Introduction

Light-emitting diodes (LEDs) have been successfully used in lighting and liquid crystal displays due to their tunable color, high efficiency, long lifetime, durability, and energy saving1,2,3. The majority of current commercial phosphor-converted LEDs can be achieved by the combination of a blue InGaN chip with single or multiple phosphors. For the display applications, the narrow emission width of phosphors is a critical factor to achieve wide display’s color gamut without sacrificing power efficiency4. In this respect, red or green phosphors with a narrow band emission have attracted a lot of attention, such as Sr2MgAl22O36:Mn2+, Ba2LiSi7AlN12:Eu2+, K2SiF6:Mn4+ (KSF), and SrLiAl3N4:Eu2+,5,6. Moreover, quantum dots (QDs), i.e. semiconductor nanocrystals (NCs) including II–VI, III–V, and perovskite NCs, have also emerged as highly credible options for displays thanks to their narrow emissions and design flexibilities (adjustable emission wavelengths)7,8. However, practical display applications not only strive for high efficiency and wide color gamut, but also for cost-competitiveness and operational stability. From the cost considerations, perovskite NCs could be one of the most cost-competitive down-conversion emitters for display applications due to their easy synthesis and cheap raw materials. But from the stability perspective, perovskite NCs including CsPbX3 (X= Cl, Br, I) NCs may be one of the worst types because their operational stability is far inferior to the ceramic phosphors, which is even worse than the conventional II–VI and III–V QDs, owing to their intrinsically moisture/heat/light sensitive ionic structures9. Obviously, achieving good stability while maintaining their excellent properties is one of the main challenges for the practical applications of perovskite CsPbX3 NCs.

So far, various strategies have been developed to stabilize CsPbX3 NCs9. A routine strategy is to coat the NCs with inert shells or incorporate them into barrier matrixes, which can isolate CsPbX3 NCs from moisture and oxygen, and also prevent ion migration and the induced inter-particles fusing9. For example, the stabilities of CsPbX3 NCs have been improved by encapsulating them into inorganic oxides (SiO210, Al2O311, SiO2/Al2O312, TiO213, ZrO214), mesoporous materials (mesoporous silica15, metal−organic frameworks16), polymer matrixes (polystyrene17, polymethyl methacrylate18, polyvinylidene fluoride19), inorganic salts (NaNO320, NH4Br21), and shell formation (CsPbBr3/CsPb2Br522, CsPbBr3/Cs4PbBr623, CsPbBr3/Rb4PbBr624). However, these shells or barrier matrixes can only slow down the degradation of CsPbX3 NCs by the external environmental factors, and their stability is still much worse than the ceramic phosphors. Generally, the failure of the protective strategy could be mainly attributed to the following three reasons: (1) the shell or matrix materials cannot completely protect CsPbX3 NCs, such as the porous matrixes, in which the pore structures are exposed, and cannot completely isolate perovskite NCs from moisture and oxygen; (2) the shell or matrix materials are not intrinsically stable, such as inorganic salts (NaNO3, NH4Br, CsPb2Br5, Rb4PbBr6) which are still sensitive to moisture and oxygen; (3) the shell or matrix materials are stable and can completely coat on CsPbX3 NCs, such as inorganic oxides (SiO2, Al2O3, SiO2/Al2O3, TiO2, ZrO2), but are not dense enough and still have some morphological pinholes, which cause the high permeable rates of external H2O/O2. Actually, these inorganic oxides require high synthesis or annealing temperature in order to achieve dense oxides with few pinholes and great barrier properties, since their densification extent is strongly dependent on the annealing temperature. Some studies suggest that high annealing temperatures above 800 °C could promote the transition from amorphous to crystalline and get much better barrier property for SiO2 and Al2O3 thin film25,26. A big challenge for CsPbX3 NCs is that they cannot withstand such a high temperature. In our previous report, the annealing temperature of CsPbBr3/SiO2/Al2O3 could not exceed 150 oC12 due to the severe surface oxidations or fusing of CsPbX3 NCs. The organic ligands on the surface of CsPbX3 NCs will be oxidized (exceed 150 oC), and higher temperatures may damage or peel off the organic ligands and accelerate ion migration of inter-particles, thus CsPbX3 NCs will be agglomerated that lead to the fluorescence quenching. It is difficult to encapsulate perovskite NCs with dense inorganic oxides at high temperature and simultaneously keep their morphology and optoelectronic properties unchanged.

