A thermally induced fluorescence enhancement strategy for efficient all-inorganic rubidium manganese halide

Abstract

Metal halides show great promise as next-generation emitting materials owing to their outstanding emission properties. However, achieving the application of white light-emitting diodes (WLEDs) based on lead-free metal halides is a challenge because of the lack of stable and efficient red emitters. In this study, we synthesized all-inorganic Rb₂MnBr₄·2H₂O crystals with isolated [MnBr₄O₂]H₄²⁻ octahedron units. By the thermo-induced partial dehydration process, the obtained Rb₂MnBr₄·0.9H₂O showed a broad red emission at 653 nm with high quantum efficiency up to 62.8%, which was ascribed to the suppressing of non-radiative recombination deriving from water molecules and the confinement excitons into polyhedron units. Moreover, Rb₂MnBr₄·0.9H₂O crystals exhibited good thermal stability, which still maintains 88% of room temperature emission intensity under 480 K. Owing to the excellent efficiency and stability of the red emitters, the as-fabricated WLED showed an excellent luminous efficacy of 84.84 lm W⁻¹ and a high color rendering index of 91.5 with a correlated color temperature of 5238 K. This work provides a new strategy to explore the red emission of metal halide materials for WLEDs.

---

1 Introduction

Metal halides, as a new class of optoelectronic materials, have evoked interest for application in the fields of light-emitting diodes (LEDs), photodetectors, solar cells, laser, X-ray scintillators, sensors, and anti-counterfeiting owing to their fascinating optical and electrical characteristics.^1–3 In particular, combining green and red emissive lead-based metal halides with blue chips for the fabrication of white LED (WLED) devices has made tremendous progress, which achieved a wide color gamut (above 125% of the National Television System Commission) and a high luminous efficacy (LE) of 116 lm W⁻¹.^4,5 Considering the intrinsic instability and toxicity of lead-based metal halides in practical applications, many efforts have been devoted to exploring environmentally friendly metal halides with outstanding properties.^6,7 Owing to structural diversity and rich elements, metal halides based on Sn²⁺,^8,9 Bi³⁺,^10,11 Sb³⁺,^12,13 In³⁺,^14,15 Cu⁺,^16,17 and Ag⁺/In³⁺ ions¹⁸ are regarded as ideal alternatives due to the attractive features of self-trapped excitons. For instance, CsCu₂I₃ with a blue emission at 446 nm (full width at half-maximum (FWHM) of 85 nm) and Cs₃Cu₂I₅ with a yellow emission at 550 nm (FWHM of 115 nm) served as emitters to fabricate white LED devices, achieving a high color-rendering index (CRI) of 91.6.¹⁶ Moreover, a WLED was assembled by combining Bi³⁺ and Ce³⁺ co-doped Cs₂Ag_0.60Na_0.40InCl₆ with a broadband emission at 570 nm (FWHM of 156 nm) and BaMgAl₁₀O₁₇:Eu²⁺ blue phosphor onto an ultraviolet (UV) LED chip, realizing a high CRI of 95.7.¹⁸ Among them, the reported lead-free metal halides mainly show blue and yellow emission upon UV excitation, leading to a poor luminous efficiency (LE) of WLED device. Commonly, commercial WLED devices composed of blue LED chip and Y₃Al₅O₁₂:Ce³⁺ phosphor show an ultrahigh LE, but the CRI value of devices is relatively low (<80) due to the lack of red-emitting component,¹⁹ which seriously impedes the development of high-quality lighting. Thus, developing stable and efficient metal halides with red emission is urgently needed for the fabrication of highly efficient blue-light excited WLEDs.

Recently, red emissive organic and inorganic hybrid Mn²⁺-based metal halides with unique d–d transition have attracted considerable attention from researchers owing to their excellent properties.²⁰ According to the crystal theory of 3d⁵ states, the octahedrally coordinated Mn²⁺ ions have a high crystal field strength, leading to red emission.²¹ For example, Hu et al.²¹ prepared C₅H₆NMnCl₃ with a red emission at 648 nm and PLQY of 40% due to the formation of linear [MnCl₆]⁴⁻ octahedron. Jian and coworkers reported C₄H₁₂NMnCl₃ single crystals with a red emission at 635 nm, full width at half-maximum (FWHM) of 85 nm, and PLQY of 91.8%.²² Moreover, green-emitting (C₈H₂₀N)₂MnBr₄ and red-emitting C₄H₁₂NMnCl₃ powders were blended onto a blue chip to fabricate a WLED device, which shows a high LE of 96 lm W⁻¹ and color rendering index (CRI) of 79. Unfortunately, these organic cation-containing Mn²⁺ halides face the issues of poor stability, such as emission intensity of (CH₆N₃)₂MnBr₄ less than 50% of the initial emission intensity at 423 K,²³ which results in the performance deterioration of photoelectric devices during long-term operation. Therefore, all inorganic Mn²⁺ halides with superior stability have been studied extensively in recent years.^24,25 Unfortunately, the reported CsMnCl₃·2H₂O single crystals exhibit a red emission centered at 650 nm with a poor PLQY of 0.5%, and CsMnBr₃ single crystals also present a red emission centered at 640 nm with a low PLQY of 6.7%.^26,27 To improve the efficiency of cesium manganese halides, many effective strategies have been developed. Almutlaq et al. prepared CsMnBr₃ nanocrystals via the hot injection method, which possessed a red emission at 643 nm with a PLQY of 54% by ligand passivation.²⁷ The PLQY of CsMnCl₃ nanocrystals can be improved from 56.3% to 77.1% by Zn²⁺ doping, which is attributed to the fact that the energy transfer between nearby Mn²⁺ ions is restrained by the doped Zn²⁺.²⁸ Regrettably, the all-inorganic manganese halides also face the problem of undesired stability, in which the emission performances of nanocrystals present obvious attenuations under high temperatures, restricting their application in WLEDs. Therefore, exploring new all-inorganic manganese halides with excellent emission properties as well as stability using conventional methods is essential to the development of WLED devices.

