Nucleation, Growth, and Structural Transformations of Perovskite

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Nucleation, growth and structural transformations of perovskite nanocrystals Udayabhaskararao Thumu, Miri Kazes, Lothar Houben, Hong Lin, and Dan Oron Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04841 • Publication Date (Web): 20 Jan 2017 Downloaded from http://pubs.acs.org on January 21, 2017

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Chemistry of Materials

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Nucleation, growth and structural transformations of perovskite nanocrystals Thumu Udayabhaskararao,† Miri Kazes, *† Lothar Houben,‡ Hong Lin§ and Dan Oron† †Department

of Physics of Complex Systems, Weizmann Institute of Science,

Rehovot, Israel 7610001 ‡Chemical

Research Support, Weizmann Institute of Science, Rehovot, Israel 7610001

§State

Key Laboratory of New Ceramics & Fine Processing, Department of Material Science and Engineering, Tsinghua University, Beijing 100084, P.R. China Supporting Information

ABSTRACT: Despite the recent surge of interest in lead halide perovskite nanocrystals, there are still significant gaps in the understanding of nucleation and growth processes involved in their formation. Using CsPbX3 as a model system, we systematically study the formation mechanism of cubic CsPbX3 nanocrystals, their growth via oriented attachment into larger nanostructures and the associated phase transformations. We found evidence to support that the formation of CsPbX3 NCs occurs through the seed-mediated nucleation method, where Pbº NPs formed during the course of reaction act as seeds. Further growth occurs through self-assembly and oriented attachment. The polar environment is a major factor in determining the structure and shape of the resulting nanoparticles, as confirmed by experiments with aged seed reaction mixtures, and by addition of polar additives. These results provide fundamental understanding of the influence of the environment polarity on self-assembly, self-healing and the ability to control the morphology and structure over the perovskite structures. As a result of this understanding we were able to extend the synthesis to produce other materials such as CsPbBr3 nanowires and orthorhombic CsPbI3 nanowires with tunable length ranging from 200 nm to several microns.

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INTRODUCTION Organic and inorganic lead halide materials have attracted significant attention in recent years, predominantly due to their outstanding photovoltaic performance. Work on thin films rapidly led to a surge of work on the formation and optical properties of colloidal lead halide based nanocrystalline materials, with the exciting prospect of using them for solution processed devices such as light-emitting diodes and lasers. Of particular interest in the context of colloidal nanoparticles is the family of all-inorganic CsPbX3 (X=Cl, Br, I) materials, which have been shown to exhibit superior stability over their organic-inorganic counterparts. Even within this much smaller family of materials, colloidal routes have led to fabrication of various nanoparticle geometries and crystalline habits under different synthetic conditions. These, in turn, may have significant effect on their optical and electronic properties. In the following we first attempt to provide a short overview of the present literature on this family of materials and identify the open questions regarding their formation mechanisms. Two routes for the synthesis of perovskite NPs were demonstrated to date: ligand assisted reprecipitation and arrested growth. In ligand assisted reprecipitation, the inorganic salt precursor and the organic amine and carboxylic acid ligands are dissolved in a polar solvent such as DMF acting as a good solvent, while a nonpolar solvent such as toluene acts as a bad solvent to promote the reprecipitation process. Previously, reprecipitation was used for preparation of hybrid organic-inorganic quantum dots1 and further extended to CsPbX3 perovskite where shape control was also demonstated.2 The formation of different shaped NPs was explained in the context of a classic micellar transition mechanism where the micellar structure obtained is dependent on the different ligands’ chain used. In addition, FTIR measurements were used to show selective adsorption of organic acid and amine ligands on the NPs facets. Spheres were shown to be capped mainly by the organic acid while rods and platelets were capped by the organic amine.2 Akkerman et al. used a slightly deferent approach developing a room temperature synthesis of CsPbBr3 NPs in octadecene (ODE) triggered by the addition of acetone.3 This allowed producing nanoplatelets (NPLs) with monolayer-level thickness control. Their explanation is that acetone destabilizes the complexes of Cs+ and Pb2+ ions in solution and therefore sets the trigger for the nucleation of the particles. The addition of protic solvents such as ethanol led to the formation of large particles (20-40nm) due to excessive destabilization. They proposed that protonated oleylamine (OLAm) ligands bind to the top and bottom facets of the NPL, effectively mimicking an


