Synergetic Effect of Postsynthetic Water Treatment on the Enhanced

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Synergetic Effect of Postsynthetic Water Treatment on the Enhanced Photoluminescence and Stability of CsPbX3 (X = Cl, Br, I) Perovskite Nanocrystals Ying Liu, Fei Li, Quanlin Liu, and Zhiguo Xia Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03330 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 5, 2018

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Chemistry of Materials

Synergetic Effect of Postsynthetic Water Treatment on the Enhanced Photoluminescence and Stability of CsPbX3 (X = Cl, Br, I) Perovskite Nanocrystals Ying Liu , Fei Li , Quanlin Liu and Zhiguo Xia* ⊥

⊥

The Beijing Municipal Key Laboratory of New Energy Materials and Technologies, School of Materials Sciences and Engineering, University of Science and Technology Beijing, Beijing 100083, China

ABSTRACT: All inorganic cesium lead halide CsPbX3 (X = Cl, Br, I) perovskite nanocrystals (NCs) are generally considered to be sensitive and even decompose when exposed to ambient humidity. Herein, we perform a well-controlled investigation of the effect of postsynthetic water treatment of the as-prepared CsPbX3 NCs on the photoluminescence quantum yields (PLQYs) and stability, and one can find that the surface defective layer of CsPbX3 NCs can be dissolved into water and we can further separate the supernatant CsPbX3 NCs hexane solution from the water, which improves the PLQYs of CsPbCl3, CsPbBr3, CsPbI3 from 5%, 69.3%, 68.1% to 78.5%, 95.5%, 91%, respectively. As a synergetic effect, a trace amount of residual water facilitated the hydrolysis of tetramethyl orthosilicate, leading to the formation of SiO2-coated CsPbX3 NCs with high stability. The stable green-emitting CsPbBr3/SiO2 composite is used as an exam4+ ple to combine with red K2SiF6:Mn to fabricate the lightemitting diodes (LEDs) device on the blue InGaN chip, which displays wide color gamut (128% of National Television Standard Committee (NTSC) standard), providing the possibility for liquid-crystal display (LCD) backlight utilization.

oleylammonium halide surface species all give opportunities for the surface defects. Attempts have been made previously to improve the PLQY of perovskite NCs by the surface modi17 fication , during or after the synthesis. Alivisatos et al. demonstrated that a post-treatment of CsPbBr3 NCs with the thiocyanate salt could improve the PLQYs of both freshly 14 synthesized and aged NCs, and this effect was attributed to the thiocyanate occupying a limited number of CsPbBr3 NCs surface sites. Bakr et al. used a bidentate ligand during the post-synthesis of CsPbI3 NCs, resulting in a narrow red photoluminescence with exceptional quantum yield (close to 18 unity) and substantially improved stability. Due to the ionic nature of the lead halide perovskite material, polar-type water is often used to control their morphology and phase. Sun et al. reported that water triggered the transformation from Cs4PbX6 NCs to CsPbX3 NCs by stripping CsX through an 19 interfacial reaction. Zhang et al. presented that the introduction of different amounts of water into the reaction mixture could prompt the crystallization of stable CsPbBr3 NCs 20 with different shapes and tunable band-gap. Eperon et al. proposed that water, in the form of ambient humidity, had a beneficial impact on fabricating high-performance me21 thylammonium lead halide perovskite films.

■ INTRODUCTION All inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite nanocrystals (NCs) have attracted mounting attention due to their excellent optical and electronic properties, such as bright photoluminescence (PL) with emission tunable across the broad visible spectral region (400-700 1-3 nm), narrow emission full width at half maximum 4-5 (FWHM), higher stability compared with the hybrid perov6-8 skites (CH3NH3PbX3 (X = Cl, Br, I)) and so on, which make them as promising luminescent materials for high9-11 performance illumination and display applications. It is widely accepted that the excellent optical properties in CsPbX3 nanocrystals come from the high defect tolerance of 12 lead halide perovskites. However, the as-prepared CsPbX3 NCs are really not defects-free, which serve as trapping centers for the exciton recombination and weaken the photolu13-15 minescence quantum yields (PLQYs) of the NCs. Possible reasons for the origin of defects should contain interior crystallinity, surface anion/cation stoichiometry, surface ligands, 16 and solvent environment. With looking deeper into the causes of defects in lead halide perovskites, the surface prop15 erties could often be stressed. The potential contribution of surface defects becomes increasingly important in NCs due to the increased surface-to-volume ratio. Lead-rich synthesis conditions, purification processing and the lability of the

