A Mechanistic Study of Phase Transformation in Perovskite

Article pubs.acs.org/cm

Cite This: Chem. Mater. 2018, 30, 84−93

A Mechanistic Study of Phase Transformation in Perovskite Nanocrystals Driven by Ligand Passivation Thumu Udayabhaskararao,† Lothar Houben,‡ Hagai Cohen,‡ Matan Menahem,§ Iddo Pinkas,§ Liat Avram,‡ Tamar Wolf,§ Ayelet Teitelboim,† Michal Leskes,§ Omer Yaffe,§ Dan Oron,*,† and Miri Kazes*,† †

Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 76100, Israel Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel § Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel Chem. Mater. 2018.30:84-93. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/23/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: Active control over the shape, composition, and crystalline habit of nanocrystals has long been a goal. Various methods have been shown to enable postsynthesis modification of nanoparticles, including the use of the Kirkendall effect, galvanic replacement, and cation or anion exchange, all taking advantage of enhanced solid-state diffusion on the nanoscale. In all these processes, however, alteration of the nanoparticles requires introduction of new precursor materials. Here we show that for cesium lead halide perovskite nanoparticles, a reversible structural and compositional change can be induced at room temperature solely by modification of the ligand shell composition in solution. The reversible transformation of cubic CsPbX3 nanocrystals to rhombohedral Cs4PbX6 nanocrystals is achieved by controlling the ratio of oleylamine to oleic acid capping molecules. High-resolution transmission electron microscopy investigation of Cs4PbX6 reveals the growth habit of the rhombohedral crystal structure is composed of a zero-dimensional layered network of isolated PbX6 octahedra separated by Cs cation planes. The reversible transformation between the two phases involves an exfoliation and recrystalliztion process. This scheme enables fabrication of high-purity monodispersed Cs4PbX6 nanoparticles with controlled sizes. Also, depending on the final size of the Cs4PbX6 nanoparticles as tuned by the reaction time, the back reaction yields CsPbX3 nanoplatelets with a controlled thickness. In addition, detailed surface analysis provides insight into the impact of the ligand composition on surface stabilization that, consecutively, acts as the driving force in phase and shape transformations in cesium lead halide perovskites.

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INTRODUCTION Hybrid organic−inorganic lead halide perovskites and their inorganic counterparts have attracted a tremendous amount of attention in the past few years, predominantly because of their superb performance as solar cell materials,1−4 but also because of their potential applications in light-emitting diodes5−7 and optical gain media.8 While studies of this family of materials (particularly the inorganics), including elucidation of their phase diagrams, have been performed for nearly 50 years, work on colloidal nanocrystals (NCs) has only recently been initiated. Here we focus on the all-inorganic cesium lead halide NCs, but the tools and methods can also likely be applied to hybrid organic−inorganic materials. The phase diagram of cesium lead halides enables the stable growth of materials with several ratios of CsX and PbX2 (where X is a halide).9 A 1:1 ratio yields the most commonly studied form, CsPbX3, featuring a three-dimensional (3D) network of corner-shared lead halide octahedra. A 1:2 ratio yields CsPb2X5, a two-dimensional (2D) layered perovskite derivate obtained from the three-dimensional (3D) analogue by slicing along crystallographic planes and insertion of PbX2 planes. Finally, a 4:1 ratio yields Cs4PbX6, a quasi-zero-dimensional (0D) perovskite derivate with a recurring motif of isolated lead © 2017 American Chemical Society

halide octahedra, exhibiting highly localized optical excitations and a bandgap in the near-ultraviolet (near-UV) region.10 As the growth conditions required to achieve either of these phases do not significantly differ, and because thin film samples are often shown to exhibit domains with a different composition, it is tempting to consider the more controlled growth and transformation of colloidal cesium lead halide NCs. To date, most reports have focused on CsPbX3 NCs,11−16 and only very recently has the synthesis of Cs4PbX6 NCs been reported.17−20 For the more commonly studied CsPbX3 form, it has been shown that size and shape control can be exercised by the adequate choice of temperature and passivating ligands, usually a combination of a carboxylic acid and an amine. In particular, the length of the carbon chain of either has been shown to affect the morphology and the optical properties of the end product.15,21 Moreover, it has been established that both the cation-stabilizing ligand shell (carboxylates) and the lead halidestabilizing ligand shell (ammonium ions) play an important role in controlling NC formation and stability.19,22 However, the Received: June 12, 2017 Revised: December 5, 2017 Published: December 5, 2017 84

