Method for manufacturing low dimensional metal halide perovskite microcrystal heterostructure using sequential growth process in one-pot solution

Abstract

The invention relates to a method for producing metal halide perovskite heterostructures comprising : - providing a first type of two-dimensional metal halide perovskite crystals; - providing a second type of two-dimensional metal halide perovskite crystals different from the first one; - providing a common solvent; - dissolving the first type of crystals in the solvent to create a first solution; - dissolving the second type of crystals in the solvent to create a second solution; - mixing the first and second solutions to obtain a third solution; - injecting an antisolvent into the third solution or inducing controlled evaporation of the solvent into the third solution; - collecting the crystals thus formed.

Description

[0001] METHOD FOR MANUFACTURING LOW DIMENSIONAL METAL HALIDE PEROVSKITE MICROCRYSTAL HETEROSTRUCTURE USING SEQUENTIAL GROWTH PROCESS IN ONE -POT SOLUTION

[0002] The present invention relates to a method for making microcrystalline heterostructures from two-dimensional layered metal halide perovskites .

[0003] Two-dimensional layered metal halide perovskites (2DLP) are emerging materials consisting of octahedral layers of alternating semiconductor metal halides and layers of organic cations as , for example, described in X. Li , J. M. Hoffman, M. G. Kanatzidis , Chem. Rev. 2021 , 121 , 2230 and M. P. Arciniegas , L. Manna, ACS Energy Lett. 2022 , 7 , 2944 to be considered part of the present disclosure. This structure results in high mobility of charge carriers in the plane of octahedral layers , but limits out-of-plane mobility due to organic cations that are mostly insulating as evidenced in S . G. Motti , M. Kober-Czerny, M. Ri ghetto, P. Holzhey, J. Smith, H. Kraus , H. J. Snaith, M. B. Johnston, L. M. Herz , S . G. Motti , M. Kober-Czerny, M. Righetto, P. Holzhey, J. Smith, H. Kraus , H. J. Snaith, M. B. Johnston, L. M. Herz , Adv. Funct. Mater. 2023, 33, 2300363 and F. Cao, D . Yu, M. Telychko, J. Lu, P. Pang, C. Su, G. Xing, ACS Energy Lett. 2023, 8 , 1236.

[0004] The lateral heterostructures in which the composition varies along the in-plane direction of the octahedral layers are interesting for controlling the charge localization in specific areas in the 2DLPplane as highlighted in A. M. Oddo, M. Gao, D . Weinberg, J. Jin, M. C. Folgueras , C . Song, C. Ophus , T. Mani , E . Rabani , P. Yang, Nano Lett. 2023, 26, 32 and A. Griesi , M. Faraji , G. Kusch, S . Khabbazabkenar , M. Borreani , S . Lauciello, A. Schleusener, R. A. Oliver, R. Krahne, G. Divitini , Nanotechnology 2023, 35 , 105204.

[0005] E . Shi , B. Yuan, S . B. Shiring, Y. Gao, Akriti , Y. Guo, C. Su, M. Lai , P. Yang, J. Kong, B. M. Savoie, Y. Yu, L. Dou, Nature 2020 , 580 , 614 describes , for the first time, a process for the formation of lateral heterostructures in 2DLPs by means of a sequential crystallisation approach in solution. The extension towards core-shell structures was instead described in A. Singh, B. Yuan, M. H. Rahman, H. Yang, A. De, J. Y. Park, S . Zhang, L. Huang, A. Mannodi -Kanakki thodi , T. J. Pennycook, L. Dou, J. Am. Chem. Capital 2023, 145 , 19885 e Y. H. Lee, J. Y. Park, P. Niu, H. Yang, D . Sun, L. Huang, J. Mei , L. Dou, ACS Nano 2023, 17 , 13840.

[0006] Sequential crystallization relies on the different solubility of the materials in the respective solvents . The solution leading to the growth of the second material is applied after the crystal of the first material has grown. Thus , it is a reaction requiring two successive steps . Furthermore, it cannot be applied for a large scale synthesis of heterostructures , since it is limited to the growth of a small amount of crystals with a wide dimensional distribution on a substrate such as silicon or glass .

