Unusual stability and temperature-dependent properties of highly

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Unusual stability and temperature-dependent properties of highly emissive CsPbBr3 perovskite nanocrystals obtained from in-situ crystallization in PVDF Panting Liang, Pan Zhang, Aizhao Pan, Ke Yan, Yongsheng Zhu, Minyan Yang, and Ling He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b06811 • Publication Date (Web): 06 Jun 2019 Downloaded from http://pubs.acs.org on June 6, 2019

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ACS Applied Materials & Interfaces

Unusual stability and temperature-dependent properties of highly emissive CsPbBr3 perovskite nanocrystals obtained from in-situ crystallization in PVDF Panting Liang,† Pan Zhang,† Aizhao Pan,*,‡ Ke Yan,*,† Yongsheng Zhu,† Minyan Yang,† Ling He‡ †

Key Laboratory of Education Ministry for Modern Design & Rotor-Bearing System, Xi'an

Jiaotong University, Xi’an 710049, China ‡

Department of Chemistry, School of Science, Xi’an Jiaotong University, Xianning West Road,

28, Xi’an 710049, China

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

ABSTRACT All-inorganic cesium lead halide perovskite nanocrystals (CsPbX3, X= Cl, Br or I) present broad applications in the field of optoelectronics due to their excellent photoluminescence (PL), narrow spectral bandwidth and wide spectral tunability. However, their poor stability limits their practical application. In this work, we successfully use an in-situ crystallization strategy for growing and cladding CsPbBr3 perovskite nanocrystals in polyvinylidene difluoride (PVDF). The CsPbBr3 nanocrystals in the as-fabricated CsPbBr3@PVDF composites have an average diameter of 16~18 nm and a strong PL emission (537nm), with a photoluminescence quantum yield (PLQY) exceeding 30%. In addition, the fabricated CsPbBr3@PVDF composites present improved resistance to heat and water preserving with remarkable optical performance, owing to the effective protection of PVDF. Moreover, the CsPbBr3 nanocrystals generated in PVDF can withstand temperatures up to 170℃ and can be completely immersed in water for 60 days while still retaining high PL intensity, which facilitate the practical application of CsPbBr3 perovskite nanocrystals. These CsPbBr3@PVDF composite films with remarkable optical performances and superior anti-interference ability have broad application prospects in optoelectronics, as well as good potential as temperature sensors in mechanical engineering. KEYWORDS: perovskite, nanocrystals, polyvinylidene difluoride (PVDF), temperature-dep endent, composite, inclusion


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INTRODUCTION In recent years, all-inorganic CsPbBr3 perovskite nanocrystals (NCs) have attracted widespread attention as a new type of photovoltaic material. CsPbBr3 NCs have many exceptional properties, such as excellent photoluminescence quantum yield (PLQY), narrow spectral bandwidth, and wide spectral tenability, which lay the foundation for the scientific and practical applications of CsPbBr3 NCs.1-5 It is worth mentioning that CsPbBr3 NCs have already found broad applicability to optoelectronic devices, such as light-emitting diodes (LED),6,7 solar cells,8-10 photodetectors,11-12 and lasers.13-15 However, they are easily decomposed by heat, oxygen, ultraviolet (UV) radiation and moisture on account of the strong ionic character and low formation energy of CsPbBr3 NCs.1617

As a consequence, CsPbBr3 perovskite nanocrystals have poor stability, greatly limiting the

practical their applications. Multiple attempts were made to improve the stability of CsPbBr3 NCs over the past few years.6,

18-22

For instance, Jing and coworkers used a surface passivation layer produced by

selective etching with acetone to improve the stability of perovskite NCs to some extent.23 Palazon and coworkers proposed to improve the stability of CsPbBr3 NCs by performing X-ray lithography on them.24 Embedding perovskite NCs into SiO2, Al2O3, TiO2, mesoporous materials and various polymers has also shown promise.25-30 Among the various approaches, encapsulation of perovskite NCs in a polymer matrix, such as poly(methyl methacrylate) (PMMA),31 polystyrene (PS),32 poly(maleic anhydride-alt-1-octadecene) (PMAO)29 is considered to be the simplest, lowest-cost and most efficient method, due to the naturally excellent chemical and mechanical properties of



