Synthesis and Characterization of YAG:Ce Phosphors for White Leds

OPTO−ELECTRONICS REVIEW 23(4), 239–251

DOI: 10.1515/oere−2015−0038

Synthesis and characterization of YAG:Ce phosphors for white LEDs

V. TUCUREANU*, A. MATEI, and A.M. AVRAM

National Institute for Research and Development in Microtechnologies, IMT Bucharest 126A Erou Iancu Nicolae Str., code 077190 Bucharest, Romania

Worldwide commercial interest in the production of cerium doped yttrium aluminium garnet (YAG:Ce) phosphors is re− flected in the widespread use of white light emitting devices. Despite of the fact that YAG:Ce is considered a “cool phosphor” it is the most important in white LED technology. This article reviews the developed techniques for producing phosphors with superior photoluminescence efficiency, including solid−state reaction, sol−gel and (co)precipitation methods. Also, by co−doping with rare earth elements, a red/blue shift is reached in the spectrum. The characteristics of YAG:Ce phosphors are investigated because the properties of the phosphors are strongly influenced by the synthesis routes and the sintering temper− ature treatment. After the phase analysis, morphology and emission studies of the phosphors there may be seen the condi− tions when the transition from the amorphous phase to the crystalline phase appears, when luminescent properties are influ− enced by the crystalline form, purity, average size of the particles, co−doping and so on.

Keywords: YAG:Ce, phosphor, solid−state, sol−gel, (co)precipitation.

1. Introduction Ce:YAG has been available since 1960s, but the YAG:Ce is considered a cool phosphor, due to the lack of the red com− Phosphors, essential for LED technologies, are used for ponent [5–9]. In the last years, the research is turning to converting the light emitted by a blue or UV chip into white co−doping the YAG:Ce, in order to shift the emission spec− light and with the most protection in terms of intellectual trum by partially substituting the Y or Al cations with differ− property. Phosphors are materials that generate lumines− ent ions. By substituting Y3+ with ions like rare−earth (lan− cence. A phosphor is a chemical compound that will emit thanide: Tb3+,Gd3+,Dy3+,La3+, etc.) a red shift is intro− radiation (photons), when bombarded with an external ener− duced. On the contrary, by the substitution of Al3+ with ions gy source (i.e., photons, electron beam), at longer wave− as Ga3+ or In3+, a blue shift can occur in the cerium emis− length than the excitation source. These types of materials, sion. A secondary peak may appear in the red spectral range known in literature as inorganic phosphors, are actually var− by co−doping ions that can act as a secondary luminescence ious types of oxides, garnets, (oxy)nitrides, (oxy)sulfides source (Pr3+). A phosphor such as YAG:Ce, Re (Re – rare etc., doped with different transition metals or rare−earths earth ions) can be used for the development of white LEDs that are able to emit light [1]. Rare−earth doped phosphors with improved optical properties. Also, an increase in the from A3B2C3O12 class (B and C may be the same or differ− Ce3+ concentration or by modifying the process parameters ent atoms) are synthetic materials with good chemical and a slight red shift can be introduced [3,10–16]. thermal stability, especially optical and mechanical proper− In this review paper we present a summary of the most ties, and they became extremely popular and have numerous used methods for phosphor synthesis. We will present the applications in different fields, from cathode−ray tubes and phase analysis, elemental composition, morphology, mi− solid−state laser, up to illumination light sources and bio− crostructure and optical properties of the garnet powders medical applications. The basis for the most phosphors used (YAG:Ce and YAG:Ce, Re). in white LEDs is the lattice of yttrium aluminium garnet (YAG, Y3Al5O12, where Y is in the dodecahedra sites (A), Al is in the octahedral sites (B) and tetrahedral sites (C) 2. Experimental section [2–4]. The YAG:Ce and YAG:Ce, Re phosphors have been manu− White LEDs, based on blue LED chips (have been deve− factured using different routes in order to obtain a nano− loped since 1991, first time by Nichia Co.) coated with yel− structured material with a uniform distribution, high effi− low emitting phosphor were first reported in 1996, and ciency emission, and high crystallinity. Among the synthe− Ce3+ doped YAG phosphor, with the general formula: sis methods, the most widely spread there are: the solid state Y Ce Al O or Y Al O :Ce abbreviated YAG:Ce or 3–x x 5 12 3–x 5 12 x reaction, the sol−gel process and the (co)precipitation meth− ods. All studies aim at improving the luminescent properties *e−mail: [email protected], [email protected] of yellow phosphorus, by choosing a particular method of

Opto−Electron. Rev., 23, no. 4, 2015 V. Tucureanu 239 Synthesis and characterization of YAG:Ce phosphors for white LEDs synthesis and/or modifying certain process parameters mixed with a solvent in the planetary balling machine. The (dopant concentration, starting materials concentration/ solvent is added into the samples to obtain homogeneous report, reaction time, flux, sintering atmosphere and mixtures. Ethanol, acetone, 1,4–butanediol and diethylene temperature, etc). glycol can be used as solvents in order to obtain homoge− neous mixtures [18,32]. Then, the precursor powder is sin− 2.1. Solid-state reaction method for phosphor tered at high temperature (over 1500°C) for long reaction synthesis times because of the refractory nature of the oxide precur− sors. The sintering process can be done in air, in H2/N2, CO, The conventional synthesis route for manufacturing the N2/CO atmosphere or combined (i.e.: first air and then re− YAG:Ce and YAG:Ce, Re phosphors is a solid state reac− ducing atmosphere). Different atmosphere can be used in tion using a mixture of oxides or carbonates as raw materi− order to change the defect structure of these crystals, to 4+ 3+ als. Y2O3 or Y2(CO3)3 can be the source for Y, and Al2O3 or facilitate the transition from Ce to Ce [21,24,26,30,33]. Al2(CO3)3 can be for Al, while Ce2(CO3)3,Ce2O3 or CeO2, The thermal treatment can be done at atmospheric pressure –3 Tb4O7,Re2O3,Re2(CO3)3 (where Re can be Gd, Eu, etc.) or under different values of pressure (i.e., 10 Pa, vacuum can be used as doping agents. But raw materials like nitrate condition). Fluxes such as NaOH, BaF2,H3BO3/BaF2/hexa− salt Y(NO ) ·6H O, Al(NO ) ·9H O and Ce(NO ) ·6H O, 3 3 2 3 3 2 3 3 2 methylenetetramine, YF3,Li2CO3,NaF,NH4NO3,SiO2, Re(NO3)3·xH2O are preferred. If the raw materials are oxi− etc., are added to obtain greater crystallinity and to decrease des or carbonates, a preliminary wet chemical preparation grain size at lower the sintering temperature [6,21,34–37]. step can be used. The oxides/carbonates are converted to the ACe3+ concentration of 0.01–10 mol% can be used. Most corresponding nitrates by dissolving them in nitric acid, and articles claim the co−dopant concentration should be maxi− the excess is removed very slowly by evaporation [6,10,13, mum half of the cerium concentration, but usually it is in the 17–24]. The chemical reactions, involved in the process, range of 0.03–0.1 mol%. Dopant and co−dopant concentra− occur between nitrates and the organic compounds from the tion influence the emission wavelength [18,35]. In the end, system. Practically, there is a reaction in which the oxygen the temperature was slowly reduced to room temperature (from metal nitrates) acts as an oxidizing agent and while C, and the cooled samples were pounded into a fine powder by H, Y are reducing agents [25]. using an agate mortar. Repeated sintering operations and The possible reactions for garnet phosphor manufactur− milling are necessary during the process [23]. Milling and ing are described below [21,26–28]. During the process dif− too higher temperature are sources of defects, i.e.: particles ferent phases may occur, including: YAlO (YAP, perov− 3 with a damaged surface, and light quality problems. Sinte− skite structure), CeAlO , ReAlO (i.e: Re = Gd – for higher 3 3 ring time can vary between 6 and 48 hours per each temper− concentrations of co−doped ions) Y Al O (YAM, monocli− 4 2 9 ature treatment step [20]. The maximum sintering tempera− nic structure) and the most important Y Al O (YAG, gar− 3 5 12 ture depends on time, pressure, atmosphere, intermediate net structure) or Re3Al5O12 (garnet structure). In the theo− retic Y O –AlO system, the ions diffuse to form YAM steps, etc. Most common temperatures are usually between 2 3 2 3 ° ° and then to form YAP and finally to YAG [24,27,29–31]. 1500 C and 1900 C. At higher sintering temperatures (over ° ® 1800 C) particle size increases beyond the desired nano− 3Y2O3 + 5Al2O3 2Y3Al5O12 (Theoretical reaction, over 1600°C) scale. Also some studies show that the starting oxides are ° ® ° present up to a specific temperature (i.e., Al2O3 to 1600 C) 2Y2O3 + Al2O3 Y4Al2O9 (900–1100 C) ® ° [26,38]. Y2O3 + Al2O3 2YAlO3 (1100–1400 C) ® ° Y4Al2O9 + Al2O3 4YAlO3 (1100–1400 C) ® ° 2.2. (Co) precipitation method for phosphor synthesis 3YAlO3 + Al2O3 Y3Al5O12 (over 1300/1400–1600 C) ® Y4Al2O9 +Y2O3 + 4Al2O3 2Y3Al5O12 (over 1300/1400– Another technique for YAG:Ce, Re and YAG:Ce, Re pow− ° 1600 C) der manufacturing with good technical results is the (co)pre− ® (3–x–y)Y2 O 3 + 2xCeO2 +yRe2 O 3 +5Al2 O 3 cipitation method which exhibits some advantages, such as 2Y(3–x–y)CexReyAl5O12 low sintering temperature, high purity products, good ho− ® (3–x–y)Y2O3 + 2xCeO2 +y/2Tb4O7 +5Al2O3 mogeneity and uniform distribution of particles, but the 2Y(3–x–y)CexTbyAl5O12 (co)precipitation processes are very complex and hard to It is believed that YAG formation occurs via the solid control. The (co)precipitation method is based on a reaction state diffusion of smaller aluminum cations into the cubic between a solution of the main reactants (source for Y and structure of Y2O3. The diffusion is very slow because of the Al), dopant/co−dopant in an acid solution and a precipitation ° high melting points of the raw materials, Al2O3 (2054 C) agent with formation of a precipitate. After aging, centrifu− ° and Y2O3 (2433 C) [17,26]. ging, filtering, washing, rinsing, drying and finally milling The main steps in the manufacturing of pure YAG gar− the precipitate is synthesized at elevated temperatures. Syn− net phase are preparation of the mixture of raw materials thesis by (co)precipitation method for YAG:Ce and YAG:Ce, and synthesis at high temperatures. The raw materials selec− Re starts with dissolving and mixing of the raw materials. ted are weighted according to the stoichiometric ratio and Oxides (Y2O3,Al2O3,Re2O3)orsalts(Y(NO3)3·6H2O,

