Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 619–623

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

journal homepage: www.elsevier .com/locate /saa

Influence of Li+ codoping on visible emission of Y2O3:Tb3+, Yb3+ phosphor

1386-1425/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.saa.2013.08.109

⇑ Corresponding author. Tel.: +91 326 223 5404/5282.E-mail addresses: vineetkrrai@yahoo.co.in, rai.vk.ap@ismdhanbad.ac.in (V.K.

Rai).

Anurag Pandey, Vineet Kumar Rai ⇑, Kaushal KumarLaser and Spectroscopy Laboratory, Department of Applied Physics, Indian School of Mines, Dhanbad 826004, Jharkhand, India

h i g h l i g h t s

� Y2O3:Tb3+, Yb3+, Li+ phosphor hasbeen prepared through combustionprocess.� Up/down conversion emission has

been investigated using 980 nm/303 nm excitation.� Codoping of lithium enhances the

emission intensity throughout thevisible region.� The purity of intense green emission

is verified by chromaticity diagram.

g r a p h i c a l a b s t r a c t

Comparison of the upconversion emission spectra of 0.1 mol% Tb3+ + 3.0 mol% Yb3+ and 0.1 mol%Tb3+ + 3.0 mol% Yb3+ + 2.0 mol% Li+ codoped Y2O3 phosphors.

a r t i c l e i n f o

Article history:Received 11 January 2013Received in revised form 21 March 2013Accepted 26 August 2013Available online 2 September 2013

Keywords:Upconversion phosphorCombustion synthesisCooperative transferTerbiumCIE coordinates

a b s t r a c t

Upon 980 nm diode laser excitation visible upconversion emission from the Tb3+ ions has been observedin combustion synthesized Tb3+–Yb3+ codoped Y2O3 phosphor. The intensity of upconversion as well asdownconversion emission bands has been increased by codoping of Li+ ions into Tb3+–Yb3+:Y2O3 phos-phor and the reason behind this increment is discussed. The pump power dependence of upconversionemission bands has shown two-photon absorption process. The cooperative energy transfer from Yb3+

to Tb3+ ions is supposed to be responsible for the upconversion emission from the Tb3+ ions on near infra-red excitation. The calculated colour coordinates indicate the purity of intense green emission from pres-ent phosphor which is suitable for various photonic applications.

� 2013 Elsevier B.V. All rights reserved.

Introduction

The research on inorganic luminescence materials having highefficiency, good thermal and chemical stability has revealed wideinterests from last few decades. The emission from rare earth ionsdoped luminescent materials in the optical region of electromag-netic spectrum has been observed through upconversion (UC) anddownconversion (DC) processes. Upconversion phosphors have a

number of properties that make them striking for photonics appli-cations such as display devices, lighting purposes, solar cells, bio-logical applications, temperature and radiation sensors etc. [1–6].The visible light emission from the upconverting phosphor materi-als has some additional advantages in the sense that the componentof its every colour is useful for eye vision. Luminescence from triplyionized lanthanides are mainly ascribed to their intra 4f or 5d–4ftransitions, characterized by narrow emission lines with high col-our purity that suits for present interest. The emission efficiencyof lanthanides in solid host material has been achieved near tothe theoretical limits, so there seems a possibility for furtherimprovement in emission efficiency of these materials. Search for

Fig. 1. XRD pattern of Tb3+–Yb3+ and Tb3+–Yb3+–Li+ codoped Y2O3 phosphors.

620 A. Pandey et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 619–623

the relatively high efficient upconverting lanthanide doped lumi-nescent materials under inexpensive 980 nm near infrared (NIR)diode laser excitation is continued till now.

The selection of suitable hosts and dopants mainly depends ontheir crystal structure, ionic radii, emissive efficiency, thermal con-ductivity, refractive index, phonon frequency, etc. Numerous oxideand non-oxide hosts have been used by the researchers but someunique features of oxide hosts make them more practical. Amongthe oxide hosts, Y2O3 is very popular due to its versatile featuressuch as high band gap, low phonon frequency, wide transmissionregion and matching of ionic radii with many rare earth ions [6,7].