Here, we propose a facile strategy to synthesize ceramic-like stable and highly luminescent CsPbBr3 NCs by template confined solid-state synthesis and in situ encapsulation, which is based on a strategical collapse of the silicon molecular sieve (MS) template at a high synthesis temperature (600–900 oC). The synthesis process is a solid-state reaction at high temperature without organic solvents and organic ligands. The collapsed MS not only confine the growth of CsPbBr3 NCs, but also block the interaction of CsPbBr3 NCs at high temperature. The as prepared CsPbBr3–SiO2 micron-size powders not only show high photoluminescence quantum yield (PLQY) up to 71% and a narrow emission with a full width at half maximum (FWHM) around 20 nm, but also show exceptional photostabilities even slightly better than the ceramic Sr2SiO4:Eu2+ green phosphor under our testing conditions. Furthermore, thanks to the complete encapsulation of dense SiO2 at high temperature, the as prepared CsPbBr3-SiO2 powders can survive in a harsh hydrochloric acid aqueous solution (1 M HCl) for 50 days without obvious photoluminescence (PL) intensity changing, which prove the excellent barrier properties of the formed SiO2 solid. Even the small Cl− ions cannot pass through the SiO2 protective layer to reach CsPbBr3 NCs. We believe the CsPbBr3–SiO2 powders have tremendous potential for LEDs and backlight display applications based on their excellent optical properties and stability.

Results

Synthesis and encapsulation of CsPbBr3 into MS

All-silicon molecular sieves (MCM-41) were chosen as the MS template (Supplementary Table 1) because of their large surface area, narrow pore size distribution (d = 3.6 nm). More importantly, their pore structures can collapse at certain high temperature27. As illustrated in Fig. 1a, the porous MS template was firstly soaked into the precursor salts (CsBr and PbBr2) solution, and then dried at 80 °C. The resultant mixture was placed into a furnace and heated up to a high temperature (600–900 oC). At this high temperature, CsPbBr3 NCs were synthesized in the confined pores of MS, meanwhile, the pore structures of MS gradually collapsed and encapsulated the CsPbBr3 NCs, finally forming a dense CsPbBr3–SiO2 solid.

Fig. 1: Growth and encapsulation of CsPbBr3 NCs into MS matrixes at high temperature.
figure 1

a The schematic diagram of synthesis CsPbBr3 NCs into MS (SiO2). b Photographs of the unwashed CsPbBr3–SiO2 powders (upper) and water washed CsPbBr3–SiO2 powders (bottom) at different calcination temperature under visible illumination, CsBr/PbBr2:MS = 1:3. c XRD patterns of unwashed CsPbBr3–SiO2 powders. d XRD patterns of water washed CsPbBr3-SiO2 powders.

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Figure 1b illustrated the colors of CsPbBr3–SiO2 (mass ratio of CsPbBr3: MS is 1:3) which gradually changed from white to deeper yellow with the synthesis temperature increasing from 400 oC to 700 oC, and then turned into lighter colors when the temperature changed to 800 oC and 900 oC. The XRD patterns (Fig. 1c) confirmed the successful formation of cubic CsPbBr3 NCs (PDF# 54-0752) when the reaction temperature was above 500 oC. In addition to the diffraction peaks from the cubic CsPbBr3 NCs, a broad shoulder around 23o C in CsPbBr3–SiO2 powders was also observed, which belonged to the amorphous phase of SiO2 (Fig. 1c and Supplementary Fig. 1a). For the sample calcined at 400 oC (CsPbBr3-SiO2–400), except the existence of the cubic CsPbBr3 structure, we also observed the sharp diffractions from CsPb2Br5 (PDF# 25-0211) and CsBr (PDF# 52-1144, red arrows), but the signal from the latter structure was weak, which was similar to the dried raw mixture (Supplementary Fig. 1b) without calcination, indicating that there was no obvious chemical reaction at 400 oC. These large CsPbBr3 and CsPb2Br5 crystals were formed in the dry process (Eqs. 1–2)28:

$${\mathrm{2PbBr}}_{\mathrm{2}} + {\mathrm{CsBr}} \to {\mathrm{CsPb}}_{\mathrm{2}}{\mathrm{Br}}_{\mathrm{5}}$$
(1)
$${\mathrm{CsPb}}_{\mathrm{2}}{\mathrm{Br}}_{\mathrm{5}} + {\mathrm{CsBr}} \to {\mathrm{2CsPbBr}}_{\mathrm{3}}$$
(2)

Upon increasing the calcination temperature at 500 oC, the diffraction peaks from CsPb2Br5 structure disappeared gradually and the peaks from cubic CsPbBr3 became more obvious because of the decomposition of CsPb2Br5 into CsPbBr3 (Eq. 3)29. It is also possible that the unreacted CsBr was converted into CsPbBr3 (Eq. 4).

$${\mathrm{CsPb}}_{\mathrm{2}}{\mathrm{Br}}_{\mathrm{5}} \to {\mathrm{CsPbBr}}_{\mathrm{3}} + {\mathrm{PbBr}}_{\mathrm{2}}$$
(3)
$${\mathrm{PbBr}}_{\mathrm{2}} + {\mathrm{CsBr}} \to {\mathrm{CsPbBr}}_{\mathrm{3}}$$
(4)

The melting point of CsPbBr3 is about 567 oC, and its sublimation starts when it is melted30,31. Therefore, when the temperature was heated up to 600 oC, the large CsPbBr3 crystals started melting and sublimating, which filled into the pores of MS and formed CsPbBr3 NCs when the temperature was cooling down. Simultaneously, the pores of MS started to collapse and finally fused into a dense SiO2 solid at high temperatures. The sealed SiO2 nanopore not only confined the growth of CsPbBr3 NCs, but also protected the formed CsPbBr3 NCs from oxidation and inter-particles fusion at high temperature.

To see if the CsPbBr3 NCs are really incorporated into the SiO2 solid, the samples were washed with water. Figure 1d showed the XRD patterns of CsPbBr3–SiO2 powders after water washing, and only the samples with higher calcination temperature (≧600 oC) maintained the diffractions features of cubic CsPbBr3 NCs. In contrast, the lower temperature synthesized samples (CsPbBr3–SiO2–400, CsPbBr3–SiO2–500) almost lost all the diffraction peaks after water washing, showing that the perovskite crystals were not sealed yet. Supplementary Fig. 2 showed the UV-Vis absorption spectra of CsPbBr3-SiO2 powders before and after water washing. Except for CsPbBr3–SiO2–400, all the other samples presented an absorption band edge around 507 nm before water washing. After water washing, similar to the observation from the XRD patterns, the samples with lower calcination temperatures (400 oC, 500 oC) almost completely lost their absorption because the water damaged or dissolved the unprotected CsPbBr3 NCs. Obviously, the samples synthesized at higher temperatures had better water resistance. In the following study, CsPbBr3–SiO2 meant the samples after water washing if there is no particular explanation.

