All-Inorganic Perovskite Nanocrystals with a Stellar Set of Stabilities and Their Use in White Light-Emitting Diodes

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1 Cite This: All-Inorganic Perovskite Nanocrystals with a Stellar Set of Stabilities and Their Use in White Light-Emitting Diodes Yajing Chang,, Young Jun Yoon, Guopeng Li, Enze Xu, Shengtao Yu, Cheng-Hsin Lu, Zewei Wang, Yanjie He, Chun Hao Lin, Brent K. Wagner, Vladimir V. Tsukruk, Zhitao Kang,, Naresh Thadhani, Yang Jiang,*, and Zhiqun Lin*, School of Materials Science and Engineering and Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, Georgia 30332, United States School of Materials Science and Engineering, Hefei University of Technology, Hefei , P. R. China Downloaded via GEORGIA INST OF TECHNOLOGY on November 17, 2018 at 16:42:17 (UTC). See for options on how to legitimately share published articles. *S Supporting Information ABSTRACT: We report a simple, robust, and inexpensive strategy to enable all-inorganic CsPbX 3 perovskite nanocrystals (NCs) with a set of markedly improved stabilities, that is, water stability, compositional stability, phase stability, and phase segregation stability via impregnating them in solid organic salt matrices (i.e., metal stearate; MSt). In addition to acting as matrices, MSt also functions as the ligand bound to the surface of CsPbX 3 NCs, thereby eliminating the potential damage of NCs commonly encountered during purification as in copious past work. Quite intriguingly, the resulting CsPbX 3 MSt nanocomposites display an outstanding suite of stabilities. First, they retain high emission in the presence of water because of the insolubility of MSt in water, signifying their excellent water stability. Second, anion exchange between CsPbBr 3 MSt and CsPbI 3 MSt nanocomposites is greatly suppressed. This can be ascribed to the efficient coating of MSt, thus effectively isolating the contact between CsPbBr 3 and CsPbI 3 NCs, reflecting notable compositional stability. Third, remarkably, after being impregnated by MSt, the resulting CsPbI 3 MSt nanocomposites sustain the cubic phase of CsPbI 3 and high emission, manifesting the strikingly improved phase stability. Finally, phase segregation of CsPbBr 1.5 I 1.5 NCs is arrested via the MSt encapsulation (i.e., no formation of the respective CsPbBr 3 and CsPbI 3 ), thus rendering pure and stable photoluminescence (i.e., demonstration of phase segregation stability). Notably, when assembled into typical white light-emitting diode architecture, CsPbBr 1.5 I 1.5 MSt nanocomposites exhibit appealing performance, including a high color rendering index (R a ) and a low color temperature (T c ). As such, the judicious encapsulation of perovskite NCs into organic salts represents a facile and robust strategy for creating high-quality solid-state luminophores for use in optoelectronic devices. KEYWORDS: CsPbX 3 NCs, perovskite, metal stearate, stability, WLED INTRODUCTION their practical applications as they suffer from intrinsic The past several years have witnessed rapid advances in chemical instability. 27 First, their ionic character suggests a synthesis and applications of all-inorganic perovskite CsPbX poor moisture tolerance. 28,29 CsPbX 3 NCs degrade rapidly in 3 (X = Cl, Br, and I) nanocrystals (NCs). 1 3 In sharp contrast to the presence of moisture. Second, the weakly bounded surface traditional semiconductor NCs [e.g., CdSe quantum dots ligands, typically in the form of an ion pair of carboxylic acid (QDs) and 4 6 InP QDs 7,8 ], the high ionic bonding character and amine, 2 are easily dissociated from the surface of NCs renders the formation of CsPbX 3 within seconds. 9 More during purification with antisolvents, thereby resulting in importantly, because of their spectacular array of attractive greatly reduced colloidal stability and PLQY of NCs. 3,30 Third, properties such as high defect tolerance, 10,11 size- and a fast yet an unavoidable anion exchange occurs when mixing composition-tunable emissions, 12,13 narrow emission bandwidth, 14,15 and high photoluminescence quantum yield (PLQY), 1,16 this class of emerging materials provide new opportunities for use in optoelectronic devices, including solar cells, 17,18 light-emitting diodes (LEDs), lasers, photodetectors, 26 and so on. Despite significant progress that has been achieved in synthesis of CsPbX 3 NCs, it remains a grand challenge toward CsPbX 3 NCs of different compositions, leading to the composition homogenization. 15 For example, a room-temperature mixing of CsPbBr 3 and CsPbI 3 NCs yields CsPbBr x I 3 x NCs, and their emission is situated between the respective Received: August 8, 2018 Accepted: October 10, 2018 Published: October 10, American Chemical Society 37267

