FAPbBr₃@GA₂PbBr₄ quantum dots: one step fabrication with improved stability for light-emitting applications†

Jiaqi Liu ^a, Feng Zhang *^b, Cuihe Fan ^a, Zhengwei Cao ^a and Yuying Hao *^a
aKey Laboratory of Advanced Transducers and Intelligent Control System (Ministry of Education), Taiyuan University of Technology, Taiyuan 030024, China. E-mail: haoyuying@tyut.edu.cn
^bSchool of Physics and Optoelectronic Engineering, Beijing University of Technology, Beijing 100124, China. E-mail: fengzhang@bjut.edu.cn

First published on 24th July 2024

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Introduction

Due to their unique properties, such as narrow emission line width, high photoluminescence quantum yields (PLQYs), tunable wavelength covering the entire visible region and low cost solution fabrication process, perovskite quantum dots (QDs) have shown great potential applications in optoelectronic devices including solar cells, light-emitting diodes (LEDs), lasers and photodetectors.^1–4 Especially in LEDs, both perovskite QD based phosphor converted LEDs and electroluminescence (EL) devices have witnessed a rapid development in the past several years.^5–7 For example, phosphor converted LEDs using perovskite QDs as color converters have achieved an ultra-wide color gamut of 140% National Television Standards Committee (NTSC).⁸ The external quantum efficiency (EQE) of electroluminescent (EL) devices based on perovskite QDs has experienced a rapid development from 0.76% to exceeding 25%, which is almost comparable to that of organic LEDs or conventional QD based LEDs.^9–13 The excellent performance of perovskite QDs in display devices attracts a broad interest and intense attention from both academic research and industrial development.¹⁴

Although the applications of perovskite QDs in display technologies have made remarkable achievements, the stability issues of QDs strongly restrict the long-term operation of QD based devices.^15,16 In essence, the ionic nature and low formation energy are the main reasons for the instability of perovskites. To overcome this, numerous encapsulation strategies are explored, including composing with polymers, growing into in organic lattice, and coating with oxide shells.^17–21 What is insufficient is that these strategies only provide a physical barrier to protecting perovskites from oxygen and moisture. Except for the optical properties, the conductivity and solution processibility are strongly affected by the above encapsulation strategies. Inspired by the research in conventional QDs, the formation of a core–shell structure is demonstrated as an effective way to regulate the optoelectronic properties and enhance the stability of QDs simultaneously.^22–24 Up to now, preliminary progress has been made on the construction of core–shell perovskite QDs, including CsPbBr₃/SiO₂ nanoparticles, CsPbBr₃/ZnS NCs, FAPbBr₃/CsPbBr₃ NCs, etc.^25–28 However, these core–shell perovskite QDs reported in the literature still face some shortages, such as poor conductivity (especially for oxide shells, like SiO₂), complex fabrication process (precisely designed technique for shell growth), and an unignored lattice mismatch between the core and shell molecules. Therefore, it is of great significance to explore core–shell perovskite QDs with new components and structures.

Here, based on the structural characteristics of perovskite materials, a new type of core–shell structure, 2D/3D core–shell structured FAPbBr₃@GA₂PbBr₄ QDs, was fabricated through a phase transfer enhanced emulsion strategy. The preparation of FAPbBr₃@GA₂PbBr₄ QDs can be accomplished in a one-step synthesis due to the selectivity of formamidine (FA) and guanidine (GA) cations in the core and shell. The influence of the GA⁺/FA⁺ ratio on the formation of FAPbBr₃@GA₂PbBr₄ QDs is also studied by varying the amount of GABr in the precursor solution. The optimal ratio of GA⁺ leading to the formation of high quality core–shell structured FAPbBr₃@GA₂PbBr₄ QDs is demonstrated through a combined analysis with X-ray diffraction (XRD), transmission electron microscopy (TEM), absorption spectra, and steady and time-resolved photoluminescence (PL) spectra. Meaningfully, the as fabricated core–shell FAPbBr₃@GA₂PbBr₄ QDs show a greatly enhanced stability under light, heat and moisture compared to original FAPbBr₃ QDs. Finally, phosphor converted LED devices are fabricated based on FAPbBr₃@GA₂PbBr₄ QDs, proving their superiority in perovskite based optoelectronics.