The structure dimensional (D) regulation can effectively modulate the PL properties of metal halides.²⁹ For Mn²⁺ halides, the PLQY can be significantly enhanced by decreasing the inorganic skeleton dimension from 1D linear to 0D isolated [MnX₆]⁴⁻ units.³⁰ For instance, (CH₆N₃)₂MnBr₄ with isolated trimeric [Mn₃Br₁₂]⁶⁻ octahedral units shows a red emission at 627 nm with PLQY of 61.91%.²³ 0D (2-aminobenzimidazole)₄MnBr₆ with an isolated [MnBr₆]⁴⁻ octahedron presents a highly efficient red emission (PLQY up to 80%).³⁰ This is attributable to the hindering of exciton transition between adjacent isolated polyhedrons by quantum constraint, elevating the possibility of radiation recombination. Recently, the reported 0D Cs₂MnCl₄·2H₂O possesses an isolated [MnCl₄O₂]H₄²⁻ octahedron, enabling a red emission at 634 nm.³¹ Unfortunately, Cs₂MnCl₄·2H₂O shows a low PLQY.

Inspired by the advantages of 0D structure, we developed a facile strategy to boost the emission of 0D all-inorganic Mn²⁺ halides with excellent stability. In this work, we successfully synthesized the all-inorganic Rb₂MnBr₄·2H₂O crystals via the facile slow solvent evaporation method. Rb₂MnBr₄·2H₂O crystals with a weak emission have a unique 0D structure with isolated [MnBr₄O₂]H₄²⁻ octahedron units. To activate their PL properties, a thermally induced fluorescence enhancement strategy was employed, resulting in a broad red emission at 653 nm with FWHM of 107 nm and PLQY up to 62.8%. This is ascribed to the suppression of non-radiative recombination from water molecules via thermo-induced partial dehydration process and the hindering of energy transfer between adjacent Mn²⁺ ions by isolated polyhedron units. Based on the results from thermogravimetric analysis (TGA) and energy-dispersive X-ray spectroscopy (EDS), the composition of red emitters is confirmed as Rb₂MnBr₄·0.9H₂O. Thermal stability tests indicate the good stability of Rb₂MnBr₄·0.9H₂O, which can remain at nearly 88% of room temperature emission intensity at 480 K. Furthermore, the as-fabricated WLED based on the Rb₂MnBr₄·0.9H₂O red emitters exhibits excellent performances with a CRI of 91.5, LE of 84.84 lm W⁻¹, and excellent stability. This work can provide a new pathway for designing highly efficient and stable Mn²⁺ halide-based WLEDs.

2 Results and discussion

Rb₂MnBr₄·2H₂O crystals were prepared by a modified slow solvent evaporation method.³² Briefly, Rb₂CO₃ and MnCO₃ with a molar ratio of 1 : 1 were dissolved in 2 mL concentrated HBr. The mixture solution became clear after being heated and stirred at 80 °C for 1 h. Then, the transparent solution was filtrated and then heated at 80 °C for three days. The Rb₂MnBr₄·2H₂O crystals were obtained by washing with ethanol and drying. The powder X-ray diffraction (XRD) analysis shows that the prepared bulk crystals belong to the triclinic-structure Rb₂MnBr₄·2H₂O (PDF# 50-0629) in the space group of P1̄(2). XRD Rietveld refinement results display that the lattice parameters of Rb₂MnBr₄·2H₂O crystals are a = 5.8916(0) Å, b = 6.8800(1) Å, c = 7.3647(2) Å, α = 66.032(7)°, β = 87.7903(14)°, γ = 84.8967(14)° and V = 271.64(1) ų (see Table S1†). The crystal structures of Rb₂MnBr₄·2H₂O are shown in Fig. 1a and b. It can be found that an Mn atom is coordinated with four Br atoms and two O atoms, resulting in the formation of [MnBr₄O₂]H₂²⁻ octahedron. The isolated [MnBr₄O₂]H₄²⁻ octahedron is surrounded by Rb⁺ cations to form a 0D structure. Scanning electronic microscope (SEM) images show that the crystals have a smooth surface, and EDS results confirm that the Rb, Mn, Br, and O elements display uniform distribution in the crystals and the molar ratio of n(Rb) : n(Mn) : n(Br) : n(O) is 26.02 : 8.70 : 40.68 : 24.59 (Table S2†), which confirms the feeding ratio (Fig. S1†). As shown in Fig. 1d, the prepared bulk Rb₂MnBr₄·2H₂O crystals present an average length of ∼50 mm, which appears irregular under ambient light.

Fig. 1 (a) Crystal structure and (b) primitive cell of Rb₂MnBr₄·2H₂O, (c) XRD Rietveld refinement plot of Rb₂MnBr₄·2H₂O, (d) photograph of Rb₂MnBr₄·2H₂O bulk under daylight.

