One-Step Preparation of Cesium Lead Halide CsPbX3 (X = Cl, Br, and

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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One-Step Preparation of Cesium Lead Halide CsPbX3 (X = Cl, Br, and I) Perovskite Nanocrystals by Microwave Irradiation Huiwen Liu,† Zhennan Wu,† Hang Gao,† Jieren Shao,† Haoyang Zou,† Dong Yao,*,† Yi Liu,† Hao Zhang,*,†,‡ and Bai Yang† †

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China ‡ Nanjing Haiyan Electric Technology Co. Ltd., Nanjing 211500, P. R. China S Supporting Information *

ABSTRACT: CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) are competitive emitting materials for illumination and display because of their outstanding photophysical properties. However, the conventional synthetic approaches suffer from low yields, complex procedures, and toxic chemicals. In this work, we demonstrate a one-step microwave-assisted approach to prepare CsPbX3 NCs. The homogeneous heating and rapid temperature increment of microwave preparation facilitate the growth of CsPbX3 NCs, producing the NCs with high photoluminescence quantum yields up to 90%, narrow emission full-width at half-maximum, and emission color tunable from blue to red. By optimizing the preparation conditions of the microwaveassisted approach, CsPbX3 NCs with cation- and halide anion-controlled emission properties, tunable reaction rate, and enhanced stability are prepared. Light-emitting diode (LED) prototypes are further fabricated by employing the as-prepared CsPbX3 NCs as the color conversion materials on commercially available 365 nm GaN LED chips. KEYWORDS: perovskite nanocrystals, CsPbX3, microwave preparation, full-color emission, light-emitting diode

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INTRODUCTION Perovskite nanocrystals (NCs) are potential emitting materials for illumination and display applications, owning to the low preparation cost, high photoluminescence quantum yields (PLQYs), narrow emission full-width at half-maximum (fwhm), and tunable PL emission across the entire visible spectral region.1−8 As highly competitive alternatives to conventional semiconductor quantum dots (QDs), perovskite NCs have been successfully employed for fabricating high performance light-emitting diodes (LEDs).9−20 For example, organic−inorganic hybrid perovskites (CH3NH3PbX3 (X = Cl, Br, and I)) have been used in LEDs, which show an external quantum efficiency over 8%.21−23 However, the hybrid perovskites are susceptible toward oxygen and moisture, meeting the challenge of poor stability.24−27 Accordingly, allinorganic perovskite CsPbX3 (X = Cl, Br, and I) NCs are emerging as a new class of metal halide perovskite NCs with higher stability than the hybrid ones.28−30 They exhibit improved photophysical properties, such as composition- and size-controlled PL emission, high PLQYs, narrow emission fwhm, and short radiative lifetimes, comparable and even superior to the conventional metal-chalcogenide QDs.31−34 Enormous efforts have been devoted to optimizing the preparation techniques of CsPbX3 NCs. The chemical vapor deposition (CVD) method has been employed for epitaxial growth of single-crystal CsPbX3 nanowires on crystalline substrates with improved crystal quality.35,36 However, the © XXXX American Chemical Society

relatively high reaction temperature and difficulty in engineering morphology hinder the NC preparation with various shapes, sizes, and dimensions.33,34 Thus, a variety of solution chemical approaches have been developed for the preparation of CsPbX3 perovskite NCs with controlled shapes and sizes, including QDs, nanowires, nanoplates, and so forth.28,30,34,37−40 These approaches produce the CsPbX3 NCs dispersible in solution and are able to systematically tune the anion composition by anion exchange, making the PL emission of NCs tunable in the whole visible region.28,34,37,38 The solution preparation of CsPbX3 NCs usually includes two separate stages, seed-mediated nucleation and the followed growth via oriented attachment and self-assembly, which can be flexibly controlled by regulating the experimental parameters, such as solvents, capping ligands, the species of cation and anion, reaction temperature, and so forth.28,31 According to the reaction temperature, CsPbX3 NCs can be prepared at both room temperature and high temperature.28,30−32 For the hightemperature preparation, the hot-injection route is the most widely used and successful method;28,30 however, the dramatically rapid growth of CsPbX3 NCs makes it hard to seize the growth process,31,33 while the microfluidic technology with rapid and controlled mass transport permits us to perform Received: September 27, 2017 Accepted: November 24, 2017

