Recent development of Eu3+-doped phosphor for white LED application: A review

The phosphor converted w-LEDs gaincrucial attention in solid state lighting (SSL) for generation of illumination owing to their numerous meritorious advantages such as superior life time, excellent efficiency, compactness, reliability and power saving consumption as well as environmental friendly. The quality of w-LEDs in lighting and display is influence by host phosphorand the choice of activator. So greatlyattemptwere dedicated to developed inventiveuni-nucleiluminescent phosphor materials compose of chromatic stability, optimum CRI and low correlated color temperature. This review elaborate theintroduction ofEu3+rare earth activated red emitting phosphor assigned to 5D0→7FJ (J= 1,2,3,4) energy levelsand its fundamental merit for w-LEDs. This article represent the analyseofcombination of different types of Eu3+ activated luminescent materials by traditional and novel methods and its impact on photoluminescence for SSL.


Introduction
In the modern society and digitalization, energy saving and environmental protection gain world-wide attention. In the past decades, luminescence materials shows crucial utilization for development of display technology [1]. On the basis of absorption and emission of radiant energy luminescence are classified into categories namely up-conversion and down conversion emission mechanism [2]. The active centre and active medium plays very important role in order to visualize and realize efficient luminescence. The phosphors materials applicable for commercial purpose must possess certain meritorious characteristics like long operational life, remarkable energy efficiency, high CRI, considerably environmental friendly. Moreover the conventional source of light includes incandescent lamp, halogen and xenon lamp owing to their large energy consumption and environmental issue are replace by light emitting diode [3]. It should be expected that widespread utilization of LED source of light in comparison with traditional light source reduce worldwide consumption of electricity and exclusion of mercury support the environment which gives additional boost to the development of LEDs for lighting [4]. The development of this eco-friendly technology help in reduction of global power requirements as well reduces utilization of fossil fuels. There are various chronology reported for the development optimum luminescence performance of phosphor for w-LED application required for solid state lighting applicable for lighting and display backlight sources [5]. Commercially, phosphor converted w-LEDs can be obtained when blue emitting InGaN based semiconductor chip coated with yellow light emitting YAG:Ce phosphor results generation of white light [6]. However, it shows poor color rendering indices (CRI~ 70-80) along with high correlated color temperature (CCT ~4000-7500 K) but due to deficiency of red component in the spectral region hinder its utilization for indoor lighting [7]. To overcome the flows alternative method is adopted involving the combination of near ultraviolet (n-UV) LED chip with tri-chromatic red, green and blue (RGB) light emitting phosphor has been proposed [8].Unfortunately phosphor mixture revealed strong re-absorption of the blue light by the green and red phosphor as well as non-uniformity in luminescent properties, resulting deficient in luminescent efficiency, but at the same time achieve high color rendering index (> 90) ,broad band emission spectrum with quite stable color output [9]. It can be further resolved by development of single phase phosphor assemble with UV or n-UV LEDs [10,11].
Rare earth ions play a very important role in the world of display technology, due to their remarkable luminescence characteristics gives extremely sharp peak emission band due to presence of numerous allowed energy levels. The various novel phosphors activated with rare earth ions exhibits significance attention in the field of solid state lighting, radiation detection [12]. The pronounce luminescence efficiency, excellent stability, long durability and environmental-friendly are the main parameter of phosphor required for its commercialization for the w-LEDs application, which can be realized by suitable doping of rare earth and hence gain extensive attention in the field of research [13]. Normally the rare earth ions exhibits two type of emission (i) broad band emission (ii) narrow band emission [14]. Among the various rare earth the europium (Eu) most commonly found in +3 and +2 oxidation state, having second lowest melting point and lower density of all lanthanides. The present review elaborate the basic aspect of transition mechanism involved in trivalent Eu3+ ions along with luminescence properties of Eu 3+ doped activated inorganic materials also focused on reliable invention and utilization of rare earth Eu 3+ phosphor for w-LEDs applications.

