RSC Advances

PAPER

Laser and Spectroscopy Laboratory, Departm

Mines, Dhanbad-826004, Jharkhand, India.

ap@ismdhanbad.ac.in; Tel: +91-326-223-54

Cite this: RSC Adv., 2014, 4, 21844

Received 18th February 2014Accepted 22nd April 2014

DOI: 10.1039/c4ra01400f

www.rsc.org/advances

21844 | RSC Adv., 2014, 4, 21844–2185

Simultaneous influence of Zn2+/Mg2+ on theluminescent behaviour of La2O3:Tm

3+–Yb3+

phosphors

Astha Kumari, Anurag Pandey, Riya Dey and Vineet Kumar Rai*

The structural characterizations of La2O3 phosphors doped/codoped with Tm3+ and Yb3+ ions synthesized

by the urea assisted solution combustion techniquewere performed using X-ray diffraction analysis, Fourier

transform infrared spectroscopy and scanning electron microscopy. Codoping with Zn2+/Mg2+ ions causes

an increase in the particle size and aggregation for the La2O3:Tm3+–Yb3+ phosphor. The upconversion (UC)

study under 980 nm excitation shows four UC emission bands centred around 476 nm (1G4/3H6), 653 nm

(1G4/3F4), 702 nm (3F2/

3H6) and 795 nm (1G4/3H5). The effect of codoping with Zn2+ andMg2+ ions in

the Tm3+–Yb3+ codoped La2O3 phosphor was investigated. Decay curve analysis of the prepared

phosphors was done to understand the mechanism of the upconversion emission intensity variation due

to codoping with Zn2+/Mg2+ ions. The purity of colour emitted from the sample does not show any

change with variations in pump power density.

1. Introduction

Owing to their excellent luminescent behaviour, luminescencematerials have found wide applications in the eld of opticaldata storage, lasers, temperature sensors, detection of infraredradiation, colour displays, biomedical diagnostics, upconvert-ers, optical ampliers, telecommunication, plasma displaypanels, high denition televisions (HDTVs), light emittingdiodes, cathode ray tubes (CRTs), nger print detection, scin-tillators, etc.1–8 In these materials, luminescence takes place dueto energy transition within the energy levels of rare earth ions.1

Rare earth (RE) ions are characterized by a partially lled 4fshell that is shielded by completely lled outer 5s2 and 5p6

orbitals, which causes sharp luminescence, followed by intra 4fshell transitions. The energy levels of the rare earth ions aretherefore largely insensitive to the host environment in whichthey are placed and that leads to their radiative transitions insolid hosts which resemble those of the free ions.9 Lanthanideelements when incorporated into crystalline or amorphoushosts, exist in a triply ionized state, or occasionally, a doublyionized oxidation state.

The upconversion emission from the singly thulium dopedluminescent materials has been observed by manyresearchers10–14 but stronger luminescence was observed whenthulium was codoped with ytterbium.12,15,16 This is because ofthe high absorption cross-section and favourable energy levelstructure of Yb3+ ions for being excited to the higher energy level

ent of Applied Physics, Indian School of

E-mail: vineetkrrai@yahoo.co.in; rai.vk.

04

1

upon 980 nm excitation.17,18 The selection of appropriate hostmaterial is important in the preparation of micro/nanocrystal-line luminescent materials with efficient optical properties. Ofthe various solid host materials, oxides and uorides are mostlypreferred because of their low cut-off phonon frequencies whichresults in lesser energy loss due to non-radiative relaxationchannels.19 La2O3, a solid material, is expected to be suitable forthe preparation of highly luminescent phosphor materials dueto its low cost, good chemical durability and thermal stability,low phonon frequency (�400 cm�1) etc.20,21 Furthermore, La2O3