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DOI: 10.1021/acsami.7b14677 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a−i) TEM images of the as-prepared CsPbX3 (X = Cl, Br, I) NCs that are prepared with microwave treatment for 20 (a, CsPbCl3), 15 (b, CsPbCl2Br1), 15 (c, CsPbCl1Br2), 10 (d, CsPbBr2I1), 10 (e, CsPbBr1.5I1.5), 8 (f, CsPbBr1.2I1.8), 8 (g, CsPbBr1I2), 8 (h, CsPbI3), and 10 min (i, CsPbBr3). Inset in (i): the HRTEM image of the CsPbBr3 NCs. (j) XRD patterns of the as-prepared CsPbX3 NCs, which show the diffraction of the monoclinic phase.

Microwave-assisted preparation is a very promising methodology to achieve high quality NCs, by virtue of the heating effect created by the interaction of the dipole moment of the liquid molecules with the high frequency electromagnetic radiation.41−45 It outweighs other methods in terms of enhanced reaction rates and yields with homogeneous heating because the homogeneous heating reduces the thermal gradient effects in the whole volume.44,45 Microwave-assisted preparation has encompassed a variety of NCs, such as metals (Au, Ag), metal oxides (FeO, Mn2O3, and CoO), semiconductors (CdS, CdSe, CuInS2, and PbSe), and so forth.41−45 Recently, microwave irradiation has been employed to promote the crystallization of hybrid perovskite films and micrometer-sized particles,46,47 showing the potentials for microwave-assisted preparation of nanometer-sized CsPbX3 NCs. Most recently, Long et al. reported the high-throughput and tunable preparation of colloidal CsPbX3 perovskite NCs by microwave irradiation.39 Full-color emission was achieved by the anionexchange process. By inheriting from the conventional hotinjection approach, the microwave-assisted reaction was conducted in a sealed system, before which the solvents and stabilizers were also pretreated and degasssed.39 Pan et al. also realized microwave-assisted preparation of high-quality allinorganic CsPbX3 (X = Cl, Br, I) perovskite NCs with full-color emission, where the precursors need to be dissolved and heated.40 In this scenario, CsPbX3 NCs with extremely fast and uncontrollable crystallization rate perfectly match the rapid and homogeneous temperature elevation of microwave-assisted

ultrafast kinetic measurements and further reaction optimization.38 It requires only small amounts of reagents but a few hours of reaction and yields the information equivalent to the information reached in 200−1000 conventional hot-injection preparations.38 Overall, although high-temperature preparation is capable to produce uniform NCs with a high degree of monodispersity, this method is tedious under an inert atmosphere, complicated in preparation device, difficult to perform large-scale preparation, and time-consuming.33,34,37 These disadvantages are considered to be solved by roomtemperature preparation because of the ionic crystal feature of perovskites, which may be free from vacuum and inert environment. In this scenario, a method named roomtemperature supersaturated reprecipitation utilizing inorganic ions transferring from polar into nonpolar organic solvents is developed, though the polar solvents can severely degrade CsPbX3 NCs and further reduce the NC yields.29 Aimed at polar-solvent-free and large-scale preparation, high-quality CsPbX3 NCs can be prepared through an ultrasonicationassisted method.37 By direct ultrasonication of the precursors in the presence of organic ligands, both the halide composition and the size of CsPbX3 NCs can be tuned. Despite the great successes, the current preparation approaches still meet several challenges, including low yields, complex procedures, toxic chemicals, and extensive energy consumption.33 Novel preparation methods that are capable to produce highly luminescent CsPbX3 NCs in a reproducible and cost-effective way are highly desired. B


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Figure 2. (a) Photographs of the colloidal dispersions of CsPbX3 NCs with different halide (X = Cl, Br, and I) composition in hexane under room light (top) and UV light (bottom, λ = 365 nm). (b) Corresponding UV−vis absorption and PL emission spectra of the samples shown in (a). (c) PL lifetimes of the NCs shown in (a).

