WO2024052929A1 - A process for preparation of stable single phase intrinsic white light emitting phosphors and application thereof - Google Patents

Abstract

The present invention relates to a process of preparation of highly luminescent single phase intrinsic white light emitting materials (phosphors) by a simple and facile approach comprising hydrothermal treatment followed by two step calcination. More particularly, the present invention provides a novel magnesium and zinc co-doped strontium cerium oxide phosphors to exhibit average lifetime ranging from 31-44 μs and a tunable photoluminescence quantum yield (PLQY) ranging from 15-95 %, tunable chromaticity coordinates (0.238 to 0.300, 0.329 to 0.372). Further, a white light emitting device is fabricated by combining a 365 nm UV-LED chip and single phase intrinsic white light emitting materials (phosphors), exhibiting highly intense broad emission over the spectral range (400-700 nm) with tunable color rendering index (CRI) from 85-95 and CCT from 4900-5700 K (cool to warm region).

Description

A PROCESS FOR PREPARATION OF STABLE SINGLE PHASE INTRINSIC WHITE LIGHT EMITTING PHOSPHORS AND APPLICATION THEREOF FIELD OF THE INVENTION [0001] The present invention relates to a process of preparation of highly luminescent single-phase intrinsic white light emitting materials (phosphors) by a simple and facile approach (hydrothermal) that exhibits tunable chromaticity coordinates (0.238 to 0.282, 0.329 to 0.372) and PLQY from 15% to a very high ~ 95%. More particularly, the present invention relates to the fabrication of white light-emitting device using developed luminescent single-phase material (phosphor) that give rise intense white light with tunable color rendering index (CRI) from 85 to 95 and CCT from 4900 to 5700 K (warm to cool region). BACKGROUND OF THE INVENTION [0002] Currently the white light emission, covering the entire visible spectral range (400-700 nm), is achieved by combining the blue light emitting phosphors (conventionally InGaN LED chip) with yellow (e.g. YAG:Ce³⁺) phosphor. Such white light emission has many shortcomings such as lower color rendering index (CRI ^ 80), high correlated color temperature (CCT >6000) and poor color reproducibility due to the lack of red-emitting component in its photoluminescence spectrum. In another method, coating of near-ultraviolet (n-UV) chips with three primary colors (blue/green/red) emitting phosphors a practiced which have certain shortcomings like poor luminous efficiency due to re-absorption of excitation light and also there are difficulties in proper balancing the ratio of different phosphors that curtail their use. Organic and other types of materials encounters stability issues. Their practical application as UV-excited phosphor in display and lighting applications in different areas also relies on the CIE, CRI, and CCT of the fabricated prototype. There is emerging thrust to design single phase white light materials that can surpass shortcomings of multicomponent based white light emitters that includes-critical issues of color balance, low color-rendering index, thermal stability and a bluish tinge that leads to damage retina cells of human eye. [0003] Among different luminescent inorganic phosphors, the luminescence of Sr2CeO4 can be tuned by doping a diverse variety of metal ions by using different synthesis procedures. [0004] Reference may be made to the article by T. Kato et al., Optical Materials, 87, 139-144, 2019 wherein the solid-state synthesis of Sr2CeO4 is reported that gives a broadband blue-emitting phosphor. [0005] Reference may be made to the article by Danielson et al., Science, 279, 1998 wherein Sr2CeO4 is synthesized by the solid-state method that is giving bluish-white light with CIE (0.198,0.292) with a long excited-state lifetime of 51.3 μs. This synthesized material has a quantum yield of 0.48. [0006] There have been various attempts to tune its emission by non-rare earth metals and using different synthesis procedures. [0007] Reference may be made to the article by T. Grzyb et al., The Journal of Physical Chemistry C, 2012, 116, 3219−3226, wherein Mg²⁺, Ca²⁺, Ba²⁺, and Zn²⁺ individually doped resulted in the blue emission from the doped Sr2CeO4. The size of dopant plays role in little shift of the emission maximum than that of bare Sr₂CeO₄. [0008] Reference may be made to the article by Zhai et al, Asian J. Chem. 2014, 26, 4767-4770 wherein individual doping of Mg²⁺and Zn²⁺ gives blue-white emission with emission maximum nearly at 466 nm. Nonetheless, the intrinsic white light emission from the non-rare earth element doped Sr2CeO4 has not been achieved. Thus, keeping in view the drawbacks of the hitherto reported prior arts (summarized in Table-1), there is a need to design an efficient single phase broad white light- emitting material with improved CIE, absolute photoluminescent quantum efficiency, and tunable emission wavelengths. OBJECTIVE OF THE INVENTION [0009] Accordingly, the main objective of the present invention is to provide a process for the preparation of Zn and Mg co-doped Sr₂CeO₄ Phosphor. [0010] Another object of the present invention is to provide tunable intrinsic emission to the white light of Sr2CeO4 by co-doping it with Mg and Zn. [0011] Another object of the present invention is to demonstrate the fluorescence property exhibited by the co-doped material using steady-state and time-resolved photoluminescence spectroscopy as a tool. [0012] Yet Another objective is to tune requisite characteristics such as CIE ranging from 0.238 to 0.282, 0.329 to 0.372 and PLQY from 15 to 95%. [0013] Yet Another objective is to tune requisite characteristics of the fabricated prototype such as CIE ranging from 0.30 to 0.34, 0.33 to 0.36, correlated color temperature (CCT) ranging 4900 to 5700 K and color rendering index (CRI) ranging from 85 to 95. [0014] Still another object of the present invention is to demonstrate the highly intense white light emission so that it is useful in designing the highly demanding UV excited white light-emitting devices (W-LEDs, display panels) SUMMARY OF THE INVENTION [0015] Additional features and embodiments of the present disclosure will better be understood through the techniques and other aspects of the disclosure. Other embodiments of the invention are described in detail herein and are considered a part of the claimed disclosure. [0016] In an aspect of the present disclosure, there is a process for the preparation of