A Mechanistic Study of Phase Transformation in Perovskite

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A mechanistic study of phase transformation in perovskites NCs driven by ligand passivation Udayabhaskararao Thumu, Lothar Houben, Hagai Cohen, Matan Menahem, Iddo Pinkas, Liat Avram, Tamar Wolf, Ayelet Teitelboim, Michal Leskes, Omer Yaffe, Dan Oron, and Miri Kazes Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b02425 • Publication Date (Web): 05 Dec 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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Chemistry of Materials

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A mechanistic study of phase transformation in perovskites NCs driven by ligand passivation Udayabhaskararao Thumu1, Lothar Houben,2 Hagai Cohen,2 Matan Menahem,3 Iddo Pinkas,3 Liat Avram,2 Tamar Wolf,3 Ayelet Teitelboim,1 Michal Leskes,3 Omer Yaffe,3 Dan Oron1* and Miri Kazes,1* 1

Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot 76100,

Israel 2

Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76100,

Israel 3

Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel

*e-mail: [email protected] ; [email protected] ABSTRACT: Active control over the shape, composition and crystalline habit of nanocrystals is a long sought-after goal. Various methods have been shown to enable post-synthesis 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. Reversible transformation of cubic CsPbX3 nanocrystals to rhombohedral Cs4PbX6 nanocrystals is achieved by controlling the ratio of oleylamine to oleic acid capping molecules. HRTEM 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 recrystaliztion process. This scheme does not only enable fabrication of high purity monodispersed Cs4PbX6 nanoparticles with controlled sizes. Rather, depending on the Cs4PbX6 nanoparticles final size as tuned by the reaction time, the back reaction yields CsPbX3 nanoplatelets with controlled thickness. In addition, detailed surface analysis provides insight on

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the impact of the ligand composition on surface stabilization which, 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 tremendous attention in the last few years, predominantly due to their superb performance as solar cell materials1,2,3,4, but also due to potential applications in light emitting diodes5,6,7 and optical gain media8. While studies on this family of materials (particularly the inorganics), including elucidation of their phase diagrams, have been carried on for nearly fifty years, work on colloidal nanocrystals has only recently been initiated. Here we focus on the all-inorganic cesium lead halide nanocrystals, but the tools and methods are likely applicable also to hybrid organic-inorganic materials. The phase diagram of cesium lead halides enables to stably grow 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 3D analog 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 halide octahedra, exhibiting highly localized optical excitations and a bandgap in the near UV region.10 As the growth conditions required to achieve either of these phases do not significantly differ, and since thin film samples are often shown to exhibit domains with different composition, it is tempting to consider the more controlled growth and transformation of colloidal cesium lead halide nanoparticles. To date, most reports have focused on CsPbX3 nanoparticles.11,12,13,14,15,16 and only very recently has the synthesis of Cs4PbX6 been reported.17,18,19,20 For the more commonly studied CsPbX3 form, it has been shown that size and shape control can be exercised by 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 halide stabilizing ligand shell (ammonium ions) play an important role in controlling the nanoparticle formation and stability.19,22 Yet, the underlying control mechanism has not been fully isolated. We recently reported23 on shape and crystal phase transformation between cubic and orthorhombic CsPbX3 by controlled self-assembly of small CsPbX3 nanoparticles via the control of the ligand shell environment through acid-base interactions. Following this, we set out here to expand this study to



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

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 nanoparticles were synthesized using standard protocols11 and purified via centrifugation following the addition of methyl acetate as an anti-solvent. After resolubilizing these nanoparticles in a hexane solution containing both oleic acid (OA) and oleylamine (OLAm), a small modification in the OA to OLAm ratio leads to the spontaneous transformation from cubic CsPbBr3 nanocrystals (NCs) to Cs4PbBr6 NCs and vice versa, even at room temperature. Evidence for the conversion is the color change from green for CsPbBr3 and red for CsPbI3 luminescent solutions, respectively, to weakly luminescent yellow-white solutions (Figures S1-S2). The conversion time is reduced in the case of CsPbI3 relative to 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 (Figure 1b), which have recently been shown to be associated with the formation of Cs4PbBr6 nanoparticles.18 We also followed this process by time dependent X-ray diffraction (XRD) spectra (Figure 1c), which clearly shows that the disappearance of cubic phase CsPbBr3 is followed by the formation of Cs4PbBr6 NCs (Figure 2). Transmission electron microscopy (TEM) images show that during the conversion the cubic morphology of CsPbBr3 NCs changes to a rhombohedral one (Figure 1a).


