Eu3+/Tb3+-codoped Y2O3 nanophosphors: Rietveld refinement, bandgap and

photoluminescence optimization

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IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 45 (2012) 415102 (11pp) doi:10.1088/0022-3727/45/41/415102

Eu3+/Tb3+-codoped Y2O3 nanophosphors:Rietveld refinement, bandgap andphotoluminescence optimizationS Som and S K Sharma

Department of Applied Physics, Indian School of Mines, Dhanbad 826004, India

E-mail: sksharma.ism@gmail.com

Received 6 June 2012, in final form 6 August 2012Published 28 September 2012Online at stacks.iop.org/JPhysD/45/415102

AbstractIn this work, Eu-doped, Tb-doped and Eu, Tb-codoped Y2O3 nanophosphors were synthesizedby the combustion synthesis method. The prepared phosphors were characterized by x-raydiffraction (XRD), Rietveld refinement and Fourier transform infrared (FTIR) spectroscopy.XRD studies and Rietveld refinement confirmed the body-centred cubic structure of doped andcodoped phosphors. FTIR studies also confirmed the formation of these compounds. Thermalanalysis results indicated that there was no phase transition for all the phosphors in the studiedtemperature range. In the optical properties, diffuse reflectance (DR) and photoluminescence(PL) measurements were performed. DR spectra were used to determine the bandgap and itincreased in the doped and codoped samples due to the crystallite size effect. A strongcharacteristic emission from Eu3+ and Tb3+ ions was identified and the influence of dopingconcentration and annealing temperature on PL properties was systematically studied. Transferof energy was observed from Tb3+ to Eu3+ ions in the codoped phosphor at room temperature.

(Some figures may appear in colour only in the online journal)

1. Introduction

Rare earth compounds, especially the rare earth oxides,are of particular interest for luminescence and photonicapplications with good thermal and chemical stability. Amongthese oxides, yttrium oxide is a promising host material fordifferent photonic applications. It is used in plasma displaypanels (PDPs), field emission displays (FEDs), cathode raytubes (CRTs), fluorescent lamps, lasers and different detectordevices, etc [1–4].

Due to the similarities of chemical properties and ionicradii of Y3+ with the rare earth ions, Y2O3 is considered asthe best host for rare earth ions. Rare earth doped Y2O3

has excellent luminescent properties, such as narrow emissionlines and long luminescent lifetime. These properties areessential for its applications in solar cells, display devices andoptical communication. The rare earth emission from dopedY2O3 can also be enhanced by codoping with another rareearth ion. In codoped compounds, the non-radiative energytransfer from one rare earth ion to another in different hostshas also gained significant attention [5]. Terbium (Tb3+) and

europium (Eu3+) codoped compounds have been extensivelystudied due to their unique spectral properties [6–8]. Colourtunability and white light from Eu3+/Tb3+-codoped Y2O3

nanophosphors were achieved by Georgios et al [9] and Donget al [10]. In these works, the phosphors were synthesizedby spray pyrolysis and co-precipitation techniques and thephotoluminescence (PL) studies were mainly reported. Butthere is no detailed quantitative analysis carried out for thestructural, bandgap and CIE parameters.

This study deals with the structural, thermal and opticalcharacterization of Eu3+/Tb3+-codoped Y2O3 phosphorsprepared by the combustion method. The combustion method[11] yields single phase compounds with high purity andhomogeneity due to high in situ temperature generationduring the combustion reaction. The process is simple,inexpensive and less time consuming compared with spraypyrolysis and co-precipitation methods. This study addssome analytical analysis of the obtained experimental datato the existing literature on the studied phosphors. Thisanalytical analysis includes Rietveld refinement, calculationof structural and CIE parameters along with bandgap. The

0022-3727/12/415102+11$33.00 1 © 2012 IOP Publishing Ltd Printed in the UK & the USA

J. Phys. D: Appl. Phys. 45 (2012) 415102 S Som and S K Sharma

Figure 1. XRD pattern of the prepared samples for (a) different dopants and (b) annealing temperatures.

bandgap of these phosphors was calculated from diffusereflectance (DR) spectra using the Kubelka–Munk (K–M)function and Tauc relation. To the best of our knowledge,detailed calculations of these parameters were made for thefirst time. The above parameters were calculated in orderto see the suitable application of these phosphors in differentlighting devices. Moreover, the phenomena of concentrationquenching and energy transfer were discussed in detail withsupporting analytical calculations/graphs. An attempt was alsomade to optimize the PL with doping and annealing conditions.

