METHOD FOR PREPARING METAL HALIDE NANOCOMPOSITES

Abstract

The invention relates to a method for the preparation of luminescent nanocomposites comprising metal halide nanocrystals co-embedded with inorganic salts in the pores of a porous metal oxide matrix comprising the steps of: a) preparing a mixture comprising at least a metal halide, a combination of inorganic salts, and porous metal oxide particles in the absence of an organic solvent; b) heating above the melting temperature of the mixture of step a); c) cooling to obtain the nanocomposite. It further relates to nanocomposites obtained with the claimed method and optoelectronic devices, tracers and tagging material comprising the same.

Claims

1. A method for a preparation of luminescent nanocomposites comprising metal halide nanocrystals co-embedded with inorganic salts in pores of a porous metal oxide matrixcomprising the steps of: a) preparing a mixture comprising at least a metal halide, a combination of inorganic salts, and porous metal oxide particles in the absence of an organic solvent; b) heating above a melting temperature of the mixture of step a); and c) cooling to obtain the nanocomposite.

2. The method according to claim 1, characterized in that said metal halide is selected from the group consisting of CsBr, CsCl, PbBr.sub.2, PbCl.sub.2, CsI, PbI.sub.2, and mixtures thereof.

3. The method according to claim 2, characterized in that said metal halide is a mixture of CsBr and PbBr.sub.2.

4. The method according to claim 1, characterized in that said inorganic salt is selected from inorganic compounds of formula M.sub.aX.sub.b, wherein M represents one or more metal cations selected from Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Al, Ce, Co, Fe, Mn, La, or Ni; X represents one or more anions selected from the group consisting of halide, nitrate, nitrite, perchlorate, carbonate, thiocyanate, sulfates, sulfites, phosphates and phosphites; and a and b are independently an integer ranging from 1 to 3.

5. The method according to claim 4, characterized in that said inorganic salt is selected from the group consisting of KNO.sub.3, KBr, KCl, NaNO.sub.3, KBr, NaBr, LiBr, RbBr, and mixtures thereof.

6. The method according to claim 5, characterized in that said inorganic salt is selected from a mixture of KNO.sub.3 and KBr or a mixture of KNO.sub.3, NaNO.sub.3 and KBr.

7. The method according to claim 1, characterized in that said porous metal oxide particles are mesoporous metal oxide particles.

8. The method according to claim 1, characterized in that said porous metal oxide particles are selected from the group consisting of silica particles, aluminum oxide particles, titanium oxide particles, zinc oxide particles and zeolite particles.

9. The method according to claim 7, characterized in that said mesoporous metal oxide particles have a particle size equal to or less than 1 μm.

10. The method according to claim 7, characterized in that said mesoporous metal oxide particles are m-SiO.sub.2.

11. The method according to claim 1, further comprising a step d), wherein step d comprises washing said nanocomposite obtained from step c) with a polar solvent.

12. The method according to claim 11, characterized in that said polar solvent is selected from the group consisting of water, dimethylsulfoxide and dimethylformamide.

13. The method according to claim 11, further comprising a step e), wherein step e comprises drying the nanocomposite of step d).

14. A luminescent nanocomposite obtained with the method according to claim 1.

15. An optoelectronic device comprising the nanocomposite of claim 14.

16. A tracer and tagging material for oil industry comprising the nanocomposite of claim 14.

17. A polymer film containing luminescent nanocomposite obtained with the method according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The present invention will be described in detail with reference to the figures in the annexed drawings, which show purely illustrative and non-exhaustive examples in which:

[0036] FIG. 1 illustrates (a) PL spectra of CsPbBr.sub.3-salt-silica composites before (thick solid curve) and after different washing steps: cleaning with the use of deionized (DI) water (solid curve); DI water followed by DMSO (dashed curve). An arrow indicates the PL emission corresponding to bulk CsPbBr.sub.3; (b,c) XRD pattern of CsPbBr.sub.3-salt-silica composites obtained after one washing step with water (b) and after a subsequent washing step with DMSO (c). The bulk reflections of PbBrOH (ICSD 98-002-83-13), CsPbBr.sub.3 orthorhombic (ICSD 98-009-7851) and KNO.sub.3 cubic (ICSD 98-000-0384) are reported by means of vertical bars.