To explore the formation process of CsPbBr3 NCs and the evolution of the pore structures of MS at high temperature, TEM images of CsPbBr3-SiO2 powders (washed) at different calcination temperatures are shown in Fig. 2a–f. TEM images of CsPbBr3–SiO2-400 and CsPbBr3–SiO2–500 exhibited no CsPbBr3 NCs, and similar pore structures as the original MS were still clearly observed (Supplementary Fig. 3), indicating that the pores of MS have not collapsed yet. As the calcination temperature reached 600 oC, the pore structures of MS started to collapse, and some tiny CsPbBr3 NCs appeared in MS matrixes (Fig. 2c). When the temperature reached 700 oC, the MS obviously became a compact solid, meantime the average sizes of CsPbBr3 NCs increased gradually with increasing calcination temperature as shown in Fig. 2c–f and Supplementary Fig. 4 (CsPbBr3–SiO2-600, d = 6.7 nm; CsPbBr3–SiO2–700, d = 9.5 nm; CsPbBr3–SiO2–800, d = 20.9 nm; CsPbBr3–SiO2-900, d = 30.1 nm). The increased sizes of CsPbBr3 NCs may be attributed to the softening and collapse of pores of MS with increasing temperature, leading to a weaker template confinement effect, which allowed the growth of larger particles. Before the water washing step, some reaction residues and salts were observed on the surface of unwashed CsPbBr3-SiO2–700 as shown in Supplementary Fig. 5a. After water washing, only those fully encapsulated CsPbBr3 NCs (d = 9.5 nm) were left, and the high-resolution TEM (HRTEM) image clearly exhibited a good crystallinity of CsPbBr3 NCs with a lattice spacing of 0.30 nm corresponding to the lattice fringes of the (200) planes (Fig. 2d, inset). Compared with the unwashed CsPbBr3-SiO2-700, the PL intensity of the water washed CsPbBr3–SiO2–700 had been improved due to the removing of reaction residues and salts from the surface of SiO2 (Supplementary Figs. 5 and 6), which usually were non-luminescent.

Fig. 2: The evolution of CsPbBr3 NCs and the pore structures of MS with temperature increasing.
figure 2

TEM images of water washed CsPbBr3–SiO2 at different calcination temperatures: a CsPbBr3–SiO2–400, b CsPbBr3–SiO2–500, c CsPbBr3–SiO2–600, d CsPbBr3–SiO2–700, e CsPbBr3–SiO2–800, f CsPbBr3–SiO2–900. g Small-angle XRD patterns of original MS and CsPbBr3–SiO2. h Surface area of original MS and CsPbBr3–SiO2 (CsBr/PbBr2: MS = 1:3) calculated with BET method.

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The collapse of the pore structures of MS was also confirmed by small-angle XRD patterns (Fig. 2g). With the increase of calcination temperature, the intensities of the major peaks from the hexagonal structure (PDF# 49-1712) of MS decreased gradually and pore structures were damaged27. When the calcination temperature reached 700 oC, the hexagonal structure of MS completely disappeared. Meanwhile, the surface area of CsPbBr3–SiO2 decreased gradually as the temperature increased (Fig. 2h). Compared with the original MS, the surface area of CsPbBr3–SiO2–700 decreased from 1037 m2 g−1 to 9.6 m2 g−1, which indicated that MS had collapsed completely at 700 oC and most of the CsPbBr3 NCs were encapsulated into the compact SiO2 solid during the collapse process. The complete collapse of MS played a key role in protecting CsPbBr3 NCs from the damages of moisture, because CsPbBr3 NCs can be completely encapsulated into the dense SiO2 solid. As we can see, no obvious PLQYs decays were observed during CsPbBr3–SiO2 (700 oC, 800 oC, and 900 oC) immersed in water for 50 days (Supplementary Fig. 7), but the PLQY of CsPbBr3–SiO2–600 decreased slightly owing to the incomplete collapse of MS at 600 oC.

Optical characterization of CsPbBr3-SiO2

The PL spectra of CsPbBr3-SiO2-700 and ceramic Sr2SiO4: Eu2+ green phosphor are shown in Fig. 3a. The FWHM of CsPbBr3–SiO2–700 is just 20 nm, which is much narrower than that of ceramic Sr2SiO4: Eu2+ green phosphor (FWHM = 62 nm), showing that narrow-band emitting CsPbBr3-SiO2-700 has a great potential to become an outstanding candidate in advanced wide-color-gamut backlight display. Figure 3b showed the PLQYs of CsPbBr3–SiO2 powders synthesized with different experimental conditions. Obviously, CsPbBr3–SiO2–700 (mass ratio of CsPbBr3: MS is 1:3, t = 700 oC) exhibited the highest PLQY of 63% (Fig. 3b and Supplementary Table 3). To better understand the change in PLQYs of CsPbBr3–SiO2 with different calcination temperatures, the PL decay curves of the CsPbBr3–SiO2 were shown in Supplementary Fig. 8 and Supplementary Table 4. The average lifetimes of CsPbBr3-SiO2 gradually increased with the synthesis temperature increasing from 400 oC to 700 oC, and then began to decrease when the temperature reached 800 oC and 900 oC. Particularly, the longest average lifetime was 21.81 ns at 700 oC. The longer lifetimes usually indicated the suppression of the nonradiative decay, and the generated excitons were more inclined to recombine with the radiative path, which was in consistent with the PLQYs with the change of synthesis temperatures. On the other hand, the mass ratios of CsPbBr3: MS (CsBr/PbBr2: MS) profoundly affect the optical properties of CsPbBr3-SiO2 because of the availability of the pores/cavities of MS that may encapsulate CsPbBr3 NCs (Supplementary Figs. 9–11).

