Materials Research Bulletin 45 (2010) 572–575

Enhanced green emission from Tb3+–Bi3+ co-doped GdAlO3 nanophosphors

Jin Young Park a, Hong Chae Jung a, G. Seeta Rama Raju a, Byung Kee Moon a,*, Jung Hyun Jeong a,Se-Mo Son b, Jung Hwan Kim c

a Department of Physics, Pukyong National University, Busan 608-737, South Koreab Division of Image Science and Information Engineering, Pukyong National University, Busan 608-739, South Koreac Department of Physics, Dong Eui University, Busan 614-714, South Korea

A R T I C L E I N F O

Article history:

Received 8 June 2009

Received in revised form 14 December 2009

Accepted 19 January 2010

Available online 28 January 2010

Keywords:

A. Optical materials

C. X-ray diffraction

D. Luminescence

A B S T R A C T

Bi3+ and Tb3+ ions co-doped GdAlO3 (GAP) nanophosphors have been synthesized by means of

solvothermal reaction method. The XRD pattern of GAP phosphor confirms their orthorhombic phase.

The luminescence properties of these phosphors have been explored by analyzing their excitation and

emission spectra along with their decay curves. The excitation spectra of GAP:Tb3+, Bi3+ phosphors

consist of a broad band in the shorter wavelength region due to the 4f8! 4f75d1 transition of Tb3+ ions

overlapped with the 6s2! 6s16p1 (1S0! 3P1) transition of Bi3+ ions and some sharp peaks in the longer

wavelength region due to f! f transitions of Tb3+ ions. The present phosphors exhibit green color due to

strong 5D4! 7F5 transition of Tb3+ ions. The emission intensity was enhanced by co-doping with Bi3+

ions under 292 nm excitation, which indicate that the efficient energy transfer occurred from Bi3+ to Tb3+

ions.

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1. Introduction

Currently, trivalent rare-earth ions (RE3+) activated inorganicluminescent phosphors have gained abundant interest in theirproduction for their potential applications in the development ofluminescent lamps, plasma display panels (PDPs), field emissiondisplays (FEDs) and light emitting diodes (LEDs). Luminescencefrom the RE3+ ions originates from the transitions between 4forbitals, these transitions are forbidden on symmetry grounds thatare (4fn) well shielded by 5s2 and 5p6 orbitals. Therefore, emissiontransitions yield sharp lines in the optical spectra. Among the RE3+

ions, Tb3+ ion shows intense green emission at 544 nm due to5D4! 7F5 transition. Tb3+ (4f8) ion consists of a broad excitationband due to 4f8! 4f75d1 transition in the shorter wavelengthregion and some weak and narrow bands due to 4f8! 4f8

transitions in the longer wavelength region. However, theabsorption of Tb3+ ion in the UV region is weak. So, a sensitizerwith strong absorption in the UV region is required for theenhancement of Tb3+ emission.

The Bi3+ ions have attracted much attention, because Bi3+ ionsplay an important role as an activator or as a sensitizer inphosphors [1–7]. It is well known that Bi3+ ion is a post-transitionmetal ion with 6s2 configuration, which consists of a strong broadabsorption in the UV region due to 1S0! 3P0 transition. Based on

* Corresponding author. Tel.: +82 51 629 5569; fax: +82 51 629 5549.

E-mail address: bkmoon@pknu.ac.kr (B.K. Moon).

0025-5408/$ – see front matter � 2010 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2010.01.016

the survey made in the literature, it has been found that a littlework has been reported on the luminescent properties of Bi3+ co-doped with RE3+ activators and energy transfer between Bi3+ andRE3+ ions such as YSiO5:Bi3+, Eu3+, YSiO5:Bi3+, Eu3+/Dy3+, YVO4:Bi3+,Sm3+, YVO4:Bi3+, Eu3+ and CaSrS:Bi3+, Tm3+ [4–8]. The excitationenergy is strongly absorbed by Bi3+ ions in the UV region, and thenthis higher energy is transferred to the lower states of RE3+ ions.