a stable luminescent single-phase intrinsic white light emitting Zn and Mg co- doped Sr2CeO4 phosphor materials with high colour rendering index (CRI) (up to 95), photoluminescence quantum yield (PLQY) (up to 95%), bright white light emission, tunable correlated colour temperature (CCT) from 4900 to 5700 K (warm to cool region), comprising the steps of: a) mixing stoichiometric amounts of Sr and Ce precursors and dopants precursors of Mg and Zn dissolved in water to obtain a solution; b) adding ammonia to the solution until pH 8-9 is achieved while stirring for 30 min to 2 h to obtain a reaction mixture; c) subjecting the reaction mixture to hydrothermal reaction at 120-200oC for 6 to 48 h and subsequently allowing it to cool down and recovering a product by repeated washing with water and alcohols until neutral pH is achieved; d) drying the obtained product at 80°C for 12h to obtain a powder; e) grinding and calcining the powder at 950-1100°C to obtain a calcined product; f) grinding the calcined product while using 5% polyvinyl alcohol (PVA) as a binder, subsequently making a pellet and subjecting it to recalcination at 1150-1250 °C to obtain the Zn and Mg co-doped Sr2CeO4 phosphor material. [0017] The present invention provides a novel co-doped strontium cerium oxide phosphor synthesized by partially substituting strontium by two divalent ions e.g., magnesium and zinc to exhibit white light emission covering entire visible spectral range with improvements in luminescent brightness, color rendering index, CIE, and CCT thereof. The developed white light emitting materials are superior to prior art materials, particularly in terms of balanced white light emission, high photoluminescence efficiency, which cannot be achieved by prior art with other rare earth and non-rare earth doped strontium cerium oxide systems. [0018] In an embodiment of the present invention provides a new luminescent material of a new system other than strontium cerium oxide system or comprises magnesium and zinc partially in place of strontium. [0019] The present invention provides a new long photoluminescence lifetime luminescent material and the manufacturing process thereof, the present material exhibits a broad-spectrum range, good stability, high photoluminescence efficiency, high CRI, CCT and stable in ambient condition, near white CIE, and outstanding luminescence performance of products containing the present material. [0020] In an embodiment of the present invention, a hydrothermal reaction process followed by two step calcination was adopted for the synthesis of co-doped Sr₂CeO₄. The precursors of each element dissolved in deionized water in certain mole ratios while stirring for 1 h and followed by sonication for 10 min. Subsequently, NH3 solution was slowly added into the above-prepared solution to adjust the pH value ^ 9 followed with stirring for 1 h and then subjected to hydrothermal reaction in a teflon-lined autoclave at 180 °C for 6h. To recover the product, repeated washing with deionized water and a little amount of absolute alcohol till pH becomes neutral and subsequently dried the recovered product at 80 °C for 12h. It is then calcined in a muffle furnace at 1100 °C for 2 h, followed by preparing a disc from the powder using 5% PVA as binder and subsequently subjected to and re-calcination at 1200 °C for 1h to get the final product- a luminescent phosphor. [0021] In an embodiment of the present invention, the main chemical composition of the luminescent material of the present invention can be expressed by formula Sr2-x-yMgxZnyCeO4 where x and y are the dopant mole ratio. [0022] According to a further preferred embodiment of the present invention, it provides luminescent material, wherein x ranges 0.01 to 0.04 in formula (1) to form a highly luminescent material, where it partially replaces strontium in the host lattice. [0023] According to a further preferred embodiment of the present invention, it provides luminescent material, wherein y ranges 0.01 to 0.04 in formula (1) to form a highly luminescent material, where it partially replaces strontium in the host lattice. [0024] According to a preferred embodiment of the present invention, it provides a luminescent material that exhibit broad emission from 400 to 700 nm spectral region with high photoluminescence quantum efficiency (ranges 15 to 95%) and CRI (ranges from 85 to 95). [0025] According to a further preferred embodiment of the present invention, it provides luminescent material, wherein different concentration of (x, y) ranges (0.01,0.01), (0.01,0.02), (0.02,0.02), (0.02,0.04), (0.04,0.02), and (0.04,0.04) in formula (1) to form highly luminescent phosphor, where strontium is partially replaced in the host lattice. [0026] In the chemical composition of the highly luminescent white light emitting material of the present invention, the lattice sites of strontium are partially replaced by magnesium and/or zinc, thereby the luminescence properties of the luminescent material and the performances of the products thereof can be greatly improved and tuned. There is drastic improvement in emission, photoluminescence quantum yield, CRI and CIE in the invented material. There is great improvement in the luminescence properties, and stability also. [0027] In an embodiment of the present invention, wherein the formation of phase was characterized by PXRD, revealing the formation of phase pure orthorhombic Sr2CeO4. The little shift of PXRD peaks for co-doped Sr2CeO4 as compared to Sr₂CeO₄ implies the incorporation of dopant in the lost lattice. The morphological analysis was performed by FESEM and TEM revealing the formation of rice (rod) like structures that are assembled in the star shape. [0028] In an embodiment of the present invention wherein the PL emission of co- doped Sr₂CeO₄ measured at ^_exc = 330 nm, exhibit broad emission spanning over entire visible spectral region and the fluorescence intensity gradually increases with the increase in the dopant concentration up to optimal concentration of Mg and Zn . These co-doped Sr2CeO4 phosphors exhibit photoluminescence excited state lifetime ranging from 31 to 44 μs and the photoluminescence quantum yield (PLQY) ranging from 15% to 95 %. [0029] In an embodiment of the present invention, the electroluminescence device containing the phosphor material [4] obtained by the process as disclosed in claim 1, the device comprising: [1] a cathode layer; [2] an electron injector layer; [3] an electron transport