2. Experimental

Eu-doped, Tb-doped and Eu,Tb-codoped Y2O3 nanophos-phors were prepared by the combustion synthesis method us-ing europium oxide (Eu2O3) (99.9%, Otto), terbium oxide(Tb4O7) (99.9%, Otto), yttrium oxide (Y2O3) (99.9%, Otto),nitric acid (HNO3) (72%, Rankem) and urea (CO(NH2)2)

(99%, SRL) as the starting raw materials. Stock solu-tions of Y(NO3)3, Eu(NO3)3 and Tb(NO3)3 were preparedby dissolving Y2O3, Eu2O3and Tb4O7 in nitric acid anddiluting with deionized water. Tb-doped samples wereprepared by mixing Y(NO3)3 and Tb(NO3)3 according tothe formula (Y1−xTbx)2O3 (x = 0.001–0.05). Similarly,Eu-doped samples were prepared by mixing Y(NO3)3 andEu(NO3)3 according to the formula (Y1−yEuy)2O3 (y =0.01–0.1). Eu,Tb-codoped samples were prepared by mixingY(NO3)3, Eu(NO3)3 and Tb(NO3)3 according to the formula(Y1−x−yTbxEuy)2O3 and by varying the x/y ratio. A suitableamount of urea was added to the mixture of the correspond-ing nitrate solution keeping urea to metal nitrate molar ratioas 2.5 [11]. The corresponding mixture was then dissolvedproperly to achieve a uniform solution and dried by heating at80 ◦C using a magnetic stirrer. Finally, the solid residue wastransferred to a silica crucible and heated at 600 ◦C in a furnacefor an hour. The synthesis reaction was as follows [11]:

(2 − 2x)Y(NO3)3 + 2xRE(NO3)3 + 5(NH2)2CO

→ (Y1−xREx)2O3 + 5CO2 + 8N2 + 10H2O.

X-ray diffractogram of the prepared phosphors was recordedin a wide range of Bragg angle 2θ (15◦ � 2θ � 85◦) usinga Bruker D8 advanced x-ray diffraction (XRD) measuringinstrument with Cu target radiation (λ = 0.154 056 nm).The Fourier transform infrared (FTIR) spectra were recordedin the wavenumber range 4000–400 cm−1 using a Perkin-Elmer make FTIR-2000 spectrometer. Thermal studieswere carried out on a Seiko make EXSTAR 6000 thermalanalyser in the range 25–1100 ◦C. The DR spectra wererecorded using a Perkin-Elmer make Lambda 950, UV–VIS–NIR spectrophotometer in the wavelength range 200–800 nm.PL studies were carried out on Hitachi make fluorescencespectrometer F-2500 in the range 220–650 nm.

3. Results and discussion

3.1. Structural analysis

3.1.1. XRD. To investigate the structure of the preparedcompounds, XRD patterns of the doped, codoped andcomercially available undoped Y2O3phosphors were recordedand compared with the JCPDS file No 83-0927, as shown infigure 1(a). The sharp and single diffraction peaks of the XRDpattern confirm the formation of single phase compounds,which is attributed to the high in situ temperature generatedduring the combustion reaction. The XRD peaks wereidentified and indexed according to the JCPDS database. The(h k l) values are labelled on the peaks. This implies thatthe doped and codoped phosphors are of body-centred cubicstructure with space group Ia-3. The peak at 2θ ∼ 29.5◦

was observed as the strongest peak corresponding to the plane(2 2 2) and no peaks were observed due to dopants.

Figure 1(b) shows the XRD pattern of Y2O3 : Tb3+ afterannealing at different temperatures from 500 to 1000 ◦C. Nochange in peak position was observed for all the annealedsamples. The 1000 ◦C annealed phosphor showed more narrowpeaks compared with the 500 ◦C annealed phosphor, which wasan indication of growth of crystallinity of the samples at hightemperatures.

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J. Phys. D: Appl. Phys. 45 (2012) 415102 S Som and S K Sharma

(a) (b) (c)(a)

Figure 2. (a) Rietveld refinement pattern, (b) schematic representation of the unit cell and (c) cationic symmetry site for the Y2O3 : Tb3+

phosphor.

Table 1. Structural parameters of Tb3+-doped Y2O3 after Rietveld refinementa.