DOI: 10.1021/acs.chemmater.7b02425 Chem. Mater. 2018, 30, 84−93

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Chemistry of Materials

Figure 1. Ligand control of the dynamic reversibility between CsPbBr3 and Cs4PbBr6. (a) Transmission electron microscopy images of cubic CsPbBr3 to Cs4PbBr6 NCs (from left to right, respectively) by means of addition of an excess of oleic acid/oleylamine for the forward reaction and the addition of oleic acid for the backward reaction. (b) Changes in the absorption spectra during the progress of the forward reaction from CsPbBr3 to Cs4PbBr6 and conversion back to CsPbBr3. (c) X-ray diffraction (XRD) of the conversion of CsPbBr3 (red) to Cs4PbBr6 NCs (blue) and back along with the intermediates (green). Bulk XRD spectra for cubic CsPbBr3 and Cs4PbBr6 are given as a bar plot. (d) Two complete CsPbBr3− Cs4PbBr6 NC cycles as followed by ultraviolet−visible spectroscopy. Plotted is the wavelength of the first excitonic absorption peak.

Figure 2. Atomic-resolution images of (a) CsPbI3 and (b) Cs4PbI6. CsPbI3 crystallizes in a perovskite crystal structure with orthorhombic distortion in which PbX6 octahedra are corner sharing. The cubic crystals are bound by facets on (001) and (100) planes. The Cs4PbI6 structure is rhombohedral in space group R3̅c. The typical 2-fold symmetry of the high-resolution images of Cs4PbX6 is produced by the projection of chains of PbX6 octahedra in the [122] viewing direction. The habit is such that the rhombohedral crystals are formed by a layering of densely packed PbX6 with interlayers of cations.

and stoichiometry of cesium lead halide NCs. We show that surface destabilization can rapidly occur and enable full conversion of NCs from CsPbX3 into Cs4PbX6 particles (and vice versa) with small changes to the nanoparticle chemical environment. Transmission electron microscopy (TEM) investigation of intermediate stages of the transformation

underlying control mechanism has not been fully isolated. We recently reported23 on shape and crystal phase transformation between cubic and orthorhombic CsPbX3 by controlled selfassembly of small CsPbX3 NCs via the control of the ligand shell environment through acid−base interactions. Following this, we set out here to expand this study to control the shape 85


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The conversion from CsPbBr3 to Cs4PbBr6 involves a dramatic change in crystal structure and must be accompanied by a vast change in atomic composition. Li et al. have demonstrated growth of CsPb2Br5 sheets from orthorhombic CsPbBr3 intermediates in the presence of excess PbBr2 during oriented attachment.24 This mechanism, however, cannot explain the CsPbBr3 to Cs4PbBr6 transformation because there is no excess CsBr in the system. However, this transformation can be viewed also as a depleted Pb perovskite from which PbX2 is being extracted to the solution either through a change in polarity or through favored complexation with an added molecule. This was later demonstrated by Liu and co-workers, with the addition of thiols serving as a strong complexing agent for the Pb.19 Notably, however, in that work, too, the addition of OLAm was necessary to obtain a stable product. Here, the Cs:Pb atom ratio calculated from inductively coupled plasma mass spectrometry (ICP-MS) measurements confirms the theoretical stoichiometry of the purified products of CsPbBr3 and Cs4PbBr6. However, the supernatant left after precipitation of Cs4PbBr6 shows both Cs and Pb atoms to be present with an excess of Pb over Cs of ∼4:1. This indicates that Cs4PbBr6 formation is at least not solely driven by favorable ligand complexation with Pb as there still is a significant amount of free Cs in the solution. Interestingly, Wu et al.25 reported on the transformation from Cs4PbBr6 to CsPbBr3 via a two-phase polar/nonpolar reaction with water, explaining it by extraction of CsBr into the water phase. Palazon et al.26 showed such a conversion by thermal annealing and by Cs extraction with a chelating agent. Our notion is that subtle changes in the acid−base equilibrium change the ratio of Cs to lead halide octahedron-stabilizing ligands. This leads to a change in the thermodynamically favored phase of the cesium lead halide. A similar ligand-mediated stabilization of structural polymorphs has recently been demonstrated for a perovskite system of FAPbI3 NPLs and hexagonal gold sheets.27,28 Surprisingly, in contrast with most colloidal systems, the kinetics of the conversion process is fast, on the order of seconds to minutes or hours even at room temperature. Transformation Mechanism. HRTEM data presented in Figure 2 reveal details about the crystal structure, orientation, and facets of the cubic CsPbX3 and the rhombohedral Cs4PbX6 NCs, shown here for the case of the iodine compound. The crystal structure of the cubic particles (Figure 2a) exhibits a characteristic tilt of the lead halide octahedra and is in agreement with orthorhombic space group Pnma. The cube facets of the CsPbI3 NCs are the trivial principal lattice planes, i.e., (100) and (001). The rhombohedral Cs4PbX6 phase (Figure 2b) corresponds to the R3̅c crystal structure.29 All rhombohedra share a common [122] viewing direction with 2fold symmetry. The rhomboidal cross section of the Cs4PbX6 crystals is determined by their (2−32) and (−2−12) side facets, while (012) planes represent the top and bottom facets. Electron nanodiffraction data further supporting the crystal structure and facet determination for the rhombohedral Cs4PbX6 phase are presented in Figures S3 and S4. In this crystal habit, the rhombohedral particles are characterized by a stack of layers of densely packed PbX6 octahedra, alternating with an interlayer of Cs cations. Truncation of rhombohedra corners is found frequently and occurs on densely packed PbX6 octahedra, as well. Therefore, the reversible transformation between the two phases involves a transition between a three-dimensional network of corner-shared PbX6 octahedra in the cubic phase

