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Visible Light-Emitting Er -Tm -Yb CodopedZrO Phosphor for Sensing and Photonics

ApplicationsAnurag Pandey, Riya Dey, Saurabh Pandey, and Vineet Kumar Rai

Abstract—The Er -Tm -Yb codoped ZrO phosphorsynthesized via chemical co-precipitation technique has beencharacterized by the X-ray diffraction (XRD) analysis. The up-conversion (UC) emission study of present material has beenperformed upon 980 nm diode laser excitation. The efficientenergy transfer between dopants has been found to be respon-sible in the UC mechanism. The tuning in color of light emittedfrom the developed phosphor has been noticed by consideringconcentration as well as power variation. The Correlated ColorTemperature (CCT) corresponding different color coordinates hasbeen calculated. The temperature sensing behavior of developedmaterial has also been explained using fluorescence intensity ratio(FIR) technique.

Index Terms—Lanthanides, co-precipitation, upconversion(UC) luminescence, color coordinates, fluorescence intensity ratio.

I. INTRODUCTION

R ARE EARTH (RE) ions doped phosphors have beenstudied extensively since last few decades due to their

unparallel luminescence properties. Proliferation in the studyof such materials is due to presence of the several energy levelsin rare earth ions which increases the number of interactionchannels with incident wave and controls over the desiredemission. The reason behind the splitting of energy levels ofrare earth ions are the electrostatic, spin-orbit and ion-latticeinteractions. In case of RE ions the splitting of energy levelsare mainly because of the electrostatic interaction, while theion-lattice interaction assisted energy level splitting is lessprobable [1], [2]. Partially filled shell in lanthanides isshielded by the completely filled outer 5s and 5p shells whichresults the inner sharp optical transitions and unaf-fected by the crystal field of host matrix [3]. RE ions dopedupconversion (UC) phosphors are demanding material owingto its property of converting the photon of lower energy intohigher energy. The emission bands from these materials canbe tuned by changing the excitation wavelength, concentration

Manuscript received December 09, 2013; revised nulldate; accepted February08, 2014. This wrok was supported byUniversity Grants Commission, NewDelhi, India, under F. No. 39-534/2010(SR)The authors are with the Laser and Spectroscopy Laboratory, Department

of Applied Physics, Indian School of Mines Dhanbad 826004, India (e-mail:rai.vk.ap@ismdhanbad.ac.in).Color versions of one or more of the figures are available online at http://

ieeexplore.ieee.org.Digital Object Identifier 10.1109/JDT.2014.2305773

variation of RE ions or codoping with other RE ions. RE iondoped materials covers a broad area in photonics applicationsnamely, field emission displays, plasma display panel, opticalamplifiers, white light-emitting diodes (W-LEDs), biologicaldiagnosis, sensors and security purposes, etc. [4]–[9].The presence of structural disorder in amaterial alters its elec-

tronic and optical properties [10]–[12]. A study involving theeffect of structure of host material on the optical properties ofdopant has drawn a significant interest for monitoring its var-ious optical properties such as quantum efficiency, radiative andnon-radiative rates, emission intensity, lifetime of dopant ions,etc. [10], [11]. A majority of RE ions in a crystal experiencesthe same crystal field resulting the very similar energy levelsand narrow linewidth of fluorescence transitions. The reducedquantum efficiency or the lifetime of an energy level is a mea-sure of an upper limit of dopant concentration due to ion-ioninteractions such as energy transfer, cross-relaxation and coop-erative energy transfer [12].The Zirconia has emerged as a promising material in the field

of optics and presents several technological applications [13].The ZrO exhibits various attractive properties such as excellenthardness, resistance to molten metal and alloys, high meltingpoint, high optical transparency, mechanical and chemical sta-bility [14], [15]. Due to low phonon frequency ( 270 cm ),large band gap ( 6 eV), high refractive index (2.13), Zirconiahas been considered as a promising candidate for the prepara-tion of efficient luminescent materials [16].Recently, the interest towards tridoped luminescent materials

has extensively increased because of its multicolor emissions.Researchers have reported the studies of tridoped materialsby taking different combinations of RE ions as dopant andobtained various color lights [17]–[22]. Every color of visiblelight emitted from the materials has its own characteristicsand application in our daily lives. Like the other primaryand secondary color lights, the cyan light is also used forlighting, display, color printing and photographic applications.So here we have tried to develop a phosphor material thatemits cyan color along with others. Several investigations havebeen performed on Er /Tm /Yb codoped systems [6],[17]–[19]. To the best of our knowledge, for the first time wehave synthesized the ZrO :Er –Tm –Yb phosphor viacoprecipitation process and its UC luminescence behaviour hasbeen studied. The reason behind the preparation of sample bycoprecipitation process is the uniform crystal morphology ofthe synthesized phosphor and less preparation time.

1551-319X © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

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In the present study, we have synthesized theEr –Tm –Yb :ZrO phosphor via chemical copre-cipitation process. The structural study has been carried outwith the help of X-ray diffraction (XRD) analysis. The UCemission properties of synthesized phosphors have beeninvestigated on 980 nm diode laser excitation. The mechanismsinvolved for the UC emissions are discussed in detail withthe help of schematic energy level diagram. The developmentof optical temperature sensors have been proposed on thebasis of fluorescence intensity ratio (FIR) variation of twothermally coupled levels of green band on increasing theexcitation power. The Correlated Color Temperature (CCT)of the materials has also been calculated in order to realize itsapplicability in LCD screen.

