Cs4PbBr6 Nanocomposites: Formation Mechanism, Large

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CsPbBr3/Cs4PbBr6 Nanocomposites: Formation Mechanism, Large-scale and Green Synthesis, and Application in White Light-Emitting Diodes Wenkang Wang, Duofa Wang, Fan Fang, Song Wang, Guohua Xu, and Tianjin Zhang Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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Crystal Growth & Design

CsPbBr3/Cs4PbBr6 Nanocomposites: Formation Mechanism, Large-scale and Green Synthesis, and Application in White Light-Emitting Diodes Wenkang Wang,† Duofa Wang,*† Fan Fang,† Song Wang,†† Guohua Xu,††† and Tianjin Zhang*† Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials Key Laboratory for the Green Preparation and Application of Functional Materials, Ministry of Education School of Material Science and Engineering, Hubei University 368 Youyi Road, Wuchang District, Wuhan 430062, China †† Hubei University of Arts and Science, Xiangyang 441053, Hubei, China ††† Yichang CSG Polysilicon Co., Ltd. Yichang 443000, Hubei, China †

Keywords: CsPbBr3/Cs4PbBr6 nanocomposites, all-inorganic perovskite, LED

ACS Paragon Plus Environment

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT CsPbBr3/Cs4PbBr6 nanocomposites are found to yield efficient luminescence recently. However, the formation mechanism of the nanocomposites are unclear and large-scale and green synthesis is still challenging. Here, we develop a self-assembly reaction to fabricate CsPbBr3/Cs4PbBr6 nanocomposites efficiently and environmental friendly. The transmission electron microscopy clearly

shows

CsPbBr3 nanocrystal

CsPbBr3/Cs4PbBr6

composite

is embedded

structure.

In-situ

in

Cs4PbBr6

characterization

matrix, reveals

forming that

a the

CsPbBr3/Cs4PbBr6 nanocomposites are formed by a two-step reaction, driven by ion concentration difference. The self-encapsulation and separation of the CsPbBr3 NCs by the host Cs4PbBr6 result in the material exhibiting a high PLQY of 83% and narrow-band emission at 517 nm with a full width at half-maximum of only 21 nm. Further, we fabricate an on-chip white light-emitting diode (LED) using the as-synthesized CsPbBr3/Cs4PbBr6 nanocomposites as a green emitter and red K2SiF6:Mn4+ phosphor on the surface of a blue LED chip. The resulting white LED exhibits a high luminous efficiency of up to 88 lm W−1 at 20 mA with an NTSC value of 131% and Rec. 2020 of 98%.

INTRODUCTION Recently, all-inorganic perovskite nanocrystals (NCs) with a composition of CsPbX3 (X = Cl, Br, and I) have been developed. These NCs offer excellent luminescence properties, including narrow-band emission and high quantum efficiency and color tunability over the entire visible spectral range.1-5 These make them promising not only as color-conversion phosphors in white


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LEDs, but also as electroluminescent materials in quantum-dot LED6,7 and as gain materials in lasers.8,9 Perovskite CsPbBr3 NCs are regarded as the most promising narrow-band greenemitting material used in wide-color-gamut backlight displays, outperforming Cd-based (CdSe) and Cd-free (InP, CuInS2) NCs systems. CsPbBr3 NCs exhibit narrow-band emission with a full width at half-maximum (FWHM) of ~20 nm,1,3,10-13 high photoluminescence quantum yield (PLQY), and a relatively simple synthesis method.14 The main drawback of perovskite NCs is their poor chemical stability under ambient conditions. The PLQY of colloidal NCs of ~90% decreases dramatically to below ~20% when they are in the solid phase (such as in a thin film); this occurs because of the loss of capping ligands, which causes further aggregation, resulting in aggregation-induced quenching of their emission.15-17 It is necessary to develop a facile and reproducible procedure to fabricate perovskite NCs with high PLQY and long-term stability. In attempts to improve their stability, perovskite NCs have been embedded in polymer matrices such as polymethyl methacrylate, polystyrene,4,6,8 and polyethylene oxide with further antisolvent vapor treatment,18 coated with strongly binding ligands19,20 or silica,14,21-23 confined in metal-organic frameworks,24 and embedded in zeolite-Y, NaCl, and NaNO3.25-33 Embedding is regarded as an effective way to improve the stability of NCs and preserve their optical properties.34,35 Nevertheless, these methods are not easy to operate and for large-scale production, and a large amount of hazardous wastes were generated during the fabrication and encapsulation process of perovskite NCs. Very recently, CsPbBr3/Cs4PbBr6 composites structure was reported to retain high PLQY and showed potential to substitute the traditional encapsulation method.31 Carroll and co-workers explored the electroluminescence (EL) devices utilizing Cs4PbBr6 films with CsPbBr3 NCs embedded in.33 More recently, Zhong’s group reported centimeter-sized Cs4PbBr6 crystal with embedded CsPbBr3 and integration of efficient luminescence device.36


