Optical Materials 34 (2011) 298–302

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Optical Materials

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Multi-photon luminescence in Er3+/Yb3+:SrO�TiO2 glass ceramic

Aditya Maheshwari a,⇑, S.B. Rai b, Om Parkash a, Devendra Kumar a

a Department of Ceramic Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, Indiab Department of Physics, Faculty of Science, Banaras Hindu University, Varanasi 221005, India

a r t i c l e i n f o

Article history:Received 3 March 2011Received in revised form 15 August 2011Accepted 1 September 2011Available online 6 October 2011

Keywords:Upconversion emission3-photon processBorosilicate glass

0925-3467/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.optmat.2011.09.001

⇑ Corresponding author. Tel.: +91 9458089522.E-mail address: aditya.maheshwari.cer09@itbhu.ac

a b s t r a c t

Er3+/Yb3+ doped strontium titanate borosilicate glass was prepared. Glass ceramic was prepared by con-trolled heat treatment (at 955 �C) of glass. Ti10O19 and Sr3Ti2O7 were found as major crystalline phases.The emission spectra of glass and glass ceramic samples were investigated under 976 nm laser excitation.In glass ceramic, the intensity of the emitted radiation was much higher (�50 times for green and�10 times for red emission) than in the glass. A new three photon process was found to be responsiblefor emission at low power which is not yet observed in Er3+/Yb3+:SrO�TiO2 glass ceramic system to thebest of our knowledge. The details of upconversion mechanisms e.g. Energy Transfer (ET) and ExcitedState Absorption (ESA) were studied by power-intensity log dependence. It is expected that Er3+/Yb3+

doped nanocrystalline (610 nm) Sr3Ti2O7 phase was responsible for the observed upconversion phenom-enon in glass ceramic.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Upconversion (UC) is the phenomenon in which system absorbstwo or more excitation photons with low energy (high k) and emitsone photon of higher energy (low k). Rare earth materials as well asthe materials, in which rare earth ions are used as the absorptionor emission centres, show upconversion phenomenon from nearinfra red to visible region. These properties make the rare earthdoped materials potential for the applications as upconversionphosphors, fluorescent labels, infrared quantum counter detectors,three dimensional displays and detectors for infrared radiation etc.Generally Er3+, Tm3+, Ho3+, and Yb3+ are used for upconversion pro-cess and all of these give different emission bands. Out of all theseions, the levels 4I11/2 (10123.6 cm�1) and 4I9/2 (12345.5 cm�1) of‘‘Er3+ ion’’ can be easily populated. It is, therefore, well suited rareearth ion for upconversion. The host matrix affects the upconver-sion performance of the rare earth ion. Oxides with low vibrationalfrequencies and excellent physical and chemical stability are gen-erally used as host matrix. Good upconversion results have beenfound in Er3+ and Yb3+ doped SrO�TiO2 materials. Guo et al. [1,2]reported an increment in the intensity of the emission and energytransfer between Er3+ and Yb3+ ions in SrTiO3 ultrafine powders.Quenching of emission was also observed beyond a certain concen-tration of Er3+ ions concentration. Lemos et al. [3] reported anefficient enhancement in the infrared to visible upconversion emis-sion in Er3+/Yb3+ codoped SrTiO3 perovskite powder prepared by apolymeric precursor method. Huang and Hsieh [4] investigated the

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.in (A. Maheshwari).

effect of crystal field as well as the concentration of Yb3+ ion onupconversion in Er–Yb codoped ferroelectrics. Battisha et al. [5]reported a decrease in the intensity of luminescence because ofthe structural disorders and defects in ultrafine Eu3+ doped SrTiO3.It is reported that the upconversion luminescence intensityincreased drastically as the rare earth doped glasses are crystal-lized into glass ceramic [6–9]. Thakur et al. [10,11] investigatedthe dielectric behaviour of the SrO�TiO2 borosilicate glass ceramicsextensively and reported the formation of Sr3Ti2O7, TiO2 andSr2B2O5 as major phases in their glass ceramic samples. We wouldlike to investigate the structural and upconversion luminescence ina similar SrO�TiO2 borosilicate glass ceramic system doped withEr3+ and Yb3+. It is observed that in the glass ceramic, the intensityof the emitted radiation is very much higher as compared to glass.A three photon process is responsible for green UC emission in theglass ceramic.