has been considered as an attractive host material for codopingof triply ionised lanthanides owing to its comparable ionicradius. It is known that the codoping with rare-earth ionsincreases the pump efficiency and hence the intensity of theupconversion (UC) emission, but the effect of codoping withnon-rare earth ions has rarely been studied. Researchers havestudied the effects of co-doping with zinc and magnesiumindividually on the electrical, structural, electrochemical andphotoluminescence properties of rare earth doped/codopedluminescent materials. The UC luminescence enhancement ofabout �37 times for the green band at �551 nm in the Nd3+–Yb3+–Zn2+ codoped Y2O3 phosphor upon excitation at 980 nmhas been observed by Dey et al.22 The effect of magnesiumdoping on the structural as well as optical properties ofZn2GeO4:Mn phosphor has been reported by Anoop et al.23 Theenhancement of about �60 times in the green UC intensity hasbeen observed due to codoping with Mg2+ ions in the Er3+–Yb3+

codoped CaAl12O19 phosphor.24 But the effects of codoping by acombination of non-rare earth ions like zinc andmagnesium onthe UC luminescence have not been reported until now to thebest of our knowledge.

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Paper RSC Advances

Here, we report the synthesis and characterization of theprepared phosphors. The near infrared (NIR) to visiblefrequency UC emission of the Tm3+–Yb3+ codoped La2O3

phosphors and the effects of Zn2+–Mg2+ codoping on the UCemissions arising from the thulium ions have been studied.XRD, FTIR, pump power dependence and decay curve analysishave been studied in order to obtain information on themechanism involved in the frequency of UC emissions. Theeffect of the variations of pump power density on the colouremitted from the developed phosphors has also been examined.

2. Experimental2.1 Material synthesis

The Tm3+, Tm3+–Yb3+, Tm3+–Yb3+–Zn2+ and Tm3+–Yb3+–Zn2+–

Mg2+ doped/codoped La2O3 phosphors have been prepared by acombustion route. The compositions taken for the synthesis ofthese phosphor powders are as follows:

(100 � p � q)La2O3 + pTm2O3 + qYb2O3 (1)

where p ¼ 0.1 mol% and q ¼ 0.0, 1.0, 3.0, 5.0 mol% and

(100 � p � q � r)La2O3 + pTm2O3 + qYb2O3 + rZnO (2)

where p ¼ 0.1 mol%, q ¼ 3.0 mol%, r ¼ 5.0, 10.0, 15.0 and 25.0mol%, and

(100 � p � q � r � s)La2O3 + pTm2O3 + qYb2O3

+ rZnO + sMgCO3 (3)

where p ¼ 0.1 mol% and q¼ 3.0 mol%, r ¼ 15.0 mol% and s ¼3.0, 5.0, 7.0 and 10.0 mol%. Stoichiometric amounts of theprecursor chemicals were taken and treated with nitric acid toform their corresponding nitrates. Aer that, they were mixedtogether and an appropriate amount of urea solution was mixedwith the nitrate mixtures. The solutions were stirred at 950 rpmfor about 3 hours at a constant temperature of 60 �C to obtaingel-like products. The gels were then transferred into differentalumina crucibles and kept inside an electric furnace at 550 �C.Within 2–5minutes, the combustion process was complete. Thewhitish products obtained were ground into ne powder andfurther annealed at 800 �C for 3 hours. The annealed sampleswere used for the structural and optical analysis.

Fig. 1 XRD pattern of (A) JCPDS file no. 74-1144, (B) Tm3+–Yb3+, (C)Tm3+–Yb3+–Zn2+, and (D) Tm3+–Yb3+–Zn2+–Mg2+ codoped La2O3

phosphors.

2.2 Characterization

For the structural study, X-ray diffraction (XRD) analysis andscanning electron microscopy (SEM) have been done usingBruker D8 advanced X-ray diffractometer and HITACHI (S-3400N) scanning electronmicroscope, respectively. To study thepresence of impurities in the samples, Fourier transforminfrared (FTIR) spectra of the samples were recorded in the 400–4000 cm�1 spectral region on a Perkin Elmer Spectrum RX1spectrophotometer. The UC emission spectra of the phosphorpowders were recorded through a monochromator attachedwith a photomultiplier tube (PMT) upon excitation with 980 nmcontinuous wave (CW) diode laser. The lifetime analysis was

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performed by chopping the 980 nm laser beam by using amechanical chopper and measurements were recorded with thehelp of a quick start digital oscilloscope. All the measurementswere performed at room temperature.