2 Figure 1. (a) UV vis absorption and PL spectra of CsPbBr 3 NCs. Inset: digital image of CsPbBr 3 NCs dispersed in toluene under the irradiation of 365 nm UV light. (b) XRD pattern of CsPbBr 3 NCs (green curve). The standard XRD pattern of the bulk CsPbBr 3 material is also included for comparison (black curve). (c) TEM image of CsPbBr 3 NCs with an average size of 10.5 nm, scale bar = 50 nm. (d) HRTEM image of CsPbBr 3 NCs, scale bar = 10 nm. emissions of the two constituents. 31,32 Such color instability disqualifies perovskite NCs for imaging applications where different colors are highly desired to span over the visible spectral range. 33 Fourth, phase instability of CsPbX 3 NCs is also a widely recognized issue. Notably, it has been reported that all CsPbX 3 NCs crystallize in their cubic phase and remain stable in this phase owing to high synthesis temperature and contributions from surface energy. 1 Particularly, for CsPbI 3 NCs, their perovskite phase (e.g., cubic phase) was found to be metastable under ambient conditions. As a result, they undergo a rapid cubic-to-orthorhombic phase transition, thus diminishing their fluorescent properties. 34,35 This is not surprising because the orthorhombic phase is thermodynamically stable for a bulk CsPbI 3 material below 315 C. 1 Finally, for allinorganic mixed-halide perovskites, for example, CsPbBr x I 3 x NCs, they are ubiquitously plagued by phase segregation, leading to the degradation of emission. 36 Clearly, there is a tremendous need to improve stability of CsPbX 3 NCs for rendering their commercially viable applications. To this end, many efforts have recently been centered on addressing the instability issues noted above, including coating CsPbX 3 NCs with polyhedral oligomeric silsesquioxane, 37 incorporating CsPbX 3 NCs in mesoporous silica, 38 and purifying CsPbI 3 NCs using methyl acetate as an antisolvent to prevent the dissociation of the surface ligand during the purification process. 17 Herein, we report an effective strategy to achieve CsPbX 3 NCs with greatly improved water, compositional, phase, and phase segregation stabilities by impregnating them within solid organic salt matrices [i.e., metal stearate (MSt)], that is, forming CsPbX 3 MSt nanocomposites. MSt used in our study includes aluminum stearate (AlSt 3 ), zinc stearate (ZnSt 2 ), and sodium stearate (NaSt). The introduction of the MSt solid into the CsPbX 3 NC solution leads to the coprecipitation of MSt (NaSt, ZnSt 2, and AlSt 3 ) and CsPbX 3 NCs. The subsequent centrifugation process readily removes the excessive ligands and precursors without the need of using antisolvents. The resulting CsPbX 3 MSt nanocomposites exhibit excellent optical properties with an impressive set of high stabilities. First, water stability is manifested in the retention of high PL of CsPbX 3 MSt nanocomposites (except NaSt as it is water soluble) in the presence of water because of the insolubility of MSt in water and the impregnation of CsPbX 3 by MSt. Second, compositional stability is demonstrated by the retention of two distinct emission peaks from CsPbBr 3 and CsPbI 3 NCs, when mixing separately prepared CsPbBr 3 MSt and CsPbI 3 MSt nanocomposites together. In other words, no anion exchange between CsPbBr 3 and CsPbI 3 NCs occurs as MSt effectively encapsulates and isolates CsPbBr 3 and CsPbI 3 NCs. Third, phase stability is revealed by the preservation of emission of CsPbI 3 NCs in MSt because of the released lattice strain by the wraparound MSt matrices, preventing it from phase transformation from the emissive cubic phase to the nonemissive orthorhombic phase. Consequently, CsPbI 3 NCs in CsPbI 3 MSt nanocomposites maintain their cubic phase and high emission for at least one month. Finally, phase segregation stability is signified in the appearance of pure and stable PL of CsPbBr 1.5 I 1.5 NCs as the possible phase segregation of CsPbBr 3 and CsPbI 3 NCs is suppressed probably by the released lattice strain as well as owing to the MSt encapsulation. A representative white light-emitting diode (WLED) composed of CsPbX 3 MSt nanocomposites excited by a blue InGaN LED chip is constructed, displaying a high color rendering index (R a ) and a low color temperature (T c ). Taken together, the results of this study exemplify that MSt salts are an attractive class of matrix materials for stabilizing perovskite CsPbX 3 NCs via coprecipitation of CsPbX 3 NCs and MSt, yielding highly stable fluorescent solid-state materials for optoelectronic devices. RESULTS AND DISCUSSION A set of CsPbX 3 NCs (X = Br, Br/I, and I) were synthesized (see the Experimental Section). Figure 1a shows the UV vis and PL spectra of CsPbBr 3 NCs in toluene synthesized at 160 C, where a band edge at 498 nm and a PL emission peak at 517 nm (i.e., green emission) are observed. These green

