Highly Stable Cesium Lead Halide Perovskite Nanocrystals through in

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Highly Stable Cesium Lead Halide Perovskite Nanocrystals through in Situ Lead Halide Inorganic Passivation Ju Young Woo,†,‡ Youngsik Kim,‡,§ Jungmin Bae,# Tae Gun Kim,∥,# Jeong Won Kim,∥,# Doh C. Lee,*,† and Sohee Jeong*,‡,§ †

Department of Chemical and Biomolecular Engineering (BK21+ Program), KAIST Institute for the NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea ‡ Nano-Convergence Research Division, Korea Institute of Machinery and Materials (KIMM), Daejeon 305-343, Korea § Department of Nanomechatronics, ∥Department of Nanoscience, Korea University of Science and Technology (UST), Daejeon 305-350, Korea # Korea Research Institute of Standards and Science (KRISS), Daejeon 305-340, Korea S Supporting Information *

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NCs,14 immobilized inorganic passivation could work better than organic passivation for effective stabilization of CsPbX3 NCs. Herein, we present in situ synthesis of inorganically passivated and extremely stable CsPbX3 NCs, where X is Br or I, by introducing metal halides. Changes in anionic elements, which accompany unwanted changes in band gap,12 are completely avoided since we use the metal halides with same anionic element as the ones in the perovskite crystals. Stability of the resulting CsPbX3 NCs was examined by monitoring PL QY, powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) over time after storing the NC solids under ambient conditions. From the studies using high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) and X-ray photoelectron spectroscopy (XPS), unprecedented stability enhancement of CsPbX3 NCs is interpreted in the context of lead halide inorganic passivation on (110) surface upon the addition of metal halides in the synthesis. From the understanding, we propose two possible models for the lead halide inorganic passivation. Pristine-CsPbBr 3 NCs were synthesized based on a previously reported method by Protesescu et al.1 In our new synthesis, metal bromide (e.g., ZnBr2) was additionally introduced in a flask containing mixture of other precursors, such as PbBr2, oleic acid, and oleylamine in octadecene. Preheated Cs-oleate was rapidly injected into the opaque reaction solution at 170 °C (Figure S1), and then green colored NC solution was immediately formed (denoted hereafter as ZnBr2-CsPbBr3 NCs, see Supporting Information (SI) for details of the synthetic procedures). We collected the NCs via multiple high-speed centrifugations. To keep the surface unaltered during collection of the NCs, we added no additional chemical, such as polar antisolvent (see SI). The resulting CsPbBr3 NCs were dispersed in toluene, and the solution was dropcast on glass substrates for further characterizations. Figure 1A shows PL spectra of pristine- and ZnBr2-CsPbBr3 NC films. Both samples showed sharp emission with full width

recent surge of interest in colloidal cesium lead halide perovskite (CsPbX3 where X is Cl, Br, or I) nanocrystals (NCs) has emerged from their high photoluminescence (PL) quantum yield (QY),1 very narrow emission line width,2 defect tolerance,3,4 and wide range of band gaps tunable by control of both composition5,6 and morphology.7,8 The promise of CsPbX3 NCs has been examined in the context of their use in optoelectronic devices, such as light emitting diodes,9 lasing,10 detection,11 and photovoltaics.12,13 Unique features, such as highly dynamic surfaces,14 rapid reaction kinetics,15 and carrier dynamics,16 have also triggered the fundamental interest of CsPbX3 NCs. Despite the mounting interest, instability of CsPbX3 NCs has impeded the utilization of the materials in practical applications.12,17−19 For example, exposure of CsPbX3 NCs to ambient atmospheric conditions (with variable humidity, heat, and/or light) leads to rapid drop of PL QY,20 shape transformation,21 and phase transition.12 The instability is generally attributed to ionic character,20,22 labile surfaces,14 and metastable structures1,12 of CsPbX 3 NCs. For reliable integration of CsPbX3 NCs in various applications, the stability issue must be addressed. The past few years have seen strides toward stable colloidal CsPbX3 NCs based on some passivation strategies. For example, CsPbX3 NCs incorporated into encapsulating matrices showed enhanced stability to some extent.18 Surface ligand modifications such as introduction of tightly bound ligands during the synthesis,20 ligand replacement,19,23 and crosslinking between ligands in solid state17 appear to help enhance stability. Recently, appropriate choice of antisolvent in washing steps was reported to result in relatively brighter and more stable NCs by minimizing damage on surface.12,22 Although study of surface-ligand interaction and its effect on the stability of CsPbX3 NCs has made progress, inorganic passivation and its impact on stability of CsPbX3 NCs remains nearly unexplored. As inorganic (atomic) passivation has proven to be exceptionally successful in terms of elevating both stability and device performance based on ionic chalcogenide NCs,24,25 such strategies rightfully deserve to be attempted in CsPbX3 NCs. Furthermore, considering intrinsically very dynamic interactions between organic ligands and surfaces of CsPbX3 © 2017 American Chemical Society

