Oxide Janus

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Interfacial Synthesis of Highly Stable CsPbX3/Oxide Janus Nanoparticles Huicheng Hu, Linzhong Wu, Yeshu Tan, Qixuan Zhong, Min Chen, Yinghua Qiu, Di Yang, Baoquan Sun, Qiao Zhang, and Yadong Yin J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11003 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 12, 2017

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Journal of the American Chemical Society

Interfacial Synthesis of Highly Stable CsPbX3/Oxide Janus Nanoparticles Huicheng Hu,†,‡ Linzhong Wu,†,§,‡ Yeshu Tan,†,‡ Qixuan Zhong,† Min Chen,† Yinghua Qiu,† Di Yang,† Baoquan Sun,†,* Qiao Zhang,†,* and Yadong Yin§,* † Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, 199 Ren’ai Road, Suzhou, 215123, Jiangsu People’s Republic of China § Department of Chemistry, University of California, Riverside, Riverside, California 92521 United States KEYWORDS CsPbX3 nanoparticle, silica, Janus, stability, light emitting diode

ABSTRACT: The poor stability of CsPbX3 (X = Cl, Br, I) nanocrystals has severely impeded their practical applications. Although there are some successful examples on encapsulating multiple CsPbX3 nanocrystals into an oxide or polymer matrix, it has remained a serious challenge for the surface modification/encapsulation using oxides or polymers at a single particle level. In this work, monodisperse CsPbX3/SiO2 and CsPbBr3/Ta2O5 Janus nanoparticles were successfully prepared by combining a water-triggered transformation process and a sol-gel method. The CsPbBr3/SiO2 nanocrystals exhibited a photoluminescence quantum yield of 80% and a lifetime of 19.8 ns. The product showed dramatically improved stability against destruction by air, water, and light irradiation. Upon continuous irradiation by intense UV light for 10 h, a film of the CsPbBr3/SiO2 Janus nanocrystals showed only a slight drop (2%) in the PL intensity, while a control sample of unmodified CsPbBr3 nanocrystals displayed a 35% drop. We further highlighted the advantageous features of the CsPbBr3/SiO2 nanocrystals in practical applications by using them as the green light source for the fabrication of a prototype white light-emitting diode, and demonstrated a wide color gamut covering up to 138% of the National Television System Committee standard. This work not only provides a novel approach for the surface modification of individual CsPbX3 nanocrystals but also helps to address the challenging stability issue, therefore it has an important implication toward their practical applications.

1. INTRODUCTION All-inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite nanocrystals (NCs) have been regarded as emerging materials for many optoelectronic applications owing to their excellent photophysical properties, such as narrow emission width, high photoluminescence quantum yield (PLQY), composition-dependent tunable bandgap, and so on.1-7 Since the pioneering work reported by Kovalenko et al. in 2015, remarkable progress in the synthesis and application of CsPbX3 NCs has been made.7 For example, CsPbX3 NCs with controllable shape and composition have been prepared through various approaches, including hot-injection,8-10 solvothermal,11 ultrasonication,12,13 post-treatment,14,15 and chemical vapor deposition (CVD).16,17 The potential applications of such NCs have also been demonstrated in diverse fields, including light emitting diode (LED), solar cell, and laser.1825

Despite the impressive achievement, the stability of CsPbX3 NCs against the destruction by water, oxygen, and light treatment is still a significant challenge.26-29 A very straightforward strategy is to modify the surface of CsPbX3 NCs with another stable material. Accordingly,

much effort has been devoted to this field, and some successful examples have been reported.30-39 For instance, Li and co-workers successfully improved the stability of CsPbBr3 NCs by incorporating them into a SiO2/Al2O3 monolith.30 Fu group embedded CsPbX3 NCs into a polymer to overcome their vulnerability to water.31 Buonsanti group improved their stability by encapsulating a film of CsPbX3 quantum dots with an amorphous alumina matrix through an atomic layer deposition (ALD) method.32 Rogach group prepared water-resistant CsPbX3 NCs by protecting them with polyhedral oligomeric silsesquioxane.33 However, up to date, all the reported methods were only successful in dealing with an ensemble of multiple CsPbX3 NCs and the products were macroscale particles. This limitation brings obstacles to some important applications. For instance, in many bio-related fields, single nanoparticles are highly preferred to allow uptake by cells or extravasation into the tissues; while in the application of light emitting diode (LED), small particles are advantageous in forming films with a high uniformity. It is therefore highly desired to develop a method that can achieve surface modification of CsPbX3 NCs at a single particle level.