these polymers. For example, Yang et al enhanced the stability of CsPbBr3 NCs by blending them into polystyrene (PS) via an electrospray method.33 Zhang and coworkers encapsulated CsPbX3 NCs with different chemical compositions into PS microhemispheres to achieve high stability perovskite NCs.34 In spite of the good stability and optoelectronic performances of perovskite NCs achieved by blending them with polymers, this approach still has drawbacks. The perovskite NCs are usually prepared first, thus they are inevitably partially decomposed by oxygen or moisture during blending.35 In order to prevent them being exposed to the air and being degraded, in-situ growth of perovskite NCs in a polymer has been developed. Meyns and coworkers produced perovskite NCs with improved stability by in situ addition of PMAO.36 Ma et al embedded CsPbBr3 NCs into PMMA by microfluidic spinning microreactors, obtaining PMMA@perovskite composites with good photoluminescence and PL stability.37 The in-situ crystallized CsPbBr3 NCs capped with polyvinylpyrrolidone (PVP) with uniform size distribution fabricated by Liu et al also exhibited improved stability.38 Polyvinylidene difluoride (PVDF) is a hydrophobic polymer with superior piezoelectric, mechanical and film forming properties,39,40 which is used as the polymer matrix for coating perovskite NCs in this paper. Herein, as depicted in Scheme 1, we use an in-situ crystallization strategy for growing and cladding CsPbBr3 perovskite NCs in PVDF. In order to obtain CsPbBr3@PVDF composite films with better performance, we optimized the preparation process by changing the heating temperature, solvents and polymer types. We used X-ray diffraction (XRD), proton nuclear magnetic resonance (1H-NMR), and X-ray photoelectron spectroscopy


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(XPS) to characterize the structure and composition of the as-fabricated CsPbBr3@PVDF. The morphology and size of CsPbBr3 perovskite NCs encapsulated in PVDF were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). We also performed water and photo stability tests by immersing the as-prepared composite films into water and exposing them to UV light. Additionally, we performed a temperature-dependent experiment to evaluate the film’s thermal stability. The CsPbBr3@PVDF composite film not only exhibits remarkable optical performances, but also presents excellent thermal and water stability, facilitating the practical application of CsPbBr3 NCs as temperature sensors in mechanical engineering. EXPERIMENTAL SECTION Materials PbBr2 (99.90%), CsBr (99%), polyvinylidene difluoride (PVDF) were purchased from Aladdin Industrial Corporation. N,N-Dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) (analytical purity) were purchased from Fuyu Fine Chemical Corporation (Tianjin, China). All other reagents were used as received without further purification. Fabrication of CsPbBr3@PVDF composite films DMF was used as solvent for the fabrication of CsPbBr3@PVDF composite films, thanks to its superior dissolving power. PVDF was pre-dissolved in DMF (20% wt.) to form a clear polymer solution under vigorous stirring. PbBr2 (139 mg) and CsBr (80 mg) were completely dissolved in DMF (5 ml) with the help of sonication. The clear precursor solution of CsPbBr3@PVDF was



prepared by thoroughly mixing the two solutions (1:1 vol.) with the addition of a small amount of DMSO (DMF:DMSO=4:1 vol.), because of the latter’s low evaporation rate and outstanding dissolving power.38 We then deposited a drop of the precursor solution onto a glass substrate and heated it to 100℃. After about 6 minutes, when DMF was completely evaporated, the color of the precursor film changed to yellow, implying the formation of the expected CsPbBr3@PVDF composite film. Notably, the resulting composite film could be successfully separated from the glass substrate. For synthesis of the CsPb(Br/Cl)3@PVDF (blue), CsPb(Br/I)3@PVDF (yellow) and CsPbI3@PVDF (red) films, PbBr2 was replaced by the PbCl2/PbBr2=1:1, PbBr2/PbI2=1:1 or pure PbI2, and CsBr were replaced by CsCl and CsI. Optimization of the fabrication process To obtain a composite film which has the best optical properties, we fabricated 6 samples under different conditions. As displayed in Table S1, different CsPbBr3@Polymer composite films were fabricated in different solvents, using different polymers, and with different heating temperatures. The CsPbBr3 NCs without polymer coating emits a weak green light under a UV lamp owing to the aggregation of NCs, as shown in sample 1. In contrast, the CsPbBr3 NCs encapsulated in PVDF matrix (sample 2) emits a brighter green light. The brightness of sample 3, to which a small amount of DMSO was added, is significantly higher than sample 2. When replacing PVDF with PMMA (sample 4), the composite film still emits bright green light, but the emission is weaker than that of sample 3. We observed that the composite film prepared at a heating temperature of 100℃ has the highest fluorescence intensity by comparing sample 3, sample 5 and sample 6, for which the