II. EXPERIMENTAL DETAILS

Chemical coprecipitation method has been used for thesynthesis of present phosphor material. The starting materialsEr O , Tm O , Yb O and ZrO are in oxide form. The hostand dopant materials are weighed in a stoichiometric ratioaccording to following equation

- - - ZrO Er O Tm O O (1)

where mol%, mol% and to 15 mol%These oxide powders were dissolved in deionized water. Toform the nitrate precursors for the coprecipitation process, con-centrated HNO was then added into the solutions. The so-lutions were heated for 10–15 min at a constant temperature(60 C). With drop wise addition of NH OH solution, precipita-tion starts. To achieve complete precipitate, the solution is leftfor about 20 hours. After that the precipitate is filtered usingfilter paper and washed twice with deionised water and twicewith ethanol to remove the impurities. The precipitate is driedat 500 C for a few minutes in order to completely remove thewater and to get final product. The prepared phosphor powdersare further annealed at 800 C for 2 hours and used for structuraland optical characterization purposes.Structural study of synthesized Er –Tm –Yb codoped

ZrO phosphor has been done with the help of XRD study toanalyze the phase composition and average crystallite size. TheUC emission spectra of phosphors excited with 980 nm powertunable diode laser have been recorded by a system comprisingof Princeton triple grating monochromator (Acton SP-2300) at-tached with a photomultiplier tube and personal computer. Allthe measurements were carried out at room temperature.

III. RESULTS AND DISCUSSION

A. XRD Analysis

Fig. 1 shows the XRD pattern of Er -Tm -Yb codopedZrO phosphor heat treated at 800 C. The presence of welldefined peaks in the XRD pattern indicates the formationof good crystalline phosphor material. All peaks of theEr -Tm -Yb :ZrO phosphor could be suitably matchedwith that of JCPDS card no. 83-0944 corresponding to standardmonoclinic ZrO . When compared with the standard peaksof monoclinic ZrO , one additional weak peak is observed at

which is due to the tetragonal phase of ZrO . The

Fig. 1. XRD pattern of Er -Tm -Yb codoped ZrO phosphor heattreated at 800 C.

Fig. 2. UC emission spectrum of 0.5 mol% Er 1.0 mol% Tm 10.0mol% Yb codoped ZrO phosphor under 980 nm laser excitation.

crystallite size of present material has been calculated usingDebye Scherrer’s formula [23]. The average value of crystallitesize has found to be nm indicating the nanocrystallinestructure of Er –Tm –Yb codoped ZrO phosphor.

B. Upconversion (UC) Emission Study

The UC emission spectra of the annealedEr –Tm –Yb :ZrO phosphors with optimumconcentrations of Er (0.5 mol%), Tm (1.0 mol%) andvarying concentration of Yb (1.0–15.0 mol%) ions arerecorded in the 450–850 nm wavelength range upon a 980nm diode laser excitation. The concentration dependence ofobserved blue, green and red bands is shown in inset of Fig. 2.From the variation in UC intensity of all bands (inset of Fig. 2),it is concluded that at 10 mol% of ytterbium concentration,the color emitted from the sample approaches cyan light (asshown in optical photograph). Thus the 10 mol% of Ybconcentration is taken as optimum one for getting the cyancolor in the codoped phosphor. Fig. 2 shows the UC emissionspectrum of 0.5 mol% Er mol% Tm mol%Yb codoped ZrO phosphor. Four UC emission bandsare observed in blue (450–508 nm), green (512–582 nm),red (634–690 nm) and near infrared (754–840 nm) regions

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Fig. 3. Logarithmic of UC emission intensity of various UC bands as a functionof logarithmic of pump powers.

Fig. 4. Schematic energy level diagram for Er -Tm -Yb ions with pos-sible channels responsible for multicolor emissions.

(Fig. 2). The sharp blue emission peak around 476 nm isattributed to the H transition of Tm ions. Thetwo green emission bands peaking around 525 nm and 546nm correspond to the H andtransitions of the Er ions, respectively. The red emissionaround 661 nm is due to both (Er ions)and H (Tm ions) transitions,respectively. The NIR emission band at 800 nm could beassigned as the H H transition of Tm ion [24].It is well known that the UC emission intensity is di-

rectly proportional to the nth power of pump excitation power(P) i.e., , where ‘n’ is the number of pump photonsresponsible for the UC emissions. A plot betweenversus (P) gives a straight line with slope n. To estimatethe number of pump photons involved in the UC process, wehave plotted the logarithmic of input excitation power versusUC emission intensity for different bands as shown in Fig. 3.The slopes corresponding to blue, green and red emission bandsare found 2.07, 1.91, and 1.51, respectively indicating theinvolvement of two photons in UC process in all the cases.The probable UC mechanisms and processes involved have

been discussed with the help of schematic energy level diagramof the Er , Tm and Yb ions as shown in Fig. 4.

When the sample is excited with 980 nm NIR radiation atlow pump power, Tm ion does not show any UC emissiondue to the absence of energy level corresponding to this energyvalue while a weak blue UC signal is observed at high excitationpower. It is possibly due to absorption of three NIR photons athigh excitation power [25]. The Er ions can be excited easilyby 980 nm radiation via ground state absorption (GSA) and ex-cited state absorption (ESA) processes and give UC emissions.Incorporation of Yb ions act as an efficient sensitizer for Erand Tm ions when irradiated with a 980 nm radiation. Thus,the main route to obtain the intense visible UC emission fromthe synthesized phosphor is the efficient energy transfer fromYb to Er and Tm ions.The H F levels of Tm are populated by two step

energy transfer process. Firstly, the H level is populatedby excitation of ground state Tm ions via energy transferfrom excited Yb ions ( F ). The ions in H state relaxnonradiatively to F state and then excited to F levelsby energy transfer from Yb ( F ) to Tm ( F ). Due tothe smaller lifetime of the F states compared to the Hstate, the H state is populated via the nonradiative relaxationfrom the F state. The radiative relaxation from F andH states to ground state ( H ) gives red and NIR emis-sions around 656 nm and 800 nm through the Hand H H transitions, respectively. Also the G level ispopulated through cooperative energy transfer from Yb toTm ions. The two NIR photons are absorbed by Yb ionsupon a 980 nm laser excitation and subsequently the populationis transferred cooperatively to the Tm ions in the G level[23]. The G level decays radiatively to the low lying F andH levels giving emissions in the blue and red regions around476 nm and 656 nm respectively.In the singly Er doped material absorption of incident