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Figure 1: Ligand control of dynamic reversibility between CsPbBr3 and Cs4PbBr6. a) Transmission electron microscopy (TEM) images of cubic CsPbBr3 to Cs4PbBr6 NCs (left to right) by means of addition of oleic acid/oleylamine excess 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 traces) to Cs4PbBr6 NCs (blue traces) and back along with the intermediates (green traces). Bulk XRD spectra for cubic CsPbBr3 and Cs4PbBr6 are given as a bar plot. d) Two complete CsPbBr3–Cs4PbBr6 NCs cycles as followed by UV-vis spectroscopy. Plotted is the wavelength of the first excitonic absorption peak.

Intriguingly, this conversion process to Cs4PbX6 NCs can be reversed by various methods such as the addition of oleic acid, subjection to heat (90 to 180 oC) 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, SI 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



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concentrations. Evidently, small changes in the acid to 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 of the ion concentrations in solution over time, the number of times this cycle could be continued is finite. Notably, however, washing and re-dissolution in a hexane/oleic acid mixture could produce back 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 coworkers,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. The conversion from CsPbBr3 to Cs4PbBr6 involves a dramatic change not only in crystal structure but also 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 where PbX2 is being extracted out to the solution either through change in polarity or through favored complexation with an added molecule. This was later demonstrated by Liu and coworkers 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 in order to get a stable product. Here, the Cs to 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 to 1. This indicates that the 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/non polar 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 acid-base equilibrium changes the ratio of Cs to lead halide octahedra 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


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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 reveals details about the

crystal structure, orientation and facets of the cubic CsPbX3 and the rhombohedral Cs4PbX6 nanoparticles, shown here for the case of the iodine compound. The crystal structure of the cubic particles (Fig. 2a) exhibits a characteristic tilt of the lead halide octahedra and is in agreement with the orthorhombic space group Pnma. The cube facets of the CsPbI3 NP are the trivial principal lattice planes, i.e. (100) and (001). The rhombohedral Cs4PbX6 phase (Fig. 2b) corresponds to the

Figure 2: Atomic resolution images of (a) CsPbI3 and (b) Cs4PbI6. CsPbI3 crystallises in a perovskite crystal structure with orthorhombic distortion in which PbX6 octahedra are corner sharing. The cubic crystal are bound by facets on (001) and (100) planes. The Cs4PbI6 structure is rhombohedral with space group R-3c. The typical two-fold symmetry of the highresolution 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.



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R-3c crystal structure.29 All rhombohedra share a common [122] viewing direction with two-fold 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 is presented in the Supporting Information (Figures SI S3-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 to a zero dimensional layered network of isolated PbX6 octahedra in the rhombohedral phase that are separated by cation planes. In relation to the NPs surface, the cubic CsPbX3 phase as seen in Fig. 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 the PbX6 octahedra and Cs atoms, and are thus expected to be closer to neutral.

Figure 3: Transient phases in the conversion from CsPbX3 to Cs4PbX6. Top row: TEM images of the initial Br perovskite nanoparticles and of intermediates upon conversion to the final Cs4PbBr6 product. Bottom row: Same as above for the I perovskite. ACS Paragon Plus Environment

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The transformation mechanism we propose involves environment,22,27

change in the surface ligand

followed by recrystallization induced by micelle formation30 or soft ligand

templating13,31,32. This mechanism is supported by the observation of intermediate stages in TEM images showing the gradual decomposition of cubic CsPbX3 NCs into lamellar structures, thin sheets and platelets and also amorphous material coexisting in solution, converted to homogenous 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, monitoring the evolution of the emission spectrum over the course of the reaction, different peaks emerge and disappear at discrete wavelengths corresponding to discrete thicknesses of several perovskite MLs (SI movie(2)). This suggests an exfoliation33,34,35,36 process aided by the excess of oleic acid followed by the ionic sphere rearrangement and recrystallization, as we further outline below.37 Noteworthy, the PL evolution of the forward CsPbX3 to Cs4PbX6 reaction show only a gradual decrease in the PL of the initial CsPbX3 sample, as anticipated for decomposition into amorphous material.