Results and discussion

The formation of perovskite–perovskite heterojunctions utilizing compositional engineering has become a versatile strategy for the fabrication of perovskite QDs with varied optical properties. In our attempt, perovskites with the A site of FA⁺ and GA⁺ were used for the construction of core–shell perovskite QDs. Due to the limitation of the tolerance factor, GA⁺ cations with a larger ionic radius will form a two dimensional (2D) GA₂PbBr₄ structure rather than the 3D structure of the FAPbBr₃ constructed by FA⁺ cations.^29–31 Therefore, it can be assumed that the addition of GA⁺ in a mixed system would not enter the 3D FAPbBr₃ core and would exist on the surface of the FAPbBr₃ crystal to further induce the formation of GA₂PbBr₄, as shown in Fig. 1. Based on this assumption, 2D/3D core–shell FAPbBr₃@GA₂PbBr₄ QDs can be formed in a one pot synthesis, which greatly simplifies the fabrication process of core–shell perovskite QDs.

To verify this, a related synthesis process was conducted based on our previously reported phase transfer enhanced emulsion reprecipitation strategy (detailed fabrication process can be related to the Experiment section).^32–34 As shown in Fig. 2a, firstly, octylamine and oleic acid were added into 1-hexane to assist the foundation of an emulsion system. Then, a fixed amount of FABr, PbBr₂, and GABr were dissolved in DMF to form the precursor solution. Under vigorous stirring, the precursor solution was swiftly injected into the mixture of 1-hexane, octylamine and oleic acid. Later, 6 mL of acetonitrile (ACN) were added into the mixture to induce the demulsify process and nucleation of perovskite QDs because of the relatively low saturated concentration of perovskite precursors in ACN. Knowing the fact that ACN is miscible with DMF but immiscible with hexane, the newly formed QDs nucleated in DMF and the ACN phase spontaneously transferred into the hexane phase, greatly avoiding the degradation of QDs by DMF. Finally, solution stratification was accomplished after removing the stirring with QDs dispersed in hexane as the top layer and an ACN/DMF mixture as the bottom layer. The top layer of the QD solution was then collected and purified with ethyl acetate for further characterizations. To explore the addition of GA⁺ on the formation of the core–shell structures, the molar ratio between GABr and FABr was adjusted from 0 to 1 with the resulting samples denoted as x = 0, x = 0.2, x = 0.4, x = 0.6, x = 0.8 and x = 1 (detailed addition amounts can be found in Table S1 in ESI†). As shown in Fig. 2b, the as-fabricated QDs dispersed in toluene with the GABr ratio varying from 0 to 0.8 exhibited similar bright green emissions. While QDs with x = 1.0 exhibited a blue emission which most possibly originated from pure GA₂PbBr₄ with a 2D layered structure. To further explore the abrupt color changes from green to blue, the ratio of GABr was finely tuned between 0.8 and 1 with x = 0.85, 0.9 and 0.95. Bluish-green samples were obtained with dual emissions in both the green and blue regions, indicating the occurrence of phase separation rather than formation of a core–shell structure with x larger than 0.8 (see Fig. S1 in ESI†). Moreover, unlike the other samples, QDs with x = 1.0 could not be well dispersed in toluene and precipitated from the solution after storage for 2 h.

TEM and high resolution TEM (HRTEM) characterizations were first applied to acquire the morphology information of the as fabricated perovskite QDs. QDs of x = 1.0 were not characterized due to their poor dispersity. As shown in Fig. 3a–e, QD samples with different amounts of GABr exhibited similar cubic shaped nanoparticles. It can be observed that the particle sizes of the QDs are slightly increased and subsequently decreased with increasing amounts of GABr (see Fig. S2 in ESI†). QDs with a GABr ratio of 0.4 show the most uniform morphology and good dispersity with an average size of 11.8 nm. With increasing the amounts of GABr to 0.6 and 0.8, the boundaries of the as fabricated QDs gradually become indistinct, implying a decreased crystallinity. To further estimate the crystallinity of the as fabricated QDs, HRTEM was applied on typical QDs with a GABr ratio of 0.4. As shown in Fig. 3f, a clear crystal lattice is observed with interplanar distances of 0.29 nm and 0.26 nm corresponding to the (200) and (210) crystal faces of FAPbBr₃, respectively. However, crystal faces that originated from GA₂PbBr₄ could not be well identified from the HRTEM image, which might be related to the instability of the GA₂PbBr₄ shell under high energy electron beams.³⁵