Occupancy Occupancy Lattice constants Lattice constantsAtoms Wyckoff Symmetry x y z (600 ◦C) (1000 ◦C) (600 ◦C) (1000 ◦C)

Y1 8b C3i 0.250 0.250 0.250 0.069 0.167 a = b = c a = b = c

Y2 24d C2 0.968 0.000 0.250 0.321 0.500 10.5896 Å 10.6108 ÅO1 48e — 0.392 0.156 0.379 1.000 1.000

a Ia-3 (206)—cubic Rp = 5.65; Rwp = 7.64; Rexp = 3.12 and GoF= 2.4 for 600 ◦C; Rp = 4.91; Rwp = 7.15; Rexp = 3.25 andGoF = 2.2 for 1000 ◦C.

3.1.2. Rietveld refinement. To confirm that the structure istruly of cubic type, a structural refinement by the Rietveldmethod [12] was performed using the Fullprof Program[13]. The structural refinement results for the Y2O3 : Tb3+

phosphor annealed at 600 ◦C are illustrated in figure 2(a)and are presented in table 1. The results indicate goodagreement between the observed and calculated XRD patterns(figure 2(a)). The quality of structural refinement data [13, 14]was checked by measuring a parameter called goodness of fit(GoF), which is defined as GOF = Rwp/Rexp. For perfectrefinement the GOF must approach unity. In the present case,the GOF was found to be 2.4.

3.1.3. Unit cell representation. Figure 2(b) illustrates a Y2O3

unit cell. This unit cell was modelled through a programcalled Visualization for Electronic and Structural Analysis(VESTA) [15] using Rietveld refinement data (lattice constantsand atomic positions) (table 1). The Y2O3 phosphor hasa cubic structure with a space group Ia-3 and point-groupsymmetry m-3. The unit cell contains 16 formula units with 32cations. This structure contains two cation symmetry sites C2

and C3i, both coordinated six-fold with oxygen, as shown infigure 2(c). These two cationic sites were distributed over twoWyckoff positions 8b with local symmetry C3i and 24d withlocal symmetry C2. Oxygen ions are located at 48e Wyckoffpositions.

The rare earth cations were substituted at the Y3+

symmetry site. The ionic radii of Y3+ (0.9 Å), Eu3+ (0.94 Å)and Tb3+ (0.92 Å) are very close and therefore, Y3+ is suitableto substitute with Eu3+ and Tb3+ ions in the doped and codopedsamples. By the incorporation of these rare earth ions, nosignificant change in the XRD pattern was observed, as shownin figure 1(a) indicating that Eu3+ and Tb3+ ions do not causeany change in the cubic structure of the host lattice.

Cation site occupancy is an important structural parameterthat determines optical and other physical properties. From anoptical point of view, the C3i site has a centre of inversionsymmetry and so according to selection rules, only themagnetic dipole transitions are allowed whereas the C2 sitedoes not have inversion symmetry and hence both electric andmagnetic dipole transitions are allowed [16]. The C3i/C2 siteoccupancies were obtained from the Fullprof program. Thecalculated values of C3i/C2 site occupancies of Tb3+ are givenin table 1. Both site occupancies increase with increase in theannealing temperature of the sample.

3.1.4. Calculation of structural parameters. The structuralparameters such as Miller indices, lattice constants, crystallitesize, density, dislocation density, microstrain and inter-planarspacing were determined from XRD data.

The Miller indices (hkl) were calculated by an analyticalmethod described elsewhere [17]. The calculated (hkl) valueswere matched with the JCPDS database.

The lattice constants were calculated using the followingformula [17]:

sin2 θ = λ2

4a2(h2 + k2 + l2) (1)

where the terms have their usual meanings.The crystallite size (D) was determined using the Debye–

Scherrer formula [18] and the Hall–Williamsons equation [19]as given below:

D = 0.9λ

β cos θ(Debye–Scherrer formula) (2)

β cos θ

λ= 1

D+

ε sin θ

λ(Hall–Williamsons equation) (3)

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J. Phys. D: Appl. Phys. 45 (2012) 415102 S Som and S K Sharma

Figure 3. Hall–Williamson plot.