The optical property of the prepared Rb₂MnBr₄·2H₂O crystals was measured. Unfortunately, a weak broad emission can be detected in the PL spectrum (Fig. S2†), which might be attributed to a strong electron–phonon coupling of H₂O molecules.²⁴ Therefore, to improve PL emission and suppress electron–phonon coupling, we have tried to heat the crystals to high temperatures. The PL emission spectra are illustrated to investigate the fluorescence properties of Rb₂MnBr₄·2H₂O bulk under different temperatures (Fig. 2a and b). Under 273 nm excitation, the Rb₂MnBr₄·2H₂O bulk show a weak emission at temperatures of 30–80 °C. With the increase in temperature to 120 °C, a strong, broad red PL emission at 653 nm with an FWHM of 107 nm can be observed. As the temperature increases to 160 °C, the red emission intensity reaches a maximum value. The PLQY of the sample reaches 62.8% (Fig. 2c). Moreover, the PL intensity deteriorates by further enhancing the temperature to 220 °C. The PL peak shows a blue-shift tendency from 653 nm to 632 nm with the increase of temperature from 120 to 220 °C. Furthermore, the PL excitation (PLE) spectrum under 653 nm emission for Rb₂MnBr₄·2H₂O crystals at 160 °C shows strong excitation bands in UV and blue light region (Fig. 2d), which is ascribed to the d–d transition of Mn²⁺.³³ Among them, the excitation peaks at 262 nm, 273 nm, 287 nm, 344 nm, 363 nm, 380 nm, 435 nm, 452 nm, and 530 nm are assigned to the electronic transitions from the ground state ⁶A₁(⁶S) to the excited states ⁴T₂(⁴F), ⁴T₁(⁴F), ⁴A₂(⁴F), ⁴T₁(⁴P), ⁴E(⁴D), ⁴T₂(⁴D), [⁴E(⁴G), ⁴A₁(⁴G)], ⁴T₂(⁴G) and ⁴T₁(⁴G), respectively.

Fig. 2 (a) Temperature-dependent PL spectra (inset is digital photographs under UV light) and (b) Pseudo color maps of temperature-dependent PL emission spectra of Rb₂MnBr₄·2H₂O, (c) PLQY and (d) PL excitation spectrum of samples at 160 °C.

To understand the boosting emission mechanism of Rb₂MnBr₄·2H₂O, the TGA curve is displayed in Fig. S3.† Fig. S3a† shows that the mass of Rb₂MnBr₄·2H₂O gradually reduces with the increase of temperature to 200 °C. The reduced mass of 6.22% originated from the water molecule vapor, which is in agreement with the theoretical value of 6.20%.³⁴ With elevating the temperature to 600 °C, there is no obvious mass loss can be found. Furthermore, the temperature-dependent XRD patterns of Rb₂MnBr₄·2H₂O crystals were carried out, as shown in Fig. S4.† When the temperature is elevated from 30 °C to 80 °C, the diffraction peaks of crystals are matched well with the standard card of Rb₂MnBr₄·2H₂O (PDF# 50-0629). The intensity of diffraction peaks gradually declines. Moreover, the strong diffraction peaks at 30.20° and 30.56° decline rapidly with the increase of temperature to 220 °C, while the peak intensity at 31.34° gradually enhances. Meanwhile, the peak at 31.34° shifts toward a higher angle and presents a broadening feature, demonstrating that the cell volume of crystals has reduced at high temperatures. In order to further study the composition of crystals at high temperatures, Fourier-transform infrared (FT-IR) spectra, TGA curve, SEM, and EDS analysis were carried out. FT-IR spectra indicate that the intensity of –OH vibration peaks at 1600 cm⁻¹ and 3343 cm⁻¹ obviously decreases with the increase of temperature from 30 °C to 160 °C (Fig. S5†).³⁵ TGA curve shows that the mass loss of the sample after 160 °C treatment is 2.80% (Fig. S3b†). The calculated chemical structure of the sample after heat treatment at 160 °C can be expressed as Rb₂MnBr₄·0.9H₂O. In addition, SEM images show that some holes appear on the surface of crystals, which might originate from the evaporation of water molecules (Fig. S6†). Furthermore, EDS results show that the molar ratio of n(Rb) : n(Mn) : n(Br) : n(O) is 30.91 : 10.41 : 46.40 : 12.28 (Table S2†), agreeing with the results of TGA. These results reveal that the shift in diffraction peak is assigned to the dehydration process of Rb₂MnBr₄·2H₂O.

Fig. S7† displays X-ray photoelectron spectra (XPS) of Rb₂MnBr₄·2H₂O and Rb₂MnBr₄·0.9H₂O. From the high-resolution XPS spectrum of Mn 2p (Fig. S7d†), the peaks centered at 640.84 and 646.28 eV are assigned to Mn²⁺ 2p_3/2, while the peaks located at 653.36 and 658.07 eV are attributed to Mn²⁺ 2p_1/2.³⁶ Meanwhile, the peak at 642.10 eV originated from Mn³⁺ 2p_3/2.³⁷ When the structural water is partially removed from the Rb₂MnBr₄·2H₂O lattice, the binding energy of Mn²⁺ 2p_3/2, Mn²⁺ 2p_1/2 and Mn³⁺ 2p_3/2 decreases to 640.77 and 646.22 eV, 653.30 and 658.01 eV, and 642.05 eV, respectively, because of the enhancing electron density around the Mn ions by lattice shrinkage.³⁸ Meanwhile, the binding energies of Rb 3d, Br 2p, and O 1s also move toward lower energy side (Fig. S7b, c and e†). The peak area percentage of Mn³⁺ exhibits a slight enhancement (from 35.81% to 38.20%) during the dehydration process under high temperature, while the peak area percentage of Mn²⁺ still keeps above 60%, demonstrating that the samples are mainly composed of Mn²⁺.

