Microwave-assisted synthesis of phase engineered cesium manganese bromide nanocrystals with color-tunable emission

Abstract

The present invention discloses a microwave-assisted process for preparation of lead-free Mn-doped CsBr [Mn2+: CsBr], a phase-engineered all-inorganic cesium manganese bromide (0D Cs3MnBr5) and 1D CsMnBr3 phase-pure perovskite nanocrystals (NCs) the process comprising (a) preparing a cesium oleate mixture by adding 1-octadecene (ODE) and oleic acid (OA) to cesium carbonate (Cs2CO3), then (b) dissolving the oleate mixture by heating at a temperature in a range from 110 to 130℃ to obtain a solution; and (c) adding a mixture of manganese bromide (MnBr2), ODE, OA and oleylamine (OAm) to the dissolved solution and treating under microwave conditions at a temperature in a range from 190 to 210℃, wherein, using cesium carbonate (Cs2CO3) to manganese bromide (MnBr2) in a mole ratio of 1:1 gives CsBr [Mn2+: CsBr] nanocrystal, using cesium carbonate (Cs2CO3) to manganese bromide (MnBr2) in a mole ratio of 1:2.5 gives 0D Cs3MnBr5 nanocrystal, and using cesium carbonate (Cs2CO3) to manganese bromide (MnBr2) in a mole ratio of 1:7 gives 1D CsMnBr3 nanocrystal.

Description

[0001] MICROWAVE-ASSISTED SYNTHESIS OF PHASE ENGINEERED CESIUM MANGANESE BROMIDE NANOCRYSTALS WITH COLOR-TUNABLE EMISSION FIELD OF THE INVENTION

[0002] The present invention generally relates to a rapid and efficient microwave-assisted synthesis of cesium manganese bromide -based nanocrystals with color-tunable emission. Specifically, the present invention relates to a microwave assisted process for preparation of lead-free manganese (Mn)-doped cesium bromide (CsBr) [Mn2+: CsBr], phase-pure all-inorganic cesium manganese bromide 0D Cs₃MnBr₅, and 1D CsMnBr₃ perovskite NCs, with tunable blue-green-red emission (RGB) colour with high colour purity.

[0003] BACKGROUND AND PRIOR ART OF THE INVENTION

[0004] Cesium lead halide perovskite CsPbX₃ (X = Cl, Br, I) nanocrystals (NCs) are known for their use as good red, green and blue (RGB) emitters in liquid crystal displays (LEDs), and perovskite NC light-emitting diodes (PLEDs) in literature. These materials are used in display applications, having high color purity and photoluminescence quantum yield (PLQY), tunable and narrow full width at half-maximum (FWHM) of the photo luminescence (PL) peak, to realize wide color gamut. However, the concerns about lead (Pb) toxicity and the poor stability of lead, resulted in the development of lead-free metal halide perovskite NCs that can deliver similar optical characteristics. Therefore, different Pb-free metal halide perovskites NCs, with structure similar to lead halide perovskites have been reported.

[0005] Several wet chemical solution-based synthetic methods have been reported till date to synthesize phase-pure cesium manganese bromide (CsMnBr₃ and Cs₃MnBr₅) nanocrystals (NCs) with high PLQY and narrow FWHM with PL decay time, from a few hundred micro / pico seconds.

[0006] Ping Gao et. al Molecules., 2022, 27, 8259(1-10)) recently reported a synthesis of lead-free all-inorganic halide materials with different manganese (Mn2+)-based crystal structures (Cs₃MnBr₅ and CsMnBr₃) having white-light emitting properties using aconvenient one-pot sonication method at room temperature. However, it appears that the sonication process disclosed is time consuming and is atmospheric conditions dependent, compared to microwave-assisted method.

[0007] Recently, Q. Kong et. al. (Angew. Chem. Int. Ed., 2021, 60, 19653- 19659) reported tunable color-pure red-green-blue (RGB) emission in lead-free cesium manganese bromide NCs through phase engineering by modulating coordination sites of the NCs. The 1D CsMnBr₃ NCs (red emission) and 0D Cs₃MnBr₅ NCs (green emission) were synthesized by hot-injection method by controlling cesium acetate (Cs(OAc)): manganese acetate (Mn(OAc)2) ratio using trimethylbromosilane (TMSBr) as a Br source; the phase-pure Cs₃MnBr₅ NCs, were obtained via isopropanol treatment for 5.5 hours. Whereas, the Cs₂MnBr₄·2H₂O NCs (blue emission) were obtained after phase-pure CsMnBr₃ / Cs₃MnBr₅ NCs were placed in environments with air humidity of 99% for several hours. Conversely by further thermal annealing, the Cs₂MnBr₄·2H₂O phase transformed into the mixture of CsMnBr₃ and Cs₃MnBr₅ phase, instead of pure CsMnBr₃ / Cs₃MnBr₅ phase during the dehydration step. Moreover, the TMSBr which is used as a bromine source is highly flammable.

[0008] Hence, there is a need in the art to develop a rapid, one pot micro wave-assisted (MW-AT) process for the preparation of non-toxic and stable lead (Pb)-free metal halide perovskite NCs with tunable RGB emission color with high color purity via a small change in a single process.

[0009] OBJECTIVES OF THE INVENTION

[0010] An object of the present disclosure is to provide a microwave-assisted process for preparation of a lead-free manganese (Mn)-doped cesium bromide (CsBr) [Mn2+: CsBr], phase-engineered all-inorganic cesium manganese bromide 0D Cs₃MnBr₅ and 1D CsMnBr₃ phase-pure perovskite nanocrystals (NCs) using MnBr₂ as both Mn and Br source, in a nonpolar solvent.

[0011] Another objective of the present invention is to provide a phase engineered cesium manganese bromide perovskite NCs using MnBr₂ as both Mn and Br source, in a nonpolar solvent.Another object of the present invention is to provide a lead-free Mn-doped CsBr, a phase-engineered all-inorganic cesium manganese bromide 0D Cs₃MnBr₅ and 1D CsMnBr₃ phase-pure perovskite NCs with tunable blue-green-red emission colour with high colour purity.

[0012] Yet another objective of the present invention is to provide a phase engineered cesium manganese bromide perovskite NCs as described above, wherein these perovskites may be used in anticounterfeiting and security applications and also for developing moisture sensors.

[0013] SUMMARY OF THE INVENTION

[0014] Accordingly, in order to accomplish an objective, the present invention provides a microwave-assisted process for preparation of lead-free Mn-doped CsBr [Mn2+: CsBr], phase-engineered all-inorganic cesium manganese bromide 0D Cs₃MnBr₅ and 1D CsMnBr₃ phase-pure perovskite NCs.