3. Results and discussion3.1 XRD analysis

The powder XRD patterns of the Tm3+–Yb3+; Tm3+–Yb3+–Zn2+

and Tm3+–Yb3+–Zn2+–Mg2+ codoped La2O3 phosphor sampleswere obtained between the range of 20 and 90 degrees (Fig. 1).All the XRD patterns have the same baselines and patterns,which shows that the crystal structure and phase of theseprepared materials have not been altered due to codoping withrare-earth as well as non-rare-earth elements. A large number ofintense diffraction peaks for different 2q values of �26.71�,�29.6�, �30.55�, �39.94�, �46.66�, �52.72�, �55.53�, �63.04�,�72.88�, �75.94�, �79.42�, �81.37�, �84.37�, �85.72� corre-sponding to the diffraction from the (100), (002), (011), (012),(110), (103), (112), (202), (203), (211), (114), (122), (015), (300)planes, respectively, were observed. The observed structure ofthe prepared phosphors was indexed suitably with JCPDS leno. 74-1144 that shows the hexagonal phase of the La2O3 withlattice parameters a ¼ b ¼ 3.93 A and c ¼ 6.12 A.

The average crystallite size of synthesized phosphors wascalculated using the Debye–Scherrer equation:20

d ¼ 0.89l/b cos q (4)

where “d” is the crystallite size, “l” is the wavelength of X-ray(�1.5405 A) used, “b” is the full-width half maxima (FWHM)and “q” is the diffraction angle. The calculated values of theaverage crystallite size were found to be �13 nm, �15 nm and�25 nm for La2O3:Tm

3+–Yb3+, La2O3:Tm3+–Yb3+–Zn2+ and

La2O3:Tm3+–Yb3+–Zn2+–Mg2+ phosphors, respectively.

In addition to the Debye–Scherrer method, the crystallite sizeof the prepared materials was also estimated with the help ofWilliamson–Hall analysis which satises the equation25

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Fig. 2 Williamson–Hall plot for (A) Tm3+–Yb3+, (B) Tm3+–Yb3+–Zn2+,and (C) Tm3+–Yb3+–Zn2+–Mg2+ codoped La2O3 phosphors.

Fig. 3 FTIR spectrum of (A) Tm3+, (B) Tm3+–Yb3+, (C) Tm3+–Yb3+–Zn2+, and (D) Tm3+–Yb3+–Zn2+–Mg2+ codoped La2O3 phosphors.

RSC Advances Paper

b cos q ¼ (0.89l/d) + 43 sin q (5)

where “3” is the strain present in the sample and other symbolshave their usual meanings.

The strain present in the material and the crystallite sizecould be extracted from the slope and the intercept, respec-tively, of the 4 sin q versus b cos q plot. The Williamson–Hallplots for Tm3+–Yb3+, Tm3+–Yb3+–Zn2+ and Tm3+–Yb3+–Zn2+–

Mg2+ codoped La2O3 phosphors are shown in Fig. 2.The values of the crystallite size as calculated byWilliamson–

Hall analysis are 15.65 nm, 16.81 nm and 28.29 nm forLa2O3:Tm

3+–Yb3+, La2O3:Tm3+–Yb3+–Zn2+ and La2O3:Tm

3+–

Yb3+–Zn2+–Mg2+ phosphors, respectively. Also, by analysing theresults it can be concluded that the crystallite size calculated byboth methods (Debye–Scherrer Method and Williamson–Hallmethod) for all three samples are comparable and a slightincrease in the crystallite size is observed as we move fromLa2O3:Tm

3+–Yb3+ to La2O3:Tm3+–Yb3+–Zn2+–Mg2+ via

La2O3:Tm3+–Yb3+–Zn2+ phosphors. The crystallite size in each

case was found to be in the nanometer range, which indicatesthat the developed phosphors are nanocrystalline in nature.

