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Journal of Science: Advanced Materials and Devices xxx (2017) 1e11

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Journal of Science: Advanced Materials and Devices

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Original Article

Facile combustion based engineering of novel white light emittingZn2TiO4:Dy3þ nanophosphors for display and forensic applications

K.M. Girish a, b, S.C. Prashantha b, c, *, H. Nagabhushana d

a Department of Physics, Dayanand Sagar Academy of Technology and Management, Bengaluru 560082, Indiab Research and Development Center, Bharathiar University, Coimbatore 641046, Indiac Research Center, Department of Science, East West Institute of Technology, VTU, Bengaluru 560091, Indiad Prof. CNR Rao Center for Advanced Materials, Tumkur University, Tumkur 572103, India

a r t i c l e i n f o

Article history:Received 18 March 2017Received in revised form2 May 2017Accepted 22 May 2017Available online xxx

Keywords:Zn2TiO4:Dy3þ

NanophosphorCIE and CCTJuddeOfeltLatent fingerprint

* Corresponding author. Research Center, DepartInstitute of Technology, VTU, Bengaluru 560091, India

E-mail address: [email protected] (S.C. PrashanPeer review under responsibility of Vietnam Nati

http://dx.doi.org/10.1016/j.jsamd.2017.05.0112468-2179/© 2017 The Authors. Publishing services b(http://creativecommons.org/licenses/by/4.0/).

Please cite this article in press as: K.M. Gnanophosphors for display and forensic apj.jsamd.2017.05.011

a b s t r a c t

Nanomaterials find a wide range of applications in surface based nanoscience and technology. To passhigh backward encumbrance, low sensitivity, complicated setup and poor universality in traditionalmethods for the enhancement of latent fingerprints and display applications, we explored the super-structures of dysprosium (Dy3þ) doped Zn2TiO4 via a facile solution combustion route. This method offersnew potentials in surface-based science comprising display, latent fingerprint, and luminescent ink foranticounterfeiting applications. The characteristic emissions of intra-4f shell Dy3þ cations in blue, yellowand red regions corresponding to 4F9/2 to

6H15/2,6H13/2, and

6H11/2 transitions respectively, showed whiteemission, and the JuddeOfelt theory was used to estimate photometric parameters. Concentrationquenching phenomenon is discussed based on possible interactions. Our study reveals a new prospect ofusing optimized Zn2TiO4:Dy

3þ nanophosphors for research in display, fingerprint detection, cheiloscopy,anti-counterfeiting technology, ceramic pigment and forensic applications.© 2017 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.

This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

1. Introduction

In recent years, luminescent phosphors have gained particularattention in white light generation for future technologies such aselectroluminescent devices, integrated optics, biomedical applica-tions, displays, X-ray detectors, solar cells, solid-state lighting,liquid crystal back lights, and white light emitting diodes (WLEDs),due to their energy efficiency, long lifetime and environmentallyfriendly properties [1e4].

Trivalent rare earth (RE3þ) cations are used as the luminescentactivators to convert Ultraviolet (UV), Near ultraviolet (NUV) orblue light radiation to visible light due to their 4f / 4f or 4f / 5dtransitions. Among the trivalent rare earth ions, dysprosium (Dy3þ)is selected as a good activator for the emission of light in blue,yellow and red regions related to the transitions 4F9/2 / 6H15/2, 4F9/2 / 6H13/2, and 4F9/2 / 6H11/2, respectively. The combination ofthese colours leads to white emission, which is suitable for the

ment of Science, East West.tha).onal University, Hanoi.

y Elsevier B.V. on behalf of Vietnam

irish, et al., Facile combustiplications, Journal of Science

manufacture of WLEDs [5,6]. It was noticed that the crystal struc-ture of the host lattice and dopant ion plays a major role on theluminescence properties of the phosphor. In this manner, titaniumbased inorganicmaterials have been studied vigorously due to theirexcellent properties and potential applications in various fields [7].Various methods such as solution combustion, solid-state re-actions, solegel, chemical co-precipitation, hydrothermal, sprayparalysis [8e13] etc., have been used for the synthesis of pure andrare earth doped ZnOeTiO2 nanophosphors.