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heating temperature is 100℃, 170℃ and 50℃, respectively. Figure S1 shows microscopic imaging results of different composite films, which further demonstrates that sample 3 has the best morphologies and optical properties. It should be noted that in this work we use the composite film fabricated under the sample 3 conditions as the experimental sample in further tests. Stability tests To measure water stability, a CsPbBr3@PVDF composite film was immersed in water for different times. For the photo stability test, a CsPbBr3@PVDF composite film was exposed to UV light for 48 h. We performed a temperature-dependent experiment to evaluate the thermal stability of the as-fabricated composite films by tracing the changes in the photoluminescence spectrum of the composite films with temperature. The composite film was heated from 30℃ to 170℃, and then cooled back to 30℃ on a digital thermostat. For fabricating the thermal sensor to monitor the bearing temperature, the as-prepared CsPbBr3@PVDF composite films were attached by thermal conductive silica gel to the inner ring of bearing.41 In this process, we used portable Ocean View fiber optic spectrometer to measure the spectrum curves of the composite films at different temperatures while in bearing rotating operation. Furthermore, we also designed a cycle test to estimate the reversibility and durability of the as-fabricated composite film. Fabrication of White-Emitting LED device A white-emitting light-emitting diode (LED) was fabricated by loading CsPbBr3@PVDF composite film (537 nm) and YAG fluorescent (660 nm) layer onto a GaN (460 nm) LED chips. The red light-emitting layer was formed by spin coating the mixture on a blue LED chip, thermally



curing at 40°C for 30 minutes, and then thermally curing at 120°C for 60 minutes. The asfabricated composite film was used to yield a green-emitting layer. Final device stacks were subjected to a thermal curing at 40°C for 30 minutes and then a thermal curing at 120°C for 60 minutes to obtain a white-emitting device. Characterization Methods 1

H NMR spectra were acquired on a Bruker Avance II 400 MHz NMR spectrometer operating at

a 1H frequency of 400 MHz and equipped with a BBFO-Z probe. Ultraviolet and visible absorption (UV-vis) spectra for CsPbBr3@PVDF composite films were observed by a Cary 5000 UV-visNIR spectrophotometer. The fluorescence spectra were measured using Edinburgh Instruments FLSP920 fluorescence spectrophotometer, equipped with an integrated sphere for measuring the absolute photo luminescent quantum yields (PLQYs). A portable Ocean View fiber optic spectrometer was used for monitoring the spectrum curves while in bearing rotating operation. 41 XRD data were obtained on a Bruker AXS D8 Discover GADDS X-ray diffractometer equipped with a Vantec-500 area detector operated at 35 kV and 40 mA at a wavelength of Cu Kα (1.79 Å). Elemental composition of X-ray photoelectron spectroscopy (XPS) measurements was processed on the air-exposed film surface by an AXIS ULTRA (England, KRATOS ANALYTICAL Ltd.) using an Al mono Kα X-ray source (1486.6 eV) operated at 150 W. SEM images were acquired using a JEOL7800F field-emission scanning electron microscope. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were recorded on a FEI G2F30 electron microscope operated at 200 kV with a Gatan SC 200 CCD camera.


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RESULTS AND DISCUSSION

Scheme 1. Illustration of the in-situ fabrication of a CsPbBr3@PVDF composite film. It is widely known that PVDF can swell and expand in DMF.40 At the same time, the CsPbBr3 perovskite precursor solution can be introduced into the PVDF matrix during the swelling process. During heating process, DMF solvent is driven out of the polymer matrix, and the in-situ crystallization of CsPbBr3 perovskite NCs begins. The growth and sizes of CsPbBr3 NCs is limited by the surrounding polymer chain clusters.6, 35 At the same time, PVDF will shrink and form a protective layer around CsPbBr3 NCs, protecting them from water, oxygen or heat from the surrounding environment.40 Therefore, the key in the in-situ crystallization of CsPbBr3 NCs encapsulated in PVDF is to regulate the crystallization processes of PVDF and of CsPbBr3 NCs, simultaneously.40