[0037] FIG. 2 illustrates a) TEM image of the empty m-SiO.sub.2 MCM-41 template; b-c) TEM images of the CsPbBr.sub.3-KNO.sub.3/m-SiO.sub.2 nanocomposites.

[0038] FIG. 3 illustrates (a) DLS curves obtained for starting m-SiO.sub.2 particles and CsPbBr.sub.3-salt/m-SiO.sub.2 composites prepared using KNO.sub.3:KBr (in a molar ratio of 15:5) and dispersed in water; (b) PLQY of CsPbBr.sub.3-salt/m-SiO.sub.2 composites washed with either DI water or DMSO, and bare CsPbBr.sub.3 NCs before (I) and after annealing at 180° C. in Argon for 3 h (II) or 2 h (II′); (c) Time-dependent absolute PLQY of CsPbBr.sub.3-salt/m-SiO.sub.2 composites (washed with either DI water or DMSO) and CsPbBr.sub.3 NCs immersed in 100% water.

[0039] FIG. 4 illustrates PL spectra of CsPb(Cl,Br).sub.3 based composites with the PL tunable of from 520 nm (curve 1) to 443 nm (curve 7) by varying the halide composition.

[0040] FIG. 5 illustrates PL spectra of a metal halide nanocomposite prepared using KNO.sub.3:NaNO.sub.3:KBr (in a molar ratio of 10:5:5) as the molten salts medium, before and after immersion in aqua regia.

[0041] FIG. 6 illustrates the normalized PL intensity of the composite of the invention recorded during different type of accelerated test conditions (HF=High Flux, HH=High Humidity, HT=High Temperature).

[0042] FIG. 7 illustrates the schematic of the LCD device:

1—light source with pink color, 2—the spectra of the light source, 3—representation of red (marked as “R”, which is 630 nm wavelength's peak) and blue light (marked as “B”, which is 450 nm wavelength's peak), 4—polymer film with luminescent nanocomposites of present disclosure, 5—representation of red (R), green(G) and blue (B) light, 6—LCD matrix, 7—color filters, 8—resulting images.

[0043] FIG. 8 illustrates the TEM image of the CsPbBr.sub.3/SiO.sub.2 nanocomposite synthesized at 320° C. PL emission of CsPbBr.sub.3/SiO.sub.2 nanocomposites synthesized at 320° C., 330° C. or 340° C.

DETAILED DESCRIPTION OF THE INVENTION

EXAMPLE

Example 1

Preparation of Emissive LHP-Salt-Silica Composite

[0044] A mixture of CsBr, PbBr, KNO.sub.3 and KBr (molar ratio of 1:1:15:5), and mesoporous silica (commercial name MCM-41, Sigma Aldrich code 643645), having particles size of ˜1 μm with a very broad size distribution, and pores size of 3.3 nm, were mixed in a ceramic crucible and heated up to 350° C. in a furnace for 60 minutes under air. The resulting product was allowed to cool down to room temperature, forming a monolith which was washed with a polar solvent (preferably water, but other solvents such as dimethylsufoxide (DMSO) or dimethylformamide (DMF) can be also used, with no major differences in optical, structural and chemical properties of the final product) in the following way: 100 mg of product was loaded in a vial with 1 ml of water and sonicated for 10 min, centrifuged at 5000 rpm, and the supernatant was discarded. This procedure was repeated 5 times in order to quantitatively remove all the inorganic salts and all the CsPbBr.sub.3 crystals that had formed outside of the m-SiO.sub.2 particles. The final powder was eventually dried in a vacuum oven at 40° C., delivering ˜90 mg of final CsPbBr.sub.3-salt-silica composite. The synthesis yield is around 90%.

[0045] The obtained composite presents partially sealed SiO.sub.2 pores.

