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Enhanced photoluminescence behaviour of Eu3+ activated ZnMoO4 nanophosphors via Tb3+ co-doping for light emitting diode
Author’s Accepted Manuscript Enhanced photoluminescence behaviour of Eu3+ activated ZnMoO4nanophosphors via Tb3+ codoping for light emitting diode Neha Jain, Bheeshma Pratap Singh, Rajan K. Singh, Jai Singh, R.A. Singh www.elsevier.com/locate/jlumin
PII: DOI: Reference:
S0022-2313(16)31892-0 http://dx.doi.org/10.1016/j.jlumin.2017.05.007 LUMIN14731
To appear in: Journal of Luminescence Cite this article as: Neha Jain, Bheeshma Pratap Singh, Rajan K. Singh, Jai Singh and R.A. Singh, Enhanced photoluminescence behaviour of Eu3+ activated ZnMoO4nanophosphors via Tb3+ co-doping for light emitting diode, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2017.05.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced photoluminescence behaviour of Eu3+ activated ZnMoO4nanophosphors via Tb3+ co-doping for light emitting diode Neha Jain1, Bheeshma Pratap Singh2, Rajan K. Singh1, Jai Singh1* , R. A. Singh1 1
Department of Physics, Dr. Harisingh Gour University, Sagar, M.P.-470003, India
2
Department of Physics, Indian Institute of Technology (BHU), Varanasi- 221005, India
*Email: [email protected], [email protected]
Abstract In the present study, structural and photo-luminescence studies of Eu3+ and Tb3+ co-doped ZnMoO4 host matrix has been investigated in detail. ZnMoO4:Eu3+/Tb3+ have been synthesized via a simple and cost effective co-precipitation method. XRD, Raman and FTIR studies confirmed its triclinic structure of Eu3+/Tb3+ co-doped ZnMoO4 host. Emission spectra of the measured samples corroborate the dominant electric dipole transition emission (5D0-7F2) intensity over magnetic dipole transition emission (5D0-7F1) intensity. Moreover, the improvement in red emission (5D0-7F2) for Eu3+ (3 at.%) and Tb3+(2 at.%) and high asymmetric ratio 10.4 is observed which dictates it as a highly red emitter. Additionally, commission international de-I’ Eclairage (CIE) values approaches to NSTC. Color purity of red emission improves after Tb3+ co-doping. Photoluminescence studies reveals that Tb3+ acts as a sensitizer for the Eu3+ activated ZnMoO4 host matrix. Study reveals that Eu3+/Tb3+ codoped ZnMoO4 can be excited by near UV device, which is bottleneck in the advancement of LEDs technology.
GRAPHICAL ABSTRACT
Schematic representation of enhancement in red emission intensity via cross relaxation process. Energy transfer mechanism from Tb3+ions to Eu3+ ion by cross relaxation.
Keywords- ZnMoO4, Energy transfer, Photoluminescence, Lifetime, Cross relaxation
1. Introduction Recently, rare earth doped phosphors have been widely investigated for their versatile applications particularly in white light emitting diodes (w-LEDs), field emission display,
scintillating detectors, temperature sensors, fluorescent lamps, solar cells, biomedical and bio-imaging are developed [1-10]. These applications of rare earth ions (lanthanides- Ln3+) occurs mainly due to 4f valence shell electrons and photoluminescence obtained due to f-f or 4f-5d transitions [11,12]. In addition, emission wavelength also depends on the splitting of energy levels Ln3+ ions, for example, Tb3+ activated phosphors are strong green emitter. Emission from Tb3+ ion is obtained due to the transitions of 5D3 - 7FJ in the blue and 5D4 - 7FJ in the green region (J = 6, 5, 4, 3, 2) and the transition intensities depend on its critical doping concentrations [5]. Also, Eu3+ activated phosphors are strong red emitter phosphors due to 5D0→7F2 electric dipole transition [2]. There are various ways for the enhancement in intensity of red emission has been reported such as annealing at higher temperature, doping of charge compensator and co-doping of rare earth ions [2, 12-15]. Annealing of samples can remove number of defects or impurities from surface of the phosphor because the rate of nonradiative decay rate decreases. Second way to enhance intensity of red emission is doping of charge compensator (Li+, Na+ and K+) with rare earth (Eu3+) ion. In this method, a deficiency created due to difference between oxidation state of metal ion (M2+) and rare earth ion (Ln3+) is nullified with charge compensator and thus non-radiative decay rate decreases. Co-doping of Ln3+ ion into doped phosphors is also an appropriate way to increase intensity since in this process energy transfer occurs from one Ln3+ ion acts as a sensitizer to another Ln3+ ion acting as an activator. The energy transfer process between Ln3+ ions is possible due to resonant energy transfer, energy transfer by non-radiative transition and quantum cutting [16]. Many combinations of sensitizer and activator of rare earth ions have been developed as Eu3+-Dy3+, Eu3+-Tb3+, Eu3+-Gd3+, Ce3+-Tb3+, Eu3+-Sm3+, Er3+-Yb3+, Tm3+-Yb3+ and Ho3+Yb3+ in which one ion transfers its energy to another ion [8,10,13,17- 20]. The energy transfer from sensitizerTb3+ to activator Eu3+ is of keen interest as in this combination multi- color emission can be observed (emission can be tuned from green-
yellow-to red) [9,18]. In addition, Tb3+ ion has strong excitation band in blue and ultraviolet (UV) region [34], which can efficiently transfer energy to Eu3+ ion thus resulting in the improved performance of w-LEDs. Although, energy transfer from Tb3+ to Eu3+ was previously reported in Y2O3, NaY(MoO4)2, NaLa(MoO4)2, CaGdAlO4, KGdPO4WO4, Ca8MgLu(PO4)7, LaPO4, Na(Y,Gd)F4 and Sr3AlO4F3 etc. [5,10,18,21-26] However, Tb3+ and Eu3+ co-doped ZnMoO4 photoluminescence (PL) properties have not been reported yet to the best of our knowledge. ZnMoO4 has wide band gap, high chemical and thermal stability [2729]. It exhibits two types of crystal phase, triclinic α-ZnMoO4 and monoclinic β-ZnMoO4, depending upon synthesis methods and environmental conditions [30-32].It has been already reported that luminescence intensity of Eu3+ activated ZnMoO4 was 93% more than CaMoO4:Eu3+, this indicates that number of defects are more in wolframite triclinic structure so more energy transfer from Mo-O to Eu3+ ion takes place and intensity of red emission is enhanced [33]. An intense red emission by energy transfer from Bi3+ to Eu3+ion into ZnMoO4 was also reported [34]. Several methods are reported earlier to prepare ZnMoO4 such as solid-state reaction, precipitation method, electrochemical assisted laser ablation and hydrothermal methods [35-38]. In this paper, variation in luminescence properties due to relative doping of Eu3+ and Tb3+ ion into ZnMoO4 are studied. Intensity of red emission corresponding to 5D0→7F2 transition improves with increasing Tb3+ ion concentration relative to Eu3+ ion in ZnMoO4. Here, we have reported the possible energy transfer mechanism from Tb3+ to Eu3+ ion and measured the energy transfer efficacy. The CIE co-ordinate for 3 at.% Eu3+-2 at.% Tb3+ codoped ZnMoO4 was (x= 0.61, y=0.32) closer to standard of NTSC (x=0.67, y=0.32). The variations in color purity with relative Eu3+ to Tb3+ concentration are also discussed.
2. Experimental details 2.1 Materials and Synthesis Zinc chloride (Merck, 99.9%), Sodium Molybdate (Merck, 99.9%), Europium oxide (Merck, 99.99%), Terbium oxide (Tb2O3, Merck, 99.99%), Concentrated Nitric acid (69%, Merck) and sodium hydroxide (Merck, 98%). The ZnMoO4:Ln3+ (Ln3+= 5at.%Eu3+-0at.%Tb3+, 4at.%Eu3+-1at.%Tb3+, 3at.%Eu3+-2at.%Tb3+, 2at.%Eu3+- 3at.%Tb3+, 1at.%Eu3+-4at.%Tb3+ and 0at.%Eu3+-5at.%Tb3+) were prepared by facile co-precipitation method. Firstly, to prepare 5at.% Eu3+ doped ZnMoO4 (E5), 1.021 gm of ZnCl2 and 0.069 gm of Eu2O3were added together into 10ml distilled water and then 2-3 drops of HNO3were added into it to dissolve Eu2O3 completely, Eu(NO3)3was formed. Excess of acid was removed by heating the solution at 80 °C with addition of deionized water (15ml) and pH of the solution was adjusted ~9-10 by adding NaOH solution. The solution was stirred continuously; 1.909 gm of Na2MoO4 was dissolved into 10ml water then added into previous prepared solution. The resulting solution was heated for 30 min. at 50°C with continuous stirring, and a white precipitate was formed. This precipitate was washed with distilled water, methanol and acetone to remove any impurity from sample and dried at 60°C for 2hrs, a white powder was formed. Finally, sample was calcined at 500°C for 6hrs for completion of oxidation and/or reduction of ions. Same procedure was followed to prepareZnMoO4: 4%Eu3+, 1%Tb3+ (E4T1),ZnMoO4:3% Eu3+, 2%Tb3+ (E3T2), ZnMoO4: 2%Eu3+, 3%Tb3+ (E2T3), ZnMoO4: 1%Eu3+, 4%Tb3+ (E1T4) and ZnMoO4: 5%Tb3+ (T5) by adding Eu2O3 and Tb2O3together into ZnCl2.
2.2 Characterizations
The structural analysis were characterized by using D8 Bruker X-ray diffractometer (XRD) with Ni-filtered Cu-Kα (1.5405Å) radiation at 40 kV and 40 mA. All patterns were recorded over the range 10° < 2θ < 80° with a step size of 0.02°. The morphology of particles was determined by using scanning electron microscope (FESEM, Model- Quanta 200 FEI, 30kv) and transmission electron microscope (TECNAI G2). Simultaneous DTA/TGA spectra were recorded using NETZSCH STA 449 F1. DTA and TGA analyses were carried out using 22 mg of the sample at a heating rate of 10 °C min-1 up to 1000 °C, in nitrogen air under a flow of 60cm3 min-1. The Raman spectra of the samples were recorded with Renishaw microRaman spectrometer attached with a laser excitation source of 633 nm. Infrared spectra were recorded on a Fourier transform infrared (FT-IR) spectrophotometer of Shimadzu (model 8400 S) with a resolution of 2 cm-1 and in the range 500–3800 cm-1. To measure IR spectra, the samples were mixed with KBr (Sigma Aldrich, 99.99%) in 1: 5 ratio and then spectra were
recorded.