[0015] In an aspect, the present invention relates to a microwave assisted process for preparation of lead-free manganese (Mn)-doped cesium bromide (CsBr) [Mn2+: CsBr], phase-engineered all-inorganic cesium manganese bromide (0D Cs₃MnBr₅) and 1D CsMnBr₃ phase-pure perovskite nanocrystal (NCs), the process comprising:

[0016] (a) preparing a cesium oleate mixture by adding 1 -octadecene (ODE) and oleic acid (OA) to cesium carbonate (Cs₂CO₃);

[0017] (b) dissolving the oleate mixture by heating at a temperature in a range from 110 to 130°C to obtain a solution; and

[0018] (c) adding a mixture of manganese bromide (MnBr₂), ODE, OA and oleylamine (OAm) to the dissolved solution and treating under microwave conditions at a temperature in a range from 190 to 210°C,

[0019] wherein,

[0020] using cesium carbonate (Cs₂CO₃) to manganese bromide (MnBr₂) in a mole ratio of 1:1 gives CsBr [Mn2+: CsBr] nanocrystal,using cesium carbonate (Cs₂CO₃) to manganese bromide (MnBr₂) in a mole ratio of 1:2.5 gives 0D Cs₃MnBr₅ nanocrystal, and

[0021] using cesium carbonate (Cs₂CO₃) to manganese bromide (MnBr₂) in a mole ratio of 1:7 gives 1D CsMnBr₃ nanocrystal.

[0022] In another aspect, the present invention relates to a process for transforming / converting the Cs₃MnBr₅ and CsMnBr₃ nanocrystal to Cs₂MnBr₄.2H₂O crystal in a moisture environment having air humidity in a range from ~98-99%.

[0023] In another aspect, the present invention relates to a process for inversely transforming the Cs₂MnBr₄.2H₂O crystal obtained by the present process to Cs₃MnBr₅ and / or CsMnBr₃ nanocrystal by thermal annealing at a temperature in a range from 80 °C to 100 °C.

[0024] ACRONYMS USED TO DESCRIBE THE INVENTION:

[0025] Cs₂CO₃: Cesium carbonate

[0026] MnBr₂: Manganese bromide

[0027] ODE: 1- Octadecene

[0028] OA: Oleic acid

[0029] OAm: Oleylamine

[0030] NCs: nanocrystals

[0031] PLQY: photoluminescence quantum yield

[0032] FWHM: Full width at half-maximum

[0033] BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 depicts the photographic images of the compounds synthesized at different ratios of cesium carbonate and manganese bromide according to Example 4.

[0035] FIG. 2 depicts the schematic presentation of the microwave-assisted synthesis of Mn-doped CsBr [Mn2+: CsBr], Cs₃MnBr₅ and CsMnBr₃ prepared according to the processof the present invention; (a) the pictures of the said compounds under ambient condition, and (b) the emission on the respective compounds under 365 nm UV light.

[0036] FIG. 3 depicts PXRD pattern of: (a) Mn-doped CsBr [Mn2+: CsBr] prepared according to Example 2, (b) CssMnBrs prepared by the process of Example 3 and (c) CsMnBr₃ prepared by the process of Example 4; along with crystal structure showing doping of Mn in (d) [Mn2+: CsBr]; in (e) Cs₃MnBr₅, and (f) CsMnBr₃ respectively.

[0037] FIG. 4 depicts the FESEM images of the solid powder of (a) Mn2+doped CsBr [Mn2+: CsBr] prepared according to Example 2 (scale 10 μm), (b) Cs₃MnBr₅ prepared by the process of Example 3 (scale 5 μm) and (c) CsMnBr₃ prepared by the process of Example 4 (scale 10 μm).

[0038] FIG. 5 depicts Confocal microscopy reflection images of (a) Mn2+: CsBr crystal prepared according to Example 2, (b) Cs₃MnBr₅ prepared according to Example 3 and (c) CsMnBr₃ prepared according to Example 4.

[0039] FIG. 6 depicts ESEM-EDAX and elemental atomic % of as-synthesized compounds: (a) Mn2+: CsBr prepared according to Example 2, (b) Cs₃MnBr₅ prepared according to Example 3 and (c) CsMnBr₃ prepared according to Example 4.

[0040] FIG. 7 depicts High resolution deconvoluted XPS spectra of Mn2+: CsBr prepared according to Example 2 showing (a) Cs 3d, (b) Mn 2p & (c) Br 3d; the XPS spectra of CssMnB prepared according to Example 3 showing (d) Cs 3d, (e) Mn 2p & (f) Br 3d; and the XPS spectra of CsMnBn prepared according to Example 4 showing (g) Cs 3d, (h) Mn 2p & (i) Br 3d core levels.

[0041] FIG. 8 depicts TGA analysis of (a) Mn2+: CsBr prepared according to Example 2, (b) Cs₃MnBr₅ prepared according to Example 3 and (c) CsMnBr₃ prepared according to Example 4.

[0042] FIG. 9 depicts UV-vis spectra of (a) Mn2+: CsBr prepared according to Example 2, (b) Cs₃MnBr₅ prepared according to Example 3 and (c) CsMnBr₃ prepared according to Example 4.FIG. 10 depicts PLE and PL spectra of (a) Mn2+:CsBr prepared according to Example 2, (b) Cs₃MnBr₅ prepared according to Example 3 and (c) CsMnBr₃ prepared according to Example 3;, (d) CIE plot of as-synthesized compounds (star denotes Mn2+:CsBr, circle denote Cs₃MnBr₅, triangle denotes CsMnBr₃ respectively; 11(e-f) shows the time-resolved PL decay curves at excitation wavelength 260 nm of (11e) Cs₃MnBr₅ prepared according to Example 3 and (11f) CsMnBr₃ prepared according to Example 4 with average lifetime 265 μs and 51.58 μs respectively.

[0043] FIG. 11 depicts Typical ‘Tauc’ plots for calculating optical band gaps of (a) Mn2+: CsBr crystal prepared according to Example 2, (b) Cs₃MnBr₅ prepared according to Example 3 and (c) CsMnBr₃ prepared according to Example 4.

[0044] FIG. 12 depicts the phase transformation process of Cs₃MnBr₅ to Cs₂MnBr₄.2H₂O according to Example 7 wherein (a)(i) to (a)(iii) shows PXRD of Cs₃MnBr₅ prepared according to Example 3 (a)(i), of Cs₂MnBr₄.2H₂O (a)(ii), wherein the asterisks denote peaks from CsBr after keeping in moisture and Cs₂MnBr₄.2H₂O (a)(iii) after heating at ~80°C; (a)(iv) to (a)(vi) shows UV and PL spectra of Cs₃MnBr₅ prepared according to Example 3 (a)(iv), Cs₂MnBr₄.2H₂O after keeping in moisture (a)(v) and after heating Cs₂MnBr₄.2H₂O (a)(vi) at ~80°C; (a)(vii) depicts the reaction of the respective phase transformation process.

