Perspective pubs.acs.org/cm
Colloidal Halide Perovskite Nanoplatelets: An Exciting New Class of Semiconductor Nanomaterials† Mark C. Weidman, Aaron J. Goodman, and William A. Tisdale* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ABSTRACT: Metal halide perovskites are a family of semiconductor materials with exciting properties such as long charge carrier diffusion lengths, ease of synthesis and composition tunability, and remarkable defect tolerance. Recently, methods have been developed to synthesize metal halide perovskites in the form of colloidal nanosheetsor nanoplateletswhich are only a few unit cells in thickness and, as a result, experience the effects of strong dielectric and quantum confinement. This leads to narrow and blue-shifted absorption/emission as compared to the bulk stateallowing lead bromide and lead iodide nanoplatelets to cover the entire visible range. In contrast to bulk crystals, nanoplatelets exhibit strongly excitonic properties and enhanced radiative recombination. In this article we present an overview of colloidal perovskite nanoplatelets: how they are made, what their capabilities are, why 2D is beneficial, and where these materials are headed. We draw analogues to solid phase layered perovskites, cadmium selenide nanoplatelets, and 2D transition metal dichalcogenides to emphasize some of the most promising attributes of 2D materials such as their penchant for directional emission, fast/directional energy transfer, strong exciton binding energy, and reduced dielectric screening effects. We discuss the interesting physics present in these materials, remaining stability issues, and the future applications for nanoplatelets in LEDs, photovoltaics, photodetectors, and lasers.
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INTRODUCTION Metal halide perovskites are an interesting family of semiconductor materials. They have the chemical formula ABX3 where A is a metal or organic cation (+1 oxidation state), B is a metal ion (+2 oxidation state), and X is a halide ion (−1 oxidation state). The name “perovskite” specifies their unit cell arrangement, which is depicted in Figure 1a. Typically, the six halide ions (X) surrounding each metal ion (B) are drawn as octahedra. By changing the chemical identity of the constituent ions, it is possible to obtain a wide range of band gap values. For example, when A = methylammonium (MA) and B = lead (Pb), single crystal MAPbCl3 has a band gap of 2.88 eV while single crystal MAPbBr3 and MAPbI3 have band gaps of 2.18 and 1.51 eV, respectively.1,2 Furthermore, mixed halide compositions can be used as a means of achieving fine band gap tunability in these materials.3−6 Bulk metal halide perovskites have also received attention due to their facile synthesis, long charge carrier diffusion lengths, and low trap state density.7−10 Because of these properties, metal halide perovskites show excellent potential for LEDs, photovoltaics, and photodetectors.1,11−15 Most notably, perovskite photovoltaics have seen a boom in power conversion efficiency values from 3.8% to greater than 20% in a remarkably short time span (2009−2016).4,16−23 Recently, synthetic capabilities have been developed to controllably form colloidal perovskite nanocrystals and nano-
platelets. The nanocrystal work typically focuses on cubic nanocrystals, with side lengths of 4 nm or larger, like that depicted in Figure 1b.24−29 The use of long, coordinating ligands in the synthesis both controls nanocrystal size and imparts colloidal stability to the products. The nanocrystals exhibit photoluminescence quantum yields (PLQY) in the range of 50−90%, and their emission can be tuned to span the visible range of the spectrum. At this size, the nanocrystals may show signs of weak quantum confinement but typically have more bulk-like absorption and emission properties. On the other hand, perovskite nanoplatelets can be synthesized with thicknesses less than 4 nm, such that they exhibit the effects of strong quantum confinement. By adjusting some of the synthesis conditions for nanocrystals, it is possible to confine the nanoplatelet growth in one dimension, leading to laterally large 2D nanoplatelets which are only a few unit cells thick (Figure 1c). As a result, the nanoplatelets show thicknessdependent absorption and emission which is blue-shifted from that of the bulk phase by ∼0.6 eV. Akin to the bulk perovskite, the nanoplatelets can be represented by the formula L2[ABX3]n−1BX4. Here, the L species represents a ligand with +1 oxidation state, which both prevents growth in one direction of the nanoplatelet because of its inability to fit within the unit cell geometry and gives the nanoplatelet colloidal stability. The Received: April 4, 2017 Revised: May 8, 2017 Published: May 9, 2017
