Spin-polarized light-emitting diodes based on CrI₃ operating without external spin injection

Domain: neuroscience
Published: 2026-04-13
Editorial outcome: major_revision

Abstract
Spin-polarized light-emitting diodes convert electron spin into circularly polarized light, enabling direct optical readout of spin information and applications in on-chip information processing. Previous demonstrations have largely relied on GaAs-based emitters integrated with spin injectors. These devices, however, require complex epitaxial growth, limiting integration, while high circular polarization remains difficult, demanding high-quality materials for efficient spin injection, coherent spin transport, and spin-conserving radiative recombination. Here, we report an alternative approach for spin-polarized light-emitting diodes by employing monolayer CrI₃ as the emitter, sandwiched between two graphene/hexagonal boron nitride tunneling contacts. Although these contacts inject unpolarized carriers into CrI₃, the resulting electroluminescence exhibits circular polarization, with helicity governed by the magnetic order of CrI₃. Notably, the electroluminescence degree of polarization reaches 20% and its helicity can be reversed with a low magnetic field (~0.17 T). Combined with the inherent integrability of heterostructures, this approach provides a promising platform for future on-chip spin-optoelectronics.

Introduction
Solid-state spin-polarized light-emitting diodes (spin-LEDs) have long been pursued for their ability to convert electron spin into circularly polarized light, enabling direct optical readout of spin information. Their inherent compatibility with integrated photonic circuits further broadens their potential for cryptography, reconfigurable optical interconnects, and advanced optical switches1,2. Typically, these solid-state spin-LEDs operate by injecting spin-polarized carriers into a bandgap semiconductor, where optical selection rules couple the non-equilibrium spin population to the circular polarization of emitted photons; reversing the external magnetic field flips the injected spin orientation, thereby switching the EL helicity1-3. Early realizations of this concept were achieved using GaAs-based semiconductors as the emitting layer, integrated with ferromagnetic metals, dilute magnetic semiconductors such as (Ga,Mn)As and BeₓMnᵧZn₁₋ₓ₋ᵧSe, or tunneling spin filters such as MgO to serve as spin injectors1,2,4-8. However, these approaches yield intricate multilayer heterostructures, and their fabrication requires sophisticated epitaxial growth, limiting device integrability. To overcome these challenges, recent efforts have turned to emerging two-dimensional (2D) materials as a new platform for device design. Their layered nature allows the assembly of complex heterostructures and direct integration on diverse substrates without lattice or thermal mismatch issues9-11, opening new opportunities for spin-LED development. For instance, monolayer transition metal dichalcogenides (TMDs), with their unique spin–valley properties12-14, have been exploited to generate circularly polarized EL through integration with spin injectors such as permalloy or (Ga,Mn)As, or via electric-double-layer transistor architectures15-18. More recently, 2D magnets have been implemented as spin injectors19-22, enabling the injection of spin-polarized carriers into TMDs and the realization of fully vdW spin-LED architectures. This can be realized by using Fe₃GeTe₂ as a spin source, yielding degree of circular polarization (DOCP) ~8% at 78 K, with nonvolatile DOCP sign reversal under a low magnetic field (~0.1 T) [19]. Alternatively, trilayer CrI₃ can function as a spin-filter barrier, producing EL from TMDs with a DOCP of up to ~40%. This is achieved at 4 K under an ~2 T magnetic field, which ensures CrI₃ converts from an antiferromagnetic to a ferromagnetic state and enables efficient spin filtering. [20].

Despite these advances, most spin-LEDs reported to date rely on external spin injectors. Such architectures demand efficient injection of spin-polarized carriers, preservation of spin coherence during transport, and spin-conserving radiative recombination in the emitter. All three processes are highly sensitive to material defects and interfacial properties, which directly impact the DOCP and render device optimization nontrivial1-3. To move beyond the above-mentioned limitations, an attractive strategy is to employ 2D magnets directly as the light-emitting medium, exploiting the intrinsic coupling between excitons and the magnetic order to produce polarized emission. This concept has recently been validated in NiPS₃, where EL exhibits an ultranarrow linewidth (~1 meV) and a high degree of linear polarization, arising from excitons coupled to the underlying antiferromagnetic order21,22.

In this context, the magnetic 2D semiconductor CrI₃ emerges as a promising platform for realizing injector-free spin-LEDs. Identified as the first experimentally demonstrated 2D ferromagnet, monolayer CrI₃ exhibits a Curie temperature of ~45 K and an out-of-plane easy axis of magnetization, while few-layer CrI₃ adopts a layered type-A antiferromagnetic ground state23-25. Additionally, unlike most bandgap semiconductors where light emission arises from Wannier–Mott excitons, CrI₃ emits via intra-atomic d–d transitions within the Cr³⁺ ions, correlated with the ligand field of the surrounding halides. This localized emission is intrinsically linked to the magnetic order, resulting in circularly polarized photoluminescence (PL) whose helicity reflects the orientation of the magnetization26-28. Provided that EL from CrI₃ can be realized, these features could in principle allow the emission helicity to be controlled solely via tuning the magnetic state of the CrI₃ layer, without relying on external spin injectors. However, this concept has yet to be experimentally demonstrated, even though it is conceptually straightforward.

Here, we report the experimental realization of spin-LEDs based on vdW heterostructures, with monolayer CrI₃ serving as the light-emitting layer. Helicity-resolved EL measurements show that, under an upward (downward) out-of-plane magnetic field, the emission exhibits stronger 𝜎− (𝜎+) circular polarization, with the EL DOCP reaching -20% (20%). Moreover, the EL DOCP reverses in a nearly square hysteresis as the magnetic field direction is swept. This behaviour closely matches the magnetic hysteresis measured by reflective magnetic circular dichroism (RMCD) and magneto-PL, providing clear evidence that the EL helicity is directly governed by the intrinsic magnetic order of CrI₃. These findings establish a new approach for designing spin LEDs and, combined with the integrability of vdW heterostructures, pave the way for future spin-optoelectronic devices enabling on-chip information processing.

Results
Device structure and characterization of 2D CrI₃

Figure 1a,b presents a schematic and an optical micrograph of the investigated vdW spin-LED heterostructures. Details of the material preparation and device fabrication are provided in the Methods section. In this device, monolayer CrI₃ serves as the light-emitting layer, encapsulated between two tunneling contacts composed of graphene and hexagonal boron nitride (hBN). Graphene acts as a transparent and highly conductive electrode29,30, while bilayer hBN flakes function as tunneling barriers, allowing electrons and holes to efficiently tunnel from graphene into CrI₃, where they persist long enough to undergo radiative recombination under an applied bias voltage31-33. The emissive capability of CrI₃ is verified by PL measurement (Fig. 1c), which exhibits an emission peak centered at 1.14 µm. The relatively broad linewidth, with a full width at half maximum (FWHM) of ~190 nm, reflects coupling between electronic excitation and lattice vibrations, in agreement with previous reports on CrI₃-based optical emission26.

