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Journal of Luminescence 122-123 (2007) 993–996

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Vacuum ultraviolet optical properties of MO–Y2O3–B2O3:Re(M ¼Mg,Sr; Re ¼ Eu,Tb,Ce,Gd)

Dan Zhou, Dawei He�, Zhongyi Liang, Tao Hou

Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology,

Beijing Jiaotong University, Beijing 100044, China

Available online 14 March 2006

Abstract

A series of phosphors MO–Re2O3–B2O3:Eu3+ (M ¼Mg,Sr; Re ¼ Eu,Tb,Ce,Gd) were prepared and studied. Excitation spectra

indicated a high absorption in vacuum ultraviolet (VUV) region. Strong emissions peaked at 591 and 613 nm, which were corresponding

to the 5D0-7FJ (J ¼ 1; 2; 3; 4) transition of Eu3+ when excited under VUV. In addition to the host absorption band of

MgO–Y2O3–B2O3, absorption bands around 172, 178, 195, 204, 225 nm, were found which were corresponding to the 4f 8–4f 75d

transition of Tb3+ of MgO–Y2O3–B2O3:Re (Ce,Tb,Gd). Their energy transfer (Gd3+–Tb3+) was further discussed.

r 2006 Elsevier B.V. All rights reserved.

Keywords: Phosphors; Vacuum-ultraviolet (VUV); Energy transfer

1. Introduction

In recent years, efficient luminescent materials undervacuum ultraviolet (VUV) radiation and low-voltageelectron bombardment are becoming more and moreimportant [1]. The efficiency of phosphor has beenadvanced by both new host materials and improvedsynthetic techniques. The luminescent properties of phos-phors are strongly dependent on the crystal structure ofhost materials.

The present work described a novel phosphorMO–Y2O3–B2O3:Re (M ¼Mg,Sr; Re ¼ Eu,Tb,Ce,Gd)which exhibited high absorption in VUV region. Mean-while, the spectral properties of Eu3+, Tb3+, Ce3+ andGd3+ and energy transfers among them in the matrix werealso studied.

2. Experiments

MO–Y2O3–B2O3:Re (M ¼Mg,Sr; Re ¼ Eu,Tb,Ce,Gd)phosphors were prepared by the conventional solid-statereaction method. Stoichiometric amounts of constituentmaterials MgO, SrCO3, Y2O3, Tb4O7, Gd2O3, CeO2,

e front matter r 2006 Elsevier B.V. All rights reserved.

min.2006.01.348

ing author. Tel.: +8610 51688018; fax: +86 10 51683933.

ess: dwhe@center.njtu.edu.cn (D. He).

Eu2O3 and H3BO3 with high purity were thoroughly mixedin agate mortar. The mixtures were put in an aluminacrucible and then calcined at 850 1C for 2 h.The powders were analyzed by X-ray diffraction (XRD)

using Cu Ka radiation to reveal the phase composition.The VUV excitation and emission spectra were measuredat the VUV Spectroscopy Station of Beijing SynchrotronLaboratory.

3. Results and discussion

3.1. X-ray diffraction spectra

Fig. 1 exhibits XRD patterns of MgO–Y2O3–B2O3:Eu3+

and MgO–Y2O3–B2O3:Gd3+ powders. The single-phasedphosphors were obtained in this synthesis process. Itindicated that the doped Eu3+ or Gd3+ had little influenceon the structure of the luminescent host.

3.2. MO–Y2O3–B2O3:Eu3+ (M ¼Mg,Sr)

Fig. 2 illustrates the VUV excitation and emissionspectra of MO–Y2O3–B2O3:Eu

3+ (M ¼Mg,Sr). It wascomposed of two broad bands in the excitation spectra.One band centered at about 172 nm was assigned to the

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Fig. 1. XRD pattern of MgO–Y2O3–B2O3:Eu3+ and MgO–Y2O3–

B2O3:Gd3+.

