Photoluminescence properties of nanocrystalline Tb3+ doped Y2O3 phosphor prepared through a novel synthetic route

Available online at www.sciencedirect.com

www.elsevier.com/locate/optmat

Optical Materials 30 (2007) 608–616

Photoluminescence properties of nanocrystalline Tb3+ dopedY2O3 phosphor prepared through a novel synthetic route

S. Ray a, A. Patra b, P. Pramanik a,*

a Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721 302, WB, Indiab Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India

Received 31 July 2006; received in revised form 21 December 2006; accepted 22 January 2007Available online 12 March 2007

Abstract

Green emitting cubic Y2O3:Tb3+ nanocrystalline phosphor having particle size of 22 nm was synthesized by TEA (triethanolamine)based precursor solution method. We have characterized our nanocrystalline sample by XRD, HRTEM and Raman spectroscopic mea-surement. We compared the photoluminescence properties of our nanocrystalline sample with the bulk one. Due to the presence of defectstates and lower phonon density of nanomaterials, cross relaxation process seems to be less effective in nanosized sample reflecting higherblue/green ratio in nanosized sample. Relative intensity of 5D4–7F5 transition with respect to 5D4–7F6 transition is reduced in nanosizedsample due to greater transition probability of the latter transition in the distorted nanostructured material. Our analysis suggests thatchange of local environments takes place with decreasing the particle size and it is reflected in the photoluminescence spectra.� 2007 Elsevier B.V. All rights reserved.

Keywords: Nanocrystalline; Photoluminescence; Cross relaxation; Local environment

1. Introduction

Rare-earth doped materials have been paid much atten-tion in the last decades due to their potential use in photon-ics applications [1]. Combining the promising opticalproperties of rare-earth ions and nanoparticles, the studyof luminescence properties of rare-earth ions in nano-envi-ronment is important [2,3]. In case of rare-earth ions, theelectronic f–f transitions involve electrons which are local-ized in atomic orbital of the ions. Therefore, no size-depen-dent quantization effect (due to confinement of delocalizedelectrons) is found in these transitions. However, confine-ment effects may be induced by inter-ionic electronic inter-action and particularly, through electron–phononinteraction [4] which has important manifestations in influ-encing the optical properties. The effect of the reduceddimensionality on the electronic relaxation and the phonon

0925-3467/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.optmat.2007.01.013

* Corresponding author. Tel.: +91 3222 283322; fax: +91 3222 255303.E-mail addresses: [email protected], [email protected]

p.ernet.in (P. Pramanik).

density-of-states of semiconductor and insulating nano-crystals has been extensively investigated using both atheoretical [5] and an experimental (Raman [6] and lumi-nescence analysis [4,7]) approach. As a consequence, thespontaneous emission probability of optical transitions(luminescence lifetime) from rare-earth (RE) ions dopedin dielectric nanoparticles may be significantly differentfrom their bulk counterparts. Generally, the luminescenceproperties of nanophosphors depend on the crystallinephase and size of the particle may be owing to changes inthe (i) photon emission probability and/or (ii) phononemission probability. Now it is essential to understandwhich mechanism (i) or (ii) dominates the luminescenceproperties in nanophosphors. Among the rare earth ions,Tb3+ ions are used for green phosphors as it is an impor-tant component for trichromatic fluorescent material [8].Nanocrystalline phosphors are suitable for high definitiontelevision (HDTV) where conventional bulk phosphorcan not be used [9]. Tb3+ has eight electrons in its f shell,represented as 4f8 (which is half filled 4f7 plus one). To havethe half filled stable 4f7 electronic configuration, Tb3+ read-

mailto:[email protected]



1500 2000 2500 3000 35000.0

0.1

0.2

0.3

0.4

(b)

(a)Abs

orba

nce

(a.u

.)

Frequency (cm-1)

Fig. 1. FTIR of (a) TEA (triethanol amine) without heat treatment and(b) solution of TEA complexed with metal ion just before decomposition.

