Luminescent nanofluids of organometal halide perovskite

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Luminescent nanofluids of organometal halide perovskite nanocrystals in silicone oils with ultrastability Hechun Lin, Pei Tian, Chunhua Luo, Hai Wang, Jungang Zhang, Jianping Yang, and Hui Peng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05489 • Publication Date (Web): 23 Jul 2018 Downloaded from http://pubs.acs.org on July 24, 2018

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ACS Applied Materials & Interfaces

Luminescent nanofluids of organometal halide perovskite nanocrystals in silicone oils with ultrastability Hechun Lin*a, Pei Tian a, Chunhua Luo a, Hai Wang a, Jungang Zhang b, Jianping Yang b, Hui Peng a,c a

Key Laboratory of Polar Materials and Devices, Ministry of Education, Department of

Electrical Engineering, East China Normal University, Shanghai, P. R. China b

Shanghai Transcom Scientific Co., Ltd. 528 Ruiqing Rd. 20A, Z. J. East Area Hi-Tech Medical

Park, Shanghai c

Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi

030006, P. R. China Corresponding author: [email protected] KEYWORDS: organometal halide perovskite, nanocrystal, nanofluid, stability, luminescence

ABSTRACT

Luminescent nanofluids are successfully prepared by directly dispersing organometal halide perovskite nanocrystals (OHP NCs) with different emission colours in silicone oils. The photoluminescence quantum yields of nanofluids with green, blue and red emission are 47 %, 32 % and 19 %, respectively. Furthermore, the nanofluids greatly enhance the stability of OHP NCs

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and show excellent resistance against moisture, heat and ultraviolet light. The luminescent nanofluids can be used as liquid colour converter for LED. By loading them onto silica aerogel, luminescent perovskite powders were achieved. Their applications as phosphor additives for preparing luminescent PMMA composites were demonstrated. 1. INTRODUCTION

Organometal halide perovskites (OHPs) are widely studied as a new class of semiconductor materials for their excellent optoelectronic properties over the past years.1-3 Among their remarkable properties, large absorption coefficient and long carrier lifetimes make the bulk crystals and films of OHPs appropriate candidates for many optoelectronic devices like solar cells, laser devices and light-emitting diodes (LEDs). 4-8 Compared to bulk OHPs, colloidal OHP nanocrystals (NCs) demonstrate their superior optical properties such as high photoluminescence quantum yield (PLQY) and size-tunable emission wavelengths throughout the whole visible spectrum.9-11 With these outstanding properties, OHP NCs have been developed rapidly within the field of halide perovskite semiconductors, recently utilized in LEDs, lasers, and photodetectors.12-14 Colloidal OHP NCs have been easily synthesized through plentiful solution-based methods, with different emitting wavelengths across the entire visible spectrum.15, 16 The emission color of OHP NCs can be tuned by alternating the halide compositions and capping ligands, or by changing their sizes and shapes.17-23 Although OHP NCs have remarkable performance in low cost solution-processed optoelectronic devices, they still suffer from fast degradation due to inherent instability (sensitive to moisture, light, or heat), which greatly hinders their practical applications.24-26 Introducing long-chain surface ligands during synthesis process can partially improve the stability of OHP NCs dispersed in