Forgery was an ever developing global crisis that encounteredsystems. Anticounterfeitingmethods thatmake specific itemsmoredifficult to repeat, and simpler approaches to validate them wereconsequently primary for the fortification of manufacturers andpriceless documents. Several efforts have been made worldwide toprotect things and currencies from being forged. Though they haveobtained positive results, improvement is still needed in fabricationand design of security ink to prevent faking. Currently, a couple ofapproaches have been used tomake latent fingerprints (LFPs), sinceLFPs provide body proof for identification of individuals in againstthe crime spot [14].

Fingerprint detection was known as a good method for identi-fying people because of its immutable uniqueness. Conventional

National University, Hanoi. This is an open access article under the CC BY license

on based engineering of novel white light emitting Zn2TiO4:Dy3þ

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Fig. 1. Powder X-ray diffraction patterns of Zn2TiO4:Dy3þ nanophosphors.

K.M. Girish et al. / Journal of Science: Advanced Materials and Devices xxx (2017) 1e112

fingerprint materials (ferric oxide, TiO2, rosin lead, gold and silver)were unable to develop latent detection on some difficult surfacesin the form of powder or suspension, such as rough materials,fabrics, wetted materials and adhesives [15,16]. Recently, re-searchers using powders based on luminescent nanophosphors inorder to overcome such difficulties due to their biocompatibility,low toxicity and observed that an enormous progress in this di-rection on personal identification for forensic purposes [17e19].Amongst, powder dusting process is used as the easiest andcommonly encountered system for enhancement of LFPs in a shortinterval of time and with none elaborate requisites. The size andshape-tunable luminescent nano powders have been talents op-tions to overhaul such obstacles like low sensitivity and selectivity,excessive background problem and low contrast fingerprints goingthrough to make LFPs visible. These factors provide new prospectsof nanomaterials in surface science as security inks to guard high-value merchandise, documents, pharmaceuticals and currency[20e22]. In the present work, we report the structural, optical,luminescent, photometric and forensic properties of Zn2TiO4:Dy3þ

nanophosphors prepared by a facile solution combustion method.

2. Experimental

Zn2TiO4 nanophosphors doped with (1e11 mol%) Dy3þ wereprepared by the solution combustion method using Zinc nitrate[(Zn(NO3)2$6H2O)], tetra butyl titanate [TiO (NO3)2], Dysprosiumnitrate [Dy(NO3)2] as analytical reagents and oxalyl di-hydrozide(ODH) [C2H6N4O2] as a fuel. The stoichiometric ratios of analyticalreagents and fuel with minimum quantity of double ionized waterwere mixed well in a petri dish using magnetic stirrer and the dishcontaining a homogeneous mixture was placed in a pre-heatedmuffle furnace. The solution boiled and catched the fire afterdehydration gave white powder finally. Details of the synthesisprocedure have been reported elsewhere [23].

The crystalline nature of the powder samples is characterized byPXRD using X-ray diffractometer (Shimadzu) (operating at 50 kVand 20 mA by means of CuKa radiation at a wavelength of 1.541 Åwith a nickel filter at a scan rate of 2� min�1). The surfacemorphology of the product is examined by Hitachi table top (SEM)(Model TM 3000) (accelerating voltage up to 20 kV using Tungstenfilament). Transmission Electron Microscopy (TEM) analysis isperformed on a JEOL, JEM-2100 (accelerating voltage up to 200 kV,LaB6 filament) equipped with EDS having 1.5 Å resolutions. Thediffuse reflectance (DR) spectral studies of the samples are per-formed in the range 200e800 nm using Shimadzu UVeVis spec-trophotometer model 2600. Photoluminescence studies are madeusing Horiba (model Fluorolog-3) Spectrofluorimeter at RT using450 W xenon lamp as an excitation source. Fluor Essence™ soft-ware is used for spectral analysis.