Density functional theory (DFT) was implemented to understand the band gap structure and electronic structure of the dehydration process in Rb₂MnBr₄·2H₂O, as shown in Fig. 3a–d. To simplify the calculation process, the crystal structures of Rb₂MnBr₄·1H₂O and Rb₂MnBr₄·2H₂O with 1 × 1 × 1 supercell are used. The optimized crystal structures via Perdew–Bruke–Ernzerhof (PBE) functional are shown in Fig. S8.† It can be found that the remaining water molecules can stabilize the structure, which serves the [MnBr₄O]H₂²⁻ units. The energy gap for Rb₂MnBr₄·2H₂O can be calculated to be around 2.45 eV at the Q point of the Brillouin zone, which is close to the calculated optical bandgap of 2.36 eV from the UV absorption spectrum (Fig. S9b†). The density states of Rb₂MnBr₄·2H₂O (Fig. 3b) display that the valence band maximum (VBM) is mainly composed of Br 4p, O 2p, and Mn 3d orbitals, while the conduction band minimum (CBM) consists of Mn 4d orbitals. The Rb 5s and H 1s orbitals do not contribute to VBM and CBM. The partial density of states of Rb exhibit a relatively large energy gap, confirming that the Rb cations have no obvious coupling with manganese and bromine. When two water molecules reduce in half, the band gap of the sample reduces to 2.13 eV, which is fairly in agreement with the calculated optical bandgap of 2.11 eV in Rb₂MnBr₄·0.9H₂O (Fig. 3c). The VBM mainly derives from the Br 4p and Mn 3d orbitals, while the CBM still comprises Mn 3d states, which lies at the G point (Fig. 3d), resulting in a direct band gap. It is worth noting that the O 2p orbitals are not contributing to VBM due to the dehydration of water molecules. These results indicate that the electrons in the dehydrated sample are favorable to transition from CBM to VBM, leading to the bright red luminescence in Rb₂MnBr₄·0.9H₂O.

Fig. 3 Band structures and the corresponding electronic structures of (a and b) Rb₂MnBr₄·2H₂O and (c and d) Rb₂MnBr₄·1H₂O, (e) PL decay curve of Rb₂MnBr₄·2H₂O after heat treatment at 160 °C, and (f) Tanabe–Sugano energy diagram of 3d⁵ electron orbital.

The PL emission lifetimes of Rb₂MnBr₄·0.9H₂O were measured, as shown in Fig. 3e. The decay curve was well fitted by the biexponential function: I(t) = A₁ exp(t/τ₁) + A₂ exp(t/τ₂), where τ₁ and τ₂ represent two lifetimes.³⁹ The calculated decay lifetimes τ₁ is 0.020 ms with a proportion of 6.61%, and τ₂ is 0.627 ms with a proportion of 93.39%. The shorter lifetime τ₁ is ascribed to the non-radiative recombination of water molecules, while the longer lifetime τ₂ is attributable to the d–d transition of Mn²⁺.^40,41 The results demonstrate that the luminescence behavior of Rb₂MnBr₄·2H₂O is closely related to the content of water molecules. When the non-radiative recombination from water molecules is suppressed, realizing the efficient emission of Rb₂MnBr₄·0.9H₂O. The average lifetime of the sample is 0.653 ms.

The d–d transition of Mn²⁺ can be described via Tanabe–Sugano (T–S) energy diagram in Fig. 3f. The ground state ⁶S and excited state ⁴G of octahedron Mn²⁺ are related to the crystal field (Δ). The Δ calculated from the PLE spectrum is 6990.39 cm⁻¹ (detailed calculation in ESI†). The larger Δ indicates that the energy levels have stronger splitting, leading to a lower energy emission.²⁷ It is shown that the distortion of [MnBr₄O₂]H₄²⁻ octahedron during dehydration causes a stronger crystal field, which is responsible for the bright red emission.

To study the effect of electron–phonon coupling on the emission properties, temperature-dependent PL spectra of Rb₂MnBr₄·0.9H₂O were carried out, as shown in Fig. 4. The PL intensity of the sample gradually decreased by enhancing the temperature from 80 K to 280 K because of the quenching of excitons.²⁸ Interestingly, there is a slight increase in emission intensity with the increase of temperature from 280 K to 360 K, which is ascribed to the loss of water molecules. Furthermore, the emission intensity continues to decrease by elevating the temperature from 360 K to 500 K. It is worth noting that the emission intensity of the sample presents a slight decrease at 440 K, in which 92.68% of emission intensity at 300 K can be maintained. In addition, only 11.98% of intensity attenuation (relative to the intensity at 300 K) can be found at 480 K, indicating the excellent thermal stability of Rb₂MnBr₄·0.9H₂O, which is superior to the previously reported Mn²⁺ halide red emitters.^20–23,42 In addition, the PL peak displays a blue shift tendency with the increase in temperature, which is attributed to the fact that the crystal field intensity and the spin–spin coupling effect between adjacent Mn²⁺ ions decrease due to polyhedral distortion and lattice expansion (Fig. 4c).⁴⁰ The emission line-width increases by enhancing the temperature, resulting from the enhancement of the interaction between the electron and phonon. The related FWHM and temperature can be expressed by the following equation:⁴³

where S, ħω_phonon, k_B, and T are the electron–phonon coupling parameter, phonon frequency, Boltzmann constant, and temperature, respectively. The calculated S and ħω_phonon are 6.60 and 25.13 meV (Fig. 4d), respectively. The small S value indicates that the electron–phonon coupling has little effect on the luminescence emission of the sample. Therefore, the PL emission of Rb₂MnBr₄·0.9H₂O is mainly ascribed to the d–d transition of Mn²⁺.