UV-Vis
spectra
were
recorded
using
UV-2700
Double
beam
spectrophotometer in the absorbance mode. The samples were dispersed in the methanol and then UV-Vis spectra were recorded. Photoluminescence measurements were carried out under ultraviolet excitation using 266 nm radiation from a Nd: YAG laser and detected by a CCD detector (Model: QE 65000, Ocean Optics, USA) attached with the fibre. Lifetime decay was recorded with Edinburg instrument F-920 equipped with 100W flash xenon lamp as the excitation source. 3. Results and discussion 3.1 XRD The XRD pattern of Eu3+/Tb3+ doped and co-doped ZnMoO4 are shown in Figure. 1. All the diffraction peaks of ZnMoO4:Ln3+can be indexed considering triclinic structure (space group P-1) of α-ZnMoO4 with
card no. 5-0
5 (a 8.
Å,
.
1Å, c
.
4Å, α
106.87, β
101. 2, γ= 96.734 and
51 . 5Å3). Slight shift in peak position and
diffraction intensity of ZnMoO4:Ln3+ in XRD pattern arises due to substitution of Zn2+ ions by Eu3+/Tb3+.The peak at 2θ
25. 0° exhi it maximum diffraction intensity for E5, E4T1,
E3T2, E2T3 and E1T4 while T5 phosphor the maximum intensity is seen for the peak at 28.64°. The changes in diffraction intensities are caused by stronger diffraction due to the higher number of electrons in Eu3+ and Tb3+ doped and co-doped phosphor. Although, change in XRD pattern in 5at% Tb3+ doped ZnMoO4 also cause due to formation of Tb-Mo-O or MoO complex. The shift in diffraction intensity also supported by previous literature, in which diffraction intensity vary with doping of Eu3+ ion into Tb2Mo3O12 [39]. Lattice parameter and unit cell volume of Eu3+ and Tb3+ co-doped ZnMoO4 are given in table 1. The variations in Lattice parameter and unit cell volume is obvious due to substitution of Zn2+ (0. 00 Å) ion y Eu3+ ion (0. 4 Å) and T
3+
ion (0. 2 Å).
The structures of various compositions were analyzed by Rietveld refinement using FullProf suite [34]. The lattice parameters calculated for E5 sample were a = 8.28946 Å, . 14 8 Å, c E2T were a
. 55
Å, α
8.28 1 Å,
10 .001°, β .
1 Å, c
101.042° and γ . 4184 Å, α
. 12°, lattice parameter of 10 .
°, β
101.222° and γ
96.89°. Whereas Zn2+ has smaller radii as compared to Ln3+, the lattice parameters and unit cell volume should increase with doping of larger radii Ln3+. But in present paper cell volume shrink with Ln3+ ion doping. This is because of difference in oxidation state of Zn2+ and Eu3+/Tb3+ ion. olyhedral representation of triclinic α- ZnMoO4 is shown in figure 1(b). The crystal structure was established by using GSAS software (general structural analysis system). It can be clearly seen from this figure that zinc ion has octahedral configuration [ZnO6] and molybdenum ion has tetrahedral [MoO4] and octahedral [MoO6] coordination. Also, the zinc ion is surrounded by Mo- clusters. Moreover, average crystallite size was calculated by using Scherrer formula,
D=
Where,
(1)
is average crystalline size, λ is wavelength of X-Rays (0.15405 nm), β is full width
at half maximum (FWHM) and θ is the diffraction angle. The strong peaks at 2θ
25. 0 2
(210), 28.4279 (2 0 1) and 34.1309 ( ̅ ̅ 2) were used to calculate crystalline size of the samples. The crystalline size for different samples is given in table 1. The strain in ZnMoO4:Eu3+, Tb3+ was calculated by using Williamson-Hall (W-H) fitting method [40] and strain plot with linear fit is shown in figure 2. Strain arises due to lattice distortion in crystal. It may results in the change in bond length and lattice parameters due to ionic size mismatch. train values under studied systems are negative strain (ε) and having range in etween 0.002 - 0.005. The negative slope for ZnMoO4:Ln3+ indicates compressive strain into crystal. This is also consistent with the decreased unit cell volume for Eu3+/ Tb3+ doped and co-doped ZnMoO4. 3.2 Morphology Study Figure 3(a), (b) and (c) illustrate SEM images of 5at.%Eu3+ doped, 3at.%Eu3+-2at.%Tb3+ codoped and 5at.%Tb3+ doped ZnMoO4. Nanoparticles are spherical in shape and agglomerated in absence of surfactant. TEM micrographs of 5at.%Eu3+ doped, 3at.%Eu3+-2at.%Tb3+ codoped and 5at.%Tb3+ doped ZnMoO4 are shown in figure 4(a), (b) and (c), respectively. It can be seen from TEM micrograph that the size of the particles around ~50 nm which is good agreement with the crystalline size ~50-70 nm estimated by Debye- Scherrer formula. The HR-TEM of 5at.%Eu3+ doped, 3at.%Eu3+-2at.%Tb3+ co-doped and 5at.%Tb3+ doped ZnMoO4 are given in figure 4 (d), (e) and (f), with their d-value 0.191 nm, 0.191nm and 0.202 nm corresponding to (231), (231) and ( ̅ 41) planes, respectively. Selected area electron diffraction pattern (SAED) patterns of these samples are depicted in figure 4(g), (h) and (i), respectively.
3.3 TGA/DTA study The thermal properties of Eu3+ and Tb3+ co-doped ZnMoO4 (E3T2) was studied by using differential thermo-gravimetric analysis are shown in figure 5. Heat flow and change in mass was recorded under nitrogen atmosphere in temperature range from 23°C to 1000°C with heating rate 10°C min-1. TG curve shows mass loss of 1.35 % at 104°C, this mass loss is attributed to loss of water molecule from surface of the sample. An endothermic peak found in DTA curve at 104°C arises due to release of water content. Another mass loss of 1.70% at 350°C obtained due to complete evaporation of water from sample. Final mass loss 2.64% is obtained at 1000°C. DTA curve have two endothermic peaks at 520°C and 574°C which is attri uted to phase transition from α-ZnMoO4 to β- ZnMoO4 phase, similar results are also reported by Zhang et al. [29] These results reveals that Eu3+/ Tb3+ co-doped sample has high thermal stability. 3.4 Raman study The Raman spectra of 5at%Eu3+ and 5at%Tb3+ doped ZnMoO4 are shown in figure 6. The bands observed because of internal and external vibration of Mo-O unit into host ZnMoO4, so bands from 300-1000cm-1 were due to internal vibration of polyhedra [MoOn] and bands observed from less than 200cm-1 were due to external vibration of host. The main bands observed in present study are at ~977, 948, 910, 888, 785, 520, 350 and ~320 cm-1. The bands between ~880- 950 cm-1 are assigned to symmetry stretching mode (υ1) of Mo-O and bands observed between ~750- 880 cm-1 range are assigned to antisymmetric stretching mode (υ3) of [MoO4]2- tetrahedral unit [41-42]. The bands at ~320 and ~350 cm-1 represent symmetric (υ2) bending mode related to Mo-O bond and band at ~520 cm-1 is assigned due to antisymmetric (υ4) bending mode of Mo-O bonds [41]. These Raman results were obtained according to phase transition study of ZnMoO4 [1,42]. A weak intensity band observed at
977cm-1 indicates vibration of [MoO6]2- octahedral unit corresponding to Mo=O bond. However, intensity for ~520 cm-1 band for Tb3+ doped ZnMoO4 is less as compare Eu3+ doped ZnMoO4 due to slight deformation in Mo-O bond. 3.5 FTIR study FTIR spectra of Eu3+ and Tb3+ ion doped ZnMoO4 are shown in figure 7 in range ~550- 3800 cm-1.
ince α- ZnMoO4 have triclinic structure having [MoO4]2- tetrahedral and [ZnO6]2-
octahedral units so bands in spectra were found corresponding to their different vibration modes. The main bands were observed at ~ 800-900, ~1666 and ~3444 cm-1. The band near ~800 cm-1 observed due to symmetry and antisymmetric stretching mode of O-Mo-O into [MoO4]2-, other bands at ~3444 and ~1666 cm-1 correspond to –OH stretching and H-O-H bending, respectively [43]. Since there was no shift in bands with doping of Eu3+ and Tb3+ ion into ZnMoO4, all bands were attributed to bands of host matrix. Therefore, IR vibrational study confirms that there is no change in phase or structure due to doping of Eu3+/Tb3+ ion. 3.5 UV-Vis spectroscopy UV-Visible absorption spectra of ZnMoO4: Ln3+ (Ln3+- E5, E4T1, E3T2, E2T3, E1T4, T5) are shown in figure 8(a). The spectra exhibit strong absorption at 328 nm owing to charge transfer from ligand (oxygen) to metal (molybdenum) atoms inside [MoO4]2- group. On increasing relative concentration of Eu3+/ Tb3+, there was no shift in absorption peaks due to host. However, doping with Ln3+ ion slightly alter band structure of host ZnMoO4 which influence number of photons absorbed by phosphors [2, 44]. It may also evident in absorption spectra, compared to single 5 at.% doped Eu3+ and Tb3+, the absorption intensity of Eu3+ and Tb3+ co-doped phosphor is increases and decreases. The optical band gap energy of ZnMoO4: Ln3+was calculated y using Wood and Tauc’s method [45]. This method gives an empirical relation to calculate the optical band gap as:
αhv = K hν - Eg
n
(2)
where, α- is the a sor ance, h is lanck’s constant, ν is the frequency, Eg is the optical band gap and n is an exponent having values 1/2, 2, 3/2 or 3 depending on type of transition such as direct allowed, indirect allowed, direct forbidden or indirect forbidden, respectively. Since α- ZnMoO4 has direct band gap, so band gap is calculated y (αhv)2 Vs photon energy plot. The band gap plot for ZnMoO4 doped with E5, E3T2 and T5 are shown in figure 8(b), (c) and (d), respectively. The estimation of band gap values using equation (2) as ~ 3.60, 3.52 and 3.50 eV, respectively and these values are matches to and gap of α-ZnMoO4 (3.30 eV) [46]. The band gap show a red shift from E5, E3T2 to T5, which may be attributed to creation of defects into crystal with doping and relative co-doping of Eu3+ and Tb3+. 3.6 Photoluminescence study The excitation and emission spectra of 5at.% Eu3+ activated ZnMoO4 (E5) is shown in figure 9(a). The excitation spectra were recorded by monitoring emission wavelength at ~613 nm. Excitation spectra consists of broad excitation band at 250-330 nm and sharp peaks are observed in range ~330- 500 nm due to f-f transition of Eu3+ ion. The broad band signature arises mainly due to charge transfer band from ligand (O- 2p) to metal ion (Mo- 4d or Eu ion). Another sharp excitation peaks at 395, 410 and 466 nm were arises due to the characteristics electronic transitions7F0→5L6, 7F0→5D3 and 7F0→5D2 of Eu3+ ion. The emission spectra recorded under 280nm excitation have maximum intensity for 613 nm emission corresponding to transition 5D0→7F2. The characteristic emission peaks due to f-f transition of Eu3+ ion were also found at 591 (5D0→7F1), 615 (5D0→7F2), 654 (5D0→7F3) and 701 nm (5D0→7F4). Similarly, excitation and emission spectra of Tb3+ activated ZnMoO4is shown in figure 9(b). A broad excitation band around 250- 350 nm recorded by monitoring emission
wavelength 543 nm, is attributed 4f-5d transition of Tb3+ ion. This band centered at 272 nm which indicates successful substitution of Tb3+ ion in place of Zn2+ ion into ZnMoO4. The emission spectra consist of the characteristics peaks of Tb3+ ion at 488 (5D4→7F4), 544 (5D4→7F4) and 548 nm (5D4→7F4) under 290nm excitation. In present investigation, green emission is dominant due to transition from 5D4 excited state of Tb3+ ion. Emission from 5D3 state is absent for higher Tb3+concentration (> 1at.%)due to cross relaxation betweenlevel of Tb3+ ion, 5D3→5D4 (dipole cross relaxation) [5]. The excitation and emission spectra of Eu3+ and Tb3+co-doped ZnMoO4 (E2T3) are shown in figure 9(c). The excitation spectra consist of both Eu3+ and Tb3+excitation peaks monitored for 613 nm emissions. A broad band ~268-305 nm is attributed toEu3+-Tb3+/ MoO42- charge transfer band. The sharp peaks at 382 (7F0→ 5G4), 395 (7F0→5L6), 465 nm (7F0→5D2) were observed due to f-f transition in Eu3+ ion. Similarly, peaks at 416 nm (7F6→5D3) and 487 nm (7F6→5D4) were observed due to f-f transition of Tb3+. These observations show that excitation can be broadened with relative co-doping of Eu3+/Tb3+ ions. The emission spectra recorded under 290nm excitation has strongest emission peak at 618 nm corresponding to Eu3+ on 5D0→7F2 transition. The other peaks were observed at 591 (5D0→7F1), 654 (5D0→7F3) and 702 nm (5D0→7F4). Another peak around ~ 550 nm has very weak intensity found due to Tb3+ ion characteristic emission corresponding to
5
D4→7F5 transition.