980 nm (NIR) photons by Er also takes place via groundstate (GSA) and excited state absorption (ESA) process. GSApopulates the I level of Er ions by absorbing onephoton. Excited ions in the I absorb another NIR photonand populate the F level via ESA Through non-radiativerelaxations, ions in the F level decays to the H andS levels. The population in these states relaxes radiativelyto give green emissions through the H and

transitions. The transitionof Er ions is also responsible for the red UC emissionfrom the sample. The population of the H S and

levels of Er ion are enhances by the energy transferfrom excited Yb ions through energy transfer process asreported elsewhere [26]. The cooperative energy transfer isnot favourable from Yb to Er ions because the Ilevel of Er ion has almost equal energy with that of Flevel of Yb ion. Also the oscillator strength correspondingto the H (Er transition is smaller than the

(Yb transition.

C. Color Tunability and CCT Analysis

The variation in color coordinates of Er -Tm -Ybcodoped ZrO phosphor on varying the concentration of Ybions is shown in the CIE chromaticity diagram (Fig. 5). Ithas been observed that the color of emitted light varies on

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Fig. 5. CIE chromaticity diagram of Er -Tm -Yb codoped ZrO phos-phor on varying concentration of Yb ions.

TABLE ICCT VALUES OF Er –Tm –Yb CODOPED ZrO PHOSPHOR AT

DIFFERENT Yb ION CONCENTRATION

varying the Yb concentration. The color corresponding toUC emissions changes from green to red via cyan color as theconcentration of Yb ions were increased from 1.0 to 15.0mol%. At 1.0 mol% of Yb concentration, the color emittedfrom sample was in green region (0.24, 0.71) and move towardscyan with increasing concentration of Yb ions upto 10.0mol%. At 15.0 mol% Yb concentration the color coordinatemoved in reddish region (0.43, 0.31) of the CIE diagram. So atuning in color of emitted light from sample was observed onvarying the Yb concentrations.The Correlated Color Temperatures (CCT) possesses a close

resemblance with color of a perfect blackbody when it is heatedto that temperature measured in Kelvin (K). Changes in the CCTvalues represent the variation in the color of a black body withdifferent temperature. As per the McCany empirical formula

(2)

where and arefixed in all cases and are different color coordinates [27].The CCT values corresponding to different CIE coordinates onvarying Yb ion concentration are listed in Table I.Not only with concentration, the present phosphor also shows

the variation in color of emitting light with pump excitationpower as visualised in Fig. 6. It has been observed that the color

Fig. 6. CIE chromaticity diagram of Er -Tm -Yb codoped ZrO phos-phor at different pump powers.

TABLE IICCT VALUES OF EREr -TM -YB CODOPED ZRO PHOSPHOR AT

DIFFERENT PUMP EXCITATION POWERS

coordinate moves from green to cyan region on increasing thepump excitation power. The color coordinates of 0.5 mol%Er 1.0 mol% Tm 10.0 mol% Yb codoped ZrOphosphor at 108 mW is (0.21, 0.58) and moves to (0.17, 0.49)at 1410 mW power. Therefore, the color emitted from samplewas tuned on varying the pump excitation power.The CCT values of the developed phosphors are also calcu-

lated at different pump excitation power as given in Table II.From the tables it has been confirmed that observed CCT valuessuitably lies in the range which is required for LCD or CRTscreen region except for 15.0 mol% ytterbium concentrations.

D. Temperature Sensing Behavior

Fig. 7 shows the UC emission intensity of green band in505–560 nm range of Er -Tm -Yb codoped ZrO phos-phor at various pump powers when irradiated with a 980 nmradiation. It was observed that the UC emission intensity corre-sponding to the H (525 nm) and(546 nm) transitions varies with increasing pump powers butnot in the same way. At very low pump power the intensity ofpeak around 525 nm was less in comparison with that of 546nm while at higher pump power reverse trend was observed.

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Fig. 7. Variation of green UC emission spectrum of Er -Tm -Ybcodoped ZrO phosphor with pump powers.

Fig. 8. A plot of UC intensity ratio of green band as a function of input exci-tation power.

This variation in intensity of observed band increases their in-tensity ratio. The energy gap between H S levels is

cm which strengthens the fact that these two levels arethermally coupled. Thus the fluorescence intensity ratio (FIR)technique can be applied to the present sample for investigatingits temperature sensing behavior [28]–[30]. The heat producedinside the sample is due to presence of the nonradiative relax-ation channels [29].It was observed that the intensity ratio of two transitionsH and nm nmvaries linearly with the pump excitation power as shown inFig. 8. Also we know that the FIR of two closely spaced levelsrelates with the temperature as,

nm nm (3)

where all the terms have their usual meanings [28].Now we have estimated the temperature of the sample corre-

sponding to different pump powers using the FIR variation cor-respondingly. At very low pump power the sample is assumedto be at room temperature (27 C). From the relation (3), the es-timated value of B is found to be . Using this value ofconstant B we have calculated the temperature of the sampleat different UC intensity ratios nm nm . The value

TABLE IIICALCULATED TEMPERATURE OF Er –Tm –Yb CODOPED ZrOPHOSPHOR AT DIFFERENT INTENSITY RATIOS CORRESPONDING TO THE

DIFFERENT PUMP POWERS

of temperature corresponding to various intensity ratios at dif-ferent pump powers are listed in Table III. From Table III, it isclear that by using (3) the calculated temperature of the devel-oped phosphor is raised on increasing the excitation power dueto the thermalization involved between the H and S ex-cited states. So, the present phosphor material may be of scien-tific interest for temperature sensing upto higher temperatures.

