Synthetic Evolution of Colloidal Metal Halide Perovskite Nanocrystals

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Invited Feature Article pubs.acs.org/Langmuir

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Synthetic Evolution of Colloidal Metal Halide Perovskite Nanocrystals Chun Kiu Ng, Chujie Wang, and Jacek J. Jasieniak*

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ARC Centre of Excellence in Exciton Science, Department of Materials Science and Engineering, Faculty of Engineering, Monash University, Clayton, VIC 3800, Australia ABSTRACT: Metal halide perovskite semiconductor nanocrystals have emerged as a lucrative class of materials for many optoelectronic applications. By leveraging the synthetic toolboxes developed from decades of research into more traditional semiconductor nanocrystals, remarkable progress has been made across these materials in terms of their structural, compositional, and optoelectronic control. Here, we review this progress in terms of their underlying formation stages, synthetic approaches, and postsynthetic treatment steps. This assessment highlights the rapidly maturing nature of the perovskite nanocrystal field, particularly with regard to their lead-based derivatives. It further demonstrates that significant challenges remain around precisely controlling their nucleation and growth processes. In going forward, a deeper understanding of the role of precursors and ligands will significantly bolster the versatility in the size, shape, composition, and functional properties of these exciting materials.

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Cs+), B (e.g., Pb2+ and Sn2+), and X (Cl−, Br−, and I−) ions, respectively.29 When t ≈ 1, the perovskite will possess a highly symmetrical cubic phase (Figure 1g) phase, while for t > 1 and t < 0.8, a hexagonal or tetragonal (Figure 1h)phase and a rhombohedral or orthorhombic (Figure 1i) phase, respectively, will be favored. This structural framework provides great flexibility for tuning the composition of metal halide perovskites and their resulting properties. The PNC field has closely followed the advances in compositional control arising from bulk perovskite films. As a result, most attention to date has been placed on lead-based candidates. These exhibit a distinct electronic structure compared to the more typical tetrahedrally coordinated semiconductors (e.g., CdSe or GaAs), with the lead halide perovskite conduction and valence bands being dictated by antibonding Pb(6p) and hybridized X(p)−Pb(6s) orbitals.10 This entails that they have rather benign defect states, which is a key reason for their exceptional optoelectronic properties. Moreover, it indicates that the modification of the X and B sites provides the most suitable strategies for modifying the band gap. Of these, the modification of the halide composition has emerged as the most facile for tuning the optical properties of PNCs across the visible spectrum,16 with the larger halides (i.e., rCl− = 1.81 Å, rBr− = 1.96 Å, rI− = 2.20 Å) yielding higher band gaps owing to their energetically deeper bonding p orbitals.10,31 In contrast, A-site modification has exhibited limited band gap tunability but has opened up hybrid and inorganic perovskite

INTRODUCTION The accelerated global reliance on optoelectronic devices requires the development of faster, more efficient. and cheaper optoelectronic materials.1,2 Colloidal metal halide perovskite nanocrystals (PNCs) have emerged as one of the most exciting nanoscale optoelectronic material candidates.3 Since being first reported in 2014,4 the field of PNCs has advanced quickly by leveraging the progress in the material design of their bulk counterparts5,6 and synthetic developments in traditional semiconductor nanocrystal (e.g., II−VI or III−V) systems.7−9 This can be seen by the dramatic growth in the number of PNC publications and the concomitant decline of those focused on more traditional semiconductor nanocrystals (Figure 1a,b). As a result of this activity, PNCs with reasonable synthetic control of size, shape, and composition have been demonstrated in only a few years.10,11 This synthetic control has led to PNCs with tunable absorption and photoluminescence (PL) across the entire visible spectrum,12,13 photoluminescence quantum yields (PLQY) approaching unity,14,15 short radiative lifetimes of ∼10 ns,16,17 narrow PL full-width at half maxima (fwhm) of 12 to 45 nm,4,18 and suppressed PL blinking statistics.19 These appealing characteristics have further led to PNCs being successfully harnessed within numerous optoelectronic applications, including light-emitting diodes (LEDs),20,21 lasers,22,23 photodetectors,24,25 amplified spontaneous emitters,26 and photovoltaics (Figure 1c−f).27,28 Metal halide perovskites have the generalized chemical formula of ABX3, where A and B are cations and X is a halide anion. Their precise crystal phase can be gauged by the Goldschmidt tolerance factor, t = (rA + rX)/[(√2)rB + (√2)rX], where rA, rB, and rX are the radii for the A (e.g., MA+, FA+, and © XXXX American Chemical Society

Received: April 24, 2019 Revised: June 2, 2019 Published: June 4, 2019 A

DOI: 10.1021/acs.langmuir.9b00855 Langmuir XXXX, XXX, XXX−XXX

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Invited Feature Article

Figure 1. (a) Accumulative and (b) per annum bibliometrics of various semiconductor nanocrystal/colloidal/quantum dot systems from 1992 to 2018 (from the Web of Science database). (c) Scanning electron microscopy (SEM) cross section of a CsPbBr3 photovoltaic device, adapted with permission from ref 28. Copyright 2016 Springer Nature. (d) Schematic of a colloidal CsPbBr3 green-emitting LED and (e) photographs of their 2 × 2 mm2 active area operating at 5 V. Reproduced from ref 20. Copyright 2018 American Chemical Society. (f) Ultrastable amplified spontaneous emission of a CsPbBr3 PNC film with improved passivation. Reproduced from ref 26. Copyright 2015 American Chemical Society. Unit cell of general (g) cubic, (h) tetragonal, and (i) orthorhombic ABX3 structures where A+ cations are green spheres and purple BX6 polyhedral comprise steel blue B2+ and purple X− spheres. Reproduced with permission from ref 30. Copyright 2015 American Physical Society.