S. Ray et al. / Optical Materials 30 (2007) 608–616 609

ily loses an electron and intraionic transition 4f8–4f75dtakes place. As the 5d orbit lies on the surface of the ionso the lattice of the host in which Tb3+ is embedded hasa strong impact on 5d electrons. It is thus believed thatthe optical and electronic properties of doped phosphorare closely related to the local environment surroundingthe dopant ion and thus a change in the local environmentin nanocrystalline sample should be reflected in their opti-cal properties. A considerable amount of work on the opti-cal properties of Tb3+ doped in different inorganic hostwith the particle in the nanometer size regime has beenreported in the literature [10–12]. Chen and Kao [13] havedemonstrated elaborately the emission process of Tb3+ inCaIn2O4 matrix. But they have discussed it in terms of theirpolycrystalline material. We have compared the photolu-minescence our nanocrystalline sample with our bulk sam-ple. Buchal and co-workers [14] have explained thedifferent electronic transition of Tb3+ in SiO2 on Si indetail. May and Sommer [15] have demonstrated nicelythe well known cross relaxation process of Tb3+ inCsCdBr3 matrix. We have compared this cross relaxationprocess in nanocrystalline as well as bulk sample. For theuse of nanocrystalline phosphor in modern devices detailunderstanding of the photoluminescence process of thesmall sized phosphor particle is necessary. Yan and co-workers [16] have shown nicely the defective microstructureof Y2O3 has a pronounced effect on the photoluminescenceproperties of Eu3+ in nanocrystalline Y2O3:Eu3+. A num-ber of preparative methods such as spray-pyrolysis, sol–gel, and co-precipitation have been developed for the syn-thesis of nanocrystalline materials which actually requirea high degree of sophistication and thus imposing seriouslimitations in the preparation of nanosized phosphor.Recently Song and Wang [17] have prepared Tb3+ dopedY2O3 by combustion procedure using glycine. But theyhave identified the presence of some unwanted Tb4+ intheir sample. We have prepared our nanocrystalline Tb3+

doped Y2O3 phosphor by an organic chelate based precur-sor solution method where TEA acts as a chelating ligand[18]. We have reduced Tb4O7 with the help of hydrazinehydrate in presence of concentrated HCl and we have gotno residue of Tb4+ in our sample. The salient feature of thisfacile synthesis is inexpensive precursor and formation oflow agglomerated particles with high homogeneity.

2. Experimental

2.1. Sample preparation

In our preparative method Y2O3 (Aldrich) was dissolvedin concentrated HNO3 and boiled on water bath to get aclear solution of Y(NO3)3. Measured amount of Tb4O7

(Aldrich) was taken in a beaker containing concentratedHCl and hydrazine hydrate and pH of the solution wasmaintained at pH 1. This entire solution was boiled onwater bath and after boiling for 1 h a clear solution ofTbCl3 was obtained. Hydrazine hydrochloride solution

helps to reduce Tb4+ to Tb3+ as Tb is present in mixedvalence state (Tb4+ and Tb3+) in Tb4O7. The redox poten-tial of Tb4+/Tb3+ system (3.10 V) is greater thanNH3NHþ2 =N2 system (0.23 V), so Tb4+ can easily oxidizehydrazine to N2 and itself can be reduced to Tb3+. pH ofthe solution plays a crucial role in this reduction process.pH of the solution is kept at pH 1 with the addition of con-centrated HCl. The reaction is as follows:

3Tb4O7 + NH2NH2 + 2Hþ+ 2e = 6Tb2O3 + N2 + 3H2O

With the addition of NH4OH to that clear solution a whiteprecipitate of Tb(OH)3 was obtained. This precipitate wascollected and dissolved in concentrated HNO3 to obtain aclear solution of Tb(NO3)3. To produce nanocrystallineTb3+ doped Y2O3 where Tb3+ is present as 0.5, 1, 3 and5 at wt% with respect to Y3+, both the nitrate solutionwas taken according to the required stoichiometry. ThenTEA (triethanolamine) was added to that nitrate solutionand the molecular ratio of metal salt:TEA was maintained1:4. Then certain amount of concentrated HNO3 solutionwas added to maintain the pH 3–4 to avoid any kind of me-tal hydroxide precipitation. TEA was used as it is an effi-cient chelating ligand and possesses good coordinationproperties with the metal ions. The clear solution of TEAcomplexed metal nitrate is evaporated on a hot plate at180 �C with constant stirring. Continuous heating of thesolution causes foaming and puffing. During evaporationof the precursor solution, the TEA present in the systemprobably led to the formation of vinyl functional groupsthat cause polymerization. as evidenced by the FTIR(Fig. 1b) study of resulting precursor solution heated at200 �C before decomposition. It showed a broad peak inthe region from 2900 to 3140 cm�1 which was due to thepolymerization through vinyl group formation during theheat treatment of the precursor solution. But only TEAwithout heat treatment showed no peaks as shown inFig. 1a in the frequency range from 3000 to 3100 cm�1

which confirms that polymerization has occurred duringthe heat treatment of TEA complexes with metal ions.