240 Opto−Electron. Rev., 23, no. 4, 2015 © 2015 SEP, Warsaw ® Y(CH3 COO)3 ,Al(NO3 ) 3 ·9H2 O, Ce(NO3 ) 3 ·6H2 O, 5[(NH4)2Al6(CO3)3(OH)14]·9[Y2(CO3)3·3H2O] 5Al6O7 Re(NO3)3·xH2O) can be used as starting materials. As in the (CO3)2 +9Y2(CO3)3 + 40H2O + 5CO2 + 10NH3 (decompo− case of the solid−state method, the oxides are preferred to be sition reaction) dissolved in dilute HNO and the surplus is removed. Salts ® 3 Al6O7(CO3)2 3Al2O3+2CO2 were dissolved in deionized water. The raw material solu− Y (CO ) ® Y O +3CO tion is dropped to the precipitating agent in order to obtain 2 3 3 2 3 2 It is important to keep a constant pH value, which can be the precipitate. The main precipitant solution/precipitation done by adding the precipitation agent in excess or by con− agent can be: triethylamine, urea, NH HCO , (NH ) CO , 4 3 4 2 3 trolling the dropwise time of the raw material solution over NH solution. The morphology and particle size can be 3 the precipitant solution. The variation in pH and the concen− influenced by the amount of precipitating agent and concen− tration of [NH +] and [CO 2–] influence the final product. In tration ratio [pp]/[M3+]. The precipitated slurry was aged 4 3 this case the purity of the obtained phosphor is higher when (15 min.–24 h), filtered and rinsed with water, acetone or using NH HCO as a precipitating agent [4,23,38,39]. ethanol. [23,27,39–40] Poly(vinylpyrrolidone) may be used 4 3 as mediator in the solution for aggregating the precipitates in order to form spherical nanoparticles and (NH4)2SO4 for 2.3. Sol-gel combustion method for phosphor grain growth promoter. The precipitation step may occur at synthesis pH ³ 6. In order to completely react it requires up to 72 h of stirring [4,36,41]. Another wet chemical process, the sol−gel method has pro− Thermal treatment starts with the dried precipitate at ven to be a viable alternative to the solid state process for 120–150°C, for 10–15 h, and finishes with a sintering step obtaining YAG:Ce phosphor. In order to produce metal at 1000–1800°C (usually 1000–1200°C), for 2–10 hours in oxide nanoparticles, the sol−gel combustion method is based on the formation of a concentrated colloidal sol oxide and air or different atmospheres (H2/N2, CO, O2). By using an autoclave for precipitate producer (in hydrothermal condi− converting it to a gel (after hydrolysis and condensation). In tions) solvothermal (co)precipitation method can be sped the end, a thermal treatment is performed in order to obtain up. Some possible reactions, which may be involved in the the phosphor powder. The use of the sol−gel combustion (co)precipitation process for YAG:Ce and YAG:Ce, Re process for YAG:Ce and YAG:Ce, Re synthesis has a num− synthesis are presented below [23,36,39,42–44]. ber of technological advantages (easier composition con− l for urea as precipitation agent: trol, good mixing of the raw materials, better homogeneity, It is possible to induce the precipitation of a hydroxycar− relatively low sintering temperature, lower manufacturing bonate with general chemical formula: time, a product with fewer defects and more benefits − high Y Al Ce (OH) (CO ) ·nH O purity, fine powders) and economic benefits (cheaper than (3–x) 5 x 24–2z 3 z 2 solid−state or (co)precipitation methods). By controlling the (CO)(NH ) ® (NH )+ + (OCN)– (hydrolysis of urea) 2 2 4 process parameters, it is easier to control the properties of + – ® 2– (NH4) +HO +H2O NH3 +CO3 (neutral and basic the product. The particle size of the powder can be con− medium) trolled by changing the concentration of the sols, the tem− – + ® + (OCN) + H + H2O (NH4) + CO2 (acidic medium) perature or time for gelation, drying, (pre)sintering. For At 100°C urea is decomposed and reacts with nitrates. YAG:Ce and YAG:Ce, Re powder, manufacturing the sol− (3–x)Y(NO3)3 + xCe(NO3)3 + 5Al(NO3)3 + 20CO(NH2)2 −gel combustion method follows these general steps: dis− ® Y3−xCexAl5O12 + 20CO2 + 40H2O + 32N2 solving raw materials, stirring, adding the chelating agents, (3–x–y)Y(NO3)3 + xCe(NO3)3 + yRe(NO3)3 + 5Al(NO3)3 + stirring, aging and thermal steps. Dissolving and mixing the ® raw materials for hydrolysis and condensation of molecular 20CO(NH2)2 Y(3–x–y)CexRebAl5O12 + 20CO2 + 40H2O + 32N precursors and sol formation followed by gelation: the gel is 2 prepared using oxides (Y O ,AlO ,ReO ) and/or salts l for carbonates as precipitation agent: 2 3 2 3 2 3 (Y(NO ) ·6H O, Y(CH COO) , Al(NO ) ·9H O, AlOOH, NH HCO (or (NH ) CO )+H O « NH OH + H CO (hy− 3 3 2 3 3 3 3 2 4 3 4 2 3 2 4 2 3 Ce(NO ) ·6H O, (NH ) Ce(NO ) , Re(NO ) ·xH O) as sta− drolysis) 3 3 2 4 2 3 6 3 3 2 rting materials. As metal ions sources alkoxides may be « + – NH4OH2 NH4 + HO used, but it is difficult to control the process. As a dispersion H CO « H+ + HCO – 2 3 3 medium several agents can be used: an acid (CH3COOH or – « + 2– HCO3 H + CO3 HNO3) for dissolving the oxides, and water or water with By accelerating the hydrolysis reaction, the pH incre− small amounts of acid for dissolving the salts. After dissolv− ases, a suspension was obtained and the formation of a pre− ing the starting materials, the chelating agents, such as citric cipitate with empirical formula were reported: Y(3–x)Al5Cex acid, acetic acid, malic acid or ethylene glycol, are added. (OH)24–2z(CO3)z·nH2O, 5[(NH4)2Al6(CO3)3(OH)14]·9[Y2 Also, (poly)acrylamide, N, N−methylene−bis−acrylamide, (CO3)3·3H2O] (by using (NH4)2CO3 as the precipitator), ammonium persulfate can be used as polymerization agents. NH4AlY0.6(CO3)1.9(OH)2·0.8(H2O) (by using NH4HCO3 as Depending on the complexing agent and the intermediate the precipitator), Al(OH)3·0.3[Y2(OH)5(NO3)·3H2O] (by phase, we can talk about citrate or Pechini (nitrates are pre− using NH3 solution as the precipitator) [39,42,44]. ferred as starting materials) sol−gel process, nitrate–citrate,