process allows us to elucidate the mechanism involved in this transformation. In particular, we show that the transformation from CsPbX3 to Cs4PbX6 involves complete dissolution of the particles into an amorphous phase, whereas back-conversion is likely associated with exfoliation of 2D layers. Finally, we show how this channel can be used for size and shape control of the end product and provide insight into the transformation mechanism via a detailed study of ligand surface passivation in the two stoichiometries.

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RESULTS AND DISCUSSION Reversible Structural Transformation between CsPbX3 and Cs4PbX6. The route for the structural transformation between CsPbX3 and Cs4PbX6 is illustrated in Figure 1a for the case of the bromide compound. CsPbBr3 NCs were synthesized using standard protocols11 and purified via centrifugation following the addition of methyl acetate as an antisolvent. After the resolubilization of these NCs in a hexane solution containing both oleic acid (OA) and oleylamine (OLAm), a small modification in the OA:OLAm ratio leads to the spontaneous transformation from cubic CsPbBr3 NCs (NCs) to Cs4PbBr6 NCs and vice versa, even at room temperature. Evidence of the conversion is the change in color from green for CsPbBr3 and red for CsPbI3 luminescent solutions to weakly luminescent yellow-white solutions (Figures S1 and S2). The conversion time is reduced in the case of CsPbI3 relative to that of CsPbBr3. The gradual reduction in the absorption and fluorescence intensity of CsPbBr3 NCs is accompanied by the emergence of new strong absorption features in the near-UV region (Figure 1b), which have recently been shown to be associated with the formation of Cs4PbBr6 NCs.18 We also followed this process by timedependent X-ray diffraction (XRD) spectra (Figure 1c), which clearly show that the disappearance of cubic phase CsPbBr3 is followed by the formation of Cs4PbBr6 NCs (Figure 2). TEM images show that during the conversion the cubic morphology of CsPbBr3 NCs changes to a rhombohedral one (Figure 1a). Intriguingly, this conversion to Cs4PbX6 NCs can be reversed by various methods such as the addition of oleic acid, subjection to heat (90−180 °C), or removal of passivating ligands. As shown in Figure 1d, following the addition of an excess of oleic acid, we observed the conversion back to CsPbX3 and were able to repeat this cycle simply by repeated additions of OLAm and OA up to two conversion cycles (Figure 1d and Movie 1). Below we provide strong evidence that this transformation is controlled by the OLAm to OA Brønsted acid−base type equilibrium15,22 shifting the carboxylate to ammonium concentrations. Evidently, small changes in the acid:base ratio lead to destabilization of the ligand shell, resulting in the conversion to particles of a different stoichiometry. Since multiple additions involve a decrease in the ion concentrations in solution over time, the number of times this cycle could be continued is finite. Notably, however, washing and redissolution in a hexane/oleic acid mixture could again produce the CsPbBr3 NCs along with remnant Cs4PbBr6 NCs. Upon addition of lost lead salts, a pure CsPbBr3 phase could be reproduced. While conversion from Cs4PbBr6 to CsPbBr3 via the addition of lead salts has recently been reported by Manna and co-workers,18 we show here that the addition of lead precursors is absolutely not essential for such a conversion process to proceed, and that the thermodynamics of the conversion process can be controlled via manipulation of the surface species alone. 86


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Figure 3. Transient phases in the conversion of CsPbX3 to Cs4PbX6: (top row) TEM images of the initial Br perovskite NCs and of intermediates upon conversion to the final Cs4PbBr6 product and (bottom row) same as the top row for the I perovskite.

