickness of ∼20 nm, while robust ferromagnetic order is retained above room temperature. The coexistence of semiconducting transport and intrinsic ferromagnetism establishes ultrathin CuCo₂S₄ as an example of 2D magnetic semiconductors realized without magnetic doping or proximity effects. Furthermore, the semiconducting nanoflakes exhibit broadband photodetection with high responsivity and fast response, underscoring the multifunctionality enabled by thickness engineering in nonlayered 2D materials. These results not only identify CuCo₂S₄ as a promising platform for integrated electronic, optoelectronic, and spintronic devices, but also highlight dimensional reduction of nonlayered magnetic conductors as an effective strategy for realizing intrinsic 2D magnetic semiconductors.

4. Experimental Section

4.1. CVD Growth and Transfer of 2D CuCo₂S₄ Nanoflakes

2D CuCo₂S₄ nanoflakes were synthesized by CVD in a horizontal tube furnace with two temperature zones. Copper(I) chloride and cobalt(II) chloride (Alfa Aesar) were mixed in a mass ratio of 1: 2.6 and thoroughly ground to form a homogeneous metal precursor. 24 mg of the mixed precursor was placed at the center of the downstream zone. To regulate precursor sublimation and stabilize metal vapor flux during growth, 20 mg of 3 Å molecular sieve powder was uniformly spread over the metal precursor. Freshly cleaved fluorophlogopite mica substrates were placed above the metal precursors. Sulfur powder (50 mg) was placed in the upstream zone. High‐purity argon (100 sccm) was used as the carrier gas throughout the growth process. The temperature for the metal chlorides was ramped to 660°C over 40 min and kept at this temperature for 20 min. The OOP‐oriented CuCo₂S₄ nanoflakes were transferred from mica to other substrates using a polymer‐free mechanical pressing method to preserve surface cleanliness and intrinsic properties.

4.2. Material Characterizations

The morphology, lateral size, and thickness distribution of the CuCo₂S₄ nanoflakes were examined using OM and AFM (Cypher ES, Oxford Instruments). Crystal structure and phase purity were characterized by XRD (APEXII, Bruker). Chemical composition and valence states were analyzed by XPS (SHIMADZU, AXIS SUPRA). HRTEM (Talos F200X G2, Thermo Fisher Scientific), together with SAED and EDS, was employed to investigate crystallinity, lattice structure, and elemental distribution. Raman spectra were collected using a confocal Raman microscope (inVia, Renishaw) with a 532 nm excitation laser. Magnetic properties of the CuCo₂S₄ nanoflakes transferred onto SiO₂/Si substrates were measured using VSM. Temperature‐dependent magnetization and magnetic hysteresis loops were recorded under both in‐plane and out‐of‐plane magnetic fields. To probe magnetic domain behavior, MFM measurements were performed using a CoCr‐coated magnetic probe.

4.3. Device Fabrication and Measurements

For transport and photodetection measurements, CuCo₂S₄ nanoflakes were transferred onto SiO₂ (300 nm)/Si substrates, where the heavily doped Si served as a global back gate. Source and drain electrodes were defined by standard photolithography, followed by thermal evaporation of Cr/Au (10/50 nm). Electrical measurements were conducted at room temperature using a probe station (CRX‐6.5K, Lake Shore) and a semiconductor parameter analyzer (4200‐SCS, Keithley). Photoresponse measurements were carried out by illuminating the devices with lasers of selected wavelengths and controlled power densities delivered through an optical fiber.

4.4. Theoretical Calculations

All the CuCo₂S₄ models were simulated using the spin‐polarized density functional theory (DFT) method through Vienna Ab initio Simulation Package (VASP5.4.4). Generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE) was adopted to describe exchange correlation interaction. The ion‐electron interaction was treated using the projector augmented wave (PAW) technique. The plane‐wave cutoff energy of 450 eV was employed. The DFT+U method was adopted to describe the transition metal d electrons strong Coulomb interactions. The U parameters were 4.0 eV for Co and 3.0 eV for Cu. The atomic positions were fully relaxed until the maximum force on each atom was less than 0.01 eV/Å. The Brillouin zone was sampled using the Monkhorst‐Pack scheme with a k‐point mesh of 3 × 3 × 1 in the Γ‐centered grids for the structural relaxation. Van der Waals interaction was considered using the semiempirical DFT‐D3 approach. A vacuum distance over 15 Å was used to avoid interactions between neighboring layers. All visualized structural data is rendered using VESTA software. The energy band structure and spin‐polarized electron density were carried out using the VASPKIT1.3.5 tool.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: smll74381‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.