The products are centrifuged and washed with hexane. The size and morphology of the as-prepared CsPbX3 NCs are characterized by transmission electron microscopy (TEM), which clearly shows monodisperse NCs with well-defined cubic and rectangular shapes (Figure 1). The average crystal sizes are in the range of 15.1−23.7 nm for different CsPbX3 NCs. The as-prepared CsPbBr3 NCs are rather monodisperse with an average size of 18.8 ± 2.1 nm, which are slightly larger than the NCs (ca. 11 nm) prepared by a conventional hot-injection method (Figure S1).28,30 High-resolution TEM (HRTEM) observation reveals that the as-prepared NCs have legible crystal lattices with an interplanar distance of 4.10 Å (Figure 1i), which consists of the (−1,1,0) plane of the monoclinic bulk CsPbBr3 (PDF#18-0364). The presence of Cs, Pb, and X in the CsPbX3 NCs is confirmed by energy-dispersive X-ray spectroscopy (EDX), which is also close to the typical stoichiometric ratio of all-inorganic perovskite (Figure S2). XPS is performed to investigate the valence states of the constituent elements in the as-prepared CsPbX3 NCs (Figure S3). The Cs 3d spectrum shows two symmetric peaks at 738.8 and 724.9 eV with a peak splitting of 13.9 eV, indicating Cs(I) (Figure S3).49,50 The Pb 4f peaks at 137.9 and 142.8 eV with a separation of 4.9 eV indicate Pb(II) (Figure S3).49,50 In terms of the valence states of Cs and Pb from CsPbX3 NCs with different composition, no significant difference is observed. The asymmetric peaks in Figure S3c and S3d and the two narrow peaks in Figure S3e indicate the halogen X (Cl, Br, I) binding energy for the lattice X−, respectively.49,50 The X-ray diffraction (XRD) patterns further confirm that the NCs possess the crystalline structure of monoclinic bulk CsPbBr3 (PDF#18-0364) (Figure 1j). When the halide composition changes from Cl to Br and I, the XRD peaks shift gradually to small angles, which is attributed to the increased radii of halide ions from Cl to Br and I.34,37 To further compare the crystallization of the CsPbX3 NCs from different preparation methods, CsPbX3 NCs with the same composition are prepared at the reaction temperature of 150

preparation. It will not only promote the nucleation and growth of high quality NCs but also reduce time and energy consumption. In addition, the wide size distribution caused by thermal gradient effects and inhomogeneous heating can be avoided. Inspired by the rapid progress in CsPbX3 NC preparation,48 the microwave-assisted approach is potentially optimized by systematically investigating the preparation conditions. In this work, we demonstrate that all-inorganic CsPbX3 (X = Cl, Br, and I) NCs can be prepared through a versatile, polarsolvent-free, and single-step microwave-assisted approach. The microwave-assisted preparation is featured with its simplicity as all the precursors are simply mixed and microwave treated in an open-air system where neither predissolving nor pretreating is required. The microwave-assisted preparation also exhibits the flexibility in composition control. Aside from halide anions, the cation composition is also controllable, such as the preparation of Mn-doped CsPbCl3 NCs. The as-prepared CsPbX3 NCs possess good emission properties, including the tunable PL emission from 410 to 691 nm, narrow fwhm, and high PLQYs up to 90%.

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RESULTS AND DISCUSSION Preparation of CsPbX3 NCs through the MicrowaveAssisted Method. As mentioned in the Experimental Section, CsPbX3 (X = Cl, Br, I, and mixed Cl/Br, Br/I) NCs are prepared through a microwave-assisted method. Typically, cesium acetate (CsOAc) and lead halide (PbX2) are mixed in a 50 mL beaker containing 1-octadecene (ODE), bis(2,4,4trimethylpentyl) phosphinic acid (TMPPA), and oleylamine (OLA). Then, the beaker is placed in a microwave oven and heated for a specific duration depending on the species of PbX2 (X = Cl, Br, and I). CsOAc and TMPPA are used as the Cs precursor and organic ligand, respectively, which are different from the conventionally used cesium carbonate (Cs2CO3) and oleic acid (OA),28,30 and the benefits will be discussed later. C


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ACS Applied Materials & Interfaces Table 1. Summary of the Optical Properties of CsPbX3 NCs with Different Halide Compositions molar feed ratio

element composition

FHWM (nm)

PLQY (%)

UV absorption peak position (nm)