3.2 FTIR Spectroscopy

The FTIR spectra of the prepared materials annealed at 800 �Ctemperature were recorded in the 400–4000 cm�1 region (Fig. 3).The presence of different impurities in the prepared materialswas traced by matching the vibrational frequencies corre-sponding to each impurity in the FTIR spectrum. From the FTIRspectrum of the prepared samples it can be concluded thatfunctional groups like –OH, –CH and –NO are present in all thesamples. The peaks observed around 3613 cm�1 and 2382 cm�1

are due to asymmetric stretching vibration of O–H. The peaksaround 2930 cm�1 and 1475 cm�1 are due to C–H and N–Ostretching vibrations, respectively. It is also observed from theFTIR spectrum that the presence of O–H impurity observedaround 3613 cm�1 was reduced in the case of La2O3:Tm

3+–Yb3+–

21846 | RSC Adv., 2014, 4, 21844–21851

Zn2+–Mg2+ phosphor compared to other cases. The peak around650 cm�1 is due to the lattice vibration of La–O.

3.3 SEM Image

The surface morphology and crystallinity of solid host materialsare important parameters which determine the emission char-acteristics of phosphors. The microstructural analysis ofLa2O3:Tm

3+–Yb3+ and La2O3:Tm3+–Yb3+–Zn2+–Mg2+ samples

was performed with the help of SEM examination (Fig. 4). FromFig. 4 it is clear that the aggregation and particle sizes of theLa2O3:Tm

3+–Yb3+ phosphors increase with codoping Zn2+/Mg2+

ions. The SEM image shows that the particles are not uniformlydistributed throughout the surface, which is mainly caused bythe inhomogeneous distribution of temperature during thesynthesis of the material by the combustion technique. Also, itis clearly visualized that the particles are in the agglomeratedform with their size in the micrometer range.

3.4 Upconversion emissions study of La2O3:Tm3+–Yb3+

phosphor

The room temperature frequency upconversion spectra of Tm3+/Tm3+–Yb3+ doped/codoped La2O3 phosphor powders uponexcitation with 980 nm CW diode laser were recorded in the450–900 nm range (Fig. 5). From Fig. 5, it is clear that no UCemission was observed under a 980 nm diode laser excitationfrom singly Tm3+ doped La2O3 phosphor. But in the case ofLa2O3:Tm

3+–Yb3+ phosphor, four UC emission bands wereobserved around 476 nm, 653 nm, 702 nm and 795 nm corre-sponding to the 1G4 /

3H6,1G4 /

3F4,3F2 /

3H6 and1G4 /

3H5 transitions, respectively.To get the optimum UC intensity, the concentration of Yb3+

ions was varied from 1.0 to 5.0 mol% with the Tm3+ ionsconcentration xed at 0.1 mol%. The maximum UC emissionintensity was observed for the 0.1 mol% Tm3+ + 3.0 mol% Yb3+

combination. The light emitted from the phosphor appearsblue which could be detectable by the naked eye. The requiredNIR photons for the origin of these UC bands can be explainedwith the help of the pump power dependence study.

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Fig. 4 SEM images of (A) Tm3+–Yb3+ codoped La2O3 phosphor and(B) Tm3+–Yb3+–Zn2+–Mg2+ codoped La2O3 phosphor.

Fig. 5 The upconversion emission spectra for Tm3+/Tm3+–Yb3+

codoped La2O3 phosphors.

Fig. 6 Energy level diagram and energy transfer mechanism in Tm3+–Yb3+ codoped La2O3 phosphors.

Paper RSC Advances

It is well known that for extremely small upconversion ratesthe number of photons involved in populating the upconver-sion emitting level can be calculated directly from the slope ofthe logarithmic plot of intensity versus pump power. But, as weincrease the pump power to a higher value the upconversionluminescence caused by ‘n’ photon absorption is found to obeythe relationship IUC < Pn.26

We know that UC emission intensity (IUC) is directlyproportional to pump excitation power (P) in the low pumppower region by the relationship,27

IUC f Pk (6)

where “k” represents the number of pump photons required toexcite the emitting states.

The slopes corresponding to 476 nm and 653 nm bands arecalculated to be �1.93 and �2.08, which shows that both the

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blue and red emissions are due to two photon absorptionprocesses.