3 Figure 2. (a) Schematic illustration of the preparation of CsPbX 3 MSt nanocomposites by impregnating CsPbX 3 in the AlSt 3 matrix via a coprecipitation strategy. (b) Digital images of CsPbBr 3 NCs (cyan, upper left), CsPbBr 1.5 I 1.5 NCs (orange, upper central), and CsPbI 3 NCs (red, upper right) dispersed in toluene and the corresponding CsPbBr 3 NCs AlSt 3 (cyan, lower left), CsPbBr 1.5 I 1.5 AlSt 3 (orange, lower central), and CsPbI 3 AlSt 3 nanocomposites (red, lower right) under 365 nm UV light irradiation. (c) PL spectra of CsPbBr 3 NCs (cyan, dashed curve), CsPbBr 1.5 I 1.5 NCs (orange, dashed curve), and CsPbI 3 NCs (red, dashed curve) dispersed in toluene. The PL spectra of the corresponding CsPbX 3 AlSt 3 nanocomposites in dry states are shown in solid curves. emitting NCs have a finite full width at half-maximum (fwhm) of 30 nm, indicating a narrow size distribution. The PLQY of CsPbBr 3 NCs is 81%. The crystal structure is shown in Figure 1b. The standard X-ray diffraction (XRD) pattern of bulk CsPbBr 3 is also included as the reference. The emergence of crystal planes of (100), (110), (200), (211), and (220) suggest the formation of the cubic phase of CsPbBr 3 NCs. Assynthesized CsPbBr 3 NCs possess the cubic shape with an average size of 10.5 nm (Figure 1c). High-resolution transmission electron microscopy (HRTEM) of CsPbBr 3 NCs is shown in Figure 1d. The interplanar distance is measured to be 0.59 nm, which coincides with the (100) plane of CsPbBr 3 NCs. For CsPbBr 1.5 I 1.5 NCs synthesized at 160 C, their toluene solution is orange-emitting under UV irradiation, and their PL emission peak is centered at 612 nm with the fwhm of 32 nm and PLQY of 67%. The average size of cubicshaped CsPbBr 1.5 I 1.5 NCs is approximately 11.0 nm. The XRD measurement shows that CsPbBr 1.5 I 1.5 NCs have cubic phase (Figure S1a,c,e). Similarly, CsPbI 3 NCs synthesized at 120 C have the PL emission peak at 664 nm with an fwhm of 39 nm and PLQY of 52%, emitting red fluorescence upon UV excitation. They are cubic shaped as well with an average size of 9.6 nm and have a cubic phase (Figure S1b,d,f). Figure 2a depicts the stepwise formation of CsPbX 3 MSt nanocomposites. The MSt powder is added into the oleylamine (OLA)-capped CsPbX 3 NCs toluene solution. The organic salts (MSt) rapidly precipitate together with CsPbX 3 NCs from the mixed dispersion solution within several minutes (i.e., coprecipitation of CsPbBr 3 NCs and MSt). We speculate that not only MSt may bind the surface of CsPbX 3 NCs via the coordination bonding of St Cs and St Pb (i.e., replacing the original OLA ligands) but also the ionic interaction between metal ions in MSt and X in CsPbX 3, as will be further discussed later, thus leading to the coprecipitation of MSt and CsPbBr 3 NCs. Such a behavior is evidenced by weak PL from the supernatant, whereas the precipitates (i.e., CsPbX 3 MSt nanocomposites) emit bright fluorescence under 365 nm UV light excitation. After collecting the precipitates by centrifugation, they were dried in an oven to yield the final product. Figure S2 shows that after adding AlSt 3 into the CsPbBr 3 NC toluene solution followed by centrifugation, the supernatant displays a light color (Figure S2a) and weak PL (Figure S2b). With more AlSt 3 added, an even lighter color can be seen in the supernatant under ambient light and weaker PL was found under 365 nm UV light irradiation. Such progressive decrease of PL intensity of the supernatant (Figure S2c) indicates that less CsPbBr 3 NCs are present in the supernatant, signifying that CsPbBr 3 NCs are impregnated within the AlSt 3 matrix. Digital images of as-prepared CsPbBr 3, CsPbBr 1.5 I 1.5, and CsPbI 3 NC toluene solutions and the corresponding CsPbI 3 AlSt 3, CsPbBr 1.5 I 1.5 AlSt 3, and CsPbI 3 AlSt 3 nanocomposites are shown in Figure 2b. Cyan-emitting CsPbBr 3 AlSt 3, orange-emitting CsPbBr 1.5 I 1.5 AlSt 3, and red-emitting CsPbI 3 AlSt 3 nanocomposites demonstrate similar colors compared to as-prepared perovskite NC toluene solutions under UV light excitation. Figure 2c shows the PL spectra of CsPbX 3 NCs in toluene and the corresponding CsPbX 3 MSt nanocomposites in the dry state, respectively. The PL peaks are centered at 521 nm for CsPbBr 3 AlSt 3, 621 nm for CsPbBr 1.5 I 1.5 AlSt 3, and 670 nm for CsPbI 3 AlSt 3, respectively, in their dry states, representing a slight red-shift compared to their NC toluene solution counterparts due likely to a slight aggregation of perovskite NCs when impregnated in AlSt 3. Figure S3 compares transmission electron microscopy (TEM) images of pure AlSt 3 and CsPbBr 3 AlSt 3 nanocomposites. It is clear that pure AlSt 3 is irregular, whereas CsPbBr 3 AlSt 3 nanocomposites contain cubic-shaped CsPbBr 3 NCs dispersed in the AlSt 3 matrix with a slight aggregation. Figure S4 displays fluorescent micrographs of CsPbBr 3 AlSt 3, CsPbBr 1.5 I 1.5 AlSt 3, and CsPbI 3 AlSt 3 nanocomposites, emitting cyan, orange, and red fluorescence, 37269