[0007] Heterostructures consisting of materials with different halides were obtained by anion exchangeprocesses using solid-gas phase processes with different halide vapours as , for example, described in C. R. Roy, D . Pan, Y. Wang, M. P. Hautzinger, Y. Zhao, J. C. Wright, Z . Zhu, S . Jin, J. Am. Chem. Capital 2021 , 143, 5212 and C. He, J. Li , Y. Bao, J. Li , H. Wang, M. Zhang, H. Li , H . Tang, Z . Sun, Q. Zhang, Y. Fang, J. Xu, Y. Yang, C. He, J. Li , Y. Bao, H. Wang, M. Zhang, H. Li , H. Tang, J. Xu, Y . Yang, Z . Sun, Y. Fang, Q. Zhang Jiangsu, Small 2022 , 18 , 2203036. Anion exchange processes are applied after fabrication of the structure and therefore also comprise at least two realization steps (crystal growth and ion exchange) leading to a wider range of parameters to be controlled than a one-pot synthesis . Furthermore, the combination of multiple realization steps typically increases costs and may create problems towards upscaling to larger quantities of such processes .

[0008] The object of the present invention is to provide a method for growing core-shell , core-frame or frame-only microcrystalline heterostructures made of two-dimensional layered metal halide perovskites by a one-pot growth process .

[0009] According to embodiments of the present invention, a method for producing metal halide perovskite heterostructures comprises :

[0010] a) providing a first type of two-dimensional metal halide perovskite crystals ;

[0011] b) providing a second type of two-dimensional metal halide perovskite crystals different from the first one;

[0012] c) providing a common solvent, such as , for example, acetonitrile (ACN) , acetone,dimethyl formamide (DMF) , dimethyl sul ide (DMSO) , y-butylactone or mixtures of these substances ;

[0013] d) dissolving the first type of crystals in the solvent to create a first solution;

[0014] e) dissolving the second type of crystals in the solvent to create a second solution;

[0015] f) mixing the first and second solutions to obtain a third solution;

[0016] g) injecting an antisolvent, such as , for example, toluene or dichlorobenzene (DCB) , into the third solution or inducing a controlled evaporation of the solvent into the third solution;

[0017] h) collecting the crystals thus formed.

[0018] The formed crystals may be collected in a container by centrifugation and subsequent washing by antisolvent or on the surface of a substrate, such as , for example, glass , a Si02 wafer, an element coated with a gold film, by evaporating the solvent at room temperature or with slight heating, for example in the range of 20 to 50 °C.

[0019] When the solvent is evaporated, the process can advantageously comprise the following steps :

[0020] providing a silicon or silicon dioxide substrate ;

[0021] dropping a few drops of the third solution onto the substrate inside a container, such as , for example, a vial , placed in a glove-box;

[0022] covering the container, for example with a perforated cover;

[0023] allowing the solvent to evaporate in the glovebox at a temperature comprised between 20 and 50 °C, in particular at room temperature.In one embodiment, mixing the second solution with the first solution occurs in conjunction with a continuous injection of the anti -solvent into the first solution. In this way the third solution is formed by a mixture of the first solution, the second solution and part of the anti -solvent .

[0024] In particular, the mixing of the second solution with the first solution takes place after injecting a part of the anti-solvent into the first solution so that the third solution comprises a mixture of the first solution, the second solution and part of the anti -solvent .

[0025] According to this variant, the step of mixing the first solution with the second solution (step f reported above) provides for an injection of part of the anti-solvent into the first solution followed by mixing the first solution, comprising the injected anti-solvent, with the second solution to form the third solution.

[0026] This allows to form nuclei with pure halides and side edges in which halides are present in varying percentages .

[0027] The sequential growth of the distinct crystalline structures that construct the crystalline heterostructure is triggered by the differing solubility of materials dissolved in a solvent, or mixtures of solvents, in common. Single microcrystals are obtained between the two different crystalline steps with well-defined epitaxial and non-epitaxial interfaces , which can be precipitated in solution by the controlled addition of an antisolvent in a vial or in a reaction flask or by evaporation of thesolvent or solvent mixture on the microcrystals deposited directly on the surface of a substrate.