The origin of the UC emission bands can be explained withthe help of an energy level diagram as shown in Fig. 6. The UCemissions observed from Tm3+–Yb3+ codoped La2O3 phosphorare due to the cooperative energy transfer (CET) from Yb3+ toTm3+ ions only, as no UC emission is observed in the singlydoped Tm3+:La2O3 phosphor. The Yb

3+ ions present in the 2F7/2ground state, aer absorption of 980 nm NIR photon, getexcited to the 2F5/2 manifold. In the 2F5/2 excited state, theexcited donor Yb3+ ion transfers its excitation energy to theacceptor Yb3+ ions in the 2F5/2 state and these acceptor ions arefurther excited to the virtual level (V). The energy of the virtuallevel ‘V’ of Yb3+ ions is in resonance with the 1G4 (�21 015 cm�1)level of Tm3+ ions. So, the Yb3+ ions in the virtual level transfertheir excitation energy to the Tm3+ ions in ground state. Theground state Tm3+ ions then get excited to the 1G4 level. Finally,the Tm3+ ions in 1G4 level transit downward to the 3H6,

3F4(�5765 cm�1) and 3H5 (�8258 cm�1) levels, emitting blue, redand NIR emissions around 476 nm, 653 nm and 795 nm,

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RSC Advances Paper

respectively. The 3H4 (�12 556 cm�1) and 3F2 (�15 240 cm�1)levels of Tm3+ ion are populated by the cross-relaxation energytransfer mechanisms, CR1 and CR2 viz. (1G4,

3H6 / 3H5,3H4)

and (3H5,3H4 /

3F4,3F2) respectively. Aer that, the Tm

3+ ionsin the 3F2 level decay radiatively to the 3H6 level giving a photonat �702 nm corresponding to the 3F2 /

3H6 transition. The3F3

(�14 627 cm�1) level is populated by non-radiative relaxationfrom the 3F2 level followed by the emission of a phonon. Theemission peak intensity ratio between the blue and red emis-sions was �30 and that between the blue to NIR emissions was�5. This large UC emission intensity corresponding to the 1G4

/ 3H6 transition is basically due to the direct cooperativeenergy transfer (CET) from the Yb3+ ions to the Tm3+ ions fol-lowed by the absorption of two NIR photons.

3.5 Effect of Zn2+ and Mg2+ codoping in La2O3:Tm3+–Yb3+

phosphor

From Fig. 7 it is clearly observed that the UC intensity of all thebands is enhanced by the addition of Zn2+ and Mg2+ ions in theLa2O3:Tm

3+–Yb3+ phosphor. The variations in UC intensity withdifferent doping concentrations of Zn2+ and Mg2+ ions wereinvestigated. The enhancement of �2 times for the blue UCbands was observed with the codoping of 15.0 mol% Zn2+ ionsin the Tm3+–Yb3+ codoped La2O3 phosphor. It was also observedthat the intensities of UC bands are further increased when theMg2+ ion is incorporated into the Tm3+–Yb3+–Zn2+ codopedLa2O3 phosphor. The optimum concentration of Mg2+ ion isfound to be 5.0 mol%. Enhancements of about 36 times, 34times and 11 times in the intensity of UC peaks at �476 nm,�653 nm and �795 nm have been observed respectively withthe codoping of 5.0 mol% Mg2+ ions compared to that of Tm3+–

Yb3+ codoped La2O3 phosphor. The intensity of upconversionemission bands lying in the blue, red and NIR regions isincreased with increasing concentration of Zn2+ and Mg2+ ionsup to 15.0 mol% and 5.0 mol% respectively. The enhancementin the UC intensity is caused by the creation of the oxygenvacancies due to codoping with the Zn2+/Mg2+ ions in the La2O3

Fig. 7 The upconversion emission spectra of Tm3+–Yb3+, Tm3+–Yb3+–Zn2+ and Tm3+–Yb3+–Zn2+–Mg2+ codoped La2O3 phosphors.

21848 | RSC Adv., 2014, 4, 21844–21851

phosphor. These oxygen vacancies act as sensitizers for energytransfer due to the mixing of charge transfer states.28 Thisenhancement in the UC emission intensity may also beexplained by the broadening in the absorption bands, followedby the overlap between the defect states produced by thecodopants with the rare earth ions charge transfer states,29

which is beyond the scope of the present study.The UC emission peak positions remain unaltered with the

codoping of Zn2+/Mg2+ ions. The non-occurrence of the spectralshi and the presence of a sharp UC emission peak are due tothe effect of the shielding of 4f electrons by the outer 5s and 5porbitals. The UC intensity for all of the bands is quenchedbeyond the optimum concentration of Zn2+ and Mg2+ ions. Thereduction in the UC intensity is most probably due to imperfectatomic arrangement in the La2O3 phosphor. These defectspresent in solid host material may lead to quenching in theemission intensity.