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Even though excellent optical property was achieved by the CsPbBr3/Cs4PbBr6 nanocomposites, many fundamental questions are still not addressed: (1) the formation mechanism of CsPbBr3/Cs4PbBr6 nanocomposites are unclear; (2) more rational and direct experimental evidences are need to reveal the CsPbBr3/Cs4PbBr6 composite structure;36,37 (3) there is still debate on PL mechanism, induced by defect or impurity?38 (4) large-scale and green synthesis is still challenging. In this study, we prepared CsPbBr3/Cs4PbBr6 nanocomposites by a facile and efficient method. The microstructure of the materials, the formation mechanism, and the PL mechanism are clearly revealed. Additionally, an on-chip white LED with wide color gamut is constructed using the as-prepared CsPbBr3/Cs4PbBr6 nanocomposites as a green phosphor. EXPERIMENTAL SECTION Materials. Lead

(II)

bromide

(PbBr2

99.999%),

Cesium

bromide

(CsBr

99.999%),

N,N-

dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were purchased from Aldrich. All chemicals were used as received without further purification. Synthesis of CsPbBr3/Cs4PbBr6 nanocomposites. CsBr (1.6 mmol) and PbBr2 (0.4 mmol) (CsBr/PbBr2 molar ratio of 4:1) were added to DMF (2 mL) in a centrifuge tube with a volume of 5 mL. The centrifuge tube was ultrasonicated for 30 minutes until all the reactant was converted to a light yellowish precipitate. To control the composition of the products accurately, the CsPbBr3/Cs4PbBr6 nanocomposites powder was obtained by evaporating the solvent in a vacuum oven at 70°C instead of centrifugation.


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Scale-up synthesis of CsPbBr3/Cs4PbBr6 nanocomposites. CsBr (16 mmol) and PbBr2 (4 mmol) (CsBr/PbBr2 = 4:1) were added to 2.5 mL DMF and 2.5 mL DMSO in a small vial with a volume of 25 mL. The vial was ultrasonicated until all the reactant was converted to a light yellowish precipitate. To improve the preparation efficiency, the precipitate was collected via centrifugation (TGL-16G) at 5000 rpm for 5 minutes and then dried in a vacuum oven at 70°C to obtain CsPbBr3/Cs4PbBr6 nanocomposites powder in abundance. Fabrication LED Devices. In a typical process, a total of 100 mg of CsPbBr3/Cs4PbBr6 nanocomposites and K2SiF6:Mn4+ powders with a weight ratio of 5:1 were mixed with thermal-curable silicone resin (100 mg, OE6550, Dow Corning Co.) under vigorous stirring for 30 min. The resulting mixture was deposited on top of an InGaN blue LED chip (460 nm, Epileds, China) and then dried at 100 °C for 4 h. Characterization. The crystal structure was determined using an X-ray diffractometer (Bruker D8 Advance) with Cu-Kα radiation (λ=1.5406 Å). HRTEM images were captured on an FEI Tecnai G2 F30 transmission electron microscope operated at 300 kV. Absorption spectra were obtained using a UV-Visible-NIR spectrophotometer (Shimadzu UV-3600). PL and PLE spectra were measured on a PerkinElmer LS55 fluorescence spectrometer. Edinburgh FLS 980 was used to measure the PL decay curves. Absolute PLQYs were measured by a Quantaurus-QY spectrometer (C1134711, Hamamatsu, Japan) with an excitation wavelength of 450 nm. The luminescent spectrum, Ra, CCT, and CIE coordinates of the LED were measured by a spectroradiometer (HAAS-2000, Everfine, China).


