Introduction

Circularly polarized luminescence (CPL) emitters are prospective for diverse applications, including three-dimensional (3D) displays1, optical anti-counterfeiting2, information encryption3,4, biological probes5, and circularly polarized light-emitting diodes (CP-LEDs)6,7,8,9,10,11,12. Recently, chiral organic-inorganic hybrid metal halides have emerged as promising CPL emitters due to their excellent chiroptical effects, high photoluminescence quantum yield (PLQY), and versatile solution processing strategies13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34. Red light, as one of the three primary colors of light, can regulate all visible light signals together with green and blue light, which is crucial for CPL emitters in high-definition 3D displays35,36,37,38. However, it remains a great challenge to construct the chiral hybrid metal halides to obtain both efficient and pure-red CPL. Existing studies on red-CPL emitters mostly focus on the chiral manganese-based hybrid metal halides39,40,41,42,43. Despite the remarkable performance achieved by these materials, they still suffer from broad emission peaks which result in insufficient color purity44. Moreover, accurately predicting the emission bands for traditional chiral hybrid metal halides is still extremely challenging, as the complicated coordination environment deeply influences the luminescent color. Thus, developing novel strategies to construct the chiral hybrid metal halides with both high color purity and predictable red emission is highly needed.

Rare-earth ions with 4f-4f transitions, as ideal luminescent centers, have garnered significant interest owing to their exceptional features, such as high color purity, large Stokes shift, and long excited state lifetime of millisecond level45,46,47,48. More importantly, the 4f orbitals of rare-earth ions are shielded by the outside 5s2 and 5p6 electrons46, weakening the influence of the surrounding environment on the 4f orbitals, resulting in the sharp and narrow line-like fingerprint emission44,49. As a result, these rare-earth materials possess predictable luminescent wavelengths together with high color purity. Among the rare-earth ions, the trivalent europium ion (Eu3+) with both large PLQY and high color purity in the red emission region has attracted the attention of broader research community in various fields, such as color television screens50, anti-counterfeiting inks2, and luminescent probes towards biochemical applications51. Therefore, it is highly desirable to construct the Eu-based chiral hybrid metal halide to achieve both the efficient and pure-red CPL.

In this work, we have designed and constructed the chiral Eu-based hybrid metal halides, namely, (R/S-3BrMBA)3EuCl6 (3BrMBA = 1-(3-bromophenyl)ethylammonium), through an anti-solvent assisted crystallization method. By coupling the 4f-4f transition properties of Eu3+ ions with chirality, these chiral Eu-based halides exhibit both efficient and narrow-band red photoluminescence (≈2 nm), high color purity and PLQY (59.8%), together with large dissymmetry factor |glum| of 1.84 × 10−2. Compared with the previously reported chiral metal halides with red CPL emission, these chiral Eu-based halides exhibit the highest CPL brightness. In addition, the degree of photoluminescence polarization (DP) of (R/S-3BrMBA)3EuCl6 can be effectively manipulated and enhanced by the external magnetic field. An unusual positive magneto-photoluminescence (MPL) at room temperature was also observed in these chiral Eu-based hybrid metal halides, which is attributed to the field-generated Zeeman splitting and spin mixing at exciton states. This work provides a feasible strategy for developing both the efficient and pure-red CPL emitter based on chiral rare-earth halides and lays the foundation for their applications in chiral optoelectronics and spintronics.