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Sol-gel method has been widely used to modify the surface of colloidal nanoparticles with various oxide materials, such as SiO2 and TiO2.40,41 However, because CsPbX3 NCs are extremely sensitive to water and alcohols, it is challenging to use the conventional sol-gel methods to modify their surfaces.30 Recently, we developed a watertriggered transformation process, in which high-quality CsPbX3 NCs can be obtained by treating Cs4PbBr6 NCs dispersed in a nonpolar solvent with water.15 The asprepared CsPbX3 NCs showed improved stability against water etching. More importantly, the formation of CsPbX3 NCs was achieved at the water/hexane interface, which may provide an opportunity for nanoparticle surface modification. Herein, we demonstrate an effective sol-gel process for modifying the surface of perovskite NCs at the interface of water/nonpolar solvent and produce highly stable monodisperse CsPbX3/SiO2 and CsPbX3/Ta2O5 Janus NCs. Thanks to the surface passivation derived from water treatment and silica modification, the CsPbX3/SiO2 Janus NCs exhibited significantly enhanced PL stability against damages by air, water, and light treatment. Furthermore, these Janus nanostructures show beneficial advantages in forming high-quality thin film owing to their excellent dispersibility and small size. By combining the green-emissive CsPbBr3/SiO2 NCs with red-emissive CdSe NCs and blue-emissive GaN LED chip, we further demonstrate the potential applications of the as-modified perovskite NCs in white light devices by fabricating a WLED encompassing 138% color gamut of the National Television System Committee (NTSC, 1913) standard. 2. EXPERIMENTAL SECTION Materials. Cesium carbonate (Cs2CO3, 99.998%), lead halide (PbX2, ultradry, 99.999%), 1-octadecene (ODE, tech. 90%), oleic acid (OA, tech. 90%), oleylamine (OAm, 80-90%) and tantalum (V) ethoxide (TTEO, 99.999%) were purchased from Alfa Aesar. Hexane and were obtained form Sigma-Aldrich. Acetylacetone (99.5%) was purchased from aladdin. Tetramethoxysilane (TMOS, 99%) was purchased from Energy Chemical and tetraethylorthosilicate (TEOS, 99.9%) was purchased from TCI. All chemicals were used as received without any further purification. Synthesis of Cs4PbX6 NCs. Cs4PbX6 NCs were prepared through a hot-injection method. A cesium-oleate solution was prepared first by mixing 0.16 g Cs2CO3 (0.49 mmol), 0.5 mL oleic acid (OA) and 8 mL 1-octadecene (ODE) in a 25-mL three-neck flask, followed by drying the solution for 1 h at 120 oC under vacuum, and then heating it under N2 atmosphere to 150 oC until all Cs2CO3 reacted with OA. In a typical synthesis of Cs4PbBr6 NCs, oleylamine (1 mL, OAm), OA (1 mL), ODE (10 mL) and PbBr2 (0.2mmol) were loaded into a 25-mL three-neck flask and dried under vacuum for 1 h. The reaction system was heated to 140 o C. Then, 2.2 mL hot cesium-oleate solution was rapidly injected into the PbBr2 solution. 10 seconds later, the reaction mixture was immediately cooled by immersed into an ice-water bath. The product was centrifuged at 7000 rpm for 5 minutes and then dispersed in 25 ml hexane.

