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

Spin-polarized light-emitting diodes based on CrI₃ operating without external spin injection | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Spin-polarized light-emitting diodes based on CrI₃ operating without external spin injection Chang-Hua Liu, Chung-Chun Lu, Li-Wei Chang, Wei-Qing Li, Po-Liang Chen, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8061031/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Spin-polarized light-emitting diodes (spin-LEDs) convert electron spin into circularly polarized light, enabling direct optical readout of spin information and opening new avenues for solid-state, on-chip information processing and cryptography. To achieve such applications, considerable research has been devoted to GaAs-based emitters integrated with spin injectors including ferromagnetic metals, dilute magnetic semiconductors, and spin-filter tunnel barriers. These conventional spin-LEDs, however, require complex epitaxial growth and their limited integrability remains a critical challenge. Additionally, achieving high circular polarization is inherently difficult, as it requires high-quality materials and interfaces to ensure efficient spin injection, coherent spin transport, and spin-conserving radiative recombination. Here, we report an alternative approach to realize spin-LEDs by employing the monolayer CrI₃ as the light-emitting layer, sandwiched between two graphene/ hexagonal boron nitride tunneling contacts. Although the exploited contacts inject unpolarized carriers into CrI₃, we show that the resulting electroluminescence could be circularly polarized, with its helicity governed by the magnetic order of CrI₃, as confirmed by helicity-resolved EL measurements and magneto-optical analysis. Notably, the EL degree of polarization reaches 20%, outperforming most conventional spin-LEDs, and its helicity can be readily reversed with a low magnetic field (~ 0.17 T). Combined with the inherent integrability of our proposed heterostructures, this approach provides a promising platform for future on-chip spin-optoelectronic devices. Physical sciences/Nanoscience and technology/Nanoscale devices/Magnetic devices Physical sciences/Engineering/Electrical and electronic engineering Figures Figure 1 Figure 2 Figure 3 Figure 4 Main text 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 switches 1 , 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 helicity 1 – 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 BeₓMn γ Zn₁₋ₓ₋ γ Se, or tunneling spin filters such as MgO to serve as spin injectors 1 , 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 issues 9 – 11 , opening new opportunities for spin-LED development. For instance, monolayer transition metal dichalcogenides (TMDs), with their unique spin–valley properties 12 – 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 architectures 15 – 18 . More recently, 2D magnets have been demonstrated to act as spin injectors 19 – 22 , and their combination with TMDs has unlocked fully van der Waals (vdW) spin-LEDs 19 , 20 . Despite these advances, spin-LEDs developed to date mostly exhibit a limited degree of circular polarization (DOCP) 1 – 3 . To achieve higher polarization, spin-polarized carriers must be efficiently injected, maintain spin coherence during transport, and undergo spin-conserving radiative recombination; however, all three processes are highly susceptible to material defects and interfacial properties, making their optimization a nontrivial task 1 – 3 . To move beyond the above-mentioned limitations, an attractive strategy is to employ the emerging magnetic 2D semiconductor CrI₃ as the emitting layer. Identified as the first 2D ferromagnet, CrI₃ has been extensively investigated, revealing that monolayer samples possess a Curie temperature of ~ 45 K and an out-of-plane easy axis of magnetization. In few-layer form, CrI₃ adopts a layered antiferromagnetic ground state, where adjacent layers have opposite spin orientations (i.e., type-A antiferromagnetism) 21 – 23 . 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 magnetization 24 , 25 . Provided that electroluminescence (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 \(\:{\sigma\:}^{-}\) ( \(\:{\sigma\:}^{+}\) ) 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. Device structure and characterization of 2D CrI Figure 1 a,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 electrode 26 , 27 , 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 voltage 28 – 30 . The emissive capability of CrI₃ is verified by PL measurement (Fig. 1 c), 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 emission 24 . 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. 1 d, 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 behaviour 5 , 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 2 a 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. 2 b). 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. 2 c-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. 