Example 2

XRD and PL/Abs Characterization of CsPbBr.SUB.3./KNO.SUB.3./SiO.SUB.2 .nanocomposites

[0046] The product of Example 1, before the washing step, showed a PL emission peak at 520 nm, arising from the CsPbBr.sub.3 NCs inside the m-SiO.sub.2, with a shoulder at longer wavelengths, which was attributed to the presence of bulk-like perovskite crystals located outside the m-SiO.sub.2 matrix (see FIG. 1a). After the washing steps with water, the PL emission of the resulting sample was peaked at 520 nm without a shoulder at lower energies, full width at half maximum (FWHM) of 22 nm (102 meV) and a PLQY as high as 87%.

[0047] The XRD pattern of the product after the cleaning with water is dominated by the peaks of the PbBrOH phase which, most likely, formed upon the reaction of Pb-Br compounds with water and which is insoluble in water (FIG. 1b). Minor reflections ascribable to KNO.sub.3 are also observed. On the other hand, it is not possible to assess if any CsPbBr.sub.3 phase is present as the corresponding reflections are covered by those of the PbBrOH phase. Thus, in order to have a more clear structural analysis, the composites was cleaned with DMSO, which does not lead to the formation of hydroxide compounds. The inventors highlight here that this washing method results in a final product having the same PLQY of that obtained by cleaning with water. The XRD pattern of the DMSO washed product indicated the absence of the PbBrOH phase and the presence of orthorhombic CsPbBr.sub.3 (ICSD 98-009-7851) and KNO.sub.3, most likely filling silica pores together with CsPbBr.sub.3 NCs (FIG. 1c). A broad peak ranging from 15° to 35° was ascribed to the SiO.sub.2 matrix, which was amorphous (see FIG. 1b).

Example 3

Electron Microscopy, EDS and HAADF Characterization

[0048] To better understand the morphology and the nanostructure of the CsPbBr.sub.3/KNO.sub.3/SiO.sub.2 product of Example 1, the inventors performed an in-depth TEM characterization. High-resolution (HR) TEM analysis revealed that the mesoporous nature of the SiO.sub.2 was preserved after the molten salts synthesis procedure, with pores having a mean diameter of 3.3 nm (FIGS. 2a-c). Moreover, it was possible to assess that the product of the method of the invention, after washing, was composed of m-SiO.sub.2 particles whose pores were filled with small NCs (FIGS. 2b-c). Scanning transmission electron microscopy (STEM) energy dispersive X-ray spectrometry (EDS) analysis indicated the presence of inorganic NCs containing Cs, Pb, Br and K inside the pores of m-SiO.sub.2, in agreement with our XRD results, and no presence of undesired inorganic particles outside the m-SiO.sub.2.

Example 4

Stability Test

[0049] In order to assess if the m-SiO.sub.2 particles aggregate or merge during the production of the composites, the inventors tested the method of the invention by employing a commercial m-SiO.sub.2 particles (Sigma Aldrich code 748161) having a mean diameter of 200 nm with a spherical shape and 4 nm pore size. This because MCM-41 m-SiO.sub.2, used in the experiment disclosed above, is characterized by a broad size distribution, making it hard to assess if aggregation occurs after the composites preparation. The dynamic light scattering (DLS) analysis, performed by dispersing m-SiO.sub.2 or CsPbBr.sub.3-salt/SiO.sub.2 particles in water, clearly indicated that the method of the invention does not lead to any aggregation or merging of SiO.sub.2 particles (FIG. 3a), contrarily to what happens when working in the absence of molten salts at higher temperatures (Q. Zhang et al., Nat. Commun., vol. 11, no. 1, pp. 1-9, 2020).

[0050] CsPbBr.sub.3/m-SiO.sub.2 composites prepared using KNO.sub.3:KBr (in molar ratio of 15:5), made with MCM-41 m-SiO.sub.2, and washed by using either water or DMSO were subjected to various tests in order to assess their stability under heating (thermal stability) or to water exposure (water stability). The bare CsPbBr.sub.3 NCs obtained via a standard colloidal approach (reference standard) were also tested in parallel (L. Protesescu et al. Nano letters 15.6 (2015): 3692-3696).

