This is an open access article published under a Creative Commons Non-Commercial No Derivative Works (CC-BY-NC-ND) Attribution License, which permits copying and redistribution of the article, and creation of adaptations, all for non-commercial purposes. pubs.acs.org/NanoLett Letter Highly Stable Perovskite Supercrystals via Oil-in-Oil Templating Yingying Tang, Leyre Gomez, Arnon Lesage, Emanuele Marino, Thomas E. Kodger, Janne-Mieke Meijer, Paul Kolpakov, Jie Meng, Kaibo Zheng, Tom Gregorkiewicz, and Peter Schall* Cite This: Nano Lett. 2020, 20, 5997−6004 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: Inorganic perovskites display an enticing foreground for their wide range of optoelectronic applications. Recently, supercrystals (SCs) of inorganic perovskite nanocrystals (NCs) have been reported to possess highly ordered structure as well as novel collective optical properties, opening new opportunities for efficient films. Here, we report the large-scale assembly control of spherical, cubic, and hexagonal SCs of inorganic perovskite NCs through templating by oil-in-oil emulsions. We show that an interplay between the roundness of the cubic NCs and the tension of the confining droplet surface sets the superstructure morphology, and we exploit this interplay to design dense hyperlattices of SCs. The SC films show strongly enhanced stability for at least two months without obvious structural degradation and minor optical changes. Our results on the controlled large-scale assembly of perovskite NC superstructures provide new prospects for the bottom-up production of optoelectronic devices based on the microfluidic production of mesoscopic building blocks. KEYWORDS: assembly, emulsion-droplet templating, supercrystals, perovskite films, stability T hree-dimensional (3D) highly ordered supercrystals (SCs) assembled from individual nanocrystals (NCs) promise the advent of novel optoelectronic devices based on reported SCs made from perovskite NCs. A method that has been used to prepare CsPbBr3 SCs is tip sonication,14 with the produced SCs displaying red-shifted emission due to the “artificial solids”, exhibiting tunable collective solid-like proper- interparticle electronic coupling. Microscale supercubes have ties from assembled semiconductor NCs, where the individual been obtained by slow drying of drop-casted CsPbBr3 NCs.15 NCs can be regarded as artificial atoms.1 The length scale and The resulting ordered superstructures showed superfluores- crystallographic orientation of the ordered arrangements of the cence such as accelerated radiative decay, photon bunching, NCs can then be controlled by either physical or chemical and delayed emission pulses at high excitation density. methods. After self-assembly, the SCs usually display novel collective properties, that is, superfluorescence,2 bathochromic However, all these efforts remain small scale and of rather shift,3 and minibands,4 prompted by their special configuration poor external control in terms of size, morphology, and crystal and periodicity. Their self-assembly offers new opportunities quality of the superstructures, leaving the controlled large-scale for optoelectronic applications, by assembling the NCs into assembly for applications a major challenge. As is well-known, ordered films.5 the structural quality of the films, including morphology, Significant research effort has gone into general self- crystalline degree, and order, directly affects their functionality assembly strategies for various NC systems to steer the system and efficiency. Furthermore, the critical issue of the above toward its crystalline ground state. The most widespread methods is the merging of individual perovskite SCs in a few strategies are drop-casting on a substrate,6 assembly at the air/ days resulting in photonic degradation driven by the liquid interface,7 and casting onto a patterned template.8 This detachment of the surface ligands. Therefore, finding a new way, many superstructures have been obtained for