Opto−Electron. Rev., 23, no. 4, 2015 V. Tucureanu 241 Synthesis and characterization of YAG:Ce phosphors for white LEDs

glycolate (acetates are preferred as starting materials) and and they may be followed by their polyesterification with acetate−nitrate−glycolate sol−gel method. Different fuel such a polyalchool (i.e. ethylene glycol). as glicin, glycerine, or triethanolamine can be used in the l for acetate−nitrate−glycolate sol−gel process process in order to initiate the combustion reaction by mix− M2O3 +6CH3COOH = 2M(CH3COO)3 +3H2O (where: ing with the salts and/or the gel [23,28,45–51]. In these M=Y, Ce, co−dopant) (dissolution of oxides) ® 3+ – steps, the most important parameters are: temperature, time M(CH3COO)3 M + 3CH3COO (dissociation into ions) ® 3+ – of stirring, pH, the ratio between the complexing agent and M(NO3)3 M + 3NO3 (where: M=Al, Y, Ce, codopant) the metal cations, and the presence of additives (glycerin, 3+ ® M +3H2O M(OH)3 (hydrolysis under elevate tempera− ethanol, polyethylene glycol, citric acid, etc.) [23,29]. The ture and week acid pH) process is carried out at a temperature of about 60–80°C, for ® 2M(OH)3 (HO)M(OO)M(OH) + 2H2O 3–12 hours, under a rigorous control of pH ( optimum 2–5) 2n(HO–CH –CH –OH) + n(HO)M(OO)M(OH) ® [–O– so as to form an oxide gel and prevent the precipitation 2 2 CH2–CH2–O–M(OO)M–O–CH2–CH2–O–]n + 2nH2O [23,45–46]. At pH < 2 the ionization of the complexing (xerogel). ³ agent, is inhibited, while at pH 5, YAM and YAP simulta− In the combustion step, salts can act as an oxidant and neously occur or a precipitate is observed, thus the emissive the complexing agent as a reductant [48]. properties are affected as a result of a non−uniform distribu− The advantages of using the sol−gel method for obtain− tion of Y and Al ions and blocking of the emissive centre ing phosphorus derived from an exact composition, the mo− due to impurity from YAG lattice. The optimal molar ratio lecular level mixing and the lower crystallization tempera− of complexing agent/metal ions can be 1–2. At a ratio > 1 ture, but the method requires long process times [29]. the agent has a double function, that of complexing agent and fuel, and it provides the necessary energy for the com− 3. Results and discussion bustion step and the phosphor obtained has superior lumi− nescence properties. At a molar ratio of complexing agent/ Optical properties of phosphors are determined by phase metal ions < 1 some Y or Ce ions remain in solution, at ratio transition from amorphous to crystalline, lattice constant, ³ 3 it may induce the impurities in the lattice and particle purity of the final product, morphology and micro− size increases. In the condensation reaction the gel is formed structure. if the temperature and pH are constant during the several hours of stirring. After that the gel can be transformed into 3.1. Fourier transform infrared (FTIR) spectrometry a xerogel by slow evaporation at temperatures of about 100–150°C, during a time which can be from 10 hours to 10 Phase analysis of the YAG:Ce and YAG:Ce, Re powders days. In the drying step elimination of toxic gases is can be performed by Fourier Transform Infrared (FTIR) possible due to the decomposition of nitrates [29,48]. spectroscopy and X−ray diffraction (XRD). As in the solid state process, the thermal treatment is The bond configuration of YAG:Ce and YAG:Ce, Re a decisive stage. Pyrolysis of organic compounds and calci− phosphor powder show only high absorption bands below –1 nations occur during the thermal treatment. Pre−sintering 900 cm (V. Schiopu, et al. present a typical FTIR spectra occurs at 200–600°C, for 5 to 10 hours. In the pre−sintering of the YAG:Ce, Gd powder in Ref. 53), demonstrating the step, decomposition of the nitrates starts at about 200°C, at transition from amorphous phase to the crystalline phase of 350°C the carbonates and finally resulting in a total elimina− YAG. These bands are features of garnet structure and attri− tion of organic compounds from the system. After an inter− butable to the stretching mode of metal–oxygen (M–O). mediate grinding the YAG:Ce, Re powders were sintered at Possible bands and assignments for each type of M–O bond 1000–1300°C for 10 to 18 h. The thermal treatment can be are presented in Table 1. Any peaks in the spectral range –1 performed in different types of atmosphere (static, inert, from 4000 to 900 cm suggest the presence of impurities oxidative or reductive), but in order to prevent the oxidation located to the particle surface and/or an incomplete sintering of Ce3+ a weak reduction atmosphere is recommended. Bel− process (Table 2) [2,32,40,42,44,49,53–62]. low some possible reaction underlying the sol−gel process for YAG:Ce and YAG:Ce, Re synthesis is presented [48, 3.2. X-ray diffraction (XRD) 52]: The crystal structures, crystallite size, lattice parameter, l for citrate sol−gel process strain, as well as phase purity of YAG:Ce and YAG:Ce, Re (9–3x)Y(NO ) + 3xCe(NO ) + 15Al(NO ) + 20C H O 3 3 3 3 3 3 6 8 7 nanocrystalline powder can be characterized by X−ray pow− ® 3Y Ce Al O + 120CO + 80H O + 36N 3−x a 5 12 2 2 2 der diffraction (XRD). At sintering temperatures below (9–3x)Y(NO3)3 + 3xCe(NO3)3 + 3yRe(NO3)3 + 15Al(NO3)3 900°C, the XRD shows an amorphous compound, while at ® + 20C6H8O7 3Y(3–x–y)CeaRebAl5O12 + 120CO2 + higher temperatures the crystalline phases appear [a typical 80H2O + 36N2 X−ray diffraction profiles of pure garnet phase are presented The citrate sol−gel process is based on the formation of by V. Schiopu, et al. in Ref. 53 and an index for all single polychelates between metal ions and C=O from citric acid, lines are presented in Table 3 (JCPDS 01–082–0575)]. [53]

242 Opto−Electron. Rev., 23, no. 4, 2015 © 2015 SEP, Warsaw Table 1. FTIR possible assignments for M–O bonds in case of pure YAG:Ce or YAG:Ce, Re powder. Wavenumber (cm–1) n(M–O) bonds(a) From Ref. 375 Gd–O YAG(b) [53] 396 Gd–O YAG [53] 430(c) Al–O YAG [49,56,59] 433 Gd–O YAG [53] 453 Y–O YAG [57] 457(c) Al–O YAG [49,53,55,59] 459 Ce–O YAG [53,56]

465 Y–O Y2O3 [49,56] 475 Y–O YAG [49,54,61] 484 Al–O YAG [49,62] 490 Y–O YAG [49,56,58] 500 Al–O YAG [49,56] 505 Y–O YAG [32,56] 510–515(c) Al–O YAG [42,53,54,56,59,61] 514–516 Ce–O YAG [49,53,56] 536 Al–O YAG [49,56]

560 Y–O Y2O3 [59] 565(c) Y–O/Al–O YAG [2,32,42,49,53,54,56,61] 570 Y–O/Y–O–Al YAG [40,49,56] 620 Al–O YAG [42,49,56,62] 640 Al–O YAG [49,56,58] 675 Al–O YAG [49, 56] 690(d) Al–O YAG [2,32,40,42,49,53,54,55,56,57,58,59,61] 720(d) Y–O/Al–O YAG [2,32,40,49,53,55,56,57,59] 730 Y–O YAG [42,49,54,56,61,62] 740 Al–O YAG [49,56]

755 Al–O Al2O3 [49] 790(d) Al–O YAG [2,32,40,42,49,53,54,55,56,57,59,61] 820(c) Al–O YAG [49,54]

823 Al–O Al2O3 [49] 830 Y–O YAG [49] 1070 Al–O Al–OH /Boemite [55] Note: (a) n = vibration stretching mode (b) YAG with respect to YAG:Ce and YAG:Ce, Re (c) Al–O from octahedral sites (d) Al–O from tetrahedral sites

All diffraction peaks correspond to pure YAG, indicating Ia–3d (230) space group. In the YAG unit cell with eight the insertion of the (co)doping ions in the YAG host lattice, Y3Al5O12 molecular units, inter connected octahedrons without changing the initial structure. The XRD pattern of (centred on Al), tetrahedrons (centred on Al) and dodeca− YAG:Ce and YAG:Ce, Re reveals a cubic garnet structure, hedrons (centred on Y) with oxygen at the corners [occupy q ° polycrystalline, with the main peak centred at 2 = 33.41 the 96(h) sites, each O is linked to two Y, one Alocta. and one 3+ and it may be attributed to (420) Miller indices. In case of Altetr.]. Al with two sites in the lattice, Alocta.occupy the incomplete thermal processes additional peaks may appear 16(a) site and Altetr. 24(d), are six and four−fold coordinated q ° ° ° ° at (2 ): 28.68 (CeO2), 28.572 and 47.51 (Ce2O3), 29.28 – an AlO6 (octa.) is connected with six AlO4 (tetr.) and an AlO4 ° ° ° (YAM with a monoclinic structure), 24.06 , 26.93 , 34.32 , is connected with four AlO6 by sharing the corners (A.B. 35.16°, 35.92°, (YAP with a perovskite structure), 25.64° Munoz Garcia present a typical YAG unit cell in Ref. 31). 3+ (Al2O3), [6,18,20,25,63]. Single Y occupies the 24(c) site, coordinated with eight 2– The YAG crystal structure has a 160 atom body−centred O inside the AlOx. By XRD analysis YAG cell parameters cubic (bcc) unit cell, with 80 atoms in primitive cell, and of 12 were found [12,22,25,31,35, 38,64,65].