[0028] Unlike heterostructures of other 2D materials such as metal dichalcogenides , the process according to the invention may be carried out at room temperature (RT) or with only slight heating, for example in the range 20 °C to 50°C.

[0029] The features of the invention and the advantages resulting therefrom will become more apparent from the description of non-limiting embodiments , illustrated in the accompanying drawings , wherein:

[0030] Fig. 1 shows the formation of heterostructures between two different halides in the PEA2PbBr4-PEA2Pbl4 system: (a) Schematic illustration of the one-pot reaction for the formation of lateral heterostructures by mixing stock solutions of PEA2PbBr4 and PEA2Pbl4, followed by recrystallization by slow injection of di chlorobenzene (DCB) ; b) Low and high magnification confocal images (Xex= 400 nm) , showing a photoluminescence (PL) distinct from the centre and the edge; c) UV-Vis absorption (shown on the left side) and PL spectra with Xex= 350 nm (right side) of PEA2PbBr4 and PEA2Pbl4 indicating the formation of bound phases with two distinct iodide (I) compositions in the core and in the edge; d) The X-ray diffraction spectrum (XRD) of the heterostructures with respect to the pure materials shows a single reflection due to the superposition of the reflections of the two distinct alloys ; and) Scanning electron microscopy (SEM) images of a single microcrystal and maps of the distribution of Energy-Dispersostrospectroscopy (EDX) elements that show a stronger sign of the edge.Fig. 2 shows 2DLP heterostructures obtained by one-pot growth process strategy: a) Confocal images of PEA2PbBr4~PEA2Pbl4 microcrystals with pure Br in the centre and a composite peripheral structure rich in I ; b) SEM and EDX maps of PEA2PbBr4-PEA2Pbl4 microcrystals without signal I from the core region.

[0031] Fig. 3 shows 2DLP lateral heterostructures in PEA2PbBr4-PEA4AgBiBrs system: a) Confocal image shows PEA2PbBr4 core emitting blue while PEA4AgBiBrs non-emissive edge remains dark under excitation with light at 400 nm wavelength; b) SEM image and corresponding EDX maps of a single PEA2PbBr4-PEA4AgBiBrs heterostructure. The EDX map confirms that lead (Pb) is mainly located in the core while silver (Ag) and bismuth (Bi) can only be detected on the edge. The bromide signal (Br) can be detected from the whole crystal since both phases contain Br; c) UV-Vis absorption of PEA4AgBiBrs; d) PL spectra of PEA2PbBr4 and PEA4AgBiBrs; e) XRD map of the heterostructures formed with respect to the pure materials .

[0032] Fig. 4 shows the formation of lateral heterostructures between PEA2SnBr4 and PEA4AgBiBrs : (a) schematic illustration of the heterostructure formation process by slow solvent evaporation; (b) optical image; (c) SEM image and (d) corresponding EDX mapping of PEA2SnBr4-PEA4AgBiBrs indicating a smooth interface between core (PEA4AgBiBrs) and edge (PEA2SnBr4) .

[0033] The starting point of the process according to the invention are two-dimensional (2D) metal halide perovskite crystals of the most varied compositions .The types of crystals used can therefore differ for different components , such as for example with different halides such as Cl , Br and I , or different divalent metal cations such as Pb, Sn, Ge, Cu, Cr, or a combination of monovalent and tetravalent metal cations such as Ag / Bi , Ag / In, Ag / Sb, Na / In and others as well as different organic spacer cations such as phenyl ethyl amine (PEA) and 2 -thiophenethylamine .

[0034] As will be reported later, the two different types of crystals used can be differentiated by the use of different two-dimensional metal halides , such as , for example, PEA2PbBr4 and PEA2Pbl4.

[0035] The crystals can be obtained by any process according to the known art, for example by the rapid crystallization process assisted by antisolvent described in X. Li , J. M. Hoffman, M. G. Kanatzidis , Chem. Rev. 2021 , 121 , 2230 , to be considered an integral part of the present description.