610 S. Ray et al. / Optical Materials 30 (2007) 608–616

During evaporation of the precursor solution whencomplete dehydration occurs, the nitrates themselves aredecomposed with the evolution of brown fumes of nitrogendioxide, leaving behind a voluminous, organic-based,black, fluffy powder, i.e., precursor powder which is thencalcined at 500 �C for 2 h and then heated at 900 �C inpresence of hydrazine vapour for 2 h to get the requirednanocrystalline Y2O3:Tb3+ sample. Hydrazine vapour isused as it is more efficient reducing agent than H2 gasand thus avoids the oxidation of Tb3+ to Tb4+. The bulksample was obtained by sintering the nanocrystalline sam-ple at 1400 �C (in presence of hydrazine vapour) for 2 h.Our nano and bulk samples both are free from Tb4+. Wehave developed a new chemical procedure for the detectionof Tb4+ using KI and starch solution and this detectionlimit works up to 20 ppm level. For the detection of Tb4+

the sample was dissolved in concentrated HCl which waspassed through Ag gas to remove O2 and then 5% KI solu-tion was added into it. Upon addition we did not find anyblue coloration indicating no Tb4+ is present in the mixtureas this blue coloration is very sensitive to the presence ofTb4+. So we conclude that we have prepared Y2O3:Tb3+

sample which is absolutely free of any Tb4+ residue.

2.2. Thermal study

The DTA curves in Fig. 2 revealed an exothermic ther-mal affect for all the precursor material with their respec-tive peak around 380–500 �C. The exothermic peak couldbe assigned to the oxidation of carbonaceous remains fromthe decomposed metal-complexes and TEA. The entirethermal effect was accompanied by the evolution of variousgases (such as CO, CO2, NH3, water vapor, etc.), whichwas manifested by a single step weight loss in the TG

Fig. 2. Simultaneously recorded

curves shown in Fig. 2. Above 500 �C, there was no signif-icant thermal effect observed in DTA curves and the corre-sponding TG curves showed no weight loss, implying thecomplete volatilization of carbon compounds.

3. Characterization

Samples have been characterized by X-ray diffraction atroom temperature using Philips PW1710 X-ray Diffrac-tometer, equipped with a vertical goniometer and CuKaradiation source of wavelength (k = 1.514 A). X-ray dif-fraction patterns for both nano and bulk sample areshown in Fig. 3a. The observed values of the diffractionangles (2h) match quite well with the standard JCPDS file(86-1326) and the patterns exhibit cubic symmetry (space-group Ia3). It has been observed that the Full width athalf maxima (FWHM) of the X-ray diffraction peakincreases as the particle size decreases. After the removalof the broadening in FWHM in the XRD peak due toinstrumental correction the breath of the diffraction peakof the samples can be interpreted in terms of the particlesize and strain [19]. It is already known that the full widthat half maxima (FWHM) can be expressed as a linearcombination of the contribution from the lattice strainand crystallite size [20]. The effect of the strain and particlesize on the FWHM can be expressed by the followingequation [19,20,9]:

b cos hk¼ 1

Dþ e sin h

kð1Þ

where b is the actual FWHM (in radians, after the instru-mental correction has been made), h is the Bragg angle ofthe peak, k is the X-ray diffraction wavelength, e is theeffective strain.

DTA/TG for the precursor.

0.16 0.17 0.18 0.19 0.20

3.703.753.803.853.903.954.004.054.10

sin θ/λ sin θ/λ

β co

sθλ

/ (x

10-3

)

β co

sθλ

/ (x

10-3

)

28 29 30 31

0

1000

2000

3000

4000 (222)

Inte

nsity

(a.u

)2θ

a

b c

0.16 0.17 0.18 0.19 0.20

1.60

1.65

1.70

1.75

1.80

1.85

1.90

Fig. 3. (a) X-ray diffraction pattern for nanocrystalline sample (dashed line) and bulk sample (solid line) and (b) Williamson–Hall plots of nanocrystallinesample and (c) Williamson–Hall plot of bulk sample.

Table 1Properties of nanocrystalline and bulk sample

Sample Size (nm) Lattice distortion

Nanocrystalline sample 24 0.034Bulk sample 178 0.0264


b can be obtained from the following equation [21]:

b2 ¼ b2M � b2

S ð2ÞbM is the broadening due to the size and strain of the sam-ple, measured can be obtained from a Gaussian fit of thediffraction line and bS can be obtained from the FWHMof the diffraction line of standard stress free Si sample. InFig. 3b and c bcosh/k is plotted versus sinh/k (which isknown as Williamson Hall plot) for the nanocrystallineas well as bulk samples. Then the extrapolation to h = 0�of the Williamson Hall’s plot [22] for the X-ray peaks givesD from the reciprocal of the y-axis intercept. The latticedistortion of the samples can be obtained from the slopeof the Williamson Hall’s plot. The detection limit lies with-in ±1% for both nanocrystalline as well as bulk samples.We also have put the error bars in this plot which was orig-inated from the accuracy of X-ray diffractometer, which is0.03� and also in the calculation of bM (measured FWHM).The X-ray diffractometer linewidth has been calibratedwith the help of stress free standard Si sample. The sizeand strain of nanocrystalline as well as bulk samples havebeen tabulated in Table 1. Tannar [23] and Igarashi et al.[9] have observed seven times more lattice distortion innanocrystalline sample with respect to bulk one. On thecontrary we have observed 1.28 times more lattice distor-tion in our nanocrystalline Y2O3:Tb3+ as compared tothe bulk one.

Samples for transmission electron microscopy (TEM)are deposited onto 300 mesh copper TEM grids coatedwith 50 nm carbon films. Nanocrystals, dissolved in ace-tone (sonicated for 3 h), are placed on the grid drop-wise.The excess liquid is allowed to evaporate in air. The gridsare examined in JEOL 2010 microscope with ultra-high res-olution (UHR) pole-piece using a LaB6 filament operatedat 200 kV.

In Fig. 4a we see the TEM picture of our nanocrystallinesample Fig. 4b is the corresponding frequency plot, Fig. 4cis the HRTEM image and Fig. 4d is the correspondingSAED pattern. The frequency plot of the size distribution,shown in Fig. 4b, is obtained by measuring size of �150particles per sample. Size distributions for the nanoparti-cles are usually found to be log-normal [24]:

P ðdÞ ¼ 1

dffiffiffiffiffiffiffiffi2prp exp � lnðd=�dÞffiffiffi

2p

r

� �2

ð3Þ

Here �d and r are related to the average size and the size dis-tribution of the particles. By fitting the frequency plot using

16 18 20 22 24 26 28 30 32 34

0

5

10

15

20

25

Fre

quen

cy

Diameter of the particle (nm)

4.2Å (211)

2.9Å (222)

211220222

a

c d

b

Fig. 4. (a) TEM picture of Y2O3:Tb3+, (b) frequency plot, (c) HRTEM image, (d) SAED pattern.


Eq. (2) (solid line in Fig. 4b), the average particle size of thenanocrystalline Sample A has been estimated to be 22 nmwith a very narrow size distribution (r = 0.14). TheHRTEM of nanocrystalline sample clearly demonstratesthe lattice fringes for the cubic phase of Y2O3:Tb3+ andfrom the micrograph we have determined the lattice spac-ing in the particle to be 2.9 A (for a particular set of planes)which corresponds to the (222) plane and 4.2 A (for an-other sets of planes) which corresponds to the (211) planeof the cubic phase of the materials. From SAED pattern ofthe micrograph we have also calculated the planes and con-firms the presence of (211), (22 0), (222) and (321) planesof cubic Y2O3.

Raman spectra of the samples were obtained using Ren-ishaw Raman System RM 1000B coupled with a LeicaDMLM microscope. Excitation of sample was achieved

350 360 370 380 390 400

4000

8000

12000

16000 1.37 cm-1 shift in nano sample

377.06

375.69

Inte

nsity

(a.u

)

Energy (cm-1)

Fig. 5. Raman spectra of nanocrystalline sample (dashed line) and bulksample (solid line).

with a 20 mW Ar ion Laser source of 514 nm wavelengths.As shown in Fig. 5 the prominent Raman line of Y2O3

shifts to lower frequencies (from 377 to 375.69 cm�1)accompanied by an increased broadening and asymmetryof peak with decreasing the size. It is already reported byZhang and Chan [25] that the lattice expansion occurs withdecreasing particle size, and they also showed that Ramanpeak shifts towards lower energy with decreasing the parti-cle size, indicating that phonon energy decreases withdecreasing the particle size. It is well established that inthe luminescence of rare-earth ions, the highest phonon fre-quency of the host lattice are responsible for nonradiativerelaxations.