Scheme 1. Illustration of the water treatment procedure of CsPbBr3 NCs, including the synergetic effect of water with the dissolution and removal of surface defective layer of CsPbX3 NCs and the promotion of the hydrolysis of tetramethyl orthosilicate (TMOS). In addition, numerous attempts to adopt surface coating strategies, such as SiO2, Al2O3 or polymer coating, have also 22-25 been performed to enhance the stability of CsPbX3 NCs . A recent example demonstrated by Yin and the co-workers found that the monodisperse CsPbX3/SiO2 and CsPbBr3/Ta2O5 Janus NCs were successfully prepared by combining a water-triggered transformation process and a sol–gel method, which showed dramatically improved stability 26 against destruction by air, water, and light irradiation. In

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our present investigation, different form previous report, the synergetic effect on postsynthetic water addition has been discussed, and then such a water treatment process can remove surface defective layer to enhance the stability and PLQY values of CsPbX3 NCs with invariable morphology. Furthermore, the existence of a trace amount of water facilitated the hydrolysis of tetramethyl orthosilicate, leading to the formation of the SiO2-coated CsPbX3 NCs with high stability. As shown in Scheme 1, pristine CsPbX3 NCs contained surface defects are related to low PLQYs. After the water treatment, surface defective layer of CsPbX3 NCs can be dissolved into water, boosting the PLQYs of CsPbCl3, CsPbBr3, CsPbI3 from 5%, 69.3% and 68.1% to 78.5%, 95.5% and 91%, respectively. Then, we separated the hexane solution from water and added tetramethyl orthosilicate (TMOS). A trace amount of residual water in the hexane hydrolyzed TMOS and the SiO2-coated CsPbX3 NCs can be obtained (Scheme 1). Finally, the green emissive SiO2-coated CsPbBr3 NCs were selected as an example to fabricate remote type LED devices. The prototype devices exhibited stable performance and wide color gamut (128% of National Television Standard Committee (NTSC) standard), providing the possibility for liquid-crystal display (LCD) backlight utilization.

■EXPERIMENTAL SECTION Materials. Cesium carbonate (Cs2CO3, 99.9%, Sinopharm), octadecene (ODE, >90%, Aladdin), oleic acid (OA, AR, Aladdin), lead (II) bromide (PbBr2, 99.9%, Aladdin), oleylamine (OLA, Aladdin, 80-90%), hexane (99.9%, Sinopharm), ethyl acetate (99%, Sinopharm). All chemicals were used as received without further purification. Preparation of cesium oleate precursors. The cesium oleate solution was prepared following the approach reported by Protesescu et al. 0.4 g Cs2CO3, 1.2 mL OA and 15 mL ODE were added into a 50 mL three-neck flask. After a 30 min stirring under vacuum at 120 °C, the mixture was heated under N2 to 150 °C to obtain a clear solution. Synthesis of CsPbX3 NCs. 10 mL ODE, OLA (4 mL for PbCl2, 1.1 mL for PbBr2 and 1.1 mL for PbI2), OA (4 mL for PbCl2, 2 mL for PbBr2 and 2 mL for PbI2) and 0.376 mmol PbX2 (PbCl2 - 0.1040 g, PbBr2 - 0.138 g, PbI2 - 0.172 g) were loaded into a 50 mL three-neck flask and dried under vacuum for 30 min at 120 °C. After complete solubilisation of PbX2, the temperature was raised to 160-170 °C and the hot (150 °C) cesium oleate precursor (0.6 mL) was quickly injected. Five seconds after the injection, the reaction mixture was quickly cooled down by an ice-water bath for the next purification process. Purification of CsPbX3 NCs. Simply, the different CsPbX3 NCs were extracted from the crude solution via centrifugation at 8000 rpm for 4 min. Then, the supernatant was poured off and the precipitate at the bottom of the centrifuge tube was redispersed in hexane. After the NCs were completely dispersed in hexane, the solution was again centrifuged at 5000 rpm for 4 min. After that, precipitates were discarded and supernatant was filtered into another centrifuge tube with athyl acetate added, forming a stable colloidal solution. Finally, the particles were collected from the colloidal solution by centrifuging at 9000 rpm for 4 min. After the purification, the NCs were stored as dispersion in hexane.

