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Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

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One Stone, Two Birds: High-Efficiency Blue-Emitting Perovskite Nanocrystals for LED and Security Ink Applications Chun Sun,*,†,‡ Zhiyuan Gao,†,‡ Hanxin Liu,†,‡ Le Wang,†,‡ Yuchen Deng,§ Peng Li,§ Huanrong Li,§ Zi-Hui Zhang,†,‡ Chao Fan,†,‡ and Wengang Bi*,†,‡

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State Key Laboratory of Reliability and Intelligence of Electrical Equipment, and ‡Tianjin Key Laboratory of Electronic Materials and Devices, School of Electronics and Information Engineering, Hebei University of Technology, 5340 Xiping Road, Tianjin 300401, P. R. China § Hebei Provincial Key Lab of Green Chemical Technology and High Efficient Energy Saving, School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, P. R. China S Supporting Information *

ABSTRACT: Blue-emitting perovskite nanocrystals (NCs) are known to exhibit low photoluminescence (PL) quantum yield (QY) compared to green- and red-emitting counterparts, which is due to the presence of localized trap states on the surface. Here, we demonstrate a new method to synthesize high-efficiency blue-emitting perovskite NCs by anion exchange of 0D and 3D perovskite NCs. The remarkable enhancement of the PL QY is caused by surface reconstruction and subsequent suppression of nonradiative recombination centers of 3D perovskite NCs during the anion exchange. Remarkably, these high-efficiency NCs have two appealing applications. First, they can be applied in backlight display, where the GaN light-emitting diode is usually used as the blue emission source. We found that the as-prepared NCs were facile to tune their emission peaks and possessed narrow linewidths. As a downconverter, the NCs realize an ultrapure blue light for backlight display. Second, benefiting from the existence of 0D perovskite, anticounterfeiting inks can also be achieved based on these blue-emitting perovskite NCs. The encoding/reading strategy relies on the conversion of 0D perovskites. These findings provide new opportunities for blue-emitting perovskite NCs and broaden their application. and broad emission linewidth because of enormous defects.22 The second one is CsPbBr3 nanoplatelets (NPLs), whose QY can be greatly enhanced by PbBr2 treatment.23,24 However, the emission peak is limited by the number of NPL layers and thus cannot be tuned delicately.25,26 The last one is Cl- and Brmixed perovskite NCs, which possess narrow linewidth and low QY. Recently, several researches about improving the QY of CsPb(Cl/Br)3 have been reported. There were two types of methods that have been adopted: (1) using salts posttreatment27−30 and (2) doping other ions into the lattice.31−34 In this work, we propose a unique method to enhance the QY of Cl- and Br-mixed perovskite NCs. Enlightened by the passivation effect of Cs4PbBr6 NCs to CsPbBr3 NCs,35−38 we mixed CsPbCl3 and Cs4PbBr6 NC solution to acquire CsPb(Cl/Br)3 and Cs4Pb(Cl/Br)6 composite materials with high QY, which could be used to fabricate high-performance LED. These composite materials not only reserve the luminescence properties of 3D perovskite but also possess the transformation ability of 0D perovskite. Therefore, we also can use these unique luminescence properties to synthesize

1. INTRODUCTION Halide perovskite nanocrystals (NCs), especially all inorganic cesium lead halides (CsPbX3, X = Cl, Br, I) NCs, have attracted significant attention owing to their extraordinary optoelectronic properties, including narrow emission linewidths, wide color gamut, and highly tunable band gaps throughout the entire visible range controlled by halide ions.1−5 Consequently, CsPbX3 NCs have demonstrated many potential applications in light-emitting diodes (LEDs),3,6−12 lasers,13,14 and photodetectors.15,16 Because perovskites possess high defect tolerance, the green- and redemitting NCs always show high quantum yields (QYs) (over 70%). However, because of the relative large band gap of blueemitting perovskite NCs, it is prone to introduce undesirable energy states that fall in the band gap of NCs, leading to a significant loss in radiative carrier recombination.17,18 The QYs of green-emitting (CsPbBr3) and red-emitting (CsPbI3) perovskite NCs were close to unity via post-treatment and modified synthesis procedures,19−21 while the development of blue-emitting perovskite NCs lagged far behind. Generally, there are three strategies to acquire blue-emitting perovskite nanomaterials. The first one is small-size CsPbBr3 NCs with a quantum confinement effect. Although small-size CsPbBr3 NCs exhibit blue emission, they usually suffer from low QY © XXXX American Chemical Society