IV. CONCLUSION

Monoclinic Er –Tm –Yb codoped ZrO phosphor ma-terial has been synthesized successfully via chemical coprecip-itation route. The NIR to visible UC emission study suggest theinvolvement of two photons for the generation of the UC bands.The efficient energy transfers from ytterbium to thulium and er-bium ions have also been discussed. The color of emitted radi-ation is changed from green-cyan-red on increasing the Ybion concentrations while the variation in green to cyan region isalso marked on increasing pump power. Correlated Color Tem-peratures on varying concentration as well as power was alsocalculated and found in the range of LCD or CRT screen. Thecyan light emitted from the sample at optimum concentrationof Yb (10.0 mol%) can be used for developing cyan bulb andin color printing. The temperature of sample was calculated atdifferent pump powers upto 658 K which indicate the opticalheating within the sample due to laser excitation. So the presentphosphor can be used inmakingmulticolor LED’s and high tem-perature sensing probe.

REFERENCES[1] L. A. Riseberg and M. J. Weber, , E. Wolf, Ed., Progress in Optics.

Amsterdam, the Netherlands: North-Holland, 1976, vol. XIV, p. 89.[2] W. J. Miniscalco, Rare Earth Doped Fiber Lasers and Amplifiers, M.

J. F. Digonnet, Ed. New York: Marcel Dekker, 1993, p. 19.[3] W. Koechner, Solid-State Laser Engineering. Berlin, Germany:

Springer-Verlag, 1992, p. 35.[4] Y. Pan, Q. Su, H. Xu, T. Chen, W. Ge, C. Yang, and M. Wu, J. Solid

State Chem., vol. 174, p. 69, 2003.[5] V. K. Rai, K. Kumar, and S. B. Rai, Opt. Mater., vol. 29, p. 873, 2007.[6] V. K. Rai, R. Dey, and K. Kumar, Mat. Res. Bull., vol. 46, p. 2232,

2013.[7] A. S. S. de Camargo, J. F. Possatto, L. A. de O. Nunes, E. R. Botero, E.

R. M. Andreeta, D. Garcia, and J. A. Eiras, Solid State Commun., vol.137, p. 1, 2006.

[8] T. Nyokong and V. Ahsen, Photosensitizers in Medicine, Environment,and Security. Dordrecht, The Netherlands: Springer Science + Busi-ness Media B. V., 2012.

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[9] A. Pandey, V. K. Rai, R. Dey, and K. Kumar, Mat. Chem. Phys., vol.139, p. 483, 2013.

[10] W. S. Brocklesby, Annu. Rev. Mater. Sci., vol. 23, p. 193, 1993.[11] M. J.Weber, Laser Spectroscopy of Solids, W.M. Yen and P.M. Selzer,

Eds. Berlin, Germany: Springer-Verlag, 1981, p. 189.[12] J. R. Simpson, Rare Earth Doped Fiber Lasers and Amplifiers, M. J.

F. Digonnet, Ed. New York, NY, USA: Marcel Dekker, 1993, p. 1.[13] F. S. De Vicente, A. C. De Castro, M. F. De Souza, and M. S. Li, Thin

Solid Films, vol. 418, p. 222, 2002.[14] N. Suzuki, Y. Tomita, K. Ohmori, M. Hidaka, and K. Chikama, Opt.

Express, vol. 14, p. 12712, 2006.[15] L. Zhang and Y. Han, Mater. Sci. Eng. C, vol. 31, p. 1104, 2011.[16] A. Patra, Chem. Phys. Lett., vol. 387, p. 35, 2004.[17] X. L. Pang, C. H. Jia, G. Q. Li, and W. F. Zhang, Opt. Mater., vol. 34,

p. 234, 2011.[18] N. K. Giri, K. Mishra, and S. B. Rai, J. Fluorosec., vol. 21, p. 1951,

2011.[19] X. Chen, Y. Li, F. Kong, L. Li, Q. Sun, and F. Wang, J. Alloys Compd.,

vol. 541, p. 505, 2012.[20] C. H. Yang, Y. X. Pan, Q. Y. Zhang, and Z. H. Jiang, J. Fluoresc., vol.

17, p. 500, 2007.[21] J. H. Chung, S. Y. Lee, K. B. Shim, and J. H. Ryu, Appl. Phys. Express,

vol. 5, p. 052602, 2012.[22] Y. L. Liu, X. M. Tang, X. D. Chen, B. F. Lei, and D. X. Feng, Chin.

Chem. Lett., vol. 10, p. 709, 1999.[23] A. Pandey and V. K. Rai, Appl. Phys. B, vol. 109, p. 611, 2012.[24] G. H. Dieke, Spectra and Energy Levels of Rare Earth Ions in Crys-

tals. : Interscience, 1968.

[25] V. K. Rai, L. de S. Menezes, and C. B. de Araujo, J. Appl. Phys., vol.103, p. 53514, 2008.

[26] V. Singh, V. K. Rai, and M. Haase, J. Appl. Phys., vol. 112, p. 063105,2012.

[27] A. K. Parchur, A. I. Prasad, S. B. Rai, and R. S. Ningthoujam, DaltonTrans., vol. 41, p. 13810, 2012.

[28] A. Pandey and V. K. Rai, Appl. Phys. B, vol. 113, p. 221, 2013.[29] R. K. Verma and S. B. Rai, J. Quant. Spectrosc. Ra., vol. 113, p. 1594,

2012.[30] T. Hayakawa,M. Hayakawa, andM. Nogami, J. Alloy. Compound, vol.

451, p. 77, 2008.

Anurag Pandey, photograph and biography not available at time of publication.

Riya Dey, photograph and biography not available at time of publication.

Sauraph Pandey, photograph and biography not available at time ofpublication.

Vineet Kumar Rai, photograph and biography not available at time ofpublication.