4 respectively, when excited with 365 nm UV light. These micrographs further corroborate that perovskite NCs are distributed within an AlSt 3 solid matrix. It is notable that in addition to AlSt 3, other stearate salts (NaSt and ZnSt 2 ) were also investigated. When NaSt was employed to encapsulate CsPbX 3 NCs, a red-shift of PL maximum over pristine CsPbX 3 NCs can be found in resulting CsPbX 3 NaSt nanocomposites (Figure S5). Similarly, such a red-shift is due likely to the aggregation of CsPbX 3 NCs during the coprecipitation process. Interestingly, for ZnSt 2, a PL blueshift was observed in resulting CsPbX 3 ZnSt 2 nanocomposites (Figure S6), compared to that of the pristine CsPbX 3 NCs. This blue-shift may result from partial cation exchange between Pb 2+ and Zn 2+. A similar phenomenon, that is, cation exchange between Pb 2+ and divalent ions (e.g., Cd 2+,Zn 2+, and Sn 2+ ), has been reported. 39 It is well known that whether an ABX 3 perovskite structure can be formed and sustained is assessable by the Goldschmidt tolerance factor 40 t = ( r + r )/[ 2( r + r )] A X B X where r A, r B, and r X are the ionic radii of the respective ions in ABX 3. An ABX 3 perovskite structure is sustainable if the tolerance factor is between 0.8 and As summarized in Table S1, the tolerance factor of CsZnBr 3 is 0.933, which is within the tolerance factor range noted above to allow for the formation of a stable perovskite structure. Thus, partial cation exchange may occur between Pb 2+ and Zn 2+, leading to a blueshift of the PL maximum. This can be rationalized as follows. The incorporation of smaller Zn 2+ results in the contraction of PbBr 6 octahedra, resulting in shorter Pb Br bonds and thus stronger interactions between Pb and Br orbitals. 39 As the conduction band minimum is composed of antibonding combinations between Pb 6p and Br 4p orbitals, 42 it shifts to higher energy with stronger interaction. As a result, the band gap of perovskite NCs increases and their PL maximum blueshifts. In contrast, for NaSt, the tolerance factors of NaPbBr 3 and NaPbI 3 are and 0.749, respectively, which are smaller than 0.8. Consequently, no cation exchange occurs between Na + and Cs +. Different mass ratios of CsPbBr 3 NCs to stearate salts were investigated to examine their influence on the PL spectra (Figure S7). For NaSt and AlSt 3, the extent of the red-shift in the PL maximum of the resulting nanocomposites decreases as the CsPbBr 3 NC/stearate salt mass ratio decreases. As noted above, the red-shift is induced by the aggregation of perovskite NCs during their impregnation into NaSt and AlSt 3. It is not surprising that a lower mass ratio results in a lower degree of aggregation, as evidenced in Figure S8, which in turn causes a smaller red-shift. For ZnSt 2, as the mass ratio increases, the PL maximum of CsPbBr 3 ZnSt 2 nanocomposites further blueshifts because more ZnSt 2 is introduced, leading to an increased cation exchange between Pb 2+ and Zn 2+. We also studied the effect of solvents (toluene and hexane) used to disperse perovskite NCs during the NC encapsulation process by MSt on their PL (Figure S9). The dispersions of pristine CsPbBr 3 NCs only are also included for comparison. Clearly, CsPbBr 3 NaSt and CsPbBr 3 AlSt 3 nanocomposites display the PL red-shift, regardless of toluene or hexane used. The PL blue-shift can be found for CsPbBr 3 ZnSt 2 nanocomposites in both toluene and hexane when they are used for CsPbBr 3 NCs dispersion. However, the PL blue-shift in toluene is larger than that in hexane. This is because of the different solubilities of ZnSt 2 in toluene and hexane. ZnSt 2 is partially dissolved in toluene, thus the cation exchange reaction between CsPbBr 3 NCs and ZnSt 2 is relatively fast and efficient. In contrast, ZnSt 2 cannot be dissolved in hexane, leading to a slow and inefficient cation exchange of CsPbBr 3 NCs and ZnSt 2. Figure 3 compares the XRD patterns of CsPbBr 3 NCs, CsPbBr 3 AlSt 3 nanocomposites, and pure AlSt 3 powders. All Figure 3. XRD patterns of CsPbBr 3 AlSt 3 nanocomposites (red curve), CsPbBr 3 NCs (blue curve), and pure AlSt 3 (black curve). diffraction peaks of CsPbBr 3 NCs can be readily identified in the XRD spectrum of CsPbBr 3 AlSt 3 nanocomposites, signifying that the crystal structures of CsPbBr 3 NCs are retained in nanocomposites. Specifically, CsPbBr 3 AlSt 3 nanocomposites