1 d). 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 3 a shows the I–V b characteristics measured by applying a bias ( V b ) 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. 3 b). 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. 3 c) 31 . 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. 3 d–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. 3 f, lower panel), and the emission peak exhibits a slight redshift from 1.14 µm to 1.17 µm (Fig. 3 f, upper panel), accompanied by a noticeable broadening of the EL linewidth (Fig. 3 d–e). These observations suggest that at high bias, the strong vertical electric field enhances direct carrier tunneling between the graphene electrodes, thereby reducing the probability of radiative recombination in CrI₃, as observed in other vdW heterostructure tunneling emitters 28 – 30 , while the modest redshift is a signature of Joule heating under elevated current. 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. 4 a, 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. 4 b). These results demonstrate that this proof-of-concept vdW spin-LED can achieve a high EL DOCP, exceeding most conventional spin-LEDs based on III–V and II–VI epitaxial materials (see Supplementary Section 3 for a comparison table) 1 , 2 . More notably, achieving high EL polarization without the need for spin injection or filtering highlights the promise of 2D CrI₃ for realizing simpler spin-LED architectures. 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. 4 c, 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. 1 d and also see Supplementary Section 1). This discrepancy likely arises from the different probing characteristics of the measurements — RMCD and PL are sensitive to local magnetization at a focused spot 21 , whereas EL integrates emission across the entire CrI₃ layer, effectively averaging over possible domain variations 21 . Moreover, the EL DOCP remains at about − 19.5% (19.5%) when the magnetic field is reduced from 0.6 T to 0 (-0.6 T to 0), showing non-volatile behaviour. At zero magnetic field, however, we observe that the absolute value of the DOCP gradually decreases from 19.5% to 2.5% as the injection current is increased from 0.5 µA to 50 µA, suggesting that Joule heating weakens the magnetic order. This trend is supported by our finite-element simulations, which show that a current of 50 µA raises the device temperature from its base of 30 K to 36 K (see Supplementary Section 4 for simulation results), close to the Curie temperature of CrI₃, where remanent magnetization and EL helicity are significantly suppressed. In addition to monolayer CrI₃ devices, the dependence of EL polarization on the magnetic state of CrI₃ is further evidenced 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 5 for magneto-EL from a spin-LED based on bilayer CrI₃), directly reflecting the layered antiferromagnetic ground state of bilayer CrI₃ (↑↓ or ↓↑) 21–23 , 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. Conclusions In summary, we demonstrate a new approach to realizing spin-LEDs by employing a 2D magnet, CrI₃, as the light-emitting layer. This design circumvents the integration of spin injectors and the intricate heterostructure engineering required to suppress spin decoherence, both of which remain major challenges for most spin-LEDs developed to date 1 , 2 . Building on this framework, we anticipate that higher degrees of polarization can be achieved by engineering CrI₃ through methods such as doping or applying mechanical strain, both of which have been shown to enhance its remanent magnetization 23 , 32 , 33 . In addition, the approach can be extended to other emerging 2D magnets with different bandgaps, such as CrBr₃, whose luminescence is similarly coupled to magnetic order as in CrI₃, thereby enabling spin-LEDs operating at different emission wavelengths 34 – 36 . Beyond these device-level advantages, the vdW nature of our proposed spin-LEDs allows seamless integration with a variety of photonic platforms 9 , 37 , supporting on-chip architectures where helicity control can be harnessed for advanced functionalities in optical communication, information processing, and quantum photonics 2 , 3 . 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 procedures 21 . The vdW heterostructures were assembled by sequentially stacking the selected flakes through a dry transfer process 10 . 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 helicities, following procedures described in previous studies 19 , 21 , 38 . 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. Declarations Competing Interests: The authors declare no competing financial interests. Author Contributions: C.-H.L. conceived the experiments and supervised the project. C.-C.L. and L.-W.C. fabricated the vdW heterostructures, assisted by P.-L.C. 