classes, which respectively use organic (e.g., MA+ or FA+) and inorganic (e.g., Cs+) cations.12,32 Both of these classes exhibit promising optoelectronic properties; however, the all-inorganic perovskites have tended to exhibit improved thermal, chemical, and photostabilities.33−36 These appealing properties have led to inorganic metal halide PNCs being the most studied to date, with CsPbX3 emerging as the archetypal system. B-site modification has been motivated by its potential for optoelectronic modification and the removal of lead, which is inherently toxic.37 Lead’s closest analogue, Sn2+, is readily used as a substituent in bulk perovskite counterparts to reduce the band gap but has proven difficult to integrate within PNCs because of its tendency to readily oxidize to Sn4+ within nanocrystalline systems.38 As a result, the more redox-stable Bi3+ and Sb3+ cations, which are also notably isoelectric with Pb2+ and Sn2+, respectively, have emerged as promising candidates for achieving Pb-free PNCs.37 In addition to these direct band-edge recombination materials, new optoelectronic functionality has been further introduced through B-site doping. The dual emission observed for Mn/CsPbCl339,40 and sensitized lanthanide emission for La/CsPbX3 are the most common examples of these.41 Beyond their enhanced optoelectronic properties, PNCs also provide greater structural flexibility compared to their bulk counterparts.16 This is most clearly demonstrated for CsPbI3, for which the bulk nonperovskite polymorph is a nonfunctional (nonluminescent, large band gap) orthorhombic γ-phase under ambient conditions42 and transforms into its desired functional cubic α-phase above ∼310 °C.43 However, in its colloidal form,

the functional phase can be metastabilized under ambient conditions through a reduction in grain size and appropriate surface passivation to modify its surface energy.44,45 Access to such metastable structural regimes provides an important avenue for expanding the structure−property landscape of perovskites. These developments in the composition and phase engineering of colloidal PNCs have been underpinned by synthetic developments across both injection and noninjection techniques.4,16,46 Injection methods involve the mixing of precursor solutions to induce supersaturation and drive the formation of PNCs. Meanwhile, noninjection methods involve the controlled reactivity of precursors during a thermal heating profile to control the nucleation and growth dynamics. Across each of these synthetic routes, the rapid nucleation and growth processes of PNCs have made their synthetic windows extremely sensitive to factors such as the reaction temperature,47,48 reaction time,49 solvent properties,50,51 acid−base equilibria,52−54 reagent species,55−57 ligand species17,58 and mixing rates.59 Studies have shown that adjustments to even one of these parameters can modify the structural properties of the resulting PNCs among quantum dots (QD), nanocube (NCu), nanoplatelet (NPL), nanosheet (NS), nanowire (NW), and nanorod (NR) morphologies.51,60,61 While this demonstrates versatility, it also highlights the challenges in achieving structural control within these emerging materials. The progress within the field of colloidal perovskite nanocrystals has been rapid but is still maturing, with many imminent research questions. In this timely review, we provide a B


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detailed synopsis of the developments that have been made toward synthesizing and postprocessing colloidal perovskites. As a way of focus, here we do not go into detail regarding the photophysics and applications of such materials but consider only nontemplated synthetic methods because these have been shown to be more suitable for the synthesis of homogeneous PNC dispersions.

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NANOCRYSTAL FORMATION Nucleation. According to the classical LaMer model, the formation of colloids involves the conversion of precursors to reactive monomers that can form metastable clusters, some of which ultimately exceed the critical radius and grow to form the final nanocrystal.62 The conversion rates of the precursors govern the dynamics of the system achieving supersaturation, which are dictated by the specific precursor chemistry. Generally speaking, the nucleation and growth events of PNCs can be distilled down to a double-displacement or salt metathesis reaction given by eq 1. A+ + B2 + + 3X− → ABX3

(1)

For low-temperature PNC injection reactions (80%) than their shorterchained derivatives.58 Phosphorus-containing ligands have also been used in PNC synthesis. Of these, trioctylphosphine (TOP) has been the most common because it forms highly soluble complexes with PbX2.16 Its use in mixed-ligand reactions involving OA and OLA has enabled the synthesis of high-quality CsPbI3 quantum dots, with PLQYs approaching 100%.14 Importantly, nuclear magnetic resonance (NMR) spectroscopy measurements have revealed its absence from the synthesized PNC surfaces, with only OLA+ and OA− species being detected. A similar observation was observed when CsPbI3 PNCs were synthesized with OA substituted with branched bis(2,4,4-trimethylpentyl)phosphinic acid (TMPPA) in the HI synthesis, only this time OLA+ was observed as the sole ligand on the surface.45 Despite the phosphinic acid not being an active surface binding species for the final CsPbI3 PNC, the resulting OLA+ surface ligand shell allowed for the successful metastabilization of the functional αphase under ambient conditions (Figure 3a−d). It remains unclear as to whether the absence of the phosphorus-containing ligands in the above NMR studies arises from weaker chemical interactions at the PNC surface and/or from steric effects from the branched ligand chains. The use of trioctylphosphine oxide (TOPO) has similarly improved size uniformity and stability of PNCs, although its presence on the surface confirms that it plays a passivating role rather than merely acting as a sacrificial additive or spectator ligand.69,71 Investigations into less common ligands have included nonprimary amines (i.e., secondary to quaternary amine substitutes),58,85 bidentate ammonium species,51,85 aniline derivatives,86 and zwitterions.81 Generally, these have yielded lower-quality PNCs compared to the more traditional ligand combinations because of factors that include poorer steric stabilization, modified precursor/monomer solubilities, and more prominent Ostwald ripening.51 However, there are a few exceptions, with the most notable being by Krieg et al.,81 who used a long-chained sulfobetaine zwitterion as the sole ligand (Figure 3e), which yielded improved chemical durability and storage lifetimes compared to the standard OA and OLA ligand pair. This was attributed to the sulfobetaine molecule being unaffected by Bronsted acid−base equilibria and, because of a kinetically stabilized chelation mechanism, had a PNC surface binding energy similar to that of OLA+.

dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or isopropanol (IPA).28,63 Owing to the solubility of the perovskite lattice in such polar solvents, Ostwald ripening is enhanced compared to syntheses where only nonpolar solvents are used.75 It has been suggested that the presence of these solvent mixtures makes for PNC formation subject to classic micellar formation theory, where the electrostatic and hydrophobic interactions dictated by the specific precursors, solvents, and ligands influence the final PNC morphology.76 Following the initial, rapid growth stage of nanocrystal formation, orientated attachment is the other noteworthy growth mechanism that is prevalent in PNC systems.77 This growth mechanism is believed to occur when the extent of surface ligand passivation is poor or is destabilized to allow for coalescence. It is considered to be the dominant mechanism for the formation of larger PNC structures, such as NWs, NPLs, and NSs.50,78 Ligand Interactions with Perovskite Nanocrystals. Ligand species used for PNCs have often been adopted from more traditional semiconductor colloids, with primary aliphatic amines and carboxylic acid species being the most favored.48,57 Surface investigations of synthesized PNCs using these ligand combinations have unambiguously confirmed that the aliphatic ammonium and carboxylate species are the dominant surface binding ligands.79,80 These form in solution when ligand pairs undergo simple acid−base reactions, such OLA and OA reacting to form oleylammonium−oleate (OLA+−OA−).53 Density functional theory (DFT) modeling has further indicated high ligand-to-surface interaction energies of these reacted species, with that for OLA+ and OA− ranging from −1.73 to −1.95 eV for OA−81 and to −1.14 eV for OLA+.15 In comparison, the binding energy of neutral Cs-oleate and Pb-oleate is approximately half that of the oleate or an alkylammonium-alkanoic acid pair, thus demonstrating that the charged species are more favorable surface passivants.79 Investigations into alkylamine−alkanoic ligand pairs of different chain lengths have further shown that lengthier ligand species (octyl- or higher) yield better-stabilized and higherquality colloidal dispersions.82−84 This stems from the improved steric stabilization provided by the longer-chained ligands,75 which also inhibit monomer uptake during PNC growth to typically result in smaller sizes and NCu formation.57,61 As a result, longer-chain-length acids, such as OA and tetradecanoic D


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Table 1. Summary of Major Classes of Injection and Noninjection Synthetic Methods for Perovskite Nanocrystals

Table 2. Summary of the Major Synthetic Procedures for Perovskite Nanocrystalsa method

composition

shape

size (nm)

PLQY

reaction conditions

ref

NT NT LARP LARP LARP LARP HI HI HI HI HI HI convection solvothermal microwave ultrasonication

MAPbX3 CsPbBr3 MAPbX3 MA3Bi2X9 CsPbX3 Cs3Sb2X9 CsPbX3 Mn:CsPbCl3 Lanthanide: CsPbCl3 CsSnX3 Cs3Sb2X9 Cs3AgBiCl6 CsPbX3 CsPbX3 CsPbX3 CsPbX3

NP NCu, NPL QD QD QD QD NCu NCu NCu NCu NR, NW NCu NCu NCu, NW NCu NP, NPL

6 8.5, 41 × 8 ∼3.3 ∼3 11 ∼3 4−15 11 6.3−7.6 ∼10 20 × ≤3400 ∼8 4.0−15 8.2−12.5 10−13 8−15

17% 31−78% 50−70% 4−12% 70−95% 20−46% 50−90% 180 min (Figure 7b).49 Beyond the reaction temperature and time, ligand and precursor engineering are the other major handles used to modify the resulting structural properties of PNCs. Pan et al. investigated a variety of aliphatic carboxylate and amine ligand pairs, ranging from acetic to oleic acid and hexyl- to oleylamine, respectively, generally observing an increased NCu size for decreasing acid chain lengths across these.57 Meanwhile, using shorter amines than OLA resulted in PNCs adopting NPL morphologies. The NPL thickness exhibited an amine chain-

length dependence, with the thinnest NPLs of 1.8 nm in thickness being grown using hexylamine. Alternative metal precursors, including lead oxide,95 lead acetate,68 cesium acetate,57 and cesium hydroxide,58 have also been explored as a way to gain finer control of the reaction conditions.56,83 In these syntheses, the use of ammonium halides or benign metal halides as halide sources allowed for better stoichiometric control over the reaction mixture.96 Typically, X/Pb ratios ranging from 2:1 to 4:1 have been shown to yield the highest-quality PNCs, with improved PLQYs, narrower fwhm’s, and higher PL lifetimes.95 The optoelectronic improvements under these halide-rich syntheses were considered to arise from reduced surface halide vacancies on the PNCs, which can act as traps. Tuning the halide composition readily modifies the spectral nature of PNCs: A- and B-site engineering can provide extended opportunities to modify their structure−property relations. Protesescu has been one of a few authors to demonstrate A-site modification using HI, incorporating FA into CsPbI3 PNCs by simply injecting the FA-oleate precursor together with Cs-oleate during synthesis.12 Using a molar ratio of 2:1 FA/Cs in the precursor solution, it was found that only 10% of the FA was incorporated into the PNC structure. The synthesized FA0.1Cs0.9PbI3 PNCs exhibited PL emission at 685 nm with a PLQY of >70%, and their cubic phase was maintained for months under ambient conditions (Figure 8a). The latter is consistent with CsPbI3 PNCs alloyed with a larger secondary A-site cation (e.g., FA+) being more stable in a cubic phase because of their Goldschmidt tolerance factor being closer to 1.97,98 G