Several rare earths and non-rare earths have been doped in var-ious host materials for getting efficient blue, green and red UCemissions [8–13]. Terbium is an interesting rare earth because ofits efficient green emission under visible and UV light excitations[7,14–16]. Due to its typical energy level structure the terbiumion cannot be directly excited by 980 nm wavelength radiation,so codoping of another rare earth is necessary to obtain the upcon-version emission from terbium ion. Yb3+ is an ideal sensitizer forthis purpose due to its strong 2F5/2 2F7/2 absorption transitionaround 980 nm. The sensitization of Tb3+ by Yb3+ ions can be ex-pressed in terms of dipole–dipole interactions [17]. The Tb3+–Yb3+ codoped glasses have been investigated vastly [18–22] butthe reports on Tb3+–Yb3+ codoped phosphors are rare [23]. Yanget al. [23] reported cooperative energy transfer and frequency UCin the Tb3+–Yb3+ and Nd3+–Tb3+–Yb3+ codoped GdAl3(BO3)4 phos-phor upon 980 and 808 nm excitations.

Tb3+–Yb3+ codoped Y2O3 down conversion phosphor has also beeninvestigated earlier [24]. Yuan et al. [24] reported energy transfermechanism upon excitation at �483 nm in the Tb3+–Yb3+ codopedY2O3 phosphor. They found that the excited Tb3+ ions in the 4f–5dstate relax down nonradiatively to the 5D4 level and transfer its exci-tation energy cooperatively to two Yb3+ ions i.e. Tb3+ (5D4) ? 2Yb3+

(2F5/2) and can be used in silicon solar cell panels to reduce thermal-ization loss. But here we desire to study the optical properties ofTb3+–Yb3+ codoped Y2O3 phosphor upon NIR excitation at 980 nm.It is reported that codoping of Li+ ions in very small quantity play animportant role in enhancement of luminescence intensity [25,26].So, we have tried to observe the effect of Li+ codoping on visible UCand DC emissions of Tb3+–Yb3+ codoped Y2O3 phosphor.

In the present work we have synthesised the Tb3+–Yb3+ andTb3+–Yb3+–Li+ codoped Y2O3 phosphors through solution combus-tion process. The phase and crystallite size of synthesized phos-phors has been determined with the help of X-ray diffraction(XRD) measurement. The presence of impurities in developedphosphor has been detected from the Fourier transform infrared(FTIR) spectroscopy. The UC and DC emission studies upon980 nm and 303 nm excitations have been performed and ex-plained. The effect of Li+ codoping and subsequent enhancementof visible emission is also discussed. The purity of intense greenemission observed from sample has been visualised by CIE coordi-nates on increasing pump power densities.

Experimental

Sample preparation

Tb3+–Yb3+ and Tb3+–Yb3+–Li+ codoped Y2O3 phosphor powdershave been prepared by mixing the starting materials, namely Y2O3,Tb2O3, Yb2O3 and Li2CO3 through solution combustion method usingurea as the organic fuel. The composition of phosphor samples were

ð100� x� yÞY2O3 þ xTb2O3 þ yYb2O3 ðiÞ

where x = 0.05, 0.1, 0.2, 0.3, 0.4, 0.5 mol% and y = 1.0, 3.0, 5.0 mol%and

ð100� x� y� zÞY2O3 þ xTb2O3 þ yYb2O3 þ zLi2CO3 ðiiÞ

where x = 0.1 mol%, y = 3.0 mol% and z = 0.0, 2.0, 5.0, 10.0 mol%.The desired amount of starting materials was dissolved in nitric

acid (HNO3) to obtain the transparent nitrate solutions of the men-tioned raw materials. All the transparent solutions were mixed to-gether and then urea was added to the solution as a reducingagent. Now the solution was heated at 80 �C and stirred simulta-neously to form the transparent gel. The transparent gel wasplaced in a crucible and rapidly heated in an electric furnace pre-heated to 650 �C. After few minutes, the solution becomes foamedand the flame was produced which lasted in 2–3 min. The cruciblewas immediately removed from the furnace. The resultant fluffymass was taken out and crushed into fine powder using pestleand mortar. The formed phosphor powders were given heat treat-ment at 800 �C for 2 h. The obtained fine phosphor powders wereused for further characterization purposes.