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organic solvent.16,

17, 27, 28

The formation of composite films by embedding OHP NCs into

polymers is another effective way to improve their stabilities against water and heat.29-33 Nanofluids are comprised of solid phase nanomaterials such as particles, fibers, rods or tubes suspended in various fluids. Different form common nanomaterial suspensions, nanofluids show some novel properties and have been widely used industrial fluid fields, including heat transfer, magnetism and lubrication.34, 35 More recently, functional nanofluids, including metals, ceramics, carbon, and semiconductor quantum dots, have emerged as engineering media for chemical reactions, thermal storage, electrochemical energy storage, solar harvesting and optic applications. 36-42 For examples, Gondal et al. prepared w-CdS quantum dots nanofluid and for the first time observed a series of well-resolved emission lines in a range of 400 nm to 750 nm.43 Phule et al. reported highly fluorescent silver nanofluid by combining silver nanoparticles with ferroelectric host of polymer molecules.44 In our previous work, we illustrated that the formation of CH3NH3PbBr3 NCs (MAPbBr3 NCs) / silicone resin composite greatly improved the stability of MAPbBr3 NCs.33 In this work, we dispersed perovskite NCs in various silicone oils to prepare highly luminescent nanofluids with ultrastability. The obtained nanofluids can be used as liquid colour converter for LED. Furthermore, the nanofluids can be loaded onto silica aerogel and results in the production of highly luminescent powders. We also demonstrate the application of these powders as phosphor additive to prepare luminescent polymethylmathacrylate (PMMA) composites with ultrastability. 2. MATERIALS AND METHODS 2.1. Materials


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PbI2 (99.0%), PbBr2 (98.0%), methylamine (33% in ethanol), HI (57%), HBr (48%), oleic acid (85%), dimethylformamide (DMF, 99.7%), dimethylsilicone oil (DMS, viscosity: 50, 100, 500 and 1000 mPa·s) and vinyl silicone oil (VS, viscosity: 100, 500, 1000 mPa·s) were obtained from Shanghai Aladdin Bio-Chem Technology Co. Ltd. Toluene and acetonitrile (ACN) were purchased from Sinopharm Chemical reagent Co. Ltd. Silica aerogel (Degussa A200) and PMMA were donated generously by Evonik Degussa. All reagents were used as received. 2.2. Synthesis of MAPbX3 NCs The precursor solution was first prepared by dissolving PbBr2 (0.1 mmol), MABr (0.1 mmol) and octylamine (0.12 mmol) in DMF (0.2 mL). The prepared solution was slowly added into 10 mL of toluene and produced the precipitate of MAPbBr3 NCs.9 The NCs were collected via centrifugation, and washed twice with toluene. MAPbBr3-xClx NCs were synthesized with the similar procedure except that 0.1 mmol of MACl was used. MAPbI3 NCs were prepared according to the previous reported method with some modifications.45 The precursor solution was first prepared by dissolving PbI2 (0.1 mmol), MAI (0.1 mmol) and octylamine (0.12 mmol) in 2 mL ACN with ultrasonication. The obtained precursor solution was added dropwise into a glass bottle containing 10 mL of toluene under vigorous stirring. The precipitate was collected by centrifugation and washed twice by using toluene. 2.3. Preparation of MAPbX3 nanofluids and stability investigation 4 mg of MAPbX3 NCs (MAPbBr3, MAPbBr3-xClx or MAPbI3) were dispersed in 6.0 mL of silicone oils with different viscosity under stirring to form stable nanofluids. For water stability test, 1.2 mL of de-ion water was added into the prepared MAPbX3 nanofluids at room temperature under shaking to ensure fully mixing. The emission intensity was monitored for a


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certain time. The test of thermal stability was carried on by keeping the MAPbX3 nanofluid at 70 °

C or 100 °C and the photoluminescence intensity of nanofluid was monitored. For testing the

resistance against UV light, the nanofluid was exposed to a UV lamp of 6 W. 2.4. Preparation of transparent PET / nanofluid laminated film Three of transparent PET films (28 cm × 21 cm) were used to prepare PET / nanofluid laminated film. One of them was cut into a rectangular frame with a hollow section of 24 cm long and 18 cm wide. Then it was stacked neatly with another PET film. The void area was filled with MAPbX3 nanofluid, and covered with the third one. Finally, the three-layered film were pressed by using a scraper, and bubbles were discharged to obtain a transparent PET / nanofluid laminated film. 2.5. Preparation of MAPbX3 / DMS / silica powders and MAPbX3 / silica / PMMA composite films 25 mg of MAPbX3 NCs (MAPbBr3, MAPbBr3-xClx or MAPbI3), 1.5 g of DMS (500 mPa·s) and 1.2 g of Degussa A200 were grinded together to generate homogenous and luminescent MAPbX3/DMS/silica powders. For preparing MAPbX3/silica/PMMA composite films, 100 mg of the obtained powder was added to the PMMA toluene solution (0.8 g PMMA in 2 mL of toluene) under magnetic stirring. After stirred for 20 minutes at room temperature, the MAPbX3/DMS/silica/PMMA composite was formed. The composite was then casted onto glass substrates.