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RESULTS AND DISCUSSION The solubility of reactants has very important effect on the chemical reaction. The CsBr is not soluble with N,N-dimethylformamide (DMF) while the solubility of PbBr2 in DMF is much higher.36 When the PbBr2 and CsBr solids are added into the solvent of DMF, the concentration of Pb2+ is much higher than Cs+. In the present work, this ion concentration difference was utilized as the driving force to synthesize CsPbBr3/Cs4PbBr6 nanocomposites. CsBr and PbBr2 (CsBr/PbBr2 = 4:1, mole ratio) were added to DMF solvent and ultrasonicated. The CsPbBr3/Cs4PbBr6 nanocomposites were obtained through collecting the precipitate. The detailed information on the fabrication is described in the experimental section.

Figure 1. XRD pattern of the products. The purple diamonds and black dots represent characteristic peaks of Cs4PbBr6 and CsBr, respectively. The structure of the products obtained was investigated by X-ray diffraction (XRD) and highresolution transmission electron microscopy (HRTEM). The XRD pattern of the products (Figure 1) contains strong diffraction peaks corresponding to Cs4PbBr6 (JCPDS 73-2478), indicating that the synthesized products are mainly Cs4PbBr6. No characteristic diffraction peaks of CsPbBr3 are observed, which is due to the low content, small crystal size, and good dispersion of CsPbBr3 in


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the Cs4PbBr6 matrix.36,37 The formation of Cs4PbBr6 was believed to occur according to equation (1), considering that the molar ratio of CsBr to PbBr2 used in our experiment was 4:1. Except for the diffraction peaks relevant to Cs4PbBr6, a small peak at 29.5° indexed to CsBr (JCPDS 730391) was observed in the XRD pattern, indicating that not all PbBr2 was converted into Cs4PbBr6 through equation (1). Therefore, it is reasonable to speculate that a small number of reactants or intermediate products such as PbBr2 or CsPbBr3, which is unable to be resolved by XRD, coexist with Cs4PbBr6. (Further characterization and analysis later in the paper revealed that it is CsPbBr3.) 퓀ᐽ鹰Ǩ ĸ

퓀鹰Ǩ

퓀ᐽ

퓀鹰Ǩ

(1)

Figure 2. (a) Normalized absorption (black dashed line) and PL (solid line, purple and green peaks denote the visual emission colors) spectra of CsPbBr3/Cs4PbBr6 nanocomposites. (b) Normalized absorption (black dashed line) and PLE (red solid line) spectra of CsPbBr3/Cs4PbBr6 nanocomposites. To reveal the composition of the incompletely converted products, we measured its absorption, photoluminescence (PL), and photoluminescence excitation (PLE) spectra. Cs4PbBr6 is an insulator with a large band gap of ~3.9 eV,39,40 whereas CsPbBr3 is a semiconductor with a band