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This Perspective is part of the Up-and-Coming series. © 2017 American Chemical Society
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meV) even if the ensemble lateral dimensions are inhomogeneous.33−35 In Figure 2, we present an overview of perovskite nanoplatelets and their properties. Figure 2a shows depictions of some common nanoplatelet thicknesses, n = 1−3, highlighting how n represents the number of octahedra layers. Figure 2b shows experimental photoluminescence spectra for L2[MAPbBr3]n−1PbBr4 nanoplatelets, where n = ∞ represents the bulk phase, MAPbBr3. The effects of quantum confinement in the nanoplatelets can blue-shift the emission compared to bulk by as much 0.75 eV in this system (MAPbBr3). The general thickness-dependent trend for perovskite nanoplatelets is illustrated in Figure 2c.36 In Figure 2d−f, we show representative transmission electron microscopy (TEM), scanning electron microscopy (SEM), and fluorescence microscopy images of nanoplatelets. The image in Figure 2f illustrates how nanoplatelets can tolerate dispersity in the lateral dimensions while maintaining the same emission peak, as long as the nanoplatelet thickness is homogeneous. Another benefit of the anisotropic nanoplatelet geometry is the potential for directional effects. For instance, directional absorption/emission has been observed in CdSe based nanoplatelets/nanorods37−39 and single-layer MoS2.40 Taking advantage of this anisotropic emission could lead to significant improvements in LED efficiency by increasing the light outcoupling efficiency.41 CdSe nanoplatelets have also demonstrated extremely fast internanoplatelet energy transfer, on the order of 10 ps,42 and directed energy transfer could be realized in stacks of nanoplatelets. The few-unit-cell thickness of perovskite nanoplatelets also contributes to large exciton binding energies (a few hundred meV or more) and reduced dielectric screening effects, as observed in other 2D semiconductors including CdSe nanoplatelets,35 transition metal dichalcogenides (TMDs),43−47 and solid-phase layered perovskites.35 In turn, these properties could result in enhanced radiative recombination in perovskite nanoplatelets48 and fast energy transfer between nanoplatelets and neighboring particles.42,46
Figure 1. Representations of the different metal halide perovskite allotropes: (a) bulk perovskite unit cell and typical constituent ions, (b) cubic nanocrystal, and (c) nanoplatelet.
value of n represents the nanoplatelet thickness in number of metal halide layers, and n − 1 represents the thickness in terms of the number of complete unit cells. In this paper, we discuss the exciting properties and opportunities in colloidal perovskite nanoplatelets as well as some remaining challenges and potential future directions for this new class of semiconductor nanomaterials.
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EARLY DEVELOPMENTS The groundwork for colloidal perovskite nanoplatelets was established in the 1990s and focused on low-dimensional, layered perovskite materials.31,49−59 These studies synthesized thin film, quasi-crystal layered perovskite structures rather than individually dispersed nanoplatelets, but nevertheless there are many parallels between the two systems. These structures are typically formed from supersaturated ionic solutions or selfassembly during solvent evaporation. Recently, Dou et al. demonstrated the synthesis of atomically thin L2BX4 nanoplatelets, bridging the gap between layered perovskites and colloidal perovskite nanoplatelets.60 More recently, methods have been developed to synthesize colloidally stable perovskite nanocrystals with all dimensions of 4 nm or larger.24−29,61−63 In some of these synthetic recipes, nanoplatelets were also formed in addition to the desired nanocrystals. Our lab first identified and characterized these colloidal perovskite nanoplatelets in 2015 as a side product of MAPbBr3 nanocrystal synthesis, which appeared as additional blue-shifted peaks in the absorption and emission spectra.64 Soon after, Sichert et al. published a method of directly synthesizing L2[MAPbBr3]n−1PbBr4 based nanoplatelets with thickness control.32 Since that time, several other works have focused solely on the synthesis and characterization of