2. Experimental

Batch corresponding to 25 g glass sample was prepared by mix-ing the appropriate amounts of SrCO3 (REIDEL India Chemicals,99%), TiO2 (Thomas Baker Chemicals Pvt. Ltd., 98%), SiO2 (LobaChemie Pvt. Ltd., 95%), H3BO3 (Thomas Baker Chemicals Pvt. Ltd.,99.5%) along with 2 wt.% Yb2O3 (Alfa Aesar, 99.9%) and 0.3 wt.%Er2O3 (ALDRICH, 99.9%) in an agate mortar using acetone as mixingmedium. The glass composition was melted in high grade aluminacrucible in electronic furnace in closed air atmosphere and thenannealed at 450 �C for 1 h. DTA plot was recorded with S60/51920-SETARAM Sci. & Ind. Equipments DTA & TGA. Glass samplewas crystallised at 955 �C for 4 h. The prepared glass ceramic was

A. Maheshwari et al. / Optical Materials 34 (2011) 298–302 299

translucent whereas glass was transparent with light wine colour.The crystalline phases in the prepared glass ceramic samples weredetermined from X-ray diffraction (XRD) patterns recorded withRigaku High Resolution Powder X-ray Diffractometer using Cu Karadiation. The surface of the glass ceramic was polished and etchedby using a solution of 2% HF + 5% HCl for 20–30 s for observing themicrostructure [12]. The surface was coated with Au–Pd by 6.2HUMMER Sputtering-Anatech Ltd. and microstructures wererecorded with SUPRA 40, Zeiss Field Emission Scanning ElectronMicroscope (FESEM). Prepared samples were excited with976 nm radiation from 500 mW InGaAs diode laser and emissionspectra were recorded using iHR320 Horiba Jobin Yvon Spectrom-eter. Power dependence and possible transitions were analysed.

Fig. 2. Indexed XRD pattern of glass ceramic.

3. Results and discussion

3.1. DTA analysis

From DTA plot, shown in Fig. 1, for glass sample it can beobserved that there are two peaks, one at 712 �C and second at955 �C. Taking the possibility into account that crystallizationmay be start from 712 �C, the glass sample was heat treated attwo different temperatures, 720 �C and 955 �C. Glass sample, heattreated at 720 �C, is shown by ‘‘GC720 �C’’ and the glass sample,heat treated at 955 �C, is shown by ‘‘GC955 �C’’.

3.2. XRD analysis

From X-ray diffraction (XRD) pattern (shown in Fig. 2) of theglass and the heat treated glass samples, It may be noticed thatno crystalline phase is present in GC720 �C and it is same as glasstherefore, 712 �C cannot be the crystallization temperature. Thepeak at 712 �C may be because of minor powder surface crystalli-zation but not because of bulk crystallization. It may be observedthat in GC955 �C, tetragonal Sr3Ti2O7 appeared as a major phase.By Scherrer formula, Sr3Ti2O7 particles are found to be in range15–20 nm. Fitting of the (h k l) data of Sr3Ti2O7 peaks in cell param-eter software, we got the deviation of 0.0052 in D-spacing, which isvery small. Another major phase formed is Ti10O19. It is found to besimilar to Ti9O17 in structure, which is triclinic. The ionic radii ofEr3+ (0.881 ÅA

0

) and Yb3+ (0.856 ÅA0

) are smaller than that of Sr2+

(1.12 ÅA0

); thus Er3+ and Yb3+ ions may replace Sr2+ ions fromSr3Ti2O7 and thus these may be doped in Sr3Ti2O7 matrix. SomeSrO remained in the sample, which appears as the glassy phase.

Fig. 1. DTA plot for glass sample.

3.3. FTIR analysis

In Fig. 3, Fourier transform infra red (FTIR) spectra for the glassand heat treated glass samples are shown. The weaker bands at�1000 cm�1 (peak1) arise due to stretching motion of Si–O–Si link-ages within the glassy network. The vibrational modes of theborate network are due to the asymmetric stretching of B–O bond[15,16]. These vibrational modes occur at 1200–1600 cm�1 (peak2). Small features at �2350 cm�1 (peak 3) are due to uncompen-sated atmospheric CO2 absorption [17]. The small peak at�2900 cm�1 (peak 4) is corresponding to C–H bond [17] and issame for GC720 �C and glass but disappeared in case ofGC955 �C. The peak at �3400 cm�1 (peak 5) is corresponding tothe O–H bond [17] present in silica network. It can be observedin FTIR spectra that percent transmittance of GC720 �C is samewhereas of GC955 �C is lesser than that of the glass because of crys-tallization. Therefore, the peak at 712 �C in DTA plot is not becauseof crystallization and we would further consider only GC955 �C asglass ceramic sample.