After

solidified

under

ambient

temperature

for

2

hours,

the

MAPbX3/DMS/silica/PMMA films were obtained. 2.6. Characterization


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The morphologies of the MAPbX3 NCs were investigated by using a transmission electron microscope (TEM, JEM-2100F, Japan). Powder X-Ray diffraction spectra were recorded on a powder diffractometer (Brucker AXS D8). The photoluminescence spectra were measured by using a fluorescence spectrofluorimeter (PerkinElmer LS 55). The excitation wavelengths used for MAPbBr3-xClx, MAPbBr3 and MAPbI3 NCs were 300 nm, 365 nm and 400 nm, respectively. The UV-vis absorption spectra were measured by using a spectrophotometer (TU 1901). The measurement of absolute PLQYs were performed on a spectrofluorimeter (HORIBA, FluroMax4) by using the integrating sphere method under an excitation light of 450 nm. The time-resolved fluorescence spectra was obtained on Picharp 300. 3. RESULTS AND DISCUSSION MAPbBr3 NCs were synthesized according to the previous method.9 The precursor solution containing octylammonium bromide as a capping ligand were added to toluene to produce the MAPbBr3 NCs. The TEM image illustrates that the obtained of MAPbBr3 NCs are nanoparticles with sizes ranging from 6 nm to 11 nm (Figure 1a). The measured XRD spectrum of MAPbBr3 NCs (Figure S1) presents characteristic diffraction peaks of cubic phase at 14.9°, 21.2°, 30.1°, 33.8°, 37.1°, 43.1° and 45.3° which agree well with the previous reports.9, 18 The absorption spectrum of MAPbBr3 NCs dispersed in toluene have three prime peaks located at 423 nm, 455 nm and 475 nm (Figure 1b). It is known that the absorption band of bulk MAPbBr3 is around 525 nm. The formation of nanostructures will cause the blue shift of the absorption band of MAPbBr3 due to the quantum confinement effect.16,

46, 47

So the three dominant absorption peaks are

assigned to MAPbBr3 NCs with different sizes which are illustrated by the result of TEM measurement. On the other hand, the absorption band corresponding to the bulk MAPbBr3 becomes non-obvious. The photoluminescence spectrum of MAPbBr3 NCs shows a narrow


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emission peak at 517 nm. The full-width at half-maximum (FWHM) of the emission peak is 18 nm. The MAPbBr3 NCs dispersed in toluene are quite sensitive to water. When water is added to this suspension, the MAPbBr3 NCs will quickly degrade and completely lose their luminescent property within 40 minutes as presented in the inset of Figure 1b. The stabilities of MAPbBr3 NCs in other unpolar solvents, namely, chlorobenzene (PhCl), perchlormethane, hexane and 1octadecene were tested as well and the results are shown in Figure S2. The emission of MAPbBr3 NCs dispersed in PhCl, perchlormethane or hexane vanishes within 1 hour. While, they show better stability in 1-octadecene with only 12 % emission decay of the initial value after 3 hours. The MAPbBr3 NCs dispersed in toluene are also sensitive to heat and UV light. The photoluminescence intensities decrease to 59 % and 51 % of the initial values after 30 min at 100 ◦C and 4 hours under UV light, respectively (Figure S3).

Figure 1. (a) The TEM image of MAPbBr3 NCs; (b) UV-vis and photoluminescence spectra of the MAPbBr3 NCs / toluene suspension. The excitation wavelength is 365 nm. The inset is the evaluation of the photoluminescence spectra of the MAPbBr3 NCs / toluene suspension after the addition of water at room temperature.