To probe the intrinsic magnetic properties of CrI₃, we performed polar RMCD measurements (see Methods) as a function of an externally applied magnetic field oriented perpendicular to the sample plane (i.e., in the Faraday geometry). Unless otherwise specified, all RMCD and subsequent optoelectronic measurements were conducted at 30 K. As shown in Fig. 1d, the resulting hysteresis loop displays a square-like shape with a coercive field of approximately 0.17 T and a finite RMCD signal at zero field, confirming that the CrI₃ used here exhibits ferromagnetic ordering with an out-of-plane easy magnetization axis characteristic of an Ising-type magnet. The fact that only a single hysteresis loop appears near zero field further indicates that the used CrI₃ is monolayer, consistent with the known layer-dependent magnetic behaviour5,20.

Next, we perform magneto-PL measurements (see Methods) on the used monolayer CrI₃ to resolve the effect of applying magnetic field on its PL helicity. Figure 2a presents the helicity-resolved PL spectra under a magnetic field of 0.6 T, where an apparent intensity contrast between the σ⁺ and σ⁻ components is observed, with σ⁻ emission stronger. The PL DOCP, defined as (I₊ − I₋)/(I₊ + I₋) with I₊ and I₋ denoting the peak intensities of measured σ⁺ and σ⁻ emission, respectively, reaches around -20%. This helicity contrast remains even as the magnetic field is reduced from 0.6 T to zero (Fig. 2b). When the field is reversed to −0.6 T, the σ⁺ emission becomes dominant, yielding DOCP around 20%, and this helicity state is retained as the field returns to zero (Fig. 2c-d). These results reveal a direct coupling between PL helicity and the magnetic order of CrI₃, because magnetic fields of 0.6 T (-0.6T) are sufficient to fully align the magnetization in the out-of-plane upward (downward) orientation. Moreover, as the magnetic field is swept, the PL DOCP shows non-volatile behaviour, reversing its sign at approximately ±0.17 T (Supplementary Section 1), matching the ferromagnetic hysteresis observed in the RMCD measurement (Fig. 1d).

EL generation from graphene/hBN/CrI₃/hBN/graphene heterostructures

With these fundamental characterizations, we then examine the electrical behaviour and the feasibility of generating EL in our proposed graphene/hBN/CrI₃/hBN/graphene heterostructures. Figure 3a shows the I–V characteristics measured by applying a bias (Vb) across the graphene electrodes, with the bottom electrode grounded. The nearly symmetric tunneling diode-like response under both bias polarities indicates the absence of band tilting across the heterostructure at equilibrium (Fig. 3b). When a bias is applied, it induces a vertical electric field that tilts the energy bands, while simultaneously shifting the Fermi levels of the two graphene electrodes due to the quantum capacitance effect (Fig. 3c)34. Once the Fermi levels align with the electronic states of CrI₃, carriers begin to tunnel efficiently through the atomically thin hBN barriers, causing the turn-on of the tunneling diode and initiating EL emission (Fig. 3d–e).

Notably, at low injected current (~few hundreds nA), the EL peak wavelength coincides with the PL peak (~1.14 µm), and the EL spatial map shows that emission is confined to the CrI₃ region (Supplementary Section 2), confirming that the observed EL originates from intra-atomic d–d transitions within CrI₃. Moreover, we note that this tunneling device structure can sustain high injected currents up to 50 µA (corresponding to a current density of 250 A/cm² for the graphene/CrI₃/graphene overlapped area of 20 µm²) while continuing to emit light. However, at higher bias, the EL intensity increases sublinearly with current (Fig. 3f, lower panel), and the emission peak exhibits a slight redshift from 1.14 µm to 1.17 µm (Fig. 3f, upper panel), accompanied by a noticeable broadening of the EL linewidth (Fig. 3d–e). The sublinear scaling of EL intensity at high bias can be attributed to a combination of device- and material-related effects. That is, the high bias creates a strong vertical electric field that enhances direct carrier tunneling between the graphene electrodes, reducing the fraction of carriers that recombine radiatively within CrI₃ [31-33], while strong carrier injection can also introduce nonradiative recombination pathways through exciton−exciton annihilation35, further limiting the EL efficiency. To clarify the origin of the bias-induced spectral changes, we first consider the possible contribution of Joule heating. Finite-element simulations reveal that increasing the current to 50 µA raises the device temperature by ~6 K, from 30 K to approximately 36 K (Supplementary Section 3). However, temperature-dependent PL measurements on monolayer CrI₃ (Supplementary Section 4) indicate that this modest temperature increase produces negligible spectral changes, suggesting that additional mechanisms may contribute. Indeed, recent studies have shown that excitonic emission in CrI₃ can be described by exciton-polaron, Frenkel-exciton, or self-trapped-exciton models28,36,37. Within these frameworks, an increasing exciton population enhances phonon sidebands and lattice distortions, leading to emission redshift and linewidth broadening, potentially accounting for the bias-dependent spectral evolution.

Evidence for graphene/hBN/CrI₃/hBN/graphene heterostructures as spin-LEDs

Following this, we demonstrate the potential of CrI₃-based heterostructures for spin-LED applications. In pursuit of this, an out-of-plane magnetic field was applied onto heterostructures in the Faraday geometry to probe the influence of magnetic order on EL helicity. As shown in Fig. 4a, helicity-resolved EL spectra measured at an injected current of 0.5 µA reveal a dominant σ⁻-polarized emission under a 0.6 T out-of-plane field. The resulting EL DOCP of -19.5%, comparable to the PL DOCP shown in Fig. 2, even though the graphene electrodes inject unpolarized carriers. Upon reversing the magnetic field to −0.6 T at the same injection current, the emission helicity switches correspondingly, resulting in a DOCP of 19.5% (Fig. 4b). These observations indicate that this proof-of-concept vdW spin-LED can achieve polarized EL performance without external spin injection or filtering, highlighting the potential of CrI₃ for simpler spin-LED architectures and yielding DOCP values that exceed most conventional III–V and II–VI epitaxial devices (see Supplementary Section 5 for a comparison table)1,2. Moreover, the EL intensity and DOCP remain stable after 48 hours of continuous operation, and measurements on multiple independently fabricated CrI₃-based spin-LEDs show consistent DOCP values of 17–23%, suggesting stable operation and good device-to-device reproducibility (Supplementary Section 6).