3.4. SEM analysis

From the SEM images, in Fig. 4, one can see that some biggergrains are seemed to be embedded in small particle regions, whichis very dense. Bigger grains are formed by Ti10O19, which is found

Fig. 3. FTIR spectra of glass and glass ceramics and magnified view in small wave-number region (500–2000 cm�1).

Fig. 4. SEM image of glass ceramic at (a) 20 K � magnification and (b) 300 K �magnification.

Fig. 5. Emission spectra for glass and glass ceramic with excitation wavelength976 nm.

Fig. 6. Log dependence of intensity on laser pumping power corresponding tovarious emission lines.

300 A. Maheshwari et al. / Optical Materials 34 (2011) 298–302

to be one major phase in XRD. The un-etched glass surface appearsas glassy phase in SEM. At 300 K � magnification, one can noticethat small particle regions of host are ‘‘nano-crystalline’’ in nature.These nanoparticles are of tetragonal Sr3Ti2O7, which is confirmedby the calculation of particle size from XRD pattern using Scherrerformula. In these regions, small particles with less than 10 nmdimensions are distributed homogeneously.

3.5. Luminescent properties

The emission spectra of the SrO�TiO2:Er3+/Yb3+ borosilicate glassand glass ceramic are shown in Fig. 5. Two UC bands correspondingto green and red emission were found in the ranges 520–565 nmand 625–740 nm respectively. Two peaks corresponding to thegreen UC emission at �525 nm and �550 nm were assigned tothe 2H11/2?

4I15/2 and 4S3/2?4I15/2 transitions of Er3+ respectively

and the peak corresponding to the red UC emission at �660 nmwas assigned to the transition 4F9/2?

4I15/2. The inhomogeneousbroadening of spectral lines in glass sample may be because ofthe surrounding matrices. It was reported that up to 12 differentsites occupied by Eu3+ have been identified in SrO�TiO2 by siteselective spectroscopy [5]. Ionic radii of Eu3+ and Er3+ are approx-imately equal; therefore, similar phenomena will happen in Er3+

doped SrO�TiO2 matrix. The glass and glass ceramic samples wereco-doped with Yb3+ to increase the absorption cross section andto enhance the UC emission. Er3+ has small but Yb3+ has largeabsorption cross section around 976 nm radiations and its 2F5/2

level has approximately equal energy to 4I11/2 level of Er3+. Becauseof large absorption cross-section more and more Yb3+ ions gotexcited from 2F7/2 to 2F5/2 level by absorbing photons and transferenergy to Er3+ ions while de-excitation and then by various mech-anisms upconversion occurs in Er3+ ions. Therefore, co-doping of

Er3+/Yb3+ would lead to intense emission due to high populationof upconversion levels.

3.6. Power dependence

Emission intensity of the photons depends upon laser pumpingpower as:

IaPk ð1Þ

where I is the intensity, P is Power and k is the Number of photonscontributing to the emission. One can determine, ‘k’ from the slopeof the linear plots between log (I) and log (P). To understandthe upconversion mechanism in the glass ceramic system, depen-dence of intensity of emission on the pump power was studied. InFig. 6 the power dependence curves for three intense lines areshown. It may be observed that green emission was a two photonprocess (k � 2) for high power but three photon process (k � 3)for low power whereas red emission was only two photon process(k � 2). The two photon processes for green and red upconversionare reported by previous workers in different material systems[1,3,4]. We shall follow the explanation of Suyver et al. [13] to inter-pret this change in photon process with change in the pumpingpower: The steady state population density in state |ii of acceptorion, Ni, may be written as:

A. Maheshwari et al. / Optical Materials 34 (2011) 298–302 301

Ni ¼P

j>i gjiRjNj þP

j<i WjiNjNs

Ri þP

j>i WijNsð2Þ

where gji is the Branching ratio from state |ii to state |ji; j < i, Ns isexcited-state population density of the sensitizer ion, Ri is totalemission rate constant from state |ii, Wij is the upconversion rateconstant; associated with upconversion from state |ii to state |jiof the acceptor ion. The low power limit of above equation will bereached when the term related to emission becomes more domi-nant for depopulation than ET upconversion of state |ii of acceptorion. When k steps of energy transfer upconversion are required toreach state |ii, power dependent population density of this statewill scale with:

Niðk� ETUÞaðNsÞkaPk ð3Þ

Therefore, at low power ‘‘k photon process’’ is responsible forluminescence. But in case of high power limit, the term relatedto the upconversion to any other state becomes more dominantmechanism for depopulation in comparison to the emission tolower lying states. Hence the power dependent population densityof this state will be,

Niðk� ETUÞaNsaP1 ð4Þ

Thus, only one photon process is responsible at very highpower. Hence the number of photons required for upconversiondecreases with increase in the laser pump power.