Fig. 3: Optical properties of CsPbBr3–SiO2.
figure 3

a Photoluminescence emission spectra of CsPbBr3–SiO2–700 and ceramic Sr2SiO4: Eu2+ green phosphor, excitation wavelength is 455 nm. b Absolute PLQYs of CsPbBr3-SiO2 (mass ratio of CsPbBr3: MS is 1:3, red mark) synthesized at different temperatures, absolute PLQYs of CsPbBr3-SiO2 synthesized at 700 °C with different mass ratios of CsPbBr3: MS (blue mark).

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HF etching CsPbBr3–SiO2 for improving the PLQY

Although the PLQYs of CsPbBr3-SiO2 have been optimized by changing the calcination temperature and the ratio of CsPbBr3: MS, the optimized PLQYs are still limited owing to thick SiO2 covered on the CsPbBr3 NCs, which may hinder the light absorption and conversion of CsPbBr3 NCs. Hence, a HF solution was used to etch SiO2 and remove incomplete encapsulated CsPbBr3 NCs on the surface of CsPbBr3–SiO2, and the treated sample was named as CsPbBr3-SiO2-HF (Fig. 4a). As shown in Fig. 4b and Supplementary Fig. 12, after etching, the CsPbBr3 NCs in the MS seemed more uniform and kept the same lattice spacing of 0.30 nm (Fig. 4b inset). As shown in the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and its corresponding elemental mappings of CsPbBr3–SiO2–HF (Fig. 4c), the Cs, Pb, Br elements were clearly distributed in the CsPbBr3 NCs, indicating that these CsPbBr3 NCs were not destroyed by HF solution because they were deeply buried into SiO2 matrixes. The scanning electron microscopy (SEM) images (Supplementary Fig. 13) showed CsPbBr3–SiO2–700 and CsPbBr3–SiO2–HF are in particle morphology with sizes around 0.5 um~1um. Compared with the smooth unetched sample, the surface of CsPbBr3–SiO2–HF appeared with some holes due to the removal of surface partially-embedded CsPbBr3 NCs, meantime the surface area of CsPbBr3-SiO2-HF increased from 9.6 m2 g−1 to 16.7 m2 g−1 (Supplementary Table 5). As expected, both PL peak position and absorption edge of CsPbBr3–SiO2-HF blue-shifted around 2 nm owing to the removal of larger partially-embedded CsPbBr3 NCs after HF etching (Fig. 4d). Therefore, CsPbBr3–SiO2–HF showed a brighter green fluorescence (Fig. 4e) with improved absolute PLQY from 63% to 71% (Supplementary Table 3) without any changes in XRD patterns (Fig. 4f). The PL decay curves of the CsPbBr3–SiO2–700 and CsPbBr3–SiO2–HF were shown in Supplementary Fig. 14 and Supplementary Table 6. There was no significant difference in average fluorescence lifetimes between CsPbBr3–SiO2–700 (τ = 21.81 ns) and CsPbBr3–SiO2–HF (τ = 22.14 ns). The HF etching did not change the structures and properties of CsPbBr3 NCs. In other words, the improved PLQY was mainly attributed to the thinning of SiO2 shell.