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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 NPLs with thickness from 1 to 10 monolayers can be produced upon the addition of OA to the Cs4PbBr6 nanocrystals, as evident by the absorption and PL spectra in Figure 4d, in agreement with previously reported emission peaks.16 In addition, the size distribution of both Cs4PbBr6 NPs and that of the CsPbBr3 obtained from the back transformation by far narrower than that of the initial CsPbBr3 NPs. This is also evident from the disappearance of multiple excitonic features seen in the absorption spectrum of the initial CsPbBr3 NPs shown in Figure 1b. Note that the emission peaks seen in SI movie(2) do not completely follow the full distinct evolution spectra shown in Figure 4 which 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

Figure 4: Selective transformation of Cs4PbBr6 NCs to CsPbBr3 NPLs. a-c) TEM images of the CsPbBr3 samples emitting at 410 nm, 432 nm and 490 nm respectively. d) Absorption and emission spectra of CsPbBr3 NPLs. Five different absorption and emission peaks correspond to five different thicknesses of the plates (1−10 unit cells). Emission peaks from left to right: 410nm (1 ML), 432 nm (2 MLs), 460 nm (5 MLs), 479 nm (8 MLs), 488 nm (10 MLs). Peak

assignments were done by fitting to Akkerman’s data, ref. 8 and taking 0.6 nm as the lattice parameter.


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(see experimental details in the SI). 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 nanoplatelets 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-and-forth transformation without the addition of extra precursors to study the effect of the transformation on the composition of mixed halide nanoparticles. 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 can be seen in Figure 5, the back-converted particles have segregated into Br-rich and I-rich particles, clearly manifested by the emergence of two separated

Figure 5: Photoluminescence spectroscopy upon back-and-forth transformation of a mixed halide perovskite through the Cs4PbX6 phase. PL spectrum of initial CsPbBr1.5I1.5 NCs in black and of segregated Br-rich and I-rich CsPbBr(3-x)Ix NCs in red after one transformation cycle. photoluminescence peaks.

Effect of surface stabilization on Cs4PbBr6 NCs phase and morphology. In some sense the perovskite nanoparticles 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 formation of disordered clusters of particularly favorable surface tension and / or surface energy.37 Upon dilution either with hexane or large additions of OLAm/OA, diamond shaped Cs4PbBr6 NCs are decomposed giving a clear solution that precipitates into an amorphous phase



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on the TEM grid as shown in Figure 6. Re-concentration by evaporation results in transformation into quasi-spherical shaped Cs4PbBr6, or more accurately, truncated rhombohedrons (Figure 6c). It is also worth to mention that a change from a diamond shape to quasi-spherical shape of Cs4PbBr6 while maintaining the rhombohedral phase is observed when washing the particles (Figure S5). Surface-bound ammonium ions are more labile and will detach more easily through the washing process.15 This means that a cooperative effect of the OA/OLAm ligand mixture plays a crucial role in determining the nanoparticle shape following the conversion through acid-base protonation.38 Recently, de Weerd at al39 showed that spherical NCs, appearing in the conformation of CsPbBr3 and Cs4PbBr6, show a component of optical bandgap absorption similar to 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 where the final particle size is determined by the amount of OA/OALm used and by temperature. Following CsOA injection at 150 ºC, an inhomogeneous mixture of Cs4PbBr6 NCs is obtained. A sequential ageing step of 5 hours will result in a homogenous sample of 13 nm rhombohedral Cs4PbBr6 NCs. Ageing at increased temperature in the range of 50 – 100 ºC will result in NCs of increased sizes of 23, 40 and 70 nm size particles 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 the supplementary information Figure S6. The Cs4PbX6 NCs exhibit a strong absorption peak at 310 nm and 370 nm for the bromide and iodide, respectively (Figure 7g, black and red lines, respectively). Mixed halide CsPbBr1.5I1.5 nanoparticles (Figure 7f) were also synthesized by using 50:50 mole ratio of the corresponding lead salt in the reaction mixture,

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

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exhibiting an excitonic absorption at 340 nm (Figure 7g, green line). These absorption spectra are indeed in agreement with the recently reported Akkerman et al work.16

Surface analysis. Gaining detailed information on the ligand shell composition and conformation is crucial for understanding the driving force in this phase transition. To this end, we performed Raman scattering, XPS and NMR measurements on highly purified CsPbBr3 and Cs4PbBr6 samples (powders) taking care to remove excess ligands.

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



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a) Intensity (a.u.)

