optical behaviour of cadmium and mercury free eco-friendly lamp nanophosphor for display devices

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Results in Physics xxx (2014) xxx–xxx

RINP 105 No. of Pages 6, Model 5G

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Contents lists available at ScienceDirect

Results in Physics

journal homepage: www.journals .e lsevier .com/resul ts - in-physics

Optical behaviour of cadmium and mercury free eco-friendly lampnanophosphor for display devices

http://dx.doi.org/10.1016/j.rinp.2014.04.0032211-3797/� 2014 The Authors. Published by Elsevier B.V.This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/).

⇑ Corresponding author. Tel.: +91 09826937919.E-mail address: [email protected] (V. Dubey).

Please cite this article in press as: Tiwari R et al. Optical behaviour of cadmium and mercury free eco-friendly lamp nanophosphor for display dResults Phys (2014), http://dx.doi.org/10.1016/j.rinp.2014.04.003

Ratnesh Tiwari a, Vikram Awate b, Smita Tolani c, Namrata Verma d, Vikas Dubey a,⇑,Raunak Kumar Tamrakar e

a Department of Applied Physics, Bhilai Institute of Technology (Seth Balkrishan Memorial), Raipur 493661 (C.G.), Indiab Department of Physics, S.B. Jain Institute of Technology Management & Research, Nagpur, Indiac Department of Physics, St. Vincent Pallotti College of Engineering and Technology, Nagpur, Indiad Department of Electronics and Telecommunication, Bhilai Institute of Technology (Seth Balkrishan Memorial), Raipur 493661 (C.G.), Indiae Department of Applied Physics, Bhilai Institute of Technology (Seth Balkrishan Memorial), Durg 491001 (C.G.), India

a r t i c l e i n f o a b s t r a c t

28293031323334353637

Article history:Received 15 April 2014Accepted 24 April 2014Available online xxxx

Keywords:Lamp phosphorPhotoluminescenceThermoluminescenceCGCD

3839404142

The present paper reports the synthesis of cadmium and mercury free lamp (Y, Gd)BO3: Eu3+ phosphorwhich is in nano range useful for display device application. The phosphor doped with Eu3+ was synthe-sized by the solid state reaction method which is suitable for large scale production and eco-friendly. Theprepared phosphor was characterized by the X-ray diffraction technique (XRD), field emission gun scan-ning electron microscopy (FEGSEM) and transmission electron microscopy (TEM). The optical behaviourof the prepared phosphor was determined by photoluminescence (PL) spectra recorded in room temper-ature. The PL excitation spectra were found at 470 nm and the emission spectra cover all visible regions(419–625 nm) which indicate that the prepared phosphor can act as a single host for white light emittingdiode (WLED) application and verified by Internationale de I’Eclairage (CIE) techniques. The thermolumi-nescence (TL) glow curve was recorded for Eu3+ doped (Y, Gd)BO3 phosphor. The TL glow curve wasrecorded for UV, beta and gamma irradiations and also the kinetic parameters were calculated. In addi-tion to this trap parameters of prepared phosphor were studied using computerized glow curve decon-volution (CGCD).� 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://

creativecommons.org/licenses/by/3.0/).

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1. Introduction them yttrium and lanthanide orthoborates of formulation LnBO 62
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Up to present, (Y, Gd)BO3: Eu3+or Y2O3: Eu3+for red, Zn2SiO4:Mn2+

or BaAl12O19:Mn2+ for green, and BaMgAl14O23: Eu2+ orBaMgAl12O17:Eu2+ for blue are the main phosphors for plasma dis-play panel (PDP) [1,2]. A chief drawback to these phosphors is lowefficiency [1,2]. Along with the great advances made in PDP andHg-free fluorescent lamps, the demand for highly efficient phos-phors under vacuum ultraviolet (VUV) excitation increases signifi-cantly [1–5]. One of the key factors controlling luminescenceefficiency is the energy transfer process from host to activator, i.e.host sensitization process [6]. The ability of host sensitization isdetermined by ionic radius, electronegativity, and type of anions/cations [1,6].