have the diffraction peaks at 2θ of 15.1, 21.4, and 30.4, corresponding to the (100), (110), and (200) crystalline planes of CsPbBr 3 NCs. The diffraction peaks at 2θ of 6.7, 11.5, 19.5, and 23.2 are from the AlSt 3 matrix. Clearly, these results suggest that the cubic structure of CsPbBr 3 NCs is well preserved after the impregnation of CsPbBr 3 NCs in the AlSt 3 matrix. We note that the peak intensities from CsPbBr 3 are relatively weak compared to those of AlSt 3 as the latter is the matrix. To investigate the interaction between CsPbBr 3 and AlSt 3 in nanocomposites, Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) studies were performed. Figure 4 depicts the FTIR analysis of CsPbBr 3 NCs and CsPbBr 3 AlSt 3 nanocomposites with different CsPbBr 3 /AlSt 3 ratios. For CsPbBr 3 NCs, the stretching vibrations of N H at 3030 cm 1,C Hat 2925 and 2854 cm 1,C Cat 1608 cm 1, and C N at 1382 cm 1 as well as the bending vibration of C C at 908 cm 1 are observed, revealing the presence of surface capping ligands (i.e., OLA). In contrast, FTIR spectra of nanocomposites show the disappearance of N H, C C and C N vibrations, indicating the absence of OLA. The vibrations of CsPbBr 3 AlSt 3 nanocomposites are at the same wavenumbers as those of AlSt 3, representing the presence of AlSt 3. Moreover, XPS studies of O 1s, C 1s, and N 1s are shown in Figure S10, where both O 1s and C 1s peaks shift to larger binding energies after the impregnation of CsPbBr 3 NCs in AlSt 3. N 1s peaks can be clearly observed in CsPbBr 3 NCs, whereas they cannot be detected in CsPbBr 3 AlSt 3 nanocomposites, suggesting the absence of OLA in CsPbBr 3 AlSt 3 nanocomposites. These results suggest that during the coprecipitation process, the original OLA ligands on the 37270

5 Figure 4. FTIR spectra of CsPbBr 3 NCs and CsPbBr 3 AlSt 3 nanocomposites with varied mass ratios (CsPbBr 3 /AlSt 3 = 3:1, 1:1, 1:2). The FTIR spectrum of pure AlSt 3 is also included as the reference. surface of CsPbBr 3 NCs were exchanged by AlSt 3 as discussed in the mechanism above. XPS studies revealed that after encapsulation by AlSt 3, all Cs 3d, Pb 4f, and Br 3d peaks are shifted to larger binding energies, particularly Pb 4f and Br 4d peaks (Figure 5). This may suggest the coordination bond of St Cs and St Pb and the ionic bond between metal ions from MSt and X on the surface of CsPbX 3. On the basis of XPS and FTIR measurements, we propose a plausible mechanism of AlSt 3 encapsulation for yielding CsPbBr 3 AlSt 3 nanocomposites. In the presence of AlSt 3, the weakly bound surface ligands OLA are exchanged by AlSt 3. As a result, Al 3+ preferentially adsorbs to the surface of NCs via Al 3+ Br interaction, accompanied by a long St chain situating on the surface of NCs or coordinating with Cs + and Pb 2+ of NCs. Such a straight St chain promotes van der Waals attraction between adjacent NCs through the interdigitation of St chains from adjacent NCs and accordingly imparts the aggregation of NCs. Similar ligand-dependent colloidal stability has been reported in CdSe QDs. 43 For the saturated solution of CdSe QDs, a dramatically decreased solubility was observed as the length of n-alkanoate ligands change from C 14 to C 22, which was ascribed to the increased van der Waals attraction of QDs via the ligand interdigitation between neighboring QDs. The PL lifetime of CsPbBr 3 NCs and CsPbBr 3 AlSt 3 nanocomposites was studied (Figure S11 and Table S2). It is clear that CsPbBr 3 AlSt 3 nanocomposites possess a longer lifetime than that of CsPbBr 3 NCs, which can be ascribed to the surface passivation of AlSt 3. In the work that follows, we demonstrate an impressive set of excellent stabilities that CsPbX 3 MSt nanocomposites carry, including water stability, compositional stability, phase stability, and phase segregation stability. First, we take CsPbBr 3 AlSt 3 nanocomposites as an example to showcase high water resistance of CsPbX 3 MSt nanocomposites (Figure 6b). Sonication was applied to assist in the homogeneous mixing between liquid/liquid (water/toluene) and liquid/solid (water/nanocomposites) phases. For CsPbBr 3 NCs in toluene, after addition of water followed by sonication, the solution emission becomes weaker and completely disappears after 25 min (Figure 6a). In stark contrast, after sonication for 25 min, CsPbBr 3 AlSt 3 nanocomposites still display bright PL and remain highly luminescent even after being stored overnight Figure 5. High-resolution XPS spectra of Cs 3d, Pb 4f, and Br 3d in CsPbBr 3 NCs (upper blue curves) and CsPbBr 3 AlSt 3 nanocomposites (lower red curves). (a) Cs 3d (b) Pb 4f (c) Br 3d, where the fitted curves are shown in purple and green. (Figure 6b). This observation clearly indicates the