Figure 1. (a) Wide-angle XRD patterns of a CsPbBr3@PVDF composite film (black line) and standard XRD patterns of CsPbBr3 (red line). The inset displays the crystal structure of CsPbBr3. (b) XPS survey scan of the as-fabricated CsPbBr3@PVDF film. (c) Optical absorption and PL emission spectra of the as-fabricated composite film. (d) Confocal microscope image of the asfabricated composite film. The inset shows an optical image of the experimental composite film under UV irradiation. The crystallographic structure of CsPbBr3@PVDF composite film was characterized by XRD. PVDF was characterized by 1H-NMR, as depicted in Figure S2. As shown in Figure 1a, the XRD pattern of the composite film shows strong scattering signals. To confirm the crystal structure, a standard XRD patterns of CsPbBr3 is also displayed in Figure 1a. The dominant diffraction peaks of the CsPbBr3@PVDF composite film at 15, 21, 31, 38 and 44 are assigned to the lattice


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distances of the (100), (200), (210), (220), and (300) planes of crystalline orthorhombic CsPbBr3, indicating that the structure of in-situ synthesized CsPbBr3 NCs capped with PVDF matrix is consistent with orthorhombic CsPbBr3, similarly to previous reported works.20, 29, 42 Figure 1b shows the XPS spectrum of CsPbBr3@PVDF composite film, from which peaks originating from Cs 3d, Pb 4f and Br 3d electrons in CsPbBr3 perovskite NCs are clearly observed, demonstrating that the elemental composition of the as-fabricated composite film contains the elements expected from CsPbBr3 NCs. In addition, CsPbBr3@PVDF composite films also include F and C elements from PVDF. Figure S3 shows the zoomed-in scans of XPS for Cs, Pb, Br, F and C elements. Figure 1c displays the typical UV-Vis absorption and PL spectra of CsPbBr3@PVDF composite film. The as-synthesized CsPbBr3 NCs encapsulated in PVDF exhibit an absorption onset at 522 nm and a PL emission peak centered at 537 nm with a narrow full width at halfmaximum (FWHM) of 24 nm and high PLQY value (65%, Figure 1c), which implies a relatively uniform size distribution of CsPbBr3 NCs, close to previous reported encapsulated NCs.31, 42-45 As exhibited in Figure 1d, the fact that most of the area of the confocal microscope image of the CsPbBr3@PVDF composite film shows green light, combined with the inset picture, which also indicates that under UV light excitation the composite film emits bright green light, together show the uniform light emission of the as-fabricated composite film.



Figure 2. (a) SEM image of CsPbBr3@PVDF composite film. (b, c, d, and e) Elemental mapping of the as-fabricated composite film wherein (b), (c) and (d) represent Cs, Pb and Br element of CsPbBr3 NCs embedded in PVDF, respectively, (e) represents F element of PVDF. To characterize the surface morphology of CsPbBr3@PVDF composite film, scanning electron microscopy (SEM) was used. Figure 2a exhibits a typical SEM image of the as-fabricated composite film with the uniform and smooth morphology. Compared with the TEM image of the composite film in Figure 3a, CsPbBr3 NCs cannot be observed on the surface of the composite film, further confirming that CsPbBr3 NCs are encapsulated in PVDF matrix. Figure S4 shows the SEM images of different samples, indicating the high crystallinity of CsPbBr3 aggregates generated on the rough and irregular surface according to the method described in this paper. Figure 2b, c, d, e shows the elemental mapping images of the as-fabricated composite film, demonstrating that Cs, Pb and Br elements of CsPbBr3 NCs are uniformly distributed within PVDF.


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Additionally, there is evidence for the presence of fluorine from PVDF in the composite film (Figure 2e), as well.

Figure 3. (a) Typical TEM images of CsPbBr3@PVDF composite film. (b) High-magnification TEM image of the as-fabricated composite film. (c) High-resolution TEM image of a typical CsPbBr3 NCs. (d) The size distribution of CsPbBr3 NCs generated in PVDF. Figure 3a provides a typical TEM image of a CsPbBr3@PVDF composite film, clearly revealing the massive formation of CsPbBr3 NCs in PVDF matrix. The high-magnification TEM image of the as-prepared composite film shown in Figure 3b indicates the relatively homogeneous distribution of CsPbBr3 NCs within the PVDF matrix. As shown in Figure 3c, the interplanar spacing corresponding to the 001 lattice plane of CsPbBr3 NCs encapsulated within PVDF is 0.58nm, which further confirms the in-situ formation of CsPbBr3 NCs in PVDF matrix. More



importantly, the high magnification TEM (HRTEM) image verifies the high crystallinity and single-crystal nature of CsPbBr3 NCs when embedded in the PVDF matrix.44, 46 In this work, the size of CsPbBr3 NCs is limited by the mutual constraints of the crystallization process of PVDF and CsPbBr3 NCs. Nevertheless, the size of CsPbBr3 NCs is somewhat larger than expected due to aggregation. As shown in Figure 3d, the average size of CsPbBr3 NCs generated in PVDF is about 16~18 nm, deriving from TEM size histograms.