Red light emitting Eu3+ phosphors are extensively used in lampand display applications, and many Eu3+ doped materials are beingexamined for use in new flat panel display technologies. Among

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(Ln = Rare Earth (RE), Y) have proved to be potential candidatesfor such applications [7]. The luminescence of the rare earth ionsin inorganic hosts has been extensively investigated during the lastfew decades. At present, the most widely used red-emitting phos-phor for plasma display panel (PDP) is (Y, Gd)BO3:Eu3+[8,9]. How-ever, the colorimetric purity of (Y, Gd)BO3:Eu3+ is not sufficient toproduce a high quality colour TV picture due to the emission5D0–7F1 (592 nm, red–orange colour) as the most prominent groupin the luminescence spectrum. Much attention has been paid forother borates for luminescence [10–13]. Over the last few years,much attention has been paid to the synthesis and luminescentproperties of Eu3+-doped rare-earth orthoborates (REBO3) due totheir desirable properties as ideal vacuum ultraviolet phosphors,whose progress is key to the development of plasma display panels(PDPs) [14]. Various synthesis techniques have been developed toprepare high-quality. In the present paper the (Y, Gd)BO3:Eu3+

phosphor was synthesized by the solid state reaction method.The structural, morphological characterizations were recorded aswell as the optical behaviour of the phosphor. Prepared phosphor

evices.

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shows very good emission spectra in all visible regions and usefulfor display applications.

2. Experimental

Eu3+ doped (Y, Gd)BO3 phosphor was synthesized with Y2O3

(99.99%), Gd2O3 (99.99%), Eu2O3 (99.99%), and H3BO3 (99.5%) as rawmaterials. Stoichiometric Y2O3, Gd2O3, Eu2O3, and 5 mol% excessiveH3BO3 were thoroughly ground by adding proper ethanol to mixhomogeneously and then sintered at 700 �C for 2 h. The mixtureswere re-ground thoroughly, and sintered at 1200 �C for another 3 h.Then the fired products were ground and washed with 80–100 �C dis-tilled water several times to remove the excess B2O3, and dried at160 �C. The prepared phosphor was Eu3+ (1 mol%) doped (Y, Gd)BO3.

Sample phases were identified by using Bruker D8 AdvanceX-ray diffractometer. The X-rays were produced using a sealedtube and the wavelength of X-ray was 0.154 nm (Cu K-alpha).The X-rays were detected using a fast counting detector basedon Silicon strip technology (Bruker LynxEye detector). Observationof particle morphology was investigated by scanning electronmicroscope (JEOL JSM-6360). The photoluminescence (PL) emis-sion and excitation spectra were recorded at room temperatureby use of a Shimadzu RF-5301 PC spectrofluorophotometer. Theexcitation source was a xenon lamp. Thermally stimulated lumi-nescence glow curves were recorded at room temperature by usingTLD reader I1009 supplied by Nucleonix Sys. Pvt. Ltd., Hyderabad[15–18]. The obtained phosphor under the TL examination is irra-diated with UV radiation using 365 nm UV source, beta irradiationusing Sr90 source and gamma irradiation using Co60 source. Heat-ing rate used for TL measurement is 6.7 �Cs�1.

3. Results and discussion

The XRD pattern of the sample is shown in Fig. 1. The width ofthe peak increases as the size of the particle decreases. Broad peak

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Fig. 1. XRD pattern of (Y, Gd)BO3:Eu3+ (1%

Please cite this article in press as: Tiwari R et al. Optical behaviour of cadmiuResults Phys (2014), http://dx.doi.org/10.1016/j.rinp.2014.04.003

shows the nano crystalline behaviour of the sample. The size of theparticle has been computed from the full width half maximum(FWHM) of the intense peak using Scherer formula. Particle sizeof sample in the range 40 nm is found. Formula used for calculationis

D ¼ 0:9kb cos h

Here D is particle sizeb is FWHM (full width half maximum)k is the wavelength of X-ray sourceh is angle of diffractionD = 0.9�1.54/0.17�Cos (27.60) = 40 nm

Fig. 1 shows the powder XRD pattern of (Y, Gd)BO3:Eu3+. Thepeaks were found to be in agreement with JCPDS card No. 16–0277 [19] reference corresponding to the [002], [100], [101],[102], [004], [110], [104], [112], [200], [202], [114], [204]and [210] planes. It confirms the formation of mixed phase hexag-onal crystalline yttrium gadolinium orthoborate phosphor [20].The formation of a mixed phase is attributed to the high tempera-tures generated during the solid state reaction method and formthe (Y, Gd)BO3 phosphor. The particle size calculated using Schererformula is 40 nm. Table 1 represents the summary of XRD with dif-fraction angle, particle size and FWHM of prepared phosphor.

4. Field emission gun scanning electron microscopy (FEGSEM)

The surface morphology of prepared phosphor was representedby FEGSEM images (Fig. 2a and b). From SEM images it is con-cluded that the prepared phosphor shows nanocrystalline behav-iour and good connectivity with grain which shows that powdersize and morphology are well controlled. No significant differenceis observed in XRD patterns and SEM micrographs, therefore, all

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) phosphor and JCPDS card 16–0277.

m and mercury free eco-friendly lamp nanophosphor for display devices.