CdSe, solution that keeps high stability while at the same time Fe3O4, Au, and other nanoparticles, like superballs (SBs), allowing for control of the crystal quality of the perovskite SC supercubes, rhombic dodecahedral, octahedral, and hexapod- film is an urgent task. shaped SCs.9−13 However, while their assembly could be well- controlled, these materials are poorly suited for photovoltaic applications due to a bandgap mismatch with solar cells. Received: May 11, 2020 Recently, inorganic perovskite NCs have emerged as a Revised: July 23, 2020 promising material for photovoltaic films,14 exhibiting excellent Published: July 23, 2020 optoelectronic properties and featuring bandgap tunability by size and composition,15 energy transfer,16 and carrier multi- plication capabilities.17 Yet, currently, only a few works have © 2020 American Chemical Society https://dx.doi.org/10.1021/acs.nanolett.0c02005 5997 Nano Lett. 2020, 20, 5997−6004 Nano Letters pubs.acs.org/NanoLett Letter Figure 1. Schematic representation of the SCs synthesis using dual-phase emulsion assembly. (a) CsPbBr3 NCs in toluene (upper) and FC-40 with 008-FS as surfactant (lower) before being vortexed. (b) Emulsion of CsPbBr3 NCs in toluene and FC-40. The enlarged part in (b) represents the droplet of CsPbBr3 NCs in toluene well-dispersed in FC-40. (c) The process of solvent evaporation. The enlarged part represents the gradual solvent evaporation followed by the shrink of the droplet. (d) After the complete evaporation of solvent, SBs form as shown in the enlarged image. For colloidal nanoparticles, emulsion templating has been and left to evaporate (Figure 1c). Upon evaporation, the shown to provide 3D superstructures with exquisite order on a emulsion droplets shrink, causing densification and self- large scale.9−11 This technique typically uses oil-in-water assembly of the NCs inside, producing highly ordered SCs emulsions with NCs dispersed in oil droplets, which crystallize (Figure 1d). The final SC suspension was centrifugated and upon slow evaporation of the oil. Since the evaporation rate dispersed into FC-40 with 008-FS as surfactant and then drop- can be very well-controlled by temperature, surfactants, or cast onto transmission electron microscopy (TEM) grids for pressure, this method offers a powerful route to control the imaging. Two different batches of CsPbBr3 NCs synthesized by assembly and produce highly ordered superstructures in large hot-injection reaction were used: one freshly synthesized and quantities. Unfortunately, the presence of water in a typical one six months after synthesis. These have average sizes of 10 emulsion templating process prohibits the use of perovskite and 14 nm, respectively, and differ in the sharpness of the NCs in this facile controlled assembly process. cubic shape, as determined by scanning transmission electron Here, we present a well-controlled technique to assemble microscopy (STEM). Both exhibit the expected pure phase of perovskite NCs into SCs and enable a high stability of the SC orthorhombic CsPbBr3, as well as the expected absorption films using oil-in-oil emulsion templating. We solve the crucial onset at 498 nm (2.49 eV) and 499 nm (2.48 eV), respectively, problem of avoiding water usage by fabricating emulsions and the PL peak position at 519 nm (2.39 eV).19 The full made of hydrophobic solvent-in-fluorinated oil (e.g., FC-40) sample details are given in Figure S4. stabilized by fluorinated surfactant and elucidate the effect of STEM images of the superstructures obtained from aged surfactant concentration, evaporation temperature, and cubic NCs by evaporation at 20 °C reveal highly ordered SBs, as NC shape on the topology of the SCs. We identify an shown in Figure 2a−c. Lattice fringes distinguished for the intriguing interplay between the spherical confinement of the individual NCs show their atomic lattice alignment. The SBs emulsion droplets and the sharpness of the cubic NCs, in exhibit a broad size distribution with the average size of ∼90 determining the structure and morphology of the final nm, as shown in Figure 2b. The high degree of ordering is superstructure. This interplay is exploited to obtain a dense reminiscent of a previous assembly of CdSe NC SBs;11 hyperstructure of SCs, arranged in a hexagonal close-packed however, these perovskite NCs exhibit a cubic shape, resulting order. The photoluminescence (PL) spectra of the SCs exhibit in a cubic instead of a face-centered cubic ordering. When the a redshift of 1−10 nm compared to the individual NCs, aliphatic ligands on the CsPbBr3 NCs are intermixed with indicating energy transfer between the NCs.18 The resulting solvent molecules, their steric stabilization prevents inter- SCs show high morphological stability for at least two months, particle agglomeration and confers colloidal stability on the in contrast to previously reported assembly methods. NCs.20 For the CdSe NCs in oil-in-water emulsions, we The formation of the SCs is illustrated in Figure 1. To observed that their assembly is determined by effective hard- obtain SCs of CsPbBr3 NCs, emulsions of NC containing sphere behavior, while the late stages are accompanied by an hydrophobic solvent in fluorinated oil are prepared by vigorous additional attractive component upon compression of the vortex mixing (Figure 1a,b). We tested different hydrophobic ligand shell due to the receding interface of the evaporating solvents (toluene, hexane, pentane, cyclohexane, octane) with droplets.11 We speculate that, in analogy, the perovskite NC the fluorinated oils FC-40 and HFE-7500 with surfactant (008- assembly may be dominated by an effective hard (rounded) FS) concentration in the range of 0.1−5 wt % for emulsion cube behavior.21−23 stability at room temperature (Figures S1−S3, Table S1) and The morphology of the superstructures changes from found that toluene and cyclohexane formed stable emulsions spherical to cubic when we use freshly synthesized NCs with FC-40; these stable emulsions were continuously stirred exhibiting sharp edges; a single supercube is shown in Figure 5998 https://dx.doi.org/10.1021/acs.nanolett.0c02005 Nano Lett. 2020, 20, 5997−6004 Nano Letters pubs.acs.org/NanoLett Letter Figure 2. STEM images and size distribution of the perovskite NC superstructures. (a) SBs-90 obtained with 0.1 wt % surfactant at 20 °C assembled from aged NCs. (inset) HRTEM image of a typical aged NC and fitting with a superellipse (dashed line). (b) Miniaturized image with size histogram of SBs (inset). (c) STEM image of SBs-90 with clear individual NCs. (d) Supercube obtained with 0.1 wt % surfactant at 20 °C assembled from fresh NCs. (inset) HRTEM image of a typical fresh NC and fitting with a superellipse (dashed line). (e) Miniaturized image with size histogram of the supercubes (inset). (f) Tilted STEM of supercubes. (g) SBs obtained from NCs stored for six weeks, assembled with 0.1 wt % surfactant at 20 °C. Scale bar: 400 nm. (h) Irregular SCs assembled with 2 wt % surfactant at 40 °C from aged NCs. (insets) Enlarged SEM image (top right), scale bar 50 nm, and FFT image of the selected area (bottom left), scale bar, 0.5 1/nm. (i) Miniaturized image with size distribution for irregular SCs (inset). (j) HRTEM image of a single NC in an SB revealing the characteristic CsPbBr3 structure. 2d, where the individual NCs are observed to exhibit near- interfacial tension of the oil-in-fluorinated (FC-40) oil perfect cubic ordering. These supercubes have an average size emulsion containing 0.1 wt % surfactant is ∼3.54 mN/m of 400 nm as shown in the inset of Figure 2e and Figure S5. (Figure S8), while that for toluene/water is ∼18 mN/m,24 a The 3D structure of the supercubes is illustrated by tilting to factor of 5 smaller. Furthermore, we characterize the sharpness different angles (Figures 2f and S6), clearly revealing their of the cubic NCs by fitting their apparent two-dimensional cubic shape. We hypothesize that, in this case, the sharper (2D) shape in STEM images to the equation of a