Received: March 12, 2019 Revised: June 19, 2019 Published: June 20, 2019 A

DOI: 10.1021/acs.chemmater.9b01010 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

solution. As can be seen from Figure 1c, with the increasing ratio of Cs4PbBr6 NCs, the peak located at 314 nm was becoming more and more intense, whereas the other absorption peak shifted from 423 to 481 nm due to anionexchange reaction of CsPbCl3 NCs. Similarly, the emission peak also exhibited distinctly red shift from 434 to 491 nm corresponding to the red shift of absorption (Figure 1d). By using the anion-exchange reaction, delicate control of peak position can be easily realized. In situ PL experiment with the Cs4PbBr6/CsPbCl3 molar ratio of 0.24:1 was conducted to analyze the luminescent process in detail. The emission spectra variation with the reaction time is shown in Figure 2a. As can be seen from this

security inks. Nowadays, anticounterfeiting technology plays an important role in protecting important and valuable items, such as brands, luxury items, and certificates. Luminescent tags or labels are the most appreciated security elements for anticounterfeiting,39−43 which is due to their facile design and implementation. Traditional luminescent materials are easy to duplicate, thus exhibiting poor anticounterfeiting performance. Therefore, it is still in high demand to develop novel photoluminescence (PL) materials with more sophisticate and deceptive features. Multifeatured inks, optically variable inks, and thermochromic inks have been developed for better protecting.44−46 Here, we use this composite material combined with blue-emitting CsPbBr3 NPLs as the security inks. These two inks were both invisible at ambient light and displayed blue emission under UV light. However, with contacting water, our composite material turned green while the luminescence of CsPbBr3 NPLs disappeared. In this way, security information can be encrypted by our composite material ink to realize the anticounterfeiting performance.

2. RESULTS AND DISCUSSION 2.1. Blue-Emitting Perovskite NCs. CsPbCl3 and Cs4PbBr6 NCs were synthesized following the established process with minor modifications.47 The absorption spectra of CsPbCl3 and Cs4PbBr6 NCs are presented in Figure 1a. For

Figure 2. In situ PL study (a), emission peak (b), fwhm (c), and QY (d) variation of the mixed solution with the Cs4PbBr6/CsPbCl3 molar ratio of 0.24:1 along with different time.

figure, at the first stage (0−3 s), the emission intensity is increased enormously (nearly 30 times) with the emission peak shifting to the red (less than 10 nm). With the reaction time (4−15 s) going on, the emission peak still exhibits a 25 nm red shift, whereas the emission intensity is almost unchanged. Next, the movement of emission peak slows down, which changes 26 nm in 85 s. However, the emission intensity is again enhanced pronouncedly (more than 3 times) in this process. In the final stage, the emission peak and intensity are stable and unchanged. The whole halide anion-exchange reaction is finished after 102 s. Therefore, the anion-exchange reaction time can be set as 4 min to ensure complete transformation. The specific variation of emission peak position, full width at half-maximum (fwhm) and QY are shown in Figure 2b−d. At the first two stages, the fwhm presents an increasing tendency because of the inhomogenous anion composition, which is also observed in the reaction of CsPbBr3 and CsPbI3.48 Next, at the third stage, the fwhm decreases down gradually. Finally, the fwhm basically remains at 16 nm in the wide time range from 102 to 600 s. Through the anion-exchange reaction, the QY increases from 1 to 90%, which is comparable with the tetrafluoroborate salt-induced PL enhancement.29 Usually, the increment of QY for 0D and 3D composite materials comes from the following three aspects. The first aspect is lattice-matching-induced surface passivation, which can enhance the radiative rate of the NCs by forming a joint

Figure 1. (a) UV/vis absorption spectra of colloidal dispersions of CsPbCl3 and Cs4PbBr6 NCs. (b) PL spectrum of CsPbCl3 NCs. (c) and (d) UV/vis absorption and PL spectra with different ratio of Cs4PbBr6/CsPbCl3.