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JOURNAL OF DISPLAY TECHNOLOGY 1

Visible Light-Emitting Er -Tm -Yb CodopedZrO Phosphor for Sensing and Photonics

ApplicationsAnurag Pandey, Riya Dey, Saurabh Pandey, and Vineet Kumar Rai

Abstract—The Er -Tm -Yb codoped ZrO phosphorsynthesized via chemical co-precipitation technique has beencharacterized by the X-ray diffraction (XRD) analysis. The up-conversion (UC) emission study of present material has beenperformed upon 980 nm diode laser excitation. The efficientenergy transfer between dopants has been found to be respon-sible in the UC mechanism. The tuning in color of light emittedfrom the developed phosphor has been noticed by consideringconcentration as well as power variation. The Correlated ColorTemperature (CCT) corresponding different color coordinates hasbeen calculated. The temperature sensing behavior of developedmaterial has also been explained using fluorescence intensity ratio(FIR) technique.

Index Terms—Lanthanides, co-precipitation, upconversion(UC) luminescence, color coordinates, fluorescence intensity ratio.

I. INTRODUCTION

R ARE EARTH (RE) ions doped phosphors have beenstudied extensively since last few decades due to their

unparallel luminescence properties. Proliferation in the studyof such materials is due to presence of the several energy levelsin rare earth ions which increases the number of interactionchannels with incident wave and controls over the desiredemission. The reason behind the splitting of energy levels ofrare earth ions are the electrostatic, spin-orbit and ion-latticeinteractions. In case of RE ions the splitting of energy levelsare mainly because of the electrostatic interaction, while theion-lattice interaction assisted energy level splitting is lessprobable [1], [2]. Partially filled shell in lanthanides isshielded by the completely filled outer 5s and 5p shells whichresults the inner sharp optical transitions and unaf-fected by the crystal field of host matrix [3]. RE ions dopedupconversion (UC) phosphors are demanding material owingto its property of converting the photon of lower energy intohigher energy. The emission bands from these materials canbe tuned by changing the excitation wavelength, concentration

Manuscript received December 09, 2013; revised nulldate; accepted February08, 2014. This wrok was supported byUniversity Grants Commission, NewDelhi, India, under F. No. 39-534/2010(SR)The authors are with the Laser and Spectroscopy Laboratory, Department

of Applied Physics, Indian School of Mines Dhanbad 826004, India (e-mail:rai.vk.ap@ismdhanbad.ac.in).Color versions of one or more of the figures are available online at http://

ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JDT.2014.2305773

variation of RE ions or codoping with other RE ions. RE iondoped materials covers a broad area in photonics applicationsnamely, field emission displays, plasma display panel, opticalamplifiers, white light-emitting diodes (W-LEDs), biologicaldiagnosis, sensors and security purposes, etc. [4]–[9].The presence of structural disorder in a material alters its elec-

tronic and optical properties [10]–[12]. A study involving theeffect of structure of host material on the optical properties ofdopant has drawn a significant interest for monitoring its var-ious optical properties such as quantum efficiency, radiative andnon-radiative rates, emission intensity, lifetime of dopant ions,etc. [10], [11]. A majority of RE ions in a crystal experiencesthe same crystal field resulting the very similar energy levelsand narrow linewidth of fluorescence transitions. The reducedquantum efficiency or the lifetime of an energy level is a mea-sure of an upper limit of dopant concentration due to ion-ioninteractions such as energy transfer, cross-relaxation and coop-erative energy transfer [12].The Zirconia has emerged as a promising material in the field

of optics and presents several technological applications [13].The ZrO exhibits various attractive properties such as excellenthardness, resistance to molten metal and alloys, high meltingpoint, high optical transparency, mechanical and chemical sta-bility [14], [15]. Due to low phonon frequency ( 270 cm ),large band gap ( 6 eV), high refractive index (2.13), Zirconiahas been considered as a promising candidate for the prepara-tion of efficient luminescent materials [16].Recently, the interest towards tridoped luminescent materials

has extensively increased because of its multicolor emissions.Researchers have reported the studies of tridoped materialsby taking different combinations of RE ions as dopant andobtained various color lights [17]–[22]. Every color of visiblelight emitted from the materials has its own characteristicsand application in our daily lives. Like the other primaryand secondary color lights, the cyan light is also used forlighting, display, color printing and photographic applications.So here we have tried to develop a phosphor material thatemits cyan color along with others. Several investigations havebeen performed on Er /Tm /Yb codoped systems [6],[17]–[19]. To the best of our knowledge, for the first time wehave synthesized the ZrO :Er –Tm –Yb phosphor viacoprecipitation process and its UC luminescence behaviour hasbeen studied. The reason behind the preparation of sample bycoprecipitation process is the uniform crystal morphology ofthe synthesized phosphor and less preparation time.

1551-319X © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

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In the present study, we have synthesized theEr –Tm –Yb :ZrO phosphor via chemical copre-cipitation process. The structural study has been carried outwith the help of X-ray diffraction (XRD) analysis. The UCemission properties of synthesized phosphors have beeninvestigated on 980 nm diode laser excitation. The mechanismsinvolved for the UC emissions are discussed in detail withthe help of schematic energy level diagram. The developmentof optical temperature sensors have been proposed on thebasis of fluorescence intensity ratio (FIR) variation of twothermally coupled levels of green band on increasing theexcitation power. The Correlated Color Temperature (CCT)of the materials has also been calculated in order to realize itsapplicability in LCD screen.