[0045] FIG. 13 depicts (a) PL spectra of consecutive moisture-heating process for Cs₃MnBr₅ crystal prepared according to Example 3, and (b) depicts CIE plots for the phase transitions of Cs₃MnBr₅ wherein the circle denotes Cs₃MnBr₅ crystal prepared according to Example 3, the star denotes after treating in moisture, and triangle denotes after heating.

[0046] FIG. 14 (a) depicts the phase transformation process of CsMnBr₃ to Cs₂MnBr₄.2H₂O according to Example 7 wherein (a)(i) to (a)(iii) shows PXRD of CsMnBr₃ prepared according to Example 4 (a)(i), of Cs₂MnBr₄.2H₂O (a)(ii), wherein the asterisks denote peaks from CsBr after keeping in moisture and Cs₂MnBr₄.2H₂O (a)(iii) after heating at ~80°C; (a)(iv) to (a)(vi) shows UV and PL spectra of CsMnBr₃ prepared according to Example 4 (a)(iv), Cs₂MnBr₄.2H₂O after keeping in moisture (a)(v) and after heatingCs2MnBr4.2H2O (a)(vi) at ~80°C; (a)(vii) depicts the reaction of the respective phase transformation process.

[0047] FIG. 15 depicts (a) PL spectra of consecutive moisture-heating process for CsMnBr₃ crystal prepared according to Example 4 and 15(b) depicts the CIE plots for the phase transitions of CsMnBr₃ wherein the circle denotes the CsMnBr₃ crystal prepared according to Example 4, the star denotes after treating in moisture, and triangle denotes after heating.

[0048] DETAILED DESCRIPTION OF THE INVENTION

[0049] The invention will now be described in detail in connection with certain preferred and optional embodiments, so that various aspects thereof may be more fully understood and appreciated.

[0050] In an embodiment, the present invention provides a microwave-assisted process for preparation of lead-free manganese doped cesium bromide nanocrystals.

[0051] More particularly, the present invention provides a microwave-assisted process for preparation of a lead-free manganese (Mn)-doped cesium bromide (CsBr) [Mn2+: CsBr], a phase-engineered all-inorganic cesium manganese bromide (0D Cs₃MnBr₅) and 1D CsMnBr₃ phase-pure perovskite nanocrystals (NCs) using manganese bromide (MnBr₂) as both Mn and Br source.

[0052] In an embodiment, the present invention provides a microwave-assisted process for preparation of lead-free manganese (Mn)-doped cesium bromide (CsBr) [Mn2+: CsBr], a phase-engineered all-inorganic cesium manganese bromide (0D Cs₃MnBr₅) and 1D CsMnBr₃ phase-pure perovskite NCs the process comprising:

[0053] (a) preparing a cesium oleate mixture by adding 1 -octadecene (ODE) and oleic acid (OA) to cesium carbonate (Cs₂CO₃);

[0054] (b) dissolving the oleate mixture by heating at a temperature in a range from 110 to 130°C to obtain a solution; and(c) adding a mixture of manganese bromide (MnBr₂), ODE, OA and oleylamine (OAm) to the dissolved solution and treating under microwave conditions at a temperature in a range from 190 to 210°C,

[0055] wherein,

[0056] using cesium carbonate (Cs₂CO₃) to manganese bromide (MnBr₂) in a mole ratio of 1:1 gives CsBr [Mn2+: CsBr] nanocrystal,

[0057] using cesium carbonate (Cs₂CO₃) to manganese bromide (MnBr₂) in a mole ratio of 1:2.5 gives 0D Cs₃MnBr₅ nanocrystal, and

[0058] using cesium carbonate (Cs₂CO₃) to manganese bromide (MnBr₂) in a mole ratio of 1:7 gives 1D CsMnBr₃ nanocrystal.

[0059] In step (a), the cesium oleate mixture is prepared by adding 1 -octadecene having concentration in a range from 5 to 7 mL and oleic acid having concentration in a range from 1 to 1.5 mL to cesium carbonate having concentration in a range from 0.5 to 0.75 mmole.

[0060] Then, in step (b), the mixture obtained is preferably dissolved under stirring at a rate of 900 rotations per minute (rpm) to 1000 rpm. Preferably, the mixture obtained is then kept under vacuum for 1 hours to 1.25 hours after degas sing / gas sing with an inert gas. Preferably, the inert gas is nitrogen.

[0061] In step (c), a mixture of MnBn, ODE having concentration in a range from 5 to 7 mL, OA having concentration in a range from 1 to 2 mL and oleylamine (OAm) having concentration in a range from 1 to 2 mL is added to the cesium oleate mixture. Advantageously, the MnB used serves as both Mn and bromide (Br) source in the process and hence the present process does not require additional reagents. Preferably, the microwave treatment is carried out for a period of 10 minutes to 15 minutes. Following microwave treatment, preferably, the mixture obtained is cooled down to a temperature in a range from 55 °C to 50 °C and then subjected to first centrifugation, sonification followed by second centrifugation. The first centrifugation may be carried out at a speed of 10000 rpm to 12000 rpm for 10 minutes to 15 minutes to obtain a precipitate. The precipitate obtained is then dissolved in a solvent selected from hexane,cyclohexane, toluene or mixtures thereof and then sonicated for 1 minute to 2 minutes. The second centrifugation may be carried out at a speed of 9000 rpm to 11000 rpm for 5 minutes to 8 minutes.

[0062] The process of the present invention is simple, does not require complex process steps, is rapid, energy efficient, and is cost effective. Further, the process of the present invention by using MnBr₂ as both Mn and Br source and by controlling the precursor’s mole ratios, in a nonpolar solvent enables the preparation of perovskite crystals having tunable red-green-blue (RGB) emission without pre-treatment of reactants, compared to a conventional perovskite synthesis method.

[0063] In the process of the present invention, the mole ratio of CsCO₃ to MnBr₂ of 1:1 provides a pure blue coloured compound (Mn2+: CsBr), the mole ratio of CsCO₃ to MnBr₂ of 1:2.5 provides a pure green coloured compound (Cs₃MnBr₅) and the mole ratio of CsCO₃ to MnBr₂ of 1:7 provides a pure red coloured compound (CsMnBr₃). Hence, the process of the present invention enables the preparation of the perovskite crystals with tunable red-blue-green (RBG) emission in a single process.

[0064] Further, the process of the present invention provides lead-free phase engineered cesium manganese bromide perovskite NCs with high colour purity and PLQY of 23.36% [Mn2+: CsBr], 34.18% (Cs₃MnBr₅) and 27.76% CsMnBr₃.