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07:37:40","extension":"png","order_by":11,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":34842,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8061031/v1/75cae71cc04f9926c99c8d09.png"},{"id":96249387,"identity":"b778b2c3-6256-453f-9836-f951aa557431","added_by":"auto","created_at":"2025-11-19 07:33:20","extension":"xml","order_by":12,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":78351,"visible":true,"origin":"","legend":"","description":"","filename":"NCOMMS25900770structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8061031/v1/e5dc834edf5a52be23cc22a4.xml"},{"id":96142141,"identity":"640aa502-b126-48aa-96df-47a4ffe0fc7f","added_by":"auto","created_at":"2025-11-18 05:39:58","extension":"html","order_by":13,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":84881,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8061031/v1/d969ceb1e65e40ffde875ecf.html"},{"id":96142127,"identity":"3b0c116f-8284-41ac-91b2-3bee7600714e","added_by":"auto","created_at":"2025-11-18 05:39:58","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":388249,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDevice structure of the vdW heterostructure spin-LED and characterization of the CrI₃ light-emitting layer.\u003c/strong\u003e a. Top-view schematic of the monolayer CrI₃ crystal structure and device configuration, showing vertically stacked graphene/hBN/CrI₃/hBN/graphene heterostructures. b. Optical microscope image of the vdW heterostructure spin-LED. The regions of top graphene (Gr\u003csub\u003eT\u003c/sub\u003e), bottom graphene (Gr\u003csub\u003eB\u003c/sub\u003e) and monolayer CrI\u003csub\u003e3\u003c/sub\u003e are defined by blue, red and white dashed lines, respectively. Scale bar, 10 µm.\u0026nbsp; c. PL spectrum measured from the used CrI₃ flake, excited with a linearly polarized 633 nm laser at 10 μW. d. RMCD as a function of applied magnetic field on the used CrI\u003csub\u003e3\u003c/sub\u003e flake. Red (Black) data points denote the RMCD value when sweeping the magnetic field in the negative (positive) direction.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8061031/v1/8e6a7f0fc8d731b3f693c69f.png"},{"id":96251234,"identity":"5dc57276-d682-458e-81d3-30b4a765da5a","added_by":"auto","created_at":"2025-11-19 07:39:32","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":189748,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMagneto-PL of monolayer CrI₃.\u003c/strong\u003e a–d. Polarization-resolved PL spectra for σ⁻ (black) and σ⁺ (red) detection, recorded sequentially at the following magnetic field stages: (a) after ramping from 0 to 0.6 T; (b) after ramping down from 0.6 T to 0; (c) after ramping from 0 to −0.6 T; (d) after ramping up from −0.6 T back to 0. All spectra were excited with a linearly polarized 633 nm laser at 10 μW. In panels (a)–(d), positive and negative magnetic fields are defined as pointing outward from and inward toward the surface of monolayer CrI\u003csub\u003e3\u003c/sub\u003e, respectively. The blue arrows indicate the magnetic field sweep direction.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8061031/v1/0d40f475bc37065d7a946c34.png"},{"id":96248531,"identity":"73956c2e-7c51-41d4-b388-5821f72ddfc8","added_by":"auto","created_at":"2025-11-19 07:28:35","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":177242,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElectrical properties, energy band diagram, and EL characteristics of vdW heterostructures.\u003c/strong\u003e a. \u003cem\u003eI–V\u003c/em\u003e\u003csub\u003e\u003cem\u003eb\u003c/em\u003e\u003c/sub\u003e characteristics of the light-emitting heterostructures measured under a bias applied across the graphene electrodes, with the bottom electrode grounded. b–c. Schematic energy band diagrams of the vdW heterostructures under (b) zero bias (𝑉\u003csub\u003e𝑏 \u003c/sub\u003e= 0) and (c) positive bias (𝑉\u003csub\u003e𝑏\u003c/sub\u003e \u0026gt; 0), showing electrons and holes tunneling from the graphene electrodes into CrI₃ (blue and red arrows), where their recombination (gray arrow) generates EL. d. EL intensity map as a function of injected current. e. Line cuts extracted from panel (d), showing representative EL spectra at different levels of injected current. The spectra exhibit several dips near 1370 nm, originating from H₂O and CH₄ absorption. f. EL peak wavelength (upper panel) and peak intensity (lower panel) versus injected current.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8061031/v1/c97728b1b7615ede595d6af9.png"},{"id":96142131,"identity":"cc7fa07e-5e4d-493e-b1ca-03571386ce08","added_by":"auto","created_at":"2025-11-18 05:39:58","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":166983,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eEvidence of spin-LED operation in graphene/hBN/CrI₃/hBN/graphene heterostructures.