4. Result and discussion

Photoluminescence (PL) and PL excitation (PLE) weremeasured at room temp by [Perkin Elmer LS55] Lumines-cence Spectrometer .The electronic transition spectra forthe range of 400–700 nm are shown in Fig. 8. The fluores-cence of Tb3+ originated from the transition of 5D3 to 7FJ

and 5D4–7FJ manifolds of Tb3+. Tb3+ doped phosphorscan be excited by 270 nm ultraviolet light where Tb3+

(4f8) are excited to higher 4f75d1 levels and then nonradiav-ely relaxes to 5D3 and 5D4 excited states. From these twoexcited states it relaxes radiatively to different 7FJ mani-folds. Due to the relatively large energy gap between 5D3,5D4 excited states and 7FJ ground states this relaxationprocess occurs radiatively and we get the spectra in the vis-ible range [13]. Among all the transitions of Tb3+ the pre-dominance one is 5D4–7F5 at 543 nm by the larger value of

2 3 4 5 6

0.05.0x103

1.0x104

1.5x104

2.0x104

2.5x104

3.0x104

3.5x104

4.0x104

4.5x104

CTS

5D35D4

7F6

7F5

7F4

7F3

Excited states}

7F

Radiative R

elaxation

Fig. 6. Energy level diagram of Tb3+ in nanocrystalline Y2O3:Tb3+.


the reduced matrix elements at J = 5 and Judd–Ofelt the-ory [14]. In Fig. 6 the energy level diagram of Fig. 7 showsthe excitation spectra of nanocrystalline Tb3+ doped Y2O3

(0.5 at wt% Tb3+) by monitoring 543 nm (5D4–7F5) emis-sion of Tb3+. The excitation spectra monitored at 543 nm(5D4–7F5) transition of nanocrystalline Tb3+ doped Y2O3

is shown in Fig. 7. Flores-Gonzalez et al. [26] have demon-strated nicely the detail excitation spectra of Tb3+ dopedY2O3. Our excitation spectra can also be divided into three

240 280 320 3600

100

200

300

400

5007F

6-5D

27F

6-5D

3

of Tb3+

346

CT bandET band

329

270231

inte

nsity

(a.u

)

wavelength (nm)

Fig. 7. Excitation spectra of nanocrystalline Y2O3:Tb3+.

400 450 500 550 600 650 700

-100

0

100

200

300

400

x = .05

x = .03x = .01x = .005

5 D 4-7 F 3

5 D 4-7 F 45 D 4-7 F 5

5 D 4-7 F 6

5 D 3-7 F 5

wavelength (nm)

Inte

nsity

(a.u

)

a b

Fig. 8. (a) Emission spectra of nanocrystalline s

parts. Wang et al. [27] have demonstrated the ET transferband from Y2O3 host to Tb3+ between 200 and 235 nm.First part of the excitation spectra of our nanocrystallinesample contains a peak at 231 nm corresponding to energytransfer from Y2O3 host to Tb3+, second part of the spectracontains two peaks, the major one is located at 270 nm andthe next peak is located at 329 nm, both of them corre-sponds to the charge transfer transition of Tb3+ from 2porbital of oxygen to 4f orbital of Tb3+, and the last partof this spectra contains a peak at 346 nm correspondingto the transition of 7F6–5D2 or 5D3 one of Tb3+ [26].Now 0.5 at wt% Tb3+ doped in Y2O3 sample (nanocrystal-line sample) is taken as the reference sample as it gives themaximum emission intensity. The emission spectraobtained by exciting the sample of Tb3+ doped Y2O3 at270 nm, which consist a series of emission lines from400 nm to 700 nm corresponding to the transitions from5D3–7FJ and 5D4–7FJ manifolds of Tb3+.

In Fig. 8a and b the luminescence of Tb3+ from 5D3–7FJ

and 5D4–7FJ levels are shown. Fig. 8 shows the emissionspectra of nanocrystalline Tb3+doped in Y2O3 whereTb3+ concentrations are 0.5, 1, 3 and 5 at wt%, respec-tively. From this figure we can see the transitions of Tb3+

which originate from 5D3 and 5D4 manifolds as tabulatedin Table 2. From the above figure it is clear that the peakintensity of 5D4–7F5 (green) transition of Tb3+ increaseswith increasing Tb3+ concentration from 0.5 at wt% upto 3 at wt%. After 3 at wt% the peak intensity of5D4–7F5 transition decreases due to concentration quench-ing effect. But the interesting phenomena is that with theincrease of Tb3+ concentration although the peak intensityof 5D4–7F5 (green) transition increase but the peak inten-sity of 5D3–7F5 (blue) transition decreases. This decreasein blue peak with increase in Tb3+ concentration is knownas cross relaxation process which dominates after the crit-ical concentration of Tb3+ needed for this cross relaxationphenomena. This cross relaxation process is induced by theresonance between the excited states and ground states oftwo Tb3+ ions as depicted in the following equation [17]:

400 450 500 550 600 650 700

0

200

400

600

800

1000

1200

wavelength (nm)

Inte

nsity

(a.u

) 5 D 4-7 F 3

5 D 4-7 F 4

5 D 4-7 F 5

5 D 4-7 F 6

5 D 3-7 F 5

x= .05

x = .01x = .03

x = .005

ample, (b) emission spectra of bulk sample.