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WHY 2D? As discussed in the introduction, perovskite nanocrystals have been highly developed to have excellent tunability and emission properties. Thus, a pertinent question is why study 2D nanoplatelets? One main reason is to better understand the effects of quantum confinement in metal halide perovskites. The exciton Bohr radius is small in these materials approximately 2−3 nm.30−32 Fabricating cubic nanocrystals with such small side lengths is synthetically challenging, and therefore a more viable option is to make nanoplatelets in which only one dimension must adhere to this limitation. Furthermore, as the degree of quantum confinement is determined by a single dimension, if that parameter can be accurately controlled then excellent absorption and emission homogeneity can be achieved regardless of the lateral dimensions. This is analogous to cadmium selenide (CdSe) nanoplatelets, which exhibit extremely narrow absorption and emission features (fwhm < 10 nm) and small Stokes shifts ( 2 thickness nanoplatelets, but we found that adding excess ligand species (L) enabled selective formation of only n = 2 nanoplatelets. In Figure 4a we present the effects of varying the metal (B) and halide (X) species in nanoplatelets. For the B species we have explored Sn and Pb, and for X we have explored Cl, Br, and I. We used formamidinium as the A species and an equimolar butylammonium/octylammonium mixture for the L species throughout. Starting with the X species, we found that the n = 1 and n = 2 nanoplatelets of L2[FAPbCl3]n−1PbCl4 have peak absorption in the UV at 3.71 and 3.50 eV, respectively. Unlike their Br and I counterparts, the pure Cl nanoplatelets unfortunately did not produce measurable photoluminescence.70 By substituting bromide for chloride, the peak absorption/emission for the L2[FAPbBr3]n−1PbBr4 n = 1 and n = 2 nanoplatelets was shifted lower in energy to 3.12/3.08 eV and 2.86/2.82 eV, respectively. Once again, substituting iodide in place of bromide, the peak absorption/emission for the L2[FAPbI3]n−1PbI4 n = 1 and n = 2 nanoplatelets was shifted to 2.45/2.41 eV and 2.19/2.16 eV, respectively. Thus, by changing the halide, it is possible to go from deep UV absorption (chlorides) to violet/blue emission (bromides) to green/yellow emission (iodides). In a similar fashion, by substituting Sn for Pb, we see again that the peak absorption/emission for the L2[FASnI3]n−1SnI4 n = 1 and n = 2 nanoplatelets was shifted to 2.05/1.97 eV and 1.83/1.80 eV, respectively. Thus, by selection of the metal (B) and halide (X) it is possible to tune the absorption/emission of the n = 1 and n = 2 nanoplatelets to span the entire visible region of the spectrum. The absorption and emission spectra are narrow with emission fwhm values of 70−90 meV and Stokes shifts of 30−50 meV. In Table 1, we summarize the emission properties of the n = 1 and 2 nanoplatelets and compare these to their analogous bulk forms to highlight the effects of quantum confinement. While changes in B and X identity allow absorption and emission tuning over a wide range of wavelengths, the step size is fairly large. To access true fine tunability, we incorporated a strategy which has been thoroughly investigated in bulk perovskitesusing halide mixtures. We found that varying halide composition in 10% increments from 100% Cl to 100% Br to 100% I allows for extensive tunability in both the n = 1 and the n = 2 nanoplatelets.70 In our case, the nanoplatelets
Figure 3. Synthetic approaches for forming nanoplatelets: (a) solid state crystallization, (b) exfoliation, (c) hot-injection crystallization, and (d) nonsolvent crystallization. Methods (b−d) produce colloidally stable nanoplatelets.
range and in many cases have shown propensity for extensive out-of-plane stacking in TEM studies.65,66 Shamsi et al. showed that the lateral dimensions could be increased to a few micrometers by addition of shorter ligands (octylamine and octanoic acid) together with the longer ones during synthesis.69 The final method, which has been used by our group to produce a wide range of nanoplatelet compositions, is the nonsolvent crystallization shown in Figure 3d.32,62,64,70 In this case, the precursor salts are dissolved in a polar solvent, typically DMF, in a ratio according to the desired thickness. For instance, n = 1 nanoplatelets require an LX:BX2:AX ratio of 2:1:0, whereas n = 2 nanoplatelets require a 2:2:1 stoichiometry. In practice, we find an excess of the ligand species (LX) helps to prevent n > 2 nanoplatelets from forming. This mixture is then added dropwise to toluene. Toluene and DMF are miscible, but the precursor salts are not soluble in toluene and therefore the nanoplatelets crystallize upon mixing. 5022
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Figure 4. (a) Solution phase absorption (dotted lines) and photoluminescence (solid lines) spectra for n = 1 and n = 2 nanoplatelets in toluene, highlighting the changes which occur when the halide (X) is changed from Cl to Br to I and when the metal (B) is changed from Pb to Sn. (b) Solution phase absorption of L2PbX4 (n = 1) nanoplatelets in toluene showing continuous tunability as a function of the halide mixture composition. Reprinted with permission from ref 70. Copyright 2016 American Chemical Society.