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ea-b. Polarization-resolved EL spectra for σ⁻ (black) and σ⁺ (red) detection, with the magnetic field at (a) 0.6 T and (b) -0.6 T, respectively. c. Change of the degree of EL polarization as a function of applied magnetic field. Red (black) data points denote the EL polarization measured when sweeping the magnetic field in the negative (positive) direction. In panels (a)–(c), positive and negative magnetic fields are defined as pointing outward from and inward toward the surface of the heterostructures, respectively, and the spin-LED was measured under an injected current of 0.5 µA. d. Current-dependent change in the EL DOCP, measured as the injected current was increased from 0.5 µA to 50 µA. The magnetic field was swept from –0.6 T to 0 T and then fixed at 0 T prior to the current-dependent measurement.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8061031/v1/8ad0b573a3788a17fa897664.png"},{"id":96256789,"identity":"9ce69a71-e94c-4727-9727-b531742545aa","added_by":"auto","created_at":"2025-11-19 07:50:34","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1500431,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8061031/v1/f7932e86-daca-4c70-80a4-057334432673.pdf"},{"id":96251183,"identity":"27e3393a-b7e5-4ddb-92e7-3161dac8f51c","added_by":"auto","created_at":"2025-11-19 07:39:28","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":682180,"visible":true,"origin":"","legend":"Supplementary Information","description":"","filename":"SI.docx","url":"https://assets-eu.researchsquare.com/files/rs-8061031/v1/09abc07b610ae938befc12e9.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Spin-polarized light-emitting diodes based on CrI₃ operating without external spin injection","fulltext":[{"header":"Main text","content":"\u003cp\u003eSolid-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 switches\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. 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 helicity\u003csup\u003e\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. 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 BeₓMn\u003csub\u003eγ\u003c/sub\u003eZn₁₋ₓ₋\u003csub\u003eγ\u003c/sub\u003eSe, or tunneling spin filters such as MgO to serve as spin injectors\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan additionalcitationids=\"CR5 CR6 CR7\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. 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 issues\u003csup\u003e\u003cspan additionalcitationids=\"CR10\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e, opening new opportunities for spin-LED development. For instance, monolayer transition metal dichalcogenides (TMDs), with their unique spin\u0026ndash;valley properties\u003csup\u003e\u003cspan additionalcitationids=\"CR13\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e, 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 architectures\u003csup\u003e\u003cspan additionalcitationids=\"CR16 CR17\" citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. More recently, 2D magnets have been demonstrated to act as spin injectors\u003csup\u003e\u003cspan additionalcitationids=\"CR20 CR21\" citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e, and their combination with TMDs has unlocked fully van der Waals (vdW) spin-LEDs\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eDespite these advances, spin-LEDs developed to date mostly exhibit a limited degree of circular polarization (DOCP)\u003csup\u003e\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. To achieve higher polarization, spin-polarized carriers must be efficiently injected, maintain spin coherence during transport, and undergo spin-conserving radiative recombination; however, all three processes are highly susceptible to material defects and interfacial properties, making their optimization a nontrivial task\u003csup\u003e\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. To move beyond the above-mentioned limitations, an attractive strategy is to employ the emerging magnetic 2D semiconductor CrI₃ as the emitting layer. Identified as the first 2D ferromagnet, CrI₃ has been extensively investigated, revealing that monolayer samples possess a Curie temperature of ~\u0026thinsp;45 K and an out-of-plane easy axis of magnetization. In few-layer form, CrI₃ adopts a layered antiferromagnetic ground state, where adjacent layers have opposite spin orientations (i.e., type-A antiferromagnetism)\u003csup\u003e\u003cspan additionalcitationids=\"CR22\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. Additionally, unlike most bandgap semiconductors where light emission arises from Wannier\u0026ndash;Mott excitons, CrI₃ emits via intra-atomic \u003cem\u003ed\u0026ndash;d\u003c/em\u003e transitions within the Cr\u0026sup3;⁺ 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 magnetization\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e. Provided that electroluminescence (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.\u003c/p\u003e\u003cp\u003eHere, 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 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\sigma\\:}^{-}\\)\u003c/span\u003e\u003c/span\u003e (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\sigma\\:}^{+}\\)\u003c/span\u003e\u003c/span\u003e) circular polarization, with the EL DOCP reaching \u0026minus;\u0026thinsp;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.