[0036] The crystals thus obtained are dissolved separately in a coordinating solvent such as acetonitrile (AON) , acetone, dimethyl formamide (DMF) , dimethyl sulfide (DMSO) , Y“butyrolactone (GBL) (and others) or mixtures thereof . Mixing these solutions in the desired amount and concentration in a single vial or beaker or reaction flask, followed by controlled injection of an anti-solvent or controlled evaporation of the solvent system, results in the desired formation of heterostructured microcrystals .

[0037] In this process , the slow addition of antisolvent triggers , first the growth of the materials with the lowest solubility (which will form the core) , and then the growth of the material withthe highest solubility, which will form the edge of the compound .

[0038] The resulting crystals can be harvested by centrifugation and subsequent washing with the antisolvent or directly on the surface of a suitable substrate (glass , SiC>2 wafer, gold (Au) film coated substrate) by evaporating the solvents at room temperature or by moderate heating.

[0039] The approach based on the evaporation of solvents on a substrate is more suitable for the direct integration of heterostructures in optoelectronic devices , while the strategy based on the injection of an anti-solvent is more suitable for photocatalytic applications where good dispersion of the active species is crucial .

[0040] We now see some examples of processes that can be used to make some heterostructures according to the present invention.

[0041] Heterostructure with different halides

[0042] In a process according to a first embodiment of the invention, perovskite crystals of different two-dimensional metal halides are used as starting materials for the reaction. These are co-dissolved in the solvent in the desired ratio with concentration imposed by solubility. To verify the formation of heterostructures consisting of crystalline blocks of two different halides , we choose, by way of example, PEA2PbBr4 and PEA2Pbl4 as starting materials . Fig. 1 summarizes the steps of the reaction and the chemi cal -physical characteristics of the products obtained.The stock solutions are prepared by dissolving 5 mg of PEA2PbBr4 and PEA2Pbl4 separately in 7.5 ml and 0.25 ml of ACN, respectively. The PEA2PbBr4 e PEA2Pbl4 stock solutions are mixed in a molar ratio of 1 :2.5 in a vial under magnetic agitation. Then 24 ml of di chlorobenzene (DCB) as anti -solvent are injected slowly (1 ml / min) into the solution. After injection of antisolvent, the solution containing the heterostructures is stirred for a further 5 min. Afterwards , the heterostructures are centrifuged at 4000 rpm for 2 min, the supernatant is discarded and the material is redispersed in 5 ml of DCB. The process is repeated two more times and the final product is dispersed in 1 ml of octane.

[0043] Low and high magnification confocal images in Fig. lb show the obtained microcrystals , of rectangular and square shape on the side up to 5 pm, where the edges have a distinct emission colour with respect to the centre of the crystal . Based on the study carried out by the present inventors on the generation of 2DLP heterostructures by anion exchange published in A. Schleusener et al . , Heterostructures via a Solution-Based Anion Exchange in Microcrystalline 2D Layered Metal-Halide Perovskites , Advanced Materials 2024 , 36, 2402924 , it is possible to state how the position of the edge in the UV-visible absorption spectrum and the band in photoluminescence (PL) spectroscopy of the heterostructures of Fig. lc indicate the presence of a Br-I phase. In addition, we identify at least two bands in the PL spectra, which we attribute to PL emission from the core and edge. The results obtained by X-ray diffraction (XRD) (Fig. Id) support thisobservation, since PEA2PbBr4-xIxphases bound within a compositional range of XBr ranging from 1 to 0.5 exhibit very similar diffraction peak positions . Scanning electron microscopy (SEM) images and EDX (SEM-energy dispersive X-ray) mapping (Fig. le) show regular-shaped microcrystals without visible cracks or pores , indicating a high quality of the interface. Further, EDX maps confirm a stronger I signal originating from the edges . Based on these results , we can conclude that the introduced procedure leads to the formation of heterostructures with a higher I content on the edge compared to the core .