3. Results and discussion

Powder X-ray diffraction (PXRD) patterns of the preparednanophosphors were recorded to study the phase, structure andinfluence of the Dy3þ ions in the host lattice. Fig. 1 illustrates thePXRD patterns of Zn2TiO4:Dy3þ (1 mol%), which show the highlycrystalline single phase cubic structure with JCPDS Card No. 77-14(space group Fd-3m (No. 227)) and no additional peaks, confirmingthat the dopant cations were well capped into the host lattice.Similar results were also observed for other concentrations whichwell match with our earlier report [23].

The lattice parameters for cubic Zn2TiO4 were estimated to be7.89 Å using 2d sin q ¼ nl for (2 2 0) plane. Further, the averagecrystallite size and other crystallographic parameters of the pre-pared nanophosphors were estimated using the following relations

Please cite this article in press as: K.M. Girish, et al., Facile combustinanophosphors for display and forensic applications, Journal of Sciencej.jsamd.2017.05.011

(Scherrer's and WilliamsoneHall method) [24,25] and all the ob-tained results are tabulated in Table 1.

D ¼ Klb cos q

(1)

3¼ b cos q4

(2)

n ¼ 43p

�D2V

� (3)

b cos ql

¼ 13þ D sin q

l(4)

d ¼ 1D2 (5)

sstress ¼ 3E (6)

Electron microscopy was carried out to investigate the surfacemorphology, orientation, size and structures of nano particles inthe prepared sample. It is clear from SEM images (not givenhere) that Dy3þ activated Zn2TiO4 nanophosphors consists ofcracks, pores, agglomerates, irregular morphology and large voids,due to escape of a large amount of gases with high pressure duringcombustion reaction [23,26]. TEM, SAED and HRTEM images (notgiven here) show the prepared samples are with an average size inthe range 20e100 nm, having polycrystalline nature (distinct ringpatterns) [27], and the high crystallinity was evidenced by thewell-



Table 1Estimated crystallite parameters of Zn2TiO4:Dy3þ nanophosphors.

Crystal planes Crystallite size (nm)Scherrer's

Dislocation density,d (�1014 m�2)

Micro-strain, 3(�10�3) sstress (�108 Pa) N Crystallite size (nm)WeH plot

Size-strain (�10�4)

220 44 5.1 0.75 1.35 11.84 ~20 ~13311 42 5.6 0.78 1.40 12.40400 34 8.6 0.97 1.74 15.32422 33 9.1 1.00 1.80 15.79511 31 10.0 1.07 1.92 16.81440 31 10.0 1.08 1.94 16.81

K.M. Girish et al. / Journal of Science: Advanced Materials and Devices xxx (2017) 1e11 3

defined lattice fringe patterns with a fringe width ~0.279 nm for(202) plane which was close to the calculated value of 0.278 nm.

Diffused reflectance spectroscopy (DRS) was performed forprepared dry powders to overcome the dispersed method whichwas popular in conventional UVeVis absorption spectroscopy. TheDR spectra of Zn2TiO4:Dy3þ nanophosphor are shown in Fig. 2a, inwhich the intense absorption band in the range 300e400 nmcorresponds to the ligand-to-metal charge-transfer (O2� to La3þ orO2� to Dy3þ) band, which consists of three absorption bands situ-ated at 454 nm due to host material, and the other bands observedwere due to the electric dipole (ED) transition from the groundstate 6H15/2 of Dy3þ to the different excited states such as 4F9/2, 6F7/2,and 6F5/2 respectively [28,29].