To further investigate the crystal structure and optical properties of the as fabricated QDs, XRD, X-ray photoelectron spectroscopy (XPS), absorption spectroscopy, and steady and time resolved PL were applied. Fig. 4a shows the XRD patterns of the as fabricated QDs with varied amounts of GABr. QDs with x = 0 show the main diffraction peaks of 15.1° and 30.2°, which are remarkable characteristics of the (100) and (200) crystal faces that originate from the FAPbBr₃ perovskite in the cubic phase.^36,37 While QDs with x = 1.0 show a typical 2D layered GA₂PbBr₄ perovskite with diffraction peaks of 8.6°, 9.5°, 12.8°and 17.1°, etc.^38–40 With x ranging from 0.2 to 0.8, the two main characteristic peaks of FAPbBr₃ located at 15.1° and 30.2° are well maintained in the corresponding XRD patterns, which indicates that the 3D FAPbBr₃ structure is dominant in QDs with a GABr content below 0.8. Besides, the obviously widened XRD pattern of x = 0.8 reflects a poor crystallinity with a GA content of 0.8, which is consistent with the unclear crystal grains observed in the TEM image (Fig. 3e). The hardly distinguishable crystal lattice in the HRTEM image (see Fig. S3 in ESI†) confirms the low crystallinity of sample x = 0.8. Except for the two main peaks, it should be noted that additional peaks at 9.6° and 12.8° are also observed in the XRD patterns with x from 0.2 to 0.8 (as indicated by the dashed frame in Fig. 4a). Specifically, an inconspicuous peak at 9.6° appeared in the XRD pattern of x = 0.2, while these two small peaks (9.6° and 12.8°) are simultaneously observed in x = 0.4, x = 0.6 and x = 0.8 (see enlarged XRD pattern in Fig. S4, ESI†), especially for x = 0.4. Through comparing with the XRD pattern of the pure GA₂PbBr₄ perovskite with x = 1.0, it can be found that the observed peaks of 9.6° and 12.8° in QDs with x from 0.2 to 0.8 are consistent with the corresponding peaks of x = 1.0. This implies that the 2D GA₂PbBr₄ structure is also formed in QDs with x from 0.2 to 0.8. To reveal the distribution or connection between FAPbBr₃ and GA₂PbBr₄, XPS analysis (Br-3d, Pb-4f, and full spectra) were applied on QDs with x = 0, x = 0.4 and x = 0.8 (see Fig. S5 in ESI†). The Pb-4f spectra are analyzed in detail as shown in Fig. 4b. All of the QDs exhibit similar XPS spectra with two main peaks of 138.1 eV and 143. 1 eV, which are attributed to Pb-4f_5/2 and Pb-4f_7/2.

Besides the main peaks, small shoulder peaks with binding energies of 136.2 eV and 141.3 eV can also be observed for QDs with x = 0 and x = 0.8. These shoulder peaks are considered to be related to Pb defects in the surfaces or interfaces.^41,42 The absence of these shoulder peaks in QDs with x = 0.4 demonstrates the reduced Pb defects, most probably due to the well passivated interfaces between FAPbBr₃ and GA₂PbBr₄. Fig. 4c and d show the absorption and PL spectra of the as-fabricated QDs. Pure FAPbBr₃ QDs (x = 0) exbibit an absorption band edge at 520 nm while pure GA₂PbBr₄ exhibits absorption peaks at 400 and 417 nm attributed to the layered structure. The absorption spectra of QDs with x = 0.2 to 0.8 simultaneously show the absorption features of FAPbBr₃ and GA₂PbBr₄, indicating the coexistence of FAPbBr₃ and GA₂PbBr₄. From Fig. 4d, QDs with x = 0 to 0.8 show similar PL spectra, with emission peaks gradually blue shifted from 534 nm to 529 nm while an emission peak of 426 nm is observed for pure GA₂PbBr₄ with x = 1.0. It should be noted that a small PL peak at 426 nm can also be observed for QDs with x = 0.8, indicating a slight phase separation of GA₂PbBr₄ and FAPbBr₃. For QDs with x from 0.2 to 0.6, the simultaneously observed absorption features from 2D GA₂PbBr₄ and 3D FAPbBr₃ as well as the single PL peak originating from 3D FAPbBr₃ indicate a strong energy transfer from 2D GA₂PbBr₄ to 3D FAPbBr₃, which also implying the formation of core–shell structure. In fact, the energy transfer from the 2D to 3D perovskites has been intensively investigated and widely recognized as an energy funneling effect.^43,44 Besides, the absolute PLQYs of these QDs were determined by an integrated sphere excited at 405 nm with 61%, 76%, 93%, 79%, 23% and 6% for x = 0 to x = 1.0, respectively (Fig. 4e). To better compare the luminescent properties of the resultant QDs, key photophysical parameters, including emission peaks, full wavelength at half maximum (FWHM), PLQYs and chromaticity coordinates in CIE-1931, are summarized in Table 1. These differences in PLQYs are further analyzed through time resolved PL spectra. As shown in Fig. 4f, the PL decay curve can be fitted to (eqn 1) and the average PL lifetimes are calculated by (eqn (2))