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gap of about 2.4 eV.41 The absorption spectrum of the products (Figure 2a) contains a strong peak at 315 nm and another weaker peak at 505 nm. The PL spectrum of the products (Figure 2a, excitation: 295 nm) displays a weak peak at 340 nm and a strong narrow peak at 517 nm (FWHM=21 nm), consistent with the absorption spectrum. The strong absorption peak near 315 nm is related to Cs4PbBr6, originating from the optical transitions between the localized states within isolated PbBr64− octahedra.39,40 It has been reported that the absorption peak of pure CsPbBr3 NCs is located at around 505 nm.1,4 Therefore, it is reasonable to speculate that the absorption peak and strong PL peak near 517 nm in Figure 2a originate from interfused CsPbBr3 NCs. This speculation is confirmed by the PLE (emission collected at 517 nm) and absorption spectra of the products presented in Figure 2b. Notably, there is a sharp dip at ~315 nm in PLE spectrum and a sharp peak in the absorption spectrum at the same wavelength. This is due to that Cs4PbBr6 exhibits very strong absorption at 315 nm,39,40 and the absorption of CsPbBr3 is comparably weak, which leads to the dip in the PLE spectrum. If the emission is related to defects in Cs4PbBr6, as mentioned in the literature,42,43 the radiative recombination at 517 nm will be detected under 315 nm excitation and no dip will appear in the PLE spectrum. Therefore, the PLE results clearly confirm that CsPbBr3 NCs coexists in Cs4PbBr6. Considering the PL peak of CsPbBr3 in Figure 2a, its intensity is strong and FWHM is as small as 21 nm, which are comparable to those of colloidal CsPbBr3 NCs.1,4,6 Moreover, the reported PLQY of bulk CsPbBr3 is very low.12,44,45 Together with the XRD results indicating that the CsPbBr3 content of the products is very low, we speculate that CsPbBr3 is present as very small NCs embedded in the host Cs4PbBr6, forming a CsPbBr3 NCs/Cs4PbBr6 matrix structure. The self-assembled matrix structure leads to the separation and encapsulation of CsPbBr3 NCs, which can thus exhibit a high PLQY and small FWHM.


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Figure 3. The TEM (a) and HRTEM (b) image of a CsPbBr3 nanocrystals in the Cs4PbBr6 matrix. The insets show the fast Fourier transform (FFT) images. HRTEM was employed to investigated the microstructure of the nanocomposites. As shown in Figure 3a, abundant of small nanopartilces are dispersed in host materials obviously indicating that the CsPbBr3 NCs/Cs4PbBr6 matrix structure is formed indeed. In the HRTEM image shown in Figure 3b, two sets of lattice fringes can be observed apparently. In the bottom right of the HRTEM image (highlighted by a white rectangle), two-dimensional lattice fringes are resolved and the intersection angle between the fringes is 79°. The d-spacing for these two fringe are measured to be 3.1 Å and 2.8 Å. The angle and the d-spacing are well consistent with those of (131) and (006) planes in hexagonal Cs4PbBr6. The fast Fourier transformation (FFT) image for this region inset at the top right of Figure 3b contains diffraction spots corresponding to the (006) and (131) planes of hexagonal Cs4PbBr6 with an intersection angle of 79°, consistent with the HRTEM results. All these results confirm that this related region is hexagonal Cs4PbBr6. The small region marked by a yellow rectangle in Figure 3b also contains two dimensional lattice fringes with an intersection angle of 90°. The measured d-spacing for these two fringes are 3.0 Å


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and 2.8 Å, corresponding to the (002) and (200) planes of monoclinic CsPbBr3. The monoclinic CsPbBr3 structure is also confirmed by FFT result as shown in the top left inset of Figure 3b. All the HRTEM and FFT results clearly reveals that the monoclinic CsPbBr3 nanocrystals are embedded in the hexagonal Cs4PbBr6 matrix.

Figure 4. Photographs of the products obtained after different reaction times under (a) sunlight and (b) UV light (365 nm) excitation. For 5 mins sample, the graph is enlarged and shown in the inset. (c) XRD patterns of the products formed after different reaction times, the purple diamonds, orange stars, and black dots represent characteristic peaks of Cs4PbBr6, CsPbBr3 and CsBr, respectively. (d) Absorption spectra of the products formed after different reaction times. (e) PL spectra of the products at the reaction time of 5 min.