II. EXPERIMENTAL DETAILS

Chemical coprecipitation method has been used for thesynthesis of present phosphor material. The starting materialsEr O , Tm O , Yb O and ZrO are in oxide form. The hostand dopant materials are weighed in a stoichiometric ratioaccording to following equation

- - - ZrO Er O Tm O O (1)

where mol%, mol% and to 15 mol%These oxide powders were dissolved in deionized water. Toform the nitrate precursors for the coprecipitation process, con-centrated HNO was then added into the solutions. The so-lutions were heated for 10–15 min at a constant temperature(60 C). With drop wise addition of NH OH solution, precipita-tion starts. To achieve complete precipitate, the solution is leftfor about 20 hours. After that the precipitate is filtered usingfilter paper and washed twice with deionised water and twicewith ethanol to remove the impurities. The precipitate is driedat 500 C for a few minutes in order to completely remove thewater and to get final product. The prepared phosphor powdersare further annealed at 800 C for 2 hours and used for structuraland optical characterization purposes.Structural study of synthesized Er –Tm –Yb codoped

ZrO phosphor has been done with the help of XRD study toanalyze the phase composition and average crystallite size. TheUC emission spectra of phosphors excited with 980 nm powertunable diode laser have been recorded by a system comprisingof Princeton triple grating monochromator (Acton SP-2300) at-tached with a photomultiplier tube and personal computer. Allthe measurements were carried out at room temperature.

III. RESULTS AND DISCUSSION

A. XRD Analysis

Fig. 1 shows the XRD pattern of Er -Tm -Yb codopedZrO phosphor heat treated at 800 C. The presence of welldefined peaks in the XRD pattern indicates the formationof good crystalline phosphor material. All peaks of theEr -Tm -Yb :ZrO phosphor could be suitably matchedwith that of JCPDS card no. 83-0944 corresponding to standardmonoclinic ZrO . When compared with the standard peaksof monoclinic ZrO , one additional weak peak is observed at

which is due to the tetragonal phase of ZrO . The

Fig. 1. XRD pattern of Er -Tm -Yb codoped ZrO phosphor heattreated at 800 C.

Fig. 2. UC emission spectrum of 0.5 mol% Er 1.0 mol% Tm 10.0mol% Yb codoped ZrO phosphor under 980 nm laser excitation.

crystallite size of present material has been calculated usingDebye Scherrer’s formula [23]. The average value of crystallitesize has found to be nm indicating the nanocrystallinestructure of Er –Tm –Yb codoped ZrO phosphor.

B. Upconversion (UC) Emission Study

The UC emission spectra of the annealedEr –Tm –Yb :ZrO phosphors with optimumconcentrations of Er (0.5 mol%), Tm (1.0 mol%) andvarying concentration of Yb (1.0–15.0 mol%) ions arerecorded in the 450–850 nm wavelength range upon a 980nm diode laser excitation. The concentration dependence ofobserved blue, green and red bands is shown in inset of Fig. 2.From the variation in UC intensity of all bands (inset of Fig. 2),it is concluded that at 10 mol% of ytterbium concentration,the color emitted from the sample approaches cyan light (asshown in optical photograph). Thus the 10 mol% of Ybconcentration is taken as optimum one for getting the cyancolor in the codoped phosphor. Fig. 2 shows the UC emissionspectrum of 0.5 mol% Er mol% Tm mol%Yb codoped ZrO phosphor. Four UC emission bandsare observed in blue (450–508 nm), green (512–582 nm),red (634–690 nm) and near infrared (754–840 nm) regions

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Fig. 3. Logarithmic of UC emission intensity of various UC bands as a functionof logarithmic of pump powers.

Fig. 4. Schematic energy level diagram for Er -Tm -Yb ions with pos-sible channels responsible for multicolor emissions.

(Fig. 2). The sharp blue emission peak around 476 nm isattributed to the H transition of Tm ions. Thetwo green emission bands peaking around 525 nm and 546nm correspond to the H andtransitions of the Er ions, respectively. The red emissionaround 661 nm is due to both (Er ions)and H (Tm ions) transitions,respectively. The NIR emission band at 800 nm could beassigned as the H H transition of Tm ion [24].It is well known that the UC emission intensity is di-

rectly proportional to the nth power of pump excitation power(P) i.e., , where ‘n’ is the number of pump photonsresponsible for the UC emissions. A plot betweenversus (P) gives a straight line with slope n. To estimatethe number of pump photons involved in the UC process, wehave plotted the logarithmic of input excitation power versusUC emission intensity for different bands as shown in Fig. 3.The slopes corresponding to blue, green and red emission bandsare found 2.07, 1.91, and 1.51, respectively indicating theinvolvement of two photons in UC process in all the cases.The probable UC mechanisms and processes involved have

been discussed with the help of schematic energy level diagramof the Er , Tm and Yb ions as shown in Fig. 4.

When the sample is excited with 980 nm NIR radiation atlow pump power, Tm ion does not show any UC emissiondue to the absence of energy level corresponding to this energyvalue while a weak blue UC signal is observed at high excitationpower. It is possibly due to absorption of three NIR photons athigh excitation power [25]. The Er ions can be excited easilyby 980 nm radiation via ground state absorption (GSA) and ex-cited state absorption (ESA) processes and give UC emissions.Incorporation of Yb ions act as an efficient sensitizer for Erand Tm ions when irradiated with a 980 nm radiation. Thus,the main route to obtain the intense visible UC emission fromthe synthesized phosphor is the efficient energy transfer fromYb to Er and Tm ions.The H F levels of Tm are populated by two step

energy transfer process. Firstly, the H level is populatedby excitation of ground state Tm ions via energy transferfrom excited Yb ions ( F ). The ions in H state relaxnonradiatively to F state and then excited to F levelsby energy transfer from Yb ( F ) to Tm ( F ). Due tothe smaller lifetime of the F states compared to the Hstate, the H state is populated via the nonradiative relaxationfrom the F state. The radiative relaxation from F andH states to ground state ( H ) gives red and NIR emis-sions around 656 nm and 800 nm through the Hand H H transitions, respectively. Also the G level ispopulated through cooperative energy transfer from Yb toTm ions. The two NIR photons are absorbed by Yb ionsupon a 980 nm laser excitation and subsequently the populationis transferred cooperatively to the Tm ions in the G level[23]. The G level decays radiatively to the low lying F andH levels giving emissions in the blue and red regions around476 nm and 656 nm respectively.In the singly Er doped material absorption of incident