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Water treatment of CsPbX3 NCs. 10 µL OLA and 1 mL H2O were introduced into a 5 mL flask containing 2 mL CsPbX3 NCs hexane solution. The flask was placed at room temperature under lighting for 8 h as an optimum treatment time and then separate the supernatant CsPbX3 NCs hexane solution from the water. Synthesis of CsPbBr3/SiO2 composite. 50 µL TMOS was introduced into a 5 mL flask containing 2 mL of water treated CsPbBr3 NCs hexane solution. The flask was placed at room temperature with continuous stirring. After stirring for 10h, the precipitates were collected through centrifugation at 9000 rpm for 10 min, and then were dried in vacuum to form the CsPbBr3/SiO2 composite. Characterization. UV-Vis absorption spectra of the colloidal solutions were collected at room temperature using a Varian Cary 5 spectrophotometer with white BaSO4 powder as a reference for absorption correction. Fluorescence spectrophotometer (FLSP9200 fluorescence spectrophotometer, Edinburgh Instruments Ltd, UK) equipped with a PMT detector and a 150 W xenon lamp was used to obtain steadystate PL spectra from solutions. The luminescence decay curves were accquired using an FLSP9200 fluorescence spectrophotometer. PLQY was measured by an Absolute Photoluminescence Quantum Yield Measurement System (C992002, Hamamatsu-Photonics) with an integrating sphere. Powder X-ray diffraction (XRD) patterns were collected with a PANalytical X’Pert3 Powder diffractometer equipped with a Cu Kα irradiation source, operating at 40 kV and 40 mA. Transmission electron microscopy (TEM) images were recorded using a JEM-2010 operated at 120 kV on 200 mesh carbon coated nickel grids. Energy dispersive X-ray (EDX) spectroscopy and mapping were performed on a probe aberration corrected microscope, JEOL JEM-ARM200CF, at 200 kV. For the white LEDs lamp fabrication, the mixed green-emitting 4+ CsPbBr3 NCs/SiO2 and red-emitting KSF:Mn phosphors coated on the blue LED chip (λ = 455 nm). And a PMS-80 Plus UV-vis-near IR Spectrophotocolorimeter system was used to characterize the electroluminescence (EL) spectrum, color-rendering index (Ra) and CIE color coordinates of the LEDs prepared.

■RESULTS AND DISCUSSION The optical data of CsPbX3 (X = Cl, Br, I) NCs is presented in Figure 1. Figure 1a and 1b display the absorption spectra and PL spectra of the pristine- and water treated CsPbCl3 (blue), CsPbBr3 (green) and CsPbI3 (red) NCs, respectively. All the samples show typical absorption spectra sharp emission spectra, and there are no obvious change in peak position before and after water treatment. However, there is a significant improvement in the PLQYs from untreated samples (CsPbCl3 at 5%, CsPbBr3 at 69.3%, CsPbI3 at 68.1%) to treated samples (CsPbCl3 at 78.5%, CsPbBr3 at 95.5%, CsPbI3 at 91%). Moreover, the water treat time is also significant and needed to be optimized. It is found that the CsPbX3 NCs will decompose if the time is too long, but the PLQYs will not be improved dramatically by short time. Figure 1c shows that the best post-treatment reaction time is 8 h. Especially for CsPbCl3 NCs, the PLQY of treated NCs is markedly higher

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Chemistry of Materials than that of freshly synthesized NCs, while previous studies indicated that the PLQYs of CsPbCl3 were mostly less than 27 60%.