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In this section, we investigate the formation mechanism of CsPbBr3/Cs4PbBr6 nanocomposites by quenching the chemical reaction at different stages and characterizing the intermediate and final products. The molar ratio of CsBr to PbBr2 was also kept at 4:1 as above. Figure 4a shows the products obtained after different reaction times. The corresponding XRD patterns, absorption and PL spectra of the products were performed on the precipitate collected at the bottom of the tube. We also prepared a reference sample by adding solid CsBr to DMF without PbBr2. For the reference sample, because CsBr is not soluble with DMF, solid CsBr accumulated at the bottom of the tube as soon as it was added, as shown on the far left of Figure 4a. In contrast, when CsBr and PbBr2 was added to the solution together, chemical reaction between CsBr and PbBr2 was clearly observed. After reaction for 1 min, a certain amount of orange precipitate was clearly observed (Figure 4a), which was confirmed to be CsPbBr3 by the characteristic diffraction peaks in the XRD pattern (Figure 4c) and characteristic absorption spectra (Figure 4d). When the reaction time was increased to 3 min, an orange dispersion appeared along with an orange precipitate at the bottom of the tube. Accordingly, the intensity of the CsPbBr3 diffraction peak and absorption peak obviously increased (see Figure 4c, d) indicating more CsPbBr3 was produced. Interestingly, an abrupt change was observed when the reaction was performed for 5 min. A light yellowish precipitate began to appear at the bottom of the tube and the corresponding region exhibited green emission under ultraviolet (UV) irradiation, as shown in Figure 4b. Figure 4e further shows that the products of 5 min exhibits a strong green emission peak at 515 nm. In the corresponding XRD pattern, the diffraction peaks indexed Cs4PbBr6 are observed. Moreover, the absorption peak at 530 nm corresponding to CsPbBr3 decreases and an additional absorption peak at 315 nm relevant to Cs4PbBr6 appears in the absorption spectrum. These results clearly reveals that the CsPbBr3/Cs4PbBr6 nanocomposites were produced within 5


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mins. After 30 min, the orange precipitate completely disappeared and light yellow precipitate accumulated at the bottom of the tube, which strongly emitted green light under UV irradiation. The XRD pattern of this sample does not contain diffraction peaks from CsPbBr3, all the diffraction peaks corresponds to Cs4PbBr6 except for a small peak from CsBr. The absorption peak at 530 nm continues to decline and the absorption peak at 315 nm is significantly increased. It indicates that most of CsPbBr3 was converted into Cs4PbBr6.

Figure 5. TEM image of the products obtained after reactant of (a) 3 min and (b), (c), (d) 5 min. (e), (f), (g), and (h) are the corresponding HRTEM images. TEM images (Figure 5) of the samples were employed to clarify the morphology evolution with reaction time. For 1 min and 3 mins, the products were mainly composed of micron-sized cube-like CsPbBr3, as could be seen from Figure S1 and Figure 5a, e. When the reaction time was extended to 5 minutes, micron-sized cube-like CsPbBr3 (Figure 5b, f), nano-sized CsPbBr3


12

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(Figure 5c, g), and CsPbBr3 NCs/Cs4PbBr6 matrix structure (Figure 5d, h) were found to be coexisted in the products.

Figure 6. (a) The two steps reaction process of the formation of CsPbBr3/Cs4PbBr6 nanocomposites.

(b)

The

schematic

illustration

for

the

formation

mechanism

of

CsPbBr3/Cs4PbBr6 nanocomposites. Based on the visual observation of the reaction process and corresponding XRD, absorption, PL and TEM characterization, we propose that the formation of CsPbBr3/Cs4PbBr6 nanocomposites involves two steps, which is schematically described in Figure 6a. At the first stage, the concentration of Pb2+ in the solution is much higher than Cs+ because CsBr is not soluble with DMF. The Pb-rich phase CsPbBr3 prefers to be formed through equation (2). With the consumption of Pb2+ and dissolution of CsBr solids into DMF, a Cs-rich environment is obtained at the second stage (3-30 mins). Most of CsPbBr3 is then transformed into Cs-rich phase Cs4PbBr6 and the CsPbBr3 NCs/Cs4PbBr6 matrix structure is formed. The phase conversion from


