Discovery of the Crystal Phase Transition on Lead-free Cesium Manganese Bromine Perovskite Nanocrystal by Solvent Concentration

doi:10.21203/rs.3.rs-805887/v1

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Discovery of the Crystal Phase Transition on Lead-free Cesium Manganese Bromine Perovskite Nanocrystal by Solvent Concentration

https://doi.org/10.21203/rs.3.rs-805887/v1

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We have been demonstrated the crystal phase transition for lead-free cesium manganese bromine perovskite nanocrystal synthesized by the modified hot-injection method due to change the concentration of solvent (trioctylphosphine; TOP). The compositions to be synthesized were determined by the amount of TOP solvent, and the structure phase of nanocrystal was changed from hexagonal CsMnBr₃ to tetragonal Cs₃MnBr₅ as the amount of TOP solvent increased. The emission peaks of CsMnBr₃ and Cs₃MnBr₅ nanocrystals were observed at 650 nm (red) and 520 nm (green), respectively. After a durability test at 85 °C and 85% humidity for 24 h, the lead-free perovskite CsMnBr₃ nanocrystal powder maintained its initial emission intensity, and the metal halide Cs₃MnBr₅ nanocrystal powder exhibited an increase in red emission due to the post-synthesis of CsMnBr₃ nanocrystals.

Optical Materials and Devices

CsMnBr3 perovskite nanocrystals

Metal halide Cs3MnBr5 nanocrystal

Phase-tunable synthesis

Lead halide perovskites, which have the general formula APbX₃ (where A = CH₃NH₃⁺ (MA), HC(NH₂)₂⁺ (FA), or Cs⁺; X = Cl, Br or I), has been studied since the middle of the 20th century [1–5]. Among these perovskites, inorganic cesium lead halide perovskite (Cs_nPbX_2+n, X = Cl, Br, I, or a mixture thereof) nanocrystals have attracted significant attention as an optical material for display and lamp applications (e.g. color converter material), as an alternative to quantum dot materials. Within the past decade, there have been rapid advances in the synthesis of cesium lead halide perovskite nanocrystals for use in solar cells, light-emitting diodes (LEDs), lasers, and photodetectors because of the excellent optoelectronic performance of these perovskite nanocrystals [6–13]. They have interesting optical, excitonic, and charge-transport properties, including an outstanding photoluminescence quantum yield (PLQY) and tunable optical bandgap.

Despite these advantages, however, the presence of Pb, a toxic element, in perovskite nanocrystals raises critical concerns with regard to commercialization. This is because the heavy metal Pb can affect human health and the environment [14, 15]. The US EPA has set the maximum allowable content of Pb in air and water as 0.15 µg/L and 15 µg/L, respectively. The European Union regulates the use of heavy and toxic materials (including Pb) in electronic devices, and the use of lead-based technology is expected to decline in the future [16, 17] Therefore, the development of a lead-free cesium halide perovskite that retains the outstanding properties of cesium lead halide nanocrystals is important, and several studies have been conducted recently in this regard.

Metallic elements that have an electronic structure similar to that of Pb and can thus form stable perovskite structures—such as Sn, Sb, Bi, Eu, and Yb—have been considered as alternatives for Pb. Various lead-free halide perovskite nanocrystals, including those containing the aforementioned metal elements, have recently been developed and have achieved optoelectronic performance comparable to that of Pb-based counterparts [17–19]. These encouraging results obtained via exploration of lead-free perovskite nanocrystals indicate a major imminent breakthrough in the fabrication of new optoelectronic devices.

In this study, to obtain lead-free inorganic halide perovskite nanocrystals, we focused on replacing Pb with the transition metal Mn. Mn has an ionic radius (0.083 nm for 6 coordination) similar to that of Pb (0.119 nm for 6 coordination), as well as a low toxicity, and has therefore attracted attention as a substitute for the B site in halide perovskites. Cesium manganese bromide perovskite nanocrystals, such as the red-emitting CsMnBr₃ and green-emitting Cs₃MnBr₅, have been reported to exhibit excellent optical properties [20–23]. In this paper, we synthesized red-emitting CsMnBr₃ and Cs₃MnBr₅ perovskite nanocrystals using the modified hot-injection method and investigated the optical properties of these perovskite nanocrystals. Furthermore, we investigated the phase-tunable synthesis from Cs₃MnBr₅ to CsMnBr₃ with controlling the amount of solvent.