Further, SchustereKubelkaeMunk (SKM) theory was used tocalculate optical band gap energy (Eg) (Fig. 2b) as [30]

FðRÞhn ¼ A�hneEg

�n (7)

where n is the nature of the sample transition (n ¼ 1/2, 2, 3/2, 3;direct allowed, indirect allowed, direct forbidden, indirectforbidden transitions respectively) and F(R) is KubelkaeMunkfunction, which can be calculated by using the following equation:

FðRÞ ¼ ð1� RÞ22R

¼ KS

(8)

where K is a molar absorption coefficient and S is the scatteringcoefficient. The energy gap of Zn2TiO4:Dy3þ (1e11 mol%) wasdetermined to be in the range 3.13e3.18 eV, which indicates thatthe present material has a suitable energy gap to create excitedelectronehole pairs, leading to an improved photoluminescence[30].

Fig. 3 illustrates the excitation spectra of Dy3þ (3 mol%) dopedZn2TiO4 nanophosphor. The spectrum was composed of two parts:the broadband in the range 250e350 nm was the charge transferband (CTB) due to the electronic excitation of O(2p) to the empty 4forbital of Dy3þ activator, known as a ligand-to-metal charge-transfer transition (LMCT), which was then resolved into threeGaussian peaks due to (a) the band to band transition, (b) the ZneOcharge transfer band (O2p electron occupy one of the empty Znorbital), and (c) the Dy3þ to O2� charge transfer [31]. In addition toCTB, the excitation bands between 350 and 450 nmwere due to feftransitions of Dy3þ ions (4f6 configuration) in the host lattice. Theexcitation maxima situated at 420 nm correspond to the electronictransition 6H15/2 /

4G11/2 and the emission spectrumwas recordedwith the same excitation energy, since, the wavelength corre-sponding to the intense excitation band can give intense emissions.It shows that the present nanophosphor can be effectively excitedby energies in the visible region, which is useful for fabrication ofWLEDs.

Characteristic emission probabilities of Zn2TiO4:Dy3þ

(1e11 mol%) nanophosphors were recorded under a 420 nmexcitation energy and the results are shown in Fig. 4. The three


main emission peaks at 496 (blue region), 572 (yellow region),and 685 nm (red region) corresponding to the transitions 4F9/2 / 6H15/2, 4F9/2 / 6H13/2, and 4F9/2 / 6H11/2 respectively werethe characteristic emissions of Dy3þ luminescence. Out of these,the transition 4F9/2 / 6H15/2 is a magnetic dipole and 4F9/2 / 6H13/2 is due to the hypersensitive transition, which isstrongly affected by the crystal field environment [32,33]. Dy3þ

ions absorb the energy and get excited to the metastable excitedstates from the ground state, and after the excitation, part of theelectrons was depopulated into the 4F9/2 level giving non-radiative (NR) transitions while the rest was depopulated asmentioned (Fig. 5).

It can be observed that the intensity of the emission peakincreased with the increase of dopant concentration up to 3 mol%and diminished beyond this concentration, due to concentrationquenching. The marvel of concentration quenching is expected tooccur in a non-radioactive resonance energy transfer process by thecreation of ion vacancies between the neighbouring dopant ionsand the host matrix or due to multipolar (Dy3þeDy3þ) interactions.The dopant ion was substituted in the host site (Zn2þ) of Zn2TiO4

when the Dy3þ was doped as follows:

2½DyO12� þ ½VZnO12�/2½DyO12�x þ�VxZnO12

�where [DyO12] is a donor while, [VznO12] is an acceptor. Thefollowing equations indicate self-trapping of electrons afterexciting the prepared sample, which suggesting that the lumines-cence intensity may affect the energy transfer process with Dy3þ

ions and Zinc vacancies.