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alkyl ammonium lead halide type of unit cell. Furthermore, they proposed that thickness control is achieved by increased protonation of OLAm (aided by the amount of HBr in the reaction). This view is also in line with recent observations on MAPbBr3 NPLs,4,5 where thickness control was achieved by tuning the acidity of the reaction mixture. The formation of ammonium may result in preferential binding, where the ammonium competes with Cs+ ions on the surface of the growing platelets and selectively slows the growth along the vertical direction, promoting anisotropic growth.3,6 A second route pioneered by Schmidt et al.7, applied the arrested growth colloidal synthesis strategy typically used for semiconductor and metal QDs for the growth of hybrid organic−inorganic CH3NH3PbBr3 NPs.7 The synthesis involved the reaction of a metal salt in the presence of oleic acid (OA) and ODE with the injection of the ammonium cation at a moderate temperature of 80 ºC. It relied on the addition of a longer ammonium cation that cannot be incorporated into the perovskite crystal structure thus arresting the crystal growth, leading to NPs formation. This procedure was later extended to the synthesis of all inorganic CsPbX3 perovskite with the cesium introduced in the form of cesium oleate (CsOA) at temperatures between 140 ºC to 200 ºC producing cubic phase nanocubes (NCs) with increased size by increased temperature.8 Interestingly, this synthesis yields cubic phase perovskite in all halide cases, even for the iodide compound which is orthorhombic in bulk at this growth temperature. Bekenstein et al.9 observed that the procedure to prepare CsPbBr3 NCs,8 when carried out at a lower temperatures range of 90-130 °C, yields cubic phase NPLs, again showing the effect of temperature on size rather than on crystallographic phase which is tetragonal for bulk in this case. TEM images of NPLs grown at 90 °C revealed very thin NPLs between lamellar structures. Therefore, they concluded that NPLs grow along and inside of these lamellar structures, suggesting that organic mesostructures serve as growth directing soft templatating10 for the 2D growth. Further work revealed the dependence of perovskite nanocrystal morphologies on the hydrocarbon chain composition of amines and carboxylic acids.11,12,13 The use of shorter chain amines leads to thinner NPLs, and the use of shorter chain carboxylic acids leads to larger size NCs. The important role of surface ligands promoted the detailed study on the surface chemistry involved in the growth process conducted by De Roo et al.14 Their findings indicate that in the early stage of the reaction (before the addition of CsOA) PbBr2 reacts with OA to form Pb-oleate



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and oleylammonium bromide (oleylamine binds HBr). Second, OA can protonate OLAm and form oleylammonium oleate. Therefore, the CsPbBr3 NCs are terminated either by oleylammonium bromide or oleylammonium carboxylate that can bind either to the metal or the cation and both are necessary to stabililze the CsPbX3 NCs in solution. Furthermore, De Roo et al. show that there is a dynamic stabilization of oleylammonium bromide in solution and the binding strength indeed relies on controlled acid−base equilibrium. This actually coincides more with the view that regards growth and shape control of colloidal NPs as a kinetically controlled process, governed by preferential ligand binding coupled with temperature-dependent dynamic surface−ligand interaction. However, a few reports also suggest growth by oriented attachment into larger selfassembled structures either by intentional destabilization of surface ligand passivation or by spontaneous gradual aggregation.5,9 In fact, a careful investigation of all reported perovskite NPs syntheses to date seems to show that all reactions actually follow the same general main attributes. First, the involvement of ammonium species in surface passivation as shown by De Roo et al.14 Second, the ammonium species can be formed by amine protonation that can be realized either by direct reaction with organic acid at high temperatures or by the addition of polar solvents even at room temperature. Third, there is preferential binding of ammonium to the perovskite surface replacing the Cs+ ion and thus affecting the growth kinetics and the obtained shape. Fourth, further growth can also carry on in oriented attachment mode promoted through surface destabilization by the addition of polar solvents. Yet, these seemingly unifying traits leave many questions regarding the formation process of CsPbX3 nanoparticles unanswered. Many reports show small spherical NPs within a larger NPLs but here are contradicting evidence regarding the nature of these particles. One report claims this is Pb metal,11 and another that it is CsPb2Br5.15 Koolyk et al investigated the growth kinetics and suggest that growth is driven by focusing and de-focusing caused by the depletion of monomers during the course of the reaction.16 However, the pathway via which very large structures are formed is still not sufficiently clear. The suggested oriented attachment growth mode has been discussed in the case of NPLs growth into sheets9 but has not been considered in a broader view of lead halide-based materials synthesis. The crystallographic structure of nanoparticles appears to differ for different synthesis specifics and is not necessarily in accordance to bulk thermodynamically stable states. Finally, a clear relation between synthesis of nanoparticles