Upon going through the literature, it has become quite clear torealize that the Bi3+ and Tb3+ co-doped aluminum perovskite(GdAlO3: GAP) phosphors by solvothermal process have not beenstudied so far, and hence the present work was undertaken basedon important properties given above. In this paper, we report onthe structural and luminescence properties of Tb3+ and Bi3+ co-doped GAP nanophosphors by means of solvothermal reactionmethod after sintering the same at 1000 8C. The concentrations ofBi3+ doping have been optimized to realize the emission properties,and to investigate the energy transfer from Bi3+ to Tb3+. Further, thestructure of these properties has been analyzed using the XRD andSEM measurements.

2. Experimental

Different concentrations (x = 0–1 mol%) of GdAlO3:Tb3+0.05,

Bi3+x were prepared by solvothermal reaction method by taking

the stoichiometric amounts of high purity grade gadoliniumnitrate hexahydrate (Gd(NO3)3�6H2O, 99.9%, Aldrich), aluminumisopropoxide ([(CH3)2CHO]3Al, 98%, Aldrich), terbium nitratehexahydrate (Tb(NO3)3�6H2O, 99.9%, Aldrich), bismuth nitrate

Fig. 2. The SEM image of GAP:Tb3+0.05, Bi3+

0.005 phosphors sintered at 1000 8C.

Fig. 3. PLE spectra of GAP:Tb3+0.05, Bi3+

x (x = 0–1 mol%) with an emission

wavelength at 544 nm (# indicates excitation peaks of Gd3+).

J.Y. Park et al. / Materials Research Bulletin 45 (2010) 572–575 573

pentahydrate (Bi(NO3)3�5H2O, 99.99%, Aldrich) as starting materi-als and 2-propanol as a solvent. The mixture was stirred with amagnetic stirrer until the formation of homogeneous solution andwas transferred into an autoclave with a Teflon liner. It was thenheated to 230 8C at a rate of 2 8C/min and maintained for 5 h at thattemperature with magnetic stirring to make stable networkbetween Gd–O–Al and Gd–O–Tb/Bi. After cooling gradually toroom temperature, the precipitate was separated by a centrifugalseparator with 3000 rpm for 3 min and then dried at 50 8C in anoven for a day. The dried powder was sintered at 1000 8C for 3 h inair and was brought to room temperature.

The XRD measurements were performed on these phosphorsusing a PHILIPS X’Pert-MPD X-ray powder diffractometer. Themorphological properties were investigated using a JEOL, JSM-6700F SEM. The luminescence properties were measured at roomtemperature using a Photon Technology International (PTI)luminescence spectrophotometer with a 60 W xenon arc lampand the lifetime was measured with a phosphorimeter attachmentto the main system with a xenon flash lamp (25 W power).

3. Results and discussion

Fig. 1 shows the diffraction patterns of the GdAlO3:Tb3+0.05,

Bi3+0.005 nanophosphors sintered at 1000 8C. The diffraction

pattern of GAP: Tb3+0.05, Bi3+

0.005 phosphor was indexed to pureorthorhombic phase (JCPDS No. 46-0395). The preparation andphase transformation of GAP phosphors were reported elsewhere[9]. In general the crystallite size is estimated from the Scherrerequation, Dh k l = 0.9l/bcos u, where D is the average crystallitesize, l is the X-ray wavelength (1.5406 A), u is the diffraction angleand b is the full-widths at half-maximum (FWHM) of observedpeak. The strongest diffraction peaks are used to calculate thecrystallite size of the samples. The calculated average crystallitesize is 100 nm.

Fig. 2 shows the morphology of GAP:Tb3+0.05, Bi3+

0.005

phosphors. The GAP powder particles are in nanometer rangeand nearly all of the particles are spherical morphology, whichagglomerate to larger lump material. The average particle size isbetween 100 and 200 nm. It is particularly well known that,phosphors with spherical shaped particles (�2 mm) are of greaterimportance because of their high packing density, lower scatteringof light, brighter luminescent performance, high definition andmore improved screen packing density [10].