prominent water resistance of the CsPbBr 3 AlSt 3 nanocomposites, suggesting the effective encapsulation and protection of CsPbBr 3 NCs by the AlSt 3 matrix. The compositional stability (i.e., anion exchange stability) of CsPbX 3 MSt nanocomposites was then evaluated. As shown in Figure 7a, immediately after mixing CsPbBr 3 and CsPbI 3 NC powders together, the two PL peaks merge into a broadened peak (mixture 1) with much reduced intensity composed of several subpeaks situated between the original CsPbBr 3 and CsPbI 3 PL emissions because of the partial anion exchange between Br and I. 15,37 After 1 h, these broad PL peaks merge into a narrow single peak (mixture 2 in Figure 7a), signifying the complete anion exchange. 15 Taking the mixing of CsPbBr 3 AlSt 3 and CsPbI 3 AlSt 3 nanocomposites as an example, it is notable that there was no salient PL peak position and intensity change even after mixing these two 37271

6 Figure 6. Digital images of (a) CsPbBr 3 NCs in toluene and (b) CsPbBr 3 AlSt 3 nanocomposites exposed to 365 nm UV light after the addition of water, sonication for different times, and stored overnight. Figure 7. (a) PL spectra of CsPbBr 3 NC powders, CsPbI 3 NC powders, and their mixtures (mixture 1 taken after mixing for 5 min, and mixture 2 taken after mixing for 1 h). (b) PL spectra of CsPbBr 3 AlSt 3 nanocomposites, CsPbI 3 AlSt 3 nanocomposites, and their mixture after mixing for one day. nanocomposites for one day. Clearly, these results corroborate that CsPbBr 3 and CsPbI 3 NCs encapsulated within the respective AlSt 3 matrix are well protected and effectively isolated from each other preventing them from anion exchange. As for phase stability of nanocomposites, CsPbI 3 AlSt 3 was used as an example. We first investigated their PL under ambient conditions. For comparison, the PL of CsPbI 3 NCs was also measured. Figure 8a shows that the PL intensity of CsPbI 3 NCs in toluene drops quickly within 10 days. The XRD measurements clearly suggest the phase transition of CsPbI 3 NCs. The freshly synthesized CsPbI 3 NCs have a cubic phase and transform into an orthorhombic phase after 10 days. Figure S12 illustrates the schematic of cubic and orthorhombic structures. In sharp contrast, the CsPbI 3 AlSt 3 nanocomposites still maintain 85% of the original PL intensity after 30 days (Figure 8b). Moreover, no orthorhombic phase can be found in the XRD pattern of nanocomposites after 30 days (Figure 8d), which is comparable to those reported in the literature (Table S3). These results demonstrate that the passivation of CsPbI 3 NCs with AlSt 3 is highly effective and greatly suppresses the phase transformation of CsPbI 3 NCs from the emissive cubic phase to nonemissive orthorhombic phase. There are two reasons which may account for the phase stability of CsPbI 3 NCs after impregnation in AlSt 3. First, AlSt 3 can effectively isolate CsPbI 3 NCs from the surrounding environment containing moisture and oxygen that accelerate the phase transition of CsPbI 3 NCs from cubic to orthorhombic. Second, AlSt 3 may release the lattice strain which is the driving force for phase transition of CsPbI 3 NCs. Notably, it has been reported that the lattice strain causes the intrinsic instability of the cubic phase at ambient temperature and drives the phase transformation into the orthorhombic phase. 27,35 Similar phase stability was also found in CsPbI 3 NaSt and CsPbI 3 ZnSt 2 nanocomposites. As shown in Figure S13, both CsPbI 3 NaSt and CsPbI 3 ZnSt 2 nanocomposites display high PL intensity after 30 days. Notably, as-prepared nanocomposites also possess remarkable phase segregation stability. In this regard, CsPbBr 1.5 I 1.5 NCs and CsPbBr 1.5 I 1.5 AlSt 3 nanocomposites were examined and compared, revealing that AlSt 3 impregnation can effectively prevent phase segregation and retain their structural stability. Figure 9a depicts that within 20 days, the PL intensity of CsPbBr 1.5 I 1.5 NCs drops quickly and the PL maximum blueshifts largely, which is consistent with the previous report in the literature. 44 By contrast, after AlSt 3 encapsulation, there is only a slight decrease in PL intensity of CsPbBr 1.5 I 1.5 AlSt 3 nanocomposites. No detectable PL peak shift can be observed, suggesting no phase segregation (i.e., forming the respective CsPbBr 3 and CsPbI 3 ; Figure 9b). This is also due probably to the AlSt 3 encapsulation offsets lattice strain which improves the phase stability. These results clearly indicate that the AlSt 3 encapsulation is robust and prevents the phase segregation of mixed-halide perovskite. As described above, CsPbX 3 MSt nanocomposites exhibit a suite of excellent stabilities. Subsequently, WLED composed of 37272