	(1)

	(2)

The fitting results demonstrate that the average PL lifetime first increases and then decreases as x increases, with the most prolonged PL lifetime achieved for QDs with x = 0.4 (detailed fitting results can be found in Table S2 in ESI†). Moreover, QDs in the solid state (typical QD samples were spin cast into thin films on glass substrates) were also tested with similar decay dynamics observed, which further confirmed the carrier recombination dynamics in these QDs (see Fig. S6 in ESI†). The longest PL lifetime of x = 0.4 implies fewer defects and well passivated non-radiative pathways, which is consistent with the XPS and PLQYs results. Based on the above analysis, it can be concluded that the FAPbBr₃@GA₂PbBr₄ core shell structure is formed in QDs with x = 0.2 to 0.6 and QDs with x = 0.4 possess a more ideal core–shell structure.

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The as fabricated FAPbBr₃@GA₂PbBr₄ QDs provide opportunities to explore the influence of the core–shell structure on the luminescent properties of perovskite QDs. Exciton binding energy, acting as an important parameter for semiconductor QDs, was estimated through temperature dependent PL measurements. The corresponding experiment was conducted on an FLS 1000 spectrometer using liquid nitrogen as the cooling medium. The PL intensities of pure FAPbBr₃ QDs and FAPbBr₃@GA₂PbBr₄ (x = 0.4) QDs were collected from 120 K to 300 K. As shown in Fig. 5a and b, both the FAPbBr₃ QDs and FAPbBr₃@GA₂PbBr₄ QDs show a decreased PL intensity with temperature increasing. Compared to core–shell FAPbBr₃@GA₂PbBr₄ QDs, the PL decline of FAPbBr₃ QDs is more rapid. The variation of PL intensity with temperature is plotted in Fig. 5c and d. The related exciton binding energy can be extracted from (eqn (3))

	(3)

In which I(T) represents the PL intensity at a certain temperature and I₀ represents the PL intensity at 0 K. E_b is the exciton binding energy and k_B is the Boltzmann constant. According to the fitting results, E_b values of 217 meV and 478 meV are obtained for the FAPbBr₃ QDs and FAPbBr₃@GA₂PbBr₄ QDs, respectively. The relatively 2.2 fold enhanced exciton binding energy indicates that the photogenerated exciton is more stable and exciton recombination occupies a higher proportion in the FAPbBr₃@GA₂PbBr₄ QDs, which is consistent with the higher PLQYs of the FAPbBr₃@GA₂PbBr₄ QDs.⁴⁵ Moreover, along with temperature increasing, the FWHM is also broadening, which can reflect the exciton phonon interactions.⁴⁶ The exciton–phonon interaction is calculated through (eqn (4)).

	(4)

In eqn 4, the first item Γ₀ is the inhomogeneous broadening contribution. σ and Γ_op describe the interactions of exciton acoustic phonon and exciton optical phonon contributions to the line width broadening, respectively. ħω_op describes the optical phonon energy. The fitting results reveal that FAPbBr₃@GA₂PbBr₄ QDs possess a larger optical phonon energy (27.7 meV) than that of pure FAPbBr₃ QDs (24.1 meV) (The fitting curves can be related to Fig. S7 in ESI†). This enhanced optical phonon interaction may well explain the narrower FWHM and strong anti-broadening effect of FAPbBr₃@GA₂PbBr₄ QDs.