For the synthesis of other Cs4PbX6 NCs, PbBr2 is simply replaced by other PbX2. Synthesis of CsPbX3/SiO2 Janus NCs. In a typical synthesis, 20 µL tetramethoxysilane (TMOS) was added into 5 mL Cs4PbBr6 NCs/hexane solution (12.5 mg/mL). 200 µL DI water was injected quickly into the mixture under vigorous oscillation using a vortexer (2800 rpm) for 5 minutes. The system was then kept undisturbed under ambient condition for 12 h. The product was centrifuged at 9500 rpm for 5 minutes to discard the precipitates. For thin film fabrication, the products were washed using 1octane at 25000 rpm for 10 minutes for three times. Synthesis of CsPbBr3/Ta2O5 Janus NCs. 5 µL tantalum (V) ethoxide (TTEO) and 1.25 µL acetylacetone were added into 5 mL Cs4PbBr6 NCs/hexane solution (12.5 mg/mL). 100 µL DI water was injected quickly into the mixture under vigorous oscillation using a vortexer (2800 rpm) for 2 minutes. The system was then kept undisturbed under ambient condition for 12 h. The product was centrifuged at 7500 rpm for 5 minutes to discard the precipitates. Anion exchange reaction. The anion exchange reaction was taken place at room temperature. First, 0.1 mmol PbCl2 or PbI2 was added into a 10 ml glass vial containing 5 mL anhydrous hexane, 0.5 mL OA and 0.25 mL OAm at 80 oC in a glovebox. The bottle was stirred for 12 h to dissolve the powders fully. Typically, 1 mL crude solution of CsPbBr3/SiO2 NCs was added into 4 mL anhydrous hexane in a glass vial under vigorous stirring. Then, the desired amount of anion sources was added into the diluted CsPbBr3/SiO2 NCs solution at room temperature to carry out the anion exchange reaction. Fabrication of thin film. The film was fabricated using a spin-coating method. Quartz substrates were cleaned in ethanol and deionized water for 20 min in sequence, the substrates were under an ozone plasma treatment for 15 min before the spin coating process. Then the solution of perovskite nanocrystals after centrifugation was spin-coated on the cleaned substrates at 2000 rpm for 45 s. Photostability test. The whole test system consists of a PhotoResearch spectrometer PR670, a Keithley 2400 SourceMeter ， a sample holder, and a computer, as shown in Figure S19a. The all-inorganic perovskite NCs were spin-coated on a quartz substrate; then the substrate was under an encapsulation to prevent the influence of air and water with optical adhesive, as shown in Figure S19b. The film was positioned in front of the PhotoResearch spectrometer and the 375 nm LED was fastened 5 mm behind the film on the holder. The PhotoResearch spectrometer was adjusted to focus on the film to accurately collect the photoluminescence signals. Figure S19c and d show that the 375 nm LED was under a continuous current of 200 mA with a power efficiency of 117 mW/cm2. The PhotoResearch spectrometer gathers signals every 5 minutes.


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Journal of the American Chemical Society Fabrication of WLED devices. Chloroform and PMMA were mixed to dissolve the CsPbBr3/SiO2 and CdSe NCs. Then, the CsPbBr3/SiO2-PMMA film and the CdSePMMA film were cast on the blue GaN chip layer by layer. Characterization methods. UV−Vis absorption spectra were recorded in a range of 300−800 nm by using an Evolution 220 spectrophotometer in transmission mode. The photoluminescence spectra and PLQY were obtained by a FLUOROMAX-4 spectrofluorometer equipped with a Xenon lamp. PLQY values were collected by using 400 nm as excitation wavelength and a calibrated sphere. The PL lifetime measurements were taken on a HORTB-FM-2015 spectrofluorometer and fitted with a triexponential decay. Powder X-ray diffraction data were collected by using an Empyrean (from PANalytical, Netherlands) equipped with monochromatic Cu Kα radiation (λ = 1.54056 Å). Transmission Electron Microscopy (TEM) images were collected by a TECNAI G2 F20 transmission electron microscope with an accelerating voltage 200 kV and a Gatan SC200 CCD camera. Scanning electron microscopy (SEM) images were acquired on a Zeiss Supra 55 from Carl Zeiss, Germany. Atomic force microscopy (AFM) image was obtained with an Asylum Research Cypher S microscopy. The electroluminance spectra of WLEDs were collected by a Keithley 2400 SourceMeter and a Photo Research spectrometer PR670. The power efficiency of commercial ultraviolet LED was measured by a Si photodiode PDBC203. For high speed centrifugation, the products were collected at 15 oC by using an Allegra 64R from Beckman Coulter, America. 3. RESULTS AND DISCUSSION

Figure 1. (a) TEM image of the obtained CsPbBr3/SiO2 Janus NCs. (b) HRTEM image of a single CsPbBr3/SiO2 NC, a lattice spacing of 0.58 nm is labeled. (c) HAADF-STEM image and (d-g) elemental mapping images showing the elemental distribution of (d) Cs, (e) Pb, (f) Br, and (g) Si. The scale bars in

(c-g) are 10 nm. (h) XRD pattern of CsPbBr3/SiO2 NCs matching with reference pattern of bulk CsPbBr3 (PDF #054-752). (i) Absorption and PL spectra (λexc = 380 nm) of pristine Cs4PbBr6 NCs (black solid line) and CsPbBr3/SiO2 NCs. The inset photograph shows a strong green PL emission under UV light (λ = 365 nm).