�VxZnO12

�complex þ e� excited/

hV1ZnO12

icomplex

hV1ZnO12

icomplex

þ e� excited/hV11ZnO12

icomplex

Fig. 6 shows a schematic diagram of the energy transfer be-tween Dy3þ ions and zinc vacancies (VZn) [34]. From the energymatch rule, the probable cross-relaxation channels (CRC1, CRC2 andCRC3) for Dy3þ ions were responsible for depopulation of 4F9/2level:

4F9=2 þ 6H15=2/6H9=2

.6F11=2 þ 6F5=2

4F9=2 þ 6H15=2/6H7=2

.6F9=2 þ 6F3=2

4F9=2 þ 6H15=2/6F1=2 þ 6H9=2

.6F11=2

In this process, the excitation energy was transferred from thehigher state Dy3þ ions into neighbouring Dy3þ ions, excited fromthe ground state to the metastable 4F9/2 level, and then was de-excited via these three cross-relaxation processes. Finally, all theDy3þ ions would go to their ground states as a result of thequenching of luminescence emission [35,36].



Fig. 2. a) Diffused reflectance spectra of Zn2TiO4:Dy3þ nanophosphors. b) Energy gap spectra of Zn2TiO4:Dy3þ nanophosphors.


Generally, the energy transfer without radiation occurs due tothe exchange or multipoleemultipole interaction among dopantions, the gap among the activator ions reduces at higher dopantdensity, leading to the decrease of emission intensity due to a non-radioactive energy transfer. For this kind of energy transfer


mechanism, it is essential to calculate the critical distance (Rc)between the dopant ions and can be estimated as

Rcz2�

3V4pXcN

�13

(9)



Fig. 3. Excitation spectra of Zn2TiO4:Dy3þ nanophosphors.


In the present case, V ¼ 604.63 Å3, N ¼ 4, Xc ¼ 0.03, and Rc ¼ 20 Åwhich is greater than 5 Å, indicating that multipolar interaction isresponsible for the concentration quenching in the presentnanophosphor.

Dexter theory was used to examine the exact kind of interactioninvolved in the quenching mechanism [37]

1X¼ k

h1þ bðXÞQ3

i�1(10)

By assuming b(X)Q/3 � 1, it can be written as [38]

log�1X

¼ K 0 � Q

3log X ðK 0 ¼ log K � log bÞ (11)

by the plot of log I/X vs log X, the value of Q was found to be 6.4(z6). This indicates that the dipoleedipole interaction plays a rolein the concentration quenching.

Themethod described by DeMello and Palsson [39,40] was usedto calculate the quantum efficiency (QE) of the optimized phosphor

QE ¼ Number of photons emittedNumber of photons absorbed

¼ Ec � EaLa � Lc

(12)

where Ec is related to the integrated luminescence caused by adirect excitation, Ea is associated with the integrated luminescencefrom the empty integrating sphere (blank, without sample), La isthe integrated excitation profile from the empty integrating sphere,Lc is the integrated excitation profile when the sample is directlyexcited by the incident beam. The QE for the Zn2TiO4:Dy3þ (3 mol%)phosphor was estimated by the integrated emission counts fromthe 450 to 700 nm. The value was found to be ~58%, indicating thehigh QE of the present sample. In our previous studies, the high QEwas also found to be 56%, 65% and 61% for MgO:Dy3þ [41],CeO2:Eu3þ [42] and YAlO3:Ho3þ [43] respectively.

The JuddeOfelt (JeO) theory has a significant achievement inthe explanation of radiative transitions in rare-earth dopeddifferent host materials [44,45]. The electric-dipole (ED) andmagnetic-dipole (MD) transitions are generally used to estimate


the line strengths of RE3þ ions, and their radiative transitions arepredominately ED in nature because the transitions of MD aremuch smaller than the forced ED transitions. As a result, MDtransitions are often neglected in the JeO calculations. The linestrength of the ED transition between J and J0 in terms of JeO in-tensity parameters can be expressed as

Sed ¼Xl¼2;4

Ul

4j

��UðlÞ��40j0

�2(13)

For RE3þ ions, it was essential to estimate radiative transitionprobabilities and radiative lifetime. The total transition probabilityof spontaneous emission from a level of angular momentum J is