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CsPbBr3 to Cs4PbBr6 in a Cs-rich environment has also been reported in previous work.46 The formation process here is controlled by the dissolution-crystallization mechanism, as illustrated in Figure 6b. When the Cs-rich circumstance is formed with the gradual dissolution of CsBr at the second stage, the Cs+, Pb2+ and Br- in the solution prefer to react and produce Cs4PbBr6 (equation 3). With the consumption of Cs+, Pb2+ and Br-, CsPbBr3 microcrystals are gradually dissolved (equation 4), since the solubility of CsPbBr3 in DMF is much larger than that of Cs4PbBr6.15 Certain amounts CsPbBr3 NCs are produced during this stage, as appeared in Figure 5c. The CsPbBr3 NCs/Cs4PbBr6 matrix structure is formed when Cs4PbBr6 are grown on the surface of CsPbBr3 NCs (observed in Figure 5d, h; illustrated in Figure 6b). Finally, the CsPbBr3/Cs4PbBr6 nanocomposites are obtained by the ripening of the CsPbBr3 NCs/Cs4PbBr6 matrix structure. 퓀ᐽĸ ĸ

퓀ᐽĸ ĸ

퓀ᐽ 퓀鹰Ǩ

퓀

퓀

ĸ

ĸ

ĸ

ĸ

鹰Ǩ

鹰Ǩ

퓀ᐽĸ ĸ

퓀ᐽ 퓀鹰Ǩ

퓀

ĸ

퓀ᐽ ĸ

퓀鹰Ǩ

鹰Ǩ


( )

( )

( )

14

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Figure 7. (a) Photographs (top: under sunlight, bottom: under UV irradiation) and (b) XRD patterns (purple diamonds, orange stars, and black dots represent characteristic peaks of Cs4PbBr6, CsPbBr3, and CsBr, respectively) of the products fabricated by the divided reaction method. (c) Absorption spectra of the products. The products are denoted by the final ratios of CsBr to PbBr2 utilized in the divided reaction. (d) PLQYs of the products. The PLQY is similar to the products obtained using un-divided method for each composition. To obtain deeper insight into the transformation process from CsPbBr3 to Cs4PbBr6 and further confirm the proposed formation mechanism for CsPbBr3/Cs4PbBr6 nanocomposites, we intentionally divided the reaction process into two steps and then characterized the composition of the final products. Initially, CsBr and PbBr2 with molar ratio of 1:1 were added to DMF and react. Then, we continued to add CsBr to the solution above, investigated the chemical reaction and characterize the composition, absorption and luminescence property of the products obtained at each stage. Figure 7a displays the photographs of the final products prepared with different ratio of CsBr to PbBr2, which showed that the color of the products under visible light changed from orange to light yellow as the CsBr content was gradually increased. The bottom row of photographs


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depicts the corresponding samples irradiated by UV light and taken in a dark room. The XRD patterns and absorption spectra of these samples are shown in Figure 7b and 7c, respectively. When the CsBr/PbBr2 ratio is 1:1, only CsPbBr3 is detected by XRD and the sample is nonluminescent under UV irradiation. With the addition of CsBr, diffraction peaks from Cs4PbBr6 appear and the intensity of diffraction peaks originating from CsPbBr3 gradually decrease, indicating CsPbBr3 is converted to Cs4PbBr6 under Cs-rich circumstance. When the samples are exposed to UV irradiation, green emission are observed with increasing Cs4PbBr6 content and the highest PLQY of 56% is obtained for a 4:1 CsBr/PbBr2 ratio, as shown in Figure 7d. The reduced PLQY at others ratios is ascribed to the existence of non-luminescent micro-sized CsPbBr3 or excessive CsBr remnant. The strong light emission indicates that the CsPbBr3/Cs4PbBr6 nanocomposites are formed. The conversion from CsPbBr3 to Cs4PbBr6 was further confirmed by UV-vis spectroscopy. As shown in Figure 7c, the intensity of the absorption peak at around 530 nm related to CsPbBr3 gradually decreases and that at 315 nm gradually increases as the CsBr/PbBr2 molar ratio is increased. Additionally, a blue shift of the absorption spectrum is observed in Figure 7c, consistent with the visual color change in Figure 7a, which is ascribed to the strengthening quantum confinement effect accompanying the decrease in the content of CsPbBr3. TEM images (Figure S2) provide more direct evidence on the dissolutioncrystallization controlled formation of the nanocomposites that partial micron-sized CsPbBr3 is dissolved with the addition of CsBr, which is consistent with the results in Figure 5. These results