We used the preparation of CsPbX3/SiO2 Janus NCs as a typical example to demonstrate this unique interfacial modification process. Monodisperse colloidal Cs4PbBr6 NCs were synthesized through a hot-injection method (see more experimental details, a TEM image (Figure S1) and XRD pattern (Figure S2) in the Supporting Information). In a typical sol-gel synthesis, 20 µL tetramethoxysilane (TMOS) was added into a 5 mL Cs4PbBr6 NCs/hexane solution (12.6 mg/mL). DI water of 200 µL was injected quickly into the mixture under vigorous oscillation using a vortexer. The system was then kept undisturbed under ambient condition for 12 h. During this process, the original colorless solution turned into bright green (Figure S3), suggesting a successful transformation from non-luminescent Cs4PbBr6 NCs to highly luminescent CsPbBr3 NCs. The products were characterized using TEM. As shown in the TEM image (Figure 1a and Figure S4), monodisperse Janus nanoparticles with an obvious contrast between two parts can be observed. The high-resolution TEM (HR-TEM) image in Figure 1b shows a clear lattice spacing of 0.58 nm on the darker part, which is in good agreement with the (110) planes of orthorhombic CsPbBr3 phase. The lighter part is an amorphous material, which could be attributed to solgel derived silica. Meanwhile, some silica oligomers can be observed on the surface of the darker part. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (Figure 1c) and energydispersive X-ray spectroscopy (EDS) (Figure 1d-g) were used to reveal the elemental distribution. Consistent with the TEM observation, Cs, Pb, and Br elements are mainly distributed on one side, while Si elements are mainly distributed on the other side. Some Si elements can also be observed on the CsPbBr3 part, which can be attributed to the adsorption of some SiO2 oligomers. The powder X-ray diffraction (XRD) pattern of obtained product is depicted in Figure 1h. The diffraction peaks can be indexed as orthorhombic CsPbBr3 phase (ICSD #01072-7929),42 further confirming the successful transformation. Sharp peaks indicate high crystallinity of CsPbBr3/SiO2 NCs. It is worth mentioning that no broad peak at 2θ = 20o− 25o (a characteristic feature of amorphous materials) has been detected, which might be ascribed to the high crystallinity of CsPbBr3 NCs and the relatively small amount of amorphous SiO2. The transformation and surface modification processes have also been monitored by using the UV-Vis absorption and PL spectra. As depicted in Figure 1i, two sharp peaks at 230 nm and 314 nm can be observed in the absorption spectrum (solid black line), which is indexed as pristine Cs4PbBr6 NCs.14 After the surface modification treatment, these two sharp peaks disappeared, and a new absorption peak at 507 nm emerged (solid green line), indicating the formation of



CsPbBr3 NCs. Corresponding PL spectrum shows a sharp peak at 517 nm with a narrow full width at half maximum (FWHM) of 18 nm. Bright green PL emission could be observed under UV light irradiation (inset in Figure 1i). The PLQY of the obtained product was measured to be 80%, which is slightly higher than that obtained through the water-triggered transformation process.15 To figure out the formation mechanism, we carried out the time-dependent experiment. During the reaction process, some aliquots of the reaction mixture were taken out from the reaction system, and the intermediates were characterized by TEM. As shown in Figure 2a, tetradecahedral Cs4PbBr6 NCs with size around 17.6 nm were used as the starting materials. After mixed with water for 0.5 h, some tetradecahedral Cs4PbBr6 NCs turned into cubic CsPbBr3 NCs (Figure 2b). The particle size is about 11.3 nm, which is consistent with our previous report.15 The shrinkage in size can be explained by the CsBrstripping mechanism. Meanwhile, a small piece of SiO2 could be observed on one end of each cubic NCs. With prolonged reaction time, more and more tetradecahedral Cs4PbBr6 NCs were converted into cubic CsPbBr3 NCs, and the size of SiO2 gradually increased (Figure 2c, 2d).

Figure 2. (a-d) TEM images obtained at different reaction time to show the morphology and composition evolution of CsPbBr3/SiO2 Janus NCs. (a) 0 h, (b) 0.5 h, (c) 2 h, (d) 12 h. (e) Schematic illustration of the whole formation process of CsPbBr3/SiO2 Janus NCs.