To further verify that the observed EL helicity originates from the ferromagnetic order of CrI₃, we measured the EL DOCP at an injected current of 0.5 µA as a function of the out-of-plane magnetic field. As shown in Fig. 4c, the EL polarization exhibits a clear hysteresis loop that generally follows the RMCD hysteresis of CrI₃, suggesting a strong connection between the EL helicity and its magnetic order. But it is worth noting that the sign reversal of the EL DOCP loop occurs at approximately ±0.1 T, lower than the coercive field obtained from RMCD and magneto-PL loop experiments (~0.17 T, Fig. 1d and also see Supplementary Section 1). This difference could be partially attributed to the distinct probing characteristics of the measurements: RMCD and PL are sensitive to local magnetization at a focused spot23, whereas EL integrates emission across the entire CrI₃ layer, effectively averaging over possible domain variations23. Another factor that could contribute to this discrepancy is the applied bias. It induces carrier injection, which raises the device temperature via Joule heating, and increases the chemical potential of the graphene electrodes34, enabling carriers to be populated into higher-energy orbitals in CrI₃ and activating antiferromagnetic exchange interactions38. Thus, the bias-related effect could weaken the ferromagnetism of CrI₃ and could account for the sign reversal of the EL DOCP loop at lower magnetic fields (see Supplementary Section 7 for further evidence) as well as the decrease in EL DOCP from 19.5% to 2.5% with increasing injection current (Fig. 4d).

In addition to monolayer CrI₃ devices, the dependence of EL polarization on the magnetic state of CrI₃ is also evident in Gr/hBN/CrI₃/hBN/Gr heterostructures employing bilayer CrI₃ as the emitter. In this spin-LED, the EL exhibits a DOCP of approximately ±22% when the bilayer is in the ferromagnetic (↑↑/↓↓) configuration, comparable to that observed in monolayer emitters. However, the EL helicity vanishes as the external magnetic field drops to zero (see Supplementary Section 8 for magneto-EL from a spin-LED based on bilayer CrI₃), directly reflecting the layered antiferromagnetic ground state of bilayer CrI₃ (↑↓ or ↓↑)23-25, in which each layer emits circularly polarized light of opposite helicity. Taken together, the results from EL DOCP loops and layer-dependent CrI₃ heterostructure experiments reveal a direct correlation between the magnetic order of CrI₃ and the helicity of its EL.

Methods
Fabrication of vdW heterostructures

All graphene, hBN, and CrI₃ flakes used in this work were obtained by mechanical exfoliation, and their thicknesses were identified using atomic force microscopy (AFM) and optical contrast methods following established procedures23. The vdW heterostructures were assembled by sequentially stacking the selected flakes through a dry transfer process10. The completed graphene/hBN/CrI₃/hBN/graphene stacks were transferred onto a Si/SiO₂ substrate prepatterned with two separated Ti/Au (5/50 nm) electrodes. During assembly, the top and bottom graphene layers were precisely aligned to make contact with the respective electrodes. All exfoliation and transfer processes were carried out in a nitrogen-filled glove box, where oxygen and water levels were maintained below 0.5 ppm to prevent material oxidation. After assembly, the heterostructures were wire-bonded onto a chip carrier and promptly mounted in a cryogenic system for subsequent measurements.

Optoelectronic measurements

All fabricated vdW heterostructures were wire-bonded onto chip carriers and mounted on the cold finger of a cryostat maintained at 30 K. An electromagnet surrounding the cryostat generated an out-of-plane magnetic field in the Faraday geometry for the following optoelectronic measurements. For RMCD characterization, the helicity-dependent reflectivity of the vdW heterostructures was measured using a 633 nm HeNe laser (10 μW), whose polarization state was periodically switched between right- and left-handed circular polarizations by a photoelastic modulator. The modulated beam was normally incident and focused to a ∼1 μm spot on the vdW heterostructure through an aspheric lens. The reflected light retraced the same optical path and was collected by a balanced amplified photodetector. The resulting signal was then sent to a lock-in amplifier to extract the differential reflectivity between the two helicities19,23,46.

For magneto-PL measurements, the same HeNe laser was used to excite the CrI₃ layer while an out-of-plane magnetic field was applied to the heterostructure. The emitted PL was collected by an aspheric lens and analyzed using a grating spectrometer (Andor Shamrock 500i) equipped with an Andor iDus InGaAs CCD camera. PL spectra were recorded as a function of the applied magnetic field to probe helicity-dependent emission behaviour. For both RMCD and PL measurements, the excitation power was kept low (10 μW) to minimize sample heating and degradation.