Results

Material properties

The crystal structures of the as-synthesized (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 were determined by the single-crystal X-ray diffraction (SCXRD), and both of them crystallized in the orthorhombic Sohncke space group of P212121. As shown in Fig. 1a, they are typical zero-dimensional (0D) structures with each [EuCl6]3– octahedron surrounded by three chiral R/S-3BrMBA+ cations. Owning to the enantiomeric nature, the lattice constants of (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 are almost the same (Supplementary Table 1). Most importantly, the space group of the inorganic frameworks for (R/S-3BrMBA)3EuCl6 after removing the chiral R/S-3BrMBA+ cations is still P212121 (Supplementary Fig. 1), which suggests that the chirality is transferred to the inorganic frameworks by chiral induction52. The powder X-ray diffraction (PXRD) patterns are consistent with the simulated results, demonstrating the high phase purity of the obtained (R/S-3BrMBA)3EuCl6 powders (Fig. 1b and Supplementary Fig. 2). The scanning electron microscopy (SEM) images and the energy dispersive spectroscopy (EDS) mappings confirm the uniform distributions of the constituting elements (C, Eu, and Cl) in the (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 microcrystals (Fig. 1c and Supplementary Fig. 3). X-ray photoelectron spectroscopy (XPS) was subsequently employed to investigate the valence state of each element in (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 (Supplementary Figs. 4, 5). The XPS peaks at approximately 200.1 and 198.4 eV are ascribed to the chloride ions (Cl 2p1/2 and Cl 2p3/2) in the inorganic skeleton. The peak at 284.8 eV is assigned to the carbon atoms on the chiral cations (C 1s). The peaks at 401.9 and 399.9 eV stem from the nitrogen atom of the ammonium group (N 1s). Meanwhile, two characteristic peaks of Eu3+ at 1136.1 and 1165.9 eV are attributed to the Eu 3d5/2 and 3d3/2 spin states, respectively53,54. In addition, the chiral Eu-based halides exhibit good thermal stability (≈520 K), as evidenced by the thermogravimetric analysis (TGA) (Supplementary Fig. 6). The ultraviolet-visible (UV-Vis) absorption spectra of (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 were also measured and shown in Supplementary Fig. 7, which exhibits almost undistinguishable absorption edges at ca. 379 nm due to their enantiomeric nature. By comparing the absorption spectra of (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 with their chiral cations (Supplementary Fig. 8a), it is confirmed that the absorption peak at 270 nm is mainly from the R/S-3BrMBA+ cations. Notably, owing to the 4f-4f transitions properties, many characteristic peaks of Eu3+ ions were observed in the absorption spectra, which are 395 nm (5L6 ← 7F0), 465 nm (5D2 ← 7F0), 536 nm (5D1 ← 7F1), and 592 nm (5D0 ← 7F1)51,55, respectively. According to the Tauc plot, the optical bandgaps of (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 are approximately 3.37 eV (Fig. 1d). To further understand the optical properties of (R/S-3BrMBA)3EuCl6, density functional theory (DFT) calculations were performed to explore their optical transitions in detail. The electronic band structure and the corresponding projected density of states (PDOS) for (R-3BrMBA)3EuCl6 were calculated and shown in Fig. 1e, f, and Supplementary Fig. 9. The negligible dispersion of the bands is consistent with the 0D structural features. The valence band maximum (VBM) consists of the unpaired 4f electrons of Eu atoms and p-orbitals of Cl atoms, while the conduction band minimum (CBM) comprises the 4f-orbitals of Eu atoms. The p-orbitals of the C atoms in the chiral organic cations with relatively higher conduction band energies indicate that the chiral cations do not contribute to the optical transitions near the band edges. Nevertheless, they may play a role in the transitions at shorter wavelengths. The results are close in agreement with the aforementioned UV-Vis absorption spectra. Therefore, the f-f transition dominates the optical properties of (R/S-3BrMBA)3EuCl6 near the band edge.

Fig. 1: Material properties for chiral Eu-based halides.
figure 1

a Crystal structures of (R/S-3BrMBA)3EuCl6. Eu, Br, Cl, C, N, and H are represented by the light blue, brown, green, dark gray, blue, and light gray spheres, respectively. b PXRD patterns of (R-3BrMBA)3EuCl6. The pink line is the experimental results, and the black line is the corresponding simulated results from the crystal structure. c The SEM image of (R-3BrMBA)3EuCl6 microcrystal and EDS mapping images of C (red), Eu (yellow), and Cl (blue) distributions. d Tauc plot to determine the optical bandgaps of (R-3BrMBA)3EuCl6 (pink) and (S-3BrMBA)3EuCl6 (blue). e The calculated electronic band structure of (R-3BrMBA)3EuCl6. f The calculated PDOS of (R-3BrMBA)3EuCl6 (Eu: red line, Cl: blue line, C: green line, N: purple line, Br: yellow line).