PL emission peak position (nm)

CsPbCl3 CsPbCl2Br1 CsPbCl1Br2 CsPbBr3 CsPbBr2I1 CsPbBr1.5I1.5 CsPbBr1.2I1.8 CsPbBr1I2 CsPbI3

CsPbCl2.8 CsPbCl1.8Br1.2 CsPbCl0.7Br2.3 CsPbBr2.7 CsPbBr1.9I1.1 CsPbBr1.3I1.7 CsPbBr1.1I1.9 CsPbBr0.9I2.1 CsPbI2.8

14 16 15 17 20 27 32 34 35

7 19 75 90 45 40 50 78 70

404 455 495 508 545 556 579 635 684

410 458 502 517 552 568 603 648 691

°C via a conventional hot-injection method. These NCs possess a single tetragonal phase with well-defined cubic shapes (Figure S4).51,52 The HRTEM image reveals legible crystal lattices with an interplanar distance of 3.96 Å, which is consistent with the (101) plane of the tetragonal bulk CsPbCl3 (Figure S4).52 For the CsPbX3 NCs, monoclinic is a metastable structure, the formation of which is thermodynamically controlled. In the microwave-assisted preparation, the reaction temperature is rather low that the total energy is insufficient to overcome the barrier to crystallizing into the tetragonal phase as can be obtained in the hot-injection approach. The existence of the stable metastable phase at low temperature is attributed to the capping ligands (OLA and TMPPA) on the surface of the asprepared NCs, which lower the surface energy.51 The as-prepared CsPbX3 NCs show bright emission and high PLQYs up to 90%. The PL emission can be tuned to cover the entire visible spectral range (from 410 to 691 nm) by adjusting the composition of anions (Figure 2a). The color change observed upon varying the halide content is quantified by UV− vis absorption and PL emission spectra (Figure 2b). The asprepared CsPbX3 solution exhibits narrow, single-peak PL emission with the fwhm of 14−35 nm (Figure 2b). The UV− vis absorption spectra show single and steep absorption peaks, and the emission spectra exhibit small Stokes shifts. These observations clearly show that the products are composed of the NCs with single halide composition rather than varying halide contents.37 The optical band gap energy can be tuned from 410 to 691 nm, across almost the entire visible range, by adjusting the halide composition. Moreover, the samples containing only Br or I exhibit high PLQYs over 90%, whereas the mixed halides CsPbBrxCl3−x (x ≥ 2) indicate considerably low PLQYs of only 10−19% (Table 1). In addition, the colloidal dispersion of CsPbX3 NCs appears to be stable within two months only with a slight reduction in the PLQYs (Table S1). Although the CsPbX3 NCs from our microwave-assisted method and conventional hot-injection method crystallize into different phases, their optical properties, including PL emission peak position, fwhm, and PLQYs, have no obvious difference (Figure S5 and Table S2). Time-resolved PL measurements of CsPbX3 NCs reveal multiexponential decay traces with average lifetimes in the range of 1.9−135.7 ns and with an inverse correlation between the halide-ion-controlled band gap and the decay lifetimes (Figure 2c). The PL decay times combined with the PLQYs show that the I- and Br-containing NCs are of high optical quality with nearly no nonradiative decay, whereas the NCs with a high Cl content undergo significant nonradiative decay. The lower PLQYs of CsPbI3 and CsPbCl3 NCs than that of CsPbBr3 NCs and the tendency of decay rates are ascribed to the intrinsic optical properties of the halide perovskites.33,34,37

Effect of the Cs Precursor. The one-step microwaveassisted preparation is featured with its simplicity as all the precursors are simply mixed and microwave treated. No precursor predissolving or preheating is required. In this context, the solubility of the raw materials appears to be crucial for determining the quality of the as-prepared CsPbX3 NCs. Herein, to increase the solubility and reactivity of the precursor, CsOAc is chosen other than conventionally used Cs2CO3.28,51 As compared in Figure S6, CsOAc is completely soluble in OA and TMPPA at room temperature, generating a transparent and stable solution. In contrary, the Cs2CO3 precursor solution is turbid and insoluble.28,51 Since acetic acid is more acidic than carbonic acid, correspondingly, it is easier for CsOAc than Cs2CO3 to coordinate with an organic ligand and form soluble Cs−ligand complex monomers. As the cation precursor, CsOAc can release much more soluble monomers, leading to supersaturation, nucleation, and growth of NCs.53,54 Enough monomer supply not only accelerates the nucleation and growth process but also decreases the defects of the NCs and improves the optical properties of NCs.53−56 To further clarify that the difference of the Cs precursor indeed influences the reaction rates and final properties of the as-synthesized NCs in microwave preparation, CsPbBr2I1 NCs are taken as an example. As shown in Figure 3a, strong luminescent CsPbBr2I1