Finally, the potential applications of the as fabricated FAPbBr₃@GA₂PbBr₄ QDs in wide color gamut displays are explored. Pure FAPbBr₃ QDs are also applied for comparison. As shown in Fig. 6a, the as-fabricated QDs are incorporated in PMMA to form a QD/PMMA composite film. A thin layer of the QD/PMMA composite film is deposited on a blue LED chip as a green emitter. Meanwhile, Mn²⁺:K₂SiF₆ (KSF) powder mixed with silica gel is also introduced to generate white light. Fig. 6b and c show the resulting EL spectra of FAPbBr₃ QD and FAPbBr₃@GA₂PbBr₄ QD based LED devices, respectively. Both the LED devices based on FAPbBr₃ QDs and FAPbBr₃@GA₂PbBr₄ QDs emit a bright white light composed of blue (blue LED chip), green (QDs/PMMA) and red (KSF/silica) with CIE coordinates of (0.30, 0.31) and (0.25, 0.26). To evaluate the operation stability, the LED devices were illuminated for 8 h. From Fig. 6b and c, it can be seen that the EL spectra in the green region originated from FAPbBr₃ QDs gradually declining after continuous operation. While almost no significant changes in the green region are observed from the EL spectra of FAPbBr₃@GA₂PbBr₄ QD based LED devices. In order to further demonstrate the enhanced stability of FAPbBr₃@GA₂PbBr₄ QDs, the PL variations of core–shell FAPbBr₃@GA₂PbBr₄ QDs and uncoated FAPbBr₃ QDs under heat, light radiation and moisture are systematically compared. Fig. 6d shows the variation of PL intensity under thermal heating at 373 K. Fig. 6e and f are the corresponding PL spectra of FAPbBr₃ QDs and FAPbBr₃@GA₂PbBr₄ QDs detected at different heating periods. It can be seen that the PL intensity of the FAPbBr₃ QDs experiences a rapid decreasing and only 40% of the initial PL is maintained. While the PL intensity of FAPbBr₃@GA₂PbBr₄ QDs is slightly decreased and over 85% of the initial intensity is preserved after heating for 30 min. Besides, compared to the severely declined PL of FAPbBr₃ QDs, over 70% of the initial PL is maintained after light radiation for 30 h and nearly 83% of the initial PL is maintained after moisture treatment for 100 min, as for FAPbBr₃@GA₂PbBr₄ QDs (see Fig. S8 in ESI†). The greatly enhanced stability of core–shell FAPbBr₃@GA₂PbBr₄ QDs can be attributed to the protection of the 2D GA₂PbBr₄ shell, which makes them more promising candidate in LED applications.

Conclusions

In summary, starting from the structural features of perovskite crystals, a new type of core–shell structure, 2D/3D FAPbBr₃@GA₂PbBr₄ QDs, was proposed and fabricated through a phase transfer enhanced emulsion strategy. Because of the self selectivity of the FA⁺ and GA⁺ cations in the core–shell structure, the fabrication of FAPbBr₃@GA₂PbBr₄ QDs can be accomplished in a one pot synthesis without a complicated regulation process. By systematically varying the addition amount of GABr, the construction mechanism of the core–shell structure was clarified. Well-defined 2D/3D FAPbBr₃@GA₂PbBr₄ QDs with maximum PLQYs up to 93% were obtained under an optimized ratio of GABr. The influence of the core–shell structure on the luminescent properties was intensively studied through absorption spectra, steady and time-resolved PL as well as temperature dependent spectroscopy. Fewer defect states and a 2.2 fold enhanced exciton binding energy compared to pure that of FAPbBr₃ QDs were revealed, which well explained the improved luminescent properties of FAPbBr₃@GA₂PbBr₄ QDs. Moreover, the as fabricated core–shell FAPbBr₃@GA₂PbBr₄ QDs exhibited a greatly enhanced stability under light, heat and moisture compared to that of the uncoated FAPbBr₃ QDs. The superiority of FAPbBr₃@GA₂PbBr₄ QDs in LED devices was demonstrated with a much prolonged operation lifetime, proving their promising prospect in lighting and display technologies.