Moreover, other characteristic emission due to f-f transition of Tb3+ion is absent because excited state of Tb3+ ion lie higher as compared to Eu3+ ion. Also, Tb3+ acts as sensitizer so it transfers its energy to Eu3+ ion via cross relaxation [5]. For instance, Tb3+ ions absorb energy and moves to its excited state (5D4 or 5D3). As there are several energy levels of Eu3+ ion in between 5D3 and 5D4 level of Tb3+ ion, so all the energy from Tb3+ ion is transferred to the Eu3+ ion and hence no Tb3+ characteristic emission was found.
The emission spectra under 290 nm excitation for all Eu3+ and Tb3+ co- doped ZnMoO4 phosphors are shown in figure 9(d). These spectra corroborate that as doping concentration of Tb3+ ion increases and Eu3+ ion concentration deceases than intensity of red emission has improved. The intensity of red emission enhanced until 3at.%Eu3+ and 2at.%Tb3+ concentration (E3T2), beyond this cross relaxation between Eu3+ and Tb3+ ion reduces intensity [47].Since in Eu3+/ Tb3+ co-doped system Eu3+ act as activator and Tb3+ act as sensitizer, so a defining limit for critical distance between Eu3+ and Tb3+ ions, it should e less than 4Å for exchange intensity.
ritical distance R c between Eu3+ and Tb3+ ions into
ZnMoO4 is estimated as [48]
(3)
Where, V is the volume of unit cell at critical concentration, n is number of sites available for doping in unit cell and χc is the total doping concentration at critical concentration. In this work volume of ZnMoO4 unit cell at critical concentration 3at.% Eu3+ and 2at.% Tb3+ is found 515.
5 Å3. Number of sites available for doping into ZnMoO4 is 6 and total optimum
concentration was χc 0.05 %. Critical distance (Rc) was calculated 14.8 Å which is much greater than critical distance 4.00 Å (maximum distance
etween two ions for energy
transfer) [49]. It may be the reason for the decrement in luminescence intensity beyond3 and 2 at.% concentration of Eu3+/Tb3+. These results reveal that by incorporating the relative concentration ratios of Eu3+ and Tb3+ ions into ZnMoO4, an enhanced red emission can be observed which is not mentioned previously in literature. The variation in intensity with concentration was calculated by curve fitting method. The curve fitted by using Gaussian distribution function and calculated area can be given by equation:
I= I0 + ∑
√
exp [2 (λ - λ0)2/w2]
(4)
Where, I be the intensity at particular concentration, I0 is background intensity, w is the full width at half maximum, A is area under curve, λ- wavelength and λ0 the mean value corresponding to the transition. The area under peak ranging from 600- 630 nm is due to electric dipole transition of Eu3+ ion without inversion and calculated by using Gaussian distribution function. Figure 10(a) presents variation in integrated area for electric dipole transition, it is maximum for E3T2. In addition, variation in full width at half maximum for Eu3+ -Tb3+ co-doped samples are shown in figure 10(b). The asymmetric ratio (A21) was used to probe the site symmetry environment of Eu3+ ions. Since, dominant electric dipole emission peak at ~615 nm over magnetic dipole emission at 590 nm is observed which is mainly due to occupancy of Eu3+ions as a site without inversion symmetry and vice versa then Eu3+ ions occupy site with inversion symmetry in the host matrices [50]. The higher value of A21 indicates dominant red emission of Eu3+iondue to electric dipole transition (5D0→7F2) transition [51]. In present investigation, asymmetric ratio for pure Eu3+activated ZnMoO4 is 10.39, which dictates ZnMoO4 host as a highly red emitter due to high asymmetric environment of Eu3+. 3.7 CIE Study The CIE chromaticity diagram of Eu3+ or Tb3+ doped and co-doped ZnMoO4 are shown in figure 11. The CIE chromaticity co-ordinates are calculated under 290 nm excitation listed in table 2. Tb3+ doped ZnMoO4 CIE co-ordinate are found in green region and Eu3+ doped ZnMoO4 co-ordinates are found in red region. The energy transfers from Tb3+ ions to Eu3+ ion are also confirmed by CIE co-ordinate. As Tb3+ concentration increases CIE co-ordinates show a red shift only for E3T2, on further increasing Tb3+concentration, CIE co-ordinate
move towards green region. CIE co-ordinates changed from red (0.55, 0.31) to green (0.28, 0.43). The color purity of color doped with Eu3+ or Tb3+ calculated by formula [52]
√
(5)
√
Where (x, y) are co-ordinate of sample point, (
,
) are co-ordinate of illuminate
wavelength and ( , ) are co-ordinate of white light in CIE diagram. In present study (
,
) = (0.67, 0.32)) and ( , ) = (0.3101, 0.3162). The color of pure Eu3+ doped sample is
66.67% enhanced with Tb3+ sensitizer and it is found maximum for E3T2 (x=0.61, y=0.32) is 83.33%. These results reveal that combination of Eu3+-Tb3+ gives an intense red emission, which is combination of Eu3+-Tb3+ gives an intense red emission, which is suitable for light emitting diodes. 3.8 Decay study Decay profiles give the signatures of energy transfer from Tb3+ to Eu3+ ion into ZnMoO4. The decay profile is shown in figure 12 under 290 nm excitation monitored for 543 nm emission of Tb3+ in Eu3+ /Tb3+ ion co-doped ZnMoO4 and for 613 nm emission for pure Eu3+ ion doped ZnMoO4. The decay data were fitted using a bi-exponential equation, which is given by
I = I1
-
+ I2
Where I1 and I2 are intensity at different time interval,
(6)
1
and
2
are there relative lifetime.
Fitted bi-exponential parameters vary with relative concentration of Eu3+/ Tb3+, while lifetime maximum for E3T2. The average lifetime of all Eu3+ and Tb3+ doped and co-doped samples were calculated by using following equation
(7) The lifetime values and average lifetimes were listed in table 3 for Eu3+/Tb3+ activated ZnMoO4. Average lifetime values changes with relative concentration of Eu3+ to Tb3+ ion. As Tb3+ ions transfer its energy to Eu3+ ion so its lifetime was decreases on increasing concentration of Eu3+ ion until 2at.% Tb3+. Energy transfer efficiency from sensitizer to activator was calculated by using formula [53]
(8)
Where, η is energy transfer efficiency from T presence of acceptor Eu3+ ion,
3+
ion to Eu3+ ion,
is average lifetime in
average lifetime of sensitizer (Tb3+ ion) in absence of
activator. For transfer maximum energy transfer luminescence lifetime should be decreased [18]. In present study, minimum average lifetime for 3at.% Eu3+ and 2at.% Tb3+ (E3T2) and for this, maximum energy transfer efficiency is 34.6%. This is the reason behind maximum PL emission is observed for E3T2.However, for low concentration of Eu3+ ion, Tb3+ -Tb3+ cross relaxation takes place which could affect energy transfer between sensitizer and activator [5]. It is also supported by lifetime measurements for 3, 4 and 5 at% Tb3+ the lifetime values were increased. Schematic representation of energy transfers and cross relaxation into MoO42-, Tb3+ and Eu3+ are shown in figure 13. Oxygen 2p valence shell electrons excite by absorbing excitation wavelength of 290 nm; this oxygen atom transfers its energy to molybdenum (Mo6+). Afterwards, Mo6+ ions transfer its energy to Tb3+ and Eu3+ by charge transfer. A discrete band into MoO42- was observed corresponding to wide band excitation. Energy transfer from MoO42-to Tb3+ and Eu3+ ions is also shown in this figure. For Tb3+, 5D4 excited state has slightly higher energy state compared to 5D1 excited state of Eu3+ ion, this attribute
energy transfers via cross relaxation process between 5D4 (Tb3+) to 5D1 (Eu3+) excited state by non-radiative (NR) transition. An electron jumps from 5D1 excited state to 5D0-excited state with non-radiative emission and thereafter relaxes to ground state by radiating orange and red emission corresponding to 5D0→7F1, 2. This corroborates the disappearance of greenemission in presence of activator (Eu3+) with sensitizer (Tb3+). 4. Conclusions In summary, triclinic ZnMoO4 phosphors doped with Eu3+ and Tb3+ ions have been successfully synthesized by co-precipitation method. Thermal studies (DTA/TG) of the phosphor features high thermal stability. The Raman, XRD and FTIR studies confirmed the tetrahedral
configuration
of
MoO42- group.