Characterization

The X-ray diffraction pattern of synthesized phosphors have beenrecorded by an X-ray powder diffractometer using Cu Ka1 radiation(k = 0.154 nm). The Fourier transform infrared (FTIR) spectra of thesamples were recorded in 500–4000 cm�1 range. The room temper-ature UC emission spectra were recorded using a Princeton triplegrating monochromator (Acton SP-2300) attached with a photomul-tiplier tube (PMT). The photoluminescence studies were carried outusing Fluorescence Spectrophotometer within the 450–750 nmwavelength range. The samples were pumped by a diode (CW) laseroperating at 980 nm with beam spot size 1.4 mm and all the mea-surements have been performed at room temperature.

Results and discussion

X-ray diffraction analysis

To investigate phase composition, crystallization and the crys-tallite size of the synthesized material, the X-ray diffraction(XRD) patterns of 0.1 mol% Tb3+ + 3.0 mol% Yb3+ and 0.1 mol%Tb3+ + 3.0 mol% Yb3+ + 2.0 mol% Li+ codoped Y2O3 phosphors wererecorded (shown in Fig. 1). From Fig. 1 no extra peak in Li+ codopedsample has been observed which assured that the Li+ ions replaceY3+ ions and occupy the sites of the lattice.

The diffraction peaks observed at 2h = 29.49� is the strongestone corresponding to the plane (222). The other main diffractionpeaks were observed at 20.93�, 34.14�, 48.92�, 57.89� and 79.09�

Fig. 3. The logarithmic dependence of visible UC emission intensity as a function oflogarithmic of input excitation power.

Fig. 4. Energy level diagram of the Tb3+ and Yb3+ ions with possible transitionscheme.

A. Pandey et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 619–623 621

which correspond to the (211), (400), (440), (622) and (662)planes respectively. The XRD pattern of the present phosphor issuitably matched with the JCPDS No. 41-1105. This shows thatsample is well crystallized, cubic shaped with the lattice parame-ters a = b = c = 10.604 Å and a = b = c = 90�. The average crystallitesize in both cases was found to be about 14 ± 2 nm using DebyeScherrer equation:

d ¼ 0:89kbCosh

ðiiiÞ

where d is crystallite size, k is wavelength of X-ray radiation, b is fullwidth half maxima (FWHM) and h is diffraction angle.

Upconversion emission study of Tb3+- Yb3+:Y2O3 phosphor

The UC emission spectra of the Tb3+–Yb3+ codoped Y2O3 phos-phor upon excitation at 980 nm with varying concentrations ofTb3+ and Yb3+ ions have been recorded and for 0.1 mol% Tb3+ + 3.0 -mol% Yb3+ combination the strongest UC emission was observed.Fig. 2 shows the UC emission spectra of the optimised Tb3+–Yb3+

codoped Y2O3 phosphor in the 450–700 nm wavelength range atdifferent pump power density.

Five UC emission bands have been observed in the blue, green,yellow, orange and red regions. These emission bands are assigneddue to the Tb3+ ion centred at 488 nm (5D4 ? 7F6), 547 nm(5D4 ? 7F5), 590 nm (5D4 ? 7F4), 623 nm (5D4 ?

7F3) and 660 nm(5D4 ? 7F0,1,2) [27]. Among these UC bands the green emission cor-responding to the 5D4 ?

7F5 transition at 547 nm is the most in-tense. The observed UC emission bands show stark splitting dueto crystal field effect of the host matrix. On increasing the pumppower density the intensity of UC emission bands is found to in-crease significantly due to increase in the population of the excitedlevel (Fig. 2).

To understand the UC mechanism we need to know the numberof NIR pump photons involved in UC process. This could be ob-tained from the slope of logarithmic plot of emission intensity ver-sus excitation power because UC emission intensity ‘I’ is directlyproportional to nth power of excitation power ‘P’ as [5]:

I / Pn ðivÞ

where ‘n’ is the number of photons involved in UC emission process.Fig. 3 shows the log–log plot of I versus P that gives a linear trendwith slopes about 2.09, 1.92, 1.79, 1.67 and 1.51 for the bands cen-tred at 488 nm, 547 nm, 590 nm, 623 nm and 660 nm respectively.