superellipse x m y m edges of the fresh NCs made the dense cubic superstructure a + b ≤ 1, where x and y are the coordinates along the more stable against the pressure of the receding spherical superellipse, a and b are the semi-axes of the superellipse (or droplet interface so that it maintained its cubic morphology. equivalently half the side lengths of the rounded rectangle), This trend is further confirmed when we repeat the assembly and m is the shape parameter (m = 2 for an ellipse, and m → after six weeks; the gradual loss of sharp edges resulted in a ∞ for a rectangle).23 Fitting ∼100 NCs, we find average shape gradual transition from regular to rounded and irregular SCs parameters of m = 5.3 for the freshly synthesized and m = 3.6 (Figure 2g and Figure S7). We thus find a systematic for the aged sample (see the insets of Figure 2a,d, and Figure dependence of the superstructure morphology on the sharp- S9), corresponding to a sharper and more rounded NC shape, ness of the cubic building blocks. This dependence is in respectively. The sharper cubic shape, together with the low qualitative agreement with previous work on colloidal cubes surface tension, makes the assembled structure more resistant reporting a strong effect of the sharpness of the cubic particles against the spherical interface. on their ordering.21,22 We find that even the morphology of the The interplay of interfacial tension and NC shape is further superstructure is affected by this interplay, possibly due to the explored by lowering the interfacial tension. Increasing the smaller NC size and lower interfacial tension: the measured surfactant concentration from 0.1 to 2 wt % results in a 5999 https://dx.doi.org/10.1021/acs.nanolett.0c02005 Nano Lett. 2020, 20, 5997−6004 Nano Letters pubs.acs.org/NanoLett Letter Figure 3. Optical properties of the NCs and superstructures. (a) UV−Visible absorption and PL spectra of colloidal CsPbBr3 NCs. (b) UV−Visible absorption and PL spectra of colloidal superballs (SBs-90) assembled from aged NCs. (c) Absorption and PL peak position of the perovskite NCs, irregular SCs, SBs assembled from old NCs, supercubes (suspended and drop-casted). (d) Corresponding PL QY and average lifetime for NCs and SCs. wrinkled morphology (Figure 2h,i), while the NCs still exhibit The presented emulsion-templated assembly method cubic ordering (see insets of Figure 2h and Figure S10). This provides a well-controlled alternative to the few existing morphology suggests that, upon drying, the surface tension was perovskite NC assembly methods based on drop-casting or no longer able to maintain a regular interface. The surfactant post-treatments of the perovskite NCs by specific solvents, for concentration also affects the superstructure size: higher example, methyl acetate.2,3,14,15 These works related to the surfactant concentrations lead to smaller droplets and thus to assembly of NCs face several challenges. (1) The SCs are only smaller superstructures with an average size of ∼150 nm, in stable for approximately two weeks due to the detachment of contrast to the average size of 420 nm (SBs-420 in Figures S11 surface ligands.15 (2) The volume ratio between the NC’s and S12). This might be a direct consequence of the lower solvent and the solvent applied for treatment should be interfacial tension, allowing for smaller droplets with higher precisely controlled to obtain a specific morphology.15 (3) curvature. We also note that, when lowering the evaporation Pretreatments of NCs sometimes are necessary to make them speed, we obtain much higher quality supercrystals, consistent more hydrophilic to the substrates.2 (4) Irregular morphology with previous work on CdSe superstructures, where a high- exists as well as individual NCs or bulklike impurities in the quality crystal (67% crystallinity, i.e., 67% of all superballs supercrystals.3 In contrast, the oil-in-oil emulsion template exhibiting crystalline order) was observed at a slow evaporation presented here provides an easily