\u003c/p\u003e\n\u003ch3\u003eDevice structure and characterization of 2D CrI\u003c/h3\u003e\n\u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea,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 electrode\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e,\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e, 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 voltage\u003csup\u003e\u003cspan additionalcitationids=\"CR29\" citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. The emissive capability of CrI₃ is verified by PL measurement (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec), which exhibits an emission peak centered at 1.14 \u0026micro;m. The relatively broad linewidth, with a full width at half maximum (FWHM) of ~\u0026thinsp;190 nm, reflects coupling between electronic excitation and lattice vibrations, in agreement with previous reports on CrI₃-based optical emission\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTo 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.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed, 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 behaviour\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eNext, 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\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea 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 (\u003cem\u003eI₊\u003c/em\u003e \u0026minus; \u003cem\u003eI₋\u003c/em\u003e)/(\u003cem\u003eI₊\u003c/em\u003e + \u003cem\u003eI₋\u003c/em\u003e) with \u003cem\u003eI₊\u003c/em\u003e and \u003cem\u003eI₋\u003c/em\u003e denoting the peak intensities of measured σ⁺ and σ⁻ emission, respectively, reaches around \u0026minus;\u0026thinsp;20%. This helicity contrast remains even as the magnetic field is reduced from 0.6 T to zero (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). When the field is reversed to \u0026minus;\u0026thinsp;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.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec-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\u0026thinsp;\u0026plusmn;\u0026thinsp;0.17 T (Supplementary Section 1), matching the ferromagnetic hysteresis observed in the RMCD measurement (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eEL generation from graphene/hBN/CrI₃/hBN/graphene heterostructures\u003c/h2\u003e\u003cp\u003eWith 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\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea shows the \u003cem\u003eI\u0026ndash;V\u003c/em\u003e\u003csub\u003e\u003cem\u003eb\u003c/em\u003e\u003c/sub\u003e characteristics measured by applying a bias (\u003cem\u003eV\u003c/em\u003e\u003csub\u003e\u003cem\u003eb\u003c/em\u003e\u003c/sub\u003e) 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.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). 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.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec)\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. 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.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed\u0026ndash;e). Notably, at low injected current (~\u0026thinsp;few hundreds nA), the EL peak wavelength coincides with the PL peak (~\u0026thinsp;1.14 \u0026micro;m), 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 \u003cem\u003ed\u003c/em\u003e\u0026ndash;\u003cem\u003ed\u003c/em\u003e transitions within CrI₃. Moreover, we note that this tunneling device structure can sustain high injected currents up to 50 \u0026micro;A (corresponding to a current density of 250 A/cm\u0026sup2; for the graphene/CrI₃/graphene overlapped area of 20 \u0026micro;m\u0026sup2;) while continuing to emit light. However, at higher bias, the EL intensity increases sublinearly with current (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef, lower panel), and the emission peak exhibits a slight redshift from 1.14 \u0026micro;m to 1.17 \u0026micro;m (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef, upper panel), accompanied by a noticeable broadening of the EL linewidth (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed\u0026ndash;e). These observations suggest that at high bias, the strong vertical electric field enhances direct carrier tunneling between the graphene electrodes, thereby reducing the probability of radiative recombination in CrI₃, as observed in other vdW heterostructure tunneling emitters\u003csup\u003e\u003cspan additionalcitationids=\"CR29\" citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e, while the modest redshift is a signature of Joule heating under elevated current.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eEvidence for graphene/hBN/CrI₃/hBN/graphene heterostructures as spin-LEDs\u003c/h3\u003e\n\u003cp\u003eFollowing 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.