Fig. 2. UC emission spectra of 0.1 mol% Tb3+ + 3.0 mol% Yb3+ codoped Y2O3 phos-phors at different excitation power density.

This result indicates that two NIR photons are required for all theseUC emission bands.

A simplified energy level diagram of Tb3+ and Yb3+ ions is shownin Fig. 4. In singly Tb3+ doped sample the Tb3+ ions excited by980 nm radiation transit upward and reach to a virtual state‘‘V1’’, from there relax to the lower 7F0,1,2 states. The Tb3+ ions inthe 7F0,1,2 states may absorb second incident NIR photon andreaches to another virtual state ‘‘V2’’ (not a real state) which is4592 cm�1 below from the real 5D4 state (�20496 cm�1). Thus,to get visible UC emission it is not possible to excite Tb3+ ions inthe 5D4 state by using a 980 nm diode laser excitation. In codopedsystem, the energy of incident photon (980 nm) corresponds to the2F7/2 ?

2F5/2 absorption transition of Yb3+ ions. Thus, the Yb3+ ionsin its ground state (2F7/2) absorb the incident NIR photons and pro-moted to its 2F5/2 excited state. In this excited state two Yb3+ ionstransfer their energy cooperatively {cooperative energy transferprocess (CET)} in such a way that one excited ion transits upwardto virtual state ‘‘V’’ and other one returns back to the 2F7/2 groundstate. As the energies of both the 5D4 state (Tb3+) and virtual state(V) of Yb3+ ions are in resonance, therefore a energy transfer (basi-cally cooperative energy transfer) from the excited Yb3+ ions to theneighbouring unexcited Tb3+ ions takes place and then by absorb-ing this excitation energy (�20400 cm�1) the ground state Tb3+

ions are directly promoted to its 5D4 excited state. Afterwards,the excited Tb3+ ions through radiative relaxations from the 5D4

Fig. 5. Upconversion emission spectra of 0.1 mol% Tb3+ + 3.0 mol% Yb3+ and 0.1 mol% Tb3+ + 3.0 mol% Yb3+ + 2.0 mol% Li+ codoped Y2O3 phosphors.

Fig. 6. Photoluminescence emission spectra of Tb3+–Yb3+ and Tb3+–Yb3+–Li+ codoped Y2O3 phosphors at 303 nm excitation.

622 A. Pandey et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 619–623

level to different low-lying states emit photons in the visible region(shown in Fig. 4).

Effect of Li+ codoping in Tb3+–Yb3+:Y2O3 phosphor

The UC emission spectra of the Tb3+–Yb3+–Li+ codoped Y2O3

phosphor upon excitation at 980 nm of optimised Tb3+ and Yb3+ con-centrations with varying concentrations of Li+ have been recordedand maximum enhancement in the upconversion emission intensitywas observed for the 0.1 mol% Tb3+ + 3.0 mol% Yb3+ + 2.0 mol% Li+

combination. A variation of upconversion emission intensity versusLi+ ions doping concentration is shown in inset of Fig. 5. Fig. 5 showsthe comparison of the room temperature UC emission spectra of0.1 mol% Tb3+ + 3.0 mol% Yb3+ and 0.1 mol% Tb3+ + 3.0 mol%Yb3+ + 2.0 mol% Li+ codoped Y2O3 phosphors under identical

conditions upon 980 nm excitation. As expected, the UC emissionintensity has been improved on addition of Li+ ions. The enhance-ment about 100% in UC emission intensity has been observed for547 nm band than that without Li+ codoping. The other bands alsoshowed enhancements but less in comparison to the green bandcentred at 547 nm. The photograph of green emission from theTb3+–Yb3+–Li+ codoped Y2O3 phosphor is shown in the inset ofFig. 5. Not only in UC process but also in DC process the intensityof emission bands is enhanced by codoping of Li+ ions. The roomtemperature photoluminescence emission spectra of as-synthe-sized 0.1 mol% Tb3+ + 3.0 mol% Yb3+ and 0.1 mol% Tb3+ + 3.0 mol%Yb3+ + 2.0 mol% Li+ codoped Y2O3 phosphors at 303 nm excitationis shown in Fig. 6. The assignment of bands appeared in Fig. 6 is sameas aforementioned transitions. There appears very slightenhancement (�20%) in green emission band on Li+ codoping which

Fig. 7. Fourier transform infrared spectra of Tb3+–Yb3+ and Tb3+–Yb3+–Li+ codopedY2O3 phosphors.