Figure 3. (a) PL images of the colloidal dispersions of CsPbBr2I1 NCs that are prepared using different Cs precursor (CsOAc and Cs2CO3) with the microwave treatment for 10, 20, 30, and 40 min. The corresponding UV−vis absorption (b) and PL emission spectra (c) of the NC dispersions with the microwave treatment for 40 min.

NCs are obtained under 20 min microwave irradiation with CsOAc as the precursor, while the luminescence of the NCs synthesized with Cs2CO3 as the precursor is much weaker. Despite the difference in the emission intensity, the UV−vis absorption spectra of the NCs prepared using different Cs precursor have no obvious difference (Figure 3b), suggesting D


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prepared NCs exhibit the band-edge absorption in the nearinfrared region, which is close to the α-CsPbI3 bulk optical bandgap of 1.73 eV (Figure 4c).57 Importantly, the absorption and emission properties of TMPPA-capped NCs are well preserved after one-month storage under ambient conditions (Figure S9). It has been demonstrated that the removal of excess OA and OLA-based precursors from the solution and in particular free from OA could be conducive toward enhancing phase stability57 since OA binds tightly to the as-prepared NCs and reveals etching effects on the NCs’ surface.48,57 Notably, TMPPA exists in an ion pair with excess OLA in the solution, instead of binding to the surface of NCs, which ensures that OLA and/or I-OLA are the predominant surface ligands.48 This surface chemistry directly associates with the optical property and phase stability of CsPbI3 NCs. Effect of Microwave Power and Duration. Since the parameters of microwave source kinetically determine the NC growth process during microwave-assisted preparation, the influences of microwave power and duration are also studied. As shown in Figure S10, the delayed microwave time exhibits no obvious influence on the morphology and PL emission properties under the same microwave power. When it comes to the microwave power with fixed duration, the morphology and emission properties of the as-prepared CsPbBr3 NCs also remain unchanged (Figure S11). We ascribe this phenomenon to the rapid nucleation and fast growth of the perovskite NCs with the collision of Cs-, Pb-, and X-ions in one pot.28,31,33 According to the relationship between microwave power and duration (Figure S12), the optimized preparation parameters with the least energy consumption can be established. For CsPbBr3 NCs, the microwave power is 800 W, and the duration is 8 min. Preparation of Mn-Doped CsPbCl3 NCs via the Microwave-Assisted Method. To prove that the microwave-assisted method is versatile and easy processing, Mndoped CsPbCl3 NCs are further prepared. By fixing the microwave power at 800 W, the influence of Pb-to-Mn molar feed ratio is studied. With the decrease of Pb-to-Mn molar feed ratio from 1:2.5, 1:5, 1:10, to 1:15, the size of as-prepared NCs almost remains the same (Figure 5). EDX analysis reveals the composition of the as-prepared NCs as CsPb0.93Mn0.07Cl3, CsPb0.90Mn0.10Cl3, CsPb0.79Mn0.21Cl3, and CsPb0.92Mn0.08Cl3, respectively (Figure S13). XRD patterns indicate that the products consist of the crystal structure of tetragonal CsPbCl3 (PDF #18-0366) (Figure 5g). Their UV−vis absorption and PL emission peaks are almost fixed because of the similar size with the decrease of Pb-to-Mn molar feed ratio (Figure 5e, f). As to the emission intensity, an obvious intensity increase is found for the broad emission around 589 nm when reducing the Pb-toMn molar feed ratio from 1:2.5 to 1:10, attributed to MnIIrelated electron coupling enhancement.58,59 When further reducing the Pb-to-Mn molar feed ratio to 1:15, the excess Mn is capable to form Cs−Mn−Cl precursors, which is not beneficial to Mn doping, thus leading to the lower Mnsubstitution ratio and PL intensity.58 The maximum PLQY belongs to the CsPb0.79Mn0.21Cl3 NCs, which is 26%. This result means that the preparation of CsPbCl3 NCs through the microwave-assisted method is facile both for halide anion and cation control. LEDs from CsPbX3 NCs. Because the as-prepared CsPbX3 NCs through the microwave-assisted method possess high PLQYs and narrow fwhm, they are tested as the color conversion materials for fabricating LEDs. After mixing with