clearly

confirms

the

two-step

formation

mechanism

of

CsPbBr3/Cs4PbBr6

nanocomposites. The CsPbBr3 is emissive only in the form of isolated NC and the bulk is non-luminescent. Therefore, the key point to enhance the PLQY of the CsPbBr3/Cs4PbBr6 nanocomposites are to


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make the CsPbBr3 NC isolated in Cs4PbBr6 matrix efficiently. Herein, 1 mL DMF and 1 mL DMSO is utilized as the co-solvent fabricate the CsPbBr3/Cs4PbBr6 nanocomposites considering that the solubility of CsPbBr3 in DMSO is much larger than that in DMF, which could improve the dissolution of micron-sized CsPbBr3 and the formation of CsPbBr3/Cs4PbBr6 nanocomposites. The ratio of reactants CsBr/PbBr2 is equal to 4:1 (mole ratio), same as the case of pure DMF. Similar to results with pure DMF, only diffraction peaks of Cs4PbBr6 are present in the XRD pattern (Figure S3(a)) and characteristic absorption/PL peak corresponding to Cs4PbBr6 and CsPbBr3 are observed in the absorption/PL spectrum (Figure S3(b)), indicating that the CsPbBr3 NC/Cs4PbBr6 matrix is obtained as well. As expected, the PLQY is increased to 83% when the co-solvent is employed. The CsPbBr3/Cs4PbBr6 nanocomposites prepared by the co-solvent exhibit longer lifetime as well (Figure S3(c)). The efficient dissolution of micron-sized CsPbBr3 and formation of CsPbBr3/Cs4PbBr6 nanocomposites leading by DMOS is supported by the absorption data (Figure S3(d)) that the characteristic absorption of CsPbBr3 is decreased when co-solvent is employed, and further decreased when pure DMSO is used. More impressively, PbBr2 is quite soluble with DMF/DMSO and CsPbBr3/Cs4PbBr6 nanocomposites forms in two steps with the gradual dissolution of CsBr. Therefore, it not necessary to use abundant solvent to dissolve two reactants and the consumption of solvent in this method is small. This fabrication method is therefore environmentally friendly and suitable for large-scale production of CsPbBr3/Cs4PbBr6 nanocomposites. Figure S4(a) illustrates photographs of about 4.7 g of the products has almost the same PLQY with the previous labscale samples displaying highly efficient luminescence, in which reaction only 5 mL solvent of DMF and DMSO was used. The optical property of the large-scale samples can be seen from Figure S4(b), which is the same with the lab-scale samples. Moreover, the yield of the product is


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very high and the ratio of CsPbBr3/Cs4PbBr6 nanocomposites product to reactants (CsBr+PbBr2) approaches 98%. Furthermore, the material shows stable light emission, with very little degradation after seven weeks of storage in air (Figure S5).