Here, β represents the full-width at half-maximum (FWHM)and ε represents the microstrain present in the samples.The crystallite size from the Hall–Williamsons equation wasdetermined from the reciprocal of the intercept of its straightline plot, as shown in figure 3. The slope of this equation ε

implies the microstrain (ε) present in the samples.The density (Dx), dislocation density (δ) and microstrain

(ε) were calculated using the following relations [20]:

Dx = 16M

Na3(4)

δ = 1

D2(5)

ε = β cos θ

4(6)

where M is the molecular mass and N is Avogadro’s number.The inter-planar spacing was calculated using the

following formula [17]:

d = a√h2 + k2 + l2

(7)

Table 2 summarizes the structural parameters of the undoped,doped and codoped phosphors. Table 3 contains the structuralparameters for the most intense diffraction peak of the annealedsamples. Table 3 indicates that the crystallite size andthe lattice constants increase with the increase in annealingtemperature.

3.1.5. FTIR. FTIR spectra of the doped, codoped andcommercially undoped phosphors are illustrated in figure 4(a).The absorption band centred at 560 cm−1 is attributed to Y–Olattice vibration. The peaks at 850 cm−1 and the ones at1060, 1395 and 1542 cm−1 are due to C–O bond bending andstretching vibration, respectively. These peaks were due toresidual carbon in the prepared samples or absorption of CO2

from the ambient atmosphere. The broad band at 3550 cm−1

is assigned to O–H stretching vibration [21]. The absorptionband due to O–H vibration is absent in the commercial undopedY2O3 phosphor. This band in the prepared doped samples wasprobably due to the adsorbed molecular water from the KBrpellet technique used for the preparation of samples, or maybe absorption of H2O from the ambient atmosphere.

The residual hydroxyl groups (–OH) at 3550 cm−1 areresponsible for the quenching of rare earth emission andthereby decreasing the luminescence intensity. This bandbecomes weaker with increase in annealing temperature anddisappears at higher temperatures. Moreover, the Y–O bandat 560 cm−1becomes stronger with increase in annealingtemperature (figure 4(b)). So, both these facts support thata high annealing temperature is required to prepare a goodluminescent Y2O3 phosphor.

3.2. Thermal analysis

Thermogravimetry (TG) and differential thermal analysis(DTA) curves of the doped and codoped phosphors wererecorded in the temperature range 25–1100 ◦C (figure 5). Thecontinuous TG curve indicates no phase change during therise of temperature up to 1100 ◦C [16] in all the samples.

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J. Phys. D: Appl. Phys. 45 (2012) 415102 S Som and S K Sharma

Table 2. Structural parameters of the doped and codoped phosphors.

Lattice Crystallite Microstrainconstants size microstrain

a (Å) D (nm) Density Dislocation ε (×10−3)(Dx) density

Sample JCPDS Calculated Debye–Schrrer Hall–Williamson’s (gm cm−3) (δ)(×1014 m−2) Calculated Hall–Williamson’s

Y2O3 : Tb3+ 10.60 10.569 83 10–60 40 5.079 2.23 1.61 5.49Y2O3 : Eu3+ 10.60 10.588 87 10–20 20 5.052 2.5 0.66 0.77Y2O3 : Tb3+, Eu3+ 10.60 10.620 32 10–30 35 5.008 1.6 1.36 3.89undoped 10.60 10.609 87 60–120 60 5.022 0.37 1.61 1.52

Table 3. Structural parameters of the annealed Tb3+-doped Y2O3 phosphors.

Lattice Interplaner Density(hkl) constants a (Å) spacing dh k l Dx (gm cm−3) Crystallite

Annealing Sizetemperature 2θ JCPDS Calculated JCPDS Calculated JCPDS Calculated JCPDS Calculated (D) (nm)

500 ◦C 29.15 (2 2 2) (2 2 2) 10.60 10.541 12 3.0599 3.042 96 5.036 5.1213 15600 ◦C 29.68 (2 2 2) (2 2 2) 10.569 83 3.051 25 5.0797 22700 ◦C 29.68 (2 2 2) (2 2 2) 10.585 23 3.055 69 5.0576 28800 ◦C 29.15 (2 2 2) (2 2 2) 10.598 76 3.055 96 5.0382 33900 ◦C 29.42 (2 2 2) (2 2 2) 10.600 08 3.059 98 5.0364 36

1000 ◦C 29.79 (2 2 2) (2 2 2) 10.611 14 3.063 17 5.0206 40

Figure 4. FTIR spectra of the prepared samples for (a) different dopants and (b) annealing temperatures.

The TG curves show a sharp fall up to 300 ◦C becauseof the weight loss due to removal of water and the auto-combustion reaction. This fall becomes lower between 300and 700 ◦C and after 700 ◦C it remains constant with nosignificant weight loss [16]. This result is in good agreementwith the observation of increase in crystallinity with annealingtemperature after 700 ◦C, as confirmed from the XRD patternat different annealing temperatures (figure 1(b)).