controllable bulk assembly rate, and a rather amorphous structure was seen for faster method that naturally supports upscaling. Choosing the quench rates.11,25 In all of the above cases, the phase of the fluorinated oil, immiscible with the hydrophobic solvent individual NCs is preserved as shown by representative high- dispersing the NCs, as continuous phase results in the resolution transmission electron microscopy (HRTEM) improved control over the assembly of the perovskite NCs. images of an NC inside a superstructure in Figure 2j. We By varying the assembly conditions, we can obtain distinct note that, depending on the preparation protocol, other superstructure morphologies such as SBs, supercubes, and specific morphologies like vesicles or hollow “donuts” are also superhexagons compared to conventional methods resulting obtained (Figure S12); these could play a role in potential only in supercubes and disordered superstructures. heterogeneous catalysis26 as well as protecting, carrying, and The optical properties of the resulting SCs and NCs, both in delivering cargo.27 colloidal form and films, are shown in Figure 3. In all cases, we 6000 https://dx.doi.org/10.1021/acs.nanolett.0c02005 Nano Lett. 2020, 20, 5997−6004 Nano Letters pubs.acs.org/NanoLett Letter Figure 4. Morphological and optical stability of the superstructures. STEM images at the same position on the TEM grid for (a) fresh sample and after (b) 22 and (c) 39 d of deposition, stored in air. (d) Wavelength-dependent PL QY for SBs film after stored at ambient condition. observe a weak red shift (1−10 nm) compared to the pronounced for the ordered, dense SBs. The fresh NCs show individual NCs, in agreement with previous work on perovskite an even more abrupt trend. Their PL QY and lifetime start at NC superstructures, demonstrating energy transfer between higher values, as expected for the better NC quality, and decay the NCs.2,3 In Figure 3a,b, we show that the peaks in the rapidly upon assembly into films and supercubes, as expected absorption and PL spectra shift from 498 and 519 nm for NCs, from their higher degree of order and coupling, and are respectively, to 508 and 522 nm for SBs after assembling the consistent with their stronger PL redshift. The detailed values colloidal NCs into SBs. This red shift is most pronounced for are reported in the Supporting Information, Tables S2−S4. the supercubes made from fresh NCs, exhibiting a PL red shift These results highlight the direct control of the optoelectronic of 10 nm with respect to the dispersed NCs (see Figure 3c). properties by the structural control afforded by the emulsion- For the SBs made from aged NCs, the absorption onset templated assembly technique. continuously red-shifts from the dispersed NCs via the Most importantly, we observe a highly increased stability of irregular SCs to the large SBs, indicating the availability of the NCs inside the superstructures. We tested the stability of lower states in the increasingly dense and ordered structures. the assembled SCs both in terms of their morphology and their These signatures of increasing coupling of the NCs are optical properties by repeated imaging by STEM and by supported by the measurement of the photoluminescence measurement of the PL QY, see Figure 4. The SCs assembled quantum yield (PL QY) and lifetime (Figure 3d). For both NC from individual NCs are still well-preserved without degrada- samples, we find a systematic decrease of both the PL QY and tion after 22 and even 39 d (Figure 4a−c), without merging of the lifetime, indicating increasing nonradiative relaxation due the NCs inside. In addition, small cubes appeared that grew to the increasing delocalization and increasing energy transfer. into bigger cubes (Figure 4c), which, by detailed chemical The PL QY decreases from 50.8% to 28.2% from the NC composition analysis by energy-dispersive X-ray (EDX), we suspension to film, which further decreases to 28% and 15%, identified to consist of the same perovskite material. These