980 nm (NIR) photons by Er also takes place via groundstate (GSA) and excited state absorption (ESA) process. GSApopulates the I level of Er ions by absorbing onephoton. Excited ions in the I absorb another NIR photonand populate the F level via ESA Through non-radiativerelaxations, ions in the F level decays to the H andS levels. The population in these states relaxes radiativelyto give green emissions through the H and

transitions. The transitionof Er ions is also responsible for the red UC emissionfrom the sample. The population of the H S and

levels of Er ion are enhances by the energy transferfrom excited Yb ions through energy transfer process asreported elsewhere [26]. The cooperative energy transfer isnot favourable from Yb to Er ions because the Ilevel of Er ion has almost equal energy with that of Flevel of Yb ion. Also the oscillator strength correspondingto the H (Er transition is smaller than the

(Yb transition.

C. Color Tunability and CCT Analysis

The variation in color coordinates of Er -Tm -Ybcodoped ZrO phosphor on varying the concentration of Ybions is shown in the CIE chromaticity diagram (Fig. 5). Ithas been observed that the color of emitted light varies on

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Fig. 5. CIE chromaticity diagram of Er -Tm -Yb codoped ZrO phos-phor on varying concentration of Yb ions.

TABLE ICCT VALUES OF Er –Tm –Yb CODOPED ZrO PHOSPHOR AT

DIFFERENT Yb ION CONCENTRATION

varying the Yb concentration. The color corresponding toUC emissions changes from green to red via cyan color as theconcentration of Yb ions were increased from 1.0 to 15.0mol%. At 1.0 mol% of Yb concentration, the color emittedfrom sample was in green region (0.24, 0.71) and move towardscyan with increasing concentration of Yb ions upto 10.0mol%. At 15.0 mol% Yb concentration the color coordinatemoved in reddish region (0.43, 0.31) of the CIE diagram. So atuning in color of emitted light from sample was observed onvarying the Yb concentrations.The Correlated Color Temperatures (CCT) possesses a close

resemblance with color of a perfect blackbody when it is heatedto that temperature measured in Kelvin (K). Changes in the CCTvalues represent the variation in the color of a black body withdifferent temperature. As per the McCany empirical formula

(2)

where and arefixed in all cases and are different color coordinates [27].The CCT values corresponding to different CIE coordinates onvarying Yb ion concentration are listed in Table I.Not only with concentration, the present phosphor also shows

the variation in color of emitting light with pump excitationpower as visualised in Fig. 6. It has been observed that the color

Fig. 6. CIE chromaticity diagram of Er -Tm -Yb codoped ZrO phos-phor at different pump powers.

TABLE IICCT VALUES OF EREr -TM -YB CODOPED ZRO PHOSPHOR AT

DIFFERENT PUMP EXCITATION POWERS

coordinate moves from green to cyan region on increasing thepump excitation power. The color coordinates of 0.5 mol%Er 1.0 mol% Tm 10.0 mol% Yb codoped ZrOphosphor at 108 mW is (0.21, 0.58) and moves to (0.17, 0.49)at 1410 mW power. Therefore, the color emitted from samplewas tuned on varying the pump excitation power.The CCT values of the developed phosphors are also calcu-

lated at different pump excitation power as given in Table II.From the tables it has been confirmed that observed CCT valuessuitably lies in the range which is required for LCD or CRTscreen region except for 15.0 mol% ytterbium concentrations.

D. Temperature Sensing Behavior

Fig. 7 shows the UC emission intensity of green band in505–560 nm range of Er -Tm -Yb codoped ZrO phos-phor at various pump powers when irradiated with a 980 nmradiation. It was observed that the UC emission intensity corre-sponding to the H (525 nm) and(546 nm) transitions varies with increasing pump powers butnot in the same way. At very low pump power the intensity ofpeak around 525 nm was less in comparison with that of 546nm while at higher pump power reverse trend was observed.

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Fig. 7. Variation of green UC emission spectrum of Er -Tm -Ybcodoped ZrO phosphor with pump powers.

Fig. 8. A plot of UC intensity ratio of green band as a function of input exci-tation power.

This variation in intensity of observed band increases their in-tensity ratio. The energy gap between H S levels is

cm which strengthens the fact that these two levels arethermally coupled. Thus the fluorescence intensity ratio (FIR)technique can be applied to the present sample for investigatingits temperature sensing behavior [28]–[30]. The heat producedinside the sample is due to presence of the nonradiative relax-ation channels [29].It was observed that the intensity ratio of two transitionsH and nm nmvaries linearly with the pump excitation power as shown inFig. 8. Also we know that the FIR of two closely spaced levelsrelates with the temperature as,

nm nm (3)

where all the terms have their usual meanings [28].Now we have estimated the temperature of the sample corre-

sponding to different pump powers using the FIR variation cor-respondingly. At very low pump power the sample is assumedto be at room temperature (27 C). From the relation (3), the es-timated value of B is found to be . Using this value ofconstant B we have calculated the temperature of the sampleat different UC intensity ratios nm nm . The value

TABLE IIICALCULATED TEMPERATURE OF Er –Tm –Yb CODOPED ZrOPHOSPHOR AT DIFFERENT INTENSITY RATIOS CORRESPONDING TO THE

DIFFERENT PUMP POWERS

of temperature corresponding to various intensity ratios at dif-ferent pump powers are listed in Table III. From Table III, it isclear that by using (3) the calculated temperature of the devel-oped phosphor is raised on increasing the excitation power dueto the thermalization involved between the H and S ex-cited states. So, the present phosphor material may be of scien-tific interest for temperature sensing upto higher temperatures.