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Chiroptical properties

Europium has been recognized as a magic element in the family of rare-earth metals, and the chiral hybrid europium halides are expected to exhibit excellent chiroptical properties. As shown in Supplementary Fig. 10, the bright red luminescence of (R/S-3BrMBA)3EuCl6 crystals was observed under UV light (365 nm). The photoluminescence excitation (PLE) spectra for (R/S-3BrMBA)3EuCl6 under different emission peaks are shown in Supplementary Figs. 11, 12, which exhibit relatively strong and sharp peaks at 395 and 465 nm in accordance with those of the UV-Vis absorption spectra (Supplementary Fig. 7). The steady-state photoluminescence (PL) spectra of (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 have identical features under 465 nm excitation at room temperature (Fig. 2a). The emission peaks at 594, 614, 654, and 704 nm are assigned to the 5D0 → 7F1, 5D0 → 7F2, 5D0 → 7F3, and 5D0 → 7F4 transitions of Eu3+ ions, respectively, which have been observed in other Eu(III) compounds as well56,57,58. Notably, the full width at half maximum (FWHM) at 594 nm is nearly 2 nm for (R/S-3BrMBA)3EuCl6. This is attributed to the 4f intra-shell transitions of Eu3+ ions shielded by the external 5s and 5p orbitals, reducing the influence of the surrounding environment on the 4f-4f transitions44. The Commission International de l’Eclairage (CIE) coordinates of (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 are (0.64, 0.36) and (0.64, 0.36), respectively (Fig. 2b), which are almost close to the standard red emission region (0.67, 0.33) in the National Television Standards Committee (NTSC) standard59. Therefore, (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 with such high color purity are promising candidates for the pure-red emitters. The time-resolved photoluminescence (TRPL) spectra as shown in Fig. 2c and Supplementary Fig. 13, the PL lifetime of different emission peaks for (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 are mostly around 2 ms at room temperature. The longer lifetime at the ms level arises from the parity-forbidden of the 4f-4f transitions45. The PLQYs of (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 microcrystals are 56.5% and 59.8% at room temperature, respectively (Fig. 2d and Supplementary Fig. 14). The higher PLQY illustrates the carrier recombination in (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 is mainly determined by the radiative recombination, while the non-radiative recombination associated with the defects is negligible60.

Fig. 2: Chiroptical properties for chiral Eu-based halides.
figure 2

a The normalized steady-state PL spectra of (R-3BrMBA)3EuCl6 (pink) and (S-3BrMBA)3EuCl6 (blue) under 465 nm excitation. b The CIE color coordinates diagram of (R/S-3BrMBA)3EuCl6. c The TRPL spectra of 594 nm emission for (R-3BrMBA)3EuCl6 (pink) and (S-3BrMBA)3EuCl6 (blue) under 465 nm excitation. τave, PL lifetime. d The PLQYs of (R-3BrMBA)3EuCl6 (pink) and (S-3BrMBA)3EuCl6 (blue) under 465 nm excitation. e The CD spectra of (R-3BrMBA)3EuCl6 (pink) and (S-3BrMBA)3EuCl6 (blue). f The CPL spectra of (R-3BrMBA)3EuCl6 (pink) and (S-3BrMBA)3EuCl6 (blue).

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To further investigate the chiral optical properties of (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6, the vibrational circular dichroism (VCD) spectra were first measured (Supplementary Fig. 15). The VCD signals of (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 are opposite, supporting their enantiomorphic structures. Then the circular dichroism (CD) spectra were measured and shown in Fig. 2e. The mirror-symmetric CD signals from 245 to 600 nm further confirm the enantiomorphic nature of (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6. The CD spectra of R/S-3BrMBA were also measured and their signals are before 283 nm (Supplementary Fig. 8b). However, when they are incorporated into (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6, the resulting chiral metal halides exhibit strong CD signals from 283 nm to 600 nm (Fig. 2e). These results indicate that the chirality of the chiral cations is transferred to the inorganic [EuCl6]3− framework. To further explore the chiral luminescent properties of (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6, their CPL spectra were measured and shown in Fig. 2f. Clear mirror-symmetric CPL spectra from 550 to 650 nm were observed, and the CPL peaks are mainly located at ca. 601 nm and 616 nm. They are also consistent with the steady-state PL spectra as discussed above, further indicating that chirality is transferred from chiral cations to the europium halides. The dissymmetry factors (glum) of CPL for (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 are calculated to be −1.84 × 10−2 and 1.18 × 10−2 at 616 nm, respectively61. To assess the performance of CPL materials, the figure of merit (FOM) is also calculated, which is defined as Eq. (1)61,62,

$${{{\rm{FOM}}}}=|{g}_{{{{\rm{lum}}}}}|\times {{{\rm{PLQY}}}}$$
(1)

The calculated FOM value of (R-3BrMBA)3EuCl6 is 10.40 × 10−3. Compared with the previously reported chiral metal halides with red CPL emission, (R-3BrMBA)3EuCl6 exhibits the largest FOM values, longer lifetimes, and narrower FWHM (Supplementary Figs. 16−18, and Supplementary Table 2)39,40,41,42,43,63. Therefore, these chiral Eu-based halides are promising candidates for high-performance red CPL emitters.