For magneto-EL measurements, the light emitted from the vdW heterostructures under applied bias was collected using the same optical setup as for PL, and helicity-resolved EL spectra were recorded as a function of the out-of-plane magnetic field to examine the correlation between the emission polarization and the magnetic order of the CrI₃ layer.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript experimentally demonstrates a spin-polarized electroluminescent diode based on monolayer and bilayer\nCrI₃. Compared with conventional GaAs-based emitters, this device enables direct control of the emission helicity through\nthe magnetic state of CrI₃, without relying on external spin injectors. This approach avoids the complex heterostructure\nengineering typically required to integrate spin injectors and suppress spin decoherence, while achieving a degree of\ncircular polarization (20%) that surpasses most traditional spin-polarized LEDs. Overall, the work provides a promising\nplatform for future on-chip spin-optoelectronic devices. After the following issues are properly addressed, I recommend its\npublication in Nature Communications.\n1. The schematic in Fig. 3(c) may be easily confused with a band-structure diagram. If possible, the authors are encouraged\nto include a corresponding schematic based on band theory or crystal-field theory to clarify the underlying physical\nmechanism.\n2. As the injection current increases, the emission peak redshifts, while both the linewidth and intensity increase. The\nauthors attribute the redshift to Joule heating, but no direct evidence is provided. Moreover, the physical origin of the\nincreased linewidth and intensity is not sufficiently explained. Additional analysis or experimental support is needed.\n3. The sign reversal in the EL DOCP loop occurs at around ±0.1 T, which is lower than the coercive field (0.17 T) obtained\nfrom RMCD and magneto-PL measurements. Is there an additional physical mechanism responsible for this discrepancy?\nFor example, could the applied bias modify the magnetic properties of CrI₃? The authors should discuss this point or provide\nsupporting evidence.\n4. The DOCP decreases with increasing current. The authors attribute this to the weakening of CrI₃ magnetic order due to\nJoule heating, supported by simulations showing that the material temperature rises from 30 K to 36 K. However, this\ntemperature is still far below the Curie temperature (45 K), making this explanation less convincing. Carrier injection can\nchange the band occupation of CrI₃ and may intrinsically affect its magnetism; the authors are encouraged to consider and\ndiscuss this possibility.\n5. Numerous studies have shown that bilayer CrI₃ undergoes an antiferromagnetic-to-ferromagnetic phase transition under\napplied bias. Since the present work involves bilayer CrI₃, the authors should discuss this effect and its potential implications\nfor their experimental observations.", "reviewer_2": "The manuscript reports electrically driven, magnetically switchable circularly polarized electroluminescence from monolayer\nCrI₃. This is, to my knowledge, the first demonstration of a spin-LED using monolayer CrI₃ as the intrinsic emitter without\nrelying on additional spin-polarizing TMD layers, and the ability to generate EL from monolayer CrI₃ at all is itself a\ntechnically nontrivial and interesting result. The integration of CrI₃ with graphene/hBN tunneling contacts is executed\ncleanly, and the optical–magnetic coupling they observe is consistent with the expected magnetic order of CrI₃. These\naspects make the work a meaningful addition to the growing effort of vdW-magnet–based optoelectronics.\nAt the same time, the manuscript appears to substantially overstate both the conceptual novelty and the performance\nsignificance of the device. While the authors emphasize achieving ~20% DOCP and frame this as addressing the “limited\nDOCP” in prior spin-LEDs, the paper does not explain why monolayer CrI₃ should fundamentally overcome those limitations\nthat affect other spin-LED architectures. No conceptual mechanism is articulated. As a result, the motivational narrative lacks\nthe scientific grounding needed to justify the authors’ claims about broader impact. Without a mechanistic understanding, the\nwork reads primarily as a materials demonstration rather than a solution to the DOCP bottleneck the authors highlight. In its\ncurrent form, I do not feel it meets the level of conceptual clarity and novelty required for publication in Nature\nCommunications.\nThis issue is especially problematic because a directly relevant prior work, Dang et al., Nat Commun 15, 6799 (2024),\nalready reports 26–40% DOCP in CrI₃/TMD spin-LEDs, substantially higher than the ~20% reported here. The absence of\nany discussion of this work creates a misleading impression that 20% DOCP represents a breakthrough in vdW spin-LEDs,\nwhen in fact the state of the art already exceeds it. Moreover, the authors repeatedly compare their results only to traditional\nIII–V and II–VI spin-LEDs, omitting the most relevant vdW-magnet–based devices, including linearly polarized EL in NiPS₃\n(Lebedev et al., Nat Commun 16, 10550 (2025)) and electrically controlled polarization in bilayer CrI₃/TMD devices. A fair,\ncomplete comparison is essential for evaluating the manuscript’s contributions. I strongly encourage the authors to discuss\ntheir results holistically within the existing body of vdW-based spin-LEDs and clearly articulate what novelty remains. For\nexample: does monolayer CrI₃ emission avoid the intervalley depolarization that limits the DOCP in CrI₃/TMD devices?\nIf so, what is the physical mechanism?\nThe experimental support for the core claims is also limited. The key DOCP value is shown for only one monolayer device\n(and one bilayer device in the SI). Prior studies, including those the authors cite, have documented significant device-to-\ndevice variability. Without multiple devices or at least a clear demonstration of reproducibility, it is difficult to evaluate how\nrepresentative the reported 20% polarization actually is. For a claim centered on “high DOCP,” this lack of statistical\nconfirmation is a notable weakness. Given that the previous investigation into CrI3/TMD devices showed considerable\nvariation in the DOCP of different devices (see Dang et al. Supplementary Table 1), it would also be beneficial for the\nauthors to include additional data from other supporting monolayer or bilayer devices to establish that the reported\nDOCP ~ 20% is reproducible across multiple samples rather than a single exceptional case.\nFinally, because EL from monolayer CrI₃ has not been previously established, a deeper discussion of the emission\nmechanism, possible defect-assisted recombination, and device stability under bias would significantly strengthen the\nmanuscript. As written, these aspects are treated only briefly, leaving the interpretation of the EL behavior incomplete.", "reviewer_3": "I have read the manuscript \"Spin-polarized light-emitting diodes based on CrI₃ operating without external spin injection\" by\nC.-C. Lu, et al. This work reports circularly polarized electroluminescence from CrI3 tunneling devices. The data is of high\nquality, and the interpretation straightforward. Although somewhat similar results have been observed in WSe2 / CrI3\nheterostructures, this is the first work to my knowledge to report spin-LED operation from a single 2D magnetic layer.\nTherefore, in my opinion, it deserves to be published in Nature Communications. Nevertheless, I have a couple comments\nthe authors may consider:\n1) Do the author's see any effect on the coercive field due to the applied bias voltage? That is, can the device be switched\nelectrically (by changing the bias) if the external field is close to the coercive field?\n2) The authors associate the drop in degree of circular polarization (DOCP) at high bias to heating effects. Yet, the\ntemperature induced phase transition of Ising-type ferromagnetic monolayer CrI3 is rather sharp, while the high-bias drop in\nDOCP is gradual. Perhaps the author's can clarify this discrepancy?\n3) In my opinion, the first two figures could probably be combined into one. These two do not really report anything new (the\ncircularly polarized CrI3 photoluminescence was reported long ago in 2018) compared to the main results of the paper.