Spontaneous Crystallization of Perovskite Nanocrystals in Nonpolar Organic Solvents: A Versatile Approach for their Shape‐Controlled Synthesis

Abstract The growing demand for perovskite nanocrystals (NCs) for various applications has stimulated the development of facile synthetic methods. Perovskite NCs have often been synthesized by either ligand‐assisted reprecipitation (LARP) at room temperature or by hot‐injection at high temperatures and inert atmosphere. However, the use of polar solvents in LARP affects their stability. Herein, we report on the spontaneous crystallization of perovskite NCs in nonpolar organic media at ambient conditions by simple mixing of precursor–ligand complexes without application of any external stimuli. The shape of the NCs can be controlled from nanocubes to nanoplatelets by varying the ratio of monovalent (e.g. formamidinium+ (FA+) and Cs+) to divalent (Pb2+) cation–ligand complexes. The precursor–ligand complexes are stable for months, and thus perovskite NCs can be readily prepared prior to use. Moreover, we show that this versatile synthetic process is scalable and generally applicable for perovskite NCs of different compositions.

Abstract: The growing demand for perovskite nanocrystals (NCs) for various applications has stimulated the development of facile synthetic methods.P erovskite NCs have often been synthesized by either ligand-assisted reprecipitation (LARP) at room temperature or by hot-injection at high temperatures and inert atmosphere.However,the use of polar solvents in LARP affects their stability.H erein, we report on the spontaneous crystallization of perovskite NCs in nonpolar organic media at ambient conditions by simple mixing of precursor-ligand complexes without application of any external stimuli. The shape of the NCs can be controlled from nanocubes to nanoplatelets by varying the ratio of monovalent (e.g. formamidinium + (FA + )a nd Cs + )t od ivalent (Pb 2+ )c ation-ligand complexes.T he precursor-ligand complexes are stable for months,and thus perovskite NCs can be readily prepared prior to use.Moreover,weshow that this versatile synthetic process is scalable and generally applicable for perovskite NCs of different compositions.
Over the last few years,h alide perovskite nanocrystals (NCs) have emerged as an ew class of semiconductor materials and efficient color-tunable light sources for aw ide range of applications such as displays,l ight-emitting devices, lasers,b roadband photodetectors,p hototransistors,a nd photovoltaics owing to their extraordinary optical and optoelectronic properties,w hich are easily tunable by their composition and morphology. [1] In addition, they offer several important features including ease of synthesis,tunable surface chemistry,n ear-unity photoluminescence quantum yields (PLQY), access to quantum confinement effects,and solution processability,w hich are crucial for most device applications. [2] Therefore,t here has been increased interest in the facile synthesis of shape-controlled perovskite NCs not only for af undamental understanding of their structure-property relationships,b ut also to meet the demand for various technological applications. [3] Since the first colloidal synthesis of perovskite NCs was reported in 2014, great efforts have been devoted to the fabrication of high-quality perovskite NCs (hybrid organicinorganic as well as all-inorganic) by various synthetic methods such as ligand-assisted reprecipitation (LARP), [4] hot injection, [1b,5] ultrasonication, [6] solvothermal, [7] microwave, [8] and ball-milling. [9] Among these methods,the LARP approach has received special attention because of its ability to produce perovskite NCs at room temperature. [4a, 10] This synthesis method is based on the crystallization of perovskite precursors to form NCs in the presence of ligands in ag ood solvent by the addition of abad solvent or vice versa. Thebad solvent triggers the aggregation of perovskite precursors to form NCs,w hile the ligands dictate their dimensions.L ARP has been widely applied to prepare perovskite NCs of different morphologies such as dots,p latelets,a nd wires. [1c, 3d, 4b] However,t he drawback of this method is that the use of polar solvents,s uch as N,N-dimethylformide (DMF), greatly affects the stability of NCs. [11] Moreover, this method may result in mixed NC morphologies,s uch as nanoplatelet dispersions of various thicknesses,l eading to broad PL spectra. In contrast, perovskite NCs can be directly synthesized in nonpolar organic media, however,atrelatively high temperature.H erein, we report an unconventional ligand-assisted reprecipitation method that does not require external stimuli such as heat, microwave irradiation, ultrasonication, mechanical force,o rp olar solvent, in which perovskite NCs are easily obtained through spontaneous crystallization upon simple mixing of precursor-ligand complexes in organic media at ambient atmosphere.The shape of the NCs can be tuned from nanocubes to nanoplatelets by varying the ratio of monovalent (e.g.f ormamidinium (FA + ) and Cs + )t od ivalent cation (Pb 2+ )p recursors in the reaction medium. Mechanistic studies reveal that the NCs are formed through seed-mediated growth. Furthermore,w es how that this simple synthetic approach is versatile.