[0044] In addition to the above-mentioned method for forming lateral heterostructures with different composition grades in the core and in the edge, the one-pot method is also suitable for preparing pure halide cores with variable composition edges . In this approach, the second component is not mixed directly with the core material , but is instead injected at a later stage during the addition of the antisolvent. Fig. 2 shows an example of PEA2PbBr4-PEA2Pbl4 heterostructures with a pure Br core and edges comprising both halides .

[0045] The heterostructures were prepared by injecting DCB into 3 mL of PEA2PbBr4 stock solution until the total injected volume of DCB antisolvent reached 10 mL. Subsequently, 0.15 mL of PEA2Pbl4 stock solution was rapidly injected and the injection of DCB was continued up to reaching a final volume of 24 mL. The pure nature of the Br core is confirmed by the extracted spectra of the core and edge, respectively (Fig. 2a) , as well as by the SEM images , in which theI signal can be seen to originate only from the crystal edges (Fig. 2b) .

[0046] Heterostructure with different metal cations

[0047] Heterostructures with different metal cations can be obtained by using the injection strategy discussed above. Fig. 3a shows the confocal image of a set of PEA2PbBr4-PEA4AgBiBrs microcrystals . Since PEA4AgBiBrs is a non-emissive 2DLP, the PL band originates exclusively from the PEA2PbBr4 core. Furthermore, SEM images and EDX maps (Fig. 3b) confirm the presence of Pb in the core of the structure, while Ag and Bi are in the edge . Absorption spectra were collected from sets of pure PEA2PbBr4 and PEA4AgBiBrs, as well as PEA2PbBr4-PEA4AgBiBrs heterostructures (Fig. 3c) . The spectrum of the heterostructures clearly shows an overlap of both spectra of the pure elements . Further, the PL spectra of the heterostructures (Fig. 3d) show only the PL band corresponding to the pure PEA2PbBr4 . XRD patterns (Fig. 3e) of the heterostructures reveal two distinct reflections which correspond to the positions of the two pure materials . A slight shift in the reflection corresponding to the PEA4AgBiBrs domain suggests an expansion of the unit cell , which could be attributed to interfacial deformation.

[0048] 2D stratified perovskite (2DLP) heterostructures with different metal cations in the core and structure were further demonstrated in the PEA2SnBr4~ PEA4AgBiBrs system (Fig. 4a) where they were directly grown onto a substrate. To prevent oxidation of Sn2+to Sn4+, all steps are performed in a nitrogen filledglovebox. Once formed, the heterostructures are stable in ambient conditions and can be handled outside the glovebox.

[0049] Stock solutions of PEA2SnBr4 and PEA4AgBiBrs are prepared by dissolving 10 mg of each compound in 4 ml of acetonitrile (ACN) . Any undissolved material remaining after 3 hours is filtered using a 450 nm syringe filter. A silicon (Si) or Si / SiO2 substrate is cleaned with 2 -propanol / acetone (1 : 1) in an ultrasonicator for 10 minutes before being placed on the bottom of a 4 ml vial . The solutions PEA2SnBr4 and PEA4AgBiBrs are mixed in a 1 : 1 ratio, creating a total volume of 2 ml . A small volume of this solution mixture (20 pl) is dropped onto the substrate. The vial is closed or capped with a perforated lid and the solvent is evaporated at room temperature inside the glovebox.

[0050] The heterostructures formed on the Si / SiO2 substrate present a clear optical contrast (Fig. 4b) between the centre (indicated with reference number 1) and the edge (indicated with reference number 2) . Scanning electron microscopy (SEM) images reveal a smooth interface between the two distinct crystalline blocks , with no signs of voids or discrepancies between the core and the edge. EDX (SEM-energy dispersive X-ray) mapping confirms the preferential position of Ag and Bi in the core and Sn in the edge. Since Br is present in both building blocks , its signal can be detected across the entire heterostructure. The minor signals in the edge of Ag and Bi indicate a slight binding of the PEA2SnBr4 phase to Ag and Bi , while the small signal of Sn inthe core may be correlated to the initial overgrowth of the PEA2SnBr4 core (Fig. 4c and 4d) .