The weight loss below 200 ◦C is attributed mainly tothe evaporation of water, accompanied by the correspondingendothermic peaks in the DTA curves. The auto-combustionprocess occurs around 250 ◦C followed by a weight loss in avery short time, releasing a large amount of heat. From thisit can be stated that the Y2O3 phase was formed by the rapidexothermic process. The other broad exothermic peak in theDTA curve near 500 ◦C in the case of doped samples is caused

by the burning of residual organic compounds present duringpreparation.

3.3. Optical properties

3.3.1. Diffuse reflectance. The DR spectra of the dopedand codoped Y2O3 powder phosphors were measured againsta reference standard BaSO4 compound. In DR spectra, asharp band at 210 nm was observed for the doped and codopedsamples, as shown in figure 6. This corresponds to the fact thatlight having this particular wavelength was absorbed. The bandat 210 nm was due to the bandgap of the doped and codopedY2O3 nanophosphors. The undoped commercial Y2O3 sampleshowed the band at 220 nm. The absorption edge of the dopedsamples was blue shifted with respect to the undoped sampledue to the crystallite size effect [22]. Other weak bands beyond

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J. Phys. D: Appl. Phys. 45 (2012) 415102 S Som and S K Sharma

Figure 5. TG/DTA curves of the prepared phosphors.

Figure 6. DR spectra of the prepared phosphors.

300 nm were also observed in the case of the Eu-doped sample.These bands were due to meta-stable energy states formedbetween the valence band and the conduction band by the Eu3+

ions. These weak bands beyond 300 nm were absent in theTb-doped and codoped samples.

3.3.2. Calculation of bandgap. The K–M theory [23] wasused to calculate the bandgap of the doped and codoped Y2O3

nanophosphors using a DR spectrum. In a DR spectrum, theratio of the light scattered from a thick layer of sample and anideal non-absorbing reference sample is measured as a functionof the wavelength λ, R∞ = Rsample/Rreference [23]. The relationbetween the DR of the sample (R∞), absorption coefficient (K)and scattering coefficient (S) is given by the K–M functionF(R∞):

F(R∞) = (1 − R∞)2

2R∞= K

S(8)

The bandgap Eg and the linear absorption coefficient α of amaterial is related through the well-known Tauc relation:

αhν = C1(hν − Eg)1/2 (9)

where ν is the photon energy and C1 is a proportionalityconstant. When the material scatters in a perfectly diffusemanner (or when it is illuminated at 60◦ incidence), theabsorption coefficient K becomes equal to 2α. Considering thescattering coefficient S as constant with respect to wavelength,and using equations (8) and (9), the following expression canbe written:

[F(R∞)hν]2 = C2(hν − Eg) (10)

From the plot of [F(R∞)hν]2 versus hν, the value ofEg is obtained by extrapolating the linear fitted regions to[F(R∞)hν]2 = 0. The curve of figure 7 exhibits nonlinearand linear portions, which are the characteristic of a directallowed transition. The nonlinear portion corresponds to aresidual absorption involving impurity states and the linearportion characterizes the fundamental absorption.

The bandgap calculated from the DR spectra using theK–M function F(R∞) was found to be 5.8 eV for the dopedand codoped samples but for the undoped sample the bandgapwas 5.6 eV. This increase in bandgap was due to the decrease inthe crystallite size of the phosphors compared with the undopedphosphor.

3.3.3. Photoluminescence. The performance of lumines-cent materials is influenced by the doping concentration andit is very important to determine the optimum concentrationfor luminescence applications. Emission spectra for differ-ent Tb3+ concentrations (0.1 mol%–5 mol%) and Eu3+ con-centrations (1 mol%–10 mol%) excited at 254 nm are shownin figures 8(a) and (b), respectively.