As shown in Figure 1a,o ur synthesis is based on simple mixing of two precursor complexes (monovalent cation (A + )ligand and lead halide (PbBr 2 )-ligand) in anonpolar solvent under ambient conditions,a nd at room temperature (see Movie S1 for visualization of shape-controlled synthesis at room temperature). This approach is different from the conventional LARP method. Here,t he precursors are directly dissolved in nonpolar solvents through metal ion complexation of ligands,a nd the crystallization occurs spontaneously when the two precursors are mixed. Importantly,w ef ound that the precursor solutions are extremely stable for months and the perovskite NCs can be readily prepared in large quantities af ew minutes prior to use.I n principle,t his facile synthetic approach could be easily applied to ar ange of ABX 3 perovskite NCs (where Ai s am onovalent cation, Bi sl ead, and Xi sahalide ion) with different types of Aa nd Xc ompositions as depicted in Figure 1b.W ef irst applied this approach to the synthesis of formamidium lead iodide perovskite NCs.I nat ypical synthesis,p re-prepared FA-oleate precursor was injected into atoluene solution containing the PbI 2 -ligand complex under continuous stirring at room temperature (see the Supporting Information for details). Thec olor of the reaction medium changes to orange and the reaction medium starts to fluoresce under UV light immediately after the two precursors are mixed without application of heat or use of polar solvent. This suggests that the precursor-ligand complexes break down and spontaneously crystallize to form FAPbI 3 colloidal perovskite NCs in the reaction medium. In addition, the orange emission of FAPbI 3 NCs indicates that charge carriers are quantum confined, since the bulk FAPbI 3 exhibits near-infrared emission ( % 800 nm). [12] Increasing the amount of FAoleate (200 mLo f0 .05 mol L À1 )a dded to the PbI 2 -ligand solution yields ad ark-brown solution with red luminescence under UV light illumination, which indicates the formation of FAPbI 3 colloidal NCs with weaker quantum confinement. Theo ptical properties of the obtained FAPbI 3 colloidal solutions were characterized by UV/Vis absorption and PL spectroscopy (Figure 1c). Thecolloidal dispersion exhibits an absorption onset at 625 nm and asingle PL emission peak at 650 nm for NCs obtained with the addition of 50 mLF Aoleate,w ith as mall Stokes shift (25 nm) and af ull width at half maximum (FWHM) of 60-65 nm. Thea bsorption edge and the emission peak are clearly redshifted to 720 nm and 750 nm, respectively,when an increased amount of FA-oleate (200 mL) was mixed with the PbI 2 -ligand solution. Therefore, the PL peak position of the NCs prepared by this simple approach is easily tunable by varying the ratio of FA to Pb precursor ( Figure S1). This is in away similar to the synthesis of CsPbBr 3 NCs which show tunable emission when the ratio of Cs to Pb precursor is varied. [3e] Additionally,the PL spectra of strongly confined perovskite NCs (PL at 650 nm) exhibit as mall shoulder at the red-side of the spectrum, suggesting am inor polydispersity of the colloidal dispersion. This has already been observed in perovskite NCs prepared by other methods.