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and the respective bulk materials is yet to be formed. The latter offers to provide us with further understanding of the unique properties of the bulk solids in the context of photovoltaic cells. Herein, we thus set out to suggest a comprehensive detailed formation mechanism of Cs lead halide nanoparticles. This is done by completely separating the nucleation phase at higher temperatures with the growth phase, which is conducted at low temperatures, offering easy access to intermediate structures due to the significant control over the reaction kinetics. This enables us to provide strong evidence for the involvement of polar solvents in the self-assembly process of these particles into larger structures and in their ‘self-healing’ nature. In addition, we are able to observe the structural transformations in CsPbI3 from cubic to orthorhombic throughout the selfassembly process. Our study not only provides fundamental aspects of NCs formation and structural changes but also provides, due to the enhanced control over growth kinetics, a pathway towards novel material geometries such as CsPbX3 NWs with a controlled aspect ratio.



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6 RESULTS AND DISCUSSION

Smaller CsPbX3 NCs were synthesized following the procedure reported by Kovalenko et al.

8

Briefly, PbX2 salt in the presence of oleylamine (OLAm) and oleic acid (OA) is reacted with Csoleate (CsOA) under moderate temperatures yielding cubic shaped CsPbX3 NCs of cubic phase crystal structure. The detailed synthesis procedure is provided in the SI. Typically, the size of CsPbX3 NCs can be tuned in the range of 4−15 nm by changing the reaction temperature. We propose that the synthesis goes through two stages with different mechanisms. In the first stage seed mediated nucleation of intermediate CsPbX3 NPLs occurs. In the second stage: growth occurs by oriented attachment. In order to investigate this proposal we succeeded in dividing the synthesis into two separate stages and systematically monitor the different stages by electron microscopy. Furthermore, for this aim the growth kinetics should be slowed down.

Seeding mediated nucleation.

Figure 1: Schematic representation of formation of intermediate CsPbX3:Pbº NPs proceeding in two steps. Step (1) nucleation of Pbº seeds followed by step (2) seeded growth of CsPbX3 NPs to form CsPbX3:Pbº NPs.

The scheme presented in figure 1 depicts the first stage of the reaction. Initially, PbX2 salt dissolved in a mixture of OLAm and OA at 120 Cº forms small spherical NPs (figure 1 step 1) which then grow into thin CsPbX3 NPLs with the addition of CsOA at 170 Cº (figure 1 step 2). In order to


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7 capture the intermediate primary perovskite particles in step 2, we quenched the reaction immediately after the injection of CsOA by a water bath. The crude product was then subjected to centrifugation at cold conditions to separate the CsPbX3 NPLs. The supernatant consists of thin CsPbX3 NPLs with uniform sizes of 5.7±0.4 nm, 6.9±0.7 nm and 13 nm for the Cl , Br, and I case, respectively as shown in the TEM images presented in figure 2 (bottom images). Absorbance and photoluminescence spectra of the different halide perovskite are presented in the SI figure S1. Careful observation of these TEM images reveals that along the edge of every NPL formed in step 2 one can identify a darker spherical particle. TEM images of the solution before injection of CsOA, shown in figure 1 (top images) clearly show that these spherical dark particles were already present in the solution prior to formation of the CsPbX3 nanoparticles.

Figure 2: Top frames: TEM images of Pbº seeds (i.e. the reaction product of step (1)) with their size statistics, for PbCl2, PbBr2 and PbI2 from left to right, respectively. Bottom frames: TEM images of intermediate NPLs product of step (2) for the different halides. For CsPbBr2, the inset show a HRTEM image of the Pbº seed within the perovskite NPL matrix. ACS Paragon Plus Environment