Opto−Electron. Rev., 23, no. 4, 2015 V. Tucureanu 243 Synthesis and characterization of YAG:Ce phosphors for white LEDs

Table 2. FTIR possible assignments in the case of incomplet process for YAG:Ce or YAG:Ce, Re phosphors manufacturing. Wavenumber (cm–1) Type of vibration Possible bonds Ref. 3500–3400 n O–H [2,40,42,44,56,57,58,61,62] 3500–3000(e) n N–H (NH4+ ) [42,49,62] n (f) 2960–2850 as+s C–H (CH2+CH3) [ 55,58,62] n 1750–1710 s COOH [57,58] n 1660–1640 s H–O–H/C–OH [58] n – 1630–1605 as C=O (COO ) [58,60,61] n – 2– 1565–1535 as COO /CO3 [32,42,44,49,55,57] (g) 1470 d C–H (CH2) [2,60] (h) 1455 sr C–H (CH2) [2,32] n – 2– 1440 as COO /CO3 [55,58,62] (i) 1430–1420 w C–H (CH3) [ 58] n – 1420–1400 s O=C–O (COO ) [2,32] n 2– 1410 as CO3 [ 44,49] 1415–1405 n N–O [57,58] – 1385 n NO3 /C=O [ 42,60,62]

1342 w C–H (CH2) [2,32] 1152 n C–O/C–C [2,58] 2– 1125 n SO4 [44] 1120 n C–OH [58] 1115 n C–O/C–C [ 32,42,58,62] 1065 n C–O [2,32] 990 d O–H [42] (j) 2– 846 d/g CO3 [ 44] 2– 741 d/g C–O/CO3 [42,44] Note: (e) Overlapping with OH bands, so is difficult to detect. (f) n = stretching mode: s = symetrical, as = asymmetric (g) d = deformation mode (h) sr = scissors mode (i) w = wagging mode (j) d/g = deformation out of plane mode

The presence of Ce [Ce3+ ion substitute a Y3+ from 24(c) a is the lattice parameter [30,70]. Compared to YAG the lat− 3+ with local D2 symmetry] or co−dopants (Re ) does not dis− tice constant increases in YAG:Ce and YAG:Ce, Re. Also, turb the crystalline structure of the samples, but certainly the lattice parameters are determined by process conditions, does modify the lattice parameters due to the difference in for example: in the sol−gel process, the increase of the the ionic radii between the dopants and the substituted ions, sintering time may cause a decrease in lattice constant, it is not necessary to compensate the ionic charge for Ce or while the same powder obtained by sintering in different co−dopants. By co−doping with rare earth it is possible to medium may have different lattice parameters. In air, the change the symmetry. Ionic radii for Y3+ (VIII coordina− lattice constants are less than in a reducing atmosphere, also tion) 1.019 A°,Al3+(IV coordination) 0.39A° and (VI coor− increasing the temperature the lattice parameter increases. dination) 0.535 A°, dopants: Ce3+ (VIII coordination) 1.143 This variation can be explained by partial or total transfor− A°,Pr3+ (VIII coordination) 1.126 A°,Eu3+ (VIII coordina− mation of Ce3+ into Ce4+. In these situations, a blue−shift in tion) 1.066 A°,Gd3+ (VIII coordination) 1.053A°,Tb3+ the spectra may occur due to the changes of the unit cell, due (VIII coordination) 1.04A°,Ga3+ (IV coordination) 0.47A° to a modification of the crystal field around Ce3+ [ionic radii and (VI coordination) 0.62 A°, etc. [25,66–69]. Ce4+ (0.97A°)

244 Opto−Electron. Rev., 23, no. 4, 2015 © 2015 SEP, Warsaw Table 3. X−Ray powder diffraction data of a YAG phosphor pow− 3.4. Scanning electron microscopy (SEM) der. The scanning electron microscope (SEM) can be used to 2q (°) Miller indices (hkl) Ref. B examine the morphology and the microstructure of the 20.91 (220) [20,49,53,59,60,61,62,63] YAG:Ce powder. The morphology of the phosphor parti− 27.77 (321) [20,49,53,57,59,60,61,62,63] cles is one of the properties that influence the light conver− 29.73 (400) [20,49,53,57,59,60,61,62,63] sion efficiency. The YAG:Ce and YAG:Ce, Re powders are 33.41 (420) [20,49,53,57,59,60,61,62,63] predominantly crystallites with the estimated average size 36.63 (422) [20,49,53,57,59,60,61,62,63] from 40–100 nm to several microns. The phosphor particles 38.19 (431) [20,49,53,61,63] size slightly increases with the sintering temperature and 41.13 (521) [20,49,53,57,61,62,63] reaction time. Also, significant changes of the particle shape ° 42.54 (440) [20,49,53,61,63] appear at a temperature above 1100–1400 C. Large parti− 46.59 (532) [49,53,57,59,60,61,62,63] cles require thicker layers producing more light scattering. The desired shape is spherical with smooth surfaces at nano− 51.58 (541) [53] scale. The main advantage of the spherical phosphors parti− 52.77 (631) [53,61] cles derives from minimum scattering area and high packing 55.09 (640) [20,49,53,57,59,60,61,62,63] density. Also it is preferred for the particles not to be agglo− 56.23 (552) [20,53] merated. In the case of phosphors obtained by the solid state 57.37 (642) [20,49,53,57,60,61,62,63] reaction, the characterization reveals agglomerate phos− 60.66 (651) [20,49,53,61,63] phors with irregular shaped particles or sharp edges and 61.73 (800) [20,49,53,60,61,62,63] inhomogeneous distribution of Ce3+ ions. Most likely, only 70.00 (840) [53] part of the total cerium ions has entered the host YAG lattice 3+ 71.98 (842) [53,60] (lower temperature). In this case the Ce ions can be pres− 72.96 (921) [53] ent on the surface. In the (co)precipitation process the parti− cle size may increase as a result of the ripening process. By 73.95 (664) [53] the sol−gel method the most uniform and spherical nano− 76.87 (932) [53] particles of yellow phosphor are obtained (the SEM micro− 78.80 (844) [53] graph are presented by D. Michalik et al. for a YAG:Ce pow− 81.67 (10.1.1) [53] ders obtained by the solid−state process [26], by Z. Le et al.for 84.51 (943) [53] a phosphor obtained by (co)precipitation process [29] and N. 87.33 (864) [53] Kaithwas et al. for YAG:Ce obtained by sol−gel methods [50]) 89.23 (10.4.2) [53] [8,17–18,23,26–27,29,36,50,55].

3.5. Fluorescence (PL) spectroscopy reported to be 20–100 nm (estimated using Scherrer’s equa− tion, Dhkl=kl/bcosq, where D is the average grain size, k is YAG is an insulator material in which, by adding dopant, the dimensionless shape factor – Scherrer’s constant, l is new acceptor/donor levels appear, thus materials are created the X−ray wavelength, b is the full width at half maximum with optical properties based on the electron transitions be− intensity (FWHM) – expressed in radians, q is the Bragg dif− tween these energy levels. The YAG:Ce and YAG:Ce, Re fraction angle) [23,27,36,48]. phosphors may convert the blue light from commercial blue chips to yellow light as a result of the Ce3+ inter−shell f−d 3.3. Energy dispersive X-ray spectroscopy (EDX) transition in the YAG lattice. The blue absorption corre− sponding to Ce3+ 4f (4f1) ® 5d (4f05d1) and the yellow The elemental composition of the YAG:Ce and YAG:Ce, emission occurring from 5d ® 4f transition of Ce3+. The 5d 2 Re phosphor can be examined by Energy dispersive X−ray are the lowest excited state split by crystal field into D3/2 2 2 2 spectroscopy (EDX). In the YAG:Ce phosphor spectra, and D5/2 levels and 4f ground state split in to F5/2 and F7/2 there can be seen peaks attributable to: O(K) at 0.525 keV, levels, which are separated by an energy difference of about Al(K) at 1.486 keV, Y(L) at 1.922 keV, Ce(L) at 4.839 keV 2200 cm–1 because of a spin−orbit coupling. The phase tran− and Ce(M) at 0.833 keV, Eu(L) at 5.845 keV and Eu(M) at sition from amorphous to crystalline of YAG:Ce and YAG: 1.131 keV, Gd(L) at 6.056 keV and Gd(M) at 1.185 keV, Ce, Re nanocrystals was brought into relief by optical spec− Tb(L) at 6.272 keV and Tb(M) at 1.240 keV, etc. The dop− troscopy. It was demonstrated, that the phosphors had lumi− ants concentration in the phosphor can be determined. The nescent properties only in a pure YAG phase. YAG:Ce and initial and final contents can be determined as the percent− YAG:Ce, Re phosphor show superior luminescence proper− age of each component, thus the presence of impurities ties, because of their high crystallinity [11,13,17,21–22, can be determined (N. Kaithwas, et al. present a EDX spec− 25–27,73–74]. trum of the YAG:Ce, Re powder in Ref. 50) [2–22,37, The excitation spectra for the YAG:Ce and YAG:Ce, Re l 50,58,72]. powders, may be recorded at em = 620 nm, and reveal three

Opto−Electron. Rev., 23, no. 4, 2015 V. Tucureanu 245 Synthesis and characterization of YAG:Ce phosphors for white LEDs

Table 4. Emission wavelength and lattice constants for phosphors. l Formula Lattice parameter () em. (nm) Ref

Y3Al5O12 12 – [18,31,35,38,68,71] Solide state method, 1000–1900°C, x = 0.001–0.12 Comercial garnet phosphor 12.016 550 [64,76]

Y3–xAl5O12:Cex 12.0267–12.0275 520 [71]

Y3–xAl5O12:Cex 12.006–12.012 530–540 [3,14,19,21,30,34–35]

Y3–xAl5O12:Cex – 550–560 [22–23]