7 Figure 8. PL evolutions of (a) CsPbI 3 NCs dispersed in toluene and (b) CsPbI 3 AlSt 3 nanocomposites in solid state under ambient condition. The corresponding XRD patterns of (c) CsPbI 3 NCs before (blue line) and after (red line) being stored under ambient conditions for 10 days (dry state) and (d) CsPbI 3 AlSt 3 nanocomposites before (blue line) and after (red line) being stored under ambient conditions for 30 days (dry state). The XRD pattern of AlSt 3 (black line) is also provided as the reference. Figure 9. PL evolutions of (a) CsPbBr 1.5 I 1.5 NCs dispersed in toluene and (b) CsPbBr 1.5 I 1.5 AlSt 3 nanocomposites in solid-state under ambient conditions. CsPbX 3 MSt nanocomposites was assembled, manifesting the appealing performance, including a high color rendering index (R a ) and a low color temperature (T c ). Commercially available WLEDs usually involve the use of a blue-emitting InGaN chip as the excitation source and Y 3 Al 5 O 12 :Ce 3+ (YAG:Ce 3+ ) yellow phosphors as the color conversion layer. However, such WLEDs suffer from low R a (<80) as YAG:Ce 3+ phosphors have limited red spectrum emission. In an attempt to solve this problem, a WLED was crafted by employing the mixture of YAG:Ce 3+ phosphors, CsPbBr 1.5 I 1.5 AlSt 3 nanocomposites, and epoxy deposited on a blue InGaN chip. Figure 10 shows the electroluminescence (EL) spectrum of the constructed WLED when electrically driven. Bright and warm white light is emitted with a high R a of 89.9, a low T c of 4588 K, a CIE color coordinate of (0.35, 0.33), and an efficacy of lm/w under 30 ma driven current. For comparison, a WLED by employing the mixture of YAG:Ce 3+ phosphors, CsPbBr 1.5 I 1.5 NCs, and epoxy deposited on a blue InGaN chip was also fabricated (Figure S14), yielding a much lower efficacy. These results signify the great potential of CsPbX 3 MSt nanocomposites for use as solid-state luminophores in WLEDs and other practical optoelectronic applications. CONCLUSIONS In summary, we have developed a simple yet robust strategy to craft a new class of CsPbX 3 MSt nanocomposites (where X = 37273

8 Figure 10. (a) EL spectrum and (b) CIE color coordinate of the WLED crafted by depositing the mixture of YAG:Ce 3+ and CsPbBr 1.5 I 1.5 AlSt 3 nanocomposites on a blue InGaN chip. Inset: WLED operated under 30 ma driven current. Br, I, and Br/I; M = Na, Zn, and Al) with a set of impressive stabilities, that is, water stability, compositional stability, phase stability, and phase segregation stability. Notably, this strategy allows for rapid and effective precipitation and purification of CsPbX 3 NCs from the crude solution, thus eliminating the need for polar solvents widely used for yielding perovskite NCs and in turn preventing the damage of CsPbX 3 NCs. Because of insolubility of AlSt 3 in water, the resulting CsPbX 3 AlSt 3 nanocomposites retain their high emission in water, that is, a demonstration of water stability. Moreover, when mixing CsPbBr 3 AlSt 3 and CsPbI 3 AlSt 3 nanocomposites together, an anion exchange between them (i.e., Br and I exchange) is virtually arrested because of the efficient coating and separation of the respective CsPbX 3 by AlSt 3, that is, a manifestation of compositional stability. Furthermore, for CsPbI 3 NCs, the cubic phase is metastable at room temperature and readily transformed into the PL-inactive orthorhombic phase. After coating with AlSt 3, the resulting CsPbI 3 AlSt 3 nanocomposites preserve the cubic phase of CsPbI 3 and high emission for at least one month, that is, a representation of phase stability. Finally, CsPbBr 1.5 I 1.5 NCs impregnated within the AlSt 3 matrix sustain high emission without degradation for at least one month, that is, a display of phase segregation stability. These optically active, highly stable nanocomposites are exploited as solid-state luminophores for the WLED, delivering the appreciable performance, including a high R a of 89.9 and a low T c of 4588 K. By extension, this simple strategy may be properly applied to other functional yet less stable NC systems, for example, organic inorganic perovskite NCs and inorganic nonlead-based perovskite NCs for a wide range of applications in optoelectronic materials and devices. EXPERIMENTAL SECTION Materials. Oleic acid (OA, 90%), OLA (90%), 1-octadecene (ODE, 90%), toluene, hexane, sodium stearate (NaSt, 98%), zinc stearate (ZnSt 2, 99%), aluminum stearate (AlSt 3, 99%), and transparent epoxy (JH-6800MA and JH-6800MB) were purchased from Sinopharm Chemical Reagents Co. Ltd. Lead bromide (PbBr 2, 99%), lead iodide (PbI 2, 98%), and cesium carbonate (Cs 2 CO 3, 99.9%) were from Aladdin. All of the raw materials were used directly without further purification. Blue InGaN LED chips (450 nm) with a power of 0.1 W were purchased from APT Electronics Ltd. Preparation of Cs-Oleate. Cs-oleate was prepared by following a previously reported procedure. 