The ionic radii of La3+, Tm3+, Yb3+, Zn2+ and Mg2+ are 103pm, 88 pm, 86.8 pm, 74 pm and 72 pm, respectively. The ionicradii of Tm3+, Yb3+, Zn2+ and Mg2+ are smaller than that of La3+

(which has largest ionic radius compared to other rare earthions). Also, there is a valency mismatch between Zn2+/Mg2+ andLa3+ ion. Therefore, the excess codoping of Zn2+/Mg2+ ions canintroduce distortion and hence defects into the La2O3 host.Thus, when the codoping concentration of Zn2+/Mg2+ ions isbeyond the optimum value, the UC emission intensity isreduced.

Although many researchers have demonstrated that theenhancement in luminescence intensity is caused by Mg2+

incorporation, no one has reported such a large enhancementwith Mg2+ codoping to the best of our knowledge.24,29,30 Anenhancement of about 10 times in the red UC band by codopingof Mg2+ ions in CaAl12O19:Er

3+:Yb3+ phosphor has beenobserved by Singh et al.24 A photoluminescence enhancementby a factor of 1.7 and 1.3 times for the red emission band in Eu3+

doped Y2O3 phosphor due to codoping with Mg2+ and Al3+ ionshas been observed by Chong et al.29 Pan et al. have reportedenhancement of more than 3 times in luminescence efficiencyof CaAl12O19:Mn4+ phosphor due to the codoping of Mg2+ ions.30

From the FTIR spectrum (Fig. 3), it is clear that the presenceof –OH impurity around 3613 cm�1 which acts as a quencher isreduced for La2O3:Tm

3+–Yb3+–Zn2+–Mg2+ phosphor comparedto other samples. This also supports why the UC emissionintensity is signicantly enhanced in the La2O3:Tm

3+–Yb3+–Zn2+–Mg2+ phosphor. Therefore, such a large enhancement inUC luminescence intensity, as observed in La2O3:Tm

3+–Yb3+–Zn2+–Mg2+, has made the present material a promising candi-date as a good blue upconverter.

Decay curves analysis corresponding to the 1G4 /3H6 (476

nm) blue transition of Tm3+ ions in the La2O3:Tm3+–Yb3+,

La2O3:Tm3+–Yb3+–Zn2+ and La2O3:Tm

3+–Yb3+–Zn2+–Mg2+ phos-phors powder samples was performed (Fig. 8). The calculatedvalues of decay time for the blue emission from the decay curveswere found to be �1.78 � 0.47 ms, �1.13 � 0.12 ms, and �0.98� 0.06 ms for the La2O3:Tm

3+–Yb3+, La2O3:Tm3+–Yb3+–Zn2+ and

La2O3:Tm3+–Yb3+–Zn2+–Mg2+ phosphors, respectively. This

observed variation in decay times of emitting level also support

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Fig. 8 The fluorescence decay curves corresponding to the 1G4 /3H6 (476 nm) blue transition of thulium ion in (A) Tm3+–Yb3+, (B)Tm3+–Yb3+–Zn2+ and (C) Tm3+–Yb3+–Zn2+–Mg2+ codoped La2O3

phosphors.

Table 1 The calculated CCT values for different excitation powerdensities

Power densities (W cm�2)

CIE (x, y) coordinates

CCT value (K)x coordinate y coordinate

1.32 0.12 0.13 5848.701.75 0.12 0.13 5848.704.63 0.12 0.12 5093.345.91 0.12 0.12 5093.347.36 0.12 0.12 5093.3410.04 0.12 0.11 4170.6413.10 0.12 0.11 4170.6416.73 0.12 0.11 4170.6417.87 0.12 0.11 4170.64

Paper RSC Advances

the increase in radiative transition probability of emitting statedue to the incorporation of Zn2+ and Mg2+ ions into theLa2O3:Tm

3+–Yb3+ phosphor. The decrease in decay time resultsin an increase of the radiative transition probability of emittingstate. The reason behind this increase in radiative transitionprobability due to Zn2+ and Mg2+ ions codoping is the sensi-tizing effect caused by the creation of oxygen vacancies, asdiscussed earlier.

