Optical Materials 35 (2012) 38–44

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Optical Materials

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

Up-conversion luminescence in Ho3+ and Tm3+ co-doped Y2O3:Yb3+ finepowders obtained through aerosol decomposition

V. Lojpur a, M. Nikolic b, L. Mancic a, O. Miloševic a, M.D. Dramicanin b,⇑a Institute of Technical Sciences of SASA, K. Mihailova 35/IV, Belgrade, Serbiab Vinca Institute of Nuclear Science, University of Belgrade, P.O. Box 522, Belgrade, Serbia

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

Article history:Received 25 May 2012Accepted 19 June 2012Available online 4 August 2012

Keywords:Spray pyrolysisUp-convertorsY2O3

LuminescenceYb3+

Ho3+

0925-3467/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.optmat.2012.06.019

⇑ Corresponding author.E-mail address: dramican@vinca.rs (M.D. Dramica

Fine yttrium oxide powders doped with Yb3+ and co-doped either with Tm3+ or Ho3+ were synthesized viaspray pyrolysis at 900 �C using 0.1 M nitrate precursor. Synthesized powders were additionally thermallytreated at 1100 �C for 24 h. The characterization was done through X-ray powder diffraction (XRPD),scanning electron microscopy (SEM) and photoluminescent measurements (PL). Generation of cubicbixbyte-structure with space group Ia-3 is confirmed in all samples. Spherical particles with the meansize of �400 nm, generated through volume precipitation of much smaller nanograins, expose certaindegree of porosity which alters particle morphology during additional thermal treatment. Powders opti-cal characterizations include measurement of up-conversion emission spectra after excitation at 978 nm,as well as determination of emission lifetimes and up-converted emission intensity dependence onexcitation power. Dominant green (5F4, 5S2 ?

5I8) and blue (1G4 ?3H6) emission is found for Ho3+ and

Tm3+ samples, respectively. The amplified emission intensities and enhanced lifetime in thermally trea-ted samples are correlated with the powders morphological and structural changes.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Over the past decades there is a great deal of interest in thetrivalent rare earth ions doped up-conversion (UPC) luminescencematerials due to the wide application in the fields of solid statelaser, color displays, cathode ray tubes, fluorescent biological labelsand so on [1–4]. These materials have ability to convert infraredradiation into the visible light [5]. In UPC mechanisms, excitedstates are populated by multiphoton processes such as energytransfer up-conversion (ET), excited state absorption (ESA) andaddition of photons by energy transfer (APTE) [6]. After de-excita-tion they emit light with higher energy, i.e. shorter wavelength.Among the trivalent ions (e.g. Er3+, Tm3+, Ho3+, Pr3+, Nd3+), Tm3+–Yb3+ and Ho3+–Yb3+ are very efficient for UPC systems [7,8]. More-over, Yb3+ ion enhance the absorption of light and transferring it tothe co-dopants improves luminescent efficiency. Trivalent Tm3+

and Ho3+ ions represent good candidates for up-conversion pro-cesses due to the many long live metastable levels that can resultto the transitions at various wavelengths in UV and visible region[9,10]. Most of the research has been concentrated on infraredand visible spectra emission, while data related to ultravioletspectral are rarely investigated [11].

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nin).

Knowing that UPC properties in great measure depend on hostmaterial, and according to the excellent physical and chemicalproperties, yttrium oxide is chosen to be the host matrix. The sub-stitution of rare earth ions in this material is enhanced due to itslarge energy band gap of about 5.8 eV and due the fact that itslow phonon energy (�430–550 cm�1) suppresses the multiphotonnon-radiative relaxations, improving in that way the resultantemission efficiency [12,13].

There are several techniques developed for controlling the sizeand morphology of luminescent particles, like sol–gel method, com-bustion method, homogenous precipitation method, hydrothermalmethod, emulsion liquid membrane method, etc. [14–18]. Besideabove mentioned, spray pyrolysis is very effective method for pro-ducing spherical particles with appropriate particle size, morphol-ogy and uniform distribution of luminescent center in the hostlattice [19]. This method uses precursor solution for the formationof aerosol which decomposition comprises drying, precipitation,nucleation, growth and aggregation of primary in the secondaryspherical particles in continuous process [20]. Obtaining sphericalparticle morphology allows the good packing density and a smallscattering of light, thus improving the material brightness and reso-lutions [21].

In this paper successful synthesis of Y2O3:Yb3+, Ho3+ andY2O3:Yb3+, Tm3+ powders via spray pyrolysis method is reported.Correlation of the particles structural and morphological character-istics is done with their improved UPC properties.