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Fig. 2. Excitation spectra and emission spectra of MO–Y2O3–B2O3:

Eu3+(M ¼Mg,Sr): (1) SrO–Y2O3–B2O3:Eu3+, (2) Mg0.5Sr0.5O–Y2O3–

B2O3:Eu3+ and (3) MgO–Y2O3–B2O3:Eu

3+.

D. Zhou et al. / Journal of Luminescence 122-123 (2007) 993–996994

host absorption, indicating that the energy transfer fromhost to activators was very efficient. The other bandcentered at 220 nm was the result of charge transferbetween Eu3+ and the neighboring O2�. The excitationpeak was shifted to longer wavelength and the relativefluorescence intensity was increased with the higher Srconcentration in Fig. 2.

Emission spectra of Eu-activated phosphor under 172excitation are shown in Fig. 2. It was noted that the peak at591 nm attributed to 5D0-

7F1 transition (magnetic-dipoletransition) had the strongest intensity. It also could be seenthat with the increasing of Sr content, there was an increasein the emission intensity. The reason was as follows: whilesubstituted for Mg partially by Sr in MgO–Y2O3–B2O3:Eu3+, which resulted in the increasing of Eu defect lumine-scence center, therefore, green relative emission intensitywas enhanced strongly.

Follow the parity selection rule: the electric dipoletransition is forbidden, the magnetic dipole transition ispermitted. Transitions between the levels in the configura-tion 4f n of rare earth free ion or rare earth ion occupycentrosymmetric site. If rare earth ions occupy non-centrosymmetric site, configuration 4f5d will mix to

configuration 4fn in case of the crystal field interaction,the electric dipole transition which has much highertransition probability than magnetic dipole transition willbe no longer forbidden [2,3]. The activator Eu3+ occupiedY3+ sites, which had inversion symmetry, so that theemission of electric dipole transition 5D0-

7F1 (591 nm)was stronger than magnetic dipole transition 5D0-

7F2

(613 nm) as shown in Fig. 2.

3.3. MgO–Y2O3–B2O3:Re(Re ¼ Tb,Gd)

Fig. 3 shows the VUV spectra of MgO–Y2O3–B2O3:Re(Re ¼ Tb,Gd). On monitoring by 545 nm emission, somestrong and broad absorption bands with a maximum atabout 172 nm were observed in Fig. 3. We considered thatthese bands were due to the host absorption bands. As canbe seen in Fig. 3, the host absorption band intensity wasenhanced with the introduction of Gd into MgO–Y2O3–B2O3:Tb. For Tb

3+ ions with 4f 8 electrons configuration,the ground state is 7F6, and the 4f 75d1 excitation levels arewith two spin states: the high-spin 9DJ and the low-spin7DJ states. The 7DJ levels are higher than the 9DJ statesin term of Hund’s rule [4]. Therefore, Tb3+ in a specifichost exhibits two groups of f–d transitions, one group is

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Fig. 3. Excitation spectra and emission spectra of Mg–Y2O3–B2O3: Tb3+

(Re ¼ Y,Gd): (1) MgO–0.2Y2O3–0.8Gd2O3–B2O3:Tb3+, (2) MgO–

0.6Y2O3–0.4Gd2O3–B2O3:Tb3+ and (3) MgO–Gd2O3–B2O3:Tb

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Fig. 4. Excitation spectra of Mg–Y2O3–B2O3:Re(Re ¼ Tb, Ce): (1) MgO–

Y2O3–B2O3:Tb3+ and (2) MgO–Y2O3–B2O3:Tb

3+,Ce3+.

D. Zhou et al. / Journal of Luminescence 122-123 (2007) 993–996 995

spin-allowed with high-energy and another is spin-for-bidden with low-energy.