IV. CONCLUSION

Monoclinic Er –Tm –Yb codoped ZrO phosphor ma-terial has been synthesized successfully via chemical coprecip-itation route. The NIR to visible UC emission study suggest theinvolvement of two photons for the generation of the UC bands.The efficient energy transfers from ytterbium to thulium and er-bium ions have also been discussed. The color of emitted radi-ation is changed from green-cyan-red on increasing the Ybion concentrations while the variation in green to cyan region isalso marked on increasing pump power. Correlated Color Tem-peratures on varying concentration as well as power was alsocalculated and found in the range of LCD or CRT screen. Thecyan light emitted from the sample at optimum concentrationof Yb (10.0 mol%) can be used for developing cyan bulb andin color printing. The temperature of sample was calculated atdifferent pump powers upto 658 K which indicate the opticalheating within the sample due to laser excitation. So the presentphosphor can be used inmakingmulticolor LED’s and high tem-perature sensing probe.

REFERENCES[1] L. A. Riseberg and M. J. Weber, , E. Wolf, Ed., Progress in Optics.

Amsterdam, the Netherlands: North-Holland, 1976, vol. XIV, p. 89.[2] W. J. Miniscalco, Rare Earth Doped Fiber Lasers and Amplifiers, M.

J. F. Digonnet, Ed. New York: Marcel Dekker, 1993, p. 19.[3] W. Koechner, Solid-State Laser Engineering. Berlin, Germany:

Springer-Verlag, 1992, p. 35.[4] Y. Pan, Q. Su, H. Xu, T. Chen, W. Ge, C. Yang, and M. Wu, J. Solid

State Chem., vol. 174, p. 69, 2003.[5] V. K. Rai, K. Kumar, and S. B. Rai, Opt. Mater., vol. 29, p. 873, 2007.[6] V. K. Rai, R. Dey, and K. Kumar, Mat. Res. Bull., vol. 46, p. 2232,

2013.[7] A. S. S. de Camargo, J. F. Possatto, L. A. de O. Nunes, E. R. Botero, E.

R. M. Andreeta, D. Garcia, and J. A. Eiras, Solid State Commun., vol.137, p. 1, 2006.

[8] T. Nyokong and V. Ahsen, Photosensitizers in Medicine, Environment,and Security. Dordrecht, The Netherlands: Springer Science + Busi-ness Media B. V., 2012.

IEEE

Pro

of

Prin

t Ver

sion

6 JOURNAL OF DISPLAY TECHNOLOGY

[9] A. Pandey, V. K. Rai, R. Dey, and K. Kumar, Mat. Chem. Phys., vol.139, p. 483, 2013.

[10] W. S. Brocklesby, Annu. Rev. Mater. Sci., vol. 23, p. 193, 1993.[11] M. J. Weber, Laser Spectroscopy of Solids, W.M. Yen and P.M. Selzer,

Eds. Berlin, Germany: Springer-Verlag, 1981, p. 189.[12] J. R. Simpson, Rare Earth Doped Fiber Lasers and Amplifiers, M. J.

F. Digonnet, Ed. New York, NY, USA: Marcel Dekker, 1993, p. 1.[13] F. S. De Vicente, A. C. De Castro, M. F. De Souza, and M. S. Li, Thin

Solid Films, vol. 418, p. 222, 2002.[14] N. Suzuki, Y. Tomita, K. Ohmori, M. Hidaka, and K. Chikama, Opt.

Express, vol. 14, p. 12712, 2006.[15] L. Zhang and Y. Han, Mater. Sci. Eng. C, vol. 31, p. 1104, 2011.[16] A. Patra, Chem. Phys. Lett., vol. 387, p. 35, 2004.[17] X. L. Pang, C. H. Jia, G. Q. Li, and W. F. Zhang, Opt. Mater., vol. 34,

p. 234, 2011.[18] N. K. Giri, K. Mishra, and S. B. Rai, J. Fluorosec., vol. 21, p. 1951,

2011.[19] X. Chen, Y. Li, F. Kong, L. Li, Q. Sun, and F. Wang, J. Alloys Compd.,

vol. 541, p. 505, 2012.[20] C. H. Yang, Y. X. Pan, Q. Y. Zhang, and Z. H. Jiang, J. Fluoresc., vol.

17, p. 500, 2007.[21] J. H. Chung, S. Y. Lee, K. B. Shim, and J. H. Ryu, Appl. Phys. Express,

vol. 5, p. 052602, 2012.[22] Y. L. Liu, X. M. Tang, X. D. Chen, B. F. Lei, and D. X. Feng, Chin.

Chem. Lett., vol. 10, p. 709, 1999.[23] A. Pandey and V. K. Rai, Appl. Phys. B, vol. 109, p. 611, 2012.[24] G. H. Dieke, Spectra and Energy Levels of Rare Earth Ions in Crys-

tals. : Interscience, 1968.

[25] V. K. Rai, L. de S. Menezes, and C. B. de Araujo, J. Appl. Phys., vol.103, p. 53514, 2008.

[26] V. Singh, V. K. Rai, and M. Haase, J. Appl. Phys., vol. 112, p. 063105,2012.

[27] A. K. Parchur, A. I. Prasad, S. B. Rai, and R. S. Ningthoujam, DaltonTrans., vol. 41, p. 13810, 2012.

[28] A. Pandey and V. K. Rai, Appl. Phys. B, vol. 113, p. 221, 2013.[29] R. K. Verma and S. B. Rai, J. Quant. Spectrosc. Ra., vol. 113, p. 1594,

2012.[30] T. Hayakawa, M. Hayakawa, andM. Nogami, J. Alloy. Compound, vol.

451, p. 77, 2008.

Anurag Pandey, photograph and biography not available at time of publication.

Riya Dey, photograph and biography not available at time of publication.

Sauraph Pandey, photograph and biography not available at time ofpublication.

Vineet Kumar Rai, photograph and biography not available at time ofpublication.