Figure 1 shows the XRD patterns of the cesium manganese bromide perovskite nanocrystals with the amount of TOP solvent as well as the standard XRD patterns of the CsMnBr₃ and Cs₃MnBr₅ from JCPDS card No. 26–0387 and No. 27–0117, respectively. The crystal structures of CsMnBr₃ and Cs₃MnBr₅ are shown in Fig. 2. The crystal structure of CsMnBr₃ consists of linear chains of distorted face-sharing [MnBr₆] octahedron with a D_3d symmetry parallel to the c-axis, that are bridged by Cs ions. Interestingly, the Mn − Mn distance is very short (~ 0.32982 nm) along the c-axis, leading to strongly coupled optical transitions. The XRD pattern of the sample prepared with 10 mL TOP solvent was indexed to the standard hexagonal CsMnBr₃ structure (JCPDS No. 26–0387); this sample contains the P6₃/mmc (#194) space group and has lattice parameters a = b = 0.785878 nm and c = 0.659641 nm. In the crystal structure of Cs₃MnBr₅, the Mn²⁺ ions are coordinated by four Br^- ions to form a [MnBr₄] regular tetrahedron geometry with D_2d symmetry. The Mn²⁺ site layers are separated by Cs⁺ site layers in the direction of the c-axis, and the adjacent [MnBr₄] tetrahedrons maintain a distance of 0.780593(6) nm. The XRD pattern of the sample prepared with 20 mL TOP solvent corresponds to a single-phase tetragonal Cs₃MnBr₅ structure (JCPDS No. 27–0117); this sample has the I4/mcm (#140) space group and lattice constants a = b = 0.992839 nm and c = 1.561187 nm. The nanocrystal samples prepared with 12.5 and 17.5 mL TOP solvent exhibit a mixed phase with hexagonal CsMnBr₃ and tetragonal Cs₃MnBr₅ phases, and the phase transition of nanocrystal phase from CsMnBr₃ to Cs₃MnBr₅ was observed as the amount of TOP solvent increased.

The morphology of the synthesized nanocrystals with the amount of TOP solvent was analyzed using high-resolution TEM, as shown in Fig. 3. The TEM images of lead-free perovskite CsMnBr₃ nanocrystals present a dispersed hexagonal morphology with an average size of ~ 20 nm. The interplanar spacing relative to the (021) crystalline planes of the CsMnBr₃ nanocrystals is about 0.3 nm. In contrast, the TEM image of Cs₃MnBr₅ nanocrystals shows a dispersed tetragonal morphology with an average size of ~ 30 nm. The interplanar spacing relative to the (213) crystalline planes of the Cs₃MnBr₅ nanocrystals is about 0.33 nm. The nanocrystals prepared with 12.5 and 17.5 mL TOP solvent coexist with the CsMnBr₃ and Cs₃MnBr₅ nanocrystals; the compositional change between the CsMnBr₃ and Cs₃MnBr₅ nanocrystals is determined by the amount of TOP solvent.

Figure 4 presents the PL spectra of the obtained nanocrystals with the amount of TOP solvent. The inset images show the emission of the nanocrystals under irradiation with a 365 nm UV lamp; their color changes from red to green as the amount of TOP solvent increases from 10 to 20 mL. The emission spectrum of the lead-free perovskite CsMnBr₃ nanocrystals exhibits a broad red emission peak centered at 650 nm with a full-width-at-half-maximum (FWHM) of 95 nm. The red PL emission which assigned ⁴T₁→⁶A₁ transition in [MnBr₆]^4- octahedrons of the CsMnBr₃ nanocrystals is considered to be due to the relaxation from the lowest excited state to the ground state of Mn²⁺ ions. This is consistent with reports of red emission from Mn²⁺ in the MnBr₆ octahedron [24, 25]. On the other hand, the emission spectrum of the Cs₃MnBr₅ nanocrystals shows green emission at 520 nm with an FWHM of 50 nm. The strong green emission is ascribed to the metal-centered d–d transition of the Mn²⁺ ion in the d⁵ configuration with a [MnBr₄] tetrahedral coordination geometry [26]. Interestingly, the PL spectra of the Cs–Mn–Br nanocrystals can be controlled by simply changing the amount of TOP solvent, as depicted in Fig. 4. With an increase of the amount of TOP solvent, the relative emission of red (CsMnBr₃) and green (Cs₃MnBr₅) shifts toward green emission, implying that our study can achieve tunable the color between green and red by simply controlling the amount of TOP solvent.

To evaluate the thermal and chemical stability of the CsMnBr₃ and Cs₃MnBr₅ nanocrystals, a durability test was performed under high-temperature and high-humidity conditions of 85°C and 85%, respectively, for 24 h. The PL behaviors of the CsMnBr₃ and Cs₃MnBr₅ nanocrystals after the durability test are shown in Fig. 5 (a). The lead-free perovskite CsMnBr₃ nanocrystal powder exhibits extremely high durability and maintains its initial PL intensity. In comparison, the PL intensity of the metal halide Cs₃MnBr₅ nanocrystal powder does not decrease but rather increases in the red emission (CsMnBr₃) region. It is considered that the materials that remained unreacted during the preparation of the nanocrystals reacted to form more CsMnBr₃ nanocrystals due to the high temperature during the durability test. Both nanocrystals exhibited no change in the PL peak wavelength and FWHM values after the durability test. The structures of CsMnBr₃ and Cs₃MnBr₅ are considered to have remained intact even after the durability test because no difference was observed between the XRD patterns before and after the test. The XRD patterns of the CsMnBr₃ and Cs₃MnBr₅ nanocrystals before and after the durability test are presented in Fig. 5 (b). Both samples exhibited identical XRD patterns before and after the test, indicating that the CsMnBr₃ and Cs₃MnBr₅ structures were not degraded during the durability test.