layer; [4] a white light emitting phosphor; [5] a hole transport layer; [6] a hole injector layer; and [7] an anode layer, wherein the cathode layer is Ag, the white light emitting phosphor is a Zn and Mg co-doped Sr₂CeO₄ phosphor material, the anode layer is selected from indium tin oxide, or fluorine tin oxide. [0030] In an embodiment of the present invention, there is provided a device comprising a white light emitting LED prototype fabricated by mixing luminescent single-phase material (phosphor) [4] with silicon resin coated on 365 nm ultraviolet-light emitting diode (UV LED). [0031] In an embodiment of the present invention, the UV excited electroluminescence device give rise to intense white light with tunable chromaticity coordinates from 0.30 to 0.34, 0.33 to 0.36 and color rendering index (CRI) from 85 to 95. [0032] In an embodiment of the present invention wherein, the tunable emission, high PLQY, and the stability make the developed co-doped Sr2CeO4 phosphors an ideal candidate for device application. Taking into account these characteristics, a LED prototype was fabricated by combining a 365 nm UV-LED chip and co-doped Sr2CeO4 phosphors, exhibiting highly intense broad emission over the spectral range (360-830 nm) with the chromaticity coordinate ranging from 0.30 to 0.34, 0.33 to 0.36, very high CRI from 85 to 95 and CCT from 4900 to 5700 K. The luminescent developed in present invention exhibiting broad excitation and emission spectrum, high CRI, PLQY and daylight CCT can be used in various optoelectronic application including the w-LEDs, display panels etc. [0033] In an embodiment of the present disclosure, there is provided a device comprising a Zn and Mg co-doped Sr2CeO4 material, wherein the device is fabricated by mixing the Zn and Mg co-doped Sr2CeO4 material with silicon resin and coated on an ultraviolet (UV)-light emitting diode (LED). [0034] In an embodiment of the present disclosure, there is provided an electroluminescence device comprising: a. [1] a Ag layer; b. [2] an electron injector layer; c. [3] an electron transport layer; d. [4] a white light emitting phosphor; e. [5] a hole transport layer; f. [6] a hole injector layer; and g. [7] an ITO layer, wherein the white light emitting phosphor is a Zn and Mg co-doped Sr2CeO4 material the anode layer is selected from indium tin oxide, or fluorine tin oxide. [0035] In an embodiment of the present disclosure, there is provided an electroluminescence device as disclosed herein, wherein the device emits intense white light with chromaticity coordinates from 0.30 to 0.34, and 0.33 to 0.36, and a color rendering index (CRI) in a range of 85 to 95, upon ultraviolet excitation. BRIEF DESCRIPTION OF THE DRAWINGS [0036] The invention has other advantages and features which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which: [0037] Figure 1 represents XRD pattern of (a) bare and Zn/Mg doped Sr2CeO4 and (b) the peak corresponding to (111) plane implying the incorporation of dopant in the host lattice, in accordance with an embodiment of the present disclosure. [0038] Figure 2 represents (a) TEM of 1% Zn,1% Mg co-doped Sr₂CeO₄(b) single particle image (c) SAED of1% Zn,1% Mg co-doped Sr2CeO4 and (d) lattice fringes for the 1% Zn,1% Mg co-doped Sr2CeO4 phosphor, in accordance with an embodiment of the present disclosure. [0039] Figure 3 represents (a) field emission scanning electron microscopy (FESEM) image of 1% Zn,1% Mg co-doped Sr₂CeO₄ (b), EDX of the 1% Zn,1% Mg co-doped Sr2CeO4, in accordance with an embodiment of the present disclosure. [0040] Figure 4 represents the steady-state photoluminescence spectra of bare and co-doped Sr₂CeO₄ (a) 330 nm, in accordance with an embodiment of the present disclosure. [0041] Figure 5 represents time-resolved photoluminescence spectra of bare and co-doped Sr₂CeO₄, in accordance with an embodiment of the present disclosure. [0042] Figure 6 Excitation dependent spectra of 1% Zn, 1% Mg co-doped Sr₂CeO₄, in accordance with an embodiment of the present disclosure. [0043] Figure 7 represents CIE plot for the photoluminescence emission spectra of bare and co-doped Sr₂CeO₄ at 330 nm excitation, in accordance with an embodiment of the present disclosure. [0044] Figure 8 represents the LED device fabricated by mixing 1% Zn-1% Mg co-doped Sr₂CeO₄ (SCO-3) with silicon resin coated on 365 nm UV LED, in accordance with an embodiment of the present disclosure. [0045] Figure 9 represents electro-luminescence spectra of fabricated WLED on excitation with 365 nm UV LED, in accordance with an embodiment of the present disclosure. [0046] Figure 10 represents inset CIE plot of the fabricated prototype of white light emitting device, in accordance with an embodiment of the present disclosure. [0047] Figure 11 represents fabrication of prototype by coating the 365 nm UV chip with the developed single phase white light emitting phosphor and a module showing white light from this prototype, in accordance with an embodiment of the present disclosure. [0048] Figure 12 represents electroluminescence device prototype design using the developed single phase white light emitting phosphor, in accordance with an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION [0049] The foregoing detailed description of the disclosure is elaborated to provide a clear understanding to the person who is skilled in the art. Additional features, embodiments and advantages of the invention will be described hereinafter which form the subject of the claims of the disclosure, However, the set forth disclosure provide in the specification will best be understood in conjunction with the appended claims and figures as provide heretofore. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present disclosure. It should also be realized by those skilled in the art that such equivalent processes do not depart from the spirit and scope of the disclosure as set forth in the appended claims. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and make part of this disclosure. [0050] While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made, and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from its scope. [0051] Throughout the specification and claims, the following terms take the meanings explicitly associated herein unless the context clearly dictates otherwise. The meaning of "a", "an", and "the" include plural references. The meaning of "in" includes "in" and "on." Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein. [0052] The term “precursor” refers to the compound which provides a metal in the form of ion to react in a chemical reaction. In an aspect of the present disclosure, the precursor of Sr and Ce are selected from the group consisting of cerium nitrate, cerium chloride, cerium acetyl acetonate, strontium nitrate, strontium chloride, and strontium acetyl acetonate. [0053] The term “hydrothermal reaction” refers to a chemical reaction at both high temperature and pressure, mediated in water in a sealed pressure vessel. In an aspect of the present disclosure, the reaction mixture is subjected to hydrothermal reaction at a temperature in a range of 120-200^oC for a time period in a range of 6 to 48 h. [0054] The term “calcining” or “calcination” refers to the process of heating a substance with a controlled supply of oxygen and/or controlled rate of heating. In an aspect of the present disclosure, there is provided a process as disclosed herein, wherein the process comprises calcining the material at a temperature in range of 950 to 1100^oC to obtain a product. In further aspects, the term “recalcination” refers to the repeated calcination of an already calcined product. [0055] The term “luminescent” refers to the category of materials that exhibits “luminescence” which is a phenomenon by which spontaneous emission of light radiation occurs from an electronically excited species (or from a vibrationally excited species) not in thermal equilibrium with its environment. [0056] The term “single-phase (phosphor)” refers to the material which exhibits luminescence by fluorescence or phosphorescence upon irradiation or activation using light. [0057] The term “chromaticity coordinates” refers to the parameters by which the quality of a color can be specified regardless of its luminance, defined by the two independent parameters, hue (h) and colorfulness (s). When a color exhibited by a material upon irradiation is plotted on CIE plot, the chromatic coordinates provide the information regarding where exactly the quality of the color lie. The hue is the angular component, and the purity is the radial component, normalized by the maximum radius for that hue. In an aspect of the present disclosure, the Zn and Mg co-doped Sr2CeO4 material exhibits tunable intense white light emission with chromaticity coordinates of (0.238 to 0.282), and (0.329 to 0.372). [0058] The term “photoluminescence quantum yield” (PLQY) refers to the number of photons emitted as a fraction of the number of photons absorbed. In an aspect of the present disclosure, the Zn and Mg co-doped Sr2CeO4 material exhibits photoluminescence quantum yield (PLQY) in a range of 15 to 95%. [0059] The term “color rendering index” (CRI) refers to the quantitative measure of the ability of a light source to reveal the colors of various objects to the maximum in comparison with a natural or standard light source. Light sources with a high CRI are desirable in color-critical applications. In an aspect of the present disclosure, the the Zn and Mg co-doped Sr2CeO4 material exhibits a color rendering index (CRI) of up to 95. [0060] The term “correlated color temperature” (CCT) refers to the parameter describing the color of a visible light source by comparing it to the color of light emitted by an idealized opaque, non-reflective body. In an aspect of the present disclosure, the Zn and Mg co-doped Sr₂CeO₄ material exhibits a correlated color temperature (CCT) in a range of 4900 to 5700 K (warm to cool region). [0061] The term “electroluminescence device” refers to the device which exhibits luminescence upon application of electric current in a particular voltage. In an aspect of the present disclosure, there is provided an electroluminescence device comprising a cathode layer, an electron conduction region, a white light emitting material, a hole conduction region and an anode layer. [0062] The term “ultraviolet excitation” refers to the process of irradiating a substance or a compound with ultraviolet (UV) radiation having wavelength in a range of 100 to 400nm to excite the electrons to a higher state. In an aspect of the present disclosure, the electroluminescence device emits intense white light with chromaticity coordinates from 0.30 to 0.34, and 0.33 to 0.36, and a color rendering index (CRI) in a range of 85 to 95, upon ultraviolet excitation. [0063] As discussed in the background, combination of the blue light emitting phosphors with yellow phosphor has lower color rendering index (CRI ^ 80), high correlated color temperature (CCT >6000) and lack of red-emitting component in its photoluminescence spectrum lead to poor color reproducibility. Coating of near- ultraviolet (n-UV) chips with three primary colors (blue/green/red) emitting phosphors face problems such as poor luminous efficiency due to re-absorption of excitation light and also there are difficulties in proper balancing the ratio of different phosphors that limit their applicability. Further, organic and other types of materials encounters stability issues. Generally, a predominant research is carried out in the development of single phase white light materials that can surpass shortcomings of multicomponent based white light emitters that includes-critical issues of color balance, low CRI thermal stability and a bluish tinge that leads to retina cells damage. [0064] Among different luminescent inorganic phosphors obtained through various routes, the luminescence of Sr₂CeO₄ can be tuned by doping a diverse variety of metal ions by using different synthesis procedures. However, the conventionally existing pathways of obtaining different Sr2CeO4 based materials for luminescent properties exhibits a wide variety of short comings, either in QY or in terms of the CRI or narrow spectrum of emission. [0065] In line with the above objectives, the present invention provides a facile process for the synthesis of co-doped Sr2CeO4 via a hydrothermal approach at temperature ranging from 150°C to 220°C from 6 h to 48 h followed by two-step calcination at temperature ranging from 800°C to 1300°C. To synthesize these co- doped Sr2CeO4 phosphors, the rationally chosen metal precursor from a group and NH₃ solution as a mineralizer are used. The below Table 1 provides detailed comparison of the process disclosed in the present invention with the conventional routes to obtain Sr2CeO4 based phosphors. [0066] Table 1: Comparison of the Sr2CeO4 based phosphors obtained via conventional routes with the process as disclosed herein.