\nHowever, I recognize that this point is entirely one of personal taste, so I leave it up to the authors to decide.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts."}, "author_response": "Point-by-point Responses to Reviewers’ Comments\nReviewers’ comments are in red; our responses are in blue.\n********************\nReviewer #1 (Remarks to the Author):\n1. This manuscript experimentally demonstrates a spin-polarized electroluminescent diode based\non monolayer and bilayer CrI₃. Compared with conventional GaAs-based emitters, this device\nenables direct control of the emission helicity through the magnetic state of CrI₃, without relying\non external spin injectors. This approach avoids the complex heterostructure engineering\ntypically required to integrate spin injectors and suppress spin decoherence, while achieving a\ndegree of circular polarization (20%) that surpasses most traditional spin-polarized LEDs.\nOverall, the work provides a promising platform for future on-chip spin-optoelectronic devices.\nAfter the following issues are properly addressed, I recommend its publication\nin Nature Communications.\nWe thank the reviewer for the positive assessment of the paper and careful review of our\nmanuscript. We have addressed all the comments below.\n2. The schematic in Fig. 3(c) may be easily confused with a band-structure diagram. If possible,\nthe authors are encouraged to include a corresponding schematic based on band theory or crystal-\nfield theory to clarify the underlying physical mechanism.\nWe appreciate the reviewer’s comment. In the revised manuscript, Fig. 3(c) has been updated to\nillustrate that the electroluminescence originates from transitions between the t₂g and eg orbitals\nof Cr³⁺. The revised figure caption also explicitly states that, due to crystal-field interactions,\nthe d orbitals of Cr³⁺ split into three lower-energy t₂g orbitals (dxy, dyz, dxz) and two higher-energy\neg orbitals (dx²−y², dz²). This clarification helps avoid confusion with a conventional band-\nstructure diagram.\n3. As the injection current increases, the emission peak redshifts, while both the linewidth and\nintensity increase. The authors attribute the redshift to Joule heating, but no direct evidence is\nprovided. Moreover, the physical origin of the increased linewidth and intensity is not\nsufficiently explained. Additional analysis or experimental support is needed.\nWe thank the reviewer for this insightful and substantive comment. We agree that attributing the\nobserved spectral evolution solely to Joule heating is not sufficient. In the revised manuscript, we\nhave included additional experiments and analyses to clarify the physical origins of the observed\nelectroluminescence redshift, linewidth broadening, and intensity enhancement under electrical\ninjection.\nSpecifically, as the injection current increases up to 50 µA, the EL peak exhibits a redshift from\n1.14 to 1.17 µm accompanied by clear linewidth broadening (Fig. 3d–f). Although our finite-\nelement simulations suggest that this current range increases the device temperature from 30 K\nto approximately 36 K (Supplementary Section 3), temperature-dependent photoluminescence\nmeasurements on monolayer CrI₃ indicate that such a temperature rise produce negligible peak\nredshift and linewidth broadening (revised Supplementary Section 4). Thus, as the reviewer\nsuggested, Joule heating alone cannot fully explain the current-induced spectral changes in\nelectroluminescence, and additional mechanisms must contribute. Notably, recent studies show\nthat excitons in CrI₃ are highly localized, with emission described by Frenkel excitons, exciton\npolarons, or self-trapped excitons. While these models differ in mechanism, all predict that\nincreasing exciton population can lead to linewidth broadening and peak redshift, via enhanced\nphonon sidebands in Frenkel excitons or increased lattice distortions in exciton polarons and\nself-trapped excitons. These exciton-related effects could thus contribute to the current-induced\nspectral changes in addition to heating.\nRegarding the current-dependent emission intensity, we observe that the EL intensity increases\nsublinearly with the injection current (Fig. 3f). From a device perspective, this sublinear\nbehavior is commonly seen in tunneling light-emitting diode structures, where a strong vertical\nelectric field at high bias enhances direct tunneling of carriers between the graphene electrodes\n(Ref. 31-33). Consequently, the fraction of carriers undergoing radiative recombination\ndecreases, reducing the contribution to light emission at high injection levels. From the\nperspective of the emitting material, CrI₃, exciton–exciton annihilation under strong injection\nmay further suppress the emission intensity, as recently reported (J. Chem. Phys. 161, 114706\n(2024)), contributing to the observed sublinear scaling. These analyses and the associated\nreferences have been incorporated into the revised manuscript.\n4. The sign reversal in the EL DOCP loop occurs at around ±0.1 T, which is lower than the\ncoercive field (0.17 T) obtained from RMCD and magneto-PL measurements. Is there an\nadditional physical mechanism responsible for this discrepancy? For example, could the applied\nbias modify the magnetic properties of CrI₃? The authors should discuss this point or provide\nsupporting evidence.\nThe sign reversal in the EL DOCP loop occurs at around ±0.1 T, which is lower than the\ncoercive field (0.17 T) obtained from RMCD and magneto-PL measurements. In original\nmanuscript, we attribute this difference as RMCD and PL are sensitive to local magnetization at\na focused spot, whereas EL integrates emission across the entire CrI₃ layer, effectively averaging\nover possible domain variations. We agree with the reviewer, however, that the applied bias can\nalso play an important role. Specifically, increasing the bias voltage not only enhances the\ncarrier injection rate but also raises the chemical potential of the graphene electrodes, enabling\ncarriers to populate higher-energy orbitals in CrI₃, including the Cr d and d orbitals as well\nz² x²−y²\nas the I p and p orbitals. As suggested by recent literature (2D Mater. 12, 045025 (2025)),\nx y\noccupation of these orbitals can introduce antiferromagnetic exchange interactions, which\nweaken the overall ferromagnetic order in CrI₃.\nNotably, this bias-induced weakening of ferromagnetic order provides an additional explanation\nfor why the EL DOCP sign reversal occurs at a lower magnetic field than that measured by\nRMCD and magneto-PL. To further support this interpretation, we measured EL DOCP loops at\ndifferent bias voltages (added in Supplementary Section 7) and observed that the EL DOCP\nreverses sign at even lower magnetic fields as the bias voltage increases. The corresponding\nexperimental results, discussion, and references have been incorporated into the revised\nmanuscript.\n5. The DOCP decreases with increasing current. The authors attribute this to the weakening of\nCrI₃ magnetic order due to Joule heating, supported by simulations showing that the material\ntemperature rises from 30 K to 36 K. However, this temperature is still far below the Curie\ntemperature (45 K), making this explanation less convincing. Carrier injection can change the\nband occupation of CrI₃ and may intrinsically affect its magnetism; the authors are encouraged to\nconsider and discuss this possibility.\nWe agree with the reviewer that raising the injection current to 50 µA increases the device\ntemperature from 30 K to ~36 K, slightly weakening the ferromagnetic order. However, this is\nstill below the Curie temperature of monolayer CrI₃ (~45 K), so the material remains\nferromagnetic. Thus, the rapid DOCP drop from 20% at 0.5 µA to 2.5% at 50 µA cannot be\nexplained by Joule heating alone.\nWe further agree that carrier injection under high bias can intrinsically modify the magnetic\nproperties of CrI₃. As discussed in our response to point #4 and supported by recent theoretical\nwork (2D Mater. 12, 045025 (2025)), the elevated chemical potential of the graphene electrodes\nenables carriers to populate higher-energy Cr d and I p orbitals. This altered band occupation\nenhances antiferromagnetic exchange interactions, thereby weakening the ferromagnetic order\nand contributing to the observed decrease in DOCP with increasing current (Fig. 4d). The\ncorresponding discussion has been added to the revised manuscript.