Co-doping
of Eu3+ andTb3+
ion
in
ZnMoO4 phosphors has been investigated by varying doping ratios of Eu3+ and Tb3+. The energy transfer from Tb3+ to Eu3+ ions confirm by lifetime measurement. The maximum energy transfer (34.6%) is found at 3/2 ratios of Eu3+ and Tb3+ whereas for other doping concentrations of Tb3+ and Eu3+ ions, energy transfer efficiency are very low. The effect of Tb3+ sensitizer to activator Eu3+ ion via energy transfer is analyzed which results in improvement in photoluminescence emission intensity and enhanced red color with 83.3% purity. A high asymmetric ratio ~10.4 is observed which dictates it as a highly red emitter. It is demonstrated that the excitation range are broadened by Tb3+ sensitizer and Eu3+ activated in ZnMoO4 which can be excited by NUV LED chip. The chromaticity coordinates (CIE) are much closed to the standard of National Television Standard Committee. These observations reveal that the above phosphors can be used as a potential red phosphor in w-LEDs and optoelectronic devices.
Acknowledgements Authors are thankful to NST Physics BHU for SEM characterization. We are thankful to Sophisticated Instrument Laboratory of the University for providing various characterization facilities (XRD, Raman, FTIR, DTA/TGA, HR-TEM). We are also thankful to Prof. S. B. Rai for providing PL facility. One of the authors (Neha) acknowledges the Central Research Fellowship provided by University Grants Commission (UGC), Govt. of India. Jai Singh would like to acknowledge UGC-India and DST for providing project under UGC Start-up Grant and DST Fast track. References 1. Hu D, Huan W, Wang Y and Wang Y 2015 J. Mater. Sci: Mater Elect. 26 7290 2. Maheshwary, Singh B P, Singh J and Singh R A 2015RSC Adv. 4 32605 3. Baur F, Glocker F, and Justel T 2015J. Mater. Chem. C 3 2054 4. Saha S, Das S, Ghorai U K, Mazumder N, Ganguly Dand Chattopadhyay K K 2015 J. Phys. Chem. C119 16824 5. Li X, Zhang Y, Geng D, Lian J, Zhang G, HouZ and Lin J 2014 J. Mater. Chem. C2 9924 6. Rai M, Singh S K, Singh A K, Prasad R, Koch B, Mishra K, Rai S B 2015 ACS Appl. Mater. Inter.7 15339 7. Parchur A K, Ansari A A, Singh B P, Hasan T N, Syed N A, Rai S B and Ningthoujam R S 2014 Integr. Biol.6 53 8. Pandey A, Rai V K, Kumar V, Kumar V and Swart H C 2015Sensors and Act. B209, 352 9. Ren L, Lei X, Du X, Jin L, Chen W and Feng Y 2013 J. of Lum.142 150. 10. Dorman J A, Choi. J H, Kuzmanich G and Chang J P 2012 J. Phys. Chem. C 116 12854 11. Wolfbeis O S 2011 Lanthanide Luminescence (Springer, New York)
12. Wybourne B G 2007 Optical Spectroscopy of Lanthanides (CRC Press, Taylor and Francis, Boca Raton, USA) 13. Singh B P, Parchur A K, Ningthoujam R S, Ansari A A, Singh P and Rai S B 2014 Dalton Trans. 43 4779 14. Singh B P, Maheshwary, Ramakrishna P V, Singh S, Sonu, V K, Singh S, Singh P, Bahadur A, Singh R A and Rai S B 2015 RSC Adv. 5 55977 15. Zhong J, Liang H, Su Q, Zhou J, Huang Y, Gao Z Tao Y and Wang 2010 J, Appl. Phys. B: Lasers Opt.98 139 16. Ningthoujam R S, Rai S B and Dwivedi Y 2012 Nova Science Publishers Inc. USA 7 145 17. Jin Y, Zhang J, Lu S, Zhao H, Zhang X and Wang X J 2008 J. Phys. Chem. C1125860 18. Yang M, Liang Y, Gui Q, Zhao B, Jin D, Lin M, Yan L, You H, Dai L and Liu Y 2015 Sci. Rep.5 11844 19. Wang F and Liu X 2008 J. Am. Chem. Soc. 130 5642 20. Niu N, Yang P, He F, Zhang X, Gai S, Li C and Lin 2012 J. Mater. Chem.22 10889 21. Xu Z, Li C, Li G, Chai R, Peng C, Yang D and J. Lin J 2010 J. Phys. Chem. C 1142573 22. Anh TM, Benalloul P, Barthou C, Giang LT, Vu N and Minh L 2007 J. of Nanomat. 48247 1 23. Wen D, Feng J, Li J, Shi J, Wu M and Su Q 2015 J. Mater. Chem. C 3 2107 24. Xie F, Li J, Dong Z, Wen D, Shi J, Yan J and Wu M 2015 RSC Adv. 5, 59830 25. Kim SY, Woo K, Lim K, Lee K and Jang HS 2013 Nanoscale 5 9255 26. Shang M, Li G, Kang X, Yang D, Geng D and Lin J, 2011 ACS Appl. Mater. Interfaces 3 2738 27. Furtes JR, Moreno SL, Solano JL, Errandonea D, Segura A, Perales RL, Munoz A, Radescu S, Rodriguez P, Hernandez, Gospodinov M, Nagornaya LL and Tu CY 2012 Phys. Rev. B 86 125202
28. Errandonea D, Somayazulu M and Hausermann D 2003 Phys. Status Solidi B 235 162 29. Zhang G, Yu S, Yang Y, Jiang W, Zhang S, Huang B 2010 J. of Cryst. Growth 312 1866 30. Abrahams SC 1967 J. Chem. Phys. 46 2052 31. Li Y, Weisheng G, Bo B and Kaijie G 2009 IEEE International Confer Energy Environ. Tech. 3 672 32. Spassky D, Vasilev A, Kamenskikh I, Kolobanov V, Mikhailin V, Savon A, Ivleva L, Voronina I and Berezovskaya L 2009 Phys. Status Solidi. A 206 1579 33. Zhou LY, Wei JS, Gong FZ, Huang JL and Yi LH 2008 J. Solid State Chem. 181 1337 34. Ran W, Wang L, Zhang W, Li F, Jiang H, Li W, Su L, Houzong R, Pan X and Shi J 2015 J. Mater. Chem. C 3 8344 35. Sotani N, Suzuki T, Nakamura K, Eda K and Hasegawa S 2001 J. Mater. Sci. 36 703 36. Sen A and Pramanik P 2001 Mater.Lett. 50 287 37. Liang Y, Liu P, Li H B and Yang G W 2012Cryst. Growth Des. 12 4487 38. Cavalcante L S, Sczancoski J C, Li M S, Longo E and Varela J A 2012 Colloids and Surfaces A, Physicochem. Eng. Aspects 396 346 39. Baur F, Glocker F and Justel T 2015 J. Mater. Chem. C 3 2054 40. Kar A and Patra A 2012 Nanoscale 4 3608 41. Maheshwary, Singh B P and Singh R A 2016 SpectrochimicaActa Part A: Mol. and Biomol. Spec. 152 199 42. Agarwal D C, Avasthi D K, Varma S, Kremer F, Ridgway V and Kabiraj D 2014 J. of Appl. Phys.115 163506 43. Pal M, Roy B and Pal M 2011 J. of Modern Phys. 2 1062 44. Verma R K, Kumar K and Rai S B 2010 Solid State Sci. 12 1146 45. Wood D L and Tauc J 1972 Phys. Rev. B: Solid State 5 3144 46. Keereeta Y, Thongtem T andThongtem S 2014 Superlattices and Microstructures 69 253
47. Dexter D L and Schulman J H 1954 J. Chem. Phys. 22 1063 48. Blasse G 1968 Phys. Lett. A 28 444 49. Ahsaine H A, Zbair M, Ezahri M, Benlhachemi A, Arab M, Bakiz B, Guinneton F and Gavarri JR 2015 Ceram. Intern. 41 15193 50. Singh L R, Ningthoujam R S, Sudarsan V, Srivastava I, Singh S D, Dey G K and Kulshreshtha S K 2008 Nanotech. 19 055201 51. Volanti D P, Rosa I L V, Paris E C, Paskocimas C A, Pizani P S, Varela J A and Longo E 2009 Opt. Mater. 31 995 52. Wu Y F, Nien Y T, Wang Y J, Chen I G and McKittrick J 2012 J. Am. Ceram. Soc. 95 1360 53. Paulose P I, Jose G, Thomas V, Unnikrishnan N V and Warrier M K R 2003 J. Phys. Chem. Solids 64 841
Figure 1. (a) XRD pattern of Eu3+ and Tb3+ doped and co-doped ZnMoO4 with JCPDS card no. 35-0765 (b) Polyhedral representation around z-axis of Tb3+ sensitized ZnMoO4:Eu3+.
Figure 2. lot for β cosθ /λ
s sinθ /λ of
at.%Eu3+ and 2at.%Tb3+ co-doped ZnMoO4
(E3T2).
Figure 3. SEM image of (a) ZnMoO4:5%Tb3+ (b) ZnMoO4:3%Eu3+, 2%Tb3+and (c) ZnMoO4:5%Eu3+
Figure 4. (a), (b) and (c) TEM image (d), (e) and (f) HRTEM image (g), (h) and (i) of ZnMoO4:5 at.%Tb3+, ZnMoO4:3at.%Eu3+2at.%Tb3+ and ZnMoO4:5at.%Eu3+, respectively.
Figure 5. DTA- TG curve of 3 at.% Eu3+ and 2 at.% Tb3+ co-doped ZnMoO4 (E3T2).
Figure 6. Raman Spectra of 5 at.%Eu3+ and 5at.%Tb3+co-doped ZnMoO4.
Figure 7. FTIR spectra of 5at.%%Eu3+ and 5 at.%%Tb3+ doped ZnMoO4.
Figure 8. (a) UV-Visible absorbance spectra of Eu3+ and Tb3+ doped and co-doped ZnMoO4 (E5, E4T1, E T2, E2T , E1T4 and T5) ( ) Band gap measurement (αhv)2 Vs hv for Eu3+ doped (c) Eu3+ and Tb3+ co-doped and (d) Tb3+ doped ZnMoO4.