Figure 8. (a) EL spectrum of a white LED containing the green-emitting CsPbBr3/Cs4PbBr6 nanocomposites and red-emitting K2SiF6:Mn4+ phosphor. Inset is a photograph of the LED device operated at 20 mA. (b) Color gamut of the white LED containing CsPbBr3/Cs4PbBr6 nanocomposites (thick red solid line) compared with those of the NTSC (dash-dotted line) and Rec. 2020 standards (dashed line). Considering the ultrahigh PLQY and extremely narrow FWHM of CsPbBr3/Cs4PbBr6 nanocomposites, which are comparable with other luminescent materials, such as the dye, the rare earth materials.47-49 We fabricated a white LED using CsPbBr3/Cs4PbBr6 nanocomposites as the green phosphor and K2SiF6:Mn4+ as the red phosphor on a commercial blue LED chip. The emission spectrum of the resulting white LED is presented in Figure 8a. Inset is a photograph of the white LED device during operation. The device displays a luminous efficiency of 88 lm W−1 at a current density of 20 mA with CIE coordinates of (0.39, 0.37). The color gamut of the white


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LED is particularly high, covering 131% of the NTSC gamut and 98% of the Rec. 2020 gamut (Figure 8b). These values are much higher than those of LEDs containing the commonly used green phosphors β-SiAlON:Eu2+ (NTSC 86%)50 and CdS NCs (NTSC 104%).51 The color gamut achieved here is pretty encouraging and much better than most recent reported WLED based on a perovskite phosphor.26,50,52,53 These results reflect the great potential of CsPbBr3/Cs4PbBr6 nanocomposites to fabricate wide-color-gamut display devices, considering its facile, environmentally friendly, and scalable synthesis method, as well as high device performance.

CONCLUSIONS

We fabricated highly emissive CsPbBr3/Cs4PbBr6 nanocomposites through a solution-based selfassembly reaction strategy that shows multiple advantages over the traditional encapsulation method, including facile fabrication, scalability, and environmental friendliness. HRTEM, XRD, UV-Vis absorption, and PLE spectroscopic analyses confirmed the CsPbBr3 NCs/Cs4PbBr6 matrix structure clearly, with isolated CsPbBr3 NCs self-encapsulated in Cs4PbBr6. The formation mechanism is revealed by characterizing the products at different stage of the reaction. It involved two-step reactions driven by ion concentration difference. At the first step, CsPbBr3 is produced. The CsPbBr3/Cs4PbBr6 nanocomposites are formed with the dissolution of CsPbBr3 and crystallization of Cs4PbBr6 at the second step. The solid-state CsPbBr3/Cs4PbBr6 showed a high PLQY of 83% and narrow-band emission at 517 nm with a FWHM value of only 21 nm. A white LED using CsPbBr3/Cs4PbBr6 nanocomposites as the green phosphor and K2SiF6:Mn4+ as the red phosphor on a blue LED chip exhibited high luminous efficiency (up to 88 lm W−1 at 20 mA) and wide color gamut (NTSC value of 131% and Rec. 2020 of 98%). These findings reveal


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that CsPbBr3/Cs4PbBr6 nanocomposites has great potential for light emission and display applications. ASSOCIATED CONTENT Supporting Information TEM and HRTEM image of the products obtained at a reaction time of 1 min; TEM imagins of the products fabricated by the divided reaction method; XRD and optical analysis (absorption, PL, Time-resolved PL decay ) of the simple synthesis with DMF and DMF/DMSO; images and absorption , PL spectra of large-scale samples. Stability of CsPbBr3/Cs4PbBr6 nanocomposites. (PDF)

AUTHOR INFORMATION Corresponding Author *Email: [email protected]. *Email: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 11174071, 11304088, and 51372180), and Special Technical Innovation Project of Hubei Province (No. 2016AAA035 and 20178ACA088).


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For Table of Contents Use Only

CsPbBr3/Cs4PbBr6 Nanocomposites: Formation Mechanism, Large-scale and Green Synthesis, and Application in White Light-Emitting Diodes Wenkang Wang,† Duofa Wang,*† Fan Fang,† Song Wang,†† Guohua Xu,††† and Tianjin Zhang*†

We prepare CsPbBr3 nanocrystal embedded in Cs4PbBr6 host matrix displaying efficient luminescence by a facile solution processed method. In-situ characterization reveals that the nanocomposites are formed by a two-step reaction, driven by ion concentration difference. A white LED with wide color gamut is developed base on the nanocomposites.


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Cs4PbBr6 Nanocomposites: Formation Mechanism, Large

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