Fig. 1. SEM images of Y2O3:Yb3+ Ho3+ particles obtained through spray pyrolysis (a) asp and (b) 24 h.

V. Lojpur et al. / Optical Materials 35 (2012) 38–44 39

2. Experimental

Yttrium oxide powders doped either with Yb3+ and Tm3+ or Yb3+

and Ho3+ were synthesized via spray pyrolysis of 0.1 M precursorsolutions that contains Y(NO3)3�6H2O, Yb(NO3)3�5H2O and Tm2O3

or Ho2O3 in desired cations stoichiometric ratio. For the purposeto get common nitrates precursors RE-oxides are dissolved sepa-rately in hot nitric acid. Mutual dopants ratio in precursors waschosen to be 5 in order to synthesize powders with following com-positions: Y1.89Yb0.1Tm0.02O3 and Y1.89Yb0.1Ho0.02O3. Ultrasonicatomization has been performed at 1.7 MHz (Profi Sonic-Prizma,Serbia). Generated aerosol was introduced afterwards into a hotwall tubular reactor with the three independently controlled tem-perature zones using the air flow of 1.6 dm3/min (correspondingdroplet/particle residence time in reactor was 21 s). Precursordecomposition was performed at 900 �C. The as-prepared powders(asp) were collected at the end of the reactor and are additionallythermally treated in air at 1100 �C for 24 h (24 h).

Powders phase composition was determined by X-ray powderdiffraction (XRPD) analysis using a Philips 1050 diffractometer,operating with Cu Ka radiation in 2h range 10–100� with a step scanof 0.02 and a counting time of 12 s per each step. Rietveld structuralrefinement was done in Topas Academic 4.1 software [22]. Fordetermination of the microstructural parameters Fundamental

Fig. 2. Particle size distribution of Y2O3:Yb3+ Ho3+ particles

Parameter Approach is used; FWHM based LVol (volume weightedmean column height) calculation to determine intermediate crys-tallite size broadening modelled by a Voigt function and FWHMbased strain calculation for the strain broadening. Morphologicalfeatures of the generated particles were investigated by means ofscanning electron microscopy (Tescan Vega TS 5130MM). Photolu-minescence emission spectra and lifetime measurements were per-formed on spectroflurometer system which comprises opticalparametric oscillator excitation source (EKSPLA NT 342, emissionrange 210–2300 nm), Cryostat (Advance Research SystemsDE202-AE) equipped with Lakeshore model 331 controller, spectro-graph FHR 1000 (Horiba Jobin–Yvon, 300 groove/mm grating) andICCD detector (Horiba Jobin–Yvon 3771). For the measurementsof up-converted emission intensity dependence on excitationpower 980 nm continuous wave laser diode (Rothner LasertechnikRLTMDL-980-3 W) is used for excitation.

3. Results and discussion

3.1. Powders structural and morphological characteristics

Typical morphology of the particles obtain by the spray pyroly-sis process is presented at Fig. 1a. Since that aerosol droplets pass

obtained through spray pyrolysis: (a) asp and (b) 24 h.

Fig. 3. Structural refinement of X-ray diffraction patterns for the as-prepared Y2O3:Yb3+, Tm3+ (a) and Y2O3:Yb3+, Ho3+ powder annealed at 1100 �C for 24 h (b).

Table 1Refined structural parameters of nanocrystalline Y2O3:Yb3+ co-doped with Ho3+ orTm3+ powders.

(Y2O3:Yb3+, Tm3+)asp

(Y2O3:Yb3+, Ho3+) 1100 �C,24 h

Unit cell parameter(Å)

10.5909(1) 10.5954(8)

Crystallite size (nm) 21.6(2) 59.5(8)Microstrain (%) 0.082(4) /⁄Y1:O bond length (Å) 2.2074(34) 2.2528(39)

2.2777(33) 2.3449(39)2.3687(35) 2.2681(40)

⁄Y2:O bond length (Å) 2.2911(36) 2.2681(40)⁄Y1(Y3+, Yb3+, Tm3+) x �0.03166(5) �0.03289(5)O2-

x 0.3937(3) 0.3090(4)y 0.1541(3) 0,1527(3)z 0.3801(3) 0.3785(4)

Occ Y1(Y3+) C2:0.936; S6:0.952 C2:0.937; S6:0.948RBragg 1.03 2.21Goodness of fit 1.002 1.633

Fig. 4. Up-conversion spectra of Y2O3:Yb3+, Ho3+ in VIS and near IR regions for as-prepared and thermally treated sample.