Figure 9. Photoluminescence excitation and emission spectra of (a) 5at.%Eu3+ doped ZnMoO4 (b) 5at.%Tb3+ doped ZnMoO4 (c)2 at.% Eu3+ and 3 at.% Tb3+ co-doped ZnMoO4 and (d) Eu3+ and Tb3+ co-doped ZnMoO4 (total doping concentration 5at.%)
Figure 10. (a) Integrated intensity A2 and (b) full width at half maximum plot of asymmetry wavelength (613nm) for Eu3+ and Tb3+ co-doped ZnMoO4
Figure 11. CIE Chromaticity co-ordinate of Eu3+/ Tb3+doped and co-doped ZnMoO4 in various ratio.
Figure 12. Decay curve of Eu3+/ Tb3+ co-doped ZnMoO4 at 290nm excitation monitored for (5D4 level) 543 nm emission of Tb3+ ion.
Figure 13. Schematic representation of energy transfer between Eu3+ and Tb3+ into ZnMoO4.
Table 1 Cell parameter, crystalline size and strain of Eu3+/Tb3+ doped and co-doped triclinic ZnMoO4
Sample
Lattice parameters ell olume a (Å)
(Å)
Crystalline Strain
c (Å) (Å3)
size (nm)
ZnMoO4 (JCPDS35-0765)
8.36700
9.69100
6.96400
519.75
5Eu
8.28946
9.61468
6.95577
514.505
50.4
0.00370
4Eu, 1Tb
8.28074
9.62494
6.94212
512.387
50.1
0.00375
3Eu, 2Tb
8.59664
9.29355
6.92732
515.335
51.7
0.00261
2Eu, 3Tb
8.28717
9.63719
6.94184
512.670
54.9
0.00437
1Eu. 4Tb
8.49974
9.5544
6.91784
505.419
55.4
0.00464
5Tb
8.31348
9.65944
6.93839
514.476
52.1
0.00433
Table 2 CIE- Chromaticity co-ordinate and color purity of Eu3+/Tb3+ doped and co-doped ZnMoO4 S.N.
Sample
CIE
Color purity (%)
1
5Eu
(0.52, 0.30)
66.67
2
4Eu, 1Tb
(0.60, 0.32)
80.55
3
3Eu, 2Tb
(0.61, 0.32)
83.33
4
2Eu, 3Tb
(0.59, 0.32)
77.77
5
1Eu, 4Tb
(0.56, 0.31)
69.45
6
5Tb
(0.28, 0.43)
32.10
Table 3 Average lifetime obtained by bi-exponentially fit and energy transfer efficiency of Eu3+/Tb3+ doped and co-doped ZnMoO4 Sample
(ms)
(ms)
Average lifetime ( (ms)
5Eu
0.0655
0.4734
0.2670
4Eu, 1Tb
0.0745
0.7250
0.3074
)
3Eu, 2Tb
0.0500
0.6113
0.1585
2Eu, 3Tb
0.0593
0.4955
0.2349
1Eu, 4Tb
0.0668
0.8275
0.2497
5Tb
0.2432
0.2432
0.2432
PII: DOI: Reference:
S0022-2313(16)31892-0 http://dx.doi.org/10.1016/j.jlumin.2017.05.007 LUMIN14731
To appear in: Journal of Luminescence Cite this article as: Neha Jain, Bheeshma Pratap Singh, Rajan K. Singh, Jai Singh and R.A. Singh, Enhanced photoluminescence behaviour of Eu3+ activated ZnMoO4nanophosphors via Tb3+ co-doping for light emitting diode, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2017.05.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced photoluminescence behaviour of Eu3+ activated ZnMoO4nanophosphors via Tb3+ co-doping for light emitting diode Neha Jain1, Bheeshma Pratap Singh2, Rajan K. Singh1, Jai Singh1* , R. A. Singh1 1
Department of Physics, Dr. Harisingh Gour University, Sagar, M.P.-470003, India
2
Department of Physics, Indian Institute of Technology (BHU), Varanasi- 221005, India
*Email: [email protected], [email protected]
Abstract In the present study, structural and photo-luminescence studies of Eu3+ and Tb3+ co-doped ZnMoO4 host matrix has been investigated in detail. ZnMoO4:Eu3+/Tb3+ have been synthesized via a simple and cost effective co-precipitation method. XRD, Raman and FTIR studies confirmed its triclinic structure of Eu3+/Tb3+ co-doped ZnMoO4 host. Emission spectra of the measured samples corroborate the dominant electric dipole transition emission (5D0-7F2) intensity over magnetic dipole transition emission (5D0-7F1) intensity. Moreover, the improvement in red emission (5D0-7F2) for Eu3+ (3 at.%) and Tb3+(2 at.%) and high asymmetric ratio 10.4 is observed which dictates it as a highly red emitter. Additionally, commission international de-I’ Eclairage (CIE) values approaches to NSTC. Color purity of red emission improves after Tb3+ co-doping. Photoluminescence studies reveals that Tb3+ acts as a sensitizer for the Eu3+ activated ZnMoO4 host matrix. Study reveals that Eu3+/Tb3+ codoped ZnMoO4 can be excited by near UV device, which is bottleneck in the advancement of LEDs technology.
GRAPHICAL ABSTRACT
Schematic representation of enhancement in red emission intensity via cross relaxation process. Energy transfer mechanism from Tb3+ions to Eu3+ ion by cross relaxation.
Keywords- ZnMoO4, Energy transfer, Photoluminescence, Lifetime, Cross relaxation
1. Introduction Recently, rare earth doped phosphors have been widely investigated for their versatile applications particularly in white light emitting diodes (w-LEDs), field emission display,
scintillating detectors, temperature sensors, fluorescent lamps, solar cells, biomedical and bio-imaging are developed [1-10]. These applications of rare earth ions (lanthanides- Ln3+) occurs mainly due to 4f valence shell electrons and photoluminescence obtained due to f-f or 4f-5d transitions [11,12]. In addition, emission wavelength also depends on the splitting of energy levels Ln3+ ions, for example, Tb3+ activated phosphors are strong green emitter. Emission from Tb3+ ion is obtained due to the transitions of 5D3 - 7FJ in the blue and 5D4 - 7FJ in the green region (J = 6, 5, 4, 3, 2) and the transition intensities depend on its critical doping concentrations [5]. Also, Eu3+ activated phosphors are strong red emitter phosphors due to 5D0→7F2 electric dipole transition [2]. There are various ways for the enhancement in intensity of red emission has been reported such as annealing at higher temperature, doping of charge compensator and co-doping of rare earth ions [2, 12-15]. Annealing of samples can remove number of defects or impurities from surface of the phosphor because the rate of nonradiative decay rate decreases. Second way to enhance intensity of red emission is doping of charge compensator (Li+, Na+ and K+) with rare earth (Eu3+) ion. In this method, a deficiency created due to difference between oxidation state of metal ion (M2+) and rare earth ion (Ln3+) is nullified with charge compensator and thus non-radiative decay rate decreases. Co-doping of Ln3+ ion into doped phosphors is also an appropriate way to increase intensity since in this process energy transfer occurs from one Ln3+ ion acts as a sensitizer to another Ln3+ ion acting as an activator. The energy transfer process between Ln3+ ions is possible due to resonant energy transfer, energy transfer by non-radiative transition and quantum cutting [16]. Many combinations of sensitizer and activator of rare earth ions have been developed as Eu3+-Dy3+, Eu3+-Tb3+, Eu3+-Gd3+, Ce3+-Tb3+, Eu3+-Sm3+, Er3+-Yb3+, Tm3+-Yb3+ and Ho3+Yb3+ in which one ion transfers its energy to another ion [8,10,13,17- 20]. The energy transfer from sensitizerTb3+ to activator Eu3+ is of keen interest as in this combination multi- color emission can be observed (emission can be tuned from green-
yellow-to red) [9,18]. In addition, Tb3+ ion has strong excitation band in blue and ultraviolet (UV) region [34], which can efficiently transfer energy to Eu3+ ion thus resulting in the improved performance of w-LEDs. Although, energy transfer from Tb3+ to Eu3+ was previously reported in Y2O3, NaY(MoO4)2, NaLa(MoO4)2, CaGdAlO4, KGdPO4WO4, Ca8MgLu(PO4)7, LaPO4, Na(Y,Gd)F4 and Sr3AlO4F3 etc. [5,10,18,21-26] However, Tb3+ and Eu3+ co-doped ZnMoO4 photoluminescence (PL) properties have not been reported yet to the best of our knowledge. ZnMoO4 has wide band gap, high chemical and thermal stability [2729]. It exhibits two types of crystal phase, triclinic α-ZnMoO4 and monoclinic β-ZnMoO4, depending upon synthesis methods and environmental conditions [30-32].It has been already reported that luminescence intensity of Eu3+ activated ZnMoO4 was 93% more than CaMoO4:Eu3+, this indicates that number of defects are more in wolframite triclinic structure so more energy transfer from Mo-O to Eu3+ ion takes place and intensity of red emission is enhanced [33]. An intense red emission by energy transfer from Bi3+ to Eu3+ion into ZnMoO4 was also reported [34]. Several methods are reported earlier to prepare ZnMoO4 such as solid-state reaction, precipitation method, electrochemical assisted laser ablation and hydrothermal methods [35-38]. In this paper, variation in luminescence properties due to relative doping of Eu3+ and Tb3+ ion into ZnMoO4 are studied. Intensity of red emission corresponding to 5D0→7F2 transition improves with increasing Tb3+ ion concentration relative to Eu3+ ion in ZnMoO4. Here, we have reported the possible energy transfer mechanism from Tb3+ to Eu3+ ion and measured the energy transfer efficacy. The CIE co-ordinate for 3 at.% Eu3+-2 at.% Tb3+ codoped ZnMoO4 was (x= 0.61, y=0.32) closer to standard of NTSC (x=0.67, y=0.32). The variations in color purity with relative Eu3+ to Tb3+ concentration are also discussed.