Figure 4. Selective transformation of Cs4PbBr6 NCs to CsPbBr3 NCs of different thicknesses. (a−c) TEM images of the CsPbBr3 samples emitting at 410, 432, and 490 nm, respectively. (d) Absorption and emission spectra of CsPbBr3 NCs. Five different absorption and emission peaks correspond to five different thicknesses (1−10 unit cells). Emission peaks from left to right: 410 nm (1 ML), 432 nm (2 MLs), 460 nm (5 MLs), 479 nm (8 MLs), and 488 nm (10 MLs). Peak assignments were made by fitting to Akkerman’s data8 and taking 0.6 nm as the lattice parameter.

structures, thin sheets, and platelets and also amorphous material coexisting in solution, converted to homogeneous sized Cs4PbX6 NCs as can be seen in Figure 3 and also supported by the absorption spectra shown in Figure 1b. For the backward transformation reaction from Cs4PbBr6 to CsPbBr3, by monitoring the evolution of the emission spectrum over the course of the reaction, we found different peaks emerged and disappeared at discrete wavelengths corresponding to discrete thicknesses of several perovskite MLs (Movie 2). This suggests an exfoliation33−36 process aided by the excess of oleic acid followed by the ionic sphere rearrangement and recrystallization, as we further outline below.37 It is noteworthy that the PL evolution of the forward CsPbX3 to Cs4PbX6 reaction shows only a gradual decrease in the PL of the initial

to a 0D layered network of isolated PbX6 octahedra in the rhombohedral phase that are separated by cation planes. In relation to the NC surface, the cubic CsPbX3 phase as seen in Figure 2a could have either a highly negatively or a highly positively charged surface (depending on the termination), while the facets of the rhombohedral Cs4PbX6 phase exhibit both PbX6 octahedra and Cs atoms and are thus expected to be closer to neutral. The transformation mechanism we propose involves a change in the surface ligand environment,22,27 followed by recrystallization induced by micelle formation30 or soft ligand templating.13,31,32 This mechanism is supported by the observation of intermediate stages in TEM images showing the gradual decomposition of cubic CsPbX3 NCs into lamellar 87