[0067] In the present disclosure,the Sr2CeO4 has been co-doped with non-rare earth dopants to give rise the red emission along with blue and green, so that the single phase co-doped Sr2CeO4 exhibit the intense white light emission and thus can be used for application in the LEDs and display devices. In order to ensure the doping of two elements and tuning the defect chemistry, such that it gives rise to white (broad) light emission, it was synthesized by hydrothermal approach followed by two steps of calcination. [0068] The in-depth investigation of CIE, photoluminescence quantum efficiency (PLQE), which are not well studied, has been reported in this invention. Moreover, the application of the developed co-doped Sr2CeO4 phosphor as UV-excited white-light- emitting phosphor (WLEDs), its potential for display device applications and requisite parameters such as CIE, CRI, and CCT have been tuned. [0069] In an embodiment of the present disclosure, particularly, for the synthesis of co-doped Sr2CeO4 phosphors, 3.2 to 2.5 mmol of Sr(NO3)2, 1.3 to 2 mmol Ce(NO3)3·6H2O and dopants precursors 0.015 to 0.06 mmol of Mg(NO3)2·4H2O, and 0.015 to 0.06 mmol of Zn(NO₃)₂·6H₂O were dissolved in 10 ml deionized water separately. The resultant solutions were mixed in a beaker and stirred for 1h and sonicated to get a homogenous reaction mixture. Subsequently, NH3 solution was slowly added dropwise into the above-prepared solution to adjust the pH value to 9. Then, after stirring for 1 h, hydrothermal reactions were carried out at 180 °C for 6 h and subsequently calcined in a muffle furnace at 1100 °C and then recalcined at 1200°C in a muffle furnace after pressing the above powder in a 13 mm dial and 5% PVA as a binder. The formation of the phase and morphology was confirmed from PXRD, FESEM and TEM. This phosphor shows CIE ranges 0.238 to 0.282,0.329 to 0.372, PLQY ranges from 15 % to 95% and photoluminescent lifetime ranges 31.8 to 43.2 μs. The fabricated device shows CIE ranging from 0.30 to 0.34, 0.33 to 0.36, very high CRI from 85 to 95 and CCT from 4900 to 5700 K. [0070] The present invention provides formation co-doped Sr2CeO4 phase, by varying concentration of Sr precursor Sr(NO₃)₂ with concentration in a range of 2.5 mmol to 3.2 mmol. [0071] In another approach formation co-doped Sr2CeO4 phase, by varying the concentration in a range of 2.5 mmol to 3.2 mmol of Sr precursor of Sr (SrCl2). [0072] In another approach co-doped Sr2CeO4 phase synthesized by varying the concentration in a range of 2.5 mmol to 3.2 mmol of Sr precursor of Sr (SrCO₃). [0073] In another approach co-doped Sr2CeO4 phase synthesized by another precursors of Ce (CeCl3,7 H2O) with varying concentration in a range of 1.3 mmol to 2.0 mmol. [0074] In another approach the doping ratio of Sr:(Mg+ Zn) varied as 99:(1), 65.665:1, 65.665:1 49:1, 32.33:1 and 24:1 to synthesize the requisite co-doped Sr₂CeO₄ phosphor. [0075] In another approach co-doped Sr₂CeO₄ phase synthesized by using Sr precursor Sr(NO3)2 with concentration in a range of 2.5 mmol to 3.2 mmol and varying the mineralizer (NH4)2CO3 solution concentration by adjusting the pH value to 9 to 12.5. [0076] The novelty of the present invention with respect to the prior art relates to the simple and low-cost process for the formation of Mg and Zn co-doped Sr2CeO4, exhibiting excellent photoluminescence quantum yield ranging from 15 % to 95 % depending upon doping concentration. It is having a CIE coordinate near to ideal white light ranging from 0.238 to 0.282, 0.329 to 0.372 that can be used as a white light emitting source in lighting and display applications with non-rare earth dopants. The fabricated prototype exhibits excellent CRI from 85 to 95 and CIE ranging from 0.30 to 0.34, 0.33 to 0.36 that could be used as prolonged lighting source for indoor and outdoor application. EXAMPLES [0077] 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. [0078] This invention involves a highly luminescent material which has white light emission, and the preparation method thereof: The present invention is further explained with reference to Example 1-6. Example 1 [0079] About 0.628 g of Sr(NO₃)₂, 0.651 g Ce(NO₃)₃·6H₂O, dopants (0.0038g of Mg(NO3)2·4H2O and 0.0044 g of Zn(NO3)2·6H2O)) were dissolved in water in separate beakers. All of the four dissolutions were mixed together, stirred for about 1h and sonicated for 10 mins to obtain a solution. To the solution, ammonia was added to maintain the pH 9 and subjected to hydrothermal synthesis for 6 h at 180 ^oC in a hot air oven. Subsequently, the autoclave naturally cooled to room temperature, the resultant precipitate was washed several times with deionized water and was dried at 80°C for 12 h The white powder was calcined at 1100 ^oC followed by pressing the above powder into a 13 mm dial after mixing with 5% PVA as a binder and recalcined at 1200 ^oC to obtain the Zn and Mg co-doped Sr₂CeO₄ material (SCO-1; 0.5% Mg and 0.5% Zn). Example 2 [0080] About 0.625 g of Sr(NO3)2, 0.651 g Ce(NO3)3·6H2O, dopants (0.0038 g Mg(NO3)2·4H2O and 0.0089 g Zn(NO3)2·6H2O)) were dissolved in water in separate beakers. All of the four dissolutions were mixed together, stirred for about 1h and sonicated for 10 mins to obtain a solution. To the solution, ammonia was added to maintain the pH 9 and subjected to hydrothermal synthesis for 6 h at 180 ^oC in a hot air oven. Subsequently, the autoclave naturally cooled to room temperature, the resultant precipitate was washed several times with deionized water and was dried at 80°C for 12 h The white powder was calcined at 1100 ^oC followed by pressing the above powder in a 13 mm dial after mixing with 5% PVA as a binder and recalcined at 1200 ^oC to obtain the Zn and Mg co-doped Sr₂CeO₄ material (SCO-2; 0.5% Mg and 1% Zn). Example 3 [0081] About 0.622 g of Sr(NO₃)₂, 0.651 g Ce(NO₃)₃·6H₂O and dopants (0.0076 g of Mg(NO3)2·4H2O and 0.0089 g of Zn(NO3)2·6H2O)) were dissolved in water in separate beakers. All of the four dissolutions were mixed together, stirred for about 1h and sonicated for 10 mins to obtain a solution. To the solution, ammonia was added to maintain the pH 9 and subjected to hydrothermal synthesis for 6 h at 180 ^oC in a hot air oven. Subsequently, the autoclave naturally cooled to room temperature, the resultant precipitate was washed several times with deionized water and was dried at 80°C for 12 h The white powder was calcined at 1100 ^oC followed by pressing the above powder in a 13 mm dial after mixing with 5% PVA as a binder and recalcined at 1200 ^oC to obtain the Zn and Mg co-doped Sr₂CeO₄ material (SCO-3; 1% Mg and 1% Zn). Example 4 [0082] About 0.615 g of Sr(NO3)2, 0.651 g Ce(NO3)3·6H2O, dopants (0.0153 g Mg(NO₃)₂·4H₂O and 0.0089 g Zn(NO₃)₂·6H₂O)) were dissolved in water in separate beakers. All of the four dissolutions were mixed together, stirred for about 1h and sonicated for 10 mins to obtain a solution. To the solution, ammonia was added to maintain the pH 9 and subjected to hydrothermal synthesis for 6 h at 180 ^oC in a hot air oven. Subsequently, the autoclave naturally cooled to room temperature, the resultant precipitate was washed several times with deionized water and was dried at 