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Table 1Summary of diffraction angle, crystallite size and FWHM of prepared phosphor.

S. No. 2h[�2Th.] Hkl Intensity[cts] FWHM [�2Th.] D (particle size) nm

1 20.53 002 8388 0.15 472 27.60 100 20,934 0.17 403 34.46 101 14,268 0.28 324 41.31 102 2100 0.25 355 48.48 004 8339 0.25 356 50.23 110 6699 0.39 257 53.02 104 5076 0.39 258 56.51 112 1779 0.30 159 60.64 200 2533 0.44 9

10 70.57 202 1296 0.55 5

Fig. 2. (a) FEGSEM image X40000 (b) FEGSEM image at X50000 resolution.

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samples are well crystallized into hexagonal (Y, Gd)BO3 structure.These manifest that the enhancement in luminescence efficiency isnot caused by grain morphology.

5. HRTEM (High resolution transmission electron microscopy)

Fig. 3(a and b) displayed the HRTEM images of samples pre-pared under different resolutions. All these samples exhibited asphere-like morphology with the particle size of about 5–47 nm.This two-dimensional growing habit coincided with the conceptof preferential nucleation in this system, when the particle sizecontinued to increase, a regular morphology of hexagonal flake

Fig. 3. (a) HRTEM image of phosphor


was also observed, indicating that the crystals were better crys-tallized. These images are in very good agreement with XRD andFEGSEM results. They clearly show the formation of nanosphere.The particle size of one atom is �15 nm.

6. Photoluminescence study

The photoluminescence excitation spectra show the blue shiftat 470 nm which indicate host absorption band (HB), Eu3+–O2�

charger transfer (CTB), and 8S7/2 ? 6PJ transitions of Gd3+ (doublet)(Fig. 4). The presence of Gd3+ absorption in Eu3+ excitation spectraindicates that there is energy transfer from Gd3+ to Eu3+.

(b) HRTEM image of phosphor.



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The prominent excitation band was observed at 470 nm (NUV)due to the transition of Eu3+ (7F0 ?

5L6) and this clearly indicatesthat (Y, Gd)BO3:Eu3+ phosphors are effectively excited by nearultraviolet light emitting diodes (NUV-LEDs) also [18].

The emission spectrum of phosphors was recorded by excita-tions with 470 nm. The emission spectrum is shown in Fig. 5 whichis composed of 5D0–7FJ (J = 1, 2, and 4) emission lines of Eu3+. Ingeneral, when the Eu3+ ion is located at crystallographic site with-out inversion symmetry, its hypersensitive forced electric-dipoletransition 5D0–7F2 red emission dominates in the emission spec-trum. If the Eu3+ site possesses an inversion centre, 5D0–7F1 orangeemission is dominant [21]. The origin of these transitions (electricdipole or magnetic dipole) from emitting levels to terminating lev-els depends upon the location of Eu3+ ion in (Y, Gd)BO3 lattice andthe type of transition is determined by selection rule [17]. Themost intense peak in the vicinity of 613 and 625 nm is ascribedto the hypersensitive transition between the 5D0 and 7F2 levelsdue to forced electric dipole transition mechanism. The weak emis-sion at 590 and 585 nm corresponds to the magnetic dipole transi-tion of 5D0 and 7F1 levels.

The peaks found at 419, 428, 467 and 495 nm may be due to theoxygen ion vacancies in host lattices. The peak at 364 nm is due tocrystal field effect in the prepared phosphor.

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Fig. 5 shows the curve of PL intensity of (Y, Gd)BO3:Eu3+, thispicture reflects that the best doping value of Eu3+ is 1%. Emissionband centred at 467 nm is attributed to recombination of a delocal-ized electron close to the conduction band with a single chargedstate of surface oxygen vacancy [22]. The emission band at534 nm can be attributed to self-trapped excitation luminescence[23]. Other weak emission peaks at 495, 513, 539 and 554 nmare due to different kinds of oxygen vacancies. Maximum peaksare due to the Gd3+ ions present in the host material and theintense peaks due to the Eu3+ ion as a dopant which is dominatingin nature. So we conclude that the present phosphor can act a sin-gle host for white light emitting diode application as well as PDPs(plasma display panels). This prepared phosphor is mercury andcadmium free lamp phosphor in nano size range and eco-friendly.