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Figure 7. Conditions that can influence the morphology of HI-synthesized CsPbBr3 PNCs with inset photographs of solutions under UV light. (a) Reaction temperature showing NPLs forming at lower temperatures as opposed to NCu at 150 °C, scale bar 50 nm. Reproduced from ref 78. Copyright 2015 American Chemical Society. (b) Reaction times of >180 min showing initial NCu transitioning into NWs and NSs with a scale bar of 100 nm. Reproduced from ref 49. Copyright 2015 American Chemical Society. Rubidium has also been shown to be an alternative to cesium for allinorganic PNCs. This was first demonstrated with the synthesis of orthorhombic RbPbI3 NWs by following the standard HI procedure and replacing Cs2CO3 with Rb2CO3.99 A reaction time of 30 min at 150 °C allowed the initially formed NRs to grow into NWs with a diameter of ∼32 nm and lengths of 80% (Figure 10e−g). The scalability of the reaction was demonstrated at gram levels. Tsiwah et al. further showed that by using reaction chemistries similar to that above, bromide PNCs exhibited a rhombohedral Cs4PbBr6 to orthorhombic CsPbBr3 phase transformation upon increased reaction temperature or time.112 By replacing the OLA with shorter-chained octylamine, NSs with a diameter of several micrometers were produced. This indicated that the shorter amine similarly induced anisotropic growth in both injection and convection syntheses. Solvothermal. The solvothermal route is a simple, scalable, and versatile method for nanocrystal production. It is performed in an isochoric environment, with the reaction being conducted in a pressurized vessel at elevated temperature. Chen et al. was the first to use the solvothermal method to prepare CsPbX3 PNCs.55 Cesium acetate (CsAc), PbX2, OA, and OLA in ODE were placed inside an autoclave, with TOP and TOPO being used to enhance the final solubility of the different lead salts. Upon heating the reaction mixture to 160 °C for 30 min in a rolling oven, high-quality perovskite NCu with a size of 8 to 12 nm was produced (Figure 11a). By using different lead halides or their mixtures, the emission properties of these PNCs could

be tuned across the visible region while maintaining high PLQYs of up to 80% and narrow fwhm’s of 12 to 36 nm. It was additionally shown that if the precursors were predissolved prior to heating the reaction mixture, then the modified reaction kinetics results in NWs with a thickness of only ∼3.2 nm and lengths of several hundred nanometers could be produced in almost 100% morphological yield (Figure 11b− e). The solvothermal synthesis of CsPbBr3 was further modified by tuning the Cs/Pb precursor ratio and the reaction time.113 Using predissolved precursors, at a Cs/Pb molar ratio of ∼1:10 and a reaction time of at least 30 min at 100 °C, ∼4.2-nm-thick NPLs with lateral lengths of ∼100 nm and widths of ∼15 nm formed (Figure 11f). As the thickness of the NPLs remained unchanged in the quantum confinement regime, the PL emission could be fine-tuned to between 452 to 465 nm by controlling the lateral NPL sizes to between 101 × 15 nm and 44 × 42 nm, respectively. At progressively longer reaction times, the NPLs exhibited coarsening behavior, demonstrated by a shortening of their lengths and an increase in their widths. Similar trends in the shape evolution were observed as the Cs/Pb molar ratio was increased, although at a nearly equimolar ratio rhombohedralphased Cs4PbBr6 PNCs with a hexagonal shape were observed (Figure 11g,h). The synthesis of Pb-free perovskites using a solvothermal method has also been demonstrated with CsSnX3 (X = Cl, Br, I) NRs.114 In this approach, SnX2 was mixed with ODE, OA, and OLA. A secondary mixture of Cs2CO3, ODE, OA, hydrochloric acid, TOPO, and diethylenetriamine was then combined with the first mixture under stirring at 80 °C for an hour. The combined mixture was further diluted with diethylenetriamine and transferred to an autoclave at 180 °C for 6 h. This resulted in NRs of 5 nm diameter and 10 nm length, with absorption onsets and photoluminescence emission peaks that could be tuned via their halide composition from 588 to 688 nm and 625 to 709 nm, respectively. Microwave. The use of a microwave as the heat source requires a polar solvent and/or ligands within a reaction medium that can absorb the microwave radiation to ensure rapid and uniform heating. Long et al. were the first to report the microwave synthesis of inorganic PNCs (Figure 12).115 The authors reacted Cs2CO3, PbX2, OA, and OLA in ODE at a Cs/Pb molar ratio of 1:3 within a 800 W household microwave oven for 4 min. Despite the rather rudimentary nature of the synthesis, 10 to 13 nm NCu CsPbX3 PNCs were achieved. The microwave irradiation time was correlated to the size of the PNCs, with times of between 2 and 7 min yielding tunable sizes of between 5 and 14 nm. By decreasing the Cs/Pb precursor ratio to 1:6 in the above synthesis, NWs with a length of >100 nm were preferentially formed.115,116 K


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Figure 13. (a) Schematic illustration of ultrasonication PNC synthesis. (b) Photographs under visible and UV light for PNCs with different halide compositions (c) and their respective absorption and PL spectra. Reproduced with permission from ref 70. Copyright 2016 John Wiley and Sons. (d) TEM micrograph for a CsPbBr3 supercrystal. (e) PL comparison for normal and supercrystal CsPbBr3 films. Reproduced with permission from ref 119. Copyright 2016 John Wiley and Sons. Pan et al. further investigated the synthesis of CsPbBr3 NCs using a microwave reactor, studying factors that included temperature, the predissolution of precursors, and heating rates.71 It was found that a synthesis temperature of ∼80 °C drove anisotropic growth, yielding NPLs with a thickness of ∼3.3 nm and edge lengths of ∼23.4 nm. Increasing the temperature to 140 °C produced homogeneous ∼10.2 nm NCu PNCs. A further temperature rise to 180 °C yielded inhomogeneous PNC dispersions. Predissolving the precursors at 60 °C and then reacting them at 160 °C under otherwise identical conditions resulted in the formation of NRs at morphological yields of >90%, with lengths of >60 nm and diameters of only ∼2.7 nm. It was noted that the presence of TOPO in the reaction mixture was critical to obtaining high-quality NRs. The authors suggested that the TOPO aided the dissolution of the precursors. Finally, a minimum heating rate of 18 °C/min was found to be required to generate homogeneous PNCs. A slower heating rate of 8 °C/min significantly broadened the PNC size distribution and produced particles >150 nm in size. The latter is consistent with the fundamental limitation of heat-up syntheses in that slow heating rates prolong the nucleation event to yield inhomogeneous dispersions.117 Ultrasonication. The formation of hybrid MAPbX3 (X = Br, I) PNCs has been demonstrated using ultrasonication, albeit in an indirect manner. Huang et al. dispersed PbX2 and MAX in a mixture of OA and OLA that acted as a coordinating solvent.118 This mixture was ultrasonicated to form bulk MAPbX3 within 5 min. After several further hours of ultrasonication, these bulk perovskite crystals were transformed into nanocrystals. It is not clear whether these were generated through a dissolution−recrystallization process and/or progressive fragmentation caused by the cavitation process. Nonetheless, an analysis of the MAPbBr3 samples indicated that the PNCs were ∼4 nm in size and possessed PLQYs of up 72%. In a more direct synthesis, Tong et al. was the first to use ultrasonication to synthesize CsPbX3 nanocrystals (Figure 13a).70 Cs2CO3, PbX2, OA, and OLA were mixed in ODE under ambient air, and then a 30 W sonication tip was applied to the mixture for 10 min. This induced the in situ formation of solubilized Cs+ and Pb2+ precursor complexes, which concurrently reacted to produce CsPbX3 (X = Cl, Br, I, Cl/Br, and Br/I) PNCs. The sizes of the CsPbBr3 and CsPbI3 PNCs were determined to be 10 to 15 nm and 8 to 12 nm, respectively, which are slightly higher than those obtained via HI. The produced PNCs exhibited visibly tunable optical absorption and emission properties, with the emission possessing QYs of up to 90% for the Br and I derivatives and narrow fwhm’s of 12 to 40 nm (Figure 13b,c).