accommodate greater stress or deviations in the unit cell geometry.77 Similar tunability is also accessible using this approach in n = 2 nanoplatelets.70 While emission efficiency was generally poor for the mixed halide n = 1 nanoplatelets, emission was detected from mixed halide n = 2 nanoplatelets with all Br/I ratios and from Cl/Br ratios of up to 50% Cl.70 In addition to the B and X species, we have also investigated the effects of the A species in nanoplatelets. As in the bulk perovskites, we found that the A species has a smaller effect on the absorption energy than either the B or the X species.4,36,57,79,86,87 In Figure 5a we present the absorption and emission from L2[APbBr3]PbBr4, n = 2, nanoplatelets where the A species was either formamidinium (FA) or methylammonium (MA). We see that substituting the slightly larger88 FA for MA causes a small red-shift in the absorption and emission energies by ∼15 meV. This is the same cation size-dependent trend observed in bulk systems. However, we found that the cation identity plays a more crucial role in other properties of the nanoplatelets. The fwhm of the emission peaks in both bromide and iodide nanoplatelets followed the trend of FA < Cs < MA. In particular, we found that MA generally leads to a more broadened emission peak than either FA or Cs. Furthermore, n = 2 nanoplatelets synthesized with Cs tended to evolve into thicker nanoplatelets more readily than those made with MA and FA. We attributed this to the small size of the cation, which may more easily enable postsynthesis structural rearrangement within the nanoplatelet. The most apparent effect of the cation species was how it influences the PLQY of the nanoplatelets. As shown in Figure 5b, the PLQY of L2[MAPbBr3]PbBr4 nanoplatelets is 6% while the PLQY of L2[FAPbBr3]PbBr4 nanoplatelets is 22%. This same trend holds in the case of iodide based nanoplatelets as well.70 Therefore, the choice of cation species is a promising strategy for maximizing radiative recombination in these materials and improving PLQY. As there are many potential
Table 1. Comparison of Bulk Perovskite Emission with Quantum Confined Nanoplatelet Emissiona emission formula
thickness, n
(nm)
(eV)
L2PbBr4 L2[APbBr3]PbBr4 APbBr3 [refs 64, 78−80] L2PbI4 L2[APbI3]PbI4 APbI3 [refs 4, 24, 81, 82] L2SnI4 L2[ASnI3]SnI4 ASnI3 [refs 83−85]
1 2 bulk 1 2 bulk 1 2 bulk
403 437 536 513 572 769 628 689 950
3.08 2.84 2.31 2.42 2.17 1.61 1.97 1.80 1.30
a The values in this table have been averaged over different cation compositions (Cs, MA, FA), when available.
were directly synthesized to have mixed halide composition by modifying the precursor compositions. In other studies, it has been shown that as-synthesized pure halide nanoplatelets can be easily exchanged in part or in full with other halides by exposure to an excess of the desired halide.65,66 In Figure 4b we present the results from directly synthesized n = 1 nanoplatelets with mixed halide compositions, showing that tuning spans the range from pure chloride to pure iodide. Overall, this is an energy range of 3.7 eV (chloride) to 2.45 eV (iodide). We note that the resulting absorption peak energy is fairly linear between the pure compositions,70 indicating that the nanoplatelets are indeed forming the expected compositions and not converging toward some thermodynamically favored state. This is in contrast to bulk perovskites, where it is found that crystallization is not possible in all proportions or favors specific compositions.4,75,76 We speculate that the increased compositional varieties accessible in nanoplatelets may be due to their reduced dimensionality, which could 5023
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Figure 5. Cation influence on electronic properties. (a) Absorption and photoluminescence of L2[MAPbBr3]PbBr4 and L2[FAPbBr3]PbBr4 in toluene, showing the slight changes in energy. (b) Image of the n = 2 samples under UV illumination, demonstrating the cationdependent PLQY. Reprinted with permission from ref 70. Copyright 2016 American Chemical Society.
cation candidates which can fit within the unit cell geometry,77,88 this is a clear path for optimizing the emissive properties while maintaining nearly the same emission energy.