[0051] The thermal stability was assessed by monitoring the variation of the PLQY of the sample before and after annealing at 180° C. for 3 h in argon atmosphere. The PLQY of the composites went from ˜85-87% to ˜75-80%, whereas the PLQY of the bare CsPbBr.sub.3 NCs dropped from 90% to 30% after annealing at 180° C. in argon for 2 h. (FIG. 3b)

[0052] The stability against water was assessed by dispersing the samples in deionized water and monitoring the resulting PLQY over time. As shown in FIG. 3c, the CsPbBr.sub.3-salts/m-SiO.sub.2 composites showed very slow PLQY decrements compared to the bare NCs. The PLQY of the composites obtained with the method of the invention was still around 50% after 7-8 days upon immersion in water (FIG. 3c). On the other hand, the CsPbBr.sub.3 NCs could not survive in water and their PL was completely quenched after only several minutes. The results indicated that, the molten salts were responsible of a partial sealing of the pores of m-SiO.sub.2, reason why the composites could be washed in water and their degradation under water was strongly reduced. Indeed, the salts present in the pores (KNO.sub.3) together with CsPbBr.sub.3 NCs, are soluble in water, and, are not able to protect the perovskite emitters.

Example 5

Extension of the Invention to Different Compositions

[0053] The method of the invention can be used for the production of CsPbX.sub.3 NCs with mixed halide compositions. In details, the inventors employed a mixture of PbCl.sub.2/PbBr.sub.2, CsBr/CsCl, molten salts (KNO.sub.3 and a mixture of KBr/KCl) and m-SiO.sub.2 for the preparation of CsPb(Cl,Br).sub.3-SiO.sub.2 composites.

[0054] A mixture of CsBr, CsCl, PbBr.sub.2, PbCl.sub.2, KBr, KCl and KNO.sub.3 was prepared in a ceramic crucible and heated up to 350° C. in a furnace for 60 minutes under air. After cooling down to room temperature, the formed monolith was washed with water and then, was dried in a vacuum oven at 40° C. to result final composite powder. Depending on the specific inorganic salts ratio different sample having different halide composition and consequently different emission wavelengths could be prepared (FIG. 4). In details, the samples with a PL emission from 500 nm to 510 nm were prepared using a CsBr:(PbBr.sub.2+PbCl.sub.2):Kbr ratio of 1:1:5 where the PbBr.sub.2:PbCl.sub.2 ratio was varied from 0.11:0.89 to 0.33:0.67. The samples emitting from 487 to 443 nm were prepared using CsCl:PbCl.sub.2:(KBr+KCl) of 1:1:5 where the KBr:KCl ratio was varied from 5:0 to 2.5:2.5.

Example 6

Preparation of Nanoparticles of Emissive LHP-Salt-Silica Composite in Customized m-SiO.SUB.2

[0055] Emissive LHP-salt-silica composite was also synthesized in customized m-SiO.sub.2. The customized m-SiO.sub.2 is composed of particles having an overall smaller average size compared to commercial one. The TEM analysis revealed that the average size of synthesized m-SiO.sub.2 particles is about 100 nm.(Figure x) Smaller size of m-SiO.sub.2 particles will expand the range of applications of the resulting nanocomposites, especially in those fields where small emissive composites, ideally not affected by scattering effects, are required, such as color conversion in display and lighting.

[0056] This Functionalized Nanocomposite samples are prepared in three steps:

[0057] 1) Synthesis of m-SiO.sub.2 nanoparticles. 1 g of Cetyltrimethylammonium bromide (CTABr) is dissolved in 480 ml of distilled water under string in a flask. Then, 3.5 ml of NaOH (2 M in distilled water) is added to flask and the resulting mixture is heated to 70° C. for 1 h. Subsequently, 5 ml of tetraethoxysilane is injected into mixture at a 10 ml/min injecting speed. The mixture is stirred for 2 h at 70° C., and the resulting product is washed five times with distilled water. 700 mg of products is eventually calcined at 550° C. for 5 h to remove the CTABr template phase.