Received: June 26, 2017 Revised: August 16, 2017 Published: August 16, 2017 7088

DOI: 10.1021/acs.chemmater.7b02669 Chem. Mater. 2017, 29, 7088−7092

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Chemistry of Materials

Figure 1. (A) PL spectra and PL QYs of pristine- and ZnBr2-CsPbBr3 NC films. (B) PL QYs of CsPbBr3 NC films recorded as a function of time after exposure to ambient condition.

at half maxima of 21 nm. Interestingly, PL QY of ZnBr2CsPbBr3 NCs (PL QY = 78%) is markedly higher than that of pristine-CsPbBr3 NCs (PL QY = 54%) by nearly 50% whereas the emission peak position is identical at around 518 nm (see Figure S2 for solution spectra). XRD analysis confirms formation of the CsPbBr3 perovskite structure and absence of discernible Zn-related impurity peaks in ZnBr2-CsPbBr3 NCs (Figure S3). The absence of Zn was also corroborated by XPS measurement (Figure S4). Surprisingly, in ambient condition with very high relative humidity (RH) up to 60%, nearly 60% of original PL QY was retained for the case of ZnBr2-CsPbBr3 NCs compared to about 20% retained in the case of pristineCsPbBr3 NCs (Figure 1B). Zn cation is not likely to contribute to enhancement of PL QY and its stability as XRD and XPS results points to absence of Zn cation in ZnBr2-CsPbBr3 NCs (Figure S3 and S4). To explore the bromide effect without metal cation, we synthesized the CsPbBr3 NCs by introducing tetrabutylammonium bromide (TBABr) in the place of ZnBr2. However, both PL QY and its stability were mediocre at best (Figure S5), suggesting that enhancement of PL QY and stability observed in ZnBr2-CsPbBr3 NCs cannot be achieved just by supplying additional any bromides. Several previous studies have reported that perovskite crystals experience structural instability, which ultimately results in uncontrollable, large swing of their physicochemical properties.12,26,27 However, investigation on structural stability of colloidal CsPbBr3 NC solids has been nearly lacking. To examine the structural stability of our CsPbBr3 NC films, we carried out XRD analysis before and after the films were exposed to ambient conditions, as shown in Figure 2A. XRD patterns of pristine-CsPbBr3 NCs show that peaks become considerably sharper and split after 5 days of aging in ambient conditions. Crystal structures of colloidal CsPbBr3 NCs have been in controversy because XRD patterns of cubic and orthorhombic structures appear nearly identical particularly in the case of NCs.8 Very recently, coexistence of orthorhombic and cubic phase in CsPbBr3-based nanomaterials was directly visualized by low dose-rate in-line holography using TEM.28 On the basis of the observation, we postulate that peak splitting shown in Figure 2A is associated with both growth occurring during aging and complete phase transition to orthorhombic phase under ambient atmosphere. High surface energy,20 dynamic surface,14 imperfect passivation, and metastable structures1 of pristine-CsPbBr3 NCs would result in uncontrolled growth under ambient atmosphere. In contrast, XRD patterns of ZnBr2-CsPbBr3 NCs are nearly intact after aging for 5 days under ambient atmosphere, indicating remarkably enhanced structural stability of ZnBr2-CsPbBr3 NCs.

Figure 2. (A) XRD patterns of CsPbBr3 NCs before and after aging. TEM images of pristine-CsPbBr3 NCs (B) before and (C) after aging under ambient condition for 6 days. TEM images of ZnBr2-CsPbBr3 NCs (D) before and (E) after aging under ambient condition for 6 days. Scale bars are 40 nm. (F) XRD patterns of CsPbBr3 NCs after annealing at 200 °C for 4 h under N2 atmosphere.

Structural instability of pristine-CsPbBr3 NCs appears more pronounced in TEM analysis: pristine-CsPbBr3 NCs became uncontrollably irregular in shape and their size increased up to hundreds of nanometers after the TEM grid was stored under ambient conditions (Figure 2B and 2C). The morphological transformation of the aged pristine-CsPbBr3 NCs implies that lateral fusion occurred even in solid state at room temperature. More specifically, in HAADF-STEM analysis, we observed that lateral fusion of pristine-CsPbBr3 NCs exclusively initiates along [110] crystal direction (Figure S6). On the other hand, the shape and size of ZnBr2-CsPbBr3 NCs showed nearly no change (Figure 2D and 2E) under the same aging conditions, which again point to markedly enhanced structural stability of ZnBr2-CsPbBr3 NCs. Also importantly, ZnBr2-CsPbBr3 NCs exhibit extreme structural stability against heat, which is crucial 7089