The emission spectrum of Y2O3 : Tb3+ is composed ofseveral sharp lines corresponding to the 5D4 → 7FJ transitionswhere J = 3, 4, 5 and 6. The strongest emission occursat approximately 545 nm due to the 5D4 →7F5 characteristictransition of green emission for Tb3+ [3]. The other peaksat 485 nm, 585 nm and 625 nm arise from the 5D4 → 7F6,5D4 → 7F4 and 5D4 →7F3 transitions, respectively. Whenthe Tb3+ concentration is varied from 0.1 mol% to 5 mol%,the emission intensity first increases up to 1 mol% and thendecreases concluding that the optimum doping concentrationof Tb3+ in the Y2O3 host is 1 mol%. The decrease in PLintensity of the phosphors for Tb3+ greater than 1 mol% mayhave been caused by the concentration quenching phenomena,which were due to the cross-relaxation between neighbouringTb3+ ions as per the following mechanism (figure 13): Tb3+

(5D3) + Tb3+ (7F6) → Tb3+ (5D4) + Tb3+ (7F0).The peak position in the emission spectra does not

change with Tb3+ concentration suggesting that the nature ofTb3+ activator remains unchanged with concentration. Theexcitation spectrum recorded at λemi = 545 nm emission iscomposed of two overlapping bands having maxima at 274 and302 nm [3]. The band located at around 274 nm is attributed tothe O2−–Tb3+ charge transfer band (CTB), which correspondsto the electronic transitions from the 2p orbital of O2− to the 4forbital of Tb3+. The origin of the band at 302 nm results fromthe absorption of incident radiation by Tb3+ ions and leads tothe excitation of electrons from the Tb3+ ground state to oneof its excited 4f levels. However, due to the parity forbidden

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J. Phys. D: Appl. Phys. 45 (2012) 415102 S Som and S K Sharma

Figure 7. Bandgap calculation of the prepared phosphors.

Figure 8. PL emission and excitation spectra for different doping concentrations of (a) Y2O3 : Tb3+ and (b) Y2O3 : Eu3+ phosphors.

character of the transition within the 4f configuration, the peaksbeyond 302 nm were too weak to observe here [5].

Figure 8(b) shows the excitation and emission spectra ofthe Eu3+-doped Y2O3 nanophosphor. The emission spectrawere recorded at an excitation wavelength of 254 nm. Thedistinct emission lines lying between 500 nm and 700 nmwere observed due to transitions from the excited 5D1 tothe 7F1 and 5D0 to the 7Fj (j = 0–3) levels of Eu3+ ions.The origin of these transitions (electric dipole or magneticdipole) from emitting levels to terminating levels dependson the location of Eu ion in the Y2O3 lattice, and the typeof transition is determined by selection rules [5]. The mostintense peak at 615 nm and a small peak at 630 nm correspond

to the hypersensitive transition between the 5D0 and 7F2 levelsof Eu3+ ion in the yttrium oxide host caused by the forcedelectric dipole transition mechanism. The weak emission inthe vicinity of 590 nm (590–600 nm) is ascribed to the magneticdipole transition of 5D0 to 7F1. Generally, when Eu3+ islocated at a low symmetry (without an inversion centre), the615 nm emission is dominant whereas the 590 nm emission isstronger when Eu3+ is located at a high symmetry (with aninversion centre). In the present case, the emission at 615 nm(5D0 →7F2) is dominant suggesting that the location of Eu3+

deviates from the inversion symmetry, i.e. at low symmetrypositions. The emission intensity increases with increase

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J. Phys. D: Appl. Phys. 45 (2012) 415102 S Som and S K Sharma

in Eu3+ concentration up to 5 mol% and then decreases dueto concentration quenching phenomena as per the followingcross-relaxation mechanism (figure 13):

Eu3+(5D1) + Eu3+(7F0) → Eu3+(5D0) + Eu3+(7F1).

The excitation spectra recorded at 611 nm emission containa wide band peaking at ∼250 nm, which is attributed to theCTB from O2− to Eu3+. In addition to the 250 nm peak, threemore weak peaks also appeared at 395 nm, 465 nm and 530 nm,respectively. These were from the absorption of incidentradiation by Eu3+ ions and led to the excitation of electronsfrom the Eu3+ ground state to its excited 4f levels. The DRspectra (figure 6) support the existence of other weak peaksin the excitation spectra for Y2O3 : Eu3+phosphors and theabsence of these peaks in the excitation spectra of Y2O3 : Tb3+

phosphors.For both rare earth ions, concentration quenching

phenomena were observed. They occur due to the non-radiative energy transfer between the same rare earth ions.This transfer of energy may occur via one of the followingmechanisms: exchange interaction, radiation reabsorptionand multipole–multipole interaction [25]. The type of theinteraction mechanism can be identified by knowing the criticaldistance (Rc) between the donor and the acceptor rare earthions. According to Blasse [24], the critical distance (Rc) canbe expressed as