Fig. 4 (a) Temperature-dependent emissive spectra of Rb₂MnBr₄·0.9H₂O, (b) illustrating the variation of PL intensity, (c) peak position and FWHM with different temperatures in the range of 80–500 K, (d) FWHM as a function of temperature.

To verify the reversible process of dehydration–hydration and investigate the heating–cooling cycle stability, Rb₂MnBr₄·2H₂O crystals were heated at 160 °C and exposed to an ambient environment for 24 h, achieving a dehydration–hydration cycle. The normalized PL intensity of Rb₂MnBr₄·2H₂O in the dehydration–hydration cycle is shown in Fig. S10a.† The PL intensity maintains about 50% of the initial value after 8 cycles (Fig. S10b†). Interestingly, XRD results demonstrate that Rb₂MnBr₄·2H₂O can be transformed to Rb₂MnBr₄·0.9H₂O under heat treatment at 160 °C, while Rb₂MnBr₄·0.9H₂O can also be recovered into Rb₂MnBr₄·2H₂O by exposure to air under a relative humidity of 65% for 24 h (Fig. S10c and d†), indicating that the process of dehydration–hydration is reversible. In addition, after 8 dehydration–hydration cycles, the sample still maintains a pristine structure, demonstrating that Rb₂MnBr₄·2H₂O has good cycle stability. Considering the unique dehydration–hydration cycle stability, excellent emission performance and thermal stability of rubidium manganese halides, they are promising to be applied as a heat/water-sensing or emitter materials.^39,44

In order to prove the application potential of Rb₂MnBr₄·0.9H₂O in the lighting field, a white LED is fabricated by combining Rb₂MnBr₄·0.9H₂O red emitters and commercial green phosphors with a blue LED chip. The electroluminescence spectrum of the as-fabricated WLED device at the current of 20 mA and the corresponding color coordinates are presented in Fig. 5a and b. The as-fabricated device exhibits bright white emission, in which the corresponding color coordinate of the device is (0.3388, 0.3454), close to the standard white coordinate of (0.33, 0.33). Furthermore, the WLED exhibits a high CRI of 91.5 and excellent LE of 84.84 lm W⁻¹ with CCT of 5238 K, which is superior to most of the reported red emissive Mn²⁺-halide LED devices.^21,22 Moreover, the driving current-dependent electroluminescence spectra show that the emission intensity of the device enhances with the increase in current (Fig. 5c). The CRI reaches the maximum value of 93.6 at a current of 60 mA (Fig. 5d). These results indicate that the as-fabricated WLED device has good stability.

Fig. 5 (a) Electroluminescence spectrum of WLED device at 20 mA (inset is a working photo of the device), (b) corresponding color coordinates, (d) electroluminescence spectra of WLED with different driving currents (c), evolution of CRI and LE at different driving currents.

3 Conclusions

In summary, Rb₂MnBr₄·2H₂O crystals were successfully synthesized via a facile slow solvent evaporation method. The mechanism of thermo-induced fluorescence enhancement of Rb₂MnBr₄·2H₂O crystals was elaborated in detail, in which the non-radiative recombination from water molecules and the energy transfer between adjacent Mn²⁺ ions were restrained due to the thermo-induced partial dehydration and isolated polyhedron units, respectively, resulting in a broad red emission at 653 nm with FWHM of 107 nm and high PLQY of 62.8%. Moreover, we confirmed that the composition of highly emissive dehydrated rubidium manganese halides was Rb₂MnBr₄·0.9H₂O, which also presented excellent thermal stability, and 88% of room temperature emission intensity could be maintained under the temperature of 480 K. As a result, a WLED based on red emissive Rb₂MnBr₄·0.9H₂O and green emissive commercial phosphors is fabricated, which exhibits a high CRI of 91.5 and excellent LE of 84.84 lm W⁻¹ with CCT of 5238 K. This work will provide a reliable way for the development of lead-free manganese halides and photoelectric applications.

4 Experimental

4.1 Materials

Rubidium carbonate (Rb₂CO₃, AR, 99%), manganese(II) carbonate (MnCO₃, AR, 99.95%), hydrobromic acid (HBr, 48 wt% in H₂O), and ethanol (C₂H₅OH, AR, 99.8%) were purchased from Aladdin. All reagents were used as received without further purification.

4.2 Synthesis of Rb₂MnBr₄·2H₂O crystal

In a typical synthesis, Rb₂CO₃ (1 mmol), MnCO₃ (1 mmol), and 2 mL HBr were added in a 10 mL bottle. After being heated and stirred at 80 °C for 1 h, the mixture solution becomes clear. Then, the mixture solution was filtrated and then heated at 80 °C for three days. The Rb₂MnBr₄·2H₂O crystals were filtered out via washing with ethanol three times and drying at 60 °C for 6 h.

4.3 WLED devices

The red-emissive Rb₂MnBr₄·0.9H₂O powders, commercial green phosphors (ZLG535, Ganzhou Zhonglan Rare Earth New Material Technology Co. Ltd), and epoxy resin were mixed and coated on a blue InGaN LED chip (460 nm). Then, the final WLED device was obtained after curing at 120 °C for 1 h.

4.4 DFT calculation

Cambridge Serial Total Energy Package (CASTEP) was used for DFT calculations.^45,46 The cutoff energy and self-consistent field tolerance of Rb₂MnBr₄·2H₂O and Rb₂MnBr₄·1H₂O were set at 580 eV and 1 × 10⁻⁵ eV per atom, respectively. The Brillouin zone was used for 4 × 4 × 4 Monkhorst-Pack grids. In order to characterize the interactions of Mn 3d electrons, the spin-polarized effect was also considered. The generalized gradient approximation (GGA)-PBE function was used to calculate the optimized structure, bandgap, and electronic structure of Rb₂MnBr₄·2H₂O and Rb₂MnBr₄·1H₂O.