Fig. 5. Up-conversion spectra of Y2O3:Yb3+, Ho3+ in UV region for as-prepared andthermally treated sample.

Fig. 6. Energy level diagrams of Yb3+ and Ho3+ ions following excitation with978 nm.

40 V. Lojpur et al. / Optical Materials 35 (2012) 38–44

through several stages during pyrolysis (drying, precipitation,decomposition and nucleation of the new phase) spherical andagglomerate-free particles were obtained. Apparent surface rough-ness is a consequence of the nanosized primary particles nucle-ation, growth and aggregation in the frame of secondary ones(inset in Fig. 1a). Their composite substructure is more exposedafter powder calcinations, Fig. 1b. Evident growth of primary

particles, provoked by additional supply of heat, decreases porosityof smaller particles due to the densification process, while in biggerones initiates generation of open pores. Resultant sponge-likestructure of particles bigger than 1 lm is probably a consequenceof contact deficiency among the primary particles through thewhole volume of secondary ones, so their growth is intermittent.Negligible change of particles size after thermal treatment and ab-sences of hard agglomerate formation is also evident from the

Fig. 7. Decay curves for Y2O3:Yb3+, Ho3+samples: (a) asp (b) 24 h.

Table 2Decay times for as-prepared and thermally treated Y2O3:Yb3+, Ho3+.

Decay time (ms) Y2O3:Yb3+, Ho3+ asp Y2O3:Yb3+, Ho3+ 1100 �C, 24 h

393 nm, 5G4 ?5I8 s = 0.017 s = 0.029

490 nm, 5F3 ?5I8 s1 = 0.016, s2 = 0.090 s1 = 0.035, s2 = 0.130

550 nm, 5F4, 5S2 ?5I8 s = 0.042 s1 = 0.072, s2 = 0.350

755 nm, 5F4, 5S2 ?5I7 s = 0.044 s1 = 0.082, s2 = 0.357

V. Lojpur et al. / Optical Materials 35 (2012) 38–44 41

particle size distribution given in Fig. 2. The mean particle size va-lue of 400 nm in as-prepared powder slightly changes in thermallytreated one (�380 nm), and is accompanied by narrowing of theparticle size distribution.

Structural refinement of X-ray diffraction patterns for the as-prepared Y2O3:Yb3+, Tm3+ and Y2O3:Yb3+, Ho3+ powder thermallytreated at 1100 �C for 24 h are presented in Fig. 3. Powders havea cubic bixbyte crystal structure, space group Ia-3. All peaks posi-tions correspond well to the reported ones for Y1.88Yb0.12O3 com-pound (PDF 87-2368). Detail powders microstructural analysis isperformed through Rietveld refinement starting from ICSD084132 data (crystal cell parameter value a = 10.5883). Table 1. re-veals small changes in the crystal cell parameters due to Tm3+ andHo3+ co-doping in accordance to the overall dopants concentration.Taking into account the difference in Shannon ionic radii for cat-ions with octahedral VI coordination (Y3+:0.9 Å, Yb3+:0.868 Å,Tm3+:0.88 Å, Ho3+:0.901 Å) slight increase of the lattice parameteris detected for both samples. As it well known, the cubic bixbyite-structure of the host matrix (Y2O3) is characterized with the coex-istence of two nonequivalent crystallographic sites for Y3+ ions:non-centrosymmetric C2 at the 24d site and centrosymmetric S6

Fig. 8. Dependence of up-conversion intensity as a function of excitation power at

(C3i) at the 8a site [23]. Selective refinement of the C2 and S6 sitesoccupation with Y3+ implies that substitution of Y3+ with Yb3+ andcorresponding co-dopants (either Tm3+ or Ho3+) ions is more pro-nounced in the position C2 then in S6 position, even in the as-pre-pared powder. Preservation of particles nanocrystalline nature, i.e.crystallite size of �60 nm, is confirmed in Y2O3:Yb3+, Ho3+ powderafter 24 h of annealing. Long heating time does not contributes sig-nificantly to preferential dopants accommodation in C2 positionbut induces internal crystal cell ordering, i.e. microstrain in heattreated sample reaches negligible value during structurerefinement.

3.2. Powders optical characterization

3.2.1. Y2O3:Yb3+ Ho3+

Up-conversion emission spectra for the as-prepared and ther-mally treated sample for Y2O3:Yb3+, Ho3+ under the 978 nm laserexcitation are shown in Fig. 4. Both samples exhibit strong greenemission centered at 550 nm and weak near infrared emission at755 nm, while the red emission at 668 nm is easily noticeable onlyin the as-prepared sample (magnified inset is given in Fig. 4). Ob-served emission peaks are assigned to the 4f electron transitions ofHo3+: multiple transitions for green (5F4, 5S2 ? 5I8) and near infra-red (5F4, 5S2 ?