3.7. Possible transitions

Various possible mechanisms for UC emission are shown inFig. 7. Er3+ ion in the ground state 4I15/2 absorbs one photon andget excited to 4I11/2 level via ground state absorption. Again thision is excited to 4F7/2 level either by absorbing another photoni.e. Excited State Absorption (ESA) or by taking the energy fromYb3+ ion Energy Transfer (ET). From 4F7/2 level the ion relaxes to2H11/2, 4S3/2 and 4F9/2 levels by nonradiative relaxation processes.The transitions from these states to the ground state give greenand red UC emission respectively. This moderate relaxation ratemay be explained as given below [2]:

Wn ¼W0½1� e�ht=kBT ��n ð5Þ

Fig. 7. Possible transitions for green and red UC emission and their mechanisms.[ ] – Excitation-emission transitions, [ ] – nonradiative transitions and[ ] – energy transfer from Yb3+ to Er3+ ions. (For interpretation of the referencesto colour in this figure legend, the reader is referred to the web version of thisarticle.)

where Wn is the rate at temperature T, W0 is rate at 0 K, n = DE/ht,DE is energy gap between the levels involved and t is the relevantphonon’s frequency. The energy gap between the levels 4F7/2 and4F9/2 is about 5000 cm�1 therefore, it requires at least five photonsto bridge this gap [2]. Therefore, above mechanism is not very muchdominant for red emission. There is another mechanism, by which4F9/2 level is populated directly. The Er3+ ion get excited from4I15/2 to 4I11/2 level and then transits to the long living (metastable)level 4I13/2 via nonradiative transition. Again it gets excited to 4F9/2

level by ESA or ET and then gives red emission. All of the above aretwo photon processes but it can be seen from the power depen-dence that there is also a three photon process responsible for greenUC emission. In this process some of the Er3+ ion in 4F9/2 level(populated by two photon process discussed earlier) transits tolevels 4I9/2 and 4I11/2 via nonradiative transitions. These ions nowexcites to 4F3/2 and 4F7/2 levels respectively by absorbing onephoton via ESA or ET. Again they transit to 2H11/2 and 4S3/2 levelsvia nonradiative transitions and thereafter give green emission. Itwas also seen that red emission was more intense than green inglass sample. It is already reported in high doped ZBLAN: Er3+ glassthat the cross relaxation (CR) process

2H11=2;4S3=2 þ 4I15=2 ! 4I9=2 þ 4I13=2

is responsible for quenching of green emission in glass [14]. The emis-sion in glass ceramic is very much intense than in the glass. This maybe because of several reasons: Glass ceramic has crystalline phase(varying from 30% to 90%) in it, therefore, ESA and ET processes con-tribute more in the glass ceramic than in the amorphous glass [6,7,9].But if it is the only cause, then the increase in the intensity of thegreen and red emission should be in the same proportion. Crystalfield also plays an important role in the UC emission. Huang andHsieh [4] reported that different structural symmetry of host materi-als resulted in different influence of crystal field on the energy levelsof Er3+. ET process will be less dominant in glass because it needs theenergy match of level space of Er3+ ion and Yb3+ ion that stronglydepends on crystal field due to structural symmetry [4]. The threephoton processes additionally contribute to the green emission caus-ing more increase of green UC emission than that of red. CR processplays an important role for quenching the green emission in glasstherefore; the green emission was less than red in the glass sample.

4. Conclusion

In the present work, major phases appeared in XRD of glassceramic sample were Sr3Ti2O7 (tetragonal) and Ti10O19 (triclinic).Er3+ and Yb3+ ions may replace Sr2+ from Sr3Ti2O7 matrix and/orTi4+ from Ti10O19 matrix. SEM confirms that the glass ceramic hasnano-crystalline (610 nm) structure. The upconversion intensitiesof red and green emissions increased drastically as the glass wascrystallized into glass ceramic. UC emission intensities wereincreased due to the structural changes of surrounding matrixand crystal field effect. The log dependence of the emission inten-sity with pumping power shows that the green UC emission was athree photon process at low power and two photon processes athigh power but the red UC emission was only two photon pro-cesses. ESA and ET were the possible mechanisms for UC emissionand CR was responsible for quenching of green emission in glass.

Acknowledgements

Authors acknowledge the help from Mr. Peria-Samy from Schoolof Material Science and Technology, IT-BHU, Varanasi for SEM test-ing and Mr. Rajesh Kumar Verma of Department of Physics, BanarasHindu University, Varanasi for emission characterizations.

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