DOI:
10.1039/D3QI00638G
(Research Article)
Inorg. Chem. Front., 2023,
10, 4221-4229
A thermally induced fluorescence enhancement strategy for efficient all-inorganic rubidium manganese halide†
Received
6th April 2023
, Accepted 10th June 2023
First published on 12th June 2023
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 Cs2Ag0.60Na0.40InCl6 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 P
(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) Å3 (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+ 2p3/2, while the peaks located at 653.36 and 658.07 eV are attributed to Mn2+ 2p1/2.36 Meanwhile, the peak at 642.10 eV originated from Mn3+ 2p3/2.37 When the structural water is partially removed from the Rb2MnBr4·2H2O lattice, the binding energy of Mn2+ 2p3/2, Mn2+ 2p1/2 and Mn3+ 2p3/2 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
|  | (1) |
where
S,
ħωphonon,
kB, and
T are the electron–phonon coupling parameter, phonon frequency, Boltzmann constant, and temperature, respectively. The calculated
S and
ħωphonon 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 Rb
2MnBr
4·0.9H
2O is mainly ascribed to the d–d transition of Mn
2+.
 |
| 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).
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