that there is no fundamental difference during the nucleation and crystallization processes.51 It should be mentioned that the kinetics of nucleation and growth of NCs are determined by the properties of the precursor solubility.55 An enhanced precursor solubility promotes the precursor consumption by growth, which shortens the nucleation and thus enhances its formation rate. Totally precursor-related reaction kinetics can play an important role in the resultant crystal formation rates and PL properties.56 In this regard, the CsOAc can serve as a reliable Cs precursor with added benefits such as easy manipulation and more compatibility with various temperature, which is more suitable for microwave-assisted preparation. Effect of Organic Acids. To the best of our knowledge, the species of organic ligands greatly determines the quality and stability of the as-prepared CsPbX3 NCs.48,57 Most recently, TMPPA is used as the capping ligand instead of the conventionally used OA to prepare more stable CsPbI3 NCs.48 Inspired by this method, TMPPA is used to explore the effect of organic acids and further optimize the reaction process. The first advantage of TMPPA is attributed to the capability to form clear and transparent Cs-TMPPA in ODE at room temperature, which makes it easy to hold and store the Cs precursor. The second advantage is the improvement of the stability of I-containing perovskite NCs because TMPPAcapped CsPbI3 NCs retain the stable α-phase in both crude and washed solution (Figure S7). A comparison of the PL images of TMPPA- and OA-capped CsPbI3 NCs over a period of microwave treatment is shown in Figure 4a. It clearly shows

Figure 4. (a) PL images of the colloidal dispersions of CsPbI3 NCs that are prepared using different organic acid ligand (OA and TMPPA) with the microwave treatment for 8, 16, and 24 min. (b) XRD patterns show the diffraction of the monoclinic phase (PDF #180364) and orthorhombic phase (PDF #18-0376) CsPbI3 NCs using TMPPA and OA, respectively. (c) UV−vis absorption spectra of the CsPbI3 NCs that are prepared using TMPPA and OA.

that the PL intensity of OA-capped CsPbI3 NCs gradually declines and even quenches, simultaneously with yellow precipitates, presenting the formation of δ-CsPbI3.48,57 The XRD pattern further confirms that the products possess an orthorhombic phase (PDF #18-0376) (Figure 4b).57 In contrast, the solution of TMPPA-capped CsPbI3 NCs is rather stable. Strong red emission appears after 8 min microwave treatment and is maintained with prolonged microwave treatment (Figure 4a). Compared to OA-capped CsPbI3 NCs, TMPPA-capped ones show an obvious redshift of the emission spectrum and much narrower fwhm (Figure S8). The asE


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Figure 5. TEM images of the as-prepared Mn-doped CsPbCl3 NCs with Pb-to-Mn molar feed ratio of 1:2.5 (a), 1:5 (b), 1:10 (c), and 1:15 (d). Insets: the corresponding PL images excited by 365 nm UV light. UV−vis absorption spectra (e), PL emission spectra (f), and XRD patterns (g) of the Mn-doped CsPbCl3 NCs that are prepared with Pb-to-Mn molar feed ratio of 1:2.5, 1:5, 1:10, and 1:15.