respectively, upon assembling NCs into irregular super- cubes were not observed at any other sample position after structures, in solution and drop-casted, which further decreases being exposed to ambient condition, even after two months, to values of ∼17% for densely packed SBs. This trend is suggesting that their appearance is induced by the electron qualitatively mirrored in the PL lifetime, which shows a beam illumination upon imaging; see Supporting Information, continuous decrease from the irregular superstructures to the Figures S14 and S15 for full details. This is different from our densely packed SBs, as shown in Figure 3d, and supported by previous work, where we rather observed the electron-beam full time-dependent data in Figure S13. The decrease of the PL irradiation to enhance the stability of NCs.28 The good QY and reduction of the lifetime indicate significant energy stability of the SCs is also reflected in their optical properties, transfer between the assembled NCs, which becomes the most as shown in Figure 4d. After 39 d at ambient conditions, ∼45% 6001 https://dx.doi.org/10.1021/acs.nanolett.0c02005 Nano Lett. 2020, 20, 5997−6004 Nano Letters pubs.acs.org/NanoLett Letter of the PL QY is preserved for all measurements. These results of four spots, indicating domains of cubic ordering in the are in stark contrast to the rather rapid decay of morphology superhexagons. These superhexagons are arranged in a and PL reported previously for perovskite cuboidal super- hexagonal order again, affording the possibility for densely structures, which merged into single nanoplatelets within two packed films for potential devices. Apparently, the “soft” weeks.15 We hypothesize that the FC-40 oil plays a crucial role, emulsion droplets after deposition on the substrate were being an inert oil resistant to high-temperature degradation. sufficiently stable to form a hexagonal lattice, which further After the complete drying of the toluene phase, the resulting densified upon drying. This way, close-packed perovskite films SCs are stabilized by the 008-FS, hindering ligand detachment, of superhexagons can be obtained. The thickness of the film is as the ligands are insoluble in the fluorinated oil, and avoiding on the order of 100 nm, which is the right order of magnitude further degradation and merging. Additionally, merging into for optoelectronic applications.31 This type of hierarchical fused cubes can be induced on purpose by electron-beam ordering could be used to cover larger areas by a simple up- irradiation and chemical treatments to enhance the perform- scalable process. Furthermore, the application of microfluidic ance of devices. techniques would allow for monodisperse droplets and highly We finally explore the emulsion droplet technique for the ordered hyperstructures. hierarchical assembly of dense films. The idea is to use the We have demonstrated the controlled assembly of stable superstructures themselves as building blocks for the assembly inorganic perovskite NC superstructures using emulsion on a larger scale. Indeed, the control over size and morphology templating based on oil-in-fluorinated oil emulsions, yielding of the superstructures can be exploited to tune them for supercubes, SBs, and superhexagons. The morphology of the assembly into dense ordered layers, hyperstructures of an superstructures results from a subtle interplay of roundness of ordered arrangement of SCs. To explore this route, we deposit the cubic NCs and surface tension of the spherical confine- the emulsion with the NC-loaded droplets onto a substrate ment. This interplay can be exploited to obtain hyper- (TEM grid) in the wet state, before they are completely dry, structures, hexagonal close-packed arrangements of super- after evaporating for 48 h (complete drying takes 72 h and hexagons. These results open new directions for the yields SBs). The resulting deposits exhibit hierarchical ordering hierarchical self-assembly of high-quality, stable, and homoge- as