80°C for 12 h The white powder was calcined at 1100 ^oC followed by pressing the above powder in a 13 mm dial after mixing with 5% PVA as a binder and recalcined at 1200^oC to obtain the Zn and Mg co-doped Sr2CeO4 material (SCO-4; 2% Mg and 1% Zn). Example 5 [0083] About 0.615 g of Sr(NO3)2, 0.651 g Ce(NO3)3·6H2O, dopants 0.0076 g Mg(NO3)2·4H2O and 0.0178 g Zn(NO3)2·6H2O)) were dissolved in water in separate beakers. All of the four dissolutions were mixed together, stirred for about 1h and sonicated for 10 mins to obtain a solution. To the solution, ammonia was added to maintain the pH at 9 and subjected to hydrothermal synthesis for 6 h at 180 ^oC in a hot air oven. Subsequently, the autoclave naturally cooled to room temperature, the resultant precipitate was washed several times with deionized water and was dried at 80°C for 12 h The white powder was calcined at 1100 ^oC followed by pressing the above powder in a 13 mm dial after mixing with 5% PVA as a binder and recalcined at 1200^oC to obtain the Zn and Mg co-doped Sr₂CeO₄ material (SCO-5; 1% Mg and 2% Zn). Example 6 About 0.609 g of Sr(NO₃)₂, 0.651 g Ce(NO₃)₃·6H₂O and dopants (0.0153g of Mg(NO₃)₂·4H₂O and 0.0178 g Zn(NO₃)₂·6H₂O) were dissolved in water in separate beakers. All of the four dissolutions were mixed together, stirred for about 1h and sonicated for 10 mins to obtain a solution. To the solution, ammonia was added to maintain the pH at 9 and subjected to hydrothermal synthesis for 6 h at 180^oC in a hot air oven. Subsequently, the autoclave naturally cooled to room temperature, the resultant precipitate was washed several times with deionized water and was dried at 80°C for 12 h The white powder was calcined at 1100 ^oC followed by pressing the above powder in a 13 mm dial after mixing with 5% PVA as a binder and recalcined at 1200^oC to obtain the Zn and Mg co-doped Sr2CeO4 material (SCO-6; 2% Mg and 2% Zn). Characterization of the phosphor material obtained by the process as disclosed above [0084] The formation of single-phase orthorhombic phase co-doped Sr2CeO4 with space group Pbam was confirmed by XRD measurements. The shift in the XRD peaks upon doping with Mg and Zn, confirm the incorporation of these dopants in Sr2CeO4 lattice. The crystallite size of the materials calculated using Scherrer’s equation is of the order of 12.1, 9.7, 7.6, 7.3, 7.2 and 7.0 nm for 0.5%Mg-0.5%Zn- Sr2CeO4, 0.5% Mg-1% Zn- Sr2CeO4, 1% Mg-1% Zn- Sr2CeO4, 2%Mg-1%Zn- Sr2CeO4, 1% Mg-2% Zn- Sr2CeO4, and 2% Mg-2% Zn- Sr2CeO4 respectively (Figure 1). The morphological analysis performed by field emission scanning electron microscopy (FESEM) and Transmission electron microscopy (TEM) formation of nanorods that are assembled in the star-shaped particles. EDX, selected area electron diffraction pattern (SAED) pattern and lattice fringes analysis confirms the doping of Zn, Mg and phase purity, respectively (Figure 2 and 3). Photoluminescence studies for characterizing CIE (Commission Internationale de l´Eclairage, colour mapping) value, lifetime and PLQY of developed single phase phosphors [0085] The photoluminescence investigation exhibited that upon doping with Zn²⁺ and Mg²⁺, broad emission spanning over the visible spectral region (400-700 nm). The fluorescence intensity gradually increases and reaches the maximum while using the doping concentration of the order of 1% Mg-1% Zn (SCO-3). Moreover, a single PL lifetime spectra (Figure 4) obtained when recorded at 492 nm emission wavelength and 330 nm excitation wavelength suggested that long lifetime which was single exponential fitted and had a lifetime of 41.7, 42.9, 43.4, 40.3, 33.5, 31.8 μs for 0.5%Mg-0.5%Zn-Sr2CeO4 (SCO-1), 0.5% Mg-1% Zn- Sr2CeO4 (SCO-2), 1% Mg-1% Zn- Sr₂CeO₄ (SCO-3), 2%Mg-1%Zn- Sr₂CeO₄ (SCO-4), 1% Mg-2% Zn- Sr2CeO4 (SCO-5), and 2% Mg-2% Zn-Sr2CeO4 (SCO-6), respectively in comparison with Sr2CeO4 without doping (SCO-0) (Figure 5). Photoluminescence quantum yield (PLQY) measured for the group of synthesized materials, ranges from 15%, 46%, 48%, 66%, 67% and 95%, for 2% Mg-2% Zn- Sr₂CeO₄, 1% Mg- 2% Zn- Sr2CeO4, 0.5%Mg-0.5%Zn- Sr2CeO4, 2%Mg-1%Zn- Sr2CeO4, 0.5% Mg- 1% Zn- Sr₂CeO₄ and 1% Mg-1% Zn- Sr₂CeO₄ respectively. The synthesized phosphor exhibited CIE coordinates ranging from 0.238 to 0.282, 0.329 to 0.372 for different doping concentrations (Figure 7). The LED porotype fabricated for the white light emitting phosphor using a UV LED chip (Figure 8) exhibited intense white light emission covering the whole visible spectral region (Figure 9) with the chromaticity coordinate ranging 0.30 to 0.34, 0.33 to 0.36 (Figure 10) and very high CRI color rendering index (CRI) from 85 to 95 and CCT from 4900 to 5700 K. [0086] Further, Figure 11 represents fabrication of prototype by coating the 365 nm UV chip with the developed single phase white light emitting phosphor and a module showing white light from the prototype. An electroluminescence device prototype was designed using the developed single phase white light emitting phosphor, as shown in Figure 12. Example 7 [0087] The emission spectrum for 0.5% Mg-0.5% Zn doped Sr2CeO4 phosphor material (SCO-1) was recorded using a Xe lamp source of the photoluminescence (PL) at 330 nm that exhibited a broad white light emission with a CIE value in the order of (0.238, 0.329) and a long PL lifetime in the order of 41.7 μs as shown in Figures 6 and 7. For the phosphor SCO-1, the absolute photoluminescence quantum yield (PLQY) was found to be in the order of 48%, higher than that of conventionally used phosphors and thus it was found that the phosphor SCO-1 possessed the potential to use as a white light emitting source in various lighting and display application. Example 8 [0088] The emission spectrum for 0.5% Mg-1% Zn doped Sr2CeO4 phosphor material (SCO-2) was recorded using Xe lamp source of the PL at 330 nm that exhibited a broad white light emission with a CIE value in the order of (0.271,0.334) and long PL lifetime in the order of 42.9 μs as shown in Figure 6 and 7. For the phosphor SCO-2, the absolute photoluminescence quantum yield (PLQY) was found to be in the order of 67%, which was higher than that of conventionally used phosphors and thus it was observed to possess the potential to be used as a white light emitting source in various lighting and display application. Example 9 [0089] The emission spectrum for 1% Mg-1% Zn doped Sr₂CeO₄ phosphor material was recorded using a Xe lamp source of the PL at 330 nm that exhibited a broad white light emission with CIE value in the order of (0.282,0.349) and long PL lifetime of the order of 43.4 μs as shown in Figure 6 and 7. For the phosphor SCO- 3, the absolute photoluminescence quantum yield (PLQY) was found to be in the order of 95%, higher than that of conventional phosphor materials and thus it was observed to have potential to be used as a white light emitting source in various lighting and display application. Example 10 [0090] The emission spectrum for 2% Mg-1% Zn doped Sr2CeO4 phosphor material SCO-4 was recorded using Xe lamp source of the PL at 330 nm that exhibited a broad white light emission with CIE value in the order of (0.276,0.364) and long PL lifetime in the order of 40.3 μs as shown