Analogous to the solvothermal method, decreasing the Cs/Pb ratio favored the formation of NPLs. By increasing the concentration 10-fold (1 mmol Cs2CO3, 3 mmol PbBr2) and extending the reaction time to 30 min, it has been shown that the ultrasonication approach can produce CsPbBr3 supercrystals of 200 to 400 nm diameter that are composed of closely assembled 10 sized NCu (Figure 13d).119 Such supercrystals exhibit PLQYs comparable to those of their isolated NC counterparts but are redshifted by ∼20 nm due to strong electronic coupling (Figure 13e).

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POSTSYNTHETIC TREATMENTS Purification. Postsynthetic purification of PNCs involves the removal of residual solvent, ligand, and precursor impurities through multiple precipitation and redispersion steps. Owing to the highly ionic perovskite structure, antisolvents with larger dipole moments of ∼4.0 D (DMF, DMSO) or protic solvents (methanol, ethanol) in which the underlying perovskite lattice exhibits a high solubility have been found not to preserve the structural properties of the nanocrystals.79 As a result, modestpolarity aprotic antisolvents have been favored. The inability of such solvents to undergo proton-mediated X-type ligand exchange further enables a better preservation of the surface chemistry during purification.75 Because of the modest binding constants of the ligands at the surface of PNCs, the required multiple purification cycles decrease the ligand density at the PNC surface.120 To mitigate ligand loss and the deterioration of PNC quality during this process, it has been shown that small amounts of additional organic acid and amine ligands are required to maintain the acid−base equilibria required for colloidal stability and the retention of PL properties.75,121 Despite this, under a large excess of amine,80,122 the irreversible degradation of PNCs was observed, as governed by eq 6. Excess acid was also found to destabilize the PNCs.122 amine

4CsPbBr3 ⎯⎯⎯⎯⎯→ Cs4PbBr6 + 3PbBr2

(6) 123

Postsynthetic additives in the form of thiocyanateand tetrafluoroborate-based124 salts have also been successfully used to improve PNCs, achieving PLQYs approaching unity. It is L


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Figure 14. Postsynthesis anion exchange for mixed PNC halide compositions. (a) Schematic of possible halide exchanges, (b) XRD spectra, (c) combined UV−vis absorption (solid lines) and PL emission spectra (dashed lines), and TEM micrograph of (d) CsPb(Br1.5Cl1.5) and (e) CsPb(Br1.5I1.5). Reproduced from ref 129. Copyright 2015 American Chemical Society.

Figure 15. Postsynthesis cation exchange for mixed PNC B-site compositions. Colloidal CsPbBr3 PNCs under UV illumination with increasing additions of (a) SnBr2, (b) CdBr2, and (c) ZnBr2. Combined UV−vis absorption (dashed lines) and PL emission (solid lines) spectra of CsPbBr3 dispersions with (d) Sn2+ and (e) Cd2+ and Zn2+ additions resulting in spectral blue shifts. Reproduced from ref 130. Copyright 2017 American Chemical Society.

wash cycles.126 Further exploration of such additives during purification processes will be necessary to achieve the optimized structural and optoelectronic properties of PNCs. Anion Exchange. The highly ionic nature of PNCs can readily undergo postsynthetic anion exchange reactions to enable tuning of their optical properties (Figure 14).16 The relatively rigid cationic sublattice and the high diffusivity of halide vacancies31 facilitate rapid Cl− ↔ Br− and Br− ↔ I− halide exchange across all PNCs upon the addition of the introduced

believed that these anions salts remove excess surface lead atoms from the surface, thereby removing shallow traps that detrimentally impact the optical properties. Similar effects have been observed with the addition of ammonium salts to enhance surface passivation and restrict PNC growth to improve their overall colloidal and PL stability.125 Moreover, the use of mixed treatment solutions, in the form of didodecyldimethylammonium bromide (DDAB) and PbBr2 in toluene has resulted in PNCs with PLQYs of >90% that were amenable to multiple M