Rc ≈ 2

[3V

4πCoN

]1/3

(11)

where V is the volume of the unit cell, Co is the optimumconcentration of the activator ions and N is the number of ionsper unit cell. In the present case, N = 32 and V = 1191 Å3.The approximate values of Rc for Tb3+ ions and Eu3+ ions wereobtained as 19.23 Å and 11.24 Å, respectively. The value ofRc greater than 5 Å for both rare earth ions indicates that themultipole–multipole interaction is dominant and is the majorcause of concentration quenching. The multipole–multipoleinteraction involves dipole–dipole (d–d), dipole–quadrupole(d–q) and quadrupole–quadrupole (q–q) interactions. Dexterproposed a theory to identify the type of multipole–multipoleinteraction. According to this theory, the emission intensity(I ) per activator ion is given by [24]

I

C= k

βCs/3(For β � 1) (12)

where C is the activator concentration, k and β are constantsfor a particular interaction, and the s values corresponding toexchange, d–d, d–q and q–q interactions are 3, 6, 8 and 10,respectively.

The plot of log (I/C) versus log C gives a straight linewith slope −s/3. In the present case, the values of slope werefound to be −1.87 and −1.84 for Y2O3 : Tb3+ and Y2O3 : Eu3+

phosphors (figure 9) indicating the s values of 5.6 and 5.5,respectively. The closeness of s values with the theoreticalvalue of 6 for the electric d–d interaction implies that theelectric d–d interaction is the main cause of the concentrationquenching phenomenon in these doped phosphors.

Figure 9. Logarithmic plot of I /C as a function of the activatorconcentration (C).

Figures 10(a) and (b) illustrate the emission spectraof the doped phosphors at different annealing temperatureskeeping the optimum concentration of Tb3+ (1 mol%) and Eu3+

(5 mol%). The emission intensity increases with the annealingtemperature. Annealing below 700 ◦C does not increase theemission intensity significantly but above 700 ◦C, the emissionintensity increases remarkably. This result is consistent withthat obtained from the XRD and FTIR. In the XRD, theprepared nanophosphors show good crystallinity for annealingtemperatures >700 ◦C and FTIR studies show the reduction of–OH band with annealing temperature [25].

To understand the phenomena of energy transfer betweenTb3+ and Eu3+, the emission spectra of the Tb, Eu-codopedphosphors were recorded by taking different mass ratiosof Eu3+/Tb3+ = 5 : 0.2, 5 : 0.4, 5 : 0.6, 5 : 0.8 and 5 : 1(figure 11(a)) as well as Tb3+/Eu3+ = 1 : 0, 1 : 1, 1 : 2, 1 : 3,1 : 4 and 1 : 5 (figure 11(b)). The excitation wavelengthof these codoped phosphors was also kept at 254 nm(figures 11(a) and (b)).

With a small increase in the Tb3+ concentration in theyttrium oxide host, the Eu3+ emission intensity increasessubstantially (figure 11(a)). This indicates that Tb3+ transfersa sufficient energy to Eu3+ in the Y2O3 host. On the other hand,the intensity of Tb3+ 545 nm emission decreases dramaticallywith an increase in europium concentration causing an increasein the intensity of Eu3+ 615 nm emission (figure 11(b)). Theabove results demonstrate that Tb3+ ions transfer energy toEu3+ ions [24]. Here, Tb3+ acts as a sensitizer and Eu3+ as anactivator.

Based on Dexter’s energy transfer formula of multipolarinteractions and Reisfeld’s approximation, the followingrelation can be obtained [26]:

η0

η≈ I0

I∝ Cn/3 (13)

where η0 and η are the luminescence quantum efficienciesof the donor in the absence and presence of an acceptor,

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J. Phys. D: Appl. Phys. 45 (2012) 415102 S Som and S K Sharma

Figure 10. PL emission and excitation spectra at different annealing temperatures of (a) Y2O3 : Tb3+ and (b) Y2O3 : Eu3+ phosphors.

Figure 11. PL emission spectra of the codoped phosphors.

respectively, and C is the sum of donor and acceptor ionconcentrations. n = 6, 8 and 10 correspond to dipole–dipole,dipole–quadrupole and quadrupole–quadrupole interactions,respectively. The values (η0/η) can be found from the ratioof I0/I where I0 and I are the fluorescence intensities of thedonor in the absence and presence of an acceptor, respectively.In the present case, log of the relative intensity ratio [log(I0/I)]of donor (Tb3+) ions was plotted as a function of the log of thetotal concentration [log(CTb + CEu)] (figure 12). The slope ofthe straight line suggests the n value to be ∼6 confirming theinvolvement of dipole–dipole interaction during the excitationenergy transfer [24, 26].