[0065] In a preferred embodiment, the present invention provides a microwave-assisted process for preparation of lead-free Mn-doped CsBr [Mn2+: CsBr], a phase-engineered allinorganic cesium manganese bromide (0D Cs3MnBrs) and ID CsMnBr3phase-pure perovskite NCs the process comprises the steps of:

[0066] a) mixing Cs₂CO₃, ODE and OA with continuous stirring at a speed of 850 rpm-950 rpm to obtain a reaction mixture;

[0067] b) gradually heating the reaction mixture to a temperature in a range from 110-130°C under vacuum to obtain a reaction mass;

[0068] c) gassing / degassing the reaction mass with nitrogen gas 3-4 times at a temperature in a range from 110-130°C and then keeping the reaction mass under vacuum for a time period in a range from 50-60 mins;d) maintaining the reaction mass of step c) under nitrogen at a temperature in a range of 110-130°C to obtain a cesium oleate mixture;

[0069] e) adding a mixture of MnBr₂, ODE, OA and OAm to the cesium oleate mixture obtained in step d) and placing the obtained reaction mixture in a microwave reactor;

[0070] f) setting the temperature of the micro wave in a range from 190-210°C and holding it for a time period in the range from 10-15 mins to obtain a reaction mass;

[0071] g) cooling the reaction mass to a temperature in a range from 50-55°C followed by centrifuging at 10,000-12, 000 rpm for a time period in a range from 10-15 mins to obtain a precipitate;

[0072] h) dispersing the precipitate obtained in a solvent selected from cyclohexane, n-hexane, and toluene or mixtures thereof and sonicating for 1-2 mins followed by collecting and vacuum drying,

[0073] wherein,

[0074] using cesium carbonate (Cs₂CO₃) to manganese bromide (MnBr₂) in a mole ratio of 1:1 gives CsBr [Mn2+: CsBr] nanocrystal,

[0075] using cesium carbonate (Cs₂CO₃) to manganese bromide (MnBr₂) in a mole ratio of 1:2.5 gives 0D Cs₃MnBr₅ nanocrystal, and

[0076] using cesium carbonate (Cs₂CO₃) to manganese bromide (MnBr₂) in a mole ratio of 1:7 gives 1D CsMnBr₃ nanocrystal.

[0077] In another embodiment, the present invention provides a process for transforming / converting the Cs₃MnBr₅ and CsMnBr₃ nanocrystal obtained by the process of the present invention to Cs₂MnBr₄.2H₂O crystal in a moisture environment having air humidity in a range from ~98-99%. Preferably, the process is carried out by keeping the Cs₃MnBr₅ and CsMnBr₃ nanocrystal in a humid environment having humidity in a range from ~98-99% for 5 minutes to 10 minutes.Advantageously, the process of the present invention enables the transformation of the Cs₃MnBr₅ nanocrystal having green emission and CsMnBr₃ nanocrystal having red emission to Cs₂MnBr₄.2H₂O nanocrystal having blue emission.

[0078] In another embodiment, the present invention provides a process for inversely transforming the-Cs₂MnBr₄.2H₂O crystal obtained by the process of the present invention to Cs₃MnBr₅ and / or CsMnBr₃ nanocrystal by thermal annealing at a temperature in a range from 80 °C to 100°C.

[0079] This enables, the blue emitting nanocrystal (Cs₂MnBr₄.2H₂O) to be transformed into nanocrystals having red (CsMnBr₃) or green emission (Cs₃MnBr₅).

[0080] In another embodiment, the above-said phase engineered cesium manganese bromide perovskite NCs, [Mn2+: CsBr], 0D Cs₃MnBr₅ and 1D CsMnBr₃ prepared by the process of the present invention can be used as a component in anti-counterfeiting and security applications and also for developing moisture sensors.

[0081] The process of the present invention is illustrated by non-limiting examples.

[0082] GENERAL INFORMATION:

[0083] The phase identification of the as-synthesized samples were performed by Powder X ray diffraction (PXRD), recorded on an analytical X’pert PRO powder X-ray diffractometer with Cu Ka radiation. The morphologies, Energy Dispersive Analysis of X-rays (EDAX) elemental ratio analysis and elemental mapping of the as-synthesized products were investigated using scanning electron microscope (FEI, ESEM Quanta 200-3D and FEI, Field Emission Scanning Electron Microscope (FESEM) NOVA NANO 450). The Confocal reflection images were taken in a Confocal Laser Scanning microscope (CLSM) (Zeiss, Oberkochen, Germany Model LSM 710). The thermo-gravimetric analysis (TGA) was performed using SDT Q600 DSC-TGA instrument in Air atmosphere at ramping rate 10 °C / min. The surface properties of the as-synthesized samples were characterized by X-ray Photoelectron Spectra (XPS) using Thermo Scientific K-Alpha+ X-ray photoelectron spectrometer analyzer chamber operating at 2×10-7mbar pressure. Optical absorption measurements were carried out by a Shimadzu UV-Vis-IR (UV-3600 Plus) spectrophotometer. Steady-statephotoluminescence excitation (PLE) and photoluminescence (PL) measurements were performed utilizing a spectrofluorometer FS5 (Edinburgh Instruments). PLQY of the compounds was measured in HORIBA FTO300G. Lifetime measurements were taken in an Edinburgh FLS1000 photoluminescence spectrometer, attached with Optistat DN cryostat.

[0084] EXAMPLES

[0085] The following examples, which include preferred embodiments, will serve to illustrate the practice of this invention, it being understood that the particulars shown are by way of example and for purpose of illustrative discussion of preferred embodiments of the invention.

[0086] Procurement details and Abbreviations used:

[0087] Cesium carbonate (CS2CO3, Sigma Aldrich, 99.9%), Manganese bromide (MnBr₂, Sigma Aldrich, 98%), 1- Octadecene (ODE,), Oleic acid (OA), Oleylamine (OAm), n-Hexane (SRL, 99%).

[0088] Example 1: Synthesis of Cs- oleate mixture

[0089] cesium (Cs)-oleate was prepared by taking 0.5 mmol (162.9 mg) Cs-carbonate in two necks round bottom flask along with addition of 5 mL ODE and 1 mL (3.15 mmol) OA, with continuous string at 900 rpm and gradually heated up to 120 °C using Schlenk line under vacuum (ref: ACS Nano., 2022, 16, 19618- 19625). After reaching this temperature, the solution was gassed / degassed 3-4 times using nitrogen gas, and then the solution was kept for 1 hour under vacuum. Then, the obtained transparent solution was kept under nitrogen gas, maintaining the same temperature.