Cs4PbBr6 (S)

CsPbBr3 (S)

OA (L)

OLAm (L)

600

800

1000

1200

1400

1600

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-1

Raman shift (cm )

b)

Intensity (a.u.)

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Cs4PbBr6 (S)

CsPbBr3 (S)

OA (L)

OLAm (L)

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3400

-1

Raman shift (cm )

Figure 8: Raman scattering spectra of Cs4PbBr6, CsPbBr3, OA and OLAm in a) 6001800 cm-1 frequency range. b) 2700-3400 cm-1 frequency range. Cs4PbBr6 and CsPbBr3 were measured as solids (powders), OA and OLAm as liquids (melts). Raman scattering spectra of CsPbBr3 and Cs4PbBr6 (solid samples) were compared with those of pure OA and OLAm (liquids) in order 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 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% of OA respectively. The Raman scattering modes measured on CsPbBr3 and Cs4PbBr6 are attributed to the intramolecular vibrations in the OA and OLAm layers. Broadening


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and shifting of modes in the spectra is expected in the NCs due to rearrangement and packing of the organic molecules as well as due to interactions with the NC surface. The spectrum of the surface ligands of CsPbBr3 resembles the OA and OLAm spectrum and is slightly broadened, clearly lacking the characteristic amine peaks. Cs4PbBr6 exhibits a peak at 3252 cm-1 which is likely associated with a redshifted amine peak. The red shift may indicate either interaction of the nitrogen with the surface or with OA. Moreover, Cs4PbBr6 spectrum 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 which is more pronounced for Cs4PbBr6 and to some extent also for CsPbBr3 as compared to pure OA and OLAm, is also pronounced for CsOA and Pb(OA)2 (see Figure S8 in Supplementary Information). This suggests dense packing of the ligands on the surface of Cs4PbBr6 while present in free form in the ligand shell of CsPbBr3. All in all, the Raman measurements indicate a more ordered packing of ligands around the Cs4PbBr6 and a change in the nature (or amount) of surface bound amines. To look deeper into the nature of surface bound species, we present solution 1H-NMR measurements on similar particles in Figure 9. The peak assignments were carried according to the De Roo et al.22 For CsPbBr3, the presence of bonded protonated OLAm is confirmed by the 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 the pure OA solution implying the presence of free OA in the ligand shell of CsPbBr3. Nuclear Overhauser effect spectroscopy (NOESY) giving spatial correlations, (SI 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 with agreement with previous publications on 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 assignment of (β) and (γ) resonances is corroborated by correlation spectroscopy (COSY) (SI, Figure S9) where is (γ) is assigned to the β-CH2 of the OLAm by its quartet splitting. The (β) resonance is similar to the one in the pure OLAm, only broader, suggesting the bonded nature of OLAm. The (α) resonance shifts to a lower ppm value but not to the extent of the amine in pure



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OLAm (which we assume to overlap with the (3) resonance peak), indicating a lesser degree of protonation or the formation of an ammonium oleate ion pair. NOESY spectrum of Cs4PbBr6 (SI Figure S10) confirms the bonded nature of the total ligands present. This is supported by the broadening of the (5) resonance assigned to the double bond protons. Moreover, In the case of Cs4PbBr6, the formation of a NOESY correlation between the (5) and (3) resonances is more intense than in CsPbBr3, suggesting restricted mobility of the aliphatic chains and and a more densely packed ligand shell. Notably, no free OA is detected as the (1) and (2) resonances of the OA are absent from the spectrum (Figure 9) although it is still possible that it is present in a bonded state, being screened by the surface.


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Figure 9: Spectra of 1H-NMR measurements carried in CDCl3 for Cs4PbBr6, CsPbBr3, OA and OLAm, top to bottom, respectively. Inset: A schematic presentation of OA and OLAm with the different proton positions marked with numbers and Greek letters. The CDCl3 resonance is marked with an asterisk. X-ray photoelectron spectroscopy (XPS) data complement our understanding of the two structures discussed here. Analysis of signals originated from the capping molecules was carried out and several representative values are depicted in Table S1 along with a detailed analysis description. Mainly, from the calculated amount of nitrogen per NC we can extract a normalized portion of Nterminated capping molecules out of the total capping. For Cs4PbBr6 we get a value close to 50% occupation by the N-terminated molecules, whereas for CsPbBr3 it points to close to 100% Ntermination, both in agreement with the NMR results. In addition, the XPS analysis reveals a carbon per NC ratio in CsPbBr3, higher than expected theoretically. This result reflects the