A. Pandey et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 619–623 623

is very smaller than that in upconversion process (�100%). Thus wesee that the Li+ codoping is more effective in UC process.

As reported earlier [25] smaller ionic radii of Li+ ions (76 pm)can easily be insert into Y2O3 lattice and occupy the Y3+ site. ThusLi+ ions comes closure to the Tb3+ ions (92 pm) and surround it. TheLi+ ions only mutate its local symmetry not the crystal structureand hence may enhance the emission intensity that shows its suit-ability for the present purpose [28]. The enhancements in emissionintensity can also be caused by reducing the number of CO and OHgroups on codoping of Li+ ions. It is reported that Li+ codoping re-duces the CO and OH groups [29]. The FTIR spectra of Tb3+–Yb3+:-Y2O3 and Tb3+–Yb3+–Li+:Y2O3 phosphors, to see the change invibrational bands have been monitored (Fig. 7). The band observedaround 560 cm�1 arises due to Y–O vibration of host matrix [30].The bands around 1500 cm�1 and 3400 cm�1 correspond to COand OH groups respectively [29] and their intensity decreases onLi+ ions codoping. This result confirmed the enhancement in UCemission intensity caused by the reduction of CO and OH groups.

The Commission Internationale de L’Eclairage (CIE) colour coor-dinates of visible emission of the Tb3+–Yb3+–Li+ codoped Y2O3

phosphor has been calculated at different pump power densityand found in the green region (Supplementary Fig. 1). The averagevalue of CIE colour coordinate is (0.29, 0.69) and do not alter muchon increasing the pump power density which indicates the purityof green colour emission from the synthesised phosphor.

Conclusion

We have synthesized successfully the cubic shaped Tb3+–Yb3+

and Tb3+–Yb3+–Li+ codoped Y2O3 phosphors via combustion syn-thesis process with average crystallite size 14 ± 2 nm. Five UCemission bands centred at 488 nm, 547 nm, 590 nm, 623 nm and660 nm wavelengths corresponding to the 5D4 ? 7F6, 5D4 ? 7F5,5D4 ?

7F4, 5D4 ? 7F3 and 5D4 ?7F0,1,2 transitions, respectively of

the Tb3+ ion have been observed upon a 980 nm diode laser excita-tion. These bands were obtained due to two photon absorptionprocess and the phenomenon responsible for UC emission is found

to be cooperative energy transfer from the Yb3+ to Tb3+ ions. The Li+

ions codoped into the Tb3+–Yb3+:Y2O3 phosphor enhanced theluminescence emission in the visible region due to reduction ofOH and CO groups as confirmed by FTIR spectra. On comparingthe enhancement observed in up and down conversion emissionintensity on Li+ codoping, it is concluded that upconversion processis efficient for present purpose. The purity of colour emission hasbeen visualised by the CIE coordinate (0.29, 0.69) and almost unal-tered on increasing the pump power density. The efficient visiblecolour emission form the sample can be used for lighting and otherphotonic application purposes.

Acknowledgement

Authors are grateful to the University Grants Commission, NewDelhi, India for providing the financial assistance {F. No. 39-534/2010(SR)}.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.saa.2013.08.109.

References

[1] V.K. Rai, Appl. Phys. B 88 (2007) 297.[2] A. Shalav, B.S. Richards, M.A. Green, Sol. Energy Mater. Sol. C 91 (2007) 829.[3] S.K. Singh, K. Kumar, S.B. Rai, Appl. Phys. B 94 (2009) 165.[4] F. Wang, X. Liu, Chem. Soc. Rev. 38 (2009) 976.[5] A. Pandey, V.K. Rai, Appl. Phys. B 109 (2012) 611.[6] V. Singh, V.K. Rai, I. Ledoux-Rak, S. Watanabe, T.K. Gundu Rao, J.F.D. Chubaci, L.