Cs4PbBr6 NCs, the absorption spectrum is dominated by a typical sharp peak at 314 nm, which is assigned to the 1S0 → 3P1 transition of Pb2+ centers. The exciton absorption peak of CsPbCl3 NCs is evident and located at 400 nm. Because neither Cs4PbBr6 nor CsPbCl3 NCs absorb visible light in the range of 400−780 nm, their solutions were colorless at ambient light. Although the luminescence of Cs4PbBr6 is still under debate, the Cs4PbBr6 NCs prepared here show no fluorescence under UV light. The CsPbCl3 NCs emit weak violet light centered at 404 nm with solution PL QY less than 1% (Figure 1b). Next, we mixed the CsPbCl3 with Cs4PbBr6 NC solution together by fixing the amount of CsPbCl3 NC B


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Chemistry of Materials structure.35 The second aspect is the type-I band structure, in which carriers will stay confined, increasing the chance of radiative recombination.36 The last one is the shell-protecting strategy, which encloses the surface of 3D perovskite NCs and protects it against corrosion to enhance the formation of excitons, thus resulting in improvement of the QY.37,38 Based on the above speculations, the increment of QY for composite materials could result from either the joint structure or the core−shell structure. To explore the mechanism of PL QY enhancement, we examined the X-ray diffraction (XRD) and transmission electron microscopy (TEM) of three representative products named S1−S3 (S1, S2, and S3 corresponding to Cs4PbBr6/ CsPbCl3 molar ratio of 0.04:1, 0.24:1, and 0.64:1, respectively) as well as the starting materials. As presented in Figure 3a, the

py (FTIR) was used to examine the presence of ligands in the as-synthesized and treated NC samples. The peaks at 2852 and 2923 cm−1 are ascribed to the symmetric and asymmetric C− H stretching vibrations,50 and the peak at 1465 cm−1 can be attributed to CH2 bending vibration (Figure S12). The signal at 1642 and 1542 cm−1 can be assigned to the N−H bending and vibration of the carboxylate group,30 respectively. As can be seen from this figure, all of them show peaks belonging to oleylamine (OLA) and oleic acid (OA), which indicate that the surface ligands have no change after the anion-exchange reaction. Besides, nuclear magnetic resonance (NMR) spectroscopy further confirms the ligands of the sample S2, where characteristic resonances (α, β, and 1) of both OLA and OA are recognized (Figure S13).51 Previous studies have already stated that the surface trap was the main obstacle for acquiring high QY.17,19,20,24,29 Based on the above experimental phenomenon, here, we propose that the surface trap of 3D perovskite NCs can be repaired in the process of anion-exchange reaction with 0D perovskite NCs, thereby resulting in the significantly improvement of QY, while the surface of 0D perovskite NCs is etched and 0D perovskites become round in this process. This process is illustrated in Figure 4. Normally, the as-prepared 0D perovskite NCs show a

Figure 3. (a) XRD patterns of CsPbCl3 NCs, Cs4PbBr6 NCs, and samples S1−S3. (b) TEM image of the S2 sample.