4.5 Characterizations

The powder XRD patterns and temperature-dependent XRD patterns of Rb₂MnBr₄·2H₂O were obtained by a Bruker D8 Advance diffractometer with Cu-Kα line. Rietveld refinements of Rb₂MnBr₄·2H₂O were carried out by the GSAS package. The microscopic morphology and element analysis of the Mn²⁺ halides were obtained by using a HITACHI SU-8010 scanning electron microscope and EDS (Model 550i, IXRF). The PL performances were collected via an Edinburgh FLS980 fluorescence spectrometer, which includes PL emission, excitation spectra, and PLQY. The absorption spectra of Mn²⁺ halides were collected via a PerkinElmer Lambda 850 UV–Vis spectrophotometer. FT-IR spectra of samples were obtained using Fourier infrared spectrometer (Nicolet 5700) with a KBr matrix. TGA curve of Mn²⁺ halides was obtained via NETZSCH STA 449C instrument under argon with a heating rate of 10 °C min⁻¹. The properties of LED devices were recorded by an Everfine ATA100 integrating sphere spectroradiometer.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research is financially supported by the National Key Research and Development Program of China (No. 2021YFB3500504), National Natural Science Foundation of China (No. 91963204 and 52262003), Innovation Program of Shanghai Municipal Education Commission (No. 2017-01-07-00-03-E00025), Jiangxi Provincial Natural Science Foundation (No. 20224BAB204020 and 20224BAB214024), the Key Project of Jiangxi Natural Science Foundation (No. 2020ACBL214008), the Science Foundation of Jiangxi Provincial Department of Education (No. GJJ211343), Jingdezhen Science and Technology Project (No. 20212GYZD009-07), Scientific Research Project of Jiangxi Province Education Department (No. GJJ2201006) and 2021 College Teacher Featured Innovation Research Project (No. 2021XCL11).