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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 dense 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 SI). Finally, Cs:Br atomic ratio was found to be 1:3 for the CsPbBr3 and 2:3 for the Cs4PbBr6, with relative error below 3% each. On the other hand, the Cs:Pb ratios showed a deviation from the bulk values in the CsPbBr3 (Cs:Pb, 1.2±0.04) while matching in the case of Cs4PbBr6 (Cs:Pb, 4.0±0.3). This suggests surface termination by Cs rich planes (which are partially substituted by ammonium ligands), which is with agreement with the findings of Ravi et al.40 Combining data from the above surface sensitive modalities, although each separately reveals only a partial description of the system, gives a rather conclusive description of the major differences between CsPbBr3 and Cs4PbBr6. The ligand shell of CsPbBr3 is composed of bonded ammonium ligands substituting the Cs atoms vacancies on the partial Cs terminating surface and free OA whose role in stabilization of CsPbBr3 is not clearly understood yet. The ligand shell of Cs4PbBr6 is likely composed of both OLAm and OA in a bonded state. In addition, Cs4PbBr6 surface ligands seem to have a more restricted spatial configuration possibly due to the rigidity of an ammoniumolate ion pair capping. This picture is in accordance with the Cs4PbBr6 surface composed of negatively charged PbBr6 octahedra which are partially balanced by in-plane Cs ions, thus requiring passivation by both negatively and positively charged ligands. In contrast, for CsPbBr3, the highly ionic nature of the surface requires a strong ionic stabilization. CONCLUSIONS We show here the impact of delicate changes in ligand environment on the stoichiometry and crystal structure of cesium lead halide perovskite nanoparticles. This is in line with our previous work where we showed that small changes in the oleate to ammonium ratio generated by the addition of a Lewis base promotes, for example, the transition from cubic to orthorhombic CsPbI3.23 Here we show how a robust reversible transformation from CsPbX3 to the rhombohedral Cs4PbX6 can be obtained via control of the OA to OLAm Brønsted acid-base type equilibrium. Surface analysis reveals that the ligand shell of CsPbBr3 is composed of bonded ammonium ligands and free OA,


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while the ligand shell of Cs4PbBr6 is composed of both OLAm and OA both in a bonded state. In addition, Cs4PbBr6 surface ligands seem to have a more restricted spatial configuration possibly due to the rigidity of an ammonium oleate ion pair capping. We show evidence for a transition mechanism that involves an exfoliation and recrystallization processes. This mechanism is supported by crystallographic data showing a zero dimensional layered rhombohedral phase of a habit showing a layering of densely packed PbX6 octahedra planes with interlayers of cations planes. The formation of a layered Cs4PbBr6 habit indicates a direct path for transformation that is controlled by thermodynamic surface stabilization provided by the ligand shell. All in all, this work presents a rich pathway to control both the stoichiometry and structure of perovskite nanocrystals post synthesis.



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Supporting Information This material is available free of charge via the Internet at http://pubs. acs.org. Details of all of the experimental methods and synthesis; HRTEM analysis; Raman scattering; measurements; solution and solid state NMR; XPS analysis.

AUTHOR INFORMATION U.B. synthesized the nanocrystals and performed XRD and TEM measurements. L.H. performed HRTEM measurements and analysis. U.B., A.T. and M.K. performed the optical characterization. M.M., I.P. and O.Y. performed Raman scattering measurements and analysis. H.C. performed XPS measurements and analysis. L.A. performed solution NMR measurements. T.W. performed ssNMR measurements. M.L., T.W. and L.A. performed analysis of NMR data. M. K. and D.O. conceived and supervised the project. The manuscript was jointly written by all authors. Corresponding Author *Email: (D. O.) [email protected] (M.K.) [email protected] Funding This research was supported by a grant from Israel Science foundation (grant no. 2012\224330*) and by the Crown Center of Photonics and the ICORE: the Israeli Excellence Center “Circle of Light”. The support of the Gerhardt M.J. Schmidt Minerva Center and the Irving and Cerna Moskowitz Center for Nano and Bionano Imaging and at the Ernst-Ruska Centre Juelich, Germany, is also gratefully acknowledged. Notes The authors declare no competing financial interests.


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A Mechanistic Study of Phase Transformation in Perovskite

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