[0090] Example 2: Synthesis of Mn-doped CsBr (Cs₂CO₃:MnBr₂ = 1:1)

[0091] 0.5 mmol (107.35 mg) MnBr₂ was taken in a microwave tube (30 mL) along with 5 mL ODE, 1 mL OA, and 1 mL OAm, then added to as-prepared Cs-oleate obtained in Example 1 and then the tube was kept in a microwave reactor (Anton Paar, mono wave 300) and set to temperature 200°C with holding time of 15 minutes. After the given time, the reaction mixture was cooled down to 55 °C with a self-cooling condenser.The crude solution obtained was then first centrifuged at 10,000 rpm for 10 min to obtain a precipitate, then the precipitate was dispersed in 7 mL hexane followed by sonication of 1-2 min (to completely disperse) and centrifuged at 10,000 rpm for 5 min. Again, the precipitate was washed with the same, and finally, the precipitate was collected and vacuum dried for further characterization.

[0092] Example 3: Synthesis of 0D Cs₃MnBr₅ (Cs₂CO₃:MnBr₂ = 1:2.5)

[0093] The synthesis procedure was similar as according to Example 2, except the MnBr₂ was taken as 1 mmol (268.3 mg).

[0094] Example 4: Synthesis of 1D CsMnBr₃ (Cs₂CO₃: MnBr₂ 1:7)

[0095] The synthesis procedure was similar according to Example 2, except the MnBr₂ is taken as 3.5 mmol (751.45 mg).

[0096] Example 5: The effect of reaction parameters on the synthesis of [Mn2+: CsBr], Cs₃MnBr₅, and CsMnBr₃ nanocrystals.

[0097] The Mn-doped CsBr [Mn2+: CsBr], Cs₃MnBr₅, and CsMnBr₃ prepared by the microwave-assisted synthesis procedure of Examples 2 to 4 was compared with Comparative Sample A to L in which the ratio of the precursors was not as per the present invention. The synthesis temperature was further varied from 180 to 240 °C, within a time period of 15 minutes for the comparative samples. The results are represented in Table 1 below.

[0098] Table 1:

[0099] Sample Ratio Temperature Time (min) Photoluminescence No. (°C) (PL)

[0100] CS2CO3:

[0101] MnBn

[0102] 1. 1:1 200 15 Blue PL, Mn2+:CsBr A 1:1.25 200 15 Blue PL

[0103]

[0104] B 1:1.5 200 15 Blue PLP_W0100794

[0105] c 1:2 180 15 No PL

[0106] D 1:2 200 15 Green PL

[0107] 2. 1:2.5 200 15 Green PL, CssMnBrs E 1:2 220 15 Green PL

[0108] F 1:2 240 15 Green PL

[0109] G 1:4 200 15 Green PL

[0110] H 1:6 200 15 Green PL

[0111] 3. 1:7 200 15 Red PL, CsMnBr₃

[0112] I 1:7 220 15 Red PL

[0113] J 1:7 240 15 Red PL

[0114] K 1:8 200 15 Red PL

[0115]

[0116] L 1:9 200 15 Red PL

[0117] As observed from the Table 1 above, at the reaction temperature of 200 °C and time of 15 minutes, by changing the mole ratio of cesium carbonate to manganese bromide, (Cs2CO3: MnBr2) from 1:1 to 1: 9, a series of different color emitting compounds were obtained. As observed from the said Table and as depicted in FIG. 1, when the mole ratio of Cs2CO3 / MnBr2 were 1:1, 1:2.5, and 1:7, the product shows blue, green, and red photoluminescence (PL), respectively, with excellent PL emission intensity. Further, as observed from the said Table and FIG. 1, for Comparative samples A-B not following the Cs₂CO₃: MnBr₂ ratio as per the present invention, resulted in a lesser blue emission intensity while for Sample C no PL was observed and for sample D, the crystals obtained gave a mixture of blue and green emission. For Comparative samples E-H, the crystals obtained were not pure phase, as mixture of green-red phases were observed while similarly, for Comparative samples I to L, the crystals obtained were not pure, as some extra peaks were observed in the PXRD data indicating the presence of impurities.This suggests that an appropriate feed ratio is essential for obtaining the desirable products having red-blue-green (RGB) emission with good intensity and purity. Based on the optimized reaction parameters, amongst the samples, the best sample obtained by the process of the present invention was at a temperature of 200 °C and time of 15 minutes, under the conditions of Cs2CO3: MnBr2 = 1:1 (blue-color emission) (Sample 1), 1:2.5 (green-color emission) (Sample 6), and 1:7 (red-color emission) (Sample 7). The schematic representation of the process of present invention at said ratios is depicted in FIG. 2.

[0118] Example 6: Characterization of the crystals obtained in Examples 2 to 4:

[0119] Powder X-ray diffraction (PXRD) and structure analysis:

[0120] To find out the crystal structure of the as-prepared compounds, the powder X-ray diffraction (XRD) analysis was carried out as depicted in FIG. 3 (a-c). FIG. 3(a) shows the XRD pattern of the blue-color emitting compound [Mn2+: CsBr] prepared according to Example 2, where almost all the diffraction peaks were well-matched with cesium bromide (CsBr) (ICDD Card no. 050588). Moreover, the XRD patterns showed no other diffraction peaks besides the CsBr, suggesting that no other by-products or impurities was formed by the process of the present invention. However, the peak shifting to lower angles was observed, where the low Mn doping is responsible for this uniformity. Thus, the blue-color emitting [Mn2+: CsBr] prepared by the process of the present invention was designated as Mn-doped CsBr [Mn2+:CsBr]. The crystal structure of green-color emitting compound 0D CssMnBrs prepared according to Example 3 having Cs2CO3: MnBr2 ratio of 1:2.5 was well-matched with the crystal structure of Cs₃MnBr₅ (PDF card no. COD ID 96-152-3977) having tetragonal (I 4 / mcm) structure with lattice parameters a = b = 9.596 Å, c = 15.57 Å, and the obvious diffraction peaks at 29=21.7, 26.9, 29.5, 35.1, 42.1, 44.2 and 51.9 corresponding to diffractions from the {202}, {213}, {310}, {224}, {420}, {404} and {406} planes as depicted in FIG. 3(b). Similarly, the crystal structure of red-color emitting compound (ID CsMnBn) prepared by the process of Example 4 having Cs₂CO₃: MnBr₂=1:7, was well-matched with CsMnBr3 (PDF card no. COD ID 96-153-2233) having a hexagonal structure (P63 / mmc) with a = b = 7.618 Å, c = 6.519 Å, and the obvious diffraction peaks at 29 =13.4, 19.1, 23.2, 27.2, 30.4, 38.8, 45.8, 47.7, 50.2, 55.8, 57.5 and 63.1 correspondingto diffractions from the {100}, {101},{110}, {200}, {201}, {202}, {212}, {220}, {203}, {222}, {401}, and {402} planes as depicted in FIG. 3(c). No detectable impurity was observed in both the cesium manganese bromide 0D Cs₃MnBr₅ and 1D CsMnBr₃ perovskite crystals prepared by the process of the present invention. Further, the crystal structure of as-synthesized [Mn2+: CsBr] prepared according to Example 2, Cs₃MnBr₅ prepared according to Example 3, and CsMnBr₃ prepared according to Example 4 are shown in FIG. 3(d-f), where the Mn-doped cesium bromide [Mn2+: CsBr] crystal structure is represented in FIG. 3(d). As shown in FIG. 3(e) in the CssMnBrs structure, Mn2+are present in the tetrahedral environment as [MnBr4]2“ anion, which usually emits in the green region; whereas in the case of CsMnBr₃ structure as depicted in FIG. 3(f), the Mn2+are present in octahedral environment as [MnBr₆]4-, which tend to give an orange to red emission, which conformed with the XRD results of FIG. 3 (a-c) and further as observed from (FIG. 1) [Molecules., 2022, 27, 8259(1-10), ACS Energy Let., 2022, 7, 1850-1858],