[0051] Some examples of realization methods based on a one-pot solution of single microcrystals having two-dimensional layered perovskite heterostructures based on the different solubility of the heterojunction materials in a solvent have been presented. The interface of the heterostructure originates from the different composition of the two-dimensional layered metal halide perovskite source materials that are dissolved in the common solvent. These materials can vary in their composition of halides, metal cations , and / or organic cations . Some non-limiting examples of heterostructures prepared by the method according to embodiments of the invention are PEA2PbBr4-PEA2Pbl4 , PEA2PbBr4-PEA4AgBiBrs and PEA4AgBiBrs-PEA2SnBr4 .

[0052] The growth process requires only room temperature or a slight heating up to a maximum of 50 °C and, by controlling the process parameters , a good control of the size of the obtained microcrystals can be obtained. The synthesis method is scalable and, therefore, can be extended towards large quantities . The design of the heterostructure composition allows to customize the electronic and structural properties of the final material . For example, the band gap is mainly determined by the composition of the halide and thus a heterostructure with different halides , e .g. bromide and iodide, will lead to a band offset that channels the electric charge carriers towards the regions of the material with lower band gap (metal-iodide perovskite in this example) . Such charge separation may be applied toincrease the efficiency and performance of 2D layered perovskites in photocatalysis , light emission and energy harvesting.

[0053] Future developments foresee that the method could be integrated by introducing a spray deposition to locally grow crystals in distinct positions , that a library of potential material combinations could be created for on-demand and application-specific heterostructure synthesis , and that it could be employed in high-throughput synthesis for the discovery of new materials .

[0054] Regarding the applications of the heterostructures directly obtainable from the process according to the invention, these can be photocatalysis , use as multi-colour light emitters , photodetection and photovol tai cs . The possibility of manufacturing microcrystals directly on different substrates offers advantages for their integration into the most varied electronic devices .

[0055] The characteristics of the processes according to embodiments of the invention can be summarized as follows :

[0056] low cost as they take place at ambient temperature or almost, with little excess of chemical substances ;

[0057] one-pot synthesis , that is , in a single step;

[0058] deterministic synthesis of heterostructures with different material composition and band gap for the desired application in optoelectronics or catalysis ;obtaining high quality crystals (regular shape, high crystallinity, well-defined and straight interfaces of the heterojunction) ;

[0059] lateral extensions from a few micrometres to hundreds of micrometres , thickness in the range from 100-1000 nm controlled by the growth process (evaporation or controlled injection of antisolvent) .

[0060] These features include the following advantages :

[0061] the crystals can be obtained in large quantities with well-defined dimensional distribution in solution and on a variety of substrates and surfaces in a single step;

[0062] the processes allow the deterministic manufacture of heterostructures of ionic compounds ;

[0063] the size and shape of the microcrystals obtained are suitable for on-chip integration with silicon-based technologies ;

[0064] - the size control makes it possible to adapt the microcrystals obtained to the desired applications , for example, an edge length of a few micrometres for photocatalysis , long crystals as optically active waveguides ;

[0065] the heterostructural interface allows the channelling of the charge carriers towards the material with a lower band gap.