Table 2% Increase of blue/green ratio of nanocrystalline sample with respect tobulk sample for different concentration of Tb3+ in Y2O3:Tb3+

Concentration of Tb3+

in Y2O3:Tb3+% Increase of blue/green ratio ofnanocrystalline sample with respectto bulk sample

0.5 at wt% 52%1 at wt% 63%3 at wt% 68%5 at wt% 50%


Tb3þð5D3Þ þ Tb3þð7F0Þ ¼ Tb3þð5D4Þ þ Tb3þð7F6Þ ð4Þ

This blue emission of Tb3+ is generally found to vanishwith higher concentration of Tb3+ in many hosts such asoxides, YAG, etc. But in a special category of Tb3+ acti-vated phosphors such as CsCdBr3:Tb [28] and LnBO:Tb[29] exhibiting no blue emission even at very low Tb3+ con-centration as Tb3+ in this matrix incorporate themselvesinto pair or clusters as a result of which energy transfer be-tween Tb3+ ion occurs even at very low concentration ofTb3+ in these matrices. So nature of matrix or host has a

48

52

56

60

64

68

% In

tens

ity (a

.u) o

f Blu

e/G

reen

pea

k

concen

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

at wt % of Tb3+ (nanocrystalline sample)

Blu

e/G

reen

ratio

a b

c

Fig. 9. (a) Plot of blue/green ratio of nanosample, (b) plot of blue/green ratio orespect to bulk sample with respect to at wt% of Tb3+.

pronounced role in cross relaxation process, which indi-rectly determines the intensity of blue (5D3–7F5) peak aswell as intensity of green (5D4–7F5) peak. We have plottedthe emission spectra of nanocrystalline as well as bulk sam-ples with Tb3+ concentration 0.5, 1, 3, 5, and 7 at wt%.From these two plots we measure the intensity ratio ofblue/green peak for both nanocrystalline as well as bulksamples. For both the cases the blue/green ratio decreaseswith increasing Tb3+ concentration but the blue/green ratioof nanocrystalline sample is greater for nanocrystallinesample with respect to bulk sample. This observation istrue for all concentration of Tb3+. This high blue/green ra-tio in nanocrystalline sample indicates that cross relaxationphenomena is less effective in nanocrystalline sample thanbulk sample. Jacobson et al. [30] have observed that in caseof concentration quenching in nanostructured Y2O3:Tb3+

takes place at a greater concentration (three times higher)with respect to that in a bulk one. In our nanocrystallinesample we observe greater blue/green ratio with respectto the bulk one. This discrepancy in the luminescencebehavior of nanocrystalline phosphor must be correlatedwith its microstructure. In nanosized material due to its

tration of Tb3+

0.08

0.12

0.16

0.20

0.24

Blu

e/G

reen

ratio

at wt % of Tb3+ (bulk sample)

f bulk sample, (c) plot of % increase of blue/green ratio of nanosample with

Table 3Comparison of 5D4–7F5/5D4–7F6 for nanocrystalline sample w.r.t. bulksample

Sample 5D4–7F5/5D4–7F6

Nano 0.5 at wt% 1.4Bulk 0.5 at wt% 1.92Nano 1 at wt% 1.48Bulk 1 at wt% 2.268Nano 3 at wt% 1.9Bulk 3 at wt% 2.52Nano 5 at wt% 2.25Bulk 5 at wt% 2.66


lower phonon density (as evidenced from Raman spectra)and also its defective structure is responsible for the lesseffective cross relaxation process. More elaborately innanosized material due to the presence of limited numberof primitive cells per particle traps (quenching centers) dis-tribute themselves very randomly as a result of which somenanoparticles contain more traps but some of them possesslesser number of traps [16,31,32], resulting in the hindranceof cross relaxation process. Due to lower phonon density ofnanomaterials (as evidenced from Raman spectra, Fig. 3)and also presence of defects in nanosample make this crossrelaxation process less effective. In Fig. 9 we plot the blue/green peak of (a) nanocrystalline sample and (b) bulk sam-ple with respect to Tb3+ concentration. (c) Percentage in-crease of blue/green ratio in nanocrystalline samples withrespect to bulk samples for all four concentrations ofTb3+. These values are also tabulated in Table 2.