Magneto-chiroptical properties

The magneto-chiroptical properties of (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 were then investigated by temperature- and magnetic-field-dependent degree of photoluminescence polarization (DP). More details about this experiment and the sample preparation method are provided in the Methods section. The DP is evaluated by Eq. (2),

$${{{\rm{DP}}}}(\%)=\frac{{I}_{{{{\rm{L}}}}}-{I}_{{{{\rm{R}}}}}}{{I}_{{{{\rm{L}}}}}+{I}_{{{{\rm{R}}}}}}\times 100\%$$
(2)

in which, \({I}_{{{{\rm{L}}}}}\) and \({I}_{{{{\rm{R}}}}}\) are the photoluminescence intensities due to the left- and right-handed circularly polarized photoexcitation, respectively. The temperature-dependent DP from 300 K to 4 K is shown in Fig. 3a, and there are two noticeable phenomena for (R/S-3BrMBA)3EuCl6. DP becomes increasingly larger with decreasing the temperature. Owing to the implementation of chirality, DP grows in the opposite directions for (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6. When carriers are moving in the helical structure of (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6, they may acquire an extra orbital angular momentum64,65. Such momentum can be energetically coupled with the spin of carriers, and lead to the formation of the chirality-induced spin-orbit coupling (CISOC)66,67. The ratio of the photoluminescence intensities under the left- and right-handed circularly polarized photoexcitation can be considered as the ratio for the different number of excitons with opposite angular momenta, such as \(\frac{{I}_{{{{\rm{L}}}}}}{{I}_{{{{\rm{R}}}}}}={{{{\rm{e}}}}}^{-\frac{\Delta {E}_{{{{\rm{soc}}}}}}{{{{{\rm{k}}}}}_{{{{\rm{B}}}}}T}}\). An energy difference \(({\Delta E}_{{{{\rm{soc}}}}})\) between the left- and right-handed spin-polarized excitons due to the CISOC is calculated by Eq. (3),

$${\Delta E}_{{{{\rm{soc}}}}}={E}_{{{{\rm{ex}}}}}({j}_{{{{\rm{z}}}}}^{{{{\rm{ex}}}}}=1)-{E}_{{{{\rm{ex}}}}}({j}_{{{{\rm{z}}}}}^{{{{\rm{ex}}}}}=-1)=2\alpha \tau {{{\bf{K}}}}=2\alpha \tau \frac{\sqrt{{{{{\rm{\varepsilon }}}}}_{0}}}{{{{\rm{c}}}}}{\omega }_{0}$$
(3)
Fig. 3: Measurements of temperature- and magnetic-field-dependent degree of photoluminescence polarization (DP) for chiral Eu-based halides.
figure 3

a The temperature-dependent DP for the (R-3BrMBA)3EuCl6 (pink circles) and (S-3BrMBA)3EuCl6 (blue squares) measured at 593 nm emission with the absence of applied magnetic fields. The solid lines are the fitting curves for the (R-3BrMBA)3EuCl6 (pink) and (S-3BrMBA)3EuCl6 (blue), respectively. b The temperature-dependent DP for the (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 measured at 593 nm emission with applied constant magnetic fields such as ±900 mT. The pink circles and pink triangles represent the temperature-dependent DP under 900 mT and −900 mT for (R-3BrMBA)3EuCl6, respectively. The blue rhombuses and blue pentagons represent the temperature-dependent DP under 900 mT and −900 mT for (S-3BrMBA)3EuCl6, respectively.