[0121] Field Emission Scanning Electron Microscope (FESEM) and Energy Dispersive Analysis of X-rays (EDAX) analysis:

[0122] To know the morphology of the synthesized nanocrystals prepared according to Examples 2 to 4, FESEM analysis was carried out. As shown in FIG. 4(a-c), the as-synthesized [Mn2+: CsBr] nanocrystal of Example 2 was observed with a polyhedral structure with size ranging from ~5-20 μm (FIG. 4a), whereas Cs₃MnBr₅ of Example 3 showed a uniform cubical-to-near spherical shape with average size ranging from ~300-400 nm (FIG. 4b). In case of CsMnBr₃ crystals of Example 4 (FIG. 4c), a flake-like structure at the micrometer scale was observed. The large size and uniform shapes of the all the three nanocrystals prepared by the process of the present invention indicated a good degree of crystallinity. Further, confocal reflection images were also taken for the as-synthesized Mn2+: CsBr, Cs₃MnBr₅, and CsMnBr₃ prepared according to Examples 2 to 4, which well matched with the FESEM images as shown in FIG. 5 (a-c), respectively. Additionally, EDAX mapping demonstrated the uniform distributions of Cs, Mn, and Br elements in the [Mn2+: CsBr] nanocrystal prepared according to Example 2 with Cs, Mn and Br, having an atom ratio of 49.54:0.93:49.53; which confirmed the ~0.02% Mn2+doping in CsBr in Mn2+: CsBr compound (FIG. 6a). Similarly, EDAX mapping in Cs₃MnBr₅ prepared according to Example 3 (FIG. 6b),suggested Cs, Mn, and Br, having an atom ratio of 2.2: 1:5.3, and in CsMnBr₃ prepared according to Example 4 (FIG. 6c), the Cs, Mn, and Br, having an atom ratio of 1:1:3.5.

[0123] X-ray photoelectron spectroscopy (XPS) analysis:

[0124] The (XPS) analysis of the Mn2+: CsBr, Cs₃MnBr₅, and CsMnBr₃ nanocrystals prepared by the process of the present invention was carried out to understand the perovskite properties and the results are depicted in FIG. 7. The 3d XPS spectrum For Mn2+: CsBr crystal prepared according to Example 2 depicts the Cs at binding energy 724.53 eV and 738.47 eV corresponded to Cs3d5 / 2and Cs3d3 / 2respectively, which were similar to those reported previously (FIG. 7a). Further, the binding energy peak of Mn 2p in the XPS spectra was at 641.72 eV and 653.29 eV with a clear separation of ~12 eV, corresponding to Mn2p3 / 2and Mn2p1 / 2with satellite peaks at 646.01 eV and 658.15 eV. This proves the existence of manganese in a divalent state in Mn-doped-CsBr crystal (FIG. 7b) Additionally, from the 3d spectrum of Br, the binding energy of 68.19 eV and 69.24 eV represented the Br3d5 / 2and Br3d3 / 2respectively (FIG. 7c). Similarly, Cs 3d, Mn 2P, and Br 3d XPS spectra of Cs₃MnBr₅ and CsMnBr₃ compounds are depicted in FIG. 7(d-f) and FIG. 7(g-i) respectively. The binding energies of Cs 3d, Mn 2P, and Br 3d, obtained from Mn2+: CsBr, Cs₃MnBr₅ and CsMnBr₃ nanocrystals prepared by the process of the present invention are presented in Table 2 below. The slight difference in binding energy may be assigned to the influence of environment and crystal structure.

[0125] Table 2:

[0126] Peak position of elements (eV)

[0127] Cs Mn Br Compounds

[0128] 3ds / 2 3d3 / 2 2p3 / 2 2pi / 2 3ds / 2 3d3 / 2 (satellite) (satellite)

[0129] Mn2+:CsBr of 724.53 738.47 641.72 653.29 68.19 69.24 Example 2

[0130] (646.01) (658.15)

[0131]

[0132] CssMnBrs of 725.38 739.32 642.71 654.18 69.44 70.49 Example 3

[0133] (647.35) (659.35)

[0134] CsMnBn of 724.06 738.00 641.50 (645.87) 653.12 68.35 69.40 Example 4

[0135] (657.83)

[0136]

[0137] Thermo gravimetric analysis (TGA) analysis:

[0138] To elucidate the thermal stability of the crystals prepared by the process of the present invention, TGA analysis was carried out as depicted in FIG. 8(a-c). The Mn2+: CsBr crystal prepared according to Example 2 was more stable up to ~700 °C than Cs₃MnBr₅ and CsMnBr₃ perovskite samples prepared according to Examples 3 and 4 respectively, with a minute weight loss of - 10 % at 250 °C (FIG. 8a). The Cs₃MnBr₅ and CsMnBr₃ perovskite samples prepared according to the process of the present invention decomposed mainly in three steps, as shown in FIGs. 8(b), and (c), which were well- matched with the previously reported literatures.

[0139] Optical properties:

[0140] UV Visible absorption spectra of UV-visible absorption spectra of the as-synthesized compounds are shown in FIG. 9(a-c). The photoluminescence (PL) and photoluminescence excitation (PLE) spectra were studied to explore the optical properties of the Mn2+: CsBr, Cs₃MnBr₅, and CsMnBr₃ compounds prepared by the process of the present invention and the results are depicted in FIG. 10(a to c). FIG.