Y3–x–yAl5O12:Cex, Gdy 12.106 550 [30,35]

Y3–x–yAl5O12:Cex, Tby 12.089 540–550 [14,24]

Y3–x–yAl5–zO12:Cex, GdyGaz 12.145–12.151 – [9]

Y3–x–yAl5O12:Cex, Pry – 550 + 610 [14] (Co)precipitation method, 1000–1200°C, x = 0.003–0.12

Y3–xAl5O12:Cex – 530–535 [23,27,36,41,49] Sol–gel method, 1000–1400°C, x = 0.005–0.12

Y3–xAl5O12:Cex 12.017–12.023 520–545 [23,29 47,48,50,54,78]

Y3–x–yAl5O12:Cex, Gdy 12.0203 567 [53]

Y3–x–yAl5O12:Cex, Smy – 540–550+617 [51] Combustion method, 500–1000°C

Y3–xAl5O12:Cex 12.0866–12.1012 528–537 [10,16,25]

Y3–xAl5–yO12:Cex, Gay 12.2207–12.2658 515–545 [16,25]

Y3–x–yAl5O12:Cex, Tby – 543 [25] phaY3–x–yAl5O12:Cex, Pry – 540 + 610 [15] Glyco−thermal method, 600–1100°C

Y3–xAl5O12:Cex – 530–550 [32,74]

Y3–xAl5–yO12:Cex, Gay – 568 [74] Spray pyrolysis, 1000–1100°C

Y3–xAl5O12:Cex 12.0267–12.0275 521 [71] Molten salt synthesis, 1000–1100°C

Y3–xAl5O12:Cex – 531 [8] bands at about 230, 340 and 450–470 nm, corresponding to emission wavelength and lattice parameters for some type 4f1®5d1 transitions in an electronic configuration of Ce3+ of phosphors used in white LED manufacturing process are and a crystal field splitting of the 5d state forms resulting presented [3,13,27,29]. from Ce3+ sites in a distorted garnet cube. The band at about By comparing with a reference light source with similar 450–470 nm makes possible the use of YAG:Ce and colour temperature, the CRI (colour−rendering index) para− YAG:Ce, Re yellow phosphors with blue InGaN chips. The meter indicates how well a light source renders colours. CRI commercial chips have an emission at about 450–480 nm, scale 0–100. CRI is influenced and improved by emission being in agreement with the excitation bands of phosphors light from the LED chips and from the yellow phosphors. (K.V.K. Gupta et al. present a general profile for excitation One of the problems of YAG:Ce refers to the poor CRI and emission spectra for YAG:Ce phosphor in Ref. 6) [6,13, (about 60–70) due to the lack of a red spectral component, 26–27,57]. but by co−doping it has managed a slight increase in The emission spectrum of YAG:Ce and YAG:Ce, Re may CRI > 80. A white LED with a CRI = 93 was obtained by be recorded under the excitation wavelength of 450–470 nm, using a powder mixture between a YAG:Ce and red−emit− 2+ and consists of a single band located between 520–550 nm, ting CaSiAlN3:Eu . Optical properties of white LEDs ba− for YAG:Ce and 550–570 nm, for YAG:Ce, Re and which sed on YAG:Ce or YAG:Ce,Re phosphors are presented in may be attributed to the electron transition from the 5d level Table 5 [4,6,13,14,22,30,41,58,74–76]. lowest crystal splitting component to the ground state (4f) of By careful control of the chemical composition of the Ce3+. The 5d ® 4f emission of the Ce is influenced by the garnet phosphor type, the emission can be tuned in order to crystal field and it is sensible to the crystallographic envi− improve CRI. Substituting the Y3+ or Al3+ introduces a red ronment, that determines the red or blue shift. In Table 4 (to a longer wavelength) or blue (to a shorter wavelength)

246 Opto−Electron. Rev., 23, no. 4, 2015 © 2015 SEP, Warsaw Table 5: Optical properties of white LEDs based on YAG:Ce or YAG:Ce, Re phosphors. CIE chromaticity Color temperature Lumen output Product CRI Ref XY (K) (lm/W) Blue LED, 450 nm 0.110 0.08 – – – [13] White LED (ideal) 0.330 0.330 – – – [4,13,41] White LED commercial (from warm to neutral white) 0.310–0.430 0.305–0.480 2700–6500 70–95 76–128 [6,79]

Y3–xAl5O12:Cex commercial 0.412 0.544 – – – [75]

Y3–xAl5–yO12:Cex, Gay 0.306 0.521 – – – [75] commercial

Y3–xAl5–yO12: Tbx, Gay 0.355 0.557 – – – [75] commercial Comercial garnet phosphor 0.449 0.555 – – – [76] White LED based on:

Y3–xAl5O12:Cex 0.290–0.300 0.296–0.333 6700–8245 67–83 23–73 [6,14,74,80]

Y3–xAl5O12:Cex 0.317 0.357 6112 134 73 [30]

Y3–xAl5O12:Cex 0.360–0.410 0.590–0.560 – – – [13]

Y3–x–yAl5O12:Cex, Pry 0.296 0.321 – 83 14–17 [14,80]

Y3–x–yAl5O12:Cex, Gdy 0.329–0.405 0.317–0.361 5256 82–70 95–109 [30]

Y3–x–yAl5O12:Cex, Tby 0.322 0.326 – 80 28 [14]

Mixture of Y3–xAl5O12:Cex and Y3–x–yAl5O12:Cex, Gdy 0.312 0.320 6564 86 – [74]

Mixture of Y3–xAl5O12:Cex 2+ and CaSiAlN3:Eu 0.431 0.395 3007 93 68 [22]

Y3–xAl5–zO12:Cex, Pry, Gaz 0.289 0.330 – 78 13 [80]

shift, but sometimes it is not enough to compensate for the asing the sintering temperature and some flux (i.e.: NH4Cl, absence of the red component in the phosphor. A solution to H3BO3 or Li2CO3) leads to a blue shift in the spectra (about this problem can be the co−doping of the YAG:Ce with 5–10 nm), which shows a shift from 2200 cm–1 to a lower a rare earth (i.e.: Eu3+,Sm3+,Pr3+) which can act like a sec− energy level and changes in a crystal field around Ce3+. ondary luminescence centre originated from an f−f transition A red shift may be induced by using sintering under differ− that is much weaker than the d−f transition form of Ce3+.In ent atmospheres. The CO favours the Ce4+ ® Ce3+ and the 3+ this situation the emission spectra are characterized by 2 or insertion in the YAG lattice. In air, Ce is oxidized in CeO2 3 emission peaks that can be attributed to Ce3+ emission and and decreases the emission intensity [10,11,13–15,27,29, to Re3+ emission (Eu3+ 590, 612 and 653 nm by excited at 51,57]. 240/355 nm and 593 nm by excited at 450 nm, Sm3+ 616 nm By co−doping, a lattice distortion is produced leading to by excited at 404 nm, Pr3+ 610 nm by excited at 450– a higher covalence and the energy difference between the 470 nm) (K. Li et al. present an emission spectra of excited and the ground state gets smaller. Rare earth ions, YAG:Ce, Re, Re – Gd for red shift [74] and Y. Pan Y et al. Re3+ may induce changes in the YAG:Ce photolumines− present a YAG:Ce, Re, and YAG:Ce, Re – Eu, Pr, Sm as cence spectra by shifting the peak from 525–530 nm to secondary luminescence centre [13]). a longer wavelength, at about 550–570 nm, but they also With increasing of the Re3+ concentration, a decrease in may produce a decrease of the emission intensity by incre− the Ce and Re (where Re is a secondary light centre, i.e.: Pr) asing the concentration of Re3+. The red shift can be explai− emission appears as a result of structural change and energy ned by the increase of the d−d orbital splitting, as a result of transfer from Ce3+ to Re3+. Also, by increasing the Ce3+ substituting Y3+ with Re3+, when around Ce3+ ions the crys− concentration (from 1 to 15 mol% of Y3+) introduces a red tal field becomes strong. But, at a higher Re3+ concentration shift, from 515 nm until 550 nm. Emission intensity de− (when b ³ 0.8) the crystal field gets soft, after increasing the creases with the increase of the Ce3+ concentration, and the 5d level by crystal splitting of Ce3+, and favours the forma− shape of the emission spectra does not change. A possible tion of YAP or other perovskite structure. The decrease in explanation for this may be Ce–Ce interactions, as a result the emission intensity can be explained by perturbating the of changes in the unit cell parameters. The maximum shift structure (lattice distortion), a possible competition between was found to appear for 5–6 mol%. As well, by varying the Ce ions and Re ions and energy transfer between lu− synthesis parameters, a shorter shift can be induced. Incre− minescence ions [10,15,18,30,41,69].