In Fig. 9c we see that the percentage increase in blue/green ratio increases with increase in the concentration ofTb3+ up to 3 at wt% of Tb3+, after which concentrationquenching starts dominating. From this plot it is clear thatblue/green ratio is higher for nanosample with respect tobulk sample which also increases with increase in the con-centration of Tb3+ because of less effective cross relaxationprocess in nanocrystalline sample. In Fig. 10 the emissionspectra of Y2O3:Tb3+ are shown (for both nanocrystallineand bulk sample) to compare the emission intensity of5D4–7F5/5D4–7F6 transition for all concentration of Tb3+.

Symmetry of the local environment around Tb3+ activa-tors has a pronounced effect on the intensity of the transi-tions between different J levels and can be explained interms of Judd Ofelt theory [33,34]. Magnetic dipole transi-tions are allowed and thus host has a little effect on theintensity of this transition. But electric dipole transitionsare forbidden and according to Judd–Ofelt theory theseforbidden transitions can be allowed by the admixture ofopposite parity 4fn�15d states by an odd parity crystal fieldcomponent [35]. Among the transitions of Tb3+ 5D4–7F5

400 450 500 550 600 650 7000

200

400

600

800

1000

1200

1400

x = .03

x = .01

x = .005

bulk

bulk

bulk

nano

nano

nano

5D4-7F5

5D4-7F5

5D4-7F5

5D4-7F6

5D4-7F6

5D4-7F6

Inte

nsity

(a.u

)

wavelength (nm)

Fig. 10. Comparison of emission spectra of nanocrystalline and bulkY2O3:Tb3+.

(DJ = 1) is a magnetic dipole allowed transition but5D4–7F6 transition (DJ = 2) is a forced electric dipoleallowed transition. So the host has greater impact on theintensity of 5D4–7F6 transition. From Fig. 10 it can beshown that the relative intensity of 5D4–7F5 transition isgreater in nanocrystalline sample than bulk sample. Thatis the ratio 5D4–7F5/5D4–7F6 is greater in bulk sample withrespect to nanocrystalline sample. It is clear from the dataof Table 3 that the ratio of 5D4–7F5/5D4–7F6 is higher forbulk sample with respect to nanosample. This observationis true for all four concentration of Tb3+ indicating that thetransition probability of 5D4–7F6 is greater in nanocrystal-line host. This anomalous behavior of nanosized powdershould be correlated with the microstructure of the nano-crystalline samples. With the reduction of particle size theproportion of atoms in different layers of surface increases[9] as well as lattice distortion also increases (as reflectedfrom the Williamson Hall plot of the nanocrystalline sam-ple). These defects and distortion may decrease the symme-try around Tb3+ in nanocrystalline sample, as a result ofwhich the transition probability of the forced electric dipoleallowed transition (5D4–7F6) increases (due to the greaterrelaxation of the parity forbidden rule in lower symmetrynanocrystalline host) and consequently the ratio of5D4–7F5/5D4–7F6 decreases in nanosized samples withrespect to bulk samples.

In conclusion, we have prepared nanocrystallineY2O3:Tb3+ through a novel complex based precursor solu-tion method using TEA as a chelating ligand. Starting withTb4O7 (mixed valence states of Tb4+/Tb3+), with the helpof hydrazine hydrochloride we have reduced Tb4+ toTb3+ and prepared the required samples. We have charac-terized our nanocrystalline and bulk Y2O3:Tb3+ samplesthrough XRD, HRTEM and Raman spectroscopic tech-nique. We have compared the photoluminescence proper-ties of nanocrystalline sample with the correspondingbulk samples and observed that the blue/green ratio isgreater in nanocrystalline samples with respect to bulkone indicating less effective cross relaxation process takesplace in nano-environment. Also 5D4–7F6/5D4–7F5 ratioshows a clear difference in nanocrystalline sample withrespect to bulk one. These two phenomena indicate thatthe nano-environment has a pronounced effect on the pho-toluminescence properties of this phosphor material.


Acknowledgement

Authors thank Council of Scientific and Industrial re-search for financial assistance.

References

[1] P.N. Prasad, Nanophotonics, John Wiley & Sons, NY, 2004.[2] A.J. Silversmith, W. Lenth, R.M. Macfarlane, Appl. Phys. Lett. 51

(1987) 1977.[3] A. Patra, C.S. Friend, R. Kapoor, P.N. Prasad, Appl. Phys. Lett. 83

(2003) 284.[4] H.-S. Yang, M.R. Geller, W.M. Dennis, Phys. Rev. B 62 (2000) 9398.[5] R.K. Soni, L.F. Fonseca, O. Resto, M. Buzaianu, S.Z. Weisz, J.