References

1. J. S. Kim, J.-M. Heo, G.-S. Park, S.-J. Woo, C. Cho, H. J. Yun, D.-H. Kim, J. Park, S.-C. Lee, S.-H. Park, E. Yoon, N. C. Greenham and T.-W. Lee, Ultra-bright, efficient and stable perovskite light-emitting diodes, Nature, 2022, 611, 688–694.
2. A. A. Zhumekenov, M. I. Saidaminov, O. F. Mohammed and O. M. Bakr, Stimuli-responsive switchable halide perovskites: Taking advantage of instability, Joule, 2021, 5, 2027–2046.
3. X. Hu, Y. Xu, J. Wang, J. Ma, L. Wang and W. Jiang, In situ fabrication of superfine perovskite composite nanofibers with ultrahigh stability by one-step electrospinning toward white light-emitting diode, Adv. Fiber Mater., 2023, 5, 183–197.
4. M. Fan, J. Huang, L. Turyanska, Z. Bian, L. Wang, C. Xu, N. Liu, H. Li, X. Zhang, C. Zhang and X. Yang, Efficient all-perovskite white light-emitting diodes made of in situ grown perovskite-mesoporous silica nanocomposites, Adv. Funct. Mater., 2023, 2215032.
5. J. Chen, H. Xiang, J. Wang, R. Wang, Y. Li, Q. Shan, X. Xu, Y. Dong, C. Wei and H. Zeng, Perovskite white light emitting diodes: progress, challenges, and opportunities, ACS Nano, 2021, 15, 17150–17174.
6. H. D. Tang, Y. Q. Xu, X. B. Hu, Q. Hu, T. Chen, W. H. Jiang, L. J. Wang and W. Jiang, Lead-free halide double perovskite nanocrystals for light-emitting applications: strategies for boosting efficiency and stability, Adv. Sci., 2021, 8, 2004118.
7. Y. Huang, Y. Pan, C. Peng, Y. Ding, H. Lian, L. Li and J. Lin, Orange/cyan emissive sensors of Sb³⁺ for probing water via reversible phase transformation in rare-earth-based perovskite crystals, Inorg. Chem. Front., 2023, 10, 991–1000.
8. Q. Zhang, S. Liu, M. He, W. Zheng, Q. Wan, M. Liu, X. Liao, W. Zhan, C. Yuan, J. Liu, H. Xie, X. Guo, L. Kong and L. Li, Stable lead-free tin halide perovskite with operational stability >1200 h by suppressing tin(II) oxidation, Angew. Chem., Int. Ed., 2022, 61, e202205463.
9. Z. Li, Z. Deng, A. Johnston, J. Luo, H. Chen, Y. Dong, R. Sabatini and E. H. Sargent, Precursor tailoring enables alkylammonium tin halide perovskite phosphors for solid-state lighting, Adv. Funct. Mater., 2022, 32, 2111346.
10. M. Leng, Y. Yang, K. Zeng, Z. Chen, Z. Tan, S. Li, J. Li, B. Xu, D. Li, M. P. Hautzinger, Y. Fu, T. Zhai, L. Xu, G. Niu, S. Jin and J. Tang, All-inorganic bismuth-based perovskite quantum dots with bright blue photoluminescence and excellent stability, Adv. Funct. Mater., 2018, 28, 1704446.
11. Z.-Z. Ma, Z.-F. Shi, L.-T. Wang, F. Zhang, D. Wu, D.-W. Yang, X. Chen, Y. Zhang, C.-X. Shan and X.-J. Li, Water-induced fluorescence enhancement of lead-free cesium bismuth halide quantum dots by 130% for stable white light-emitting devices, Nanoscale, 2020, 12, 3637–3645.
12. J.-L. Li, Y.-F. Sang, L.-J. Xu, H.-Y. Lu, J.-Y. Wang and Z.-N. Chen, Highly efficient light-emitting diodes based on an organic antimony(III) halide hybrid, Angew. Chem., Int. Ed., 2022, 61, e202113450.
13. Z. Ma, Z. Shi, D. Yang, F. Zhang, S. Li, L. Wang, D. Wu, Y. Zhang, G. Na, L. Zhang, X. Li, Y. Zhang and C. Shan, Electrically-driven violet light-emitting devices based on highly stable lead-free perovskite Cs₃Sb₂Br₉ quantum dots, ACS Energy Lett., 2020, 5, 385–394.
14. L. Zhou, J.-F. Liao, Z.-G. Huang, J.-H. Wei, X.-D. Wang, W.-G. Li, H.-Y. Chen, D.-B. Kuang and C.-Y. Su, A highly red-emissive lead-free indium-based perovskite single crystal for sensitive water detection, Angew. Chem., 2019, 131, 5331–5335.
15. Z. Gong, W. Zheng, P. Huang, X. Cheng, W. Zhang, M. Zhang, S. Han and X. Chen, Highly efficient Sb³⁺ emitters in 0D cesium indium chloride nanocrystals with switchable photoluminescence through water-triggered structural transformation, Nano Today, 2022, 44, 101460.
16. Z. Ma, Z. Shi, D. Yang, Y. Li, F. Zhang, L. Wang, X. Chen, D. Wu, Y. Tian, Y. Zhang, L. Zhang, X. Li and C. Shan, High color-rendering index and stable white light-emitting diodes by assembling two broadband emissive self-trapped excitons, Adv. Mater., 2021, 33, 2001367.
17. H. Peng, Y. Tian, X. Wang, T. Huang, Z. Yu, Y. Zhao, T. Dong, J. Wang and B. Zou, Pure white emission with 91.9% photoluminescence quantum yield of [(C₃H₇)₄N]₂Cu₂I₄ out of polaronic states and ultra-high color rendering index, ACS Appl. Mater. Interfaces, 2022, 14, 12395–12403.
18. C.-Y. Wang, P. Liang, R.-J. Xie, Y. Yao, P. Liu, Y. Yang, J. Hu, L. Shao, X. W. Sun, F. Kang and G. Wei, Highly efficient lead-free (Bi, Ce)-codoped Cs₂Ag_0.4Na_0.6InCl₆ double perovskites for white LEDs, Chem. Mater., 2020, 32, 7814–7821.
19. S. Wang, Y. Xu, T. Chen, W. Jiang, J. Liu, X. Zhang, W. Jiang and L. Wang, A red phosphor LaSc₃(BO₃)₄: Eu³⁺ with zero-thermal-quenching and high quantum efficiency for LEDs, Chem. Eng. J., 2021, 404, 125912.
20. S. Wang, X. Han, T. Kou, Y. Zhou, Y. Liang, Z. Wu, J. Huang, T. Chang, C. Peng, Q. Wei and B. Zou, Lead-free MnII-based red-emitting hybrid halide (CH₆N₃)₂MnCl₄ toward high performance warm WLEDs, J. Mater. Chem. C, 2021, 9, 4895–4902.
21. G. Hu, B. Xu, A. Wang, Y. Guo, J. Wu, F. Muhammad, W. Meng, C. Wang, S. Sui, Y. Liu, Y. Li, Y. Zhang, Y. Zhou and Z. Deng, Stable and bright pyridine manganese halides for efficient white light-emitting diodes, Adv. Funct. Mater., 2021, 31, 2011191.
22. T. Jiang, W. Ma, H. Zhang, Y. Tian, G. Lin, W. Xiao, X. Yu, J. Qiu, X. Xu, Y. Yang and D. Ju, Highly efficient and tunable emission of lead-free manganese halides toward white light-emitting diode and X-ray scintillation applications, Adv. Funct. Mater., 2021, 31, 2009973.