Experimental section

Materials

All the reagents were used as received without further purification. PbBr₂ (lead(II) bromide 99%, Aladdin), formamidine bromide (FABr, 99.5%, Xi'an Yuri Solar Co., Ltd), guanidine bromide (GABr, Xi'an Yuri Solar Co., Ltd), n-octylamine (99%, Aladdin), oleic acid (OA, 90%, Alfa Aesar), N,N-Dimethylformamide (DMF, analytical grade, Beijing Chemical Reagent Co., Ltd, China), acetonitrile (ACN, analytical grade, Beijing Chemical Reagent Co., Ltd, China), toluene (analytical grade, Beijing Chemical Reagent Co., Ltd, China), ethyl acetate (analytical grade, Beijing Chemical Reagent Co., Ltd, China), K₂SiF₆:Mn⁴⁺ (KSF, Beijing Yuji Science & Technology Co., Ltd, China), and poly(methyl methacrylate) (PMMA, average MW ∼35000, Alfa Aesar).

Preparation of FAPbBr₃/GA₂PbBr₄ QDs

Colloidal FAPbBr₃/GA₂PbBr₄ QDs were fabricated through modifications of the phase transfer enhanced emulsion reprecipitation strategy. For pure FAPbBr₃ QDs, 0.2 mmol FABr and 0.2 mmol PbBr₂ were dissolved in 1 mL of DMF to form a precursor solution. Before injection of the precursor solution, 15 µL of octylamine and 200 µL of OA were added into 10 mL of hexane to help the formation of the emulsion. Then, with a monitor of the UV light radiation (365 nm), 500 µL of the precursor solution was slowly dropwise added into hexane under vigorous stirring. Then 6 mL of ACN, acting as a demulsifier, was slowly injected into the emulsion system. After the addition of ACN, the mixture spontaneously separated into two layers. After removing the stirring, the top layer was carefully collected as a crude solution. The crude solution was further purified twice by ethyl acetate and redispersed in toluene. For the -shell FAPbBr₃/GA₂PbBr₄ QDs, a portion of FABr was replaced by GABr. The molar ratio between FABr and GABr was systematically adjusted from 10:0, 8:2, 6:4, 4:6, 2:8 to 0:10 to investigate the influence of GABr on the formation of the core–shell structure.

Fabrication of LED devices

A certain amount of PMMA powder was dissolved in toluene after continuous stirring for 24 h to form a PMMA/toluene solution with a fraction of 10% wt. Then, 2 mL of QDs in toluene was mixed with 1 mL of the PMMA/toluene solution and drop cast into QD/PMMA composite films after evaporation of the toluene. KSF powder (0.5 g) was blended with an equivalent (1 g) of silicone gel A and B followed by waiting for 30 min to form a KSF/silicone layer. Then, the KSF/silicone layer and FAPbBr₃ QD/PMMA composite film were successively painted on the surface of a blue chip for further measurements.

Characterizations

The XRD data were collected on a Bruker/D8 FOCUS X-ray diffractometer. TEM images were captured on a JEOL-JEM 2100F system operating at 200 kV. UV-vis absorption spectra of the QDs were measured using a Shimadzu UV-2600. A fluorescence spectrometer (FLS980, Edinburgh Instruments, E I) was used to measure the steady-state PL and TRPL spectra. XPS spectra were detected using X-ray photoelectron spectroscopy (Thermo Scientific ESCALAB250Xi). Temperature-dependent PL spectra were performed on a fluorescence spectrometer (FLS980, Edinburgh Instruments, E I) equipped with a liquid nitrogen cooler. The absolute PLQYs of the QDs were determined using an integrated sphere (C9920-02, Hamamatsu Photonics, Japan) excited at a wavelength of 450 nm.

Author contributions

The manuscript was written through contributions of all the authors. All the authors have given approval to the final version of the manuscript.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (52303219), Beijing Postdoctoral Research Foundation (2023-zz-53), NSFC-Joint Foundation Program for Regional Innovation and Development (U21A20102), Major Special Projects of Shanxi Province in Science and Technology (202202060301003), and Fundamental Research Program of Shanxi Province (20210302124391).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Analysis of the size distribution of the core–shell QDs, XPS spectra for samples x = 0, x = 0.4 and x = 0.8, plots of FWHM as a function of temperature for FAPbBr3 QDs and FAPbBr3@GA2PbBr4 QDs, results of the photostability and moisture stability tests, detailed addition amounts of FABr, GABr, and PbBr2 for fabrication of the core–shell QDs and fitting results of the average PL lifetime. See DOI: https://doi.org/10.1039/d4qm00354c

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