5I7) emission, as well as 5F5 ? 5I8 for red emission.Change in the peaks intensity and disappearance of individual oneswith the rise of the particle size (from 40 to 200 nm) is already re-ported in the literature for the Y2O3, Yb, Ho particles obtainedthrough homogenous precipitation [16]. Having in mind that sub-micronic spherical particles synthesized through spray pyrolysishave composite substructure, clear evidence of the red emission

978 nm in a double logarithmic plot for Y2O3:Yb3+, Ho3+: (a) asp and (b) 24 h.

Fig. 10. Up-conversion spectra of Y2O3:Yb3+, Tm3+ in UV region for as-prepared andthermally treated sample.

Fig. 9. Up-conversion spectra of Y2O3:Yb3+, Tm3+ in VIS and near IR regions for as-prepared and thermally treated sample.

Fig. 11. Up-conversion mechanism in Y2O3:Yb3+ Tm3+.

42 V. Lojpur et al. / Optical Materials 35 (2012) 38–44

peak at 668 nm in the as-prepared powder and its diminishing inheat treated sample could be associated to the size of primarygrains which can be estimated from the SEM (inset in Fig. 1a) tobe less than 100 nm.

With the longer emission accumulation in the range of 350–550 nm additional spectral lines are observed for both samplesand they are presented in Fig. 5. The emission bands positionedat 393, 420, 428 and 490 nm can be attributed to the followingtransitions: 5G4 ? 5I8, 3D3 ? 5I6, 5G5 ? 5I8, 5F3 ?

5I8, respectively.The successive energy transfer from excited Yb3+ ions populate

5I6, 5F4, 5S2, 5G4, and 3D3 energy levels of Ho3+. The 5F5 level is pop-ulated by two competing processes; the first process is non-radia-tive decay from 5F4, 5S2 and the second one is non-radiative decayfrom 5F4, 5S2 to 5I7 followed by excitation by energy transfer to 5F5.The 5G5 energy level is populated by non-radiative decay from 5G4

level. Populated energy levels radiatively de-excite to the ground5I8 and intermediate 5I6 states, schematically shown in Fig. 6.

The emission decays for Y2O3:Yb3+, Ho3+ asp (a) and annealled(b) samples are given on Fig. 7 and calculated lifetimes are shownin Table 2. For all wavelengths, values of lifetime in thermally trea-ted sample are greater then those obtained in asp sample, espe-cially for green and near infrared emissions. In asp sample onedecay exhibits double exponential behavior, while in annealedsample that is the case for all decays except ones at 393 nm.

In order to investigate UPC mechanism in Y2O3:Yb3+, Ho3+ sam-ples emission intensity dependence on the pumping power is re-corded, Fig. 8. For an unsaturated UPC process the logarithm ofthe emission intensity (I) is linearly proportional to logarithm ofthe pumping power (Ipump), with proportionality constant, n, that

represents the number of the excitation photons absorbed peremitted photon [24]. In asp and heat treated sample two photonsare involved in UPC processes for all transitions (5F4, 5S2 ? 5I8;5F5 ?

5I8; 5F4, 5S2 ? 5I7). Values obtained for green (n = 1.93) andnear-infrared (n = 1.92) UPC emission obtained for the heat treatedsample are higher in comparison to related ones founded in the as-prepared sample (n = 1.79 for green and n = 1.77 for red). This indi-cates that heat treated sample is more efficient UPC material. Con-trary, the red UPC process shows smaller n value in heat treatedsample (n = 1.18) than in the as-prepared one (n = 1.37), which isin agreement with the previous findings related to the size of theparticles [16].

3.2.2. Y2O3:Yb3+ Tm3+

The up-conversion emission spectra of as prepared (asp) andthermally treated (24 h) samples for Y2O3:Yb3+ Tm3+ under the978 nm laser excitation are given on Fig. 9. Both spectra showthe same typical transitions within 4f configuration of Tm3+ ions.One can easily observes three intensive emission bands: the stron-gest, blue emission band in range 450–500 nm (1D2 ?

3F4,1G4 ? 3H6), weak red in range 650–680 (1G4 ?