The Tb3+ emission peaks were found at 491, 547, 590and 621 nm, respectively, which were assigned to the 5D4 to7FJ (J ¼ 6; 5; 4; 3) transitions of Tb3+. We could see that5D4-

7F5 transition peaked at 547 nm was responsible forthe green color observation. It could also be found thatunder 172 nm excitation, there was an increasing in theemission intensity with higher Gd3+ concentration, in-dicating the increasing energy transfer efficiency from hostlattice to the activator (Tb3+). This result suggested thatGd3+ play an intermediate role in the energy transfer fromhost lattice to the activator (Tb3+) [5].

3.4. MgO–Y2O3–B2O3:Re(Re ¼ Tb,Ce)

It is known that the green emission of Tb3+ can beefficiently sensitized by Ce3+ under 254 nm excitation inmany phosphors. However, in this paper, the problem wasquite different. On monitoring by 540 nm emission, threebands with the maximum at about 210, 245 and 275 nmwhich were corresponding to the lowest f–d transition of

Ce3+ were found. There was no absorption in VUV region(o200 nm) for the Ce3+. Fig. 4 shows the excitationspectra of MgO–Y2O3–B2O3:Tb

3+ and MgO–Y2O3–B2O3:Tb3+,Ce3+. The absorptions of host lattice and theactivator (Tb3+) were found in VUV region. There existedabsorption bands peaked at 170, 178, 195, 204 and 225 nm,which were corresponding to the 4f 8–4f 75d transition ofTb3+. It was obvious that the intensity of the host andTb3+ absorption decreased by about 50% and the intensityof Ce3+ absorption (centered at 245 nm) was enhancedwhen Ce3+ activator was doped in MgO–Y2O3–B2O3:Tb.With the increasing Ce3+ content, the possibility of energytransfer from Tb to Ce increased and the luminescentintensity of Tb3+ decreased under VUV excitation. So theexistation of Ce3+ should be avoided in MgO–Y2O3–B2O3:Tb when excited under VUV.

4. Conclusions

1.

The excitation peak was shifted to longer wavelengthand the intensities of absorption (172 nm) forMO–Y2O3–B2O3:Eu

3+ (M ¼Mg,Sr) were enhancedwith the increasing Sr content . It was noted that thepeak at 591 nm attributed to 5D0-

7F1 transition(magnetic-dipole transition) had the strongest intensity.

2.

The emission intensity of MgO–Y2O3–B2O3:Re(Re ¼ Tb,Gd) increased when Tb3+ and Gd3+ ionswere co-doped in the phosphor when excited underVUV. With the increase of Gd3+ concentration, theefficiency of energy transfer between Gd3+ and Tb3+

increased gradually. It suggested that Gd3+ play anintermediate role in the energy transfer from host latticeto the activator (Tb3+).

3.

In comparison with MgO–Y2O3–B2O3:Tb3+, the inten-

sity of the host and Tb3+ absorption decreased greatlyand the intensity of Ce3+ absorption increased remark-ably in MgO–Y2O3–B2O3:Tb

3+,Ce3+. The existation of

ARTICLE IN PRESSD. Zhou et al. / Journal of Luminescence 122-123 (2007) 993–996996

Ce3+ should be avoided in MgO–Y2O3–B2O3:Tb whenexcited under VUV.

Acknowledgements

This work was supported by the National ScienceFoundation of Beijing (2052019) and the Foundation ofLaboratory of Beijing Synchrotron Radiation, Institute ofHigh Energy Physics, Chinese Academy of Sciences.

References

[1] P. Cheol-Hee, P. So-Jung, G.Y. Hu, J. Mater. Sci. Lett. 19 (2000) 335.

[2] N. Kristianpoller, A. Shmilevich, D. Weiss, R. Chen, Nucl. Inst. Meth.

B 166&167 (2000) 556.

[3] H.E. Hoefdraad, J. Solid State Chem. 15 (1975) 175.

[4] E. Nazakawa, J. Lumin. 100 (2002) 89.

[5] X.Y. Wu, H.P. You, H.T. Cui, X.Q. Zeng, G.Y. Hong, C.H. Kim,

C.H. Pyun, B.Y. Yu, C.H. Park, Mater. Res. Bull. 37 (2002) 1531.