Opto−Electron. Rev., 23, no. 4, 2015 V. Tucureanu 247 Synthesis and characterization of YAG:Ce phosphors for white LEDs

The emission intensity is influenced by the sintering tem− vices. Since 1996, when YAG:Ce was reported in white LEDs perature and other process parameters (i.e.: raw material con− technology for the first time, some problems (i.e: the lack of centration). The spectra show a direct correlation between the the red component) have been observed and scientists who sintering temperature and brightness [26,77]. Emission wa− have tried to solve them by choosing different methods of syn− velength is not influenced by the sintering temperature, but thesis, by parameters variation of the process, etc. This article the emission intensity is proportional to the temperature vari− has provided an overview of the most important synthesis ation in case of all the processes. With the increase of temper− methods and characteristic parameters of the garnet phosphors ature, the intensity of emission increases too from no emis− powders. The solid−state reaction is the most commonly used sion (at a temperature below 900°C) to a strong intensity (at method, the most controversial but the most efficient one to a temperatures higher than 1000– 1500°C). At lower temper− obtain phosphors at industrial level for optoelectronic applica− atures the Ce4+ ® Ce3+ transition may be incomplete, or tions. The main problems for solid−state technology are higher Ce3+ cannot enter totally in the YAG lattice – in this case, sintering temperatures (usually 1500–1900°C) and intermedi− Ce3+ ions can be present on the surface. Higher temperatures ate milling steps that lead to defects and to production price in− of synthesis increase the solubility of dopant ions in the YAG crease. By introducing wet methods, such as (co)precipitation crystal lattice, thus leading to a larger number of ions active and sol−gel, a number of advantages in technical and economic in the luminescence process [26,48,57]. Thus, the emission terms, at a research scale have been achieved, but they are dif− intensity may improve by increasing the homogeneous distri− ficult to use at an industrial scale. The main advantage of these bution of Ce3+ ions in YAG. methods is the sintering temperature (usually 1000–1200°C) The emission intensity may be improved by using the that is lower than in the solid−state process. In this review we fluxes in the sintering steps, due to the improvement of present the phase analysis with FTIR spectrometry, X−ray dif− YAG crystallization processes and the stabilization of triva− fraction and energy dispersive X−ray spectroscopy. The phos− lent ions in YAG lattice at high temperature [26,47]. This phor FTIR spectra demonstrate the transition from the amor− shows the influence of particle size due to the increased phous phase to the crystalline one of YAG, showing only the crystalline area and light emitting region with increasing peaks below 900 cm–1, also the diffraction pattern of phos− particle size. An increase of the brightness and improve− phors confirms the amorphous phase below 900°C, coexis− ment of the resolution have been observed while using tence of intermediate stages in case of incomplete thermal pro− spherically shaped phosphors due to lower scattering and cesses, phases that affect the luminescence properties. For higher packing densities. Defects on the particles surface a complete process the phosphor XRD spectrum reveal a pure could affect the luminescent properties. Increasing the con− garnet phase having a crystal structure with 160 atom body− centration of co−doped ions, above a certain limit, can lead −centred cubic (bcc) unit cell, 80 atoms in primitive cell, and to the red shift while the emission intensity decreases [27]. Ia−3d (230) space group. EDX analysis shows the elementary 3+ Some types of co−dopant ions (i.e.: Dy ) lead to a sharp composition. The SEM data confirm the desired spherical decrease in emission intensity [14]. shape with smooth surfaces at nanoscale. Also, by SEM the It can be said that the variation of emission intensity is undesired agglomeration tendency may be observed. Finally, it solid state > sol−gel > co(precipitation), under CO flow at was shown that the optical properties were determined by the ° 1500 C/10 h and sol−gel > co(precipitation) > solid state, synthesis method, reaction conditions (from raw materials ratio under CO flow at 1000°C/5 h, due to the variation of parti− up to the sintering temperature), the purity of the product, the cle size and morphology [23,26]. crystal structures, lattice parameter crystallite size, surface All the synthesis steps determine the wavelength and morphology, the LED chips, and so on. Also, for improving emission intensity. It was found an increasing of the emission the optical properties, a shift may be introduced in the spec− intensity from the powder obtained by the solid state reaction, trum by substituting Y3+ or Al3+ for a co−dopant (YAG:Ce, up to (co)precipitation and the highest intensity appears in the Re), by adding ions as secondary sources and varying the Ce3+ phosphors obtained by sol−gel methods [23]. It has been ob− concentration and other process parameters. Generally, the ex− served that scattering effects are less important if compared to citation spectra for the YAG:Ce powders reveal three bands at the emission yield of the phosphor [26]. YAG:Ce and about 230, 340 and 450–70 nm and the emission spectra con− YAG:Ce, Re phosphor showed high photoluminescence sists of a single band located between 520–550 nm. By a care− quantum yield efficiency (QY) of above 35–70% upon ful control of the garnet phosphor chemical composition the 450–470 nm excitation. The solid state is used to manufac− emission can be tuned and by substituting the Y3+ or Al3+ ared ture the commercial YAG:Ce powder (Phosphor Tech) with or blue shift may be introduced. the best QY (about 90%) [43].

4. Conclusions Acknowledgement

Yttrium aluminium garnet (YAG) is an A3B2C3O12 class insu− This work was supported by National Basic Funding Pro− lator material, in which new acceptor/donor levels appear by gram CONVERT− Project No. PN09290301 (2008–2013). adding dopants. Thus, materials are created with optical prop− Authors would like to acknowledge all the support from erties suitable for the manufacturing of white light emitting de− Ileana Cernica.