Figure 6. Fluorescent images (a−d) and the corresponding emission spectra (e−h) of the LEDs from the as-prepared CsPbX3 (X = Cl, Br, I) NCs with the emission centered at 480 nm (a, e, CsPbCl2Br1), 536 nm (b, f, CsPbBr3), 578 nm (c, g, CsPbBr1.5I1.5), and 699 nm (d, h, CsPbBr1I2). Insets in (a−d): Optical images of the LEDs.

curable resin, CsPbCl 2Br1, CsPbBr3 , CsPbBr1.5 I1.5, and CsPbBr1I2 NCs are coated onto commercially available GaN LED chips with 365 nm excitation and cured under UV light for 2 min, respectively (see Experimental Section). Photographs of the LED devices with blue, green, yellow, and red emissions are shown in Figure 6a−d, and the corresponding emission spectra are shown in Figure 6e−h. The LEDs exhibit bright PL emission with the color coordinate of (0.15, 0.26), (0.37, 0.61), (0.47, 0.42), and (0.68, 0.30), respectively. For the performance parameters of blue, green, yellow, and red LEDs, the maximum external quantum efficiencies (EQEs) are determined to be 0.09%, 0.17%, 0.12%, and 0.10% (Table S3). The relatively low EQE is attributed to the low quality of the commercially available 365 nm GaN LED chips because the EQE of the GaN LED chip is tested to be only 0.23% with relatively high current

of 199 mA (Table S3). The quality of the LED chips also limits the current efficiency and power efficiencies. The performance of the LED devices is expected to improve further if better LED chips are applied.

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CONCLUSIONS

In summary, we demonstrate a single-step microwave-assisted method for preparing highly luminescent CsPbX3 (X = Cl, Br, and I) NCs with controlled composition and tunable emission properties across the visible region (400−700 nm). The microwave-assisted method can achieve rapid reaction rate and high yields by supplying homogeneous heating, which is an efficient way for better nucleation and growth of NCs with less time and energy consumption. By optimizing the experimental parameters, high quality CsPbX3 NCs with the fwhm of 14 nm F


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microscope (Hitachi) with a charged coupled device camera with a 200 kV acceleration voltage. High-resolution TEM (HRTEM) images were recorded using a JEM-2100F electron microscope (Jeol) at 200 kV. Elemental analysis is characterized by an energy-dispersive X-ray spectroscopy (EDX) detector coupled with an XL30 ESEM FEG scanning electron microscope (FEI). Inductively coupled plasma was performed with an OPTIMA 3300 DV analyzer (PerkinElmer). X-ray diffraction (XRD) was carried out on an X-ray diffractometer (Rigaku) using Cu K radiation (λ = 1.5418 Å). X-ray photoelectron spectroscopy (XPS) was investigated using a VG ESCALAB MKII spectrometer with a Mg Kα excitation (1253.6 eV). Binding energy calibration was based on C 1s at 284.6 eV. Fourier transform infrared (FTIR) spectra were performed with a Nicolet AVATAR 360 FTIR instrument. Edinburgh FLS 920 (excited at 365 nm) equipped with an integrating sphere was used for the measurement of the absolute QYs of QDs. A FLS 980 spectrofluorometer was used for the characterization of the luminescence decay curves. The spectra of the LEDs were measured by an integrating sphere and a computer-controlled direct-current power configured Spectra scan PR-650 spectrophotometer under ambient condition at room temperature. The CIE (Commission Internationale de L’Eclairage 1931) calorimeter system was used for the identification of the color of the light.

and PLQY over 90% are prepared. Besides halide anion control, this polar-solvent-free microwave-assisted method can also perform cation control, such as the preparation of Mn-doped CsPbCl3 NCs with controlled Mn content. LED prototypes are further fabricated using the as-prepared NCs as color conversion materials. Since the microwave-assisted method can produce highly luminescent CsPbX3 NCs through a simple and cost-effective way, the current work may promote the practical applications of all-inorganic perovskite NCs in illumination and display.