Figure 4. (a) Water stability test: the PL intensity ratio plots of CsPbBr3 (black line), CsPbBr3@PMMA (red line) and CsPbBr3@PVDF (blue line) films upon immersion in water over time. The insets show the digital photographs of CsPbBr3@PVDF composite film immersed in water for 0 day (left) and 60 days (right) under UV irradiation. (b) Photo stability test: PL spectra


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of CsPbBr3@PVDF composite film under UV irradiation for 0 h (black line) and 48 h (red line). (c) PL spectra of CsPb(Br/Cl)3@PVDF, CsPb(Br/I)3@PVDF and CsPbI3@PVDF films immersed in water for 0 day and 7 days. The insets in (c) show the digital photographs of CsPbX3@PVDF films immersed in water for 0 day and 7 days under UV irradiation. CsPbBr3@PVDF films were immersed in water for a period of time in order to measure their water stability, as shown in Figure 4a. To highlight the remarkable water stability of the asfabricated composite film, CsPbBr3 and CsPbBr3@PMMA films were tested concurrently and displayed together in Figure 4a. Immediately after being immersed in water, degradation occurs on the CsPbBr3 film, primarily owing to its strong ionic characteristics.1,

19, 47

The relative PL

intensity of CsPbBr3@PMMA composite film decays to 50% of its initial value after being immersed in water for 30 days. As a comparison, much higher PL intensity was observed for the CsPbBr3@PVDF composite film. The PL intensity of CsPbBr3@PVDF only slight decreases after 30 days of water immersion, and even after 60 days, more than 80% of its initial emission intensity is still maintained (Figure S5), proving the superior stability of CsPbBr3@PVDF composite film, similarly with previous reported encapsulated NCs.28-32 Analogous to the water stability of the CsPbBr3@PVDF composite film, the thin films of CsPb(Br/Cl)3@PVDF (emission at 471 nm, PLQY of 18%), CsPb(Br/I)3@PVDF (emission at 560 nm, PLQY of 42%) and CsPbI3@PVDF (emission at 669 nm, PLQY of 37%) also remain highly fluorescent without PL peak shift even after immersion in water for 7 days (Figure 4c and Figure S6). It is necessary to carry out a photo stability test due to the fact that aggregation and reconstruction of perovskite NCs can be induced by irradiation.21 As shown in Figure 4b and Figure S7, no



significant decrease for PL intensity of the CsPbBr3@PVDF, CsPb(Br/Cl)3@PVDF, CsPb(Br/I)3@PVDF and CsPbI3@PVDF films are detected after 48 h of UV light irradiation, which further supports the protective effect of the PVDF matrix on CsPbX3 NCs. In addition, no PL peak shift is detected, demonstrating that CsPbX3 NCs remain well dispersed within PVDF matrix and display high size stability, even after UV light irradiation.

Figure 5. Thermal stability characterization. (a) Schematic diagram of a CsPbBr3@PVDF composite film applied to bearings as a thermal sensor. (b) Optical images of CsPbBr3@PVDF composite film under UV irradiation while temperature is increased from 30℃ to 170℃. (c) Temperature-dependent PL spectra of the composite film. (d) Comparison of fluorescence peak intensity of a CsPbBr3@PVDF composite film during heating and cooling process. (e) 30℃ and 170℃ cycle tests of the composite film. In order to broaden the engineering applications made possible by incorporating CsPbBr3 into PVDF, it is worthwhile to measure their thermal stability. The thermal stability of CsPbBr3@PVDF composite films was evaluated via a temperature-dependent experiment as described in the Experimental Section, and the PL intensity was recorded as temperature was