By the above-mentioned observation, a plausible mechanism has been proposed, as illustrated in Figure 2e. The surface of Cs4PbBr6 NCs was hydrophobic because of the coverage of hydrophobic capping ligands, e.g., oleic acid and oleylamine, which in principle prevent the deposition of hydrophilic silica to the NC surface. When the hexane solution containing Cs4PbBr6 NCs encountered water, CsBr would be stripped out through the hexane/water interface because of its high solubility in water (123 g in

100 g water at 25 oC).43 Meanwhile, the hydrophobic capping ligands in contact with the interface would be removed, allowing the deposition of the silica species. In achieving a successful silica deposition, the whole system was kept into being undisturbed during the reaction process, and TMOS was used as the silica precursor because of its high hydrolysis rate. When tetraethylorthosilicate (TEOS) was used as the silica precursor, only cubic CsPbBr3 NCs were obtained after reacting for 12 h (Figure S5). This phenomenon can be attributed to the much lower hydrolysis rate of TEOS than that of TMOS. When the reaction system was magnetically stirred (400 rpm), and the other parameters were identical, free silica nanoparticles with few CsPbBr3/SiO2 Janus NCs were obtained, which can be explained by the quick separation of product from the interface, resulting in insufficient time for the nucleation and growth of silica (Figure S6). It is worth noting that silica deposition would not occur if Cs4PbBr6 NCs were not converted (Figure 2b). To verify this mechanism, CsPbBr3 NCs obtained after the watertransformation process was used to replace original Cs4PbBr6 NCs. Under the same reaction condition, only separated CsPbBr3 NCs and silica nanospheres were obtained (Figure S7).

Figure 3. (a) UV-Vis absorption and (b) PL spectra of CsPbX3/SiO2 Janus NCs ( λexc = 380 nm for all but 420 nm for CsPbI3/SiO2 sample). Corresponding TEM images of (c) CsPb(Cl/Br)3/SiO2, (d) CsPb(Br/I)3/SiO2 and (e) CsPbI3/SiO2 NCs. Insets showing colloidal solutions in hexane under UV light (λ = 365 nm). All scale bars are 30 nm.

Because this water-triggered transformation process is versatile for other halide NCs, CsPbX3/SiO2 Janus NCs with controllable halide composition can be obtained through the same treatment. Cs4PbX6 NCs with different halide compositions were synthesized first (see absorption spectra (Figure S8) and XRD patterns (Figure S9) in Supporting Information). The absorption and PL spectra of CsPbX3/SiO2 Janus NCs were shown in Figure 3a and 3b, respectively. It is worth pointing out that CsPbCl3/SiO2 Janus NCs could not be produced. Instead, free silica nanospheres and CsPbCl3 NCs were prepared. As we reported previously, the transformation process from


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Journal of the American Chemical Society Cs4PbCl6 NCs to CsPbCl3 NCs is much faster than that of Cs4PbBr6 and Cs4PbI6 NCs.15 As a result, silica nucleation could not happen on CsPbCl3 NCs, suggesting that our proposed mechanism is plausible. Figure 3c-e show TEM images of obtained CsPbX3/SiO2 Janus NCs with controllable halide composition. Bigger silica nanoparticles were formed on CsPbI3/SiO2 Janus NCs, which might be resulted from slow transformation process from Cs4PbI6 NCs to CsPbI3 NCs. Corresponding XRD patterns of obtained CsPbX3/SiO2 Janus NCs also show orthorhombic phase (Figure S10). Under UV light irradiation, bright PL emission from blue-green to red color can be clearly observed, as demonstrated by the photographs (insets in Figure 3c3e). CsPbX3/SiO2 Janus NCs with controllable halide composition and tunable PL emission can also be obtained through a facile anion exchange reaction process. By adding desired amounts of halide ions into CsPbBr3/SiO2 Janus NCs solution, CsPbX3/SiO2 Janus NCs with tunable PL emission covering the full visible range (411-687 nm) were obtained (Figure S11). In addition, the size of silica nanoparticles on the Janus NCs can be manipulated by varying the addition amount of TMOS. As shown in Figure S12, the size of silica part can be tuned from ~3.4 nm to ~12.6 nm. When too much silica precursor was added, free silica particles would form (Figure S12c).