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EXPERIMENTAL SECTION

Materials. CsOAc (cesium acetate, 99%), PbCl2 (lead(II) chloride, 98%), PbBr2 (lead(II) bromide, 98%), PbI2 (lead(II) iodide, 99%), 1octadecene (ODE, technical grade, 90%), oleic acid (OA, technical grade, 90%), and oleylamine (OLA, technical grade, 90%) were purchased from Sigma-Aldrich. MnCl2 (manganese(II) chloride, 99.9%) was purchased from Aladdin Industrial Corporation, China. Bis(2,4,4-trimethylpentyl) phosphinic acid (TMPPA, 85%) was purchased from Adamas Reagent Co., Ltd. Hexane and ethyl acetate were purchased from Beijing Chemical Reagent Ltd., China. All chemicals were used as received. Preparation of CsPbX3 Perovskite NCs. In a typical preparation, 5 mL of ODE together with 0.5 mL of TMPPA and 0.5 mL of OLA (additional 0.5 mL of TMPPA and 0.5 mL of OLA were added for CsPbCl3 preparation) were added to CsOAc (0.1 mmol) and PbX2 (X = Cl, Br, and I) (0.3 mmol) precursor powders. Then the reaction mixture was subjected to the microwave oven at a power of 800 W for a few minutes. During the course of the reaction, the color change of the reaction mixture can be clearly seen, indicating the formation of perovskite NCs, and they exhibit strong PL emission under UV-light irradiation (Figure 2). For example, the colorless reaction mixture gradually transforms into the color “orange-yellow” for the case of CsPbBr3 (Figure S14a). Similarly, the NCs made of different halide composition were prepared by simply using appropriate precursor powders in the reaction mixture. Preparation of Mn-Doped CsPbCl3 Perovskite NCs. In a typical preparation, 5 mL of ODE together with 2 mL of TMPPA and 2 mL of OLA were added to CsOAc (0.1 mmol), PbCl2, and MnCl2 with specific molar ratio. Then the reaction mixture was subjected to a microwave oven at a power of 800 W for 20 min. Purification. The CsPbX3 (X = Cl, Br, and I) NCs and Mn-doped CsPbCl3 NCs were extracted from the crude solution by centrifuging at 9000 rpm for 10 min to discard the supernatant containing unreacted precursor and byproducts. After that, 5 mL of hexane was first added into the precipitates to disperse under mild sonication, and subsequently the obtained NC dispersions were further centrifuged at 1000 rpm to remove large NCs. For CsPbX3 (X = Cl, Br, and I) NCs and Mn-doped CsPbCl3 NC powders, 10 mL of ethyl acetate was added to the 5 mL hexane solution used to induce aggregation and followed by centrifugation at 8500 rpm for 5 min. After centrifugation for two times, the supernatant was discarded, and the precipitates were used for further characterization. Fabrication of LEDs. Commercially available GaN LED chips with 365 nm excitation and 4.0 V operating voltage were purchased from Advanced Optoelectronic Technology Inc. In the preparation of LEDs’ color conversion layer, CsPbCl2Br1, CsPbBr3, CsPbBr1.5I1.5, and CsPbBr1I2 NCs, respectively, with blue, green, yellow, and red emission were foremost milled to fine powder and mixed with the curable resin according to our previous method and put in a vacuum chamber to remove bubbles.60 After that, the mixtures were used to fill the cup-shaped void of an LED chip. After curing under UV light for 2 min, the LEDs from CsPbX3 (X = Cl, Br, I) NCs were fabricated. Characterization. A Shimadzu 3100 UV−vis spectrophotometer was used for obtaining UV−visible absorption spectra. Fluorescence spectroscopy was obtained with a Shimadzu RF-5301 PC spectrophotometer. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were recorded using an H-800 electron

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b14677. Additional TEM images, photographs under sunlight and UV, EDX analysis, XPS spectra, XRD patterns, UV−vis absorption spectra, PL emission spectra, and PLQYs of CsPbX3 NCs and Mn-doped CsPbCl3 NCs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Fax: +86 431 85193423. E-mail: [email protected]. *E-mail: [email protected]. ORCID

Dong Yao: 0000-0003-4594-3515 Yi Liu: 0000-0003-0548-6073 Hao Zhang: 0000-0002-2373-1100 Bai Yang: 0000-0002-3873-075X Author Contributions

H.Z. proposed and supervised the project. H.Z., H.W.L., Z.N.W., D.Y., Y.L., and B.Y. designed and performed the experiments and cowrote the paper. J.R.S, H.G., and H.Y.Z. participated in most experiments. All authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National key research and development program of China (2016YFB0401701), the 973 Program of China (2014CB643503), NSFC (21773088, 51425303), JLU Science and Technology Innovative Research Team 2017TD-06, the Postdoctoral Innovation Talent Support Project (BX201700099), the Postdoctoral Science Foundation of Jilin University (801171050411), and the Special Project from MOST of China.

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One-Step Preparation of Cesium Lead Halide CsPbX3 (X = Cl, Br, and

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