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increased from 30℃ to 170℃ at intervals of 20℃. As shown in Figure 5c, the photoluminescence of CsPbBr3 NCs encapsulated in PVDF was quenched substantially as the temperature increased. This can be visually verified by the optical images of the CsPbBr3@PVDF composite film under UV irradiation during the heating process presented in Figure 5b. Notably, photoluminescence can no longer be detected at 170℃, whereas the PL intensity is far higher at 30℃. Interestingly, as shown in Figure 5d, the relatively low PL intensity and wide FWHM of the CsPbBr3@PVDF composite film after being heated up to 170℃ can be almost completely reverted when cooled back to 30℃, indicating good reversibility of the composite film. To further test the performance of the as-fabricated composite film, temperature dependent PL intensity was measured between 30℃ and 170℃ for 6 cycles. Figure 5e shows that the PL intensities of the composite film at 30℃ and 170℃ for each cycle test are consistent within acceptable error limit, further demonstrating the good durability and reversibility of CsPbBr3@PVDF. We also conducted a temperature-dependent experiment on CsPbBr3@PMMA and CsPbBr3 films, as depicted in Figure S8. Compared with CsPbBr3@PVDF composite film, the PL intensity of CsPbBr3@PMMA and CsPbBr3 films at 30℃ is comparatively lower, and their reversibility is not as good as CsPbBr3@PVDF. In addition, it can be seen that the upper temperature limit for the CsPbBr3 film is only 110℃: this is lower than that of CsPbBr3@PVDF. The above analysis indicates that CsPbBr3@PVDF composite films have excellent thermal stability due to the excellent protection afforded by PVDF to CsPbBr3 NCs. The superior thermal stability of



CsPbBr3@PVDF composite films makes it an excellent candidate for use as a thermal sensor for bearing temperature measurement, as shown in Figure 5a.

Figure 6. (a) Electroluminescence (EL) spectra of the designed WLED. The inset shows a scheme of the as-fabricated WLED, composed of a GaN blue chip, a YAG layer, and a CsPbBr3@PVDF composite film. (b) CIE chromaticity coordinates of a WLED fabricated with a CsPbBr3@PVDF composite film. As a verification experiment, a CsPbBr3@PVDF composite film was employed as the green light source to fabricate a down-converting white-light-emitting device.21, 48,49 As shown in Figure 6a (inset), the white-light-emitting LED (WLED) is composed of a GaN backlight, a CsPbBr3@PVDF composite film, and a YAG powder mixed with silicone resin, which are used as the blue, green and red emission sources, respectively. The electroluminescence (EL) spectra of the fabricated WLED device in Figure 6a show a blue emission band attributed to GaN chip, a narrow green emission band from CsPbBr3 NCs, and a broad red emission band assigned to the YAG phosphor. In operation, the optimized WLED emits bright white light (Figure 6b). As


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illustrated by the Commission Internationale de L’Eclairage (CIE) chromaticity diagram displayed in Figure 6b, the CIE color coordinates of the as-fabricated WLED (0.34, 0.36) are very close to those of standard white emission (0.33, 0.33). The enhanced stability of these CsPbBr3@PVDF composite films demonstrates the great potential of this material in fabrication of wide color gamut lighting and display devices. CONCLUSION In this work, we successfully fabricated CsPbBr3 NCs encapsulated in PVDF via an in-situ crystallization strategy. We observed that CsPbBr3 NCs are uniformly distributed in PVDF matrix. Due to the protection of PVDF, CsPbBr3 NCs not only retain excellent optical performance, but also gain remarkable thermal and water stability. We demonstrated that CsPbBr3 NCs encapsulated in PVDF can withstand temperatures up to 170℃ and can be completely immersed in water for 60 days while retaining high PL intensity. In addition, a white-emitting LED was fabricated to demonstrate that CsPbBr3@PVDF composite films are suitable for optoelectronic applications. We believe that improving the stability of CsPbBr3 perovskite nanocrystals can broaden their applications to optoelectronic devices, and may make them a potential candidate for use as temperature sensors in mechanical engineering applications. ASSOCIATED CONTENT Supporting Information Different composite film samples fabricated under different preparation conditions; microscope images of different composite film samples; NMR spectrum of the as-fabricated PVDF and



PMMA; more XPS spectra; SEM image of different composite film samples; plots based on temperature-dependent experiment of CsPbBr3 and CsPbBr3@PMMA films. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no conflict of interest. ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant NO. 51802254, 51873173, 51875439), National Key R&D Program of China, New lubrication technology for high speed and high precision bearing (No. 2018YFB2000603), the Fundamental Research Funds for the Central Universities (xjj2018053) and Shaanxi province Youth Foundation (2018JQ5011). The author wish to express their gratitude to the MOE Key Laboratory for Nonequilibrium Condensed Matter and Quantum Engineering of Xi’an Jiaotong University. The authors also thank Qi Wang, for helpful discussion and experimental assistance. The authors also thank Jiao Li at the Instrument Analysis Center of Xi’an Jiaotong University for the assistance with TEM analysis. REFERENCES (1) Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M, V. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X


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