shown in Figure 5. Hexagonal-shaped superstructures neous films, offering new opportunities for phosphor layers and efficient perovskite NC-based devices after chemical or electron irradiation treatments, which has been realized in CdSe supercrystals.32 In particular, the choice of a hydro- phobic liquid, toluene, in a fluorinated oil (FC-40) droplet allows for the existing expertise garnered by research in microfluidic technologies to apply this approach in a rapid and timely manner to inorganic perovskite NC assembly.33 ■ ASSOCIATED CONTENT * Supporting Information sı The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.0c02005. The synthesis process, characterization methods, stabil- ity of the emulsion system, XRD, STEM, HRTEM images, quantification modeling, surface tension and optical measurements (UV, PL, PL QY, lifetime), additional analysis and discussions (PDF) Figure 5. STEM images of the perovskite NC superstructures on TEM grids. (a) Superhexagons obtained by drop-casting the emulsion (with 0.1 wt % surfactant) on the TEM grid before complete drying, after evaporation of 48 h at 20 °C. (inset) The FFT image for the ■ AUTHOR INFORMATION Corresponding Author selected area. Scale bar: 0.5 1/nm. (b) Miniaturized image showing Peter Schall − Institute of Physics, University of Amsterdam, the ordered packed superhexagons. 1098 XH Amsterdam, The Netherlands; Email: P.Schall@ uva.nl arrange into a hexagonal close-packed lattice, while preserving the cubic or hexagonal ordering of the NCs inside. As shown in Authors Figure S16, the superhexagons appear in the center of the Yingying Tang − Institute of Physics, University of Amsterdam, deposited droplet, while single superballs are found along the 1098 XH Amsterdam, The Netherlands; orcid.org/0000- outer edge of the structure. Upon deposition of more material 0003-4603-9499 (larger or more drops on the substrate), the hyperstructure will Leyre Gomez − Institute of Physics, University of Amsterdam, cover larger areas. Another factor is the polydispersity of the 1098 XH Amsterdam, The Netherlands; Catalan Institute of starting droplets leading to size-sorting during the drying Nanoscience and Nanotechnology, CSIC, BIST, and CIBER- step.29,30 This can be addressed by using microfluidics to BBN, 08193 Bellaterra Barcelona, Spain generate uniform droplets. From the FFT images of the Arnon Lesage − Institute of Physics, University of Amsterdam, selected area, as shown in the inset of Figure 5a and Figure 1098 XH Amsterdam, The Netherlands; orcid.org/0000- S17, crystalline ordering can be identified from the appearance 0002-3102-3980 6002 https://dx.doi.org/10.1021/acs.nanolett.0c02005 Nano Lett. 2020, 20, 5997−6004 Nano Letters pubs.acs.org/NanoLett Letter Emanuele Marino − Department of Chemistry, University of (7) Dong, A.; Chen, J.; Vora, P. M.; Kikkawa, J. M.; Murray, C. B. Pennsylvania, 19104-6323 Philadelphia, Pennsylvania, United Binary nanocrystal superlattice membranes self-assembled at the States; orcid.org/0000-0002-0793-9796 liquid-air interface. Nature 2010, 466, 474−477. Thomas E. Kodger − Physical Chemistry and Soft Matter, (8) Saboktakin, M.; Ye, X. C.; Chettiar, U. K.; Engheta, N.; Murray, Wageningen University & Research, 6708 WE Wageningen, C. B.; Kagan, C. R. Plasmonic enhancement of nanophosphor The Netherlands upconversion luminescence in Au nanohole arrays. ACS Nano 2013, Janne-Mieke Meijer − Institute of Physics, University of 7, 7186−7192. (9) de Nijs, B.; Dussi, S.; Smallenburg, F.; Meeldijk, J. D.; Amsterdam, 1098 XH Amsterdam, The Netherlands Groenendijk, D. J.; Filion, L.; Imhof, A.; van Blaaderen, A.; Paul Kolpakov − Institute of Physics, University of Amsterdam, Dijkstra, M. Entropy-driven formation of large icosahedral colloidal 1098 XH Amsterdam, The Netherlands clusters by spherical confinement. Nat. Mater. 