conds. When the silica precursor of TMOS was replaced by TTEO and the other reaction conditions were kept identical, the main products were free hollow Ta2O5 nanoshells and CsPbBr3 NCs although a small amount of CsPbBr3/Ta2O5 Janus NCs could be observed (Figure S13). The formation of free hollow Ta2O5 shells could be attributed to the fast hydrolysis and the resulted selfnucleation. It has been reported that acetylacetone can slow down the hydrolysis rate of some sol-gel processes through the chelating effect.45 Accordingly, here we found that the presence of acetylacetone could dramatically improve the yield of CsPbBr3/Ta2O5 Janus nanoparticles. As presented in Figure 4a and 4b, uniform CsPbBr3/Ta2O5 Janus NCs were successfully prepared. Different from the CsPbBr3/SiO2 Janus NCs where a solid silica sphere was formed on the end of each CsPbBr3 NC (Figure 1a), a hollow nanostructure of Ta2O5 can be found on each CsPbBr3 NC. The HAADF-STEM and EDS have also been used to verify the formation of CsPbBr3/Ta2O5 Janus NCs, as shown in Figure 4c-h. Being consistent with the TEM observation, Cs, Pb, and Br elements are mainly distributed on one side, while Ta elements are mainly distributed on the other side. The hollow nanoshell of Ta2O5 can also be confirmed by the Ta distribution. The prepared CsPbBr3/Ta2O5 Janus NCs have a strong green emission under UV light emission. As depicted in Figure 4i, an absorption peak at 510 nm can be observed (black line). Corresponding PL spectrum shows a sharp peak at 519 nm with a FWHM of 19 nm. The PLQY of the obtained product was measured to be 85%. Since this approach is versatile for the preparation of CsPbBr3/oxide Janus NCs, preparation of other oxide materials based Janus NCs is currently on the way in our laboratory.

Figure 4. (a, b) Representative TEM images of CsPbBr3/Ta2O5 Janus NCs. (c) HAADF-STEM image and (d-h) elemental mapping images showing the elemental distribution of (d) Cs, (e) Pb, (f) Br, (g) Ta and (h) elements overlap. The scale bars in (c-h) are 20 nm. (i) Absorption and PL spectra (λexc = 380 nm) of CsPbBr3/Ta2O5 NCs.

According to the proposed formation mechanism, this method is expected to work for the preparation of Janus NCs containing other oxides. Here, we demonstrate that CsPbBr3/Ta2O5 Janus NCs with a hollow Ta2O5 nanoshell can be obtained through the same approach. We recently reported that hollow Ta2O5 nanoshells could be prepared by adding TTEO (tantalum (V) ethoxide) into a mixture of water and alcohol due to the rapid hydrolysis of TTEO precursor.44 The reaction could finish within several se-

Figure 5. (a) Photographs showing the stability against water of (I) CsPbBr3/SiO2 NCs, (II) WT-CsPbBr3 NCs and (III) HICsPbBr3 NCs. The top layer is hexane, and the bottom layer is deionized water. Photographs were taken under daylight (left) and UV light (right, λ = 365 nm). (b) AFM image of CsP-



bBr3/SiO2 NCs thin film. (c) Photographs of (I) CsPbBr3/SiO2 NCs, (II) WT-CsPbBr3 NCs and (III) HI-CsPbBr3 NCs thin o film stored in humid air (40 C and humidity of 75%). (d) Photo-stability of different thin films under irradiation of 375 nm UV light.

The stability of CsPbBr3/SiO2 Janus NCs against the detrimental effects of air, water, and light irradiation was tested. The as-prepared CsPbBr3/SiO2 NCs were dispersed in hexane and placed on the top of equal volume water in air. For comparison, CsPbBr3 NCs obtained through the water-triggered transformation process (denoted as WT) and hot-injection method (denoted as HI) were treated with the same way (Figure 5a). The CsPbBr3/SiO2 NCs solution still showed bright PL after water treatment for seven days, while the emission intensity of WT-CsPbBr3 NCs solution became much lower. Furthermore, HICsPbBr3 NCs solution became almost colorless. The corresponding PL spectra are shown in Figure S14. After being treated with water for 7 days, the initial PL peak intensity of WT-CsPbBr3 NCs and HI-CsPbBr3 NCs declined to 17.8% and 9.2% of their original intensities, respectively. As a comparison, the PL intensity of CsPbBr3/SiO2 NCs can still remain around 80%. WT-CsPbBr3 NCs showed enhanced stability against water etching compared with HI-CsPbBr3 NCs due to the surface passivation during the water treatment process.15,46 After surface modification with silica, the CsPbBr3 NCs showed much better PL stability compared with those without modification, which can be attributed to the protection effect of the silica including the attached silica particles and the oligomeric silica species (Figure 1b). More importantly, thanks to the surface modification, the average decay lifetime of CsPbBr3/SiO2 NCs was determined to be ~19.8 ns which was longer than that of WTCsPbBr3 NCs (~11.6 ns) and HI-CsPbBr3 NCs (~6.7 ns, Figure S15). The longest lifetime can be attributed to the presence of oxide that can possibly passivate the NCs via reducing surface dangling bonds. As reported previously,[47] τ1, τ2 and τ3 represents trap-related recombination, radiative recombination and Auger recombination, respectively. Among the three samples, CsPbBr3/SiO2 NCs show the largest τ1 and the lowest ratio of trap-related recombination, indicating the least surface defect (Table S1). SiO2 and other polymers have been used to modify the surface of CsPbX3 NCs.30-39 However, only large particles that contain multiple NCs have been prepared, resulting in low dispersity in the solvent and difficulty in preparing a high quality thin film. In this work, the as-prepared CsPbBr3/SiO2 NCs have a much smaller particle size and better dispersibility in some frequently-used solvents (hexane, octane, chloroform, and so on). A CsPbBr3/SiO2 NCs thin film is fabricated successfully through a spincoating approach. The corresponding photograph shows uniform green emission under UV light (Figure S16). A high quality film with smooth and uniform morphology is indispensable for its future application in optoelectronics. A root mean square roughness value of 8.19 nm is obtained from the atomic force microscopy (AFM) height topography image of CsPbBr3/SiO2 NCs, suggesting the formation of a uniform