AR ¼(64p4e2n3

3hð2J þ 1Þn�n2 þ 2

�29

)XJUJ

��UðJÞ��2 (14)

where n is the refractive index of the medium, nðn2 þ 2Þ2=9 is the

Lorentz local field correction,��UðJÞ�� is the doubly reduced matrix

elements and e is the the charge of an electron. The total transitionprobability AT and the radiative lifetime can be estimated by [46]

AT ¼X

AR (15)

trad ¼ 1AT

(16)

Also, the branching ratio, or the fraction of a population that willdecay to a given lower level, can be calculated from

b�4j� ¼ A

4j;4

0j0

�AT

�4j;

� (17)

The calculated values of the JeO parameters for theZn2TiO4:Dy3þ nanophosphors are summarized in Table 2. Itis noticed thatU2 >U4, which indicates amore symmetric structure



Fig. 4. Emission spectra of Zn2TiO4:Dy3þ (1e11 mol%) nanophosphors: a) 2D view (inset: variation of PL intensity with Dy3þ concentration), b) 3D view.


of the environment, the ionic nature of the bonding between theactivator and the surrounding ligands.

The optimized Zn2TiO4:Dy3þ (3 mol%) nanophosphor may beutilized as a luminous naming marker for an upgraded latent


fingerprint detection on an assortment of surfaces in forensicscience for individual identification. Initially, the finger markswere collected from the washed hand by pressing the fingers onthe surfaces of various substrates systematically and very little



Fig. 5. Energy level diagram indicating emission probabilities of Dy3þ in Zn2TiO4 nanophosphors.

Fig. 6. Schematic diagram of the energy transfer mechanism between Dy3þ ions and zinc vacancies in the Zn2TiO4:Dy3þ nanophosphors.


amount of the prepared sample was sprinkled on the substrate,when the sprinkled substrate exposed to UV light reveals thefinger impression giving intense white emission as shown inFig. 7.

Fig. 8 depicts a magnified spatial image of the black backgroundfingerprint developed by Zn2TiO4:Dy3þ nanophosphor. The white


luminescent nanophosphor was checked for being used in thelatent fingerprint identification and the image shows a bettercontrast to the ridges of finger mark with the background, since theparticles were nano in size as confirmed by TEM [47]. It consists ofclear ridges and mainly secondary details like ridge termination,ridge splitting, crossover and, lake bifurcation which are unique to



Table 2JuddeOfelt intensity parameters (U2, U4), radiative transition probability (AT),calculated radiative (trad) lifetime and branching ratio of Zn2TiO4:Dy3þ

nanophosphors.

Dy3þ conc(mol%)

JeO intensity parameters(�10�20 cm2)

Wavelength(nm)

AT trad(ms)

b

U2 U4

1 2.77 1.68 495 31 3.2 74.1570689

3 2.89 1.78 495 33 3.0 74.5570689

5 2.81 1.69 495 32 3.1 73.6570689

7 2.69 1.64 495 30 3.3 76.0570689

9 2.75 1.67 495 31 3.2 75.2570689

11 2.75 1.67 495 31 3.2 74.6570689

Fig. 7. Illustration of the development of latent finger mar

Fig. 8. Magnified spatial images of



form the basics of personal identification and are helpful in theidentification of fingerprints [17,18].

Further, it is noticed that lip prints are permanent and un-changeable for individuals like fingerprints, due to which lipgrooves, provides information and evidence in personal identifica-tion, criminal investigation in dentistry, sex determination and ageestimation. Lip prints were used in Cheiloscopy to identify personson the basis of the arrangement of lines on red parts of the lips[48,49]. Lip prints were labeled based on geometric dominance oflines present like vertical, intersected, branched, reticular, unde-termined and the general pattern of lip print is shown in Fig. 9a.Sharma et al. noticed that vertical and intersected patterns aredominant in females whereas branched and reticular patterns arepredominant in males [50]. In the present study, the different pat-terns on different parts of the lips were identified and, marked asshown in Fig. 9b, in which the female dominant nature wasobserved.