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toluene mixture to directly dope CsPbBr3 NPLs and NCu.134 In this case, sensitized emission from the Mn was observed only at room temperature for the NPLs, with back energy transfer from the Mn to the host PNCs causing quenching for the NCu. Notably, the same authors showed that a similar cosolvent approach, albeit using toluene and methyl acetate, could successful dope CsPbX3 with Yb3+, as evidenced by its characteristic 2F5/2 → 2F7/2 IR emission being observed at ∼990 nm. Importantly, the doping of both Mn and Yb into the CsPbX3 nanocrystals narrowed their Urbach tail, indicating a favorable reduction in their intrinsic defect states. Attempts to substituting the A-site Cs+ cations with Rb+, Ag+, Cu+, or Ba2+ or the B-site Pb2+ cations with Ge2+ or Bi3+ have resulted in the decomposition of the original CsPbX3 PNCs.31 This incompatibility could stem from oxidative instabilities, incompatible ion sizes, or competition with more thermodynamically stable phases.10

halide source (e.g., PbX2 solution or an ammonium halide solution). The extent of anion exchange can be fully controlled by the ratio between the original and introduced halides.127 The exchange process retains monodisperse colloidal PNCs with similar optical properties compared to directly synthesized PNCs of the same composition.31,115 Furthermore, this process is morphologically independent, as has been demonstrated on NW- and NPL-shaped PNCs.52,128 However, the exchange process is not universal across halide-based perovskites. Because of the large difference in ionic radii of Cl− (1.81 Å) and I− (2.20 Å), attempts to perform a Cl− ↔ I− halide exchange process have resulted in only the slow conversion to a full trihalide perovskite material.31 To circumvent this limitation, it has been shown that Cl− ↔ I− exchanges can be achieved through intermediate exchange processes using Br− (1.96 Å). This unique property of PNC systems to undergo anion exchange is the key reason for its ability to easily tune its emission across the visible spectrum. In comparison, traditional metal chalcogenide nanocrystal systems undergo only cation exchanges and are limited to tuning their emissions via the quantum confinement effect through particle size control.3 Cation Exchange. Analogous to anionic exchange processes, cation exchange at both the A and B sites can similarly tune the optoelectronic properties of PNCs. Akkerman et al. observed that for CsPbX3, the A-site Cs+ (1.67 Å) could be exchanged for the larger MA+ (2.17 Å) cation.129 Similarly, Cs+ and FA+ (2.53 Å) cations could be interchanged to form (FA,Cs)PbI3 PNCs.12 Partial exchange of the B-site Pb2+ cation in CsPbBr3 was first demonstrated by van der Stam et al. by mixing a PNCs solution and M2+ cation precursor solutions (MBr2, M = Sn2+, Zn2+, and Cd 2+ ) in toluene. 130 The emission of the resultant CsPb1−xMxBr3 PNCs exhibited a blue shift proportional to the M2+ precursor concentration (Figure 15). The blue shift of the emission was attributed to the contraction of the PbBr6 polyhedra, after partial substitution of Pb2+ cations by the smaller M2+ cations, which caused a closer and stronger Pb−Br orbital interaction that raised the conduction band minimum. Despite M2+ being present in large excess, there was only partial (≤10%) substitution of the M2+ cation into the PNC. It was reasoned that the perovskite crystal structure is primarily stabilized by the rigid cationic sublattice and that cation substitution, especially at the B site, required higher activation energies than for anion exchange.31 This is consistent with the findings from studies on the solid state ion exchanges in singlecrystal perovskite NWs.131 To overcome this B-site stability, Mondal et al. showed that ultrasonication facilitated enhanced alloying of CsPbCl3 PNCs with Cd2+ using a saturated solution of CdCl2 in ethanol.132 Doping levels of up to 40% were claimed, with the PLQY increasing from 3% to near unity. The significant PLQY increase was attributed to the removal of nonradiative defect states caused by chloride vacancies and a reduction in the PbCl6 polyhedral tilting through the incorporation of the smaller Cd2+ ions. This also gave Cd/CsPbCl3 PNCs improved photostability, with PLQYs of up to 90% being retained after several months. Postsynthetic Mn2+ doping of PNCs has also been explored. Huang et al. used a concurrent halide exchange approach through the addition of MnCl2 in DMF to CsPbBr3 PNCs in toluene to yield PNCs with characteristic emission peaks of the sensitized Mn and CsPb(Cl,Br)3.133 In a more direct approach, Nag’s group simply used MnBr2 dissolved in an acetone and

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SUMMARY Here we have assessed the progress toward the synthesis of colloidal metal halide perovskite nanocrystals. These materials have leveraged advances from traditional semiconductor colloids to achieve modest size and morphological control across various hybrid and inorganic compositions within a span of only 5 years. This structural and compositional control has enabled the exceptional optoelectronic properties of perovskites in their nanocrystalline forms to be showcased, with visibly tunable absorption and photoluminescence properties, narrow emission bands, and near-unity photoluminescence quantum yields being demonstrated. These properties make perovskite nanocrystals a remarkable candidate for many optoelectronic applications, where high efficiency and low-temperature processing are advantageous. The hot-injection method has emerged as the most common for synthesizing high-quality perovskite nanocrystals. Relying on a high-temperature mixing process under aliphatic chemical conditions, control of the rapid nucleation and growth stages through ligand, precursor, solvent, and temperature profile tuning has yielded reasonable size- and shape-controlled perovskite nanocrystals. Lower-temperature injection syntheses that mostly use aliphatic and polar solvent mixtures have been concurrently developed. These have taken advantage of the slower kinetics to scale the reactions from tens of mL up to >1 L. A number of noninjection methods have also emerged as suitable synthetic alternatives that are more suitable for scale up. By adopting similar reaction mixtures to the injection methods, perovskite nanocrystals of varying size and shape have been synthesized through these, although with lesser structural tunability and quality compared to those of the injection approaches. In reviewing all of the reported synthetic approaches to date, it is apparent that a greater understanding of the nucleation and growth processes, the precursor evolution, and the interactions at the perovskite nanocrystal surface is still required for each method. The fast dynamics in these systems have largely masked the intricate details around the nucleation process and early growth stages. Further uncertainties around specific solvent and ligand interactions at the perovskite nanocrystal surface, which include the role that stereochemistry, polarity, conjugation, and chemical functionality play in mediating ligand−surface interactions, provide an incomplete picture of what is occurring at the surface and how the specific interactions can be tuned. Progress in these areas will require the expansion of the currently N


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limited library of precursors and ligands as well as more detailed theoretical studies to understand the synthetic evolution of the reaction mixtures. These insights will concurrently assist in further developing postsynthetic ligand and ion-exchange processes to provide greater structural and optical versatility of these materials. In going forward, the overarching goal in the field of perovskite nanocrystals is the development of stable and nontoxic perovskite derivatives that maintain the lucrative optoelectronic properties of existing lead-based derivatives. Preliminary works on hybrid and inorganic systems, such as MA3Bi2X9 and Cs3Sb2X9, respectively, have shown great promise toward achieving this goal. However, the low-quantum yields, high intrinsic defects, and structural instabilities that are currently observed within such materials remain major hurdles. To overcome these, tremendous effort in tailored ligand and precursor design and compositional engineering will be required to achieve the degree of synthetic and structural control necessary to yield high-quality perovskite nanomaterials with the desirable optoelectronic properties. Through these continuing advances, this exciting field of perovskite nanocrystals will reach a state of maturity, one that will deepen our understanding of structure−property relations at the nanoscale and foster a paradigm shift in the application of nanocrystals across many fields.