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in which, \({E}_{{{{\rm{ex}}}}}\) and \({j}_{{{{\rm{z}}}}}^{{{{\rm{ex}}}}}\) are the exciton’s energy and the projection of the exciton total angular momentum along the z-axis. \(\alpha\) represents the CISOC strength, \(\tau=\pm\)1 denote the helicities, K denotes the exciton wavevector, \({{{\rm{c}}}}\) is the speed of light, \({{{{\rm{\varepsilon }}}}}_{0}\) is the electric permittivity, \({\omega }_{0}\) is the angular frequency. By including the thermal perturbation effect, it can be further expressed by Eq. (4),

$${\Delta E}_{{{{\rm{total}}}}}=2\alpha \tau \frac{\sqrt{{{{{\rm{\varepsilon }}}}}_{0}}}{{{{\rm{c}}}}}{\omega }_{0}+A{{{{\rm{k}}}}}_{{{{\rm{B}}}}}T$$
(4)

where A is the coefficient due to the thermal perturbation. \({{{{\rm{k}}}}}_{{{{\rm{B}}}}}\) is the Boltzmann constant. T is the temperature. Since all microcrystals are randomly distributed over a large surface area in the experiment, the orientation average should also be considered. The complete expression for DP is defined as Eq. (5),

$${{{\rm{DP}}}}=\int _{0}^{{{{\rm{\pi }}}}/2}\frac{\cos \theta \sin \theta {{{\rm{d}}}}\theta }{1+{\cos \theta }^{2}}\frac{{{{{\rm{e}}}}}^{-\frac{2\alpha \tau \frac{\sqrt{{{{{\rm{\varepsilon }}}}}_{0}}}{{{{\rm{c}}}}}{\omega }_{0}+A{{{{\rm{k}}}}}_{{{{\rm{B}}}}}T}{{{{{\rm{k}}}}}_{{{{\rm{B}}}}}T}-1}}{{{{{\rm{e}}}}}^{-\frac{2\alpha \tau \frac{\sqrt{{{{{\rm{\varepsilon }}}}}_{0}}}{{{{\rm{c}}}}}{\omega }_{0}+A{{{{\rm{k}}}}}_{{{{\rm{B}}}}}T}{{{{{\rm{k}}}}}_{{{{\rm{B}}}}}T}+1}}$$
(5)

here\(\,\theta\) is the angle between the crystal orientation (i.e., c-axis) and the observing direction. The experimental results were fitted by Eq. (5) to extract the \(\alpha\) value. After fittings, \(\alpha\) was found to be approximately 0.152 eV·Å for (R/S-3BrMBA)3EuCl6. These 0D chiral rare-earth halide microcrystals have a competitive \(\alpha\) value comparable with that of the prototypical two-dimensional chiral lead halides, for instance \(\alpha\) = 0.400 eV·Å for (R/S-MBA)2PbI4 polycrystalline thin films65,68,69. Besides, we also found that DP can be effectively tuned by applying the external magnetic field (Fig. 3b and Supplementary Fig. 19). Depending on the chirality of (R/S-3BrMBA)3EuCl6, the application of the constant magnetic field such as 900 mT has a positive effect on elevating DP with respect to the zero field for (R-3BrMBA)3EuCl6. Meanwhile, the negative field such as −900 mT leads to the reduction of DP. Such field-dependent DP is vice-versa when the chirality is reversed70.

Apart from the temperature- and magnetic-field-dependent DP, we also performed the magneto-photoluminescence (MPL) to further investigate the manipulation of the photoluminescence of (R/S-3BrMBA)3EuCl6 by sweeping the magnetic field. The MPL in percentage is evaluated by Eq. (6),

$${{{\rm{MPL}}}}(\%)=\left(\frac{I(B)}{I(0)}-1\right)\times 100\%$$
(6)

where, I(B) and I(0) represent photoluminescence intensities measured with and without magnetic fields (B represents the magnetic field). The MPL was measured at room temperature and 4 K, respectively, and shown in Fig. 4a, b, and Supplementary Figs. 20, 21. Surprisingly, both (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 exhibit an unusual positive MPL effect for the photoluminescence at 591 nm and 612 nm. This MPL effect reaches ≈3% for (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 at the field strengths of ±900 mT at room temperature. In fact, this is comparable to the traditional hybrid metal halides, (PEA)2PbI4, measured under the same magnetic field at 15 K71. The reduced temperature (~4 K) has no significant impact on the line shape, sign, and magnitude for this chiral rare-earth metal halide system (Supplementary Fig. 21). Such behavior is indeed very different from the prototypical chiral lead halides such as (R-/S-MBA)2PbI468, and achiral metal halides such as (PEA)2PbI4 and (BA)2PbI471. In those cases, the effect usually shows a negative sign and it is strongly temperature dependent. Here, we postulate that the positive MPL effect may be ascribed to the spin mixing of the dark and bright exciton states (Fig. 4c). They correspond to the spin singlet (J = 0, dark states) and triplet (J = 1, bright states) configurations respectively. Without the magnetic field, these two states are determined by the exchange effect and the Bychkov-Rashba spin-orbit coupling72,73. The applied magnetic field may effectively trigger the Zeeman splitting for the bright states, leading to the mixing of these two states. Consequently, it gives rise to a transfer of excitons from the dark to the bright states. An increase in the bright exciton density may result in the enhancement of the photoluminescence intensity. In this case, the MPL signals were fitted by Eq. (7),