2015, 14, 56−60. Jie Meng − Department of Chemistry, Technical University of (10) Vanmaekelbergh, D.; van Vugt, L. K.; Bakker, H. E.; Rabouw, F. Denmark, DK-2800 Kongens Lyngby, Denmark; orcid.org/ T.; de Nijs, B.; van Dijk-Moes, R. J. A.; van Huis, M. A.; Baesjou, P. J.; 0000-0002-3813-5221 van Blaaderen, A. Shape-dependent multiexciton emission and Kaibo Zheng − Department of Chemistry, Technical University whispering gallery modes in supraparticles of CdSe/multishell of Denmark, DK-2800 Kongens Lyngby, Denmark; Department quantum dots. ACS Nano 2015, 9, 3942−3950. of Chemical Physics and NanoLund, Lund University, 22100 (11) Marino, E.; Kodger, T. E.; Wegdam, G. H.; Schall, P. Revealing Lund, Sweden driving forces in quantum dot supercrystal assembly. Adv. Mater. ζ Tom Gregorkiewicz − Institute of Physics, University of 2018, 30, 1803433. Amsterdam, 1098 XH Amsterdam, The Netherlands; (12) Dreyer, A.; Feld, A.; Kornowski, A.; Yilmaz, E. D.; Noei, H.; orcid.org/0000-0003-2092-8378 Meyer, A.; Krekeler, T.; Jiao, C. G.; Stierle, A.; Abetz, V.; Weller, H.; Schneider, G. A. Organically linked iron oxide nanoparticle SCs with Complete contact information is available at: exceptional isotropic mechanical properties. Nat. Mater. 2016, 15, https://pubs.acs.org/10.1021/acs.nanolett.0c02005 522−528. (13) Ushakova, E. V.; Cherevkov, S. A.; Volgina, D. -O. A.; Author Contributions Zakharov, V. V.; Komissarenko, F. E.; Shcherbakov, A. A.; Hogan, B. Y.T., L.G., E.M., T.K., T.G., and P.S. conceived the project and T.; Baldycheva, A.; Fedorov, A. V.; Nabiev, I. R.; Baranov, A. V. From designed the experiments. Y.T. prepared the samples and colloidal CdSe quantum dots to microscale optically anisotropic SCs analyzed the data. A.L. helped with the optical measurements through bottom-up self-assembly. J. Mater. Chem. C 2018, 6, 12904− and analyzed the data. J.-M.M. and Y.T. performed the 12911. modeling of the sharpness of the materials. P.K. performed the (14) Tong, Y.; Yao, E.-P.; Manzi, A.; Bladt, E.; Wang, K.; Döblinger, M.; Bals, S.; Müller-Buschbaum, P.; Urban, A. S.; Polavarapu, L.; surface tension experiment. J.M. and K.B.Z. performed the Feldmann, J. Spontaneous self-assembly of perovskite nanocrystals HRTEM measurements. All authors discussed the results and into electronically coupled SCs: toward filling the green gap. Adv. their interpretation. Mater. 2018, 30, 1801117. Notes (15) van der Burgt, J. S.; Geuchies, J. J.; van der Meer, B.; The authors declare no competing financial interest. Vanrompay, H.; Zanaga, D.; Zhang, Y.; Albrecht, W.; Petukhov, A. V.; ζ Filion, L.; Bals, S.; Swart, I.; Vanmaekelbergh, D. Cuboidal The author passed away on the seventh of July, 2019. ■ supraparticles self-assembled from cubic CsPbBr3 perovskite nano- crystals. J. Phys. Chem. C 2018, 122, 15706−15712. ACKNOWLEDGMENTS (16) Ghimire, S.; Takahashi, K.; Takano, Y.; Nakamura, T.; Biju, V. We are grateful for the Dutch Technology Foundation STW, Photon recycling by energy transfer in piezochemically synthesized The Netherlands Organization for Scientific Research (NWO), and close-packed methylammonium lead halide perovskites. J. Phys. and the Joint Solar Program (JSP III, 680-91-011) of The Chem. C 2019, 123, 27752−27758. (17) de Weerd, C.; Gomez, L.; Capretti, A.; Lebrun, D. M.; NWO for financial support. We acknowledge P. Koppejan, H. Matsubara, E.; Lin, J. H.; Ashida, M.; Spoor, F. C. M.; Siebbeles, L. D. Agrawal, M. van der Laan, C.-C. Huang, H. R. de Latour, E. A.; Houtepen, A. J.; Suenaga, K.; Fujiwara, Y.; Gregorkiewicz, T. Zheng, and D. van der Gon for their kind help. Efficient carrier multiplication in CsPbI3 perovskite nanocrystals. Nat. ■ REFERENCES (1) Kagan, C. R.; Murray, C. B. Charge transport in strongly coupled Commun. 2018, 9, 4199. (18) de Weerd, C.; Gomez, L.; Zhang, H.; Buma, W. J.; Nedelcu, G.; Kovalenko, M. V.; Gregorkiewicz, T. Energy transfer between quantum dot solids. Nat. Nanotechnol. 2015, 10, 1013−1026. inorganic perovskite nanocrystals. J. Phys. Chem. C 2016, 120, (2) Rainò, G.; Becker, M. A.; Bodnarchuk, M. I.; Mahrt, R. F.; 13310−13315. Kovalenko, M. V.; Stöferle, T. 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