\n6. Numerous studies have shown that bilayer CrI₃ undergoes an antiferromagnetic-to-\nferromagnetic phase transition under applied bias. Since the present work involves bilayer CrI₃,\nthe authors should discuss this effect and its potential implications for their experimental\nobservations.\nWe agree with the reviewer that numerous studies have reported an antiferromagnetic-to-\nferromagnetic phase transition in bilayer CrI₃ induced by electrostatic gating that tunes the\ndoping level in CrI₃. In the literature, a common approach employs\ngraphene/hBN/CrI₃/hBN/graphene heterostructures, in which the two graphene layers serve as\ntop and bottom gates to electrostatically tune the doping in CrI₃. Although this device\narchitecture is similar to that used in our spin-polarized emitters, the hBN layers in our device\nare only two layers thick and are introduced to enable efficient carrier tunneling rather than to\nfunction as gate dielectrics. Consequently, the graphene electrodes in our device do not provide\neffective electrostatic gating of CrI₃, and the structure cannot be regarded as a capacitive gating\ngeometry. An alternative route for electrostatic gating could, in principle, be realized via the\nsilicon back gate. In our device, however, the gate electric field is strongly screened by the\ngraphene bottom electrode as well as by the large density of carriers injected into CrI₃ under\nbias. As a result, sweeping the silicon back-gate voltage does not produce observable changes in\nthe EL DOCP hysteresis loop.\nWe note, however, that replacing SiO₂ with a high-κ dielectric could enhance the gating\nefficiency and potentially enable the antiferromagnetic-to-ferromagnetic transition, thereby\nproviding a viable route for actively tuning the DOCP in spin-polarized light-emitting diodes.\nThis perspective has been included in the revised conclusion section.\nAdditional Note: During the manuscript revision, we noticed that the y-axis in Fig. 3a (I–V\ncharacteristics) was mislabelled. An applied bias of ±3 V corresponds to a current injection of\n±50 µA, rather than ±5 µA as shown in the original manuscript. This has been corrected in the\nrevised version, and we apologize for any confusion this may have caused.\nReviewer #2 (Remarks to the Author):\n7. The manuscript reports electrically driven, magnetically switchable circularly polarized\nelectroluminescence from monolayer CrI₃. This is, to my knowledge, the first demonstration of a\nspin-LED using monolayer CrI₃ as the intrinsic emitter without relying on additional spin-\npolarizing TMD layers, and the ability to generate EL from monolayer CrI₃ at all is itself a\ntechnically nontrivial and interesting result. The integration of CrI₃ with graphene/hBN tunneling\ncontacts is executed cleanly, and the optical–magnetic coupling they observe is consistent with\nthe expected magnetic order of CrI₃. These aspects make the work a meaningful addition to the\ngrowing effort of vdW-magnet–based optoelectronics.\nWe thank the reviewer for the positive assessment of the paper and careful review of our\nmanuscript.\n8. At the same time, the manuscript appears to substantially overstate both the conceptual\nnovelty and the performance significance of the device. While the authors emphasize achieving\n~20% DOCP and frame this as addressing the “limited DOCP” in prior spin-LEDs, the paper\ndoes not explain why monolayer CrI₃ should fundamentally overcome those limitations that\naffect other spin-LED architectures. No conceptual mechanism is articulated. As a result, the\nmotivational narrative lacks the scientific grounding needed to justify the authors’ claims about\nbroader impact. Without a mechanistic understanding, the work reads primarily as a materials\ndemonstration rather than a solution to the DOCP bottleneck the authors highlight. In its current\nform, I do not feel it meets the level of conceptual clarity and novelty required for publication\nin Nature Communications.\nWe thank the reviewer for this important comment. We agree that the achieved DOCP of ~20%\nis not, by itself, exceptionally high compared with conventional III–V/II–VI spin-LEDs, and that\nsome van der Waals spin-LEDs have reported DOCP values approaching ~40%. Accordingly,\nwe have removed the statement claiming that our work addresses the “limited DOCP” in prior\nspin-LED architectures.\nWe clarify that the novelty of our device does not lie in the absolute magnitude of the DOCP, but\nin the physical mechanism by which circularly polarized electroluminescence is generated. In\nour device, the DOCP originates from the intrinsic properties of the emitting layer, monolayer\nCrI₃. Specifically, the strong coupling between excitons and the magnetic order in CrI₃ leads to\nspin-dependent splitting of the ground-state excitons (Adv. Mater. 35, e2209513 (2023)). This\nintrinsic magneto-excitonic coupling directly produces circular polarization in the emitted light.\nAs a result, circularly polarized electroluminescence is achieved without the use of an external\nspin injector, which is typically required in conventional spin-LEDs and imposes stringent\ndemands on spin-polarized carrier injection and spin coherence preservation during transport.\nWe further emphasize that the reported ~20% EL DOCP should be regarded as an initial\ndemonstration of this mechanism. Its potential improvement is discussed in detail in the point-to-\npoint response to Comment #9, and the characteristics of excitonic emission and the underlying\nmechanisms in CrI₃-based devices are further elaborated in the point-to-point response to\nComment #11.\n9. This issue is especially problematic because a directly relevant prior work, Dang et al., Nat\nCommun 15, 6799 (2024), already reports 26–40% DOCP in CrI₃/TMD spin-LEDs, substantially\nhigher than the ~20% reported here. The absence of any discussion of this work creates a\nmisleading impression that 20% DOCP represents a breakthrough in vdW spin-LEDs, when in\nfact the state of the art already exceeds it. Moreover, the authors repeatedly compare their results\nonly to traditional III–V and II–VI spin-LEDs, omitting the most relevant vdW-magnet–based\ndevices, including linearly polarized EL in NiPS₃ (Lebedev et al., Nat Commun 16, 10550\n(2025)) and electrically controlled polarization in bilayer CrI₃/TMD devices. A fair, complete\ncomparison is essential for evaluating the manuscript’s contributions. I strongly encourage the\nauthors to discuss their results holistically within the existing body of vdW-based spin-LEDs and\nclearly articulate what novelty remains. For example: does monolayer CrI₃ emission avoid the\nintervalley depolarization that limits the DOCP in CrI₃/TMD devices? If so, what is the physical\nmechanism?\nWe appreciate the reviewer’s comment and agree that a fair and comprehensive comparison\nwith prior van der Waals spin-LEDs is essential. In the revised Introduction, we have expanded\nthe discussion of earlier studies using two-dimensional magnetic materials as spin injectors to\ngenerate spin-polarized carrier injection into TMDs and realize fully vdW spin-LED\narchitectures. This can be realized by using Fe₃GeTe₂ as a spin source, yielding degree of\ncircular polarization (DOCP) ~8% at 78 K, with nonvolatile DOCP sign reversal under a low\nmagnetic field (~0.1 T). Alternatively, trilayer CrI₃ can function as a spin-filter barrier,\nproducing EL from TMDs with a DOCP of up to ~40%, achieved at 4 K under a 2 T magnetic\nfield, which ensures CrI₃ converts from an antiferromagnetic to a ferromagnetic state and\nenables efficient spin filtering (Dang et al., Nat. Commun. 15, 6799 (2024)).\nIn addition, our revision emphasizes that prior vdW-magnet–based and conventional III–V/II–VI\nspin-LEDs rely on an external spin injector, which requires efficient spin-polarized carrier\ninjection, preservation of spin coherence during transport, and spin-conserving radiative\nrecombination in the emitter (a process affected by intervalley depolarization in TMDs). To\novercome these limitations, an attractive strategy is to employ 2D magnets directly as the light-\nemitting medium, exploiting the intrinsic coupling between excitons and the magnetic order to\nproduce polarized emission. We explicit mention that this concept has recently been validated in\nNiPS₃, where electroluminescence exhibits an ultranarrow linewidth (~1 meV) and a high degree\nof linear polarization, arising from excitons coupled to the underlying antiferromagnetic order\n(Lebedev et al., Nat. Commun. 