2. Experimental details 2.1 Materials and Synthesis Zinc chloride (Merck, 99.9%), Sodium Molybdate (Merck, 99.9%), Europium oxide (Merck, 99.99%), Terbium oxide (Tb2O3, Merck, 99.99%), Concentrated Nitric acid (69%, Merck) and sodium hydroxide (Merck, 98%). The ZnMoO4:Ln3+ (Ln3+= 5at.%Eu3+-0at.%Tb3+, 4at.%Eu3+-1at.%Tb3+, 3at.%Eu3+-2at.%Tb3+, 2at.%Eu3+- 3at.%Tb3+, 1at.%Eu3+-4at.%Tb3+ and 0at.%Eu3+-5at.%Tb3+) were prepared by facile co-precipitation method. Firstly, to prepare 5at.% Eu3+ doped ZnMoO4 (E5), 1.021 gm of ZnCl2 and 0.069 gm of Eu2O3were added together into 10ml distilled water and then 2-3 drops of HNO3were added into it to dissolve Eu2O3 completely, Eu(NO3)3was formed. Excess of acid was removed by heating the solution at 80 °C with addition of deionized water (15ml) and pH of the solution was adjusted ~9-10 by adding NaOH solution. The solution was stirred continuously; 1.909 gm of Na2MoO4 was dissolved into 10ml water then added into previous prepared solution. The resulting solution was heated for 30 min. at 50°C with continuous stirring, and a white precipitate was formed. This precipitate was washed with distilled water, methanol and acetone to remove any impurity from sample and dried at 60°C for 2hrs, a white powder was formed. Finally, sample was calcined at 500°C for 6hrs for completion of oxidation and/or reduction of ions. Same procedure was followed to prepareZnMoO4: 4%Eu3+, 1%Tb3+ (E4T1),ZnMoO4:3% Eu3+, 2%Tb3+ (E3T2), ZnMoO4: 2%Eu3+, 3%Tb3+ (E2T3), ZnMoO4: 1%Eu3+, 4%Tb3+ (E1T4) and ZnMoO4: 5%Tb3+ (T5) by adding Eu2O3 and Tb2O3together into ZnCl2.
2.2 Characterizations
The structural analysis were characterized by using D8 Bruker X-ray diffractometer (XRD) with Ni-filtered Cu-Kα (1.5405Å) radiation at 40 kV and 40 mA. All patterns were recorded over the range 10° < 2θ < 80° with a step size of 0.02°. The morphology of particles was determined by using scanning electron microscope (FESEM, Model- Quanta 200 FEI, 30kv) and transmission electron microscope (TECNAI G2). Simultaneous DTA/TGA spectra were recorded using NETZSCH STA 449 F1. DTA and TGA analyses were carried out using 22 mg of the sample at a heating rate of 10 °C min-1 up to 1000 °C, in nitrogen air under a flow of 60cm3 min-1. The Raman spectra of the samples were recorded with Renishaw microRaman spectrometer attached with a laser excitation source of 633 nm. Infrared spectra were recorded on a Fourier transform infrared (FT-IR) spectrophotometer of Shimadzu (model 8400 S) with a resolution of 2 cm-1 and in the range 500–3800 cm-1. To measure IR spectra, the samples were mixed with KBr (Sigma Aldrich, 99.99%) in 1: 5 ratio and then spectra were
recorded.
UV-Vis
spectra
were
recorded
using
UV-2700
Double
beam
spectrophotometer in the absorbance mode. The samples were dispersed in the methanol and then UV-Vis spectra were recorded. Photoluminescence measurements were carried out under ultraviolet excitation using 266 nm radiation from a Nd: YAG laser and detected by a CCD detector (Model: QE 65000, Ocean Optics, USA) attached with the fibre. Lifetime decay was recorded with Edinburg instrument F-920 equipped with 100W flash xenon lamp as the excitation source. 3. Results and discussion 3.1 XRD The XRD pattern of Eu3+/Tb3+ doped and co-doped ZnMoO4 are shown in Figure. 1. All the diffraction peaks of ZnMoO4:Ln3+can be indexed considering triclinic structure (space group P-1) of α-ZnMoO4 with
card no. 5-0
5 (a 8.
Å,
.
1Å, c
.
4Å, α
106.87, β
101. 2, γ= 96.734 and
51 . 5Å3). Slight shift in peak position and
diffraction intensity of ZnMoO4:Ln3+ in XRD pattern arises due to substitution of Zn2+ ions by Eu3+/Tb3+.The peak at 2θ
25. 0° exhi it maximum diffraction intensity for E5, E4T1,
E3T2, E2T3 and E1T4 while T5 phosphor the maximum intensity is seen for the peak at 28.64°. The changes in diffraction intensities are caused by stronger diffraction due to the higher number of electrons in Eu3+ and Tb3+ doped and co-doped phosphor. Although, change in XRD pattern in 5at% Tb3+ doped ZnMoO4 also cause due to formation of Tb-Mo-O or MoO complex. The shift in diffraction intensity also supported by previous literature, in which diffraction intensity vary with doping of Eu3+ ion into Tb2Mo3O12 [39]. Lattice parameter and unit cell volume of Eu3+ and Tb3+ co-doped ZnMoO4 are given in table 1. The variations in Lattice parameter and unit cell volume is obvious due to substitution of Zn2+ (0. 00 Å) ion y Eu3+ ion (0. 4 Å) and T
3+
ion (0. 2 Å).
The structures of various compositions were analyzed by Rietveld refinement using FullProf suite [34]. The lattice parameters calculated for E5 sample were a = 8.28946 Å, . 14 8 Å, c E2T were a
. 55
Å, α
8.28 1 Å,
10 .001°, β .
1 Å, c
101.042° and γ . 4184 Å, α
. 12°, lattice parameter of 10 .
°, β
101.222° and γ
96.89°. Whereas Zn2+ has smaller radii as compared to Ln3+, the lattice parameters and unit cell volume should increase with doping of larger radii Ln3+. But in present paper cell volume shrink with Ln3+ ion doping. This is because of difference in oxidation state of Zn2+ and Eu3+/Tb3+ ion. olyhedral representation of triclinic α- ZnMoO4 is shown in figure 1(b). The crystal structure was established by using GSAS software (general structural analysis system). It can be clearly seen from this figure that zinc ion has octahedral configuration [ZnO6] and molybdenum ion has tetrahedral [MoO4] and octahedral [MoO6] coordination. Also, the zinc ion is surrounded by Mo- clusters. Moreover, average crystallite size was calculated by using Scherrer formula,
D=
Where,
(1)
is average crystalline size, λ is wavelength of X-Rays (0.15405 nm), β is full width
at half maximum (FWHM) and θ is the diffraction angle. The strong peaks at 2θ
25. 0 2
(210), 28.4279 (2 0 1) and 34.1309 ( ̅ ̅ 2) were used to calculate crystalline size of the samples. The crystalline size for different samples is given in table 1. The strain in ZnMoO4:Eu3+, Tb3+ was calculated by using Williamson-Hall (W-H) fitting method [40] and strain plot with linear fit is shown in figure 2. Strain arises due to lattice distortion in crystal. It may results in the change in bond length and lattice parameters due to ionic size mismatch. train values under studied systems are negative strain (ε) and having range in etween 0.002 - 0.005. The negative slope for ZnMoO4:Ln3+ indicates compressive strain into crystal. This is also consistent with the decreased unit cell volume for Eu3+/ Tb3+ doped and co-doped ZnMoO4. 3.2 Morphology Study Figure 3(a), (b) and (c) illustrate SEM images of 5at.%Eu3+ doped, 3at.%Eu3+-2at.%Tb3+ codoped and 5at.%Tb3+ doped ZnMoO4. Nanoparticles are spherical in shape and agglomerated in absence of surfactant. TEM micrographs of 5at.%Eu3+ doped, 3at.%Eu3+-2at.%Tb3+ codoped and 5at.%Tb3+ doped ZnMoO4 are shown in figure 4(a), (b) and (c), respectively. It can be seen from TEM micrograph that the size of the particles around ~50 nm which is good agreement with the crystalline size ~50-70 nm estimated by Debye- Scherrer formula. The HR-TEM of 5at.%Eu3+ doped, 3at.%Eu3+-2at.%Tb3+ co-doped and 5at.%Tb3+ doped ZnMoO4 are given in figure 4 (d), (e) and (f), with their d-value 0.191 nm, 0.191nm and 0.202 nm corresponding to (231), (231) and ( ̅ 41) planes, respectively. Selected area electron diffraction pattern (SAED) patterns of these samples are depicted in figure 4(g), (h) and (i), respectively.
3.3 TGA/DTA study The thermal properties of Eu3+ and Tb3+ co-doped ZnMoO4 (E3T2) was studied by using differential thermo-gravimetric analysis are shown in figure 5. Heat flow and change in mass was recorded under nitrogen atmosphere in temperature range from 23°C to 1000°C with heating rate 10°C min-1. TG curve shows mass loss of 1.35 % at 104°C, this mass loss is attributed to loss of water molecule from surface of the sample. An endothermic peak found in DTA curve at 104°C arises due to release of water content. Another mass loss of 1.70% at 350°C obtained due to complete evaporation of water from sample. Final mass loss 2.64% is obtained at 1000°C. DTA curve have two endothermic peaks at 520°C and 574°C which is attri uted to phase transition from α-ZnMoO4 to β- ZnMoO4 phase, similar results are also reported by Zhang et al. [29] These results reveals that Eu3+/ Tb3+ co-doped sample has high thermal stability. 3.4 Raman study The Raman spectra of 5at%Eu3+ and 5at%Tb3+ doped ZnMoO4 are shown in figure 6. The bands observed because of internal and external vibration of Mo-O unit into host ZnMoO4, so bands from 300-1000cm-1 were due to internal vibration of polyhedra [MoOn] and bands observed from less than 200cm-1 were due to external vibration of host. The main bands observed in present study are at ~977, 948, 910, 888, 785, 520, 350 and ~320 cm-1. The bands between ~880- 950 cm-1 are assigned to symmetry stretching mode (υ1) of Mo-O and bands observed between ~750- 880 cm-1 range are assigned to antisymmetric stretching mode (υ3) of [MoO4]2- tetrahedral unit [41-42]. The bands at ~320 and ~350 cm-1 represent symmetric (υ2) bending mode related to Mo-O bond and band at ~520 cm-1 is assigned due to antisymmetric (υ4) bending mode of Mo-O bonds [41]. These Raman results were obtained according to phase transition study of ZnMoO4 [1,42]. A weak intensity band observed at
977cm-1 indicates vibration of [MoO6]2- octahedral unit corresponding to Mo=O bond. However, intensity for ~520 cm-1 band for Tb3+ doped ZnMoO4 is less as compare Eu3+ doped ZnMoO4 due to slight deformation in Mo-O bond. 3.5 FTIR study FTIR spectra of Eu3+ and Tb3+ ion doped ZnMoO4 are shown in figure 7 in range ~550- 3800 cm-1.