characteristic diffraction peaks of both Cs4PbBr6 and CsPbCl3 NCs are found in all three products, indicating no other materials or phases are formed like PbBr2 and CsPb2Br5. With the increase of the Cs4PbBr6/CsPbCl3 molar ratio from 0.04:1, 0.24:1, and 0.64:1, the peak located at 32.1° is less pronounced and shifts to low angle, representing that CsPbCl3 NCs are subjected to the anion-exchange reaction. On the contrary, with the increasing ratio of Cs4PbBr6 NCs, the peak located at 28.8° becomes more evident. The microstructure and morphology can be further confirmed by TEM results. Figure 3b shows the TEM image of sample S2. As can be seen, both 3D and 0D perovskite NCs of S2 are observed with a distinct structure. The 3D perovskite NCs show a regular cubic morphology with an edge length of 9 nm, while the 0D perovskite NCs exhibit a spherical morphology with a diameter of 15 nm. However, we should notice that regular hexagon with the diameter of 22 nm was observed for 0D perovskite NCs before mixing together. After the anion-exchange reaction, the size of 0D perovskite NCs shrinks, and their corner becomes round. The high-resolution transmission electron microscopy (HRTEM) images (Figures S3 and S8) show variation of lattice spacing from 0.27 to 0.28 nm for the (002) plane of CsPbCl3 after mixing due to anion exchange, which also implies the CsPb(Cl/Br)3 formed. Similarly, the lattice spacing of Cs4PbBr6 decreases from 0.68 to 0.65 nm for the (110) plane, indicating that Cs4Pb(Cl/Br)6 NCs are produced. The selected area electron-diffraction (SAED) pattern also demonstrates that the NCs include Cs4Pb(Cl/ Br)6 and CsPb(Cl/Br)3 phase together with corresponding (200) and (110) planes of each (Figure S9). Other independent 0D and 3D perovskite structures can also be found in the samples of S1 and S3 (Figures S10 and S11), which is not similar to previous QY enhancement reports with related materials.37,38,49 Fourier transform infrared spectrosco-

Figure 4. Schematic of the anion-exchange and surface reconstruction process.

regular hexagon shape. Based on the principle of thermodynamics, the chemical reactivity of the corner is higher than those of the other places. Besides, in 0D perovskites, [PbBr6]4− octahedra is not connected with each other, which is different from the 3D perovskite. Because of this, the bonding of these individual [PbBr6]4− octahedra is weaker than that of 3D perovskites. Therefore, the [PbBr6]4− octahedra can be easily dissociated from 0D perovskites and entered into 3D perovskites. As can be seen from the figure, the corners of 0D perovskites were dissolved in the anion-exchange process, whereas the surface of 3D perovskites was reconstructed along with anion exchange. In the end, the traps of 3D perovskites were repaired, which results in high QY. For 0D perovskites, because of the etch of their corner, their size shrunk and their C


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2020 standard blue light (0.131, 0.046), which, to the best of our knowledge, represents the purest blue backlight ever reported (Figure 6b). The luminous efficiency of the device is 4.2 lm/W, which is very high for UV-chip based perovskite LED devices. Besides, the blue-emitting LED possesses excellent stability, which can maintain 80% of the original intensity after continues working 90 h (Figure S15). All these suggest the great potential of the composite NCs in backlight display application. 2.3. Anticounterfeiting Inks of Perovskite NCs. In view of the inherent merits of perovskites, the composite may hold a great promise for advanced information security applications such as anticounterfeiting inks. 3D perovskites were prone to decompose along with losing their luminescence when encountering a polar solvent such as water due to its ionic characteristic, while the colorless 0D perovskite Cs4PbBr6 could transform into bright green 3D perovskite CsPbBr3.53 It is commonly accepted that Cs4PbBr6 NCs can be regarded as a CsBr-rich perovskite structure. As Yin et al. have demonstrated, CsBr possesses high solubility (1243 g/L at 25 °C) in water.53 After treating with water, stripping of CsBr occurs. In the end, CsPbBr3 NCs can be formed. We carried out this encoding/reading experiment using solutions of sample S2 and treated CsPbBr3 NPLs as encryption (E) and confusion (C) inks, respectively (Figure 7). First, the CsPbBr3 NPLs with the emission peak at 456 nm were prepared as in the previous report54 and treated with PbBr2 solution to improve their QY over 85% (detailed characterization and analysis about the PbBr2 treatment are shown in Figures S17− S20 and Table S4). Next, different kinds of encryption systems

shape became spherical, which is in good accordance with the TEM results. To confirm this hypothesis, we performed timeresolved photoluminescence (TRPL) spectroscopy to investigate the variation of trap states. As seen from Figure 5b and

Figure 5. (a) PL decay and fitted curves of the PL emission at 434 nm by the HI method and sample S1. (b) PL decay and fitted curves of the PL emission at 463 nm by HI method and sample S2. (c) PL decay and fitted curves of the PL emission at 486 nm by HI method and sample S3.