Fig. 3 shows the excitation spectra of Bi3+ and Tb3+ co-dopedGAP at a definite Tb3+ concentration (5 mol%) and various Bi3+

concentrations by monitoring the emission wavelength of 544 nm.

Fig. 1. XRD patterns of (a) JCPDS Card and (b) GAP:Tb3+0.05, Bi3+

0.005 phosphors.

The excitation spectra consist of the intense broad bands in theshorter wavelength region and some weak sharp peaks in thelonger wavelength region. The broad band in the spectra rangingfrom 220 to 320 nm occurs due to 4f8! 4f75d1 transition (227 and279 nm) of Tb3+ ions and 6s2! 6s16p1 transition (292 nm) of Bi3+

ions. The Bi3+ configuration of the ground state is 1S0 and that of theexcited state is 3P0, 3P1, 3P2 and 1P1. For the electron transition ofthe Bi3+ ions, 1S0$ 1P1 is allowed for DS = 0 according to the spin-selection rule. Also, the 1S0! 3P0 transition is strongly forbiddenbecause total angular momentum is not changed (DJ = 0). Although1S0! 3P1 transition is forbidden for spin-selection rule, thistransition is partially allowed by mixing with singlet and tripletstates, and for the DJ = 1 [11,12]. So, the broad excitation band ofthe 1S0! 3P1 was only observed in the excitation spectra. Somesharp peaks in the longer wavelength region from 300 to 400 nmare due to 4f8! 4f8 transitions of Tb3+ ions, which are located at303, 313, 343, 353, 369, and 382 nm corresponding to theelectronic transitions from 7F6 ground state to 5H6, 5H7, 5L8, 5L9,5L10 and 5D3 excited states, respectively. The excitation spectrumof GAP:Tb3+ shows some sharp peaks located at 308 and 313 nmcorresponding to the electronic transition of Gd3+ ions from the 8S7/

2 ground states to 6P5/2 and 6P7/2 excited states, respectively[13,14]. However, Gd3+ transitions in the excitation spectra ofGAP:Tb3+, Bi3+ disappeared due to the overlapping of Bi3+

transitions.

Fig. 4. The excitation peak position of Tb3+ and Bi3+ co-doped GdAlO3 phosphors as a

function of Bi3+ concentration.

Fig. 6. The comparison of PLE and PL spectra of Tb3+ and Bi3+ co-doped GdAlO3

phosphors as a function of Bi3+ concentration.

J.Y. Park et al. / Materials Research Bulletin 45 (2010) 572–575574

Fig. 4 shows the red shifting of excitation wavelength of Bi3+

and Tb3+ co-doped GAP as a function of the amount of Bi3+. Withincreasing the Bi3+ concentration, excitation peak shifts towardslonger wavelength region. Because the ionic radius of Bi3+ (1.17 A)is larger than that of Gd3+ (1.053 A) ions, Bi3+ ions substitutes forthe Gd3+ site and s–p transition of the Bi3+ ion is sensitive for hostlattice environment.

Fig. 5 shows PL spectra of Bi3+ and Tb3+ co-doped GAP at thedefinite Tb3+ (5 mol%) and Bi3+ (0.5 mol%) concentrations. Ingeneral, the emission of Tb3+ ions mainly occurs due to thetransitions of 5D3 and 5D4 excited states to the 7FJ (J = 0, 1, . . ., 6)ground states. The Bi3+ ions show the luminescence correspondingto the 3P1! 1S0 transition in the UV or deep blue regions [5]. Butthis emission was not observed because emission range is out ofthe measurement limit. When the phosphors were excited by UVradiation of wavelength 279 nm, the Tb3+ ion (4f8) would be raisedto the higher 4f75d1 level and would feed afterward to the 5D3 or5D4 excited states [15]. When the phosphors were excited by UVradiation of wavelength 292 nm, the Bi3+ ion (6s2) would be raisedto the higher 6s16p1 level and then would feed afterward to the 5D3

or 5D4 excited states of Tb3+ through the energy transfer from Bi3+

ions to Tb3+ ions. The PL spectra of Tb3+ and Bi3+ co-doped GAPphosphors reveal several emission peaks at 493, 544, 591 and625 nm, which are attributed to the electronic transitions5D4! 7F6, 5D4! 7F5, 5D4! 7F4 and 5D4! 7F3 respectively. With