thin film. The high-quality film is also verified by scanning electron microscopy (SEM) characterization (Figure S17). The PLQY of the CsPbBr3/SiO2 NCs film is around 70%. To test the stability, thin films fabricated by CsPbBr3/SiO2 NCs, WT-CsPbBr3 NCs, and HI-CsPbBr3 NCs were placed under humid air (40 oC and humidity of 75%) for 4 days. As shown in Figure 5c, very weak PL is observed in HICsPbBr3 NCs thin film after 4 days, indicating poor stability. The WT-CsPbBr3 NCs thin film shows slightly higher stability as evidenced by the weak PL, while the CsPbBr3/SiO2 NCs thin film presented the highest stability, confirming the significantly improved stability against etching by air and water. The photo-stability of CsPbBr3 NCs is a challenging issue for their practical applications. It has been reported that light illumination could induce the surface decomposition and aggregation of CsPbBr3 NCs, leading to the degradation of their PL properties.48,49 To enhance their stability against photo-degradation, several strategies have been proposed.30,35,50,51 To date, most of the reports were focusing on the photo-stability of perovskite NCs powder or solution. Although thin films are more suitable for practical applications, fewer efforts have been devoted to studying their photo-stability, mainly because of the difficulty in fabricating high-quality thin films. Here we proposed an in situ test strategy to study the photostability of perovskite NCs thin film under continuous light illumination. The testing setup is illustrated in Figure S18. All of the films were encapsulated in an inert atmosphere so that the influence of moisture and oxygen can be avoided. The PL intensity of three thin films was tested under continuous illumination with a 375 nm LED light (117 mW/cm2) at 40 oC (Figure 5c). UV light acted as both light source for photo-stability test and excitation light for perovskite NCs film for the in situ PL intensity detection. The PL spectra were collected every five minutes for all samples (Figure S19). As depicted in Figure 5d, obvious PL enhancement was observed at the beginning for the three samples. This phenomenon has been widely observed in well-defined NCs, in which the UV light illumination can cure the surface defect sites, leading to the increase of PLQY.[25, 29, 30] With continuous light irradiation, a dramatic PL drop has been observed for the HI-CsPbBr3 and WT-CsPbBr3 NCs thin films. After continuously irradiated for 10 h, the initial PL peak intensity of HI-CsPbBr3 and WT-CsPbBr3 NCs film NCs film declined to 65% (blue line ), and 84% (red line), respectively, suggesting low photo-stability. As a comparison, the as-prepared CsPbBr3/SiO2 NCs film showed much higher stability, as confirmed by the slight drop (~2%) after the same treatment. The variation of PLQY of these three films before and after photo-stability test were recorded (Figure S20), which further demonstrated that CsPbBr3/SiO2 NCs film displayed the highest photostability.