[0141] 11(a) showed that the Mn2+: CsBr powder prepared according to Example 2 exhibited a sharp excitation wavelength at 260 nm with a bright blue emission with a peak center at 361 nm and FWHM - 59 nm with 23.36% PLQY, which arised due to the emission from the Mn2+ions. The bright blue emission in Mn2+: CsBr was observed because of doping of Mn2+in CsBr crystal, as CsBr has no emission, as reported earlier. [RSC Adv., 2021, 11, 2437–2445] Whereas, FIG. 10(b) showed that the Cs₃MnBr₅ compound prepared according to Example 3 exhibited a bright green emission with a peak centered at 521 nm. The excitation spectra of Cs₃MnBr₅ had several transitions such as6

[0142]

[0143] A₁→4A2(F),6A₁→4T₁(P),6A₁→4E(D),6A₁→4A₁,4E(G),6A₁→4T2(G) which belongedto 277 nm, 364 nm, 377 nm, 439 nm and 457 nm, respectively, (Angew. Chem. Int. Ed., 2021, 60, 19653 -19659). On exciting 277nm (highest intensity band), green emission at 521nm along with a little broad low intensity peak at - 655nm was observed as represented in FIG. 10(b). FWHM of the peak was calculated to be ~ 44 nm, with a PLQY of 34.18% for the UV excitation. Similarly, the PL and PLE obtained from the CsMnBr₃ nanocrystal is shown in FIG. 10(c). A bright red emission peak centered at ~ 652 nm was observed with a narrow FWHM of ~ 77 nm and PLQY of 27.76%. Even for CsMnBr₃ compound several transitions such as6AI^4A2(F),6AI— >4E(D),6

[0144]

[0145] A₁→4T(D),6A₁→4A₁,4E(G),6A₁→4T2(G),6A₁→4T₁(G) which belonged to 260 nm, 337 nm, 381 nm, 434 nm, 458 nm, and 545 nm, respectively, was observed. The emission spectra obtained was further plotted on Commission Intenationale de I’Eclairage (CIE) 1931 color coordinates, which is depicted presented in FIG. 10(d). The coordinates of the bright blue emission from Mn2+: CsBr prepared according to Example 2 was at (0.18, 0.12), bright green emission from Cs₃MnBr₅ prepared according to Example 3 was at (0.23, 0.65) and the coordinates obtained for CsMnBr₃ prepared according to Example 4 was at (0.50, 0.28), which is in the red region. Further, the time-resolved photoluminescence (TRPL) of the Cs₃MnBr₅ and CsMnBr₃ nanocrystals prepared according to the process of the present invention was examined to monitor the PL lifetime. The TRPL was measured by probing the PL peaks at 521 and 652 nm for Cs₃MnBr₅ and CsMnBr₃, respectively, as demonstrated in FIG. 10(e, f). The Cs₃MnBr₅ nanocrystal had a longer lifetime of 265 μs, as shown in FIG. 10(e), while the CsMnBr₃ nanocrystal had a shorter lifetime of 51.58 μs, observed by exciting at 260 nm wavelength, as shown in FIG. 10(f). The PL decay of CsMnBn followed a dual exponential trend, as shown in the equation below, [Adv. Sustainable Syst., 2024, 2400092(1-10)]

[0146] A(t) = Ai. exp (— t / xi) + A2.exp (-t / r2)

[0147] where, A(t) represents the quantity at time t, while A₁ and A2are constants that determine the initial value of the exponential decay components between τ₁ and τ2. Two PL decay components were 26.84 μs (74%) and 76.32 μs (26%), so the average lifetime for CsMnBr₃ was calculated to be 51.58 μs.Furthermore, the Tauc’s plot are shown in FIG. ll(a-c) for all the crystals prepared according to Examples 2 to 4, and the band gap values for Mn2+: CsBr, Cs₃MnBr₅ and CsMnBr₃ was calculated to be 3.95, 3.33 and 4.33, respectively.

[0148] Example 7: Process for phase Transitioning / transforming / converting the Cs₃MnBr₅ and CsMnBr₃ nanocrystal prepared according to Example 3 and 4

[0149] The Cs₃MnBr₅ and CsMnBr₃ nanocrystal prepared according to Examples 3 and 4 respectively was subjected to phase transition, in a moisture environment. When the Cs₃MnBr₅ nanocrystal prepared according to Example 3 was kept in an environment where air humidity was -98-99% for 5-10 min, clear changes in XRD patterns were noticed as depicted in [FIG. 12a(i) —> 12a(ii)]. The diffraction peaks were majorly indexed to triclinic Cs2MnBr4·2H2O phase (JCPDS 00-050-0630) and some residue of CsBr, as represented in FIG. 12a(ii). The result signifies that from CssMnBrs structure, where Mn2+is present in the tetrahedral environment as [MnBr4]2“ anion, a transformation to an octahedron [MnBr4(H2O)2]2’ was observed in the presence of moisture. The shift in absorbance was also noticed in [FIG. 12a(iv) → 12a(v)] and on excitation at 250 nm, a blue PL emission at ~391 nm with FWHM ~66 nm was observed as represented in FIG. 12b(v). Interestingly, the Cs2MnBr4-2H2O could inversely transform into the CssMnBrs phase during the thermal annealing dehydration step. When the Cs2MnBr4-2H2O compound was heated at ~80°C for 5-10 min under ambient atmosphere, the Cs₂MnBr₄.2H₂O phase again converted back to its original phase of Cs₃MnBr₅, as observed from the PXRD pattern as presented in FIG. 12a(iii) with PL spectra having green PL emission at 521 nm with FWHM of ~41 nm as shown in FIG. 12a(vi). The probable phase transition reaction is represented in FIG. 12a(vii). Even though the PL emission wavelength remained same as 521 nm for the CssMnBrs phase and after consecutive moisture-&-heating process as shown in FIG. 13(a); to further verify the change in emission colour co-ordinates, the PL data was plotted in Commission Internationale de 1'Eclairage (CIE) chromaticity diagram. A slight change in color coordinates was observed as shown in FIG. 14(b). The bright green emission from Cs₃MnBr₅ nanocrystal prepared by the process of the present invention was observed at (0.23, 0.65), which, on exposure to moisture, changed to bright blue emission at (0.22, 0.19) because of formation of Cs₂MnBr₄.2H₂O phase, which onsubsequently heating transformed back to bright green emission from Cs3MnBr5nanocrystal at (0.27, 0.60).