ince α- ZnMoO4 have triclinic structure having [MoO4]2- tetrahedral and [ZnO6]2-
octahedral units so bands in spectra were found corresponding to their different vibration modes. The main bands were observed at ~ 800-900, ~1666 and ~3444 cm-1. The band near ~800 cm-1 observed due to symmetry and antisymmetric stretching mode of O-Mo-O into [MoO4]2-, other bands at ~3444 and ~1666 cm-1 correspond to –OH stretching and H-O-H bending, respectively [43]. Since there was no shift in bands with doping of Eu3+ and Tb3+ ion into ZnMoO4, all bands were attributed to bands of host matrix. Therefore, IR vibrational study confirms that there is no change in phase or structure due to doping of Eu3+/Tb3+ ion. 3.5 UV-Vis spectroscopy UV-Visible absorption spectra of ZnMoO4: Ln3+ (Ln3+- E5, E4T1, E3T2, E2T3, E1T4, T5) are shown in figure 8(a). The spectra exhibit strong absorption at 328 nm owing to charge transfer from ligand (oxygen) to metal (molybdenum) atoms inside [MoO4]2- group. On increasing relative concentration of Eu3+/ Tb3+, there was no shift in absorption peaks due to host. However, doping with Ln3+ ion slightly alter band structure of host ZnMoO4 which influence number of photons absorbed by phosphors [2, 44]. It may also evident in absorption spectra, compared to single 5 at.% doped Eu3+ and Tb3+, the absorption intensity of Eu3+ and Tb3+ co-doped phosphor is increases and decreases. The optical band gap energy of ZnMoO4: Ln3+was calculated y using Wood and Tauc’s method [45]. This method gives an empirical relation to calculate the optical band gap as:
αhv = K hν - Eg
n
(2)
where, α- is the a sor ance, h is lanck’s constant, ν is the frequency, Eg is the optical band gap and n is an exponent having values 1/2, 2, 3/2 or 3 depending on type of transition such as direct allowed, indirect allowed, direct forbidden or indirect forbidden, respectively. Since α- ZnMoO4 has direct band gap, so band gap is calculated y (αhv)2 Vs photon energy plot. The band gap plot for ZnMoO4 doped with E5, E3T2 and T5 are shown in figure 8(b), (c) and (d), respectively. The estimation of band gap values using equation (2) as ~ 3.60, 3.52 and 3.50 eV, respectively and these values are matches to and gap of α-ZnMoO4 (3.30 eV) [46]. The band gap show a red shift from E5, E3T2 to T5, which may be attributed to creation of defects into crystal with doping and relative co-doping of Eu3+ and Tb3+. 3.6 Photoluminescence study The excitation and emission spectra of 5at.% Eu3+ activated ZnMoO4 (E5) is shown in figure 9(a). The excitation spectra were recorded by monitoring emission wavelength at ~613 nm. Excitation spectra consists of broad excitation band at 250-330 nm and sharp peaks are observed in range ~330- 500 nm due to f-f transition of Eu3+ ion. The broad band signature arises mainly due to charge transfer band from ligand (O- 2p) to metal ion (Mo- 4d or Eu ion). Another sharp excitation peaks at 395, 410 and 466 nm were arises due to the characteristics electronic transitions7F0→5L6, 7F0→5D3 and 7F0→5D2 of Eu3+ ion. The emission spectra recorded under 280nm excitation have maximum intensity for 613 nm emission corresponding to transition 5D0→7F2. The characteristic emission peaks due to f-f transition of Eu3+ ion were also found at 591 (5D0→7F1), 615 (5D0→7F2), 654 (5D0→7F3) and 701 nm (5D0→7F4). Similarly, excitation and emission spectra of Tb3+ activated ZnMoO4is shown in figure 9(b). A broad excitation band around 250- 350 nm recorded by monitoring emission
wavelength 543 nm, is attributed 4f-5d transition of Tb3+ ion. This band centered at 272 nm which indicates successful substitution of Tb3+ ion in place of Zn2+ ion into ZnMoO4. The emission spectra consist of the characteristics peaks of Tb3+ ion at 488 (5D4→7F4), 544 (5D4→7F4) and 548 nm (5D4→7F4) under 290nm excitation. In present investigation, green emission is dominant due to transition from 5D4 excited state of Tb3+ ion. Emission from 5D3 state is absent for higher Tb3+concentration (> 1at.%)due to cross relaxation betweenlevel of Tb3+ ion, 5D3→5D4 (dipole cross relaxation) [5]. The excitation and emission spectra of Eu3+ and Tb3+co-doped ZnMoO4 (E2T3) are shown in figure 9(c). The excitation spectra consist of both Eu3+ and Tb3+excitation peaks monitored for 613 nm emissions. A broad band ~268-305 nm is attributed toEu3+-Tb3+/ MoO42- charge transfer band. The sharp peaks at 382 (7F0→ 5G4), 395 (7F0→5L6), 465 nm (7F0→5D2) were observed due to f-f transition in Eu3+ ion. Similarly, peaks at 416 nm (7F6→5D3) and 487 nm (7F6→5D4) were observed due to f-f transition of Tb3+. These observations show that excitation can be broadened with relative co-doping of Eu3+/Tb3+ ions. The emission spectra recorded under 290nm excitation has strongest emission peak at 618 nm corresponding to Eu3+ on 5D0→7F2 transition. The other peaks were observed at 591 (5D0→7F1), 654 (5D0→7F3) and 702 nm (5D0→7F4). Another peak around ~ 550 nm has very weak intensity found due to Tb3+ ion characteristic emission corresponding to
5
D4→7F5 transition.
Moreover, other characteristic emission due to f-f transition of Tb3+ion is absent because excited state of Tb3+ ion lie higher as compared to Eu3+ ion. Also, Tb3+ acts as sensitizer so it transfers its energy to Eu3+ ion via cross relaxation [5]. For instance, Tb3+ ions absorb energy and moves to its excited state (5D4 or 5D3). As there are several energy levels of Eu3+ ion in between 5D3 and 5D4 level of Tb3+ ion, so all the energy from Tb3+ ion is transferred to the Eu3+ ion and hence no Tb3+ characteristic emission was found.
The emission spectra under 290 nm excitation for all Eu3+ and Tb3+ co- doped ZnMoO4 phosphors are shown in figure 9(d). These spectra corroborate that as doping concentration of Tb3+ ion increases and Eu3+ ion concentration deceases than intensity of red emission has improved. The intensity of red emission enhanced until 3at.%Eu3+ and 2at.%Tb3+ concentration (E3T2), beyond this cross relaxation between Eu3+ and Tb3+ ion reduces intensity [47].Since in Eu3+/ Tb3+ co-doped system Eu3+ act as activator and Tb3+ act as sensitizer, so a defining limit for critical distance between Eu3+ and Tb3+ ions, it should e less than 4Å for exchange intensity.
ritical distance R c between Eu3+ and Tb3+ ions into
ZnMoO4 is estimated as [48]
(3)
Where, V is the volume of unit cell at critical concentration, n is number of sites available for doping in unit cell and χc is the total doping concentration at critical concentration. In this work volume of ZnMoO4 unit cell at critical concentration 3at.% Eu3+ and 2at.% Tb3+ is found 515.
5 Å3. Number of sites available for doping into ZnMoO4 is 6 and total optimum
concentration was χc 0.05 %. Critical distance (Rc) was calculated 14.8 Å which is much greater than critical distance 4.00 Å (maximum distance
etween two ions for energy
transfer) [49]. It may be the reason for the decrement in luminescence intensity beyond3 and 2 at.% concentration of Eu3+/Tb3+. These results reveal that by incorporating the relative concentration ratios of Eu3+ and Tb3+ ions into ZnMoO4, an enhanced red emission can be observed which is not mentioned previously in literature. The variation in intensity with concentration was calculated by curve fitting method. The curve fitted by using Gaussian distribution function and calculated area can be given by equation:
I= I0 + ∑
√
exp [2 (λ - λ0)2/w2]
(4)
Where, I be the intensity at particular concentration, I0 is background intensity, w is the full width at half maximum, A is area under curve, λ- wavelength and λ0 the mean value corresponding to the transition. The area under peak ranging from 600- 630 nm is due to electric dipole transition of Eu3+ ion without inversion and calculated by using Gaussian distribution function. Figure 10(a) presents variation in integrated area for electric dipole transition, it is maximum for E3T2. In addition, variation in full width at half maximum for Eu3+ -Tb3+ co-doped samples are shown in figure 10(b). The asymmetric ratio (A21) was used to probe the site symmetry environment of Eu3+ ions. Since, dominant electric dipole emission peak at ~615 nm over magnetic dipole emission at 590 nm is observed which is mainly due to occupancy of Eu3+ions as a site without inversion symmetry and vice versa then Eu3+ ions occupy site with inversion symmetry in the host matrices [50]. The higher value of A21 indicates dominant red emission of Eu3+iondue to electric dipole transition (5D0→7F2) transition [51]. In present investigation, asymmetric ratio for pure Eu3+activated ZnMoO4 is 10.39, which dictates ZnMoO4 host as a highly red emitter due to high asymmetric environment of Eu3+. 3.7 CIE Study The CIE chromaticity diagram of Eu3+ or Tb3+ doped and co-doped ZnMoO4 are shown in figure 11. The CIE chromaticity co-ordinates are calculated under 290 nm excitation listed in table 2. Tb3+ doped ZnMoO4 CIE co-ordinate are found in green region and Eu3+ doped ZnMoO4 co-ordinates are found in red region. The energy transfers from Tb3+ ions to Eu3+ ion are also confirmed by CIE co-ordinate. As Tb3+ concentration increases CIE co-ordinates show a red shift only for E3T2, on further increasing Tb3+concentration, CIE co-ordinate
move towards green region. CIE co-ordinates changed from red (0.55, 0.31) to green (0.28, 0.43). The color purity of color doped with Eu3+ or Tb3+ calculated by formula [52]
√
(5)
√
Where (x, y) are co-ordinate of sample point, (
,
) are co-ordinate of illuminate
wavelength and ( , ) are co-ordinate of white light in CIE diagram. In present study (
,
) = (0.67, 0.32)) and ( , ) = (0.3101, 0.3162). The color of pure Eu3+ doped sample is
66.67% enhanced with Tb3+ sensitizer and it is found maximum for E3T2 (x=0.61, y=0.32) is 83.33%. These results reveal that combination of Eu3+-Tb3+ gives an intense red emission, which is combination of Eu3+-Tb3+ gives an intense red emission, which is suitable for light emitting diodes. 3.8 Decay study Decay profiles give the signatures of energy transfer from Tb3+ to Eu3+ ion into ZnMoO4. The decay profile is shown in figure 12 under 290 nm excitation monitored for 543 nm emission of Tb3+ in Eu3+ /Tb3+ ion co-doped ZnMoO4 and for 613 nm emission for pure Eu3+ ion doped ZnMoO4. The decay data were fitted using a bi-exponential equation, which is given by
I = I1
-
+ I2
Where I1 and I2 are intensity at different time interval,
(6)
1
and
2
are there relative lifetime.