Forster [5] defined the energy transfer efficiency (ηT) interms of fluorescence intensities in the presence and absenceof acceptors as

ηT = 1 − η

η0= 1 − I

I0(14)

The calculated transfer efficiencies are summarized in table 4.The possible cross-relaxation transitions from Tb3+ to

Eu3+ are indicated in figure 13. Phonons are required tocompensate for the small energy mismatches between theTb3+ and Eu3+ levels but the cross-relaxations are favouredby the overlap between donor and acceptor transitions. The

Figure 12. Logarithmic plot of I0/I as a function of the totalconcentration.

energy transferred to Eu3+ cascades rapidly via non-radiativetransitions to the 5D1 and 5D0 states. Subsequently, a non-radiatve energy transfer occurs from 5D1 to 5D0. Therefore,all the radiative transitions in Eu3+ occur from 5D0–7FJ

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J. Phys. D: Appl. Phys. 45 (2012) 415102 S Som and S K Sharma

Table 4. Energy transfer efficiencies.

Cdonor Cacceptor ηT

(mol%) Tb3+ (mol%) Eu3+ I0 Iη

η0(%)

1 0 2261 — 11 1 2166 0.96 0.041 2 1634 0.72 0.281 3 996 0.44 0.561 4 778 0.34 0.661 5 542 0.24 0.760.8 5 1005 0.44 0.560.6 5 789 0.35 0.650.4 5 540 0.24 0.760.2 5 457 0.20 0.80

Figure 13. Representation of energy transfer phenomena.

(J = 1, 2) emissions in the region 580–620 nm [7]. The cross-relaxation mechanisms for Tb3+ and Eu3+ are also shown infigure 13.

3.3.4. Calculation of CIE parameters. Figure 14 illustratesthe CIE chromaticity diagram of the codoped phosphors fordifferent Tb3+/Eu3+ mass ratios. The CIE parameters suchas colour coordinates, colour correlated temperature (CCT),colour rendering index (CRI) and luminescence efficacyof radiation (LER) were calculated in order to know thephotometric characteristics of the prepared phosphors. Theseparameters were calculated by the spectrophotometric methodusing the spectral energy distribution of the chromaticitydiagram [27]. The colour coordinates traverse a wide rangefrom green to the extreme red region on varying the Tb3+/Eu3+

mass ratios. The results show that the tuning of emissioncolour is possible by the change in the Tb3+/Eu3+ mass ratio.Near white light was obtained from the phosphor having aTb3+/Eu3+ mass ratio of 1 : 4 with good CRI for solid-statelighting applications. The values of CIE parameters for thedifferent codoped phosphors are summarized in table 5.

Figure 14. CIE chromaticity diagram of the codoped phosphors.

Table 5. CIE parameters.

(Y1−x−yTbxEuy)2O3 Colour coordinates CCT CRIx : y x y (K) (Ra) LER

1 : 1 0.24 0.54 N/A 13 4011 : 2 0.32 0.50 N/A 53 3861 : 3 0.37 0.48 4733 69 3781 : 4 0.43 0.44 3406 85 3661 : 5 0.54 0.40 1821 78 361

4. Conclusions

• XRD studies, Rietveld refinement and FTIR studies ofthe annealed samples confirm the increase in crystallinity,crystallite size, occupancy of RE3+ ions at the symmetrysites of the Y2O3 host and reduction of –OH group.These structural changes with annealing temperature areresponsible for an increase in the luminescence efficiency.

• Thermal analysis results indicate no phase change in thestudied temperature range. The bandgap of the phosphorscalculated from DR studies was found to be 5.8 eV.

• The optimum concentration of Tb3+ and Eu3+ in the Y2O3

host was achieved for photoluminescence emission as1 mol% and 5 mol%, respectively. A non-radiative energytransfer took place from Tb3+ to Eu3+ in the Y2O3 host dueto dipole–dipole interaction. A wide colour range wasachieved starting from green to extreme red as confirmedby the colour coordinate diagram indicating its applicationin different lighting devices.

Acknowledgment

The authors are grateful to the University Grants Commission,New Delhi, Government of India, for funding this work(Project F. No 37-200/2009 (SR)).

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J. Phys. D: Appl. Phys. 45 (2012) 415102 S Som and S K Sharma

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