248 Opto−Electron. Rev., 23, no. 4, 2015 © 2015 SEP, Warsaw 3+ References Y3Al5O12:Ce phosphor via Pr co−doping and Tb substitu− tion for the application to white LEDs”, J. Lumin. 126, 1. M. Cates, S. Allison, Phosphor Thermometry, OAK Ridge 371–377 (2007) doi:10.1016/j.jlumin.2006.08.09 National Laboratory, US Departament of Energy, http://web. 15. S. Chawla, T. Roy, K. Majumder, and A. Yadav “Red en− ornl.gov/sci/phosphors/Pdfs/tutorial.pdf. hanced YAG:Ce, Pr nanophosphor for white LEDs”, J. Exp. 2. P. Rai, M.K. Song, H.M. Song, J.H. Kim, Y.S. Kim, I.H. Nanosci. 1–9 (2012) doi:10.1080/17458080.2012.714481 Lee, and Y.T. Yu, “Synthesis, growth mechanism and photo− 16. R.A. Hansel, S.W. Allison, and D.G. Walker, “Temperature luminescence of monodispersed cubic shape Ce doped YAG dependent fluorescence of Ce−doped garnets for use as ther− nanophosphor”, Ceram. Int., 38, 235–242 (2012) doi:10. mographic phosphors” (2008) http://telab.vuse.vanderbilt. 1016/j.ceramint.2011.06.057. edu/docs/hansel08−MRS.pdf 3. R. Marin, G. Sponchia, P. Riello, R. Sulcis, and F. Enrichi, 17. K. Toda, “New processing of LED phosphors”, Trans. Elec− “Photoluminescence properties of YAG:Ce3+,Pr3+ phos− trical And Electronic Materials 13, 225–228 (2012) doi: phors synthesized via the Pechini method for white LEDs”, http://dx.doi.org/10.4313/TEEM.2012.13.5.225. J. Nanopart. Res. 14, 886, 1–13 (2012) doi: 10.1007/ s11051 18. A. Saat, H. Harun, and Z. Hamzah, “Synthesis and character− –012–0886–5. ization of YAG:Ce prepared by solid state reaction method”, Malaysian J. Anal. Sci. 15 4. S. Nishiura, S. Tanabe, K. Fujioka, Y. Fujimoto, and M. , 1, 101–105 (2011). Nakatsuka, “Preparation and optical properties of transparent 19. S.C. Huang, J.K. Wu, and W.J. Hsu, “Particle size effect on Ce:YAG ceramics for high power white LED” in IUMRS− the packaging performance of YAG:Ce phosphors in white −ICA 2008 Symposium, “AA. Rare−earth related material LEDs”, Int. J. Appl. Ceram. Technol. 6, 465–469 (2009). processing and functions”, IOP Conf. Series: Mater. Sci. 20. C.S. Chou, C.Y. Wu, C.H. Yeh, R.Y. Yang, and J.H. Chen, Eng. 1, 012031 (2009) doi:10.1088/1757–8981/1/1/012031. “The optimum conditions for solid−state−prepared (Y3–xCex) Al O phosphor using the Taguchi method”, Adv. Powder 5. A.A. Setlur, “Phosphors for LED−based solid−state lighting”, 5 12 Technol. 23, 97–103 (2012) doi:10.1016/j.apt.2010.12.016. The Electrochemical Society Interface 16, 32–36 (2009), 21. C.W. Won, H.H. Nersisyan, H.I. Won, J.H. Lee, and K.H. http://www.electrochem.org/dl/interface/wtr/wtr09/wtr09_ Lee, “Eficient solid−state route for the preparation of spheri− p032−036.pdf. cal YAG:Ce phosphor particles”, J. Alloy. Compd. 509, 6. K.V.K. Gupta, A. Muley, P. Yadav, C.P. Joshi, and S.V. 2621–2626 (2011) doi:10.1016/j.jallcom.2010.11.143. Moharil, “Combustion synthesis of YAG:Ce and related 22. C.C. Lin, Y.S. Zheng, H.Y. Chen, C.H. Ruan, G.W. Xiao, phosphors”, Appl. Phys. B, 105, 479–484 (2011) doi: 10. and R.S. Liu, “Improving optical properties of white LED 1007/s00340−011−4685−y. fabricated by a blue LED chip with yellow/red phosphors”, J. 7. C.A. Geiger, “Garnet: A key phase in nature, the laboratory, Electrochem. Soc. 157, H900–H903 (2010). and technology”, Elements 9, 447–452, (2013) doi: 10.2113/ 23. Y. Pan, M. Wu, and Q. Su, “Comparative investigation on gselements.9.6.447. synthesis and photoluminescence of YAG:Ce phosphor”, 8. C. Wu, A. Luo, G. Du , X. Qin, and W. Shi, “Synthesis and Mat. Sci. Eng., 106 3+ , 251–256 (2004) doi:10.1016/j.mseb. luminescent properties of nonaggregated YAG:Ce phos− 2003.09.031. phors via the molten salt synthesis method”, Mat. Sci. Semi− 24. Y.S. Lin, and R.S. Liu, “Chemical substitution effects of con. Proc. 16, 679–685 (2013) http://dx.doi.org/10.1016/j. Tb3+ in YAG:Ce phosphors and enhancement of their emis− mssp.2012.12.008. sion intensity using flux combination”, J. Lumin. 122–123, 9. M. Faheem and K. Lynn, “Structural and thermal properties 580–582 (2007) doi:10.1016/j.jlumin.2006.01.230. of Tb, Ce Doped Y2.97Gd0.03Al2Ga3O12 single crystals”, 25. M. Upasani, B. Butey, and S.V. Moharil, “Luminescence American J. Anal. Chem. 5, 695–700 (2014), http://dx.doi. studies on lanthanide ions (Gd3+,Tb3+) doped YAG:Ce org/10.4236/ajac.2014.511078. phosphors by combustion synthesis”, IOSR–JAP 6, 28–33 10. P.J. Yadav, C.P. Joshi, and S.V. Moharil, “Combustion synthe− (2014) www.iosrjournals.org sis of multicomponent ceramic phosphors for solid state ligh− 26. D. Michalik, M. Sopicka−Lizer, J. Plewa, and T. Pawlik, ting”, Int. J. of Self−Propagating High−Temperature Synthesis, “Application of mechanochemical processing to synthesis of 21, 1, 32–37 (2012) doi: 10.3103/S1061386212010 13X. YAG:Ce garnet powder”, Archives Of Metallurgy and Mate− 11. H.K. Yang, H.M. Noh, and J.H. Jeong, “Low temperature rials 56, 1257–1264 (2011). synthesis and luminescence investigations of YAG:Ce, Eu 27. K. Zhang, H. Liu, Y. Wu, and W. Hu, “Synthesis of (Y, nanocomposite powder for warm white light−emitting di− Gd)3Al5O12:Ce nanophosphor by co−precipitation method ode”, Solid. State Sci., 27, 43–46 (2014) http://dx.doi.org/ and its luminescence behavior”, J. Mater. Sci. 42, 9200– 10.1016/j.solidstatesciences.2013.11.007. 9204 (2007) doi: 10.1007/s10853–007–1913–2. 12. O. Milosevic, L. Mancic, M.E. Rabanal, J.M. Torralba, B. 28. Z. Na, W. Dajian, L. Lan, M. Yanshuang, Z. Xiaosong, and Yang, and P. Townsend, “Structural and luminescence prop− M. Nan, “YAG:Ce Phosphors for WLED via Nano−Pesudo− 3+ 3+ erties of Gd2O3:Eu and Y3Al5O12:Ce phosphor particles boehmite Sol−Gel Route”, J. Rare Earth 24, 294–297 (2006). synthesized via aerosol”, J. Electrochem. Soc. 152,9, 29. Z. Le, L. Zhou, Z. Jinzhen, Y. Hao, H. Pengde, C. Yan, and G707–G713 (2005). Z. Qitu, “Citrate sol−gel combustion preparation and photo− 13. Y.Pan, M.Wu, and Q.Su, “Tailored photoluminescence of luminescence properties of YAG:Ce phosphors”, J. Rare YAG:Ce phosphor through various methods”, J. Phys. Earth 30, 289–296 (2012) doi: 10.1016/S1002–0721(12) Chem. Solids, 65, 845–850 (2004) doi:10.1016/jpcs.2003. 60040–4. 08.018. 30. H. Shi, C. Zhu, J. Huang, J. Chen, D. Chen, W. Wang, F. 14. H.S. Janga, W.B. Ima, D.C. Leeb, D.Y. Jeona, and S.S. Kim, Wang, Y. Cao, and X. Yuan, “Lumine scence properties of “Enhancement of red spectral emission intensity of YAG:Ce, Gd phosphors synthesized under vacuum condi−

Opto−Electron. Rev., 23, no. 4, 2015 V. Tucureanu 249 Synthesis and characterization of YAG:Ce phosphors for white LEDs

tion and their white LED performances”, Opt. Mater. Ex− 44. Y. Lv, W. Zhang, H. Liu, Y. Sang, H. Qin, J. Tan, and L. press 4, 449–655 (2014) doi:10.1364/OME.4.000649 Tong, “Synthesis of nano−sized and highly sinterable Nd: 31. A.B. Munoz Garcia, “First−principles studies on Ce−doped YAG powders by the urea homogeneous precipitation me− 3+ Powder Techn. 217 YAG (Y3Al5O12), Codoping, antisite defects and Ce 4f−5d thod”, , 140–147 (2012) doi:10.1016/j. transitions”, Autonomous University of Madrid, Faculty of powtec. 2011.10.020 Science, Department of Chemistry, Dissertation Thesis (2011) 45. E. Garskaite, D. Jasaitis, and A. Kareiva, “Sol−gel prepara− https://repositorio.uam.es/bitstream/handle/10486/6278/ tion and electrical behaviour of Ln:YAG (Ln=Ce,Nd,Ho, 38176_mu%C3%B1oz_garcia_ana_belen. pdf?sequence=1 Er)”, J. Serb. Chem. Soc. 68, 8–9, 677–684 (2003). 32. N. Pradal, G. Chadeyron, A. Potdevin, J. Deschamps, and R. 46. D. Jia, “Nanophosphors for white light LEDS”, Chem. Eng. Mahioub, “Elaboration and optimization of Ce−doped Commun. 194, 1666–1687 (2007) doi: 10.1080/009864407 Y3Al5O12 nanopowder dispersions”, J. Eur. Ceram. Soc. 33, 01446359. 1935–1945 (2013) http://dx.doi.org/10.1016/j.jeurceramsoc. 47. C.J. Liu, R.M. Yu, Z.W. Xu, J. Cai, X.H. Yan, and X.T. Luo, 2013.02.004 “Crystallization, morphology and luminescent properties of 33. S.M. Kaczmarek, G. Domianiak−Dzik, W. Ryba−Romanow− YAG:Ce3+ phosphor powder prepared by polyacrylamide ski, J. Kisielewski, and J. Wojtkowska, “Changes in optical gel method”, Trans. Nonferrous Met. Soc. China 17, properties of Ce: YAG crystals under annealing and irradia− 1093–1099 (2007). tion processing”, Cryst. Res. Technol. 34, 1031–1036 (1999). 48. X.H. Yan , S.S. Zheng, R.M Yu, J. Cai, Z.W. Xu, C.J. Liu, 34. H.M. Lee, C.C. Cheng, and C.Y. Huang, “The synthesis and and X.T. Luo, “Preparation of YAG:Ce3+ phosphor by sol− optical property of solid−state prepared YAG:Ce phosphor −gel low temperature combustion method and its luminescent by a spray−drying method”, Mat.Res.Bull. 44, 1081–1086 properties”, Trans. Nonferrous Met. Soc. China 18, 648–653 (2009) doi:10.1016/j.materresbull.2008.10.006. (2008). 35. A. Lakshmanan, R.S. Kumar, V. Sivakumar, P.C. Thomas, 49. P.A. Tanner, L. Fu, L. Ning, B.M. Cheng, and M.G. Brik, and M.T. Jose, “Synthesis, photoluminescence and thermal “Soft synthesis and vacuum ultraviolet spectra of YAG:Ce3+ quenching of YAG:Ce phosphor for white light emitting di− nanocrystals: reassignment of Ce3+ energy levels”, J. Phys.: odes”, Indian J. Pure Ap. Phy. 49, 303–307 (2011). Condens. Matter 19, 216213–216227 (2007) doi:10.1088/ 36. T. Kim and J.K. Lee, “Template−free Synthesis and charac− 0953–8984/19/21/216213 3+ terization of spherical Y3Al5O12:Ce (YAG:Ce) nanoparti− 50. N. Kaithwas, M. Dave, S. Karb, and K.S. Bartwal, “Struc− cles”, Bull. Korean Chem. Soc. 35, 2917–2921 (2014) http: tural features of Ce doped YAG nanoparticles synthesized //dx.doi.org/10.5012/bkcs.2014.35.10.2917 by modified sol–gel method”, Physica E 44, 1486–1489 37. P.C. Lin, C.H. Huangb, and W.R. Liu, “An efficient nitrida− (2012) http://dx.doi.org/10.1016/j.physe.2012.03.015. tion approach to enhance luminescent intensity of YAG : 51. H. Sun, X. Zhang, and Z. Bai, “Synthesis and characteriza− Ce3+ phosphor by using hexamethylenetetramine”, J.Ceram. tion of nano−sized YAG:Ce,Sm spherical phosphors”, J. Process. Res. 15, 185–188 (2014). Rare Earths, 31, 3, 231–234 (2013 ). 38. C.Q. Li, H.B. Zuo, M. F. Zhang, J.C. Han, and S.H. Meng, 52. E.M. Loiko, L. Lipinska, J.Cz. Dobrowolski, and A. Rzepka, “Fabrication of transparent YAG ceramics by traditional “Studies on sol−gel processes accompanying formation of the solid−state−reaction method”, Trans. Nonferrous Met. Soc. yttrium aluminum garnet nanocrystals”, Materiały Elektro− China 17, 148–153 (2007). niczne T. 34, 3/4 (2006) http://rcin.org.pl 39. M. Zeng, Y. Ma, Y. Wang, and C. Pei, “The effect of precipi− 53. V. Schiopu, A. Matei, A. Dinescu, M. Danila, and I. Cernica, tant on co−precipitation synthesis of yttrium aluminum gar− “Ce, Gd codoped YAG nanopowder for white light emitting net powders”, Ceram. Int. 38, 6951–6956 (2012) http://dx. device”, J. Nanosci. Nanotechnol. 12, 8836–8840 (2012). doi.org/10.1016/j.ceramint.2012.05.066. 54. A. Lukowiak, R.J. Wiglusz, M. Maczka, P. Gluchowski, and 40. Y.T. Niena, Y.L. Chena, I.G. Chena, C.S. Hwanga, Y.K. W. Strek, “IR and Raman spectroscopy study of YAG nano− Sub, S.J. Changb, and F.S. Juang, “Synthesis of nano−scaled ceramics”, Chem. Phys. Lett. 494, 279–283 (2010) doi:10. yttrium aluminum garnet phosphor by co−precipitation me− 1016/j.cplett.2010.06.033. thod with HMDS treatmen”, Mat. Chem. Phys. 93, 79–83 55. A. Sahraneshin, S. Takami, K. Minami, D. Hojo, T. Arita, (2005) doi:10.1016/j.matchemphys.2005.02.017 and T. Adschiri, “Synthesis and morphology control of sur− 41. S. Nishiura, S. Tanabe , K. Fujioka, and Y. Fujimoto, “Prop− face functionalized nanoscale yttrium aluminum garnet par− erties of transparent Ce:YAG ceramic phosphors for white ticles via supercritical hydrothermal method”, Prog. Cryst. LED”, Opt. Mater. 33, 688–691 (2011) doi:10.1016/j.opt− Growth. Ch. Mat. 58, 43–50 (2012) doi:10.1016/j. pcrys− mat.2010.06.005 grow.2011.10.004. 42. R. Hana, D. Gaob, and K. Chen, “Y3Al5O12 nanocrystallites 56. M. Zarzecka−Napierala and K. Haberko, “Synthesis and cha− prepared from a novel crystalline precursor”, Adv Mat Res racterization of yttrium aluminium garnet (YAG) powders” 306–307, 1142–1147 (2011). Process. Appl. Ceram. 1, 69–74 (2007). 43. A. Potdevin, N. Pradal, M.L. François, G. Chadeyron, D. 57. G. Xia, S. Zhou, J. Zhang, and J. Xu, “Structural and optical Boyer, and R. Mahiou, “Microwave−induced combustion properties of YAG:Ce3+ phosphors by sol–gel combustion synthesis of luminescent aluminate powders”, (cap 9), pp. method”, J.Cryst. Growth 279, 357–362 (2005) doi:10. 189–212, Sintering – Methods and Products, Dr. Volodymyr 1016/j.jcrysgro.2005.01.072. Shatokha (Ed.), ISBN: 978–953–51–0371–4, InTech, 2012 58. M. Veith, S. Mathur, A. Kareiva, M. Jilavi, M. Zimmer, and http://www.intechopen.com/books/ sintering−methods−and− V. Huch, “Low temperature synthesis of nanocrystalline −products/microwave−induced−combustion−synthesis−of−lu− Y3Al5O12 (YAG) and Ce−doped Y3Al5O12 via different sol− minescent−aluminate−powders −gel methods”, J. Mater. Chem. 9, 3069–3079 (1999).