$${{{\rm{MPL}}}}(\%)\propto \left(\frac{1}{4}-\frac{{\Delta }_{{{{\rm{B}}}}-{{{\rm{D}}}}}}{4\sqrt{{{\Delta }_{{{{\rm{B}}}}-{{{\rm{D}}}}}}^{2}+{(\Delta g{{{{\rm{\mu }}}}}_{{{{\rm{B}}}}}B)}^{2}}}\right)\exp \left(-\frac{E-{E}_{{{{\rm{F}}}}}}{{{{{\rm{k}}}}}_{{{{\rm{B}}}}}T}\right)$$
(7)

where Δg is the effective Landé g-factor, μB is the Bohr magneton (5.788 × 10−5 eV/T), ΔB−D is the energy splitting of bright states and dark states at 0 mT, E is the energy of bright states at 0 mT, EF is the Fermi energy level. After fittings, we found ΔB−D is around 11.7 meV, smaller than the reported values for other hybrid metal halides (18~30 meV)74. This indicates that (R/S-3BrMBA)3EuCl6 has relatively smaller exchange effects, and thus the magnetic field could elevate their photoluminescence intensities.

Fig. 4: Magneto-photoluminescence (MPL) measurements for chiral Eu-based halides.
figure 4

a, b The results of MPL for the (R-3BrMBA)3EuCl6 (pink) and (S-3BrMBA)3EuCl6 (blue) measured at 300 K. The solid lines are the fitting curves. c The schematic drawing for the fine structure of exciton states with and without the magnetic field (bright states: red, dark states: dark blue).

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Discussion

In summary, for obtaining high-performance red-CPL emitters, chiral Eu-based halides, (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 were designed and constructed by using an anti-solvent assisted crystallization method. Their chiroptical properties were systematically investigated by VCD, CD, and CPL spectra. Combining the 4f-4f transition characteristics of Eu3+ ions with chirality, (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 exhibit both efficient (PLQY of 59.8%) and narrow red emission (FWHM of ≈2 nm), long lifetime (≈2 ms), together with high |glum| of 1.84 × 10−2. Compared with the previously reported chiral metal halides with red CPL emission, these chiral Eu-based halides show the highest CPL brightness. Moreover, the degree of photoluminescence polarization in (R/S-3BrMBA)3EuCl6 was further tuned by applying the external magnetic field. Particularly, they exhibit an unusual positive magnetic field effect of ≈3% at room temperature, which is comparable to the traditional hybrid metal halide, (PEA)2PbI4, measured under the same magnetic field at 15 K. This work offers a strategy to develop the highly-efficient CPL emitter with pure red and sheds light on further exploration of studies on chiral rare-earth halides in spin-optoelectronics.

Methods

Materials

The chemicals including EuCl3·6H2O (99.99%, MREDA), (R/S)-1-(3-Bromophenyl)ethylamine (R/S-3BrMBA, 98%, Leyan), hydrochloric acid (HCl, 37%, Aladdin), diethyl ether (99.8%, Tianjin Bohua Chemical Reagent), and methanol (99.9%, SuperDry, J&K Scientific) were used as received without any further purification.

Synthesis of (R/S-3BrMBA)3EuCl6

EuCl3·6H2O (0.036 g, 0.1 mmol), R/S-3BrMBA (0.06 g, 0.3 mmol), and 1 ml HCl were mixed in a flask with 1 mL of methanol. The mixture was heated to 70 °C and stirred for 1 h. Then, the temperature was increased to evaporate the solvent and obtain a solid precursor fully. Afterward, the precursor was dissolved in methanol and crystallized using diethyl ether as the antisolvent to obtain (R-3BrMBA)3EuCl6 and (S-3BrMBA)3EuCl6 crystals.