Table S3, all of the products show two-component decay curves. The average lifetime of sample S2 is faster than that of hot injection (HI) method, indicating a higher ratio of exciton recombination and less transition at defect states.52 Typically, the radiative and nonradiative decay rates can be estimated from the PL QYs and the corresponding average lifetimes. Compared with S2, the nonradiative decay rate of the HI method is reduced from 0.11 to 0.02 ns−1 and the radiative decay rate is enhanced from 0.05 to 0.25 ns−1, indicating that better channels of radiative recombination can be acquired (Table S3). A similar phenomenon of radiative and nonradiative decay rates also can be found in S1, S3, and their counterparts of the HI method, which imply that nonradiative recombination centers on the surface of the 3D perovskite are successfully suppressed in our samples.17,19,52 2.2. LEDs from Blue-Emitting Perovskite NCs. Considering that the composites of 0D and 3D perovskite solution possess high PL QY up to 90%, we can use these materials as a color conversion layer of LEDs. For example, the composite material with the Cs4PbBr6/CsPbCl3 ratio of 0.24:1 were mixed with 15 wt % poly(methyl methacrylate) (PMMA)/ toluene solution in the glove box and then coated on a commercially available 365 nm LED chip (the QY of perovskite film is 74%). As presented in Figure 6a, the LED device exhibits bright blue emission with the emission peak at 463 nm. Because of the narrow fwhm (16 nm), the color coordinate of the device (0.134, 0.046) is very close to the Rec.

Figure 7. (a) Photograph of the painting paper under ambient light. (b) Trademark pattern of the Lancôme written by security inks under UV light. (c) Decryption information of the trademark pattern of (b) after being treated with water. (d) PL spectra of NPLs before and after water treatment. (e) PL spectra of sample S2 before and after water treatment. (f) PL decay curves of NPLs before and after water treatment. (g) PL decay curves of sample S2 before and after water treatment.

Figure 6. (a) PL spectrum of UV LED chip coated with sample S2 (inset is the photograph of LED operated at 20 mA). (b) Color coordinates of the LED device together with the Rec. 2020 standard. D