Basic of transition mechanism of Eu 3+ in the phosphor
Europium is one of the rarest of rare earth elements on earth and is the most reactive rare-earth element. It exists in trivalent oxidation state under most conditions. Eu 3+ is a one of potential activator responsible for the development of red phosphors owing to 5 Do→ 7 FJenergy levels and hence consider as promising candidate for red emitting w-LEDs phosphor. The schematic energy level diagram of Eu 3+ ions is shown in figure 1. The PLE spectrums Eu 3+ doped phosphor normally exhibits broad excitation in range of 250 nm-350nm attributes to charge transfer band corresponds to O 2-→Eu 3+ transitions in the host [15]. Moreover existence of several excitation peak corresponds to ( 7 Fo→ 5 DJ, 5 LJ) transition. Figure 1 depict some excitation centred of Eu 3+ at 395 nm, 466 nm and 535nm attributes to 7 Fo→ 5 L6, 7 Fo→ 5 D2, 7 Fo→ 5 D1 transition [16]. In Eu 3+ ions Under suitable excitation wavelength PL spectrum consist of several emission peaks centred at 595 nm, 618 nm, 656 nm, 700 nm attributes to 5 Do→ 7 FJ (J=1, 2, 3, 4) transition [17]. Moreover the intense red of Eu 3+ activated phosphor (613 nm, 702 nm) is owing to ff orbital transition forbidden the sharp emission is purely host independent. As the trivalent 3 europiumgives deep orange-red emission hence shows wide application in a television sets and fluorescent lamps, which also increase the general efficiency of fluorescent lamp.

Synthesis Technique for the development of Eu 3+ activated Phosphor
The various synthesis methods are adopted for the development novel inorganic phosphor materials in order to obtain excellent luminescent performance of rare earth activated phosphor for w-LEDs application few of them are elaborate in details 3.1 Solid State diffusion Method This technique also term as high temperature synthesis method. This conventional technique is the highly popular and favourable route for the manufacture of rare earth activated phosphors such as oxide, aluminosilicates, vanadates, fluorides, chlorides and nitrides based phosphors for the mass production. [18] The traditional synthesis of several phosphates, sulphates phosphors requires temperature greater than 1000 o C, because it cannot be prepared smooth by the SSD at the temperatures below 1000 o C. This method involves the precursors in solid form which are mixed to form solid solution. The heat treatment generally ranging from 500 o C to 2000 o C required to carried out homogeneous diffusion between cation and the anion by overcoming the lattice energy.

Combustion Method
The combustion synthesis is the method use for development of oxide-based phosphors with nano range particle size. This method composed of precursors normally taken in nitrate form and urea, carbohydrazide or hydrazine-based compounds and glycine etc are used as reducer (fuel). Allstarting materialsare keep instoichiometricallyamountswith the calculation of oxidizer and reducer valences which are further crushed by mortar and pestle for 1hr. The resultant past is kept in the furnace at the temperature of 550 o C-600 o C. The vigorous flame is produced with a temperature of about 1600 o C. The resultant material is in the form of homogeneous and foamy phase.

Sol-gel method
It is techniques widely used for the fabrications of nano structured phosphor materials and also thin films or ceramics prepared by alkoxide materials which are generally organometallic compound. This process involved formation of sol by hydrolysis and polymerization reaction result in the formation of porous gel and hence term as sol-gel method. The resultant material is obtained by drying and annealing. This method is a very attractive method because of less time duration, low process temperature, low cost, good quality products and obtained maximum purity to making a chemical material.  [23]. This phosphor shows body-centered cubic (BCC) structure with space group I43d (002) confirm from XRD data and Rietveld study. PL characteristics exhibits that at excitation wavelength of 394nm shows several emission peaks however strong emission centred at 593nm ascribe to 5   ), undoped CBV shows weak broad band emission in the range of400-680nm ascribe to 3 T1→ 1 A1 transition of [VO4] 3group. Whereas Eu 3+ activated CBV material revealed intense red emission in blue excitation may be due to energy transfer from [VO4] 3groups to the 5 D0 energy level of Eu 3+ ions in the host. At the excitation wavelength 464nm CBV: xEu 3+ (0.1  x  6.0) phosphor shows most intense emission at 593nm (orange region) attributes to magnetic dipole 5 Do→ 7 F1 and other located at 613nm (red region) ascribe to electric dipole 5 Do→ 7 F2 transition. The concentration quenching beyond 4 mole% in Eu 3+ ions in host. The CIE co-ordinates at excitation wavelength 464 nm are obtained at (0.639, 0.358) which are resemble to that of commercially available red phosphor.