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, helicity-resolved EL spectra measured at an injected current of 0.5 \u0026micro;A 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.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, even though the graphene electrodes inject unpolarized carriers. Upon reversing the magnetic field to \u0026minus;\u0026thinsp;0.6 T at the same injection current, the emission helicity switches correspondingly, resulting in a DOCP of 19.5% (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb). These results demonstrate that this proof-of-concept vdW spin-LED can achieve a high EL DOCP, exceeding most conventional spin-LEDs based on III\u0026ndash;V and II\u0026ndash;VI epitaxial materials (see Supplementary Section 3 for a comparison table)\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. More notably, achieving high EL polarization without the need for spin injection or filtering highlights the promise of 2D CrI₃ for realizing simpler spin-LED architectures.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTo 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 \u0026micro;A as a function of the out-of-plane magnetic field. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec, 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\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 T, lower than the coercive field obtained from RMCD and magneto-PL loop experiments (~\u0026thinsp;0.17 T, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed and also see Supplementary Section 1). This discrepancy likely arises from the different probing characteristics of the measurements \u0026mdash; RMCD and PL are sensitive to local magnetization at a focused spot\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e, whereas EL integrates emission across the entire CrI₃ layer, effectively averaging over possible domain variations\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. Moreover, the EL DOCP remains at about \u0026minus;\u0026thinsp;19.5% (19.5%) when the magnetic field is reduced from 0.6 T to 0 (-0.6 T to 0), showing non-volatile behaviour. At zero magnetic field, however, we observe that the absolute value of the DOCP gradually decreases from 19.5% to 2.5% as the injection current is increased from 0.5 \u0026micro;A to 50 \u0026micro;A, suggesting that Joule heating weakens the magnetic order. This trend is supported by our finite-element simulations, which show that a current of 50 \u0026micro;A raises the device temperature from its base of 30 K to 36 K (see Supplementary Section 4 for simulation results), close to the Curie temperature of CrI₃, where remanent magnetization and EL helicity are significantly suppressed. In addition to monolayer CrI₃ devices, the dependence of EL polarization on the magnetic state of CrI₃ is further evidenced in Gr/hBN/CrI₃/hBN/Gr heterostructures employing bilayer CrI₃ as the emitter. In this spin-LED, the EL exhibits a DOCP of approximately\u0026thinsp;\u0026plusmn;\u0026thinsp;22% when the bilayer is in the ferromagnetic (\u0026uarr;\u0026uarr;/\u0026darr;\u0026darr;) configuration, comparable to that observed in monolayer emitters. However, the EL helicity vanishes as the external magnetic field drops to zero (see Supplementary Section 5 for magneto-EL from a spin-LED based on bilayer CrI₃), directly reflecting the layered antiferromagnetic ground state of bilayer CrI₃ (\u0026uarr;\u0026darr; or \u0026darr;\u0026uarr;)\u003csup\u003e21\u0026ndash;23\u003c/sup\u003e, 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.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eIn summary, we demonstrate a new approach to realizing spin-LEDs by employing a 2D magnet, CrI₃, as the light-emitting layer. This design circumvents the integration of spin injectors and the intricate heterostructure engineering required to suppress spin decoherence, both of which remain major challenges for most spin-LEDs developed to date\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. Building on this framework, we anticipate that higher degrees of polarization can be achieved by engineering CrI₃ through methods such as doping or applying mechanical strain, both of which have been shown to enhance its remanent magnetization\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. In addition, the approach can be extended to other emerging 2D magnets with different bandgaps, such as CrBr₃, whose luminescence is similarly coupled to magnetic order as in CrI₃, thereby enabling spin-LEDs operating at different emission wavelengths\u003csup\u003e\u003cspan additionalcitationids=\"CR35\" citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. Beyond these device-level advantages, the vdW nature of our proposed spin-LEDs allows seamless integration with a variety of photonic platforms\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e, supporting on-chip architectures where helicity control can be harnessed for advanced functionalities in optical communication, information processing, and quantum photonics\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003eFabrication of vdW heterostructures\u003c/p\u003e\u003cp\u003eAll 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 procedures\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e. The vdW heterostructures were assembled by sequentially stacking the selected flakes through a dry transfer process\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. 