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ELECTRONIC STRUCTURE AND PHOTOPHYSICS The transition from 3D bulk crystal to 2D nanoplatelet is accompanied by a dramatic change in the optical absorption spectrum. In Figure 6a we contrast the absorption spectrum of a MAPbBr3 polycrystalline film and the solution phase absorption of L2[MAPbBr3]PbBr4 nanoplatelets. In addition to the significant blue-shift which occurs due to quantum confinement effects in nanoplatelets, we observe a stark change in the absorption spectrum from one which is band-edge-like (bulk) to one which is highly excitonic (nanoplatelet). The sharp peak in the nanoplatelet absorption spectrum is indicative of strong electron−hole Coulomb interaction, which results from dielectric confinement in these atomically thin 2D materials.35,89 As illustrated in Figure 6b, the organic medium surrounding the nanoplatelet, which has a much smaller dielectric constant than the perovskite layer, is less effective in screening the electron−hole Coulomb interaction. This has the effect of increasing the exciton binding energy by an order of magnitude compared to the bulk phase. While 3D bulk perovskites are thought to have an exciton binding energy in the range 5−60 meV, experimental and theoretical evidence points to binding energies in the 200−500 meV range for 2D layered perovskites (measured by temperature-dependent optical absorption spectroscopy).31,89 These values are consistent with exciton binding energies in other 2D systems, including CdSe nanoplatelets35 and TMDs such as single-layer MoS2 and WS2.43,45,90 The strong electron−hole Coulomb interaction is also expected to increase the optical absorption coefficient and radiative decay rate in perovskite nanoplatelets relative to the corresponding bulk phase.87 Such strong light−matter inter-
Figure 6. Difference in electronic structure between bulk and nanoplatelets. (a) Comparison of the absorption spectra of a polycrystalline bulk MAPbBr3 thin film and a L2[MAPbBr3]PbBr4 colloidal suspension. (b) Illustration of how dielectric screening affects the electronic states differently in bulk crystals versus nanoplatelets.
actions are particularly attractive for photonic, nonlinear optical, and lasing applications. While it is difficult to disentangle increases in the radiative rate from the parallel trend of decreasing PLQY, the theoretically predicted trend of increasing radiative rate with decreasing thickness was recently reported in methylammonium lead iodide nanoplatelets (radiative rate of 25 ns for bulk and 5 nm for n = 1 nanoplatelets).48 Another consequence of dielectric confinement in 2D materials is polarization of the exciton transition dipole orientation within the plane of the nanoplatelet.40 In-plane dipole orientation can increase the optical coupling efficiency in planar optoelectronic devices such as solar cells and LEDs. Additionally, excitonic energy transfer can be exceptionally fast between 2D donors and acceptors. The Förster energy transfer rate scales as r−2 for nanoplatelets stacked face-to-face,91 where r is the separation distance, and energy transfer time constants as fast as 10 ps have been reported in other nanoplatelet systems.42 Interest in metal halide perovskite nanostructures is motivated, in part, by the ideal electronic properties exhibited by 3D bulk perovskites. Despite a prevalence of structural defects, 3D perovskite crystals exhibit a remarkably clean band gap characterized by an Urbach energy comparable to 5024
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Figure 7. Instability introduced to the colloidal nanoplatelet system in toluene by (a) exposure to ambient conditions and (b) exposure to UV light (∼10 mW). Reprinted with permission from ref 70. Copyright 2016 American Chemical Society.
crystalline silicon,87 with correspondingly long charge carrier diffusion lengths and low trap state densities.7−9 It has been suggested that low phonon and defect scattering rates in metal halide perovskites may result from the formation of large polaronslocal polarizations of the lattice surrounding mobile charge carriers.92 These lattice polarons, whose formation is facilitated by the rotational freedom and permanent dipole moment of the A site cation, screen the interaction of mobile charges with defects and polar longitudinal optical (LO) phonons, slowing down charge trapping processes and extending the lifetime of hot carriers.93 Paradoxically, the lattice polarization effect that is responsible for ideal optoelectronic behavior in 3D bulk crystals may also be responsible for low PLQY in the thinnest nanoplatelets. While thicker nanoplatelets (n > 3) and bulk-like nanocrystals routinely reach PLQY > 70%,24,48,66,68 the PLQY of n = 2 and n = 3 nanoplatelets is typically ∼5−20%,32,65,66,70 and n = 1 nanoplatelets typically exhibit PLQY < 1%.32,70 We propose that reduced dielectric screening effects are responsible for this trend: charges in thinner nanoplatelets are less effectively screened from scattering with defects and polar optical phonons due to incomplete formation of a lattice solvation spherei.e., a polaronleading to faster nonradiative recombination. This mechanism would explain the surprisingly large effect of the A cation on PLQY shown in Figure 5, as well as the precipitous drop in PLQY for n = 1 nanoplatelets, which are devoid of the A cation entirely.