in Figure 6 and 7. For the phosphor SCO-4, the absolute photoluminescence quantum yield (PLQY) was found to be in the order of 66%, higher than that of conventionally used materials and thus the phosphor had potential to be used as a white light emitting source in various lighting and display application. Example 11 [0091] The emission spectrum for 1% Mg-2% Zn doped Sr2CeO4 phosphor material SCO-5 was recorded using Xe lamp source of the PL at 330 nm that exhibit a broad white light emission with CIE value of the order of (0.266,0.372) and long PL lifetime of the order of 33.5 μs as shown in Figure 6 and 7. For this phosphor, the absolute photoluminescence quantum efficiency (PLQY) was found to be in the order of 46%, higher than that of in the prior arts and thus the phosphor had the potential to be used as a white light emitting source in various lighting and display application. Example 12 The emission spectrum for 2% Mg-2% Zn doped Sr₂CeO₄ phosphor material SCO- 6 was recorded using Xe lamp source of the PL at 330 nm that exhibited a broad white light emission with CIE value in the order of (0.258,0.362) and long PL lifetime in the order of 31.8 μs as shown in Figure 6 and 7. For the SCO-6 phosphor material, the absolute photoluminescence quantum efficiency (PLQY) was found to be in the order of 15%, higher than that of conventionally used phosphor materials. Thus, the phosphor SCO-6 material had the potential to be used as a white light emitting source in various lighting and display application. Example 13 [0092] Among the phosphor materials 1% Mg-1% Zn doped Sr2CeO4 phosphor material (SCO-3) possessed maximum Photoluminescence intensity as well as spectral shift towards red region and hence excitation dependent study done at different excitation wavelength ranging from 270 to 350 nm as shown in Figure 5 to analyze the charge carrier dynamics in the SCO-3 phosphor. It was observed that there was a minor shift in the position of the maximum with increase in excitation wavelength from 270 to 350 nm and the intensity of the peak decreased in an orderly manner. This showed that with the increase in excitation wavelength there is a minor change in the energetics in the energy level that was supported by the single lifetime value. The color characteristics of white light emitting prototype fabricated using the of developed single phase phosphors Example 14 [0093] Owing to the interesting properties such as high quantum efficiency, lifetime, near white CIE coordinate, the Mg-Zn co-doped Sr2CeO4 phosphor material SCO-3 was chosen to study the parameters that are essential for the practical applicability of the material in different lighting applications. Hence, a white light emitting diode (LED) prototype device was fabricated by mixing the phosphor SCO-3 with a silicon epoxy resin in 1:1 ratio and coated on a 365 nm ultra-violet (UV) LED with 3.5 V, 1 A and 4 W input specifications. The fabricated LED prototype emitted white light as shown in Figure 8. Example 15 [0094] To fabricate a practical electroluminescent device, indium tin oxide (ITO) coated on a polyethylene terephthalate (PET) sheet as anode and silver as a cathode were used. In between, PEDOT:PSS (poly(3,4- ethylenedioxythiophene):polystyrene sulphonate) was spin-coated over the anode as a hole injector layer with thickness ranging from 45 to 100 nm under nitrogen atmosphere in a glove box. Above the hole injector layer, a hole transporting layer, poly TPD (poly ( N, N ′-bis (4-butylphenyl)- N, N ′-bisphenylbenzidine)) in THF (tetrahydrofuran), toluene or chloroform was coated by spin coating at 3000 rpm under nitrogen atmosphere. Further, upon this layered anode, a layer of developed nano phosphor material SCO-3 mixed with PH-745 phosphor binder was coated. Over the white light emitting phosphor layer, ZnO nano arc ink was spin-coated at 3000 rpm as an electron transporting layer with varied thickness in the range of 20 to 80 nm. Each coated layer was dried in hot oven at a temperature ranging from 70 to 90 ^oC. Finally a silver electrode was coated by thermal evaporation with a thickness ranging from 3 to 10 nm. The design is shown in Figure 12. Example 16 [0095] Luminescence spectra of the fabricated white light emitting prototype as explained in example 14 by coating gives electroluminescence spectra was taken with the spectrometric arrangement in Oceanview Pvt. Ltd. detector as well as software. The fabricated prototype on power on condition gave a broad band in the visible region as in shown in Figure 9. This comprised of the blue, green as well as red region and thus giving a balanced white light emission for a long time without degradation and quenching in luminescence. Example 17 [0096] From the emission spectra of the white light emitting prototype fabricated by the process as explained in example 14, the CIE coordinate was calculated to obtain the values from 0.30 to 0.34, and 0.33 to 0.36 which was excellent as it was very near to the ideal CIE of the white light at (0.333, 0.333). Exhibiting white light nearing CIE values was understood to be a necessary condition for indoor and outdoor application. The fabricated prototype also possessed a correlated color temperature (CCT) from 4900 to 5700 K, which was highly desired for various lighting application. Example 18 [0097] The distinguishing capacity of a material was found to be a very essential condition to distinguish the actual color of an object like sun in day light. The fabricated prototype exhibited a color rendering index (CRI) from 85 to 95 which was excellent than the available sources having CRI less than 85. Thus the synthesized phosphor fabricated prototype was ideal for different lighting application arena. [0098] The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obviously, many modifications and variations are possible in the light of the above teaching. [0099] The embodiments were chosen and described to best explain the principles of the present invention and its practical application, to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. [0100] It is understood that various omission and substitutions of equivalents are contemplated as circumstance may suggest or render expedient, but such are intended to cover the application or implementation without departing from the spirit or scope of the present invention. ADVANTAGES OF THE INVENTION [0101] The main advantages of this invention are: 1. Formation of stable co-doped luminescent material via facile hydrothermal approach followed by in situ calcination processes. 2. Stable single phase intrinsic is simple, cost effective and does not require any special equipment or safety devices. 3. Utilizes steady-state and time-resolved photoluminescence spectroscopy as a tool to demonstrate the fluorescence property exhibited by the co-doped material. 4. Synthesized materials demonstrate highly intense and broad emission over the entire visible spectral range and very high photoluminescence quantum efficiency up to 95%. 5. Synthesized materials show excellent CIE, CCT and CRI, therefore, it has tremendous potential in many optical applications such as indoor and outdoor lighting, display devices, etc.