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Monitoring the optical density in step 1 shows increased color with longer ageing time suggesting the gradual increase in number of Pbº NPs while TEM analysis shows that their size does not change significantly. The insets in figure 2, top images, show the size statics performed on TEM images of the Pbº NPs formed in step 1 after ageing of one hour, giving diameters of 1.9±0.3 nm, 3.6±0.5 nm and 4±0.7 nm for the Cl, Br, and I case, respectively. The size of the Pbº seed seems also to influence the final size of the CsPX3 NPLs. For example, CsPbI3 NPLs of size 8.8 ±0.8 nm and 18±1.6 grow from seeds with a size of 2.5 ±0.4 nm and 4±0.6 nm respectively (figure S3). These small spherical NPs are confirmed to be Pbº by the crystal lattice spacing measured from high-resolution TEM (see SI figure S2). We note that careful consideration for possible radiation damage was taken and the hardly avoidable radiation damage that we observe in TEM did not manifest in nucleation or growth of Pbº (see SI for details). The presence of Pbº NPs in the solution prior to injection of the Cs salt as well as the observation of a spherical lead particle in every CsPbX3 NP following it provides strong evidence that the perovskite particles are indeed seeded by metallic lead. This may suggest that control of the properties of the seed Pbº NPs may enable further control over the growth of the perovskite. We therefore study the effect of ageing of the reaction mixture (step 1) before the addition of CsOA on the final morphology of the reaction product (step 2). A significant change in morphology of the final products is seen for the different halides. The reaction mixture of step 1 was subjected to 1 hour, 7 hours and 16 hours ageing at 120 ºC, whereas the rest of the growth procedure was left unaltered, i.e. the reaction was quenched immediately after the injection of CsOA. This is demonstrated in figure 3 for the synthesis of CsPbI3 where ageing for 1 hour led to the growth of perfectly cubic shaped nanocrystals of size 13±2.5 nm (figure3, top). Ageing for 7 hours produces a combination of cubes, needles and wires (figure 3, middle). Ageing for 16 hours produces a mixture of wires (figure 3, bottom) and tubes (figure 3, bottom, inset).


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Figure 3: The effect of seed ageing time (i.e. step 1) on the morphology of intermediate CsPbI3 (i.e. step2). As the ageing time is increased the reaction product of stage 1 change from cubes for 1h ageing time to mixed cubes and thin wires for 7h ageing and big wires and tubes for 16h ageing time.

A variation in the concentration of seeds, which may serve as an explanation, did not play a role in the morphology change. We verified this by multiplying the amount of PbX2 reacted up to 3 times. This indeed produced a higher concentration of Pbº seeds but had no effect on morphology: the synthesis product of step 2 remained cubic shaped for all halides. Instead, we assign the ageing effect to the formation of molecular species that gradually form over time under heating and are not present in the initial solution. In particular, as it is known that ligand polarity dramatically affects the growth process, we consider the possibility that polar moieties are gradually produced during ageing. In order to check the second assumption we investigated the effect of a controlled addition of polar agents on the evolution of the growth reaction.



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To mimic the formation of polar moieties during ageing, one must add them prior to the formation of the CsPbX3 seeds. On the other hand, it is likely that their effect on the seeding process as presented in figure 2, which seems to be dominated by the Pbº seed, is small. We therefore first consider the introduction of polar moieties after the formation of the seeds. To slow down the reaction kinetics, we first attempt this at room temperature. Growth by oriented attachment mediated by polar agents. Indeed, we found that the further growth of the intermediate perovskite NPL, which are otherwise stable in solution, into larger particles can be induced by the addition of polar solvents even at room temperature. This is observed for both the bromide and the iodide compounds. Notably, in the case of the chloride compound growth could not be arrested even at room temperature and in the absence of added polar molecules. This is due to increased polarity originated by trioclylphosphine which is added to the reaction in order to facilitate the dissolution of PbCl2 and is still present in solution. Figure 4 shows the progress in the growth process with the addition of 20% of ethanol to the crude solution intermediate NPLs species (product of step 2) for the different halides. For CsPbCl3, the Initial CsPbCl3 NPLs of size of ~5 nm self-assemble into larger NPLs of size of ~40 nm (figure 4a) along with wire-like structures in between still decorated with the Pbº seeds (figure S4). Next, these thin larger NPLs further grow in thickness by attaching face to face to gradually form submicron size particles (figure 4b,c). For CsPbBr3, the addition of ethanol resulted in the fast (5 min) formation of ultrathin long (hundreds of nm) wires along with large cubes tens of nm in size (figure 4e,f). From 10 min onwards there is a gradual disappearance of wires and the formation of larger cubes (figure 4g). A more detailed record of the gradual oriented attachment of wires into rods and eventually into platelets is presented in figure S5. It appears that while the chloride and bromide products maintain the cubic morphology, in the iodide case, the initial squared shaped NPLs first transform into spherical nanocrystals of same size of ~13nm in diameter (figure 4i) followed by one-dimensional arrangement of these spherical NCs to wires of 50nm in length and ~10nm in diameter (figure 4j) and finally smooth surface wires of several micron size length are formed (figure 4k) with the diameter varying from 14 to 30 nm. Dynamic light scattering measurements presented in figure S6 was used to follow the growth in solution through the progress of the oriented attachment process over time. The first growth stage into short wires which is depicted by color change from red to brownish is slower but once it occurs it is followed by