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Beijing Chemical Factory. Polymethylmethacrylate (PMMA, Mw = 120 000) was purchased from Sigma-Aldrich. All of the chemicals were used without further purification. 4.2. Synthesis of CsPbCl3 NCs. Ten milliliters of ODE, 1 mL of OA, 1 mL of OLA, 0.0325 g of Cs2CO3, and 0.0758 g of (CH3COO)2Pb were loaded into a 50 mL three-neck round-bottom flask. Then, the flask was degassed and dried under vacuum for 1 h at 120 °C. Next, the flask was switched to N2 and elevated to 180 °C. TMSCl (0.08 mL) was quickly injected at this temperature. After 5 s, the flask was immersed in an ice-water bath to finish the reaction. For purification, the solution was centrifuged at 7000 rpm for 10 min. Toluene was added to the precipitate and then centrifuged at 4000 rpm for 2 min. Finally, the supernatant with NCs was collected. 4.3. Synthesis of Cs4PbBr6 NCs. The procedure of Cs4PbBr6 was similar to that of CsPbCl3. Only the amount of (CH3COO)2Pb was changed to 0.015 g, and the injecting halogen was changed to TMSBr. 4.4. Anion-Exchange Reaction. 3D perovskite (50 μL, 0.5 mg/ mL) was added into a 10 mL centrifuge tube which contained 4 mL of toluene. Then, the tube was vibrated for 30 s. Another 11 samples could also be prepared by the same method. Next, different volumes (detailed numbers were outlined in the Table S1) of 0D perovskite (0.2 mg/mL) were added into the 12 centrifuge tubes and vibrated for 4 min. The above solution (0.1 mL) was taken out and added into 2 mL of toluene to measure the absorbance and PL spectra. 4.5. Preparation of CsPb(Cl/Br)3 NCs by HI Method. The synthesis of CsPb(Cl/Br)3 NCs by TMSCl and TMSBr was similar to that of CsPbCl3. To prepare NCs with 463 nm emission, 0.04 mL of TMSCl and 0.05 mL of TMSBr were mixed with 0.3 mL of ODE, and then, the solution was injected at 180 °C. For NCs with 434 nm emission, 0.053 mL of TMSCl and 0.03 mL of TMSBr were injected. TMSCl (0.03 mL) and TMSBr (0.063 mL) were used to synthesize NCs with 486 nm emission. All other steps were the same. 4.6. LED Fabrication. Unpackging UV LED chip (SP3-S6PD-1) with the peak emission wavelength centered at 365 nm and electrical power of 200−300 mW was used for the fabrication of LED, which is purchased form Skybright Inc. CsPbCl3 toluene solution (1 mL, 0.5 mg/mL) and Cs4PbBr6 toluene solution (6 mL, 0.2 mg/mL) were dispersed in 15 wt % toluene/PMMA solution. The mass ratio of 0D to 3D was about 2.4:1. The mixture was vibrated for 10 min and coated onto the UV LED chip directly drop-by-drop, allowing the toluene to evaporate under vacuum between drops. The optical density of the final coated film was about 0.9 (measured at 365 nm). Finally, 10 μL of toluene/PMMA solution was coated onto the LED chip to enhance the stability. 4.7. Synthesis of Security Inks. First, the CsPbBr3 NPLs were synthesized according to previous reports.54 Specifically, 20 mL of ODE, 3 mL of OA, 3 mL of OLA, and 0.758 g of PbBr2 were loaded into a 50 mL flask. Then, the flask was heated to 120 °C for 1 h. After that, the solution was cooled down to room temperature to obtain a PbBr2 precursor. As-prepared PbBr2 precursor (26 mL) and cesium oleate solution (2.5 mL) were loaded into a 50 mL autoclave and heated at 100 °C for 1 h. Then, the solution was centrifuged at 9000 rpm for 10 min. The precipitate was washed by ethyl acetate two times, and then, NPL particles were obtained. PbBr2−ligand solution was prepared by dissolving 0.1 mmol PbBr2, 100 μL of each of OLA and OA in 10 mL of hexane at 100 °C. Then, the NPLs were redispersed in 2 mL of the PbBr2−ligand solution to synthesize the confusion ink. The encryption ink was prepared by adding 1 mL of CsPbCl3 toluene solution (0.5 mg/mL) into 6 mL of Cs4PbBr6 toluene solution (0.2 mg/mL). 4.8. Characterizations. Fluorescence emission spectra were acquired using an Ocean Optics spectrometer. Absorbance spectra were measured by using a Shimadzu UV-2550 spectrophotometer. The morphology of the NCs was obtained by a FEI Tecnai G2 Spirit TWIN TEM operating at 200 kV. XRD patterns of NCs were carried out using a Bruker D8 ADVANCE X-ray diffractometer (Cu Kα: λ = 1.5406 Å). FTIR spectroscopy was obtained by a Thermo-Nicole iS50 FTIR-spectrometer. The absolute PL QYs of the samples were performed on a fluorescence spectrometer (FLS920P, Edinburgh Instruments) equipped with an integrating sphere with its inner face