To compare the colours emitted by the Tm3+–Yb3+, Tm3+–

Yb3+–Zn2+ and Tm3+–Yb3+–Zn2+–Mg2+ codoped phosphors, theCIE colour coordinates corresponding to these three samples at10.04 W cm�2 power density were calculated and represented inFig. 9(a), from which it is clearly visible that all these threephosphors are emitting blue colour but with the incorporationof Zn2+ and Mg2+ ions the colour is tuned towards the morebluish region. This indicates that the purity of the colouremitted by the sample is improved by codoping with Zn2+ and

Fig. 9 (a) Comparison of CIE colour coordinates corresponding to Tm3+

phosphors, (b) CIE color coordinates of Tm3+–Yb3+–Zn2+–Mg2+ codop

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Mg2+ ions. In order to check the tunability of the colour emittedfrom the Tm3+–Yb3+–Zn2+–Mg2+ codoped La2O3 phosphor, theCIE colour coordinates of the sample were calculated atdifferent excitation power densities as shown in Fig. 9(b), whichshows that the present material is an efficient blue emitter.Based on the observed results, we can say that the presentphosphor can be used as an efficient NIR to blue upconverter. Itis observed from Fig. 9(b) that the blue colour emitted by thesample is independent of the pump power density, so it can beused in the fabrication of display devices. Also, the intense bluelight emitted from the Tm3+–Yb3+–Zn2+–Mg2+ codoped La2O3

phosphor may be used in photodynamic therapy (PDT) for thediagnosis and localization of cancer cells.7

To examine the quality of white light, we calculated theColour Correlated Temperature (CCT) values at different exci-tation powers densities using McCany empirical formula31

which is expressed as,

CCT ¼ �449n3 + 3525n2 � 6823n + 5520.33 (7)

where n ¼ [(x � xe)/(y � ye)] is the inverse slope line with xe ¼0.332 and ye ¼ 0.186.

The calculated values of CCT corresponding to differentcolour coordinates are presented in Table 1. The colour

–Yb3+, Tm3+–Yb3+–Zn2+ and Tm3+–Yb3+–Zn2+–Mg2+ codoped La2O3

ed La2O3 phosphors at different pump power densities.

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correlated temperatures for different power densities fall in theregion of �4171 K to �5512 K. So, from the results it can beconcluded that the CCT values lie in the cold white light region.

4. Conclusion

The Tm3+–Yb3+, Tm3+–Yb3+–Zn2+ and Tm3+–Yb3+–Zn2+–Mg2+

codoped La2O3 phosphors have been successfully prepared bythe combustion method. The XRD patterns of the synthesizedmaterials shows that these phosphors exhibit the hexagonalphase. The crystallite sizes of the materials calculated byScherrer's formula as well as by the Williamson–Hall analysisshow that the phosphor materials are nanocrystalline. The SEMimages of the prepared materials reveal agglomerations withinthe materials with particle size in the micrometer range. TheSEM analysis shows an increase in the particle size due to theintroduction of Zn2+/Mg2+ ions in the codoped phosphors. Theimpurities present in the prepared materials are conrmed byFTIR analysis. The analysis of the upconversion spectra and thedecay curve collectively reveals that the maximum enhancementof the blue UC band is observed for the Tm3+–Yb3+–Zn2+–Mg2+

codoped La2O3 phosphor upon excitation with a 980 nm NIRlaser source. The CIE colour coordinates corresponding to theTm3+–Yb3+–Zn2+–Mg2+ codoped La2O3 phosphor does not showany signicant change with variations in the pump powerdensity. As a result, the present phosphor can be used inmaking blue light upconverters, display devices and devices forthe diagnosis and localization of cancer cells in photodynamictherapy.

Acknowledgements

The authors are thankful to the University Grants Commission(UGC), New Delhi, India and Indian School of Mines, Dhanbad,India for providing nancial support.

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