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immediate precipitation of micron size wires. We believe that the self-assembly process takes place through the destabilization of surface ligands caused by the addition of the polar solvent as was also observed in other 2D colloidal systems.17 Indeed the addition of a higher concentration of ethanol seems to accelerate the self-assembly process which is agreement with the explanation given above. It was shown by Pan et al. that purification with acetone removes the ammonium but retains the carboxylates.12 Oleic acid is known to pack in a dense ordered layer on the surface thus stabilizing the 2D growth. In addition, Bekenstein et al. experiment with the addition of alkenehalides as polar destabilizers to the synthesis at an elevated temperature of 110 °C observed only 2D thin sheets. These observations suggest that sufficient removal of oleate ligand along with the ammonium ligand has also major role in the oriented attachment growth mode from 2D into 3D structures. XRD measurements presented in figure 4 show that the initial chloride and bromide NPLs have a cubic crystal structure that is retained in the course of the self-assembly process. In the iodide case, the crystal structure transforms to orthorhombic already in the stage of “ripening” into spheres. Bulk CsPbX3 exhibit a cubic perovskite crystallographic structure in the highest temperature phase but are not stable for long periods at room temperature. The Goldschmidt ratio of the ionic radii is close to 0.8, hence CsPbX3 show a tendency for octahedral distortion18 and tetragonal or orthorhombic structures are stable at lower temperatures. For CsPbI3 another orthorhombic phase belonging to the structure type of NH4CdCl3/Sn2S3 is reported. In this phase, space group No. 62, Pnma, the lead atoms are surrounded by edge-sharing iodine octahedral line up to double chains and nine iodine atoms form ditrigonal pyramides around a central cesium atom.19 The internal energy of the cubic phase is rather close to the distorted phases and the phase transitions occurs below 130 °C for CsPbBr3 and below 47 °C for CsPbCl3.19,20,21 For bulk CsPbI3, a phase transition from cubic to orthorhombic occurs at a much higher temperature of 328 °C.8,19,22 Therefore, we suspect that with the removal of surface ligands the highly metastable phase of cubic CsPbl3 readily converts to the stable orthorhombic phase.8 Thus the growth mode is a combination of crystal rearrangement followed by oriented attachment. There have been few reports on the synthesis of the orthorhombic NWs via extending the reaction time and varying the length of the amine ligands.6,23,24,25 The advantage of the route we present here is the high chemical yield and the ability to control the aspect ratio of the NWs. The



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amount of the additional ethanol and intermediate NPLs concentration allows controlling the growth of NWs from 200 nm and up to several microns (figure S7).


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Figure 4: TEM images displaying the progress in self-assembly by the addition of ethanol to the initial NPLs for CsPbCl3, CsPbBr3 and CsPbI3, left to right, respectively. The times reported are measured from the time of ethanol addition. Bottom panel: XRD spectra for the early product in the self-assembly process in (red, bottom) and for the final product in (blue, top). ACS Paragon Plus Environment


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14 Finally, we have studied the effect of polar additives before the addition of CsOA on the final morphology of the reaction product (step 2). Ethanol (300 µL) and CsOA were injected to the reaction mixture at 170 °C, almost simultaneously, and the reaction was quenched after 5 seconds. The final product obtained for CsPbBr3 is dominated by nanosheets (figure 5a). The Pb° seeds decorating the NPL and voids within it are clearly seen. The presence of clear double peak at ~30° in XRD data confirms that the CsPbBr3 nanosheets are distorted perovskites with octahedral tilt and orthorhombic crystal structure. The voids indicate that the growth is by accelerated incomplete oriented attachment. However, this alone cannot explain the phase change. In CsPbBr3, the orthorhombic phase derives from a tilt of corner shared octahedra, which makes only a minor change to the surface than in the cubic phase but which still is more energetically favorable.8 For CsPbI3 (figure 5b), orthorhombic large sheets were formed. There the effect is more pronounced because, in CsPbI3 the orthorhombic structure is different than for CsPbBr3 and it derives from the edge-shared octahedra forming chains along the short unit cell dimension making a only halide surface.