Them orphology of the FAPbI 3 NCs has been characterized by transmission electron microscopy (TEM). The corresponding images of the NCs obtained with the addition of 50 mLa nd 200 mLF A-oleate show nearly monodisperse nanoplatelets and nanocubes (Figure 1d,e), respectively.This is in agreement with our previous study showing the transformation of morphology from 3D nanocubes to 2D nanoplatelets when the ratio of monovalent to bivalent precursor was decreased in an ultrasonication-assisted synthesis of perovskite NCs. [6a] As shown in Figure 1d,e,the nanoplatelets and nanocubes are highly monodisperse and tend to selfassemble on the TEM grid with face-face stacking and cubic close packing,r espectively.T he average thickness of the nanoplatelets determined from TEM is % 2.3 nm, which corresponds to four octahedral monolayers;t his suggested the PL peak wavelength of 650 nm was expected for nanoplatelets with at hickness of four monolayers.I ti sw orth mentioning that the nanoplatelets obtained here exhibit higher quality and monodispersity compared to the nanoplatelets synthesized by the LARP method (Figure 1d,s ee also Figure S2 for large-area STEM images). [13] Thea verage edge length of the FAPbI 3 nanocubes is % 12-14 nm (size distribution is shown in Figure S3 inset) with aP Lp eak at 751 nm. Theh igh-resolution TEM image shows that the nanocubes are single-crystalline (inset of Figure 1e,l argearea STEM images are shown in Figure S3). TheXRD spectra of FAPbI 3 nanoplatelets and nanocubes show that both exhibit ac ubic perovskite crystal structure ( Figure S4). The nanoplatelets have broader XRD peaks compared to the nanocubes indicating as maller grain size.T he PLQYs of FAPbI 3 nanoplatelets and nanocubes were found to be 30 % and 65 %, respectively.The lower PLQY of the nanoplatelets is because those are more susceptible to surface defects compared to nanocubes as they have larger surface-to-volume ratio.F urthermore,w ed emonstrate the scalability of this simple synthesis method by increasing each reaction componentsv olume by 50 times (Figure 1f), producing 100 mL of FAPbI 3 colloidal nanoplatelet dispersion in one run. The absorption, PL spectra, and PLQY are nearly same for the particles obtained from the larger scale synthesis ( Figure S5). Our results clearly suggest that simple mixing of two precursors in different ratios results in shape control with excellent monodispersity and scalability.A dditionally,w e found that the NCs prepared by this approach exhibit higher long-term stability than the NCs prepared by conventional LARP synthesis under UV illumination (l = 365 nm, 12 W power) at ambient conditions (see Figure S6).
In general, the growth of most metal and semiconductor NCs is seed-mediated or stimulated by at emplate,w hile the shape of the NCs can be controlled through precursor concentrations,l igands,a nd reaction temperature.I no rder to elucidate the growth of FAPbI 3 perovskite NCs upon mixing the two precursors,w ep erformed the reaction in ac uvette and monitored the PL evolution of nanocrystals formed in the reaction medium over ap eriod of time.A s depicted in Figure 2a,two sharp PL peaks appear at % 620 nm and % 640 nm immediately after the precursors are mixed, suggesting that the reaction is extremely fast and that the quantum-confined perovskite NCs are already formed at the early stages of the reaction. These two PL peaks correspond to nanoplatelets with athickness of three and four octahedral monolayers,r espectively. [3e, 6a] After 2s ,t he PL peak at 640 nm redshifts to % 660 nm, while the peak at 620 nm remains unchanged. In addition, an ew peak at 680 nm emerges and redshifts gradually to 730 nm over the next 5s corresponding to the final product of nanocubes.Atthis stage, all other peaks at shorter wavelengths corresponding to strongly quantum-confined NCs have vanished, meaning that the smaller particles grow faster than the larger ones,and thus eventually the reaction yields NCs of only one size,a s evidenced from TEM images (Figure 1e). It is difficult to characterize the morphology of intermediate particles due to the extremely fast reaction. However,the PL evolution of the nanocubes suggests ab imodal size distribution of clusters formed at early stages of the reaction, which then disappear as time progresses owing to their transformation into nanocubes through size focusing,a ss chematically shown in Figure 2b.