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Chujie Wang received his BE (Honors) in materials science and engineering from Monash University and Central South University in 2015. During his bachelor’s degree, he worked on different projects in several research groups: mass production of reduced graphene (Prof. Dan Li) and tissue engineering of artificial skin (Prof. Neil Cameron and Alfred Hospital). In 2016, he continued at Monash University with a Ph.D. in the Department of Materials Science and Engineering under the supervision of Assoc. Prof. Jacek Jasieniak. His current research interests are focused on the phase transformation and surface chemistry of inorganic perovskite nanocrystals.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Chun Kiu Ng: 0000-0001-5456-1020 Jacek J. Jasieniak: 0000-0002-1608-6860 Author Contributions

The manuscript was conceived by J.J. and written through the contributions of all authors. Notes

The authors declare no competing financial interest. Biographies Jacek Jasieniak completed a Bachelor of Science (First Class Honours) from Flinders University (2003) and a Ph.D. from the University of Melbourne (2008) under the supervision of Prof. Paul Mulvaney. He then undertook postdoctoral work at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) with Dr. Scott Watkins and Dr. Ezio Rizzardo (2008−2011), and was a Fulbrigh Scholar with Prof. Alan Heeger at the University of CaliforniaSanta Barbara (2011 to 2012). In 2012, he returned to CSIRO, progressing to a senior research scientist and then group leader. In 2015, he moved to Monash University as an associate professor, where he was also appointed as the director of the cross-disciplinary Monash Energy Materials and Systems Institute. His research interests include the development of nanomaterials and their use in various next-generation energy technologies.

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ACKNOWLEDGMENTS The authors thank the ARC Centre of Excellence in Exciton Science (CE170100026) for financial support.

Chun Kiu received his BE(Hons)/BSc at Monash University in 2015. During his bachelor’s degree, he worked on numerous projects under several academics: cobalt electrolytes for dye-sensitized solar cells (Prof. Udo Bach), perovskite photovoltaic-driven water splitting (late Prof. Leonne Spiccia), and organic photovoltaics (Prof. Chris McNeill). In 2016, he continued at Monash University with a Ph.D. in the Department of Materials Science and Engineering under the supervision of Assoc. Prof. Jacek Jasieniak in the area of perovskite nanocrystal synthesis and its incorporation into solar cells.

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ABBREVIATIONS °C, degrees Celsius; μm, micrometer; Ag, silver; Ba, barium; Bi, bismuth; Br−, bromide; Cd, cadmium; CdBr2, cadmium bromide; CdSe, cadmium selenide; Ce, cerium; Sm, samarium; Cl−, chloride; cm, centimeter; Cs, cesium; Cs2CO3, cesium O