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Figure 2. (a) UV-vis and photoluminescence spectra of the MAPbBr3 / DMS (500 mPa·s) nanofluid. Inset: Photograph of the MAPbBr3 / DMS (500 mPa·s) nanofluid under ambient conditions (left) and a UV lamp with 365 nm emission (right); (b) Time-resolved photoluminescence decay curves of the MAPbBr3 NCs / toluene suspension (curve I) and MAPbBr3 / DMS (500 mPa·s) nanofluid (curve II). The dash lines are fitting data. Due to its good chemical stability, thermal resistance and non-toxic property and so forth, silicone oil is widely applied in lots of fields, such as coatings, cosmetics and pharmaceutical processes.48 In view of these properties, we tried to disperse MAPbBr3 NCs into DMS (500 mPa·s) to prepare luminescent nanofluids. As given in the inset of Figure 2a, a homogenous and luminescent nanofluid can be obtained. The absorption and photoluminescence spectra of this nanofluid are similar to those of MAPbBr3 NCs in toluene (Figure 2a). Three prime absorption peaks are located at 424 nm, 455 nm and 476 nm, respectively, and the emission peak is at 519 nm with a FWHM of 22 nm. The relative quantum yield of the obtained MAPbBr3 / DMS nanofluid reaches 47 % (Figure S4b), which is a little bit higher than 42 % of MAPbBr3 NCs in toluene (Figure S4f). Figure 2b gives time-resolved photoluminescence decay curves of MAPbBr3 / DMS nanofluid and MAPbBr3 NCs in toluene. Both of the decay curves can be well fitted by using a biexponential function with a short-lived PL lifetime (τ1) and long-lived PL


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lifetime (τ2). For the MAPbBr3 NCs in toluene, τ1 and τ2 are 1.1 ns and 11.0 ns, respectively. The calculated average PL lifetime is 7.17 ns. For the MAPbBr3 / DMS nanofluid, τ1 and τ2 increase to 1.2 ns and 11.5 ns, respectively. The average PL lifetime also increases to 7.99 ns. The increase in the lifetime means the decrease in the decay rate of excitons, indicating the surface traps of MAPbBr3 NCs may be further passivated by DMS.

Figure 3. Photoluminescence spectra evaluation of the MAPbBr3 / DMS (500 mPa·s) nanofluid. (a) After the addition of water at ambient temperature. The insets are photographs of the MAPbBr3 / DMS (500 mPa·s) nanofluid mixed with water under ambient conditions (left) and a UV lamp with 365 nm emission (right); (b) Curing at 70 ◦C; (c) Curing at 100 ◦C; (d) Exposure to UV irradiation. The MAPbBr3 / DMS nanofluid exhibits ultrastability against water, thermal and UV light. Exposing the nanofluid to ambient air for at least six months, no noticeable PL decay is observed. For in-depth exploration of the stability, water was added into the nanofluid with


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strong shake. The photoluminescence intensity variation was recorded regularly. The emission intensity of the nanofluid decreases to 93 % and 52 % of the initial value after 8 days and 10 days, respectively (Figure 3a). With elevated temperature, the intensities decrease to 85 % and 79 % of the initial values after 48 hours at 70 ◦C and 14 hours at 100 ◦C, respectively, as shown in Figure 3b and 3c. The resistance of the nanofluid to UV light is also given in Figure 3d. The emission intensity of the nanofluid still keeps up to 76% of the initial value after exposure to a UV lamp with 365 nm emission (6 W) for 214 hours. These results clearly illustrate the stability of MAPbBr3 / DMS nanofluid exhibits are greatly improved than that of MAPbBr3 dispersed in toluene. Other DMS with different viscosities (50, 100 and 1000 mPa·s) and VS (100, 500, 1000 mPa·s) were also used to prepare MAPbBr3 nanofluids. All the tested silicone oils produce homogenous nanofluids of MAPbBr3 NCs with green luminescence, which are shown in Figure S5. With the increasing of viscosity, the nanofluid presents more excellent stability against water. Especially, the emission of the nanofluid prepared by using DMS with a viscosity of 1000 mPa·s only decays to 82 % of the initial value after 10 days in the existence of water. This may be due to the better water resistance of higher viscosity silicone oil. In order to illustrate versatility of this strategy to improve the stability of perovskite NCs, we synthesized MAPbBr3-xClx NCs with blue emission and MAPbI3 NCs with red emission (their XRD patterns and TEM images are given in Figure S1 and Figure S6). All these NCs can be dispersed in silicone oil to form homogenous nanofluids with blue or red emission as shown in the insets in Figure 4. The relative quantum yields for MAPbBr3-xClx and MAPbI3 nanofluids are 32 % and 19 %, respectively (Figure S4c-e). The water resistance of these nanofluids are given in Figure 4. The blue emission decays to 88 % in 10 days and the red emission decays to 44% in