1 Cs 2 CO 3 (0.8 g), OA (2.5 ml), and ODE (30 ml) were added into a 100 ml three-necked flask and degassed for 1 h at 120 C, and then heated to 150 C under N 2 flow until the solution turned clear. We note that the Cs-oleate ODE solution has to be preheated to 100 C prior to injection to the PbX 2 ODE solution as Cs-oleate precipitates out of ODE at room temperature. Synthesis of CsPbX 3 NCs. ODE (5 ml) and PbX 2 [0.188 mmol; i.e., PbI 2 (0.087 g), PbBr 2 (0.069 g), or a mixture of PbI 2 ( g) and PbBr 2 ( g)] were added into a 25 ml three-necked flask and degassed for 1 h at 120 C. Dried OLA (0.5 ml) and OA (0.5 ml) were injected into the PbX 2 ODE solution at 120 C under N 2. After the solution turned clear, the temperature was raised to C, and the Cs-oleate solution (0.4 ml) was quickly introduced, and 5 s later, the reaction mixture was cooled by an ice-water bath. Impregnation of CsPbX 3 NCs within MSt Salts. A crude solution of as-synthesized CsPbX 3 NCs was dissolved in toluene and centrifuged at 6500 rpm for 10 min. The supernatant was discarded, and the precipitation was redispersed in toluene and centrifuged at 6500 rpm for 10 min. Then, the precipitate was discarded, and 50 mg MSt was added to the supernatant (25 mg/ml) under vigorous stirring. The MSt used in this work includes aluminum stearate (AlSt 3 ), zinc stearate (ZnSt 2 ), and sodium stearate (NaSt). After 10 min, the stirring was stopped, and the mixed solution was allowed to precipitate. After centrifugation at 5000 rpm for 5 min, the supernatant was discarded, whereas the precipitate was dried in an oven at 60 C overnight. Fabrication of WLED. Typically, epoxy resin was used as the encapsulating material in the packaging of LEDs. Equal amounts of transparent epoxy JH-6800MA and JH-6800MB were mixed together, after which cerium-doped yttrium aluminum garnet (YAG:Ce 3+ ) phosphors and as-prepared CsPbBr 1.5 I 1.5 NCs-AlSt 3 nanocomposites were added. The mixture was stirred thoroughly and then dispersed on the InGaN LED chip. The coated WLED chip was solidified in an oven at 80 C for 2 h. Characterization. All measurements were performed at room temperature. Ultraviolet visible (UV vis) absorbance spectra of the CsPbX 3 NCs toluene solution were obtained using a Shimadzu UV UV vis spectrophotometer. PL spectra of the CsPbX 3 NCs toluene solution and CsPbX 3 MSt nanocomposites were recorded using a HitachiF-4600 fluorescence spectrophotometer. TEM and HRTEM images of CsPbBr 3 NCs, as well as TEM images of CsPbBr 3 AlSt 3 nanocomposites were obtained by a JEM-2100 field emission source TEM. Optical micrographs of CsPbX 3 AlSt 3 nanocomposites were taken using an Olympus SZX16 fluorescence microscope. XRD studies were performed using a RigakuD/MaxrB X- ray diffractometer to identify the crystalline phases and structures of CsPbX 3 NCs and CsPbX 3 AlSt 3 nanocomposites. FTIR spectra of CsPbX 3 NCs, AlSt 3, and CsPbX 3 AlSt 3 nanocomposites were acquired by a Nicolet 6700 FTIR spectrometer. XPS studies on 37274

9 CsPbBr 3 NCs and CsPbBr 3 AlSt 3 nanocomposites were conducted using a Thermo Escalab-250 X-ray photoelectron spectrometer. PL lifetime measurements of CsPbX 3 NCs and CsPbX 3 AlSt 3 nanocomposites were performed using an Edinburgh FL 900 photocounting system with a time-correlated single-photon counting module. Optical properties of the WLED were obtained using an integrating sphere (Everfine Photo-E-Info Co., Ltd). ASSOCIATED CONTENT *S Supporting Information The Supporting Information is available free of charge on the ACS Publications website at. Experimental details and additional figures (PDF) AUTHOR INFORMATION Corresponding Authors * apjiang@hfut.edu.cn (Y.J.). * zhiqun.lin@mse.gatech.edu (Z.L.). ORCID Shengtao Yu: Vladimir V. Tsukruk: Yang Jiang: Zhiqun Lin: Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported by the Air Force Office of Scientific Research (MURI FA ; FA ), DOD-DTRA (HDTRA ), DOE-STTR (DE- SC ), and National Natural Science Foundation of China (grant nos. U and ). Y.C. gratefully acknowledges financial support from the China Scholarship Council. REFERENCES (1) Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX 3, X= Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15, (2) Akkerman, Q. A.; Raino, G.; Kovalenko, M. V.; Manna, L. Genesis, Challenges and Opportunities for Colloidal Lead Halide Perovskite Nanocrystals. Nat. Mater. 2018, 17, 394. (3) De Roo, J.; Ibańẽz, M.; Geiregat, P.; Nedelcu, G.; Walravens, W.; Maes, J.; Martins, J. C.; Van Driessche, I.; Kovalenko, M. V.; Hens, Z. 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