We discovered the crystal phase transition of lead-free cesium manganese bromine perovskite nanocrystal from hexagonal CsMnBr₃ to tetragonal Cs₃MnBr₅ due to change the concentration of TOP solvent for the first time. The CsMnBr₃ nanocrystals had a hexagonal structure and exhibited a dispersed hexagonal morphology with an average size of ~ 20 nm and an interplanar spacing of 0.3 nm. The Cs₃MnBr₅ nanocrystals had a tetragonal structure and exhibiting a dispersed tetragonal morphology with an average size of ~ 30 nm and an interplanar spacing of 0.33 nm. The compositional change between the CsMnBr₃ and Cs₃MnBr₅ nanocrystals was determined by the amount of TOP solvent, the nanocrystal phase is changed from CsMnBr₃ to Cs₃MnBr₅ as the amount of TOP increases. The emission peaks of the CsMnBr₃ and Cs₃MnBr₅ nanocrystals were observed to be at 650 nm (FWHM = 95 nm) and 520 nm (FWHM = 50 nm), respectively. With an increase in the amount of TOP solvent, the relative emission of red (CsMnBr₃) and green (Cs₃MnBr₅) shifted toward green emission. To examine the stability of the lead-free cesium manganese bromine perovskite nanocrystals, durability tests were performed. The CsMnBr₃ and Cs₃MnBr₅ nanocrystal powders fabricated from the nanocrystal solutions were subjected to high-temperature and high-humidity testing at 85°C and 85% humidity for 24 h. After the durability test, no change was observed in the PL intensity of the lead-free perovskite CsMnBr₃ nanocrystal however, an increase in red emission was observed for the metal halide Cs₃MnBr₅ nanocrystal. Our study demonstrates that phase-tunable synthesis between lead-free perovskite CsMnBr₃ and metal halide Cs₃MnBr₅ nanocrystals can be easily achieved by controlling the amount of TOP solvent; it’s material represents a potential next-generation luminescence material for mini LED devices and applications in optoelectronic devices.

Materials Cesium carbonate (Cs₂CO₃, purity; 99.995%), manganese(II) bromide (MnBr₂, purity; 98%), 1-octadecene (ODE, technical grade; 90%), trioctylphosphine (TOP, technical grade; 90%), oleic acid (OA, technical grade; 90%), and oleylamine (OLA, technical grade; 90%) were purchased from Aldrich.

Synthesis of CsMnBr ₃ and Cs₃MnBr₅ nanocrystal Cs-oleate was prepared by mixing 0.13 g Cs₂CO₃, 0.55 mL OA, 0.4 mL OLA, and 12 mL ODE in a 30 mL glass vial for 1 h at 170°C for dissolution. The solution containing MnBr₂ was prepared by mixing 0.5369 g MnBr₂ and 5–20 mL TOP in a 30 mL glass vial at 170°C for 1 h; then, Cs-oleate was quickly injected into the solution containing MnBr₂. The CsMnBr₃ or Cs₃MnBr₅ nanocrystal solutions reacted at 170°C for 1 min and were cooled in an ice-water bath to suppress the crystal growth of the nanocrystals. The obtained CsMnBr₃/Cs₃MnBr₅ nanocrystal solution was separated via centrifugation at 4000 rpm for 5 min.

Characterizations The crystal structure of the CsMnBr₃ and Cs₃MnBr₅ nanocrystal was identified using X-ray powder diffraction (XRD; Rigaku SmartLab) analysis, and the microstructure and morphology of the nanocrystals was characterized using transmission electron microscopy (TEM; JEOL). The photoluminescence (PL) spectrum was recorded at room temperature using a fluorescence spectrophotometer (PSI, DARSA PRO-3400), and the emission spectrum was obtained at an excitation wavelength of 365 nm. The lead-free cesium manganese bromine perovskite nanocrystal powder was prepared using the nanocrystal solution; a durability test was performed at 85°C and 85% humidity for 24 h.

Acknowledgement

This research was financially supported by the Ministry of Trade, Industry and Energy (MOTIE) and Korea Institute for Advancement of Technology (KIAT) through the International Cooperative R&D program (P0006844_Development of color conversion nanocrystal luminescence materials for next generation display).

Author Contributions

T. W. Kang, J. Hwang, B. S. Bae and S. W. Kim, contributed to the design experiments, interpreted the data and prepared the paper. E. J. Choi and Y. J. Park contributed to the experiments. All authors discussed the results and reviewed the manuscript.

Additional Information

The authors declare no competing financial interest.

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Discovery of the Crystal Phase Transition on Lead-free Cesium Manganese Bromine Perovskite Nanocrystal by Solvent Concentration

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