Fig. 4: Improving PLQYs by HF etching.
figure 4

a The schematic diagram of CsPbBr3-SiO2-700 by HF etching. b TEM image of CsPbBr3–SiO2–HF. c HAADF-STEM image of CsPbBr3–SiO2–HF and the corresponding elemental mapping of Cs, Pb, Br, O, and Si. d Photoluminescence emission and UV-Vis absorption spectra (insert) of CsPbBr3–SiO2–700 and CsPbBr3–SiO2–HF. e Photographs of the CsPbBr3–SiO2–700 powders (left) and CsPbBr3–SiO2–HF powders (right) under visible illumination (upper) and UV excitation at 365 nm (bottom). f XRD patterns of CsPbBr3–SiO2–700 powders, and CsPbBr3–SiO2–HF powders.

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Water stability, chemical stability, and photostability

As is known to all, the stability of CsPbBr3 NCs is challenged by ambient factors such as light irradiation, water, oxygen, and heat, which restrict their applications9. In this work, CsPbBr3 NCs were completely encapsulated into the high temperature annealed SiO2 solid, and the water washing step already proved the good water-resistance of the as prepared CsPbBr3-SiO2 powders. To further quantify their water-resistant capability, CsPbBr3–SiO2–700 and CsPbBr3–SiO2–HF were dispersed into water with PLQY monitoring. For comparison, ceramic Sr2SiO4:Eu2+ green phosphor (Intermtix.co), KSF red phosphor (Intermtix.co), and colloidal CsPbBr3 NCs were used as references. As shown in Fig. 5a, immediate degradation of KSF red phosphor and colloidal CsPbBr3 NCs were observed in one hour. As contrast, Sr2SiO4: Eu2+ phosphor, CsPbBr3–SiO2–700, and CsPbBr3–SiO2–HF were dispersed in water for 50 days and still exhibited bright green fluorescence (Supplementary Figs. 15–16a). No obvious PLQYs reduction was observed in CsPbBr3–SiO2–700 and CsPbBr3–SiO2–HF (Fig. 5b), but the PLQYs of Sr2SiO4:Eu2+ phosphor, KSF red phosphor, and colloidal CsPbBr3 NCs decreased to 88%, 17%, and 13% of their initial PLQYs, respectively. More surprisingly, CsPbBr3–SiO2–700 and CsPbBr3–SiO2–HF showed no change even testing in water under strong illumination of a 450 nm LED light (175 mW cm−2) for 50 days (Fig. 5c and Supplementary Figs. 15–16b). Except the excellent water resistance and photostability, the samples also showed a robust chemical stability because they could even survive in the strong acid aqueous solution (1 M HCl) for 50 days (Fig. 5c and Supplementary Figs. 15–16c), which indicated that even small Cl− ions cannot pass through the dense SiO2 shell and cause ion exchange. XRD patterns of the CsPbBr3–SiO2–700 and CsPbBr3–SiO2–HF also remained unchanged after 50 days in the water with light illumination or in 1 M HCl solution (Supplementary Fig. 17), which meant that the dense SiO2 shell can effectively protect CsPbBr3 NCs from water-induced structure collapse, photodegradation, and ion migration.

Fig. 5: Water resistance and acid resistance of CsPbBr3-SiO2.
figure 5

Photographs (a) and relative PLQYs (b) of the CsPbBr3–SiO2–700, CsPbBr3–SiO2–HF, ceramic Sr2SiO4:Eu2+ green phosphor, KSF red phosphor, and colloidal CsPbBr3 NCs after immersed in water for various times. c Relative PLQYs of the CsPbBr3–SiO2–700 and CsPbBr3–SiO2–HF after immersed in various solvents for 50 days, extra light source: a 450 nm LED light (175 mW cm−2).