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.\u003c/p\u003e\u003cp\u003eOptoelectronic measurements\u003c/p\u003e\u003cp\u003eAll 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 \u0026micro;W), 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 \u0026sim;1 \u0026micro;m 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 helicities, following procedures described in previous studies\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eFor 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 \u0026micro;W) to minimize sample heating and degradation.\u003c/p\u003e\u003cp\u003eFor 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.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eCompeting Interests:\u003c/h2\u003e\u003cp\u003eThe authors declare no competing financial interests.\u003c/p\u003e\u003ch2\u003eAuthor Contributions:\u003c/h2\u003e\u003cp\u003eC.-H.L. conceived the experiments and supervised the project. C.-C.L. and L.-W.C. fabricated the vdW heterostructures, assisted by P.-L.C. C.K. and K.-H.P. C.-C.L. and L.-W.C. performed the measurements, assisted by W.-Q.L. and C.-H.L. Y.-J.L. provided numerical simulations. All authors contributed to the discussion of the data in the manuscript and Supplementary Information.\u003c/p\u003e\u003ch2\u003eAcknowledgements:\u003c/h2\u003e\u003cp\u003eC.-H.L. acknowledges support from the National Tsing Hua University (114Q2708E1) and the National Science and Technology Council (NSTC 114-2628-M-007 -002, 113-2223-E-007-008-MY3).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eHolub M, Bhattacharya P (2007) Spin-polarized light-emitting diodes and lasers. J Phys D 40:179\u0026ndash;203\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eZutic I, Fabian J, Das Sarma S (2004) Spintronics: fundamentals and applications. 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Jpn J Appl Phys 20:2403\u0026ndash;2409\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-8061031/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8061031/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSpin-polarized light-emitting diodes (spin-LEDs) convert electron spin into circularly polarized light, enabling direct optical readout of spin information and opening new avenues for solid-state, on-chip information processing and cryptography. To achieve such applications, considerable research has been devoted to GaAs-based emitters integrated with spin injectors including ferromagnetic metals, dilute magnetic semiconductors, and spin-filter tunnel barriers. These conventional spin-LEDs, however, require complex epitaxial growth and their limited integrability remains a critical challenge. Additionally, achieving high circular polarization is inherently difficult, as it requires high-quality materials and interfaces to ensure efficient spin injection, coherent spin transport, and spin-conserving radiative recombination. Here, we report an alternative approach to realize spin-LEDs by employing the monolayer CrI₃ as the light-emitting layer, sandwiched between two graphene/ hexagonal boron nitride tunneling contacts. Although the exploited contacts inject unpolarized carriers into CrI₃, we show that the resulting electroluminescence could be circularly polarized, with its helicity governed by the magnetic order of CrI₃, as confirmed by helicity-resolved EL measurements and magneto-optical analysis. Notably, the EL degree of polarization reaches 20%, outperforming most conventional spin-LEDs, and its helicity can be readily reversed with a low magnetic field (~\u0026thinsp;0.17 T). Combined with the inherent integrability of our proposed heterostructures, this approach provides a promising platform for future on-chip spin-optoelectronic devices.\u003c/p\u003e","manuscriptTitle":"Spin-polarized light-emitting diodes based on CrI₃ operating without external spin injection","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-18 05:39:53","doi":"10.21203/rs.3.rs-8061031/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"827d2cf3-98f0-46db-97b5-3ee9c46de9ca","owner":[],"postedDate":"November 18th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":58018388,"name":"Physical sciences/Nanoscience and technology/Nanoscale devices/Magnetic devices"},{"id":58018389,"name":"Physical sciences/Engineering/Electrical and electronic engineering"}],"tags":[],"updatedAt":"2026-03-31T05:46:01+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-18 05:39:53","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8061031","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8061031","identity":"rs-8061031","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}