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compared with pristine-CsPbBr3 NCs. To obtain more surface sensitive signal from XPS, we carried out angle-resolved XPS measurement. Take-off angle was changed from 90° to 20° to reduce the photoelectron signal depth.36 The measurement more pronouncedly confirms increased Br/(Pb+Cs) and Pb/Cs ratio (Figure S9). The spectral shift and quantitative elemental analysis (increase of anion/cation and Pb/Cs ratio) in XPS strongly suggest that surface of CsPbBr3 NCs became lead bromide-rich upon the addition of ZnBr2 in the synthesis; therefore, the stability of ZnBr2-CsPbBr3 NCs is markedly enhanced. Taking into account [110] directional NC fusion of pristine-CsPbBr3 NCs (Figure S6), we can specify the lead bromide enriched surface of ZnBr2-CsPbBr3 NCs to (110) surfaces. We note that both pristine- and ZnBr2-CsPbBr3 NCs have comparable surface bound ligand density as revealed in nuclear magnetic resonance (NMR) spectroscopy (Figure S10). For the formation of lead bromide enriched surface of ZnBr2CsPbBr3 NCs, we propose two possibilities: (i) lead bromide adlayer formation25 and (ii) native lead bromide surface termination37 on CsPbBr3 perovskite (110) surfaces. Previously, in lead chalcogenide NCs, (i) lead halide adlayer formation was proposed when impurity halides are added to the reaction solutions.25 Favorable formation energetics were proven by density functional theory calculations on the specific facets of ionic lead chalcogenide NCs. Similarly, lead bromide adlayer can be formed on the surface of CsPbBr3 NCs due to the favorable formation energetics when ZnBr2 is introduced to the reaction solutions. For (ii) native lead bromide surface termination, it has been demonstrated that crystallographic surface termination of perovskite strongly depends on growth conditions (e.g., precursor ratio).37 Presence of ZnBr2 during the synthesis possibly leads to the lead bromide-rich surface termination by significantly altering growth conditions, thus changing thermodynamic surface diagram of CsPbBr3 perovskites. After all, both proposed surface structures indicate inorganically passivated and highly stable surfaces of ZnBr2CsPbBr3 NCs. In an attempt to extend the list of metal bromide, we used several other metal bromides, such as InBr3 (InBr3-CsPbBr3 NCs) and CuBr2 (CuBr2-CsPbBr3 NCs), all of which resulted in similarly enhanced stability (Figure S11). Higher BE shift in Pb 4f and Br 3d XPS spectra, anion-rich composition, and increased Pb/Cs ratio were also confirmed in InBr3- and CuBr2CsPbBr3 NCs, strongly supporting the fact that inorganic PbBrx passivating layer is formed (Figure S12). Very interestingly, PbBr2 also results in enhanced structural stability (PbBr2CsPbBr3 NCs), underlining the significance of precursor stoichiometry on the stability of CsPbBr3 NCs (Figure S13). We also examined the effect of metal halide in the case of CsPbI3 NCs, which are known to be the most sensitive CsPbX3 perovskite against ambient atmosphere.12 We prepared CsPbI3 NCs by adding ZnI2 (ZnI2-CsPbI3 NCs, see SI). After synthesis, CsPbI3 NCs were collected by high-speed multiple centrifugations without adding any external agent (see SI). As shown in Figure 4A, as prepared prisitine-CsPbI3 NCs exhibited the mixture of predominant cubic phase and minor portion of orthorhombic phase. After being stored under ambient conditions for 3 days, the crystal structure of pristine-CsPbI3 NCs fully transformed into orthorhombic phase and their average size increased as evidenced by significant diffraction peak narrowing (Figure 4B). On the other hand, XRD patterns of ZnI2-CsPbI3 NCs show that the cubic phase of fresh sample

for stable operation of optoelectronic devices working at elevated temperature (Figure 2F). Given that pristine- and ZnBr2-CsPbBr3 NCs show little to no recognizable difference in crystallographic structure and morphology when they are fresh (Figure 2 and S2), the contrast in stability is overly noticeable. One obvious possibility is a difference in surface passivation25,29,30 when ZnBr2 is introduced in the synthesis. Hence, having the hypothesis that drastic contrast in stability would result from the surface passivation, we performed XPS analysis to unveil the origin of stability. After measurement, all XPS core level spectra were calibrated using C 1s peak at 285.0 eV (Figure S7).31 In Pb 4f XPS core level spectra, higher binding energy (BE) shift of 0.2 eV was observed for ZnBr2-CsPbBr3 NCs in reference to pristine-CsPbBr3 NCs (Figure S8). On the basis of the NC model proposed by De Roo et al.,14 we assume that Pb atoms in CsPbBr3 NCs are in two chemical environments, and we assigned higher BE and lower BE regions to Pb-Br and Pboleate, respectively.32,33 Therefore, the higher BE shift of Pb 4f spectrum in ZnBr2-CsPbBr3 NCs manifests the increase of PbBr species compared to pristine-CsPbBr3 NCs as evidently visualized by peak fitting (Figure 3A,B).34 Increase of Pb-Br