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To verify the potential of CsPbBr3–SiO2 in backlight displays, we tested the operational stability of CsPbBr3–SiO2–HF powders sealed with Norland-61 on the blue LED chips (peak at 455 nm, 20 mA, 2.7 V) under room temperature. As for comparison, colloidal CsPbBr3 NCs, CdSe/CdS/ZnS NCs, ceramic Sr2SiO4: Eu2+ green phosphor, and commercial KSF red phosphor were used as control groups (Supplementary Fig. 18). As shown in Fig. 6a, the relative PL intensity of CsPbBr3–SiO2–HF still maintained above 100% under illumination for 1000 h, but the relative PL intensities of ceramic Sr2SiO4: Eu2+ green phosphor and commercial KSF red phosphor decreased to 82% and 67% of the initial intensity after 1000 h. Meanwhile, the relative PL intensity of CdSe/CdS/ZnS NCs dropped to 38% after 360 h under illumination and the relative PL intensity of colloidal CsPbBr3 NCs sharply dropped to 15% after 40 h. It was confirmed that CsPbBr3–SiO2–HF exhibited comparable operation stability as the commercial ceramic phosphors. To further prove the temperature and moisture resistance of the CsPbBr3–SiO2 in practical display applications, we performed the accelerated operational stability tests for the above device samples under high temperature (HT 85 °C) and high humidity (HH 85%) conditions (Fig. 6b). After aging for 168 h, the PL intensity of CsPbBr3–SiO2–HF retained almost unchanged, which is more superior to ceramic Sr2SiO4: Eu2+ green phosphor that only remained 80% of the initial PL intensity. As a contrast, the PL intensities of commercial KSF red phosphor, CdSe/CdS/ZnS NCs and colloidal CsPbBr3 NCs sharply decreased to 43% (aging for 168 h), 60% (aging for 20 h), 7% (aging for 20 h) of the initial intensities, respectively. These significant differences undoubtedly proved that the facile strategy for fully encapsulating CsPbBr3 NCs into the dense SiO2 solid can effectively protect CsPbBr3 NCs against the damages from oxygen, moisture, light irradiation, and heat. To our knowledge, the stabilities (such as, water stability, chemical stability, and photostability) of CsPbBr3-SiO2 are much superior to the reported results of conventional perovskite composites that protected by different coating materials and methods (Supplementary Table 7). Next, to verify the universality of this method, CsPbBr3 NCs confined in different types of molecular sieves (such as ZSM, NaY, and Y-Zeolite, the pore size distribution from 0.5 nm to 3.6 nm) were further explored (Supplementary Table 1 and Supplementary Fig. 19). As we can see, luminescent CsPbBr3 NCs can be synthesized in different MS templates with a wide pore size range.

Fig. 6: Photostability of the CsPbBr3–SiO2–HF.
figure 6

a Photostabilities of the CsPbBr3–SiO2–HF, ceramic Sr2SiO4: Eu2+ green phosphor, KSF red phosphor, colloidal CsPbBr3 NCs and CdSe/CdS/ZnS NCs under illumination, sealed with Norland-61 on the LED chips (20 mA, 2.7 V) and b aged at 85 °C and 85% humidity conditions on the LED chips (20 mA, 2.7 V).

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Discussion

In summary, we have introduced a facile approach for the in situ growth and encapsulation of CsPbBr3 NCs into SiO2 at high temperature for improving stability. Based on the specific porous structure of molecular sieves (MCM-41), we were able to synthesize CsPbBr3 NCs by a nano-confined growth at high temperatures. By smartly applying the specific collapse behavior of MCM-41 at high temperature, we successfully encapsulated the CsPbBr3 NCs into the dense SiO2 solid, which offered ceramic-like stability to CsPbBr3 NCs. Particularly, the PL intensity of CsPbBr3–SiO2 remained 100% of its initial value under illumination on blue LED chips (20 mA, 2.7 V) for 1000 h, even better than the ceramic silicate phosphor. The robust stability, ultra-narrow emission, and high PLQY make the CsPbBr3–SiO2 powders an ideal active material for many optoelectronic applications, particularly as down-conversion emitters for wide color gamut display. Their excellent water/acid-resistance will extend the applications of perovskite NCs, such as in vitro bioimaging/biosensing fluorescent labels even in an acid aqueous medium (stomach), or allow us to perform long term in-vivo tracking of the labeled target if we can decrease the size of CsPbBr3–SiO2 particles to nanoscale.