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Efficient Perovskite Solar Cells. Energy Environ. Sci. 2017, 10 (3), 710− 727. (7) Yin, Y.; Alivisatos, A. P. Colloidal Nanocrystal Synthesis and the Organic-Inorganic Interface. Nature 2005, 437 (7059), 664−670. (8) Reiss, P.; Protière, M.; Li, L. Core/Shell Semiconductor Nanocrystals. Small 2009, 5 (2), 154−168. (9) Norris, D. J.; Efros, A. L.; Erwin, S. C. Doped Nanocrystals. Science (Washington, DC, U. S.) 2008, 319 (5871), 1776−1779. (10) Kovalenko, M. V.; Protesescu, L.; Bodnarchuk, M. I. Properties and Potential Optoelectronic Applications of Lead Halide Perovskite Nanocrystals. Science (Washington, DC, U. S.) 2017, 358 (6364), 745− 750. (11) Swarnkar, A.; Ravi, V. K.; Nag, A. Beyond Colloidal Cesium Lead Halide Perovskite Nanocrystals: Analogous Metal Halides and Doping. ACS Energy Lett. 2017, 2 (5), 1089−1098. (12) Protesescu, L.; Yakunin, S.; Kumar, S.; Bär, J.; Bertolotti, F.; Masciocchi, N.; Guagliardi, A.; Grotevent, M.; Shorubalko, I.; Bodnarchuk, M. I.; et al. Dismantling the “Red Wall” of Colloidal Perovskites: Highly Luminescent Formamidinium and Formamidinium−Cesium Lead Iodide Nanocrystals. ACS Nano 2017, 11 (3), 3119−3134. (13) Gonzalez-Carrero, S.; Espallargas, G. M.; Galian, R. E.; PérezPrieto, J. Blue-Luminescent Organic Lead Bromide Perovskites: Highly Dispersible and Photostable Materials. J. Mater. Chem. A 2015, 3 (26), 14039−14045. (14) Liu, F.; Zhang, Y.; Ding, C.; Kobayashi, S.; Izuishi, T.; Nakazawa, N.; Toyoda, T.; Ohta, T.; Hayase, S.; Minemoto, T.; et al. Highly Luminescent Phase-Stable CsPbI3 Perovskite Quantum Dots Achieving Near 100% Absolute Photoluminescence Quantum Yield. ACS Nano 2017, 11 (10), 10373−10383. (15) Pan, J.; Shang, Y.; Yin, J.; De Bastiani, M.; Peng, W.; Dursun, I.; Sinatra, L.; El-Zohry, A. M.; Hedhili, M. N.; Emwas, A. H.; et al. Bidentate Ligand-Passivated CsPbI3 Perovskite Nanocrystals for Stable Near-Unity Photoluminescence Quantum Yield and Efficient Red Light-Emitting Diodes. J. Am. Chem. Soc. 2018, 140 (2), 562−565. (16) Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15 (6), 3692−3696. (17) Sun, S.; Yuan, D.; Xu, Y.; Wang, A.; Deng, Z. Ligand-Mediated Synthesis of Shape-Controlled Cesium Lead Halide Perovskite Nanocrystals via Reprecipitation Process at Room Temperature. ACS Nano 2016, 10 (3), 3648−3657. (18) Lignos, I.; Stavrakis, S.; Nedelcu, G.; Protesescu, L.; Demello, A. J.; Kovalenko, M. V. Synthesis of Cesium Lead Halide Perovskite Nanocrystals in a Droplet-Based Microfluidic Platform: Fast Parametric Space Mapping. Nano Lett. 2016, 16 (3), 1869−1877. (19) Swarnkar, A.; Chulliyil, R.; Ravi, V. K.; Irfanullah, M.; Chowdhury, A.; Nag, A. Colloidal CsPbBr3 Perovskite Nanocrystals: Luminescence beyond Traditional Quantum Dots. Angew. Chem., Int. Ed. 2015, 54 (51), 15424−15428. (20) Shi, Z.; Li, S.; Li, Y.; Ji, H.; Li, X.; Wu, D.; Xu, T.; Chen, Y.; Tian, Y.; Zhang, Y.; et al. Strategy of Solution-Processed All-Inorganic Heterostructure for Humidity/Temperature-Stable Perovskite Quantum Dot Light-Emitting Diodes. ACS Nano 2018, 12 (2), 1462−1472. (21) Song, J.; Li, J.; Li, X.; Xu, L.; Dong, Y.; Zeng, H. Quantum Dot Light-Emitting Diodes Based on Inorganic Perovskite Cesium Lead Halides (CsPbX3). Adv. Mater. 2015, 27 (44), 7162−7167. (22) Yakunin, S.; Protesescu, L.; Krieg, F.; Bodnarchuk, M. I.; Nedelcu, G.; Humer, M.; De Luca, G.; Fiebig, M.; Heiss, W.; Kovalenko, M. V. Low-Threshold Amplified Spontaneous Emission and Lasing from Colloidal Nanocrystals of Caesium Lead Halide Perovskites. Nat. Commun. 2015, 6, 8056. (23) Fu, Y.; Zhu, H.; Stoumpos, C. C.; Ding, Q.; Wang, J.; Kanatzidis, M. G.; Zhu, X.; Jin, S. Broad Wavelength Tunable Robust Lasing from Single-Crystal Nanowires of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, I). ACS Nano 2016, 10 (8), 7963−7972.

carbonate; Cs4PbBr6, zero-dimensional cesium lead bromide; CsAc, cesium acetate; CsBr, cesium bromide; CsOA, cesium oleate; CsPbBr3, cesium lead bromide; CsPbCl3, cesium lead chloride; CsPbI3, cesium lead iodide; CsPbX3, cesium lead halide; CsX, cesium halide; Cu, copper; DDAB, didodecyldimethylammonium bromide; D, dipole moment in debye; DFT, density functional theory; DMF, dimethylformamide; DMSO, dimethyl sulfoxide; Dy, dysprosium; E0, standard reduction potential; EDX, energy-dispersive X-ray; Er, erbium; EtOH, ethanol; QDs, quantum dots; Eu, europium; eV, electronvolt; FA+, formamidinium; FACsI3, formamidinium cesium iodide; FAPbBr3, formamidinium lead bromide; FAPbI3, formamidinium lead iodide; FTIR, Fourier transform infrared spectroscopy; fwhm, full width at half-maximum; min, minutes; g, gram; GaAs, gallium arsenide; Ge, germanium; HI, hot injection; I−, iodide; IPA, isopropanol; LARP, ligand-assisted reprecipitation; LEDs, light-emitting diodes; M, moles per liter; MA+, methylammonium; MABr, methylammonium bromide; MAPbBr3, methylammonium lead bromide; MAPbCl3, methylammonium lead chloride; MAPbI3, methylammonium lead iodide; MAPbX3, methylammonium lead halide; MASnCl3, methylammonium tin chloride; MASnI3, methylammonium tin iodide; MAX, methylammonium halide; mL, millimeters; mm, millimeter; Mn, manganese; MnCl2, manganese chloride; MnX2, manganese halide; N2, nitrogen gas; NCu, nanocubes; NH3+, ammonium; NH4X, ammonium halide; nm, nanometer; NMR, nuclear magnetic resonance; NPL, nanplatelet; NR, nanorod; ns, nanoseconds; NS, nanosheet; NT, nontemplate; NW, nanowire; OA−, oleate; OA, oleic acid; ODE, 1octadecene; OLA, oleylamine; OLA + , oleylammonium; OLAX, oleylammonium halide; Pb, lead; PbBr2, lead bromide; PbCl2, lead chloride; PbI2, lead iodide; PbO, lead oxide; PbOA2, lead oleate; PbS, lead sulfide; PbSe, lead selenide; PbX2, lead halide; PL, photoluminescence; PLQY, photoluminescence quantum yield; PNC, perovskite nanocrystal; ref, reference; Rb, rubidium; rpm, revolutions per minute; s, seconds; SAXS, small-angle X-ray scattering; Sb, antimony; SbCl3, antimony chloride; SEM, scanning electron microscopy; Sn, tin; SnBr2, tin bromide; t, Goldschmidt tolerance factor; Tb, terbium; TEM, transmission electron microscopy; TMPPA, bis(2,4,4-trimethylpentyl)phosphinic acid; TOP, trioctylphosphine; TOPO, trioctylphosphine oxide; UV−vis, ultraviolet−visible; V, volt; W, watts; mmol, millimole; X, halide; XPS, X-ray photoelectron; XRD, X-ray diffraction; Yb, ytterbium; Zn, zinc; ZnBr2, zinc bromide; ZnX2, zinc halide.

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