VII.
Ba2GdVO6:Eu 3+ A series of white emitting Eu 3+ doped Ba2GdVO6luminescent material was reported by SSD technique [26]. The band gap Eg of prepared material is 3.78eV obtained from diffuse reflection spectra. The excitation spectrum of prepared phosphor monitor at 612nm shows broad band in region 200-300nm ascribe to the Eu 3+ -O 2charge transfer transition and host absorption with sharp excitation peak at 395nm and 465 nm corresponds to 7 Fo→ 5 L6 and 7 Fo→ 5 D2 respectively transition of Eu 3+ ions in the host. Moreover, excitation spectra consist of two sharp peak at 553nm and 612nm ascribe to MD transition  6-→Eu 3+ in the host [27].

IX.
Sr4Al14O25:Eu 3+ The photoluminescence and thermoluminescent properties of Sr4Al14O25:Eu 3+ phosphor was studied by Emen et al [28]. As-synthesize materials displayorthorhombic structure with Pmmm(space group) confirm from XRD analysis. This phosphor monitor at 616nm gives four excitation band. The intense broad excitation band located at 308nm ascribe to charge transfer transition of Eu 2+ -O2 and other three are position at 360nm, 383nm, 411nm corresponds to 7 Fo→ 5 LJ (J=6, 3, 2) transition of Eu 2+ ions respectively in the host. The emission spectrum corresponds to excitation wavelength 308nm exhibits four emission peak cantered at 592nm, 616nm, 651nm, 707nm assigned to 5 Do→ 7 FJ (J=1, 2, 3, 4) transition of Eu 3+ ions. The emission peak at 616 nm and 595nm referred as ED and MD transition etc. The larger emission of electric dipole (ED) as compare to magnetic dipole (MD) reveals that lower symmetry around the Eu 3+ ions does not contain in inversion symmetry centre [29]. Activation energy range from 0.562-0.886 eV indicate the creditability of phosphor for display and LED application. X.
Ba2YAlO5: Eu 3+ Duan et al. studied Eu 3+ singly doped Ba2YAlO5 red emitting phosphor [30]. PLE excitation spectrum shows prepared phosphor is not only UV excitable but also gives excellent absorption in blue region. The prepared phosphor is highly sensitive to Eu 3+ ions leads to dominance of electric dipole transition exhibits intense emission in red region at 614nm attributes to 5 D0→ 7 F2 transition with CIE co-ordinate shift to (0.590, 0.398) and addition to shows 74.4% thermal stability at 473k.

Applications and future prospective
White light emitting diode received prime position and attention which are in position to replace the convention source of light include incandescent and fluorescent lamps in the field of lighting industry and made tremendous revolution in next generation light source. The world-wide utilization of w-LEDs from house hold to street lighting support environmental issue and heavy power consumption shows great the impact on economic and ecological aspect of society.  [43]. It exhibits the PL emission in the range of 600nm to 720nm shows its probablematerials for red emission.

Conclusion
The exclusion of conventional light sources such as incandescence lamp, fluorescent triphosphor, tungsten halogen, xenon arc lamp etc with the development of low power consumption pc-wLED green light source for the next generation illumination is in crucial progress owing to their numerous characteristics like exceptional luminous efficiency, low power consumption, desired CCT and CRI adjustable by change of physical properties of selected phosphor are elaborate which is fundamental aspect for economical and ecological growth. This paper discusseswith the europium-(Eu 3+ ) dopedluminescent phosphors materials for SSLapplication in various field of technology. The phosphor must avoid blue color reabsorption by green and red component for fabrication of LED. The emission from the rare earth activated phosphor is describe the various parameter such as crystal symmetry, coordination geometry, bond length and crystalfield strength however still the miles to go to obtain the phosphor with desire excitation and emission spectra, excellent luminescent efficiency.