Methods

Chemicals

Cesium bromide (CsBr, 99.5%), lead bromide (PbBr2, 99%), Cesium carbonate (Cs2CO3, 99.9%), 1-octadecene (ODE, 90%), oleylamine (OAm, 90%) were purchased from Aladdin. Oleic acid (OA, 90%) was purchased from Aldrich. Methyl acetate (98%), toluene (99.5%) were purchased from Sinopharm Chemical Reagent. Molecular sieves (MS) were purchased from Tianjin Yuanli Chemical Co., Ltd. All the chemicals were used without further purification.

Preparation of CsPbBr3–SiO2

Briefly, CsBr and PbBr2 (the mole ratio was 1:1) were dissolved into 50 mL ultrapure water in the 250 mL beaker and stirred constantly for 30 min at 80 °C. Then, a certain amount of MS (the mass ratio of CsBr/PbBr2: MS = 1:3) was added into the above solution and the mixture was stirred for 1 h. The as-obtained mixture was dried at 80°C. The collected mixture was ground and calcined at the set temperature for 0.5 h with a heating rate of 5 °C min−1 in the muffle furnace under an air atmosphere. After cooling to room temperature, the sample was ground and washed with ultrapure water for several times to remove external CsPbBr3 or other salts. Finally, the washed-sample was obtained by centrifugation and drying at 60 °C. The products obtained at 400°C, 500°C, 600°C, 700°C, 800°C, and 900 °C were denoted as CsPbBr3–SiO2–400, CsPbBr3–SiO2–500, CsPbBr3–SiO2–600, CsPbBr3–SiO2–700, CsPbBr3–SiO2–800, and CsPbBr3–SiO2–900, respectively. The nominal compositions of control groups were shown in Supplementary Table 2.

HF etching CsPbBr3–SiO2

50 mg of CsPbBr3-SiO2-700 was added into 15 mL HF (c = 0.04 M) solution and stirred for 0.5 h. The obtained product was then centrifuged and washed several times with ultrapure water. Finally, the product was dried overnight at 60 oC. The obtained product was denoted as CsPbBr3–SiO2–HF.

LED package

The used UV-cured optical adhesive was Norland-61. In brief, 10 mg of CsPbBr3–SiO2 was mixed with 200 mg of UV-cured optical adhesive. To remove the bubbles from the optical adhesive, the resulting mixture was heated at 40 °C for 0.5 h under vacuum. After that, the mixture was deposited on a 455 nm InGaN LED chip and then UV cured for 50 s (365 nm, 80 W cm−2). Then the green CsPbBr3–SiO2 LED was obtained. According to the above similar procedure, other LED packages (e.g., CsPbBr3–SiO2–HF, Sr2SiO4:Eu2+ green phosphor, KSF red phosphor, colloidal CsPbBr3 NCs, and CdSe/CdS/ZnS NCs) were also obtained.

PLQYs measurements

The absolute PLQYs were calculated by using a fluorescence spectrometer (HAAS-2000) with an integrated sphere excited at a wavelength of 395 nm using a LED chip source.

Characterization

The powder X-ray diffraction (XRD) patterns of CsPbBr3–SiO2 and CsPbBr3 NCs were performed by a Bruker D8 Advance X-ray Diffractometer at 40 kV and 30 mA using Cu Kα radiation (λ = 1.5406 Å). The morphologies and elemental distributions and high-angle annular dark−field scanning transmission electron microscopy (HAADF−STEM) images were analyzed by Mira3/MIRA3 (SEM) field emission scanning electron microscope (FESEM) and FEI (TALOS F200X) transmission electron microscope (TEM) instruments. PL emission spectra of the samples were recorded on an Ocean Optics LS-450 spectrometer and a fluorescence spectrometer (HAAS-2000). N2 adsorption-desorption experiments were undertaken isothermally at 77 K on QUADRASORB SI. The photostability measurements of the samples were performed in a temperature and humidity chamber using a fluorescence spectrometer (HAAS-2000). UV-Vis absorption spectra were measured by a UV-Vis spectrophotometer (PerkinElmer Lambda 950). The chemical compositions were determined by X-Ray fluorescence spectroscopy (PANalytical Epsilon 3x). The PL decay curves were recorded on an Edinburgh FLS1000 spectrophotometer with the excitation wavelength at 365 nm.