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formation of disordered clusters of particularly favorable surface tension and/or surface energy.37 Upon dilution with either hexane or large additions of OLAm/OA, diamond-shaped Cs4PbBr6 NCs are decomposed, giving a clear solution that precipitates into an amorphous phase on the TEM grid as shown in Figure 6. Reconcentration by evaporation results in transformation into quasi-spherically shaped Cs4PbBr6 or, more accurately, truncated rhombohedra (Figure 6c). It is also worth mentioning that a change from a diamond shape to a quasispherical shape of Cs4PbBr6 while maintaining the rhombohedral phase is observed when the particles are washed (Figure S5). Surface-bound ammonium ions are more labile and will detach more easily during the washing process.15 This means that a cooperative effect of the OA/OLAm ligand mixture plays a crucial role in determining the NC shape following the conversion through acid−base protonation.38 Recently, de Weerd et al.39 showed that spherical NCs, appearing in the coformation of CsPbBr3 and Cs4PbBr6, show a component of optical bandgap absorption similar to that of the cubic CsPbBr3 phase and conclude on a hybrid structure of both phases. To this end, these structures are possibly nanocomposites of phases growing in competition rather than transients of a solid phase transformation. Cs4PbBr6 Size Control. Improved size control can be achieved by a direct synthesis method in which the final particle size is determined by the amount of OA/OALm used and by temperature. Following CsOA injection at 150 °C, an inhomogeneous mixture of Cs4PbBr6 NCs is obtained. A sequential aging step of 5 h will result in a homogeneous sample of 13 nm rhombohedral Cs4PbBr6 NCs. Aging at increased temperature in the range of 50−100 °C will result in NCs with increased sizes of 23, 40, and 70 nm as shown in Figure 7a−d. Similar, well-defined sizes are also observed for Cs4PbI6 NCs (Figure 7e). The composition of Cs4PbI6 is confirmed by XRD as presented in Figure S6. The Cs4PbX6 NCs exhibit a strong absorption peak at 310 and 370 nm for the bromide and iodide, respectively (Figure 7g, black and red lines, respectively). Mixed halide CsPbBr1.5I1.5 NCs (Figure 7f) were also synthesized by using a 50:50 molar ratio of the corresponding lead salt in the reaction mixture, exhibiting an excitonic absorption at 340 nm (Figure 7g, blue line). These absorption spectra are indeed in agreement with the recently reported work of Akkerman et al.16 Surface Analysis. Gaining detailed information about the ligand shell composition and conformation is crucial for understanding the driving force in this phase transition. To this end, we performed Raman scattering, X-ray photoelectron spectroscopy ( XPS), and nuclear magnetic resonance (NMR) measurements on highly purified CsPbBr3 and Cs4PbBr6 samples (powders), taking care to remove excess ligands. Raman scattering spectra of CsPbBr3 and Cs4PbBr6 (solid samples) were compared with those of pure OA and OLAm (liquids) to elucidate the role and configuration of the ligands via the comparison between CsPbBr3 and Cs4PbBr6 (Figure 8). The only clear differences between the spectra of the two pure ligands are the amine peaks at 3325 and 3387 cm−1 that are present in OLAm and absent in OA, attributed to the NH2 symmetric and antisymmetric stretch modes, respectively. Raman scattering spectra for solutions of OLAm with an increasing volume fraction of OA presented in Figure S7 evidently show that the amine peaks at 3387 and 3325 cm−1 vanish at 33 and 50% OA, respectively. The Raman scattering modes measured on CsPbBr3 and Cs4PbBr6 are attributed to

CsPbX3 sample, as anticipated for decomposition into amorphous material. The observed reversible, room-temperature, phase transformation from Cs4PbX6 to CsPbX3 enables a new degree of control over the synthesis of CsPbX3. Highly quantized CsPbBr3 NCs with thicknesses from 1 to 10 monolayers (MLs) can be produced upon addition of OA to the Cs4PbBr6 NCs, which is evident in the absorption and photoluminescence (PL) spectra in Figure 4d, in agreement with previously reported emission peaks.16 In addition, the size distribution of both Cs4PbBr6 NCs and that of the CsPbBr3 obtained from the back transformation is far narrower than that of the initial CsPbBr3 NCs. This is also evident from the disappearance of multiple excitonic features seen in the absorption spectrum of the initial CsPbBr3 NCs shown in Figure 1b. Note that the emission peaks seen in Movie 2 do not completely follow the full distinct evolution spectra shown in Figure 4 that result from inhomogeneous thickness distribution in the course of the exfoliation process and recrystallization. The volume and concentration of the ligands added change the kinetics of the transformation, and thus, the final CsPbBr3 thickness depends on the particular reaction conditions (see experimental details in the Supporting Information). We note, however, that the presented results do not entirely rule out other alternatives for the back-conversion process, including that of nucleation of the CsPbX3 from ions lost to the solution. To further support the decomposition and recrystallization mechanism as shown above for the CsPbX3 to Cs4PbX6 transformation, we utilize the ability to perform the back-andforth transformation without the addition of extra precursors to study the effect of the transformation on the composition of mixed halide NCs. We start from a mixed halide CsPbBr1.5I1.5, convert it to the corresponding Cs4PbX6 species, and then back-convert it to CsPbX3 while monitoring the optical properties. As shown in Figure 5, the back-converted particles have segregated into Br-rich and I-rich particles, clearly manifested by the emergence of two separated PL peaks. Effect of Surface Stabilization on the Phase and Morphology of Cs4PbBr6 NCs. In some sense, the perovskite NCs discussed above can be viewed in the more general context of phase change materials36 where both crystalline and amorphous phases exist and recrystallization is affected by the

Figure 5. Photoluminescence spectroscopy upon back-and-forth transformation of a mixed halide perovskite through the Cs4PbX6 phase. The PL spectrum of initial CsPbBr1.5I1.5 NCs in black and that of segregated Br-rich and I-rich CsPbBr3−xIx NCs in red after one transformation cycle. 88


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Figure 6. Effect of surface stabilization on the phase and morphology of Cs4PbBr6 NCs. Upon dilution with hexane, diamond-shaped Cs4PbBr6 NCs (a) are decomposed into an amorphous phase (b). Reconcentration by evaporation results in transformation into spherically shaped Cs4PbBr6 (c).