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Journal of the American Chemical Society through perovskite layer without being absorbed, as shown in Figure 6e and Figure 6f. The color changes from white to blue, as shown in Figure S21b and S21c. The slight decrease of red peak exhibits the stablity of CdSe NCs in the three kinds of WLEDs. The great stability of WLED indicats the promising application of CsPbBr3/SiO2 NCs. 4. CONCLUSIONS

Figure 6. (a) Schematic illustration of the configuration of the WLED device. (b) PL spectra of CsPbBr3/SiO2 NCs based WLED device operated a current level of 5 mA. Inset shows a photograph of the dvice. (c) CIE color diagram of the WLED device and color triangle of blue LED, green CsPbBr3/SiO2 NCs and CdSe NCs (solid line) and NTSC standard (dashed line). The label of “+” stands for the CIE coordinate of the CsPbBr3/SiO2 based device operated a current level of 5 mA. (d-f) Time-dependent PL spectra of WLED under continuous current of 1 mA based on (d) CsPbBr3/SiO2 NCs, (e) WTCsPbBr3 NCs and (f) HI-CsPbBr3 NCs.

Because of their outstanding photophysical properties, CsPbX3 NCs have been regarded as one of the best candidates for next-generation lighting and display technology. Since the as-prepared CsPbBr3/SiO2 Janus NCs show excellent film uniformity and improved stability, their potential application in the white light emitting diode (WLED) has been demonstrated. As depicted in Figure 6a, a WLED prototype device was fabricated by combining three emissive layers. On a blueemissive GaN chip, the green-emissive composite of CsPbBr3/SiO2-polymethyl methacrylate (PMMA) and a red-emissive composite of CdSe-PMMA were deposited. The CsPbBr3/SiO2 NCs based WLED device exhibited a luminous efficiency of 56 lm W-1 and an external quantum efficiency (EQE) of 16.14% when it was operated at a current of 5 mA, as shown in Figure 6b. Figure 6c presents the Commission Internationale de L'Eclairage (CIE) diagram. The CsPbBr3/SiO2 NCs based WLED has a color coordinate of (0.3, 0.32), which is close to the standard white emission coordinate (0.33, 0.33). The correspongding value of color rendering index (CRI) is 63. The color gamut of CsPbBr3/SiO2 NCs based device is about 138% of NTSC standard gamut. In addition, the stabilities of WLEDs based on CsPbBr3/SiO2 NCs, WTCsPbBr3 NCs and HI-CsPbBr3 NCs were investigated. Figure 6d shows the time-dependent spectra of WLED based on CsPbBr3/SiO2 NCs. After one-hour, the color coordinate is still in the white emission area (Figure S21a). Meanwhile, time-dependent spectra of WLEDs based on WT-CsPbBr3 and HI-CsPbBr3 are illustrated in Figure 6e and Figure 6f, respectively. Because of the low photostablity of WT-CsPbBr3 and HI-CsPbBr3 NCs, continuous blue illumination can deteriorate the performance of perovskite NCs. The intensity of green peak decreased sharply in both WT-CsPbBr3 and HICsPbBr3 based WLEDs. More blue light is transmitting

In conclusion, monodisperse CsPbX3/SiO2 and CsPbBr3/Ta2O5 Janus NCs have been successfully prepared by combining a water-triggered transformation process and a sol-gel process. A systematic study has been carried out, and a plausible growth mechanism has been proposed. It is believed that the transformation process from Cs4PbX6 to CsPbX3 NCs and the growth of oxide occurred simultaneously at the hexane/water interface. The surface modification can dramatically improve the stability of NCs against destruction by air, water, and light. After silica modification, the Janus NCs show excellent film uniformity and exhibit longer lifetime. A WLED device with excellent performance and enhanced stability has been fabricated by using CsPbBr3/SiO2 NCs as the green light source. This work is important to the practical applications of the CsPbX3-based nanomaterials because it provides a novel and unique approach for the surface modification of such perovskite nanocrystalsNCs at a single particle level and ensures their high stability.

ASSOCIATED CONTENT Supporting Information. Additional TEM images, photographs, UV-vis absorption spectra, and PL spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (B.S.). * E-mail: [email protected] (Q.Z.). * E-mail: [email protected] (Y.Y.).

Author Contributions The manuscript was written through contributions of all authors. All authors have approved the final version of the manuscript. ‡These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Ministry of Science and Technology of China (2016YFE0129600), National Natural Science Foundation of China (21401135, 21673150). We also acknowledge the financial support from the 111 Project, Collaborative Innovation Center of Suzhou Nano Science and Technology (NANO−CIC), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and SWC for Synchrotron Radiation Research. Acknowledgment is also made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research (55904-ND10).



ABBREVIATIONS NCs, nanocrystals; TMOS, tetramethoxysilane; TTEO, tantalum (V) ethoxide; TEOS, tetraethylorthosilicate.

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Journal of the American Chemical Society SYNOPSIS TOC


9

Oxide Janus

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