Claims

CLAIMS1 . Method for producing metal halide perovskite heterostructures , characterized by comprising :a) providing a first type of two-dimensional metal halide perovskite crystals ;b) providing a second type of two-dimensional metal halide perovskite crystals different from the first one;c) providing a common solvent;d) dissolving the first type of crystals in the solvent to create a first solution;e) dissolving the second type of crystals in the solvent to create a second solution;f) mixing the first and second solutions to obtain a third solution;g) injecting an antisolvent into the third solution or inducing controlled evaporation of the solvent into the third solution;h) collecting the crystals thus formed, further steps being included:providing a silicon or silicon dioxide substrate ;dropping a few drops of the third solution onto the substrate inside a container, such as , for example, a vial , placed in a closed chamber such as a glove-box;covering the container, for example with a perforated cover;allowing the solvent to evaporate in the closed chamber at a temperature comprised between 20 and 50 °C, in particular at room temperature.2 . Method according to claim 1 , wherein the formed crystals are collected in a container by centrifugation and subsequent washing by antisolvent or on the surface of a substrate, such as , for example, glass , a Si02 wafer, an element coated with a gold film, evaporating the solvent at room temperature or with slight heating, for example in the range of 20 to 50°C.3 . Method according to claim 1 or 2 , wherein the mixing of the second solution with the first solution takes place after injecting a part of the anti-solvent into the first solution so that the third solution comprises a mixture of the first solution, the second solution and part of the anti-solvent, i .e. step f) provides for an injection of part of the anti-solvent into the first solution followed by mixing the first solution comprising the injected anti-solvent with the second solution to form the third solution.4 . Method according to one or more of the preceding claims , wherein the solvent comprises acetonitrile (ACN) , acetone, dimethyl formamide (DMF) , dimethyl sulfide (DMSO) , Y“butyrolactone (GBL) or mixtures of these substances .5 . Method according to one or more of the preceding claims , wherein the anti-solvent comprises toluene or dichlorobenzene (DCB) .6 . Method according to one or more of the preceding claims , wherein perovskite crystals of metal halides comprising chloride, iodide or bromide and metal cations comprising lead, tin, germanium, copper, chromium, silver, bismuth, sodium, indium orantimony are used as starting materials for the reaction .7 . Method according to one or more of the preceding claims , wherein perovskite crystals of different two-dimensional metal halides, such as , for example, PEA2PbBr4 and PEA2Pbl4, are used as starting materials for the reaction.8 . Method according to one or more of the preceding claims , wherein perovskite crystals with different metal cations , such as for example, PEA2PbBr4 and PEA4AgBiBrs , PEA2SnBr4 and PEA4AgBiBrs are used as starting materials for the reaction.9 . Method according to one or more of the preceding claims , wherein the first and second solutions have different concentrations to compensate for the different solubility of the compounds , said different concentrations being obtained, for example, by dissolving the same amounts of perovskite crystals in different volumes of solvent, for example by dissolving 5 mg of PEA2PbBr4 and PEA2Pbl4, respectively, in 7.5 ml and 0.25 ml of acetonitrile.

10. Method according to claim 9, wherein the first and the second solution are solutions of PEA2PbBr4 and PEA2Pbl4 which are mixed in a molar ratio of 1 :2.5 in a container, such as a vial , under magnetic stirring, wherein 24 ml of di chlorobenzene as anti-solvent are injected slowly, for example at a rate of 1 ml / min, the solution being stirred for 5 min, centrifuged at 4 000 rpm for 2 min, wherein the supernatant is removed and the residual material is redispersed in 5 ml of dichlorobenzene by repeating the process for one or more times until thecollection of the final product which is dispersed in 1 ml of octane .

11. Method according to claim 9, wherein the first and second solutions are solutions of PEA2PbBr4 and PEA2Pbl4, the heterostructures being prepared by first injecting dichlorobenzene into 3 mL of PEA2 PbBr4 stock solution until the total injected volume of the dichlorobenzene anti-solvent reached 10 mL, then rapidly injecting 0.15 mL of PEA2Pbl4 stock solution by continuing the injection of dichlorobenzene until reaching a final volume of 24 mL .12 . Method according to one or more of the preceding claims , characterized in that it comprises :preparing PEA2SnBr4 and PEA4AgBiBrs stock solutions by dissolving 10 mg of each compound in 4 ml of acetonitrile;filtering the materials not dissolved in solution ;cleaning the silicon or silicon dioxide substrate, for example with 2 -propanol / acetone (1 : 1) , for example in an ultrasonicator for 10 minutes before being placed on the bottom of a vial ;mixing the solutions PEA2SnBr4 and PEA4AgBiBrs in a 1 : 1 ratio, creating a total volume of 2 ml ;dropping 20pl of this solution mixture onto the substrate ;closing or capping the vial with the perforated cap;allowing the acetonitrile to evaporate; collecting the crystals formed on the substrate.

13. Method according to one or more of the preceding claims , characterized in that it is used tomake photocatalysts for use in organic synthesis processes and / or optoelectronic devices such as , for example , variable colour light emitters , light detectors , photovoltaic cells , waveguides .

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