Fitted bi-exponential parameters vary with relative concentration of Eu3+/ Tb3+, while lifetime maximum for E3T2. The average lifetime of all Eu3+ and Tb3+ doped and co-doped samples were calculated by using following equation
(7) The lifetime values and average lifetimes were listed in table 3 for Eu3+/Tb3+ activated ZnMoO4. Average lifetime values changes with relative concentration of Eu3+ to Tb3+ ion. As Tb3+ ions transfer its energy to Eu3+ ion so its lifetime was decreases on increasing concentration of Eu3+ ion until 2at.% Tb3+. Energy transfer efficiency from sensitizer to activator was calculated by using formula [53]
(8)
Where, η is energy transfer efficiency from T presence of acceptor Eu3+ ion,
3+
ion to Eu3+ ion,
is average lifetime in
average lifetime of sensitizer (Tb3+ ion) in absence of
activator. For transfer maximum energy transfer luminescence lifetime should be decreased [18]. In present study, minimum average lifetime for 3at.% Eu3+ and 2at.% Tb3+ (E3T2) and for this, maximum energy transfer efficiency is 34.6%. This is the reason behind maximum PL emission is observed for E3T2.However, for low concentration of Eu3+ ion, Tb3+ -Tb3+ cross relaxation takes place which could affect energy transfer between sensitizer and activator [5]. It is also supported by lifetime measurements for 3, 4 and 5 at% Tb3+ the lifetime values were increased. Schematic representation of energy transfers and cross relaxation into MoO42-, Tb3+ and Eu3+ are shown in figure 13. Oxygen 2p valence shell electrons excite by absorbing excitation wavelength of 290 nm; this oxygen atom transfers its energy to molybdenum (Mo6+). Afterwards, Mo6+ ions transfer its energy to Tb3+ and Eu3+ by charge transfer. A discrete band into MoO42- was observed corresponding to wide band excitation. Energy transfer from MoO42-to Tb3+ and Eu3+ ions is also shown in this figure. For Tb3+, 5D4 excited state has slightly higher energy state compared to 5D1 excited state of Eu3+ ion, this attribute
energy transfers via cross relaxation process between 5D4 (Tb3+) to 5D1 (Eu3+) excited state by non-radiative (NR) transition. An electron jumps from 5D1 excited state to 5D0-excited state with non-radiative emission and thereafter relaxes to ground state by radiating orange and red emission corresponding to 5D0→7F1, 2. This corroborates the disappearance of greenemission in presence of activator (Eu3+) with sensitizer (Tb3+). 4. Conclusions In summary, triclinic ZnMoO4 phosphors doped with Eu3+ and Tb3+ ions have been successfully synthesized by co-precipitation method. Thermal studies (DTA/TG) of the phosphor features high thermal stability. The Raman, XRD and FTIR studies confirmed the tetrahedral
configuration
of
MoO42- group.
Co-doping
of Eu3+ andTb3+
ion
in
ZnMoO4 phosphors has been investigated by varying doping ratios of Eu3+ and Tb3+. The energy transfer from Tb3+ to Eu3+ ions confirm by lifetime measurement. The maximum energy transfer (34.6%) is found at 3/2 ratios of Eu3+ and Tb3+ whereas for other doping concentrations of Tb3+ and Eu3+ ions, energy transfer efficiency are very low. The effect of Tb3+ sensitizer to activator Eu3+ ion via energy transfer is analyzed which results in improvement in photoluminescence emission intensity and enhanced red color with 83.3% purity. A high asymmetric ratio ~10.4 is observed which dictates it as a highly red emitter. It is demonstrated that the excitation range are broadened by Tb3+ sensitizer and Eu3+ activated in ZnMoO4 which can be excited by NUV LED chip. The chromaticity coordinates (CIE) are much closed to the standard of National Television Standard Committee. These observations reveal that the above phosphors can be used as a potential red phosphor in w-LEDs and optoelectronic devices.
Acknowledgements Authors are thankful to NST Physics BHU for SEM characterization. We are thankful to Sophisticated Instrument Laboratory of the University for providing various characterization facilities (XRD, Raman, FTIR, DTA/TGA, HR-TEM). We are also thankful to Prof. S. B. Rai for providing PL facility. One of the authors (Neha) acknowledges the Central Research Fellowship provided by University Grants Commission (UGC), Govt. of India. Jai Singh would like to acknowledge UGC-India and DST for providing project under UGC Start-up Grant and DST Fast track. References 1. Hu D, Huan W, Wang Y and Wang Y 2015 J. Mater. Sci: Mater Elect. 26 7290 2. Maheshwary, Singh B P, Singh J and Singh R A 2015RSC Adv. 4 32605 3. Baur F, Glocker F, and Justel T 2015J. Mater. Chem. C 3 2054 4. Saha S, Das S, Ghorai U K, Mazumder N, Ganguly Dand Chattopadhyay K K 2015 J. Phys. Chem. C119 16824 5. Li X, Zhang Y, Geng D, Lian J, Zhang G, HouZ and Lin J 2014 J. Mater. Chem. C2 9924 6. Rai M, Singh S K, Singh A K, Prasad R, Koch B, Mishra K, Rai S B 2015 ACS Appl. Mater. Inter.7 15339 7. Parchur A K, Ansari A A, Singh B P, Hasan T N, Syed N A, Rai S B and Ningthoujam R S 2014 Integr. Biol.6 53 8. Pandey A, Rai V K, Kumar V, Kumar V and Swart H C 2015Sensors and Act. B209, 352 9. Ren L, Lei X, Du X, Jin L, Chen W and Feng Y 2013 J. of Lum.142 150. 10. Dorman J A, Choi. J H, Kuzmanich G and Chang J P 2012 J. Phys. Chem. C 116 12854 11. Wolfbeis O S 2011 Lanthanide Luminescence (Springer, New York)
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Figure 1. (a) XRD pattern of Eu3+ and Tb3+ doped and co-doped ZnMoO4 with JCPDS card no. 35-0765 (b) Polyhedral representation around z-axis of Tb3+ sensitized ZnMoO4:Eu3+.
Figure 2. lot for β cosθ /λ
s sinθ /λ of
at.%Eu3+ and 2at.%Tb3+ co-doped ZnMoO4
(E3T2).
Figure 3. SEM image of (a) ZnMoO4:5%Tb3+ (b) ZnMoO4:3%Eu3+, 2%Tb3+and (c) ZnMoO4:5%Eu3+
Figure 4. (a), (b) and (c) TEM image (d), (e) and (f) HRTEM image (g), (h) and (i) of ZnMoO4:5 at.%Tb3+, ZnMoO4:3at.%Eu3+2at.%Tb3+ and ZnMoO4:5at.%Eu3+, respectively.
Figure 5. DTA- TG curve of 3 at.% Eu3+ and 2 at.% Tb3+ co-doped ZnMoO4 (E3T2).
Figure 6. Raman Spectra of 5 at.%Eu3+ and 5at.%Tb3+co-doped ZnMoO4.
Figure 7. FTIR spectra of 5at.%%Eu3+ and 5 at.%%Tb3+ doped ZnMoO4.
Figure 8. (a) UV-Visible absorbance spectra of Eu3+ and Tb3+ doped and co-doped ZnMoO4 (E5, E4T1, E T2, E2T , E1T4 and T5) ( ) Band gap measurement (αhv)2 Vs hv for Eu3+ doped (c) Eu3+ and Tb3+ co-doped and (d) Tb3+ doped ZnMoO4.
Figure 9. Photoluminescence excitation and emission spectra of (a) 5at.%Eu3+ doped ZnMoO4 (b) 5at.%Tb3+ doped ZnMoO4 (c)2 at.% Eu3+ and 3 at.% Tb3+ co-doped ZnMoO4 and (d) Eu3+ and Tb3+ co-doped ZnMoO4 (total doping concentration 5at.%)
Figure 10. (a) Integrated intensity A2 and (b) full width at half maximum plot of asymmetry wavelength (613nm) for Eu3+ and Tb3+ co-doped ZnMoO4
Figure 11. CIE Chromaticity co-ordinate of Eu3+/ Tb3+doped and co-doped ZnMoO4 in various ratio.
Figure 12. Decay curve of Eu3+/ Tb3+ co-doped ZnMoO4 at 290nm excitation monitored for (5D4 level) 543 nm emission of Tb3+ ion.
Figure 13. Schematic representation of energy transfer between Eu3+ and Tb3+ into ZnMoO4.
Table 1 Cell parameter, crystalline size and strain of Eu3+/Tb3+ doped and co-doped triclinic ZnMoO4
Sample
Lattice parameters ell olume a (Å)
(Å)
Crystalline Strain
c (Å) (Å3)
size (nm)
ZnMoO4 (JCPDS35-0765)
8.36700
9.69100
6.96400
519.75
5Eu
8.28946
9.61468
6.95577
514.505
50.4
0.00370
4Eu, 1Tb
8.28074
9.62494
6.94212
512.387
50.1
0.00375
3Eu, 2Tb
8.59664
9.29355
6.92732
515.335
51.7
0.00261
2Eu, 3Tb
8.28717
9.63719
6.94184
512.670
54.9
0.00437
1Eu. 4Tb
8.49974
9.5544
6.91784
505.419
55.4
0.00464
5Tb
8.31348
9.65944
6.93839
514.476
52.1
0.00433
Table 2 CIE- Chromaticity co-ordinate and color purity of Eu3+/Tb3+ doped and co-doped ZnMoO4 S.N.
Sample
CIE
Color purity (%)
1
5Eu
(0.52, 0.30)
66.67
2
4Eu, 1Tb
(0.60, 0.32)
80.55
3
3Eu, 2Tb
(0.61, 0.32)
83.33
4
2Eu, 3Tb
(0.59, 0.32)
77.77
5
1Eu, 4Tb
(0.56, 0.31)
69.45
6
5Tb
(0.28, 0.43)
32.10
Table 3 Average lifetime obtained by bi-exponentially fit and energy transfer efficiency of Eu3+/Tb3+ doped and co-doped ZnMoO4 Sample
(ms)
(ms)
Average lifetime ( (ms)
5Eu
0.0655
0.4734
0.2670
4Eu, 1Tb
0.0745
0.7250
0.3074
)
3Eu, 2Tb
0.0500
0.6113
0.1585
2Eu, 3Tb
0.0593
0.4955
0.2349
1Eu, 4Tb
0.0668
0.8275
0.2497
5Tb
0.2432
0.2432
0.2432