Figure 5: TEM images of the CsPbBr3 nanosheets and CsPbI3 folded nanosheets a) and b), respectively, formed upon the addition of ethanol to the synthesis. Inset of a) XRD spectrum of the nanosheets showing the presence of orthorhombic structure.


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In addition, in order to check that the promotion of growth is not merely an effect of ligand surface removal, we conduct one other experiment where either OA or OLAm only where added to the purified “crude” CsPbBr3 NPL (product of step 1) solution in hexane. This resulted in self-assembly into large elongated sheets in the OA case but a different large bunched wires structure in the OLAm case (see SI figure S8). Thus, we postulate that the passivating ligands stabilize both metal and cation, which under destabilized conditions a crystal rearrangement can occur in order to minimize surface energy. It is noted that in all cases, without addition of ethanol the products are preferably nanocubes with cubic crystal structure. We suggest that controversy in the literature regarding the crystal structure of CsPbBr3 NCs arises from the high sensitivity of the crystal structure to slight changes in synthetic and purification conditions as demonstrated here.26

Self-healing Process. During the self-assembly process, the Pbº NPs that left decorating the growing crystals, gradually leach out. It is noted that the size of the initial Pbº seeds (step 1) increases slightly from 2±0.5 nm to 3.7±0.8 nm in diameter, indicating some fusion of Pbº NPs. The Pbº NCs do not completely decompose and are still present in solution of the final product. When the lead seeds leach out, they leave clearly observable voids within the self-assembly grown perovskite particles. Yet, upon further growth, the formed perovskite particles appear free of such voids. This is a direct observation of a property of self-healing, cited as one of the causes of the high performance of perovskite-based PV cells.

CONCLUSIONS We provide evidence supporting the notion that the formation of CsPbX3 NPs follow through two separate stages. Seed mediated nucleation followed by growth through oriented attachment. By identifying and separating the two processes we were able to determine the role of each step in the protocol. It is apparent that Pbº seeds offer stable nucleation sites for perovskite layers. Controlled assembly of these structures resulted in the formation of cubic CsPbX3 NCs with octahedral distortion. Furthermore, we show that the major factor that substantially affects the size, shape and crystallographic structure of the CsPbX3 NPs is the presence of polar moieties. We show that as a result of polar products (from OLAm and OA) due to ageing of the precursor solution the reaction



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products change from CsPbI3 nanoplatelets to nanocubes and nanowires. Room temperature formation of cubic and orthorhombic nanostructures through the self-assembly process under the influence of polar additives supports our claims. Further, single step formation of NPLs and nanowires by injecting small amounts of polar additives to the reaction flask further confirms our conclusions. In addition, we also demonstrated the synthesis of novel materials such as CsPbBr3 nanowires, CsPbCl3 bulk-like crystals and CsPbI3 orthorhombic nanowires of length ranging from 200 nm to several microns. Insights gained from understanding the inorganic perovskite NCs formation process can aid in rational design of novel materials, leading to their enhanced performance in application such as photovoltaics and optoelectronics.

ASSOCIATED CONTENT 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; additional TEM characterization and beam damage considerations; absorption and emission spectra and DLS measurements. AUTHOR INFORMATION Corresponding Author *Email: (M.K.) [email protected] Funding Sources This research was supported by The Israel Science Foundation as part of the ISF-NSFC joint program. Notes The authors declare no competing financial interests. ACNOLEDGENTS REFERENCES (1)

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Perovskite Nanocrystals. ACS Nano 2015, 9, 2948–2959. (2)

Sun, S.; Yuan, D.; Xu, Y.; Wang, A.; Deng, Z. Ligand-Mediated Synthesis of ShapeControlled Cesium Lead Halide Perovskite Nanocrystals via Reprecipitation Process at Room Temperature. ACS Nano 2016, 10, 3648–3657.

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TOC figure


Nucleation, Growth, and Structural Transformations of Perovskite

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