Figure 3. Pb 4f core level XPS spectra of (A) pristine- and (B) ZnBr2CsPbBr3 NCs. The spectra were calibrated using C 1s peak. Statistical (C) Br/(Cs+Pb) and (D) Pb/Cs atomic ratios of CsPbBr3 NCs calculated from XPS data.

species in ZnBr2-CsPbBr3 NCs was also evidenced in Br 3d spectrum. We assigned the higher BE and lower BE regions of Br 3d spectra to Pb-Br and Cs-Br, respectively, assuming that Br is composed of two chemical species. Because Pb-Br has higher BE than Cs-Br,32,35 the higher BE shift in Br 3d spectra of ZnBr2-CsPbBr3 NCs (Figure S8) also corroborates the fact that population of Pb-Br species increased in the case of ZnBr2CsPbBr3 NCs. Quantitative XPS analysis reveals that pristine-CsPbBr3 NCs show marginally cation-rich composition (Cs:Pb:Br = 1.0:1.0:2.8). However, for ZnBr2-CsPbBr3 NCs, the composition becomes slightly anion-rich (Cs:Pb:Br = 1.0:1.2:3.4), which was also repeatedly observed (Figure 3C). Furthermore, Pb/Cs ratio increased in ZnBr2-CsPbBr3 NCs (Figure 3D) 7090


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Figure 4. XRD patterns of (A and B) pristine- and (C and D) ZnI2-CsPbI3 NCs before and after aging under ambient atmosphere for 3 days. TEM images of pristine-CsPbI3 NCs (E) before and (F) after exposure to ambient condition. TEM images of ZnI2-CsPbI3 NCs (G) before and (H) after exposure to ambient condition. Scale bars are 50 nm.

remains nearly unchanged after 3 days of aging under ambient conditions with very high RH up to 60% (Figure 4C,D). The striking contrast in structural stability of the two CsPbI3 NC samples is more vivid in TEM images. In the case of pristineCsPbI3 NCs, large, irregular shaped aggregates form after aging under ambient condition (Figure 4E,F). On the contrary, size and morphology of ZnI2-CsPbI3 NCs stay the same after aging, as shown in Figure 4G,H. In addition, quantitative XPS analysis indicates inorganically passivated lead iodide enriched surface in ZnI2-CsPbI3 NCs (Figure S14). To the best of our knowledge, this is the first demonstration of universal in situ stabilization of CsPbX3 NCs, where X is Br or I, by inorganic passivation. In conclusion, we have developed a very simple and highly effective in situ protocol to make CsPbX3 NCs significantly more stable by inorganic passivation. Our in situ synthetic route employs metal halides with identical halide composition compared to PbX2 precursors; therefore, undesirable energy gap changes are completely avoided. Optical and structural characterizations (e.g., PL QY, XRD, and TEM) reveal that our metal halide-CsPbX3 NCs are highly stable without drastic drop of PL QY, changes in size and morphology, and transformation of crystalline structures under ambient condition with high RH up to 60%. The XPS and HAADF-STEM results confirm that (110) surface of metal halide-CsPbX3 NCs is enriched by lead halides and exceptionally stable, highlighting the importance of inorganic surface passivation of CsPbX3 NCs as shown in other types of semiconductors such as II-VI, III-V, and IV-VI NCs. Inorganically passivated, unprecedentedly stable CsPbX3 NCs shine the light on CsPbX3-based optoelectronic devices with prolonged lifetime.

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Detailed information on synthesis, characterizations (UV−vis, PL, XRD, XPS, TEM, and HAADF-STEM) and statistical analysis (PDF)

AUTHOR INFORMATION

Corresponding Authors

*S.J. E-mail: [email protected]. *D.C.L. E-mail: [email protected]. ORCID

Jeong Won Kim: 0000-0002-5881-9911 Doh C. Lee: 0000-0002-3489-6189 Sohee Jeong: 0000-0002-9863-1374 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Global R&D program (1415134409) funded by KIAT and the Global Frontier R&D program by the Center for Multiscale Energy Systems (2017M3A6A7051087). This work was also supported by the NRF grants funded by the Korean government (2015H1A2A1034211, 2016M3A7B4910617, 2014M3A7B6020163 and 2016R1A2B3014182).

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02669. 7091


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Highly Stable Cesium Lead Halide Perovskite Nanocrystals through in

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