Although the PL evolution of the product gives insight into the possible growth mechanism, it is unclear how the precursors crystallize into perovskite nuclei when they are mixed in an onpolar organic solution. To gain ab etter understanding of this process,o ne has to consider the dissolution of precursor salts in organic media. Generally, precursor metal ions are not soluble in organic solvents. However they can be solubilized using coordinating ligands through metal-ligand complexation, meaning that the ligand solution acts like ag ood solvent for the precursors.H ere, perovskite complexes are made by dissolving corresponding salts in al igand solution, which is then injected into toluene, ab ad solvent for metal ion precursors.T herefore,t he precursors crystallize into perovskite nuclei immediately when they are mixed in an organic solvent, like in the conventional LARP method (Figure 2b). Further evidence comes from the fact that the crystallization does not occur when both precursor-ligand complexes are mixed in the ligand (oleylamine/oleic acid) solution due to the fact that ligand solution acts as coordinating solvent. It is likely that the energy difference between precursor-ligand complexes and perovskite NCs coated with ad ense ligand shell is so small that the crystallization takes place spontaneously at room temperature immediately when they are mixed in organic media. As schematically depicted in Figure 2c,the morphology of NCs changes from 3D nanocubes to 2D nanoplatelets when the amount of the FA precursor added to the reaction medium is decreased. This suggests that anisotropic growth is favorable at al ower ratio of monovalent to divalent cation (for instance,F A/Pb ratio). In general, decreasing the concentration of reactants decreases the rate of reaction, and thus nanocrystal growth rate.M oreover,aslow growth rate favors the formation of anisotropic NCs,which has been also observed previously for metal nanoparticles. [14] We further verified the versatility of this facile synthesis approach by applying it to different kinds of perovskite NCs, such as CsPbI 3 and CsPbBr 3 .T his has been achieved by selecting the respective monovalent cation-oleate (e.g.C soleate,which was prepared using cesium acetate) and PbX 2ligand complexes as reactants.Asshown in Figure 3, CsPbBr 3 and CsPbI 3 nanoplatelets were successfully prepared by simply mixing Cs-oleate with aP bX 2 -ligand complex (X = Br or I) in toluene.T he TEM images (Figure 3a,b) show that these nanoplatelets are nearly monodisperse with average thicknesses of 1.1 nm and 1.2 nm, respectively,and they tend to self-assemble on the TEM grid in af ace-face stacking fashion. Theabsorption spectra show well-resolved excitonic peaks with the PL centered at % 475 nm and 620 nm for CsPbBr 3 and CsPbI 3 nanoplatelets,respectively (Figure 3c,d). These PL peak positions correspond to four octahedral monolayers for both materials.I nterestingly,aslightly higher reaction temperature ( % 80 8 8C) was necessary to prepare CsPbX 3 nanocubes when the two precursors were mixed in toluene ( Figure S7), in contrast to the roomtemperature formation of FAPbI 3 NCs.T his is likely due to ah igher energy barrier between perovskite precursors and CsPbBr 3 nanocubes.N evertheless,t his reaction temperature is still far lower than the temperature (typical 180 8 8C) of the hot-injection synthesis used to produce CsPbBr 3 nanocubes in organic solvents. [1b] Furthermore,w eh ave shown that this versatile approach can be applicable to the preparation of FAPbCl 3 ,F APbBr 3 ,a nd MAPbBr 3 NCs ( Figure S8).
In conclusion, we have presented af acile scalable synthesis of perovskite NCs at ambient conditions by spontaneous crystallization in anonpolar organic solvent. Compared to the classical LARP method, no polar solvent is needed to dissolve the precursors;i nstead, ligands act as coordinating solvents.The morphology of perovskite NCs is easily tunable from 3D nanocubes to 2D nanoplatelets by decreasing the ratio between the monovalent cation and Pb 2+ precursors. Importantly,w eh ave demonstrated the versatility of this synthesis approach by applying it to both organic-inorganic hybrid and all-inorganic perovskite NCs of different halide compositions.W ef oresee that this facile method could be easily extended to obtain other morphologies such as nanorods or nanowires by varying the concentration and temperature of the Pb 2+ precursor.This versatile and facile synthesis method not only opens new avenues toward the shapecontrolled perovskite NCs with excellent scalability,b ut also expands our current understanding of the crystallization of perovskite NCs directly in nonpolar solvents.