General characterization

SCXRD data were collected at 100 K on a Rigaku XtaLAB PRO MM007 DW single-crystal X-ray diffractometer (Mo-Kα, λ = 0.71073 Å). The crystal structures were solved using Olex2 software. PXRD patterns were obtained on a Rigaku Smart Lab 3 kW (Cu Kα) diffractometer. XPS images were obtained on a Thermo Scientific ESCALAB 250Xi (Al Kα) spectrometer. TGA curve was recorded on a METTLER TOLEDO TGA/DSC1 apparatus under N2 atmosphere with a heating rate of 20 K min−1. SEM images and elemental distributions of the synthesized samples were collected on a JEOL JSM-7800F field-emission scanning electron microscope with an attached energy dispersive spectrometer. VCD spectra were measured on BRUKER PMA50, INVENIO-R vibration circular dichroic-infrared spectrometer by using a mixture of KBr and (R/S-3BrMBA)3EuCl6 pellets. UV-Vis absorption spectra were collected on a Shimadzu UV-2600 UV-Vis spectrometer. An Edinburgh FLS 1000 spectrometer (with a 450 W xenon lamp) was used to measure the steady-state PL spectra. The TRPL spectra were measured on an Edinburgh FLS1000 fluorescence spectrometer using a microsecond flash lamp at 465 nm. The PLQY was measured by Edinburgh FS5 spectrometer with an integrating sphere, via the absolute method. The samples were excited at 465 nm and the PLQYs were calculated by integrating the PL emission peak. CD spectra were obtained on a JASCO J-1700 CD spectrometer. The mixture of KBr powder and (R/S-3BrMBA)3EuCl6 powder was used and pressed into pellets for CD analysis. The scanning speed is 500 nm min−1, the data pitch is 0.5 nm, and D.I.T is 1 s. CPL spectra were conducted on a JASCO CPL-300 spectrometer. The mixture of KBr powder and (R/S-3BrMBA)3EuCl6 powder was used and pressed into pellets for CPL analysis. The scanning speed is 500 nm min−1, the data pitch is 0.5 nm, and D.I.T is 1 s. The magnetic susceptibility curves of the samples were recorded using a Quantum Design MPMS3 SQUID magnetometer.

Temperature-dependent PL polarization measurement under circularly polarized photoexcitation

The experiment was conducted in a helium-free closed-cycle cryogenic system. Prior to the measurement, some microcrystals either (R-3BrMBA)3EuCl6 or (S-3BrMBA)3EuCl6 were placed on a pre-cleaned glass substrate. They were randomly distributed and encapsulated by a thin transparent glass. The sample was fixed on a cold finger in the cryogenic system. After sealing the system and pumping it down to a desirable vacuum level, the entire system was inserted in-between a pair of electromagnets with the maximum field strengths of ±1 T. For every decrement of the temperature from 300 K to 4 K, the microcrystals were photoexcited by a 405 nm continued wave semiconductor laser diode, while the laser beam passed through a polarizer followed by a quarter-wave plate. Fluorescence spectra were captured by a photoluminescence spectrometer with an integrated optical fiber (Model: Horiba Fluoroog-3).

MPL measurements

Before the measurement, the sample was mounted on a cold finger in a closed-cycle cryogenic system. The system was pre-pumped to a low vacuum stage. The system could be placed between a pair of electromagnets with a maximum field strength of 1 T. During MPL measurements, the sample is continuously irradiated by a 405 nm laser diode. The PL spectra are collected via an optical fiber and connected to a photoluminescence spectrometer (Model: Horiba Fluorolog-3).

Theoretical calculations

The Vienna Ab-initio Simulation Package (VASP) software is used to accomplish the density functional theory (DFT) calculations75,76. The projector augmented wave (PAW) method77 with the Perdew-Burke-Ernzerhof functional (PBE)78 is selected and the plane-wave cutoff energy is set to 500 eV. Geometric structures are fully relaxed until the energy and total forces are converged to 10−5 eV and 10−3 eV/Å, respectively. Spin-orbit coupling is considered for accurate electronic structure analysis. VASPKIT code79 is used for post-processing analysis.

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.