Figure 1. (a) UV-vis absorption and (b) PL emission spectra of the untreated (dash line) and treated (solid line) CsPbCl3 (blue), CsPbBr3 (green) and CsPbI3 (red) NCs. (c) Variation in PLQYs of CsPbX3 (X = Cl (blue), Br (green), I (red)) NCs with prolonged reaction time. Time-resolved PL decay curves of CsPbCl3 (d), CsPbBr3 (e) and CsPbI3 (f) NCs before and after water treatment, and the insets show the corresponding optical images of untreated (left) and treated (right) samples in hexane under ambient light. Taking into account the remarkable improvement in PLQYs, the PL decay curves should present a consistent change resulting from the water treatment process. When the photoluminescence lifetime is multi-exponential, the interpretation is often difficult without a well-defined kinetic model of the PL decay process, and even the interpretations may be tenuous. Time-resolved PL decay curves of CsPbCl3 (d), CsPbBr3 (e) and CsPbI3 (f) NCs before and after water treatment are given in Figure 1d-f. Following the water treatment, we can find that the PL lifetime is closer to monoexponential consistent with the higher PLQY, compared to the multi-exponential lifetime presented in untreated samples. Therefore, it also confirms that the water treatment is capable of reducing the non-radiative pathways that do not luminesce. As also shown from the insets in Figure 1d-f, one can find the change of the optical performance with transparent characters, which also indicate the enhanced PLQY values for all the samples. Since the proposed water treatment strategy is effective in improving the luminescence property, it is of interest to figure out whether there are microstructural variations. Figure 2 presents the transmission electron microscopy (TEM) images, X-ray diffraction (XRD) analyses and energy dispersive X-ray spectroscopy (EDS) of untreated and treated CsPbBr3 NCs, respectively. Both the untreated and treated samples show regular morphologies and uniform size distributions (Figure 2a,c), however, particle boundary can be improved and observed following the water treatment. The high-resolution TEM (HRTEM) images shown in Figure 2b,d, further reveal the same lattice spacings of 0.41 nm before and after treatment, which are both in good agreement with the (100) lattice spacing of cubic CsPbBr3. In addition, the pow-

der XRD patterns given in Figure 2e, present that CsPbBr3 NCs samples still maintain cubic phase after water treatment. XRD patterns of the as-synthesized NCs are well coincident with cubic CsPbCl3 phase (JPCDS No. 75-0411). Taken together, the water treatment does not cause macroscopic structural changes of the as-prepared and as-treated NCs. EDS is further performed to confirm the elemental composition ratios of untreated and treated CsPbBr3 NCs, indicating that the untreated sample with cation-rich composition, as shown in Figure 2f,g. And the previous studies have shown that these anion-poor (cation-rich) surfaces are negative for 28-31 the luminescent performance due to the trap states local− ized on the 4p orbitals of the Br ions added to compensate the excess charge of the perovskite NCs, providing a pathway to nonradiative exciton recombination and then result in a low PLQY. The elemental compositions of pristine CsPbBr3 + 2+ NCs are measured to be 29.73% Cs , 24.08% Pb and 46.19% Br , while the compositions of treated samples are 27.98% + 2+ Cs , 20.16% Pb and 51.86% Br , respectively, which indicates that the content of Br relatively increased by water treatment. The EDS results suggest that the water treatment removes some excess cation from the surface, which in turn reduces the shallow electron traps that are deleterious to the luminescent performance of the CsPbBr3 NCs.

Figure 2. TEM and high-resolution TEM images of CsPbBr3 NCs before (a,b) and after (c,d) the water treatment. (e) Xray diffraction patterns of the untreated (green line) and treated (blue line) CsPbBr3 NC samples. (f) EDS data and (g) atomic ratios of untreated (green line), treated (orange line) CsPbBr3 NC samples and water dissolved materials (blue line), and the Cu and C signals in EDS spectra are originated from copper-supported carbon grid. In order to understand the mechanism on the improvement of optical performance, it is instructive to figure out the interactions between the as-prepared CsPbX3 NCs and the added water. Based on the results discussed above, we propose the reaction mechanism as an interface dissolution process, as shown in Figure 3a-c. Such a process contains three different stages, and the original CsPbX3 NCs with surface defects (Figure 3a) can firstly contact with the added water molecular. As a polar solvent, water can dissolve some