16, 10550 (2025)). We then highlight that, building on this\nparadigm, our work uses CrI₃ itself as the emitter, leveraging exciton–magnetization coupling to\nproduce circularly polarized electroluminescence without the need for an external spin injector.\nWe believe this revision clarifies the distinctions and highlights the novelty of our work within\nthe context of vdW-based spin-LEDs.\nIn addition to revising the Introduction, we have updated the Conclusion to discuss potential\nstrategies for increasing DOCP in our device. One approach is lowering the device temperature,\nwhich enhances CrI₃ magnetization and reduces thermally populated carriers between exciton\nstates split by the magnetic order (Adv. Mater. 35, e2209513 (2023)), thereby increasing DOCP.\nAnother strategy is to increase CrI₃ magnetization through p-type doping, which has been\ndemonstrated experimentally and theoretically (2D Mater. 12, 045025 (2025) and Nat.\nNanotech. 13, 549–553 (2018)).\nFinally, we emphasize that although Dang et al. (Nat. Commun. 15, 6799 (2024)) report EL\nDOCP values up to ~40%, there are several key differences with our work. First, the ~20% PL\nand EL DOCP reported here were measured at 30 K—higher than the 4 K used in Dang et al.—\nand are limited by our current setup. In fact, prior study shows CrI₃ can reach PL DOCP above\n50% at 15 K (Nat. Phys. 14, 277–281 (2018)), with even higher values expected at lower\ntemperatures. Second, the device-to-device variation of EL DOCP in our work is relatively small\ncompared with previous reports, reflecting that the DOCP in our devices is solely determined by\nmagneto-excitonic coupling rather than by the more complex spin injection, transport, or\nrecombination processes. Third, the ~40% DOCP reported in Dang et al. was obtained under an\n~2 T magnetic field. The high field is required to convert the layered antiferromagnetic CrI₃ into\na ferromagnetic state for efficient spin filtering. In contrast, in our devices, EL DOCP can be\nswitched with only 0.1 T, and due to the non-volatile nature of the switching, ~20% DOCP can\nbe sustained even in the absence of an applied magnetic field, facilitating practical device\noperation.\n10. The experimental support for the core claims is also limited. The key DOCP value is shown\nfor only one monolayer device (and one bilayer device in the SI). Prior studies, including those\nthe authors cite, have documented significant device-to-device variability. Without multiple\ndevices or at least a clear demonstration of reproducibility, it is difficult to evaluate how\nrepresentative the reported 20% polarization actually is. For a claim centered on “high DOCP,”\nthis lack of statistical confirmation is a notable weakness. Given that the previous investigation\ninto CrI3/TMD devices showed considerable variation in the DOCP of different devices (see\nDang et al. Supplementary Table 1), it would also be beneficial for the authors to include\nadditional data from other supporting monolayer or bilayer devices to establish that the reported\nDOCP ~ 20% is reproducible across multiple samples rather than a single exceptional case.\nWe agree with the reviewer that demonstrating device-to-device reproducibility is important for\nassessing the representativeness of the reported DOCP. To address this concern, we fabricated\nand characterized five independent monolayer CrI₃ devices, all measured under identical\noperating conditions at 30 K. All devices exhibit clear circularly polarized electroluminescence,\nwith DOCP values consistently ranging from ~18% to ~23%, indicating good device-to-device\nreproducibility rather than an isolated exceptional case. The corresponding data and discussion\nhave been added to the revised manuscript and the Supplementary Section 6.\n11. Finally, because EL from monolayer CrI₃ has not been previously established, a deeper\ndiscussion of the emission mechanism, possible defect-assisted recombination, and device\nstability under bias would significantly strengthen the manuscript. As written, these aspects are\ntreated only briefly, leaving the interpretation of the EL behavior incomplete.\nWe appreciate the reviewer’s comment and agree that a more in-depth discussion of the emission\nmechanism is necessary. In the revised manuscript, we have updated Fig. 3c to schematically\nillustrate that the emission originates from intra-atomic transitions between the t and e\n2g g\norbitals of Cr³⁺. The revised figure caption explicitly states that, due to crystal-field interactions,\nthe Cr³⁺ d orbitals split into three lower-energy t orbitals (d , d , d ) and two higher-energy e\n2g xy yz xz g\norbitals (d , d ).\nx²−y² z²\nIn the revised main text, we further discuss that these intra-atomic d–d transitions are highly\nlocalized, and their emission may exhibit characteristics associated with exciton polarons,\nFrenkel excitons, or self-trapped excitons, as suggested by recent studies (Adv. Mater. 35,\ne2209513 (2023), Nano Lett. 22, 8755–8762 (2022) and Phys. Rev. Lett. 133, 046903 (2024)).\nWhile these models differ in mechanism, all predict that increasing exciton population can lead\nto linewidth broadening and peak redshift, via enhanced phonon sidebands in Frenkel excitons\nor increased lattice distortions in exciton polarons and self-trapped excitons. These exciton-\nrelated effects could thus contribute to the current-induced EL spectral changes (Fig. 3e-f) in\naddition to heating. The corresponding discussion and references have been included in the\nrevised manuscript.\nBeyond the spectral evolution, we also expand the discussion of the current dependence of the\nEL intensity (Fig. 3f,lower panel). In the original manuscript, this behavior was attributed to\nenhanced carrier tunneling between the graphene electrodes under strong vertical electric fields\nat high bias, which reduces the fraction of injected carriers contributing to radiative\nrecombination. In the revised manuscript, we further note that exciton–exciton annihilation in\nthe emitting CrI₃ layer may also occur under intense carrier injection, as suggested by recent\nstudies (J. Chem. Phys. 161, 114706 (2024)), providing an additional nonradiative decay\nchannel. Taken together, these effects can account for the observed sublinear scaling of the EL\nintensity with injection current.\nExtending this discussion, we also discuss how increasing carrier injection can affect the\nmagnetization of CrI₃ and thus EL DOCP (Fig. 4d) in the revised manuscript. Specifically, we\nelaborate that increasing the bias voltage not only enhances the carrier injection rate, which can\nlead to heating effects, but also raises the chemical potential of the graphene electrodes,\nallowing higher-energy states in CrI₃ to be populated by injected carriers, including the Cr d\nz²\nand d orbitals as well as the I p and p orbitals. As suggested by recent theoretical work (2D\nx²−y² x y\nMater. 12, 045025 (2025)), occupation of these states can introduce antiferromagnetic exchange\ninteractions that weaken the overall ferromagnetic order of CrI₃. Given that the exciton splitting\nin CrI₃ is closely tied to its magnetization, these effects provide a plausible explanation for the\nobserved decrease in EL DOCP with increasing injection current (Fig. 4d).\nRegarding device stability, we include Supplementary Section 6 to show that both EL intensity\nand DOCP remain stable after 48 hours of continuous operation. While longer-term evaluation\nwill be important for practical device applications, these results demonstrate the preliminary\nrobustness of our devices.\nAdditional Note: During the manuscript revision, we noticed that the y-axis in Fig. 3a (I–V\ncharacteristics) was mislabelled. An applied bias of ±3 V corresponds to a current injection of\n±50 µA, rather than ±5 µA as shown in the original manuscript. This has been corrected in the\nrevised version, and we apologize for any confusion this may have caused.