Lumin. 83–84 (1999) 187.[6] H.-S. Yang, K.S. Hong, S.P. Feofilov, B.M. Tissue, R.S. Meltzer,

W.D. Dennis, J. Lumin. 83–84 (1999) 139.[7] R.S. Meltzer, K.S. Hong, Phys. Rev. B 61 (2000) 3396.[8] B. Yan, X.Z. Xiao, Opt. Mater. 28 (2006) 498.[9] T. Igarashi, M. Ihara, T. Kusunoki, K. Ohno, T. Isobe, M. Senna,

Appl. Phys. Lett. 76 (2000) 1549.[10] J. Wang, H. Song, B. Sun, X. Ren, B. Chen, Wu Xu, Chem. Phys.

Lett. 379 (2003) 507.[11] Y.L. Soo, W. Huang, Y.H. Kao, V. Chhabra, B. Kulkarni, J.V.D.

Vekiadis, R.N. Bhargava, Appl. Phys. Lett. 75 (1999) 2464.[12] Y.L. Soo, W. Huang, Z.H. Ming, Y.H. Kao, G.C. Smith, E.

Goldburt, R. Hodel, B. Kulkarni, J.V.D. Vekiadis, R.N. Bhargava, J.Appl. Phys. 83 (1998) 5404.

[13] F.S. Kao, Teng-Ming Chen, J. Lumin. 96 (2002) 261.[14] H. Amekura, A. Eckau, R. Carius, Ch. Buchal, J. Appl. Phys. 84

(1998) 3867.[15] P. Stanley May, K. Devon Sommer, J. Phys. Chem. A 101 (1997) 9571.

[16] Zhenggui Wei, Lingdong Sun, Chunsheng Liao, Jialu Yin, XiaochengJiang, Chunhua Yan, J. Phys. Chem. B 106 (2002) 10610.

[17] Hongwei Song, Jiwei Wang, J. Lumin. 118 (2006) 220.[18] A. Pathak, S. Mahapatra, S. Mahapatra, S. Biswas, D. Dhak, N.

Pramanik, A. Taraphder, P. Pramanik, Am. Ceram. Soc. Bull. 83(2004) 9301.

[19] S.B. Qadri, J.P. Yang, E.F. Skelton, B.R. Ratna, Appl. Phys. Lett. 70(1997) 1020.

[20] Feng Gu, Shu Fen Wang, Meng Kai Lu, Guang Jun Zhou, Dong Xu,Duo Rong Yuan, Langmuir 20 (2004) 3528.

[21] B.D. Cullity, Elements of X-ray Diffraction, Addison – Westley, 1978,p. 284.

[22] Masayuki Nishi, Setsuhisa Tanabe, Masashi Inoue, Masaru Takah-ashi, Koji Fujita, Kazuyuki Hirao, Opt. Mater. 27 (2005) 655.

[23] P.A. Tanner, J. Nanosci. Nanotechnol. 5 (2005) 1455.[24] A. Roy, A.K. Sood, Phys. Rev. B 53 (1996) 12127.[25] Feng Zhang, Siu-Wai Chan, Appl. Phys. Lett. 80 (2002) 127.[26] M.A. Flores-Gonzalez, G. Ledoux, S. Roux, K. Lebbou, P. Periat, O.

Tillement, J. Solid State Chem. 178 (2005) 989.[27] Jiwei Wang, Hongwei Song, Baojuan Sun, Xinguang Ren, Baojiu

Chen, Wu Xu, Chem. Phys. Lett. 379 (2003) 507.[28] M.J.J. Lammers, G. Blasse, Chem. Phys. Lett. 126 (1986) 405.[29] H.T. Hintzen, PhD Thesis, State University Utrecht, 1990.[30] L.G. Jacobson, B.L. Bennett, R.E. Muenchausen, J.F. Smith, D.W.

Cooke, Proc. SPIE 6321 (2006) 63210J.[31] Zheng-Gui Wei, Ling-Dong Sun, Chun-Sheng Liao, Xiao-Cheng

Jiang, Chun-Hua Yan, J. Appl. Phys. 93 (2003) 9783.[32] Zhenggui Wei, Lingdong Sun, Chunsheng Liao, Chunhua Yan,

Shihua Huang, Appl. Phys. Lett. 80 (2002) 1447.[33] B.R. Judd, Phys. Rev. 127 (1962) 750.[34] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511.[35] S. Ray, P. Pramanik, A. Singha, A. Roy, J. Appl. Phys. 97 (2005)

094312.

Documents

Photoluminescence properties of nanocrystalline Tb3+ doped Y2O3 phosphor prepared through a novel synthetic route