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excessive ions of surface anion-poor NCs to remove the defects (Figure 3b), leaving the perfect unit cell in the hexane 32 (Figure 3c) . This result is also linked to the previous EDS results (Figure 2f and 2g) that the Cs:Pb:Br ratio is closer to the ideal ratio for CsPbBr3 perovskite, and furthermore leading to the decrease in the surface trap states and the im33 provement of PLQY . The reaction should happen at the interface between water and hexane that are immiscible with water via ionic movement, attributed to the local structural instability of NCs surface and the strong ionic feature of CsPbBr3 perovskites. Additionally, because of the low solubility of hexane in water (only 9.5 mg/L), further dissolution of CsPbBr3 NCs with no defects during the reaction does not happen temporarily. But when the time is prolonged, the NCs will be damaged by the synergistic effect of oxygen, il34 lumination and moisture , so the luminescent NCs should be separated from the water after the reaction to avoid the decomposition of the formed CsPbBr3 NCs. Moreover, Figure 3d and 3e presents the comparative TEM images of untreated and treated typical CsPbBr3 NCs, respectively. From the insets in the corresponding TEM images, we can find the decrease in the dimensions of the NCs, and the mean sizes of untreated and treated CsPbBr3 NCs are 13.02 nm, 10.10 nm respectively, which can also support the proposed model herein.

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of water treatment process, we choose TMOS as precursor to form and coated SiO2 shell on the water treated CsPbBr3 NCs, which can improve not only the PLQY, but also the stability. Previous studies have shown that the hydrolysis possesses via TMOS only need small amount of water con35 sumption with fast hydrolysis rate , and fortunately, after water treatment, there will be relatively more amount of water in the CsPbBr3 NCs hexane solution, which can be used and accelerate the hydrolysis of TMOS and the formation of SiO2-coated CsPbBr3 NCs (CsPbBr3/SiO2). And in turn, the water in the hexane solution can be also consumed by TMOS, thereby minimizing the water-driven decomposition of CsPbBr3 NCs, which can be regarded as “killing two birds with one stone” and is different from previous strategies on SiO2 coating. Figure 4 shows the relevant structural, optical and stability characterization of as-prepared CsPbBr3/SiO2 sample. Figure 4a shows the XRD pattern of CsPbBr3/SiO2, exhibiting the same peaks as those of CsPbBr3 NCs with cubic structure along with the amorphous phase attributing to SiO2. The presence of CsPbBr3 NCs in the SiO2 is also confirmed by the TEM and HRTEM images shown in Figure 4b,c. The HRTEM images show the interplanar distance of 0.58 Å, which is consistent with the (100) crystal faces of CsPbBr3. EDS mapping images exhibited in Figure 4d support that the CsPbBr3 NCs are embedded into the SiO2 nanoparticles. The UV−vis absorption and PL emission spectrum of the CsPbBr3/SiO2 are shown in Figure 4e, and a narrow green emission peak at 517 nm are observed. To test the stability of CsPbBr3/SiO2, the photostability tests of CsPbBr3 NCs and CsPbBr3/SiO2 are implemented under continuous ambient light, as shown in Figure 4f and 4g. In Figure 4f, CsPbBr3 NCs and CsPbBr3/SiO2 are dispersed in hexane and exposed to ambient light for 24 h. With prolonged time, the PL intensities of both samples decrease under further illumination. However, the PL decay rate of CsPbBr3 NCs is much faster than that of the CsbBr3/SiO2 sample. After 24 h, the remnant relative PL intensity of CsPbBr3 NCs is reduced to 30%, while the CsPbBr3/SiO2 solution maintains a higher value of 80%. In addition, as shown in Figures 4g, when immersed in water, the CsPbBr3/SiO2 composite coated on the glass surface exhibits a striking improvement in stability as they are stable for at least 30 min, while for the pristine CsPbBr3 NCs coated on the glass surface, there is an immediate degradation.

Figure 3. Illustration of the proposed mechanisms for water treatment of perovskite NCs in three different stages, (a) the original CsPbX3 NCs with surface defects, (b) the as-treated CsPbX3 NCs with surface ions dissolved into water and (c) the treated CsPbX3 NCs in hexane with perfect unit cell. The TEM images of CsPbBr3 NCs before (d) and after (f) the water treatment, and their corresponding size distribution in the insets demonstrating the decrease in the dimensions of the NCs. Ever since the water treatment improving the optical performance has been verified in our study, it can be recognized as an important feature for future applications in hydrolysis reactions for CsPbX3 (X = Cl, Br, I) NCs. In addition, CsPbX3 (X = Cl, Br, I) NCs have exhibited excellent optical properties, but they are unstable due to their sensitivity to oxygen, moisture and light during the application. Therefore, on the basis