process and one which can greatly impact properties like the PLQY of the products. Another hurdle for perovskite nanoplatelets is their tendency to evolve toward more bulk-like structures, thus losing their thickness-dependent quantum confinement. In Figure 7a we present the effects on nanoplatelets left as a colloid under ambient laboratory conditions (exposed to air and humidity). The nanoplatelets, as-synthesized, are L2[FAPbBr3]PbBr4 with single absorption and emission peaks. However, after 3 days in ambient conditions, the nanoplatelet emission has red-shifted to more bulk-like emission ranges, and the emission from n = 2 nanoplatelets is diminished. In the absorption spectrum, we see that the majority of absorption is still from n = 2 nanoplatelets, but a peak corresponding to n = 1 has also developed. The discrepancy between the absorption and emission in the aged sample may indicate fast energy transfer between remaining n = 2 nanoplatelets and bulk-like particles but requires further experimental investigation. Imaging these samples with TEM clearly shows that the nanoplatelets have grown in lateral dimensions and the increased contrast in the aged sample is an indication that nanoplatelet thickness had also increased. For these experiments we focused on lead-based nanoplatelets as the tin-based nanoplatelets showed extremely poor stability and degraded within minutes of being exposed to ambient conditions.70 Additionally, we find that the nanoplatelets are sensitive to high intensities of UV light, as has been observed by others.95 We tested this by monitoring the photoluminescence from L2[MAPbBr3]PbBr4 nanoplatelets in toluene during exposure to ∼10 mW of UV light (Figure 7b). Initially, the photoluminescence (438 nm) indicated the presence of only n = 2 nanoplatelets; however, after a few minutes of UV exposure an additional peak at 455 nm (n = 3) began to emerge. This was followed by a peak at 475 nm (n = 4) and then by one at 489 nm. The relative intensities of these peaks indicate that the nanoplatelets are progressively getting thickereither by coalescence of nearby nanoplatelets or by structural rearrangement of individual nanoplatelets. This behavior may be related to recent observations of enhanced ion diffusion in bulk metal halide perovskites under visible or UV photoexcitation.96,97 De Roo et al. recently published a comprehensive study of ligand binding dynamics in CsPbBr3 nanocrystals using solution 1 H nuclear magnetic resonance (NMR).98 While their work was performed on bulk-like cubic nanocrystals, their findings should extend to surface ligand dynamics in perovskite nanoplatelets as well. Ligand binding to the perovskite nanocrystal surface was
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STABILITY One of the main challenges facing perovskite photovoltaics is their poor stability when exposed to air and humidity. Perovskite nanocrystals/nanoplatelets have the potential to overcome some of these issues due to the ligands which passivated their surfaces.94 However, their colloidal nature can also be problematic. As in other colloidal syntheses, it is often desirable to precipitate the nanocrystals or nanoplatelets to separate them from the growth solution. This is typically done by adding a polar antisolvent to destabilize the nanoparticles in their nonpolar colloidal solvent. However, with perovskite nanomaterials, precaution must be taken that this polar antisolvent does not redissolve the ionic perovskite nanomaterials or needlessly introduce water, which may be dissolved in the antisolvent.66 To combat this, Bekenstein et al. have proposed the use of antisolvents with low water solubility, such as ethyl acetate and methyl−ethyl ketone.66 Still, the purification of perovskite nanomaterials remains a delicate 5025
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Figure 8. Examples of two emerging applications for quantum confined perovskites: light-emitting diodes and photovoltaics. Reprinted with permission from ref 102. Copyright 2017 American Chemical Society. Adapted by permission from ref 103. Copyright 2016 Macmillan Publishers Ltd.
strong quantum confinement regime in nanoplatelets. This could be especially useful in the blue region of the spectrum,102 where LED efficiency is typically low. Additionally, we believe that the anisotropy of nanoplatelets could lead to improved outcoupling of light and to boost efficiency as compared to isotropic emitters.41 Like perovskite nanocrystals, nanoplatelets may also show a propensity for lasing and photodetection applications.99,108−110 Perovskite nanoplatelets are also an interesting material from a more fundamental perspective. Their excitonic nature and low dielectric screening/strong exciton binding energy could lead to fast or directional exciton dynamics.42,46 Their narrow absorption, emission, and small Stokes shift make them an ideal material for studying strong coupling and light−matter interactions. As the nanoplatelets are