[0058] 2) Growth of CsPbBr.sub.3 inside the pores of SiO.sub.2. CsBr (0.6 mmol), PbBr.sub.2 (0.6 mmol), KBr (0.5 mmol), KNOB (1 mmol), NaNO.sub.3 (0.5 mmol) and m-SiO.sub.2 (0.5 mmol) are mixed via grinding with a mortar and pestle. Then, the mixture is heated in a crucible at 320° C. for 60 minutes under air and allowed to cool down to room temperature (it is also possible to run the reaction at 330 or 340° C.). The final CsPbBr.sub.3/SiO.sub.2 composites are cleaned with DMSO and ethanol 3 times.

[0059] 3) Surface functionalization of the composites. 9 mg of nanocomposites are suspended in 5 mL of octadecene under stirring at 70° C. An excess of trimethoxy(octadecyl)silane (42 μl, 0.1 mmol) is added to the suspension which is stirred (at 70° C.) for 4 hours. The resulting solid product is isolated by centrifugation and then washed with anhydrous toluene three times.

[0060] TEM image of the CsPbBr.sub.3/SiO.sub.2 nanocomposite synthesized at 320° C. and PL emission of CsPbBr.sub.3/SiO.sub.2 nanocomposites synthesized at 320° C., 330° C. or 340° C. are illustrated in FIG. 8.

Example 7

Stability Tests in Polymer Films

[0061] The emissive LHP-salt-silica composite reported in the example 6, was subjected to stability tests in polymer films for colour conversion applications.

[0062] To produce a polymer film containing emissive LHP-salt-silica composite material, the emissive LHP-salt-silica particles were dispersed in IBOA (Isobornyl acrylate monomer) together with a certain amount of a photoinitiator, TiO.sub.2 nanoparticles, acting as a light scattering agent, and SiO.sub.2 nanoparticles, acting as a viscosity modifier. The concentration of nanocomposite particles was kept at 0.5 wt %. The mixture was blade coated in between barrier films (3M™ FTB3-50 with 50 μm thickness and VWTR<0.001 g/m2-day@20° C.) and cured under UV light (800 mW/cm.sup.2 light intensity) for 1 minute. The formed film had a total thickness of 300 μm. The resulting polymer composite was characterized by an emission peak centered at 522 nm with a FWHM 18 nm and a PLQY of ˜62%.

[0063] The obtained emissive polymer composite films were tested under High Flux (HF), High Humidity (HH) and High Temperature (HT), also called accelerated reliability tests. High flux test consists in exposing the composite film under 100 mW/cm.sup.2 of blue light (450 nm wavelength) and 60° C. High humidity test is done by exposing the composite film to an environment with 90% RH at 60° C. High temperature test is done by exposing the composite film to 10 mW/cm.sup.2 of blue light (450 nm wavelength) at 60° C. In all these accelerated test conditions, the film need to maintain more than 50% from its original PL intensity. Results are illustrated in FIG. 6.

[0064] During the accelerated test, the emission intensity of the films was monitored and recorded. FIG. 6 below shows the variation of the PL intensity of the films under the three tests as a function of the test time. The composite films show stable PL intensity under these harsh condition for more than 500 hours.

[0065] Thanks to their high stability, the emissive composite films could be used for color conversion in displays. For this application, the emissive LHP-salt-silica composite will down convert the blue light to green to obtain white light (RGB color) when combined with magenta LED backlight. as shown in FIG. 7.

[0066] FIG. 7 illustrates one embodiment of display device. The polymer composite with the emissive LHP-salt-silica composite 4 is used to down convert blue light 3 from the pink LED source, which emits blue (450 nm) and red (630 nm) spectrum 1. The part of blue light is converted into green light with emission center >522 nm and FWHM<25 nm and the red emission just pass through the polymer composite. The resulting three RGB colors 5 are passing through LCD matrix 6 and then color filters 7 giving the image 8.