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Chemistry of Materials

Figure 5. (a) EL spectrum of the fabricated white LEDs showing the emission from blue chip, CsPbBr3/SiO2 and 4+ K2SiF6:Mn . Insets show the fabricated white LED without (left) and with (right) applied current of 20 mA. (b) CIE color coordinates of the fabricated white LEDs showing the corresponding color coordinates and the wide color gamut. ■ CONCLUSIONS Figure 4. (a) XRD pattern of as-prepared CsPbBr3/SiO2 and the standard pattern. (b) TEM and (c) high-resolution TEM images of CsPbBr3/SiO2. (d) EDS mapping of CsPbBr3/SiO2 composite for different elements. (e) UV-vis absorption (red line) and PL emission (green line) spectra of the colloidal CsPbBr3/SiO2 solution. (f) Remnant PL emissions of pristine CsPbBr3 and the colloidal CsPbBr3/SiO2 solutions as a function of the illumination time and (g) optical images under UV illumination (λ = 365 nm) after 30 min of soaking in water. Finally, in order to evaluate the stability in the practical application, light-emitting devices are constructed by coupling green-emitting the representative CsPbBr3/SiO2 composite prepared in this work and commercial available red4+ 4+ emitting K2SiF6:Mn (KSF:Mn ) phosphors with a blueemitting InGaN chip to demonstrate their promising usage in backlight units in modern liquid-crystal display (LCD) technique. Electroluminescence (EL) spectrum of the designed 4+ LEDs composed of CsPbBr3/SiO2 composite, KSF:Mn phosphor, and the blue-emitting chip, was presented in Figure 5a, which consisted of blue, green, and red emissions, yielding pure white-light. The color coordinates of blue-chip (0.1586, 4+ 0.314), CsPbBr3/SiO2 (0.1349, 0.773), and KSF:Mn (0.6859, 0.314) are labeled in the Commission Internationale de L'Eclairage (CIE) 1931 chromaticity diagram, respectively, showing a wide color gamut compared to the standard range (Figure 5b). Thus, the NTSC value of the as-fabricated device is calculated for backlight display. The color gamut overlap of the NTSC space is approximately as high as 128%, benefited from the narrow emission wavelength of the green perovskite nanocrystals and red phosphors with the narrow emission spectra.

In summary, we have demonstrated a facile post-treatment strategy with water to investigate the stability and luminescent properties of CsPbX3 NCs. The synergetic effect on water addition has been discussed, and the postsynthetic water treatment can remove surface defective layer to enhance the PLQY of CsPbX3 NCs. Simultaneously, the existence of a trace amount of water facilitated the hydrolysis of tetramethyl orthosilicate, leading to the formation of the SiO2-coated CsPbX3 NCs with high stability. Optical and structural characterizations reveal that water treated CsPbX3 NCs shows enhanced PLQY values at 78.5%, 95.5%, 91% for CsPbCl3, CsPbBr3 and CsPbI3, respectively, while maintaining the colloidal stability, phase and morphology. Accordingly, we have proposed a mechanism about how the PLQY can be enhanced by the water treatment strategy. Furthermore, the water permeating the hexane is used by hydrolysis of TMOS and the as-obtained CsPbBr3/SiO2 composite has been used to fabricate the light-emitting devices to evaluate the potential application. Our study proposed the significance of surface chemistry related to optical properties and provided a simple treatment with water to boost the utility of CsPbX3 (X = Cl, Br, I) perovskite NCs in high performance optoelectronic devices.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Z. Xia)

Author Contributions ⊥

These authors contributed equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundations of China (Grant Nos. 51722202, 91622125 and

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51572023), and Natural Science Foundations of Beijing (2172036).

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Synergetic Effect of Postsynthetic Water Treatment on the Enhanced Photoluminescence and Stability of CsPbX3 (X = Cl, Br, I) Perovskite Nanocrystals Ying Liu , Fei Li , Quanlin Liu and Zhiguo Xia* ⊥

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For Table of Contents Only

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Synergetic Effect of Postsynthetic Water Treatment on the Enhanced

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