Badie, F. Pelle, S. Ivanova, J. Phys. D 42 (2009) 65104.[7] T.K. Anh, P. Benalloul, C. Barthou, L.T.K. Giang, N. Vu, L.Q. Minh, J. Nanomater.

(2007) 1–10, http://dx.doi.org/10.1155/2007/48247 (Article ID 48247).[8] Z.G. Nie, J.H. Zhang, X. Zhang, S.Z. Lu, X.G. Ren, G.B. Zhang, X.J. Wang, J. Solid

State Chem. 180 (2007) 2933.[9] X. Qin, T. Yokomori, Y. Ju, Appl. Phys. Lett. 90 (2007) 073104.

[10] X. Wei, J. Zhao, W. Zhang, Y. Li, M. Yin, J. Rare Earth 28 (2010) 166, http://dx.doi.org/10.1016/S1002-0721(09)60073-9.

[11] V.K. Rai, C.B. de Araujo, Y. Ledemi, B. Bureau, M. Poulain, Y. Messaddeq, J. Appl.Phys. 106 (2009) 103512.

[12] Z. Liu, Y. Feng, D. Jiao, N. Zhang, H. Jiao, J. Mater. Sci. 43 (2008) 1619.[13] T. Murata, T. Tanoue, M. Iwasaki, K. Morinaga, T. Hase, J. Lumin. 114 (2005)

207.[14] V.K. Rai, S.B. Rai, D.K. Rai, J. Mater. Sci. 39 (2004) 4971.[15] R.F. Wei, H. Zhang, F. Li, H. Guo, J. Am. Ceram. Soc. 95 (2012) 34.[16] P. Psuja, D. Hreniak, W. Strek, J. Nanomater. (2007) 7 (Article ID 81350).[17] I.R. Martin, A.C. Yanes, J. Mendez-Ramos, M.E. Torres, V.D. Rodriguez, J. Appl.

Phys. 89 (2001) 2520.[18] B. Lai, J. Wang, Q. Su, Appl. Phys. B 98 (2010) 41.[19] Q. Zhang, G. Chen, Y. Xu, X. Liu, B. Qian, G. Dong, Q. Zhou, J. Qiu, D. Chen, Appl.

Phys. B 98 (2010) 261.[20] N.K. Giri, D.K. Rai, S.B. Rai, Appl. Phys. B 89 (2007) 345.[21] R.K. Verma, D.K. Rai, S.B. Rai, J. Alloys Compd. 509 (2011) 5591.[22] X. Qiao, X. Fan, Z. Xue, X. Xu, Q. Luo, J. Lumin. 131 (2011) 2036.[23] C.H. Yang, Y.X. Pan, Q.Y. Zhang, Z.H. Jiang, J. Fluoresc. 17 (2007) 500.[24] J.L. Yuan, X.Y. Zeng, J.T. Zhao, Z.J. Zhang, H.H. Chen, X.X. Yang, J. Appl. Phys. 41

(2008) 105406.[25] L. Sun, C. Qian, C. Liao, X. Wang, C. Yan, Solid State Commun. 119 (2001) 393.[26] H. Zhang, X. Fu, S. Niu, G. Sun, Q. Xin, J. Lumin. 115 (2005) 7.[27] G.H. Dieke, Spectra and Energy Levels of Rare Earth Ions in Crystals,

Interscience Publishers, USA, 1968. 253-261 [SBN 470 21390 6].[28] Y. Bai, Y. Wang, G. Peng, W. Zhang, Y. Wang, K. Yang, X. Zhang, Y. Song, Opt.

Commun. 282 (2009) 1992.[29] H. Guo, N. Dong, M. Yin, W. Zhang, L. Lou, S. Xia, J. Phys. Chem. B 108 (2004)

19205.[30] K. Mishra, N.K. Giri, S.B. Rai, Appl. Phys. B 103 (2011) 863.