250 Opto−Electron. Rev., 23, no. 4, 2015 © 2015 SEP, Warsaw 59. R. Choudhary, K. Laishram, and R.K. Gupta, “Rapid synthe− 70. B. Huang, Y. Ma, S. Qian, D. Zou, G. Zheng, and Z. Dai, sis of Nd:YAG nanopowder by microwave flash combus− “Luminescent properties of low−temperature−hydrothermal− tion”, Mat. Sci.–Poland 27, 1025–1031 (2009). ly−synthesized and post−treated YAG:Ce (5%) phosphors”, 60. J.A. Jeong, K. Park, D.H. Lee, H.G. Kim, and Y.Y. Kim, Opt. Mat. 36, 1561–1565 (2014) http://dx.doi.org/10.1016/j. “The Characteristics of YAG:Ce phosphor powder prepared optmat.2014.04.025. – using a NO3 – Malonic Acid–NH4NO3–NH3·H2O system”, 71. L. Mancica, K. Marinkovica, B.A. Marinkovicb, M. Drami− Bull. Korean Chem. Soc. 33, 1141–1146 (2012) http://dx. caninc, and O. Milosevica, “YAG:Ce3+ nanostructured parti− doi.org/10.5012/bkcs.2012.33.4.1141. cles obtainedvia spray pyrolysis of polymeric precursor solu− 61. G. He, G. Liu, S. Guo, Z. Yang, and J. Li, “Crystallization of tion”, J. Eur. Ceram. Soc. 30, 577–582 (2010) doi:10.1016/ 3+ Y3Al5O12:Ce glass microspheres prepared by flame−spray− j.jeurceramsoc.2009.05.037 ing synthesis”, J Mater Sci: Mater Electron 26, 72–76 72. www.jeol.com. (2015) DOI 10.1007/s10854–014–2365–5. 3+ 73. L. Seijo, B. Zarandiaran,“4f and 5d levels of Ce in D2 eigh− 62. K. Zhang, W. Hu, Y. Wu, and H. Liu, “Influence of process− tfold oxygen coordination”, Opt. Mater. 35, 1932 (2013). ing techniques on the properties of YAG:Ce nanophosphor”, 74. K. Li, C and Shen, “White LED based on nano−YAG:Ce3+/ Ceram. Int. 35, 719–723 (2009). YAG:Ce3+,Gd3+ hybrid phosphors”, Optik 123, 621–623 3+ 63. H.M.H. Fadlalla and C.C. Tang, “YAG:Ce nano−sized par− (2012) doi:10.1016/j.ijleo.2011.06.005. ticles prepared by precipitation technique”, Mat. Chem. 75. http://www.phosphor−technology.com/products/crt.htm, Phys. 114, 99–102 (2009). PTL Grade: QMK58/N−C1, QMK58/F−C1, MPK58/S−C1, 64. Y.S. Cho, Y.D. Huh, C.R. Park, and Y.R. Do, “Preparation QMPK65/N−C1. with laser ablation and photoluminescence of Y Al O :Ce 3 5 12 76. http://www.phosphortech.com. nanophosphors”, Electron. Mater. Lett. 10, 461–465 (2014) 77. J.D. Furman, G. Gundiah, K. Page, N. Pizarro, and A.K. DOI: 10.1007/s13391–014–4024–7. Cheetham, “Local structure and time−resolved photolumi− 65. W. Peng, S. Jun, T. Hua, L. Qi−Fei, and W. Da−Jian, “Ther− nescence of emulsion prepared YAG nanoparticles”, Chem. mal stability of luminous YAG: Ce bulk ceramic as a remote Phys. Lett. 465, 67–72 (2008) doi:10.1016/j.cplett.2008.09. phosphor prepared through silica−stabilizing valence of acti− 045. vator in air”, Optoel. Lett. 8, 0201–0204 (2012). 78. D.N. Chung, D.N. Hieu, T.T. Thao, V.V. Truong, and N.N. 66. http://abulafia.mt.ic.ac.uk/shannon/ptable.php. Dinh, “Synthesis and characterization of Ce−doped Y Al O 67. http://www.knowledgedoor.com/. 3 5 12 (YAG:Ce) nanopowders used for solid−state lighting”, J. Na− 68. S. Mukherjee, V. Sudarsan, R.K. Vatsa, and A.K. Tyagi, nomat. Hindawi Publishing Corporation, 1–7 (2014) http://dx. “Luminescence studies on lanthanide ions (Eu3+,Dy3+and doi.org/10.1155/2014/571920. Tb3+) doped YAG:Ce nano−phosphors”, J. Lumin. 129, 69–72 (2009) doi:10.1016/j.jlumin.2008.08.003. 79. http://www.osram−os.com. 69. F. Huang, L. Dong, Z. Fua, H. Wanga, W. Wanga, and Y. 80. H. Yang, D.K. Lee, and Y.S. Kim, “Spectral variations of Wang, “Study of co−excited green emission of Tb3+,Ce3+ nano−sized Y3Al5O12:Ce phosphors via codoping/substitution and Gd3+ in yttrium aluminum garnet”, J. Ceram. Process. and their white LED characteristics”, Mat. Chem. Phys. 114, Res. 10, 807–811 (2009). 665–669 (2009) doi:10.1016/j.matchemphys.2008.10.019.

Opto−Electron. Rev., 23, no. 4, 2015 V. Tucureanu 251