7. CIE coordinate

The results indicate that (Y, Gd)BO3:Eu3+ (1%) phosphors can beselected as a potential candidate for LED (light emitting diode)application as well as for FL (fluorescent lamp) and compact fluo-rescent lamp (CFL) (Ex.470). However, the relative intensity ofthe emission bands which provide the fundamental colour balancefor white-light emission was achieved with the 1 mol% samplewith the spectrum (Fig. 5) providing the CIE 1931 chromaticitycoordinates much closer to the equal-energy white-light. The CIEis the spectrophotometric determination (Fig. 6). It shows the 1–8 different peaks of PL spectra up to blue to red region.

8. Thermoluminescence glow curve study

The TL glow curves of prepared phosphor for UV, beta andgamma irradiations were recorded in room temperature for30 min UV and beta irradiations as well as 0.5 kGy for gamma irra-diation. It shows well resolved peak at 200–500 �C as experimentaland fitted curve. The dose–response of the phosphor was studiedby irradiating the phosphor to different doses of UV, beta andgamma radiations and measuring the TL. The shift in peak temper-ature for first peak (low temperature) is more compared to the

Fig. 6. CIE coordinate for (Y, Gd)BO3:Eu3+ phosphor.



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shift in peak temperature of second peak (high temperature). Thissuggests that the low temperature peak corresponds to kineticorder close to 2 and high temperature peak will have a kineticorder lesser than the low temperature peak.

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9. Kinetic parameters by the glow curve de-convolution method

Computerized glow curve deconvolution was carried out usingthe Spreadsheet program. The experimental glow curve recordedfor (Y, Gd)BO3:Eu3+ was fitted into individual glow peaks by inter-facing the solver utility of Microsoft Excel as reported by Afouxeni-dis et al. [24]. This simple glow curve was fitted to five glow peakswith two major peaks and three satellite peaks. The general orderkinetics (GOK) equation suggested by Kitis et al. [24] was used forthe fitting. The constraint that was used during the fitting was0 < b < 2. The solver is an Excel Add-in general purpose softwarepackage which minimizes the sum of squares of the residuals toperform a least square fitting. The experimentally determinedquantities like glow peak intensity (Imax), glow peak temperature(Tmax) and the guess values of trap depth (E) and order of kinetics(b) were used as fitting parameters. By changing the values of theseparameters, the experimental glow curve was fitted such that thefigure of merit (FOM) percentage is at minimum. Figs. 7–9 show

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the experimental glow curve (black line) along with the calculatedcurve (red line) using the GOK equation. The residual TL intensityversus temperature after curve fitting is shown in inset. The figureof merit was 0.63%. The lower value of FOM suggests that the anal-ysis process was accurate. The values of the trap depth and theorder of kinetics obtained for the two dominant peaks were 0.67,1.44, 1.4 and 1.77 eV for UV, beta and gamma irradiations.

In Fig. 7(a) the CGCD curve of the experimental TL glow curve(Fig. 7(b)) which shows the three TL glow curve which is fittedfrom experimental data. The experimental TL glow curve showsthe 2 distinct peaks at 152 and 325 �C which indicate that for UVirradiation the formation of shallow traps occurs and higher tem-perature peak is a hidden form. The hidden peak is calculated bycurve fitting techniques. Similarly for beta irradiation the experi-mental glow curve is shown in Fig. 8(b) which shows the two glowpeaks at 144 and 297 �C. The peak is shifted towards the lowertemperature side as compared to UV irradiation means that theformation of deep trap for beta irradiation and formation of shal-low trap for UV irradiation. Also for gamma irradiation the TL glowcurve shows very good TL peak at 247 and 290 �C. When we com-pare all those experimental peaks it is concluded that for beta andgamma irradiations the intense peak shows the lower temperaturepeak as compared to UV irradiation.

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9 May 2014

Formation of traps is due to host lattice and the Eu3+ dopantwhich shows the second order kinetics. The two traps form inthe luminescence centre.

10. Conclusion

It is concluded that the Eu3+ doped (Y, Gd)BO3 phosphor wassynthesized by the solid state reaction method. The sample showsthe hexagonal structure and particle size calculated by Scherer for-mula found in the range 5–47 nm. The surface morphology wasconfirmed by FEGSEM through the nano sphere formation. TheTEM images also satisfy the XRD and FEGSEM results which indi-cate the nanospheres. Optical behaviour of mercury and cadmiumfree lamp phosphor (Y, Gd)BO3 doped with Eu3+ shows all theemission spectra in visible range which indicate that the phosphorcan act as a single host for WLED application and useful for certaindisplay devices. The thermoluminescence and CGCD show thegood thermoluminescence study and the value of trap depth isfound between 0.67 and 1.77 eV.

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optical behaviour of cadmium and mercury free eco-friendly lamp nanophosphor for display devices

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