\nReviewer #3 (Remarks to the Author):\n12. I have read the manuscript \"Spin-polarized light-emitting diodes based on CrI₃ operating\nwithout external spin injection\" by C.-C. Lu, et al. This work reports circularly polarized\nelectroluminescence from CrI3 tunneling devices. The data is of high quality, and the\ninterpretation straightforward. Although somewhat similar results have been observed in WSe2 /\nCrI3 heterostructures, this is the first work to my knowledge to report spin-LED operation from a\nsingle 2D magnetic layer. Therefore, in my opinion, it deserves to be published\nin Nature Communications. Nevertheless, I have a couple comments the authors may consider:\nWe thank the reviewer for the positive assessment of the paper and careful review of our\nmanuscript.\n13. Do the author's see any effect on the coercive field due to the applied bias voltage? That is,\ncan the device be switched electrically (by changing the bias) if the external field is close to the\ncoercive field?\nWe thank the reviewer for this insightful question. By measuring the EL DOCP hysteresis loops\nunder different applied bias voltages, we indeed observe that the EL DOCP switches sign at a\nlower magnetic field as the bias voltage on the graphene electrodes increases (added in\nSupplementary Section 7). This behavior indicates that increasing the applied bias reduces the\ncoercive field of CrI₃. Consequently, when the external magnetic field is tuned close to the\ncoercive field, modulation of the applied bias can indeed switch the sign of the EL DOCP.\nHowever, higher bias also increases the EL intensity while reducing the amplitude of the DOCP,\nwhich may limit the practical utility of bias-controlled DOCP switching.\nIn addition to this experimental demonstration, we discuss the physical origin in the revised\nmanuscript. Specifically, the bias-induced reduction in coercivity can be attributed to the\nelevated chemical potential of the graphene electrodes under high bias, which allows injected\ncarriers to populate higher-energy orbitals in CrI₃, including the Cr dz² and dx²−y² orbitals as\nwell as the I px and py orbitals. As suggested by recent theoretical studies (2D Mater. 12,\n045025 (2025)), occupation of these states can introduce antiferromagnetic exchange\ninteractions, weakening the overall ferromagnetic order. This weakening provides a physical\nbasis for the bias-dependent reduction in coercivity and also links to the observed EL DOCP\nreduction at higher bias (relevant to point-to-point response #14).\nIn addition to varying the bias across the graphene electrodes, we also investigated EL DOCP\nhysteresis loops under different silicon back-gate voltages but did not observe any discernible\nchanges. This lack of gate dependence can be attributed to strong electrostatic screening by the\nbottom graphene electrode and the high carrier density injected from the graphene electrodes,\nwhich limits the effectiveness of back-gate modulation of coercivity.\n14. The authors associate the drop in degree of circular polarization (DOCP) at high bias to\nheating effects. Yet, the temperature induced phase transition of Ising-type ferromagnetic\nmonolayer CrI3 is rather sharp, while the high-bias drop in DOCP is gradual. Perhaps the\nauthor's can clarify this discrepancy?\nWe thank the reviewer for pointing this out. Indeed, as the reviewer notes, the measured DOCP\ngradually decreases from 20% to 2.5% as the injection current increases up to 50 μA (Fig. 4d).\nThis gradual behaviour is inconsistent with the typical temperature dependence of CrI₃\nmagnetization, which shows a sharp drop near the Curie temperature (~45 K). It is also notable\nthat our finite-element simulations indicate the applied current raises the device temperature\nonly from 30 K to ~36 K, well below the Curie temperature, suggesting that Joule heating alone\ncannot fully account for the observed reduction in DOCP.\nIn the revised manuscript, we attribute this discrepancy to an additional mechanism closely\nrelated to the bias-induced effects described in response #13. Specifically, the elevated chemical\npotential of the graphene electrodes under high bias allows carriers to populate higher-energy\nCr d and I p orbitals, enhancing antiferromagnetic exchange interactions. This effect weakens\nthe overall ferromagnetic order in CrI₃. Together with modest current-induced heating, these\neffects plausibly explain the gradual decrease in EL DOCP with increasing injection current.\nThe revised manuscript now incorporates this explanation along with supporting references.\n15. In my opinion, the first two figures could probably be combined into one. These two do not\nreally report anything new (the circularly polarized CrI3 photoluminescence was reported long\nago in 2018) compared to the main results of the paper. However, I recognize that this point is\nentirely one of personal taste, so I leave it up to the authors to decide.\nWe thank the reviewer for this suggestion. We agree that Fig. 2 presents results similar to those\nalready reported in the literature. However, combining it with Fig. 1 into a single figure would\nrequire reducing the figure size to fit the journal’s format, which could compromise clarity and\nreadability. For this reason, we have kept them as separate figures.\nAdditional Note: During the manuscript revision, we noticed that the y-axis in Fig. 3a (I–V\ncharacteristics) was mislabelled. An applied bias of ±3 V corresponds to a current injection of\n±50 µA, rather than ±5 µA as shown in the original manuscript. This has been corrected in the\nrevised version, and we apologize for any confusion this may have caused.\nReviewer #4 (Remarks to the Author):\n16. I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This\nis part of the Nature Communications initiative to facilitate training in peer review and to\nprovide appropriate recognition for Early Career Researchers who co-review manuscripts.\nWe thank the reviewer for co-reviewing this manuscript and for the valuable input."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have satisfactorily addressed all of my questions and concerns. I\ntherefore recommend the manuscript for publication.", "reviewer_2": "The authors have substantially revised the manuscript in response to my previous report. In particular, they have moderated\nseveral of the claims regarding device performance and clarified that the primary novelty lies in demonstrating circularly\npolarized electroluminescence originating from the intrinsic magneto-exciton coupling of monolayer CrI₃, rather than in\nachieving a record degree of circular polarization. The revised manuscript now more clearly positions the work as an\ninjector-free spin-LED architecture, where circular polarization arises from the magnetic order of the emitting layer itself.\nThis clarification addresses a central concern raised in my previous review report.\nThe additional discussion regarding current-induced spectral changes and the possible influence of carrier injection on the\nmagnetic properties of CrI₃ is also helpful. While some of these interpretations remain somewhat speculative, such as the\nbias-induced modification of exchange interactions, the authors now acknowledge these limitations and present them more\ncautiously.\nOverall, the manuscript presents a technically solid experimental demonstration of electrically driven circularly polarized\nemission from monolayer CrI₃ heterostructures with graphene tunneling contacts. Although the achieved DOCP (~20%) is\nnot exceptional compared to other vdW spin-LED platforms, the injector-free architecture represents an interesting design\nconcept for spin-optoelectronic devices based on 2D magnets.\nI therefore recommend publication after possible, but not mandatory, minor revisions. In particular, the authors may consider\nfurther clarifying the microscopic mechanism linking magnetic order to EL helicity, and discussing whether the\nrecombination process under electrical injection is expected to preserve the same polarization behavior observed in\nphotoluminescence.", "reviewer_3": "In my opinion, the authors suitably addressed my concerns. In particular, I appreciate that they made five additional\nmonolayer CrI3 devices - these are challenging devices to make and I commend their effort.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts."}, "author_response": ""}]

Decision letter