can be designed with the CsPbBr3 NPL solution and our inks. To enhance the paper resistance to water, sulfuric paper was used to replace the ordinary paper. Here, cotton soaking water is adopted to wipe the patterns not dropping water directly because the NCs treated by water is still not stable under excess water (the green color disappears in 10 min), and large amounts of water will destroy the paper. As shown in Figure 7a, the encrypted information on the sulfuric paper is invisible under ambient light. A trademark pattern of the Lancôme was fabricated (Figure 7b,c). The flower and letter can be identified and show blue fluorescent signals under UV light because of the blue fluorescence emitted from both of ink E and ink C. To distinguish the authenticity of the trademark, the Lancôme pattern anticounterfeiting tag was wiped by cotton soaking with water. If the flower disappears and the letters turn green under UV light, they are authentic, otherwise they are fake. The anticounterfeiting tag is single-use, meaning that it will be destroyed once used. That can guarantee that it will not be replicated and provides more protection to the product. PL and TRPL spectra were measured to analyze the color change. After water treatment, the lifetime of NPLs decreases and their PL peak disappears, demonstrating that the optical properties of NPLs are destroyed. Conversely, the lifetime of S2 increases after treating with water, meaning that new radiative recombination channels appear. In addition to the change of the lifetime, the PL peak of S2 located at 463 nm disappears and a new peak located at 507 nm shows up (Figure 7d−g). All these results demonstrate that the original CsPb(Cl/Br)3 NCs of 463 nm are destroyed and new 3D perovskites with more Br content appear from 0D perovskites. XRD experiment for S2 after treating with water can also verify this. A pure 3D perovskite phase is acquired because of the reaction of stripping CsX from Cs4PbX6 (Figure S20). In addition to this, security information coding with numbers, binary codes, Morse code, English letters, and Chinese characters can also be realized as shown in Figures S22−S27, demonstrating that this brand new optical authentication method could potentially be applied in advanced anticounterfeiting information projects.

3. CONCLUSION In summary, a simple strategy was presented to develop blueemitting perovskite NCs with high QY. After the simple anionexchange reaction, the QY of perovskite NCs can be enhanced to 90%. Unlike the previous reports, the extraordinary improvement of QY originated from the surface reconstruction. Two advanced applications were achieved using this highperformance material. First, blue LEDs with an emission peak at 463 nm were fabricated with the perovskite NCs. Because of the narrow fwhm, the color quality of the as-prepared LED is very closed to the standard blue light source, which becomes the best ideal light. In addition, combined with other blue luminescence perovskites, these unique perovskite NCs can act as new smart concealed inks for information encryption and decryption, suggesting the potential application in anticounterfeiting and information security. 4. EXPERIMENTAL SECTION 4.1. Chemicals. Cs2CO3 (99.9%), bromotrimethylsilane (97%, TMSBr) and chlorotrimethylsilane (99%, TMSCl) were attained from J&K. OA (90%) and octadecene (90%, ODE) were purchased from Alfa Aesar. OLA (80−90%, OLA) and (CH3COO)2Pb (99.99%) were purchased from Aladdin. Toluene (99.5%) was obtained from E


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coated with BENFLEC. Time-resolved PL lifetime measurements were carried out using a time-correlated single-photon counting lifetime spectroscopy system with a picosecond pulsed diode laser (EPL-380 nm) as the single wavelength excitation light source. NMR measurements were measured on a Bruker AVANCE 400 Spectrometer operating at a 1H frequency of 400 MHz. LED luminous efficiency was conducted by an ATA-1000 electroluminescence measurement system (made by Everfine).

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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/acs.chemmater.9b01010. Calculation of the molar ratio for 0D and 3D perovskites; discussion of the content of 0D perovskite; TEM images, SAED of 0D and 3D perovskites; FTIR spectra of the samples S1−S3 and 0D and 3D perovskites; calculation of radiative and nonradiative decay rate; PLE spectrum of S2; stability of the blueemitting LED; XRD pattern after water treatment; PL, TRPL, XRD, and TEM of CsPbBr3 NPLs and PbBr2treated NPLs; and security application of the inks (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.S.). *E-mail: [email protected] (W.B.). ORCID

Chun Sun: 0000-0003-0046-8407 Huanrong Li: 0000-0001-7400-7668 Zi-Hui Zhang: 0000-0003-0638-1118 Wengang Bi: 0000-0002-3231-7980 Author Contributions

The manuscript was written through contributions of all authors. Funding

C.S. received funding from Foundation of Hebei Education Department (BJ2019027) and the Natural Science Foundation of Hebei Province (F2018202046). W.B. received funding from State Key Laboratory of Reliability and Intelligence of Electrical Equipment (EERIZZ2018003). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Prof. Shu Xu and Dr. Chong Geng for the expert assistance. REFERENCES

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