3F4, 3F2,3 ?3H6)

and near infrared 765–840 nm (1D2 ?3F3,

1G4 ? 3H6, 3H4 ? 3H6).In heat treated sample higher intensities and narrower peaks arerecorded due to the higher powder crystallinity and preferentialdopants ions accommodation at C2 site, Table 1. With the longeremission data accumulation, the UV up-conversion emissions ofthe higher order are also detected for both samples and presentedon Fig. 10. The UV spectra are recorded in the range of 250–400 nmfor the as-prepared and heat treated sample, Fig. 10. Obtainedemission bands are very similar with peaks centered at 298, 362and 392 nm, and are assigned to the following transitions:1I6 ? 3H6, 1D2 ? 3H6 and 1I6 ?

3H5, respectively.The UPC mechanism can be described in the following way: un-

der 978 nm laser excitation Yb3+ ions goes from 2F7/2 to 2F5/2 energylevels. Then three successive energy transfers from Yb3+ to Tm3+

populate 3H5, 3F2 and 1G4 levels. The 1D2 level of Tm3+ is not popu-lates with fourth photon from Yb3+ via energy transfer due to thelarge energy mismatch. It is populates by the cross relaxation pro-cess within Tm3+ ions: 1G4 (Tm) + 3F4(Tm) ? 1D2 (Tm) + 3H4 (Tm).In addition, 1D2 state may be excited to 1I6 state via another energytransfer from excited Yb3+. Population of 3H4 occurs via non-radia-tive decay from 3F2 level. Then these populated energy levels radi-atively de-excite to the ground 3H6 and intermediate 3F4, 3H5 and3F3 states like the scheme presented in Fig. 11 shows.

The emission decays for asp (a) and heat treated (b) samples aregiven on Fig. 12, while calculated lifetimes are shown in Table 3. All

Fig. 12. Decay curves for Y2O3:Yb3+ Tm3+ powders: (a) asp and (b) 24 h.

Table 3Decay time for as-prepared and calcined Y2O3:Yb3+, Tm3+ .

Decay time (ms) Y2O3:Yb3+, Tm3+ asp Y2O3:Yb3+, Tm3+ 1100 �C, 24 h

362 nm,1D2 ?3H6 s = 0.057 s = 0.071

488 nm,1G4 ? 3H6 s = 0.144 s = 0.142654 nm, 1G4 ?

3F4 s = 0.155 s = 0.136810 nm, 3H4 ? 3H6 s = 0.155 s1 = 0.143, s2 = 0.733

Fig. 13. Dependence of up-conversion intensity as a function of excitation power at 978 nm in a double logarithmic plot for Y2O3:Yb3+,Tm3+ powders: (a) asp and (b) 24 h.

V. Lojpur et al. / Optical Materials 35 (2012) 38–44 43

decay exibit single exponential behavior, except one at 810 nm forthermally treated sample, which also shows much higher value oflifetime then in asp sample. For all the other wavelengths, values oflifetime are similar for both samples.

The UCP emission intensity dependence on excitation power forthe as prepared and heat treated Y2O3:Yb3+,Tm3+ samples are pre-sented on Fig. 13. These dependences confirm that two photons areinvolved in near-infrared UCP emission, three photons in red andblue UCP emissions, and four in UV UCP emission. Values ofn = 1.81, 2.54, 2.87 and 3.42 in heat treated sample are higher com-pared to those found in the as-prepared, n = 1.75, 2.48, 2.75, 3.18,indicating in this way more efficient UCP mechanism in thermallytreated sample.

4. Conclusion

Fine yttrium oxide powders doped with Yb3+ and co-doped eitherwith Tm3+ or Ho3+ showing intense green and blue up-conversion

emission (kexc = 978 nm) are successfully prepared via spray pyroly-sis method. This method provides well crystalline, spherical parti-cles that have composite nano substructure (primary particles lessthan 100 nm) and average size of about 400 nm. Particles stay un-agglomerated even after long heat treatment (24 h). The XRPD struc-tural refinement shows that substitution of Y3+ with Yb3+ and corre-sponding co-dopants (either Tm3+ or Ho3+) ions is more pronouncedin the C2 then in S6 crystal site position. Detail optical characteriza-tion, which includes UV, VIS and near-infrared spectra analysis, life-time determination and measurements of UCP emissiondependence on pumping power, indicates that efficient UCP pro-cesses are present in synthesized powders.

Acknowledgements

This research is financially supported through the Projects45020 and 172035 of the Ministry of Science and Education of theRepublic of Serbia. The authors gratefully acknowledge dr Zorica

44 V. Lojpur et al. / Optical Materials 35 (2012) 38–44

Marinkovic Stanojevic from the Institute of MultidisciplinaryResearch, University of Belgrade, Serbia for a SEM analysis.

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