Figure 7. (a−d) TEM images of 14 ± 0.7, 23 ± 0.8, 40 ± 1, and 70 ± 3 nm Cs4PbBr6 NCs. (e) TEM image of 50 nm Cs4PbI6 NCs. (f) TEM image of mixed halide 50 nm Cs4PbBr1.5I1.5 NCs. (g) Absorption spectra of Cs4PbBr6, Cs4PbBr3I3, and Cs4PbI6 NCs showing a clear shift in the excitonic peak with composition.

ligand shell of CsPbBr3. Altogether, the Raman measurements indicate a more ordered packing of ligands around the Cs4PbBr6 and a change in the nature (or amount) of surfacebound amines. To look deeper into the nature of surface-bound species, we present solution 1H NMR measurements of similar particles in Figure 9. The peak assignments were made according to the work of De Roo et al.22 For CsPbBr3, the presence of bonded protonated OLAm is confirmed by broad resonances (α) and (β), ascribed to the NH3+ and the α-CH2 of the protonated OLAm, respectively. However, the other resonances are almost identical to those of the pure OA solution, implying the presence of free OA in the ligand shell of CsPbBr3. Nuclear Overhauser effect spectroscopy (NOESY) giving spatial correlations (Figure S9) confirms that OA is not bonded to the CsPbBr3 surface as it features positive (Figure S8, blue) cross peaks, while protonated OLAm features only negative (black) cross peaks, which is a clear confirmation that it is all in a bonded state. Overall, the NMR results are in agreement with the results of previous studies of CsPbBr3.22,40 In contrast, for Cs4PbBr6, the presence of unprotonated OLAm is evident by the high-field chemical shift of the (α) and (β) resonances. The

the intramolecular vibrations in the OA and OLAm layers. Broadening and shifting of modes in the spectra are expected in the NCs because of the rearrangement and packing of the organic molecules as well as the interactions with the NC surface. The spectrum of the surface ligands of CsPbBr3 resembles the spectrum of OA and OLAm and is slightly broadened, clearly lacking the characteristic amine peaks. Cs4PbBr6 exhibits a peak at 3252 cm−1 that is likely associated with a red-shifted amine peak. The red shift may indicate interaction of the nitrogen either with the surface or with OA. Moreover, the spectrum of Cs4PbBr6 exhibits unique peaks, which are not observed in the pure ligands, at 1368 and 1399 cm−1. The distinctive Cs4PbBr6 modes at 1368 and 1399 cm−1, which are very weak or absent in the other materials investigated, may result from packing and reorganization of the ligand layer around the NCs resulting in new Raman active modes. The peak at 1063 cm−1 that is more pronounced for Cs4PbBr6 and to some extent also for CsPbBr3 as compared to those of pure OA and OLAm is also pronounced for CsOA and Pb(OA)2 (see Figure S8). This suggests dense packing of the ligands on the surface of Cs4PbBr6 while it is present in its free form in the 89


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terminated capping molecules out of the total capping. For Cs4PbBr6, we obtain a value close to 50% occupation by the Nterminated molecules, whereas for CsPbBr3, it points to close to 100% N-termination, both in agreement with the NMR results. In addition, XPS analysis reveals a carbon per NC ratio in CsPbBr3 that is higher than expected theoretically. This result reflects the presence of extra carbon-based molecules, situated on top of the capping layer, again, in accordance with the free OA revealed by NMR. The much lower carbon per NC ratio in Cs4PbBr6 rules out the notion of a densely packed surface while better fitting the option of an ammonium−olate ion pair capping. This is supported by the XPS-derived binding energy of the carboxylic carbon in Cs4PbBr6, which tends to be ∼0.5 eV lower than the corresponding one in CsPbBr3 and is likely an indication of the bounded nature of the carboxylate in Cs4PbBr6 (for details, see the Supporting Information). Finally, the Cs:Br atomic ratio was found to be 1:3 for CsPbBr3 and 2:3 for Cs4PbBr6, with a relative error of

A Mechanistic Study of Phase Transformation in Perovskite

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