Generally the colour of the phosphor can be identified byCommission International de I' Eclairage (CIE) colour coordinates.The CIE plot is drawn using a colour calculator software, and the CIEcoordinates (x, y) can be calculated based on the following

ks using Zn2TiO4:Dy3þ nanophosphors during process.

black background fingerprint.



Fig. 9. a) Patterns of lip prints. b) Various lip print patterns identified using Zn2TiO4:Dy3þ nanophosphors.


equations and the calculated coordinates lie in the white region ofthe CIE spectra (Fig. 10) show a possible candidature for WLEDs.

x ¼ XX þ Y þ Z

(18)

y ¼ YX þ Y þ Z

(19)

In addition to this, Correlated Colour Temperature (CCT) wasestimated by CIE coordinates, used to define the colour tempera-ture of a light source. CCT was calculated by transforming the (x, y)coordinates of the light source to (U0, V0) using the followingequations. By determining the temperature of the closest point ofthe Planckian locus to the light source on the (U0, V0) uniform


chromaticity diagram, the average CCT was found to be 4499 K,indicating the warm white light used for residence appliances,since it is less than 5000 K [51,52] and all the photometric valuesare summarized in Table 3.

U0 ¼ 4x(20)
�2xþ 12yþ 3
V 0 ¼ 9y�2xþ 12yþ 3

(21)

Also, colour purity (CP) was calculated to check the colour po-tentiality [5] of the prepared phosphor as



Fig. 10. CIE diagram of Zn2TiO4:Dy3þ (1e11 mol%) nanophosphors.

Table 3Photometric characteristics of the Zn2TiO4:Dy3þ nanophosphors.

Phosphor Concentration(mol%)

CIE coordinates CCT coordinates AverageCCT (K)

X Y U0 V0

Zn2TiO4:Dy3þ 1 0.36905 0.40246 0.20817 0.51078 44493 0.36593 0.40153 0.20655 0.509955 0.36961 0.40885 0.20628 0.513427 0.36600 0.40119 0.20671 0.509829 0.36635 0.40156 0.2068 0.5100211 0.36874 0.40184 0.20636 0.50991


color purity ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðxs � xiÞ2 þ ðys � yiÞ2

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðxd � xiÞ2 þ ðyd � yiÞ2

q � 100% (22)

where (xs, ys), (xi, yi) and (xd, yd) are the chromaticity coordinates ofthe sample point, coordinates of the illuminant point, and thedominant wavelength point respectively. The CP ofZn2TiO4:Dy3þ(1e11 mol%) nanophosphors was found to be in therange of 10e15%, which is close to the white light source, indicatingthat the Dy3þ doped Zn2TiO4 phosphor is a prominent candidate foruse in WLEDs and display applications.

4. Conclusion

Zn2TiO4:Dy3þ (1e11 mol%) nanophosphors were successfullyprepared by the facile solution combustion method. Pure, singlecubic phase, porous, agglomerated and nanocrystallites wereconfirmed by PXRD and electron microscopy studies. The intenseabsorption band in the range 300e400 nm in the diffused reflec-tance studies was shown to correspond to the ligand-to-metalcharge-transfer (O2� to Zn2þ or O2� to Dy3þ) band. All the charac-teristic emissions 4F9/2 / 6Hj (j¼15/2, 13/2, 11/2) of Dy3þ ion in aZn2TiO4 matrix were confirmed by the PL emission studies. Theestimated branching ratio was found to be ~74%, indicating theusefulness of the present nanophosphor for display device appli-cations. From the CIE chromaticity coordinates, the detection offingerprint marks on different surfaces and lip print images in-dicates that Zn2TiO4:Dy3þ is very promising for warmWLEDs, solidstate lighting, forensic sciences and Cheiloscopy applications.

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