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5 days of the initial value in the existence of water, respectively. In contrast, the emission of their dispersion in toluene will decay quickly, especially that the red emission will disappear in several minutes (Figure S7).

Figure 4. Photoluminescence spectra evaluation of MAPbX3 nanofluids. (a) MAPbBr3-xClx / DMS (1000 mPa·s) nanofluid and (b) MAPbI3 / DMS (1000 mPa·s) nanofluid after the addition of water at ambient temperature. Insets: Photograph of the nanofluids in the presence of water under ambient conditions (left) and a UV lamp with 365 nm emission (right). To illustrate the potential applications of the nanofluids in optic systems, the green emission nanofluid was laminated as transparent film in visible light (Figure 5a), which showed green emission under UV light (Figure 5b). In another case, a blue GaN LED chip (0.6 watt) with the emission at 455 nm was immersed into the nanofluids. The blue emitted light of the GaN LED chip is completely transformed to green or red emission. The emission peaks are located at 516 nm with a FWHM of 31 nm and at 708 nm with a FWHM of 62 nm, respectively. These results demonstrate the potential application of perovskite nanofluids in LED colour converter.


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Figure 5. Photographs of transmittance film laminated with MAPbBr3 nanofluid under (a) ambient conditions and (b) a UV lamp with 365 nm emission; The colour conversion of a blue GdN LED to (c) green and (d) to red emission. The photoluminescence spectrum of the blue LED was given for reference. Insets in Figure 5c and 5d: Photographs of the colour conversion of blue LED by using the MAPbBr3 and MAPbI3 nanofluids, respectively. As discussed above, high luminescent nanofluids with ultrastability can be prepared via simply dispersing MAPbX3 NCs in silicone oils and their potential application in LED colour convertor is proved. However, they are not convenient for fabricating solid devices. It is well known that silica aerogel can absorb liquids like a sponge and turn them into flowable powders which greatly benefits the handle of liquid. On the other hand, mesoporous silica has been reported to be used as template to synthesize various sized perovskite NCs mediated by the pore size of the templates with quantum yields of up to 52 %.49 But it suffered from the low wet


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stability and poor dispersion in common organic solvents. Degussa A200, a fumed silica with an average size of 12 nm and a specific surface area of 200 m2/g, has been broadly used as additive in paints, rubber, detergent additives, components of dry-mix mortar blends. So Degussa A200 was adopted to absorb the luminescent nanofluids. The MAPbX3 NCs, DMS (500 mPa·s) and Degussa A200 were mixed by grinding, which results in the production of homogenous powders as shown in Figure 6a, 6b and 6c (right). The obtained MAPbBr3-xClx / Degussa A200, MAPbBr3 / Degussa A200 and MAPbI3 / Degussa A200 powders show blue, green and red emission under UV light (Figure 6a, 6b and 6c, left), respectively. Their emission peaks are located at 459 nm, 528 nm and 712 nm with FWHM of 25 nm, 26 nm, and 60 nm, respectively. The PL quantum yields of the powders with blue, green and red emission are 6.2 %, 49.2 % and 8 %, respectively. Additionally, these powders can be pressed to relatively transparent tablets as shown in Figure 6a.