23. G. Zhou, J. Ding, X. Jiang, J. Zhang, M. S. Molokeev, Q. Ren, J. Zhou, S. Li and X.-M. Zhang, Coordination units of Mn²⁺ modulation toward tunable emission in zero-dimensional bromides for white light-emitting diodes, J. Mater. Chem. C, 2022, 10, 2095.
24. Q. Kong, B. Yang, J. Chen, R. Zhang, S. Liu, D. Zheng, H. Zhang, Q. Liu, Y. Wang and K. Han, Phase engineering of cesium manganese bromides nanocrystals with color-tunable emission, Angew. Chem., Int. Ed., 2021, 60, 19653–19659.
25. H. B. Jalali, A. Pianetti, J. Zito, M. Imran, M. Campolucci, Y. P. Ivanov, F. Locardi, I. Infante, G. Divitini, S. Brovelli, L. Manna and F. D. Stasio, Cesium manganese bromide nanocrystal sensitizers for broadband Vis-to-NIR downshifting, ACS Energy Lett., 2022, 7, 1850–1858.
26. J.-H. Wei, J.-F. Liao, X.-D. Wang, L. Zhou, Y. Jiang and D.-B. Kuang, All-inorganic lead-free heterometallic Cs₄MnBi₂Cl₁₂ perovskite single crystal with highly efficient orange emission, Matter, 2020, 3, 892–903.
27. J. Almutlaq, W. J. Mir, L. Gutiérrez-Arzaluz, J. Yin, S. Vasylevskyi, P. Maity, J. Liu, R. Naphade, O. F. Mohammed and O. M. Bakr, CsMnBr₃: lead-free nanocrystals with high photoluminescence quantum yield and picosecond radiative lifetime, ACS Mater. Lett., 2021, 3, 290–297.
28. X. Hao, H. Liu, W. Ding, F. Zhang, X. Li and S. Wang, Zn²⁺-doped lead-free CsMnCl₃ nanocrystals enable efficient red emission with a high photoluminescence quantum yield, J. Phys. Chem. Lett., 2022, 13, 4688–4694.
29. M. Lia and Z. Xia, Recent progress of zero-dimensional luminescent metal halides, Chem. Soc. Rev., 2021, 50, 2626.
30. S. Yan, W. Tian, H. Chen, K. Tang, T. Lin, G. Zhong, L. Qiu, X. Pan and W. Wang, Synthesis of 0D manganese-based organic-inorganic hybrid perovskite and its application in lead-free red light-emitting diode, Adv. Funct. Mater., 2021, 31, 2100855.
31. H. Xiao, P. Dang, X. Yun, G. Li, Y. Wei, Y. Wei, X. Xiao, Y. Zhao, M. S. Molokeev, Z. Cheng and J. Lin, Solvatochromic photoluminescent effects in all-Inorganic manganese(II)-based perovskites by highly selective solvent induced crystal-to-crystal phase transformations, Angew. Chem., 2021, 133, 3743–3751.
32. K. Carrander, Magnetic phase diagram of Rb₂MnBr₄·2H₂O, Phys. Scr., 1973, 7, 295.
33. S. Zhang, Y. Zhao, J. Zhou, H. Ming, C.-H. Wang, X. Jing, S. Ye and Q. Zhang, Structural design enables highly-efficient green emission with preferable blue light excitation from zero-dimensional manganese(II) hybrids, Chem. Eng. J., 2021, 421, 129886.
34. Q.-C. Yang, Y. Li, Y.-L. Dang, R.-J. Liu and Z.-P. Qiao, Ternary diagram of RbX–MnX₂–H₂O (X = Cl, Br) at T = 298.15 K and the standard enthalpies of dissolution and formation of their double salts, J. Chem. Eng. Data, 2022, 67, 2800–2807.
35. R. Liu, W. Zhang, G. Lia and W. Liu, An ultraviolet excitation anti-counterfeiting material of Sb³⁺ doped Cs₂ZrCl₆ vacancy-ordered double perovskite, Inorg. Chem. Front., 2021, 8, 4035–4043.
36. A. M. Anthony, M. K. Pandian, P. Pandurangan and M. Bhagavathiachari, Zero-and one-dimensional lead-free perovskites for photoelectrochemical applications, ACS Appl. Mater. Interfaces, 2022, 14, 29735–29743.
37. Z. Han, J.-J. Feng, Y.-Q. Yao, Z.-G. Wang, L. Zhang and A.-J. Wang, Mn, N, P-tridoped bamboo-like carbon nanotubes decorated with ultrafine Co₂P/FeCo nanoparticles as bifunctional oxygen electrocatalyst for long-term rechargeable Zn-air battery, J. Colloid Interface Sci., 2021, 590, 330–340.
38. L. Shao, D. Zhou, N. Ding, R. Sun, W. Xu, N. Wang, S. Xu, D. Liu and H. Song, Broadband ultraviolet photodetectors based on cerium doped lead-free Cs₃MnBr₅ metal halide nanocrystals, ACS Sustainable Chem. Eng., 2021, 9, 4980–4987.
39. J.-B. Luo, J.-H. Wei, Z.-Z. Zhang and D.-B. Kuang, Water-molecule-induced emission transformation of zero-dimension antimony-based metal halide, Inorg. Chem., 2022, 61, 338–345.
40. X. Zhu, S. Meng, Y. Zhao, S. Zhang, J. Zhang, C. Yin and S. Ye, Mn²⁺-Mn²⁺ magnetic coupling effect on photoluminescence revealed by photomagnetism in CsMnCl₃, J. Phys. Chem. Lett., 2020, 11, 9587–9595.
41. C. Li, X. Bai, Y. Guo and B. Zou, Tunable emission properties of manganese chloride small single crystals by pyridine incorporation, ACS Omega, 2019, 4, 8039–8045.
42. K. Li, W. Zhang, L. Niu, Y. Ye, J. Ren and C. Liu, Lead-free cesium manganese halide nanocrystals embedded glasses for X-ray imaging, Adv. Sci., 2023, 10, 2204843.
43. T. Huang, Q. Wei, W. Lin, H. Peng, S. Yao and B. Zou, High-efficient yellow-green emission in (TDMP)MnBr₄ single crystal with modulation of spin-phonon-charge interactions, Mater. Today Phys., 2022, 25, 100703.
44. J.-F. Hong, B. Wang, X.-Y. Zhang, H. Xu, H.-X. Zhao, L.-S. Long and L.-S. Zheng, Water-driven successive structural transformation in a two-dimensional (2D) lead-free hybrid double perovskite, Inorg. Chem., 2022, 61, 20531–20537.
45. S. J. Clark, M. D. Segall, C. J. Pickard, P. J. Hasnip, M. I. J. Probert, K. Refson and M. C. Payne, First principles methods using CASTEP, Z. Kristallogr., 2005, 220, 567–570.
46. W. Setyawan and S. Curtarolo, High-throughput electronic band structure calculations: challenges and tools, Comput. Mater. Sci., 2010, 49, 299–312.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qi00638g

---