Claims

We claim: 1. A process for the preparation of a stable luminescent single-phase intrinsic white light emitting Zn and Mg co-doped Sr2CeO4 phosphor materials with high colour rendering index (CRI) (up to 95), photoluminescence quantum yield (PLQY) (up to 95%), bright white light emission, tunable correlated colour temperature (CCT) from 4900 to 5700 K (warm to cool region), comprising the steps of: a) mixing stoichiometric amounts of Sr and Ce precursors and dopants precursors of Mg and Zn dissolved in water to obtain a solution; b) adding ammonia to the solution until pH 8-9 is achieved while stirring for 30 min to 2 h to obtain a reaction mixture; c) subjecting the reaction mixture to hydrothermal reaction at 120-200^oC for 6 to 48 h and subsequently allowing it to cool down and recovering a product by repeated washing with water and alcohols until neutral pH is achieved; d) drying the obtained product at 80^oC for 12h to obtain a powder; e) grinding and calcining the powder at 950-1100^oC to obtain a calcined product; f) grinding the calcined product while using 5% polyvinyl alcohol (PVA) as a binder, subsequently making a pellet and subjecting it to recalcination at 1150-1250 ^oC to obtain the Zn and Mg co-doped Sr₂CeO₄ phosphor material.

2. The process as claimed in claim 1, wherein Sr and Ce precursors are selected from the group consisting of cerium nitrate, cerium chloride, cerium acetyl acetonate, strontium nitrate, strontium chloride, strontium acetyl acetonate.

3. The process as claimed in claim 1, wherein dopant precursors of Mg and Zn are selected from the group consisting of zinc nitrate, zinc chloride, zinc acetyl acetonate, magnesium nitrate, magnesium chloride, and magnesium acetyl acetonate.

4. The process as claimed in claim 1, wherein the phosphor material exhibits highly intense broad emission over the entire visible spectral range, which further intensifies with doping.

5. The process as claimed in claim 1, wherein the phosphor material exhibits intense white light emission with chromaticity coordinates ranging from (0.238 to 0.282, 0.329 to 0.372), and is tunable by changing the dopant concentration.

6. The process as claimed in claim 1, wherein the phosphor material exhibits tunable photoluminescence quantum yield (PLQY) from 15 to a very high ~ 95%.

7. A electroluminescence device containing the phosphor material [4] obtained by the process as claimed in claim 1, comprising: [1] a Ag layer; [2] an electron injector layer; [3] an electron transport layer; [4] the phosphor material; [5] a hole transport layer; [6] a hole injector layer; and [7] a flexible ITO.

8. A device comprising the single white light emitting material (phosphor) obtained by the process as claimed in claim 1, wherein the device is fabricated by mixing the phosphor material [4] with silicon resin and coated on 365 nm UV LED.

9. The device as claimed in claim 8, wherein the device is UV excited, give rise to intense white light with tunable chromaticity coordinates from (0.30 to 0.34, 0.33 to 0.36), and a color rendering index (CRI) from 85 to 95.