[0150] Similarly, moisture can also trigger the phase transformation of CsMnBr3to Cs2MnBr4·2H2O phase as depicted in (FIG. 14a). When the CsMnBr3compound was kept in humid environment (air humidity -98-99%) for 5-10 min, a clear change in XRD patterns was noticed [FIG. 14a(i) — 14a(ii)]. The diffraction peaks were majorly indexed to triclinic Cs2MnBr4·2H2O phase (JCPDS 00-050-0630) with some residue of MnBr2 as represented in FIG. 14a(ii). The shift in absorbance was also noticed [FIG.

[0151] 14a (iv) — 14a(v)] and on excitation at 250 nm, a blue PL emission at -391 nm with FWHM -61 nm was observed as represented in FIG. 14a(v). Excitingly, the Cs2MnBr4·2H2O could inversely transform into the CsMnBr3phase during the thermal annealing dehydration step. When the Cs2MnBr4·2H2O compound was heated at ~ 80°C for 5-10 min in an ambient atmosphere, the Cs2MnBr4·2H2O phase again converted back to its original phase of CsMnBr3, as observed from the PXRD pattern as represented in FIG. 14a(iii) with PL spectra having red PL emission at 650 nm with FWHM -74 nm as shown in FIG. 14a(vi). The probable phase transition reaction is represented in FIG. 14a(vii). Even though the PL emission wavelength remained almost unchanged at -650 nm for the as- synthesized CsMnBr3phase and after the consecutive moisture-&-heating processes as shown in FIG. 15 (a), to further verify the change in emission color co-ordinates, the PL data was plotted in the CIE chromaticity diagram. A slight change in colour co-ordinates was observed, as shown in FIG. 15(b). As observed from the said FIG., the bright red emission from as- synthesized CsMnBn was at (0.50, 0.28), which, on exposure to moisture, changed to bright blue emission at (0.25, 0.22), because of the formation of Cs2MnBr4·2H2O phase; which on heating transform back to bright red emission observed from CsMnBr3at (0.42, 0.28). These results suggest the successful reversible transformation from green-luminescent Cs3MnBr5phase and red-luminescent CsMnBr3phase to blue-luminescent Cs2MnBr4·2H2O phase and vice versa with relatively narrower PL band with high color purity, which shows a great potential for advanced anti-counterfeiting.ADVANTAGES OF THE PRESENT INVENTION:

[0152] • Mn-doped CsBr [Mn2+: CsBr], phase-engineered all-inorganic cesium manganese bromide 0D Cs3MnBr5and 1D CsMnBr3phase-pure perovskite NCs were synthesized for the first time by a simple, rapid, high-throughput microwave- assisted synthesis strategy at a temperature of 200 °C in 15 min by controlling the precursor’s mole ratios in a single process.

[0153] • The process of the present invention provides several advantages such as less time consuming, easily reproducible and also can be easily scaled up with pure phase as compare to reported procedures.

[0154] • Pure phase of perovskite can be obtained as compared to reported procedures.

[0155] • The phase-pure Mn2+: CsBr, 0D Cs3MnBr5, and 1D CsMnBr3perovskite NCs show blue, green, and red color emission with high color purity and PLQY of 23.36%, 34.18%, and 27.76% respectively.

[0156] • In addition, under the moisture environment, both the 0D Cs3MnBr5and 1D CsMnBr3NCs were transformed into blue-emitting 0D Cs2MnBr4·2H2O phase, which can be inversely transformed back to their initial cesium manganese bromide phase during thermal annealing.

[0157] • This facile, cost-effective, energy-efficient, high-throughput, and good repeatability synthesis strategy of the present invention paves a new opportunity for large-scale lead-free perovskite NCs preparation.

[0158] • These nanocrystals obtained by the process of the present invention can be used in anti-counterfeiting and security applications and also for developing moisture sensor.

Claims

We claim:

1. A microwave-assisted process for preparation of lead-free manganese (Mn)-doped CsBr [Mn2+: CsBr], a phase-engineered all-inorganic cesium manganese bromide (0D Cs3MnBr5) and 1D CsMnBr3phase-pure perovskite nanocrystals (NCs), the process comprising:(a) preparing a cesium oleate mixture by adding 1 -octadecene (ODE) and oleic acid (OA) to cesium carbonate (Cs₂CO₃);(b) dissolving the cesium oleate mixture by heating at a temperature in a range from 110 to 130°C to obtain a solution; and(c) adding a mixture of manganese bromide (MnBr2), ODE, OA and oleylamine (OAm) to the dissolved solution and treating under microwave conditions at a temperature in a range from 190 to 210°C,wherein,using cesium carbonate (Cs₂CO₃) to manganese bromide (MnBr₂) in a mole ratio of 1:1 gives CsBr [Mn2+: CsBr] nanocrystal,using cesium carbonate (Cs₂CO₃) to manganese bromide (MnBr₂) in a mole ratio of 1:2.5 gives 0D Cs₃MnBr₅ nanocrystal, andusing cesium carbonate (Cs₂CO₃) to manganese bromide (MnBr₂) in a mole ratio of 1:7 gives 1D CsMnBr₃ nanocrystal.

2. The process as claimed in claim 1, wherein the cesium oleate mixture is prepared by adding 1-octadecene (ODE) having concentration in a range from 5mL to 7mL and oleic acid (OA) having concentration in a range from 1 mL to 1.5 mL to cesium carbonate having concentration in a range from 0.5 mmol to 0.75 mmol.

3. The process as claimed in claim 1, wherein the dissolving of the cesium oleate mixture is carried out under stirring at a rate of 900 rotations per minute (rpm) to 1000 rpm.P_W01007944. The process as claimed in claim 1, wherein in step (c), 1 -octadecene (ODE) having concentration in a range from 5 mL to 7 mL, oleic acid (OA) having concentration in a range from 1 mL to 2 mL and oleylamine (OAm) having concentration in a range from 1 mL to 2 mL is added to the dissolved solution.

5. The process as claimed in claim 1, wherein the microwave treatment is carried out for a time period in a range from 10 minutes to 15 minutes.

6. The process as claimed in claim 1, wherein the mole ratio of Cs2CO3to MnBr2of 1:1 provides a pure blue coloured compound (Mn2+: CsBr), the mole ratio of Cs2CO3to MnBr2of 1:2.5 provides a pure green coloured compound (Cs3MnBr5) and the mole ratio of Cs2CO3to MnBr2of 1:7 provides a pure red coloured compound (CsMnBr3).

7. A process for transforming / converting the Cs3MnBr5and / or CsMnBr3nanocrystal obtained by the process as claimed in claim 1 to Cs2MnBr4·2H2O crystal in a moisture environment having air humidity in a range from -98-99%.

8. A process for inversely transforming the Cs2MnBr4·2H2O crystal obtained by the process as claimed in claim 7 to Cs3MnBr5and / or CsMnBr3nanocrystal by thermal annealing at a temperature in a range from 80 °C to 100 °C.

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