Fig. 5. Emission spectra of GAP:Tb3+0.05 and GAP:Tb3+

0.05, Bi3+0.005 phosphors.

the increase of Bi3+ ions concentration up to 0.5 mol%, the Tb3+

emission increases and then intensity reaches its maximum value.If the concentration of Bi3+ is further increased above 0.5 mol%, theTb3+ emission intensity begins to decrease. If the concentration ofan activator or sensitizer is higher than the appropriate value, theemission of the phosphor is usually lowered and this effect is calledconcentration quenching. Intensity of the Tb3+ emission reached anoptimum at 0.5 mol% concentration of Bi3+ ions and the brightnessof GAP:Tb3+

0.05, Bi3+0.005 nanophosphors can be increased by a

factor of 4.8 in comparison with that of GAP:Tb3+0.05 nanopho-

sphors. The comparison of the excitation and emission intensitiesis shown in Fig. 6. This result indicates that Bi3+ ions play a key roleas sensitizers and the sensitization effect of Bi3+ ions depends onthe concentration within the quenching limit. Fig. 7 shows theenergy level scheme of Bi3+ and Tb3+ ions, which presents themechanism involved in the absorption process of Bi3+, energytransfer from Bi3+ to Tb3+ ions and emission of Tb3+ ions.

The luminescence decay curves of the emission at 544 nm(5D4! 7F5) for GAP:Tb3+

0.05 and GAP:Tb3+0.05, Bi3+

0.005 phosphorshave been recorded under the different excitation at 279 nm(4f8! 4f75d1:Tb3+), 292 nm (1S0! 3P1:Bi3+), respectively. The

Fig. 7. The configurational coordinate model used in the absorption and emission

processes for the Bi3+–Tb3+ system.

Fig. 8. Decay curves for the GAP:Tb3+0.05 and GAP:Tb3+

0.05, Bi3+0.005 phosphors by

monitoring the emission of the 5D4–7F5 transition (Tb3+).

J.Y. Park et al. / Materials Research Bulletin 45 (2010) 572–575 575

decay curves have been presented in Fig. 8. The decay curves forthe Tb3+ emission can be fitted to single exponential function as:

I ¼ K exp�t

t

� �

where K is the constant and value of t is decay time of Tb3+. Theobtained lifetimes are 2.4 and 2.2 ms for the GAP:Tb3+

0.05 andGAP:Tb3+

0.05, Bi3+0.005 phosphors, respectively.

4. Conclusion

Trivalent terbium and bismuth co-doped gadolinium aluminumperovskite nanophosphors with orthorhombic phase were suc-cessfully synthesized by a solvothermal reaction method uponannealing at 1000 8C. The excitation spectra consist of broad bandsdue to 4f8! 4f75d1 transition of Tb3+ and 1S0! 3P1 transition of

Bi3+ and some sharp peaks due to 4f8! 4f8 transition of Tb3+.When the Bi3+ concentration increases, the excitation band shiftsto longer wavelength side (276–292 nm). In the emission spectra,the green emission intensity of Tb3+ was increased by doping Bi3+

ions thus using Bi3+ ions doping as sensitizers. The intensity of Bi3+

ions co-doped GAP:Tb3+0.05, Bi3+

0.005 nanophosphors was increasedby a factor of 4.8 in comparison with that of GAP:Tb3+

0.05

nanophosphors. These results indicate that the excitation energywas absorbed by Bi3+ ions and it transferred to Tb3+ ions efficiently.From the detailed luminescent properties, we are able to concludethat Tb3+ and Bi3+ co-doped GAP nanophosphors are promisingmaterials in green region for optical display systems.

Acknowledgements

This work was supported by the Korea Research Foundation(KRF) grant funded by the Korea government (MEST) (No. 2009-0076967) and also this work was partially supported by a grant-in-aid for the National Core Research Center Program from MOST andKOSEF (No. R15-2006-022-03001-0).

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