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Figure 6. Photographs of prepared MAPbX3 / Degussa A200 powders and tablets. (a) MAPbBr3 NCs; (b) MAPbBr3-xClx NCs and (c) MAPbI3 NCs. Left: under ambient conditions. Right: under a UV lamp with 365 nm emission; (d) Corresponding PL spectra of luminescent powders. The luminescent powders can be conveniently used as phosphor additive. To illustrate this, the MAPbBr3 / Degussa A200 powders was blended with PMMA to give rise to a homogenous composite. The composite was then laminated onto a glass sheet and solidified at room temperature to produce a luminescent film with a quantum yield of 45 %. For in-depth study of the stability, the film was immersed into water at room temperature or 70 °C. The change of photoluminescence intensity was monitored regularly and the obtained results are given in Figure 7a and 7b. The photoluminescence intensity of the film keeps up to 95 % of the initial value after being immersed into water for 40 days. When the film is kept at 70 ◦C for 300 minutes, the photoluminescence intensity decreases 64 % compared to the initial value (Figure 7b). The film also exhibits good resistance to UV irradiation, as presented in Figure 7c. Upon exposure to UV lamp with 365 nm emission (6 W) for 168 hours, the intensity still keeps up to 81 % of the initial value. For comparison, a MAPbBr3 / PMMA film was also prepared by blending MAPbBr3 NCs with PMMA and immersed in water at room temperature to test the water stability. The photoluminescence intensity deceased to 53 % after 5 days as shown in Figure 7d. The powders with blue and red emission powders also exhibit excellent stability against water after the formation of composites with PMMA. During experiments, an unusual phenomenon was observed. That is, the photoluminescence intensity of MAPbBr3-xClx / Degussa A200 / PMMA film first increases after exposed to water at ambient temperature. After 48 hours, it starts to decrease, as shown in Figure 7e. This phenomenon is repeatable and not an experimental error. The specific mechanism is not clear at this moment. The photoluminescence intensities decline


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to 47 % and 44 % compared to the initial after immersed in water for 6 days and 30 hours for the MAPbBr3-xClx/ Degussa A200 / PMMA and MAPbI3/ Degussa A200 / PMMA films (Figure 7e and 7f), respectively.

Figure 7. Photoluminescence spectra of the MAPbBr3 / Degussa A200 / PMMA film immersed in water (a) at ambient temperature; (b) curing at 70 ◦C; (c) before and after the exposure to a UV lamp with 365 nm emission; (d) Photoluminescence spectra of the MAPbBr3 / PMMA film immersed in water at ambient temperature for comparison; (e) Photoluminescence spectra of


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MAPbBr3-xClx / Degussa A200 / PMMA film; f) MAPbI3 / Degussa A200 / PMMA film immersed in water at ambient temperature. Insets in Figure 7a, 7e and 7f: Photographs of MAPbBr3 / Degussa A200 / PMMA, MAPbBr3-xClx/ Degussa A200 / PMMA and MAPbI3/ Degussa A200 / PMMA films under ambient conditions (left) and a UV lamp with 365 nm emission (right), respectively. 4. CONCLUSIONS In summary, perovskite NCs nanofluids were successfully obtained via directly dispersing perovskite NCs in silicone oils. The luminescent nanofluids have a good relative quantum yield of 47 %, 32 % and 19 % for green, blue and red emission, respectively. These nanofluids exhibit excellent resistance against water, heat and light. They can be used as liquid colour converters for LED. Furthermore, luminescent perovskite powders were achieved by loading nanofluids onto silica aerogel, which can be used as phosphor additives to prepare luminescent PMMA composites with surperstability. The results suggest that luminescent nanofluids have enormous potential in optoelectronic applications. ASSOCIATED CONTENT Supporting Information Additional figures and tables (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENTS


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TOC Organometal halide perovskite nanocrystals were dispersed in silicone oil to form nanofluids with different color emission and ultrastability against water, heat and UV exposure. The luminescent nanofluids can be used as liquid convertor for LED or loaded onto silica aerogel to form luminescent perovskite powders as phosphor additives.


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Luminescent nanofluids of organometal halide perovskite

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