Luminescence of Gd3+ ions doped in oxyfluoroborate glass

PERGAMON

Solid State Communications 117 (2001) 387±392

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Luminescence of Gd 31 ions doped in oxy¯uoroborate glass A. Kumar, D.K. Rai, S.B. Rai* Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221005, India Received 22 May 2000; accepted 3 October 2000 by C.N.R. Rao

Abstract This paper presents the results of one- and two-photon absorption followed by ¯uorescence for the Gd 31 ions doped in oxy¯uoroborate glass. Fluorescence from the levels lying near 60,000 cm 21 has been reported. The lifetime of the high lying levels has also been measured. The concentration quenching and the mechanism responsible for energy transfer are discussed. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Rare earth ions; D. Optical properties; D. Quantum cutting; E. Luminescence PACS: 42.70.Ce

1. Introduction The Gd 31 ion has 4f 7 electronic con®guration and the energy gap between the ground state 8S7/2 and the ®rst excited state 6P7/2 is 32,000 cm 21. Theoretical and experimental studies on the spectroscopy of Gd 31 ions both in the free state and in crystalline environment have been reported [1,2]. Theoretical calculations show that 4f 7 energy levels of Gd 31 ion extend up to 150,000 cm 21 [3], but levels up to 67,000 cm 21 only have been probed experimentally [4]. Studies of Gd 31 in glass hosts are limited [5,6] because of two reasons. The ®rst and important reason is the strong absorption by the glass host in the UV which masks all the ¯uorescence except those from the few low lying excited states. The second reason is the limited availability of high intensity tunable sources for UV/VUV regions. The two photon absorption technique which has been successfully used for various rare earth ions doped in various matrices [7,8] has been applied to study the absorption spectrum of Gd 31 ions doped in crystals [9,10]. The search of highly ef®cient phosphors has led to the discovery of the so-called quantum cutters which emit two or more visible photons when one UV/VUV photon is * Corresponding author. Tel.: 191-542-318990; fax: 191-542368174. E-mail address: [email protected] (S.B. Rai).

absorbed [11±15]. The recent work of Wegh et al. [14] on visible quantum cutting in Eu 31 doped gadolinium ¯uorides has shown a quantum ef®ciency close to 200% in the red region. This has prompted us to investigate the spectrum

Fig. 1. (a) IR absorption spectrum of oxy¯uoroborate glass. (b) Fluorescence spectrum of Gd 31 ion in oxy¯uoroborate glass.

0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(00)00430-0

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A. Kumar et al. / Solid State Communications 117 (2001) 387±392

Table 1 Energy level of Gd 31 ion in different lattices Multiplet

8

S7/2 P7/2 6 P5/2 6 P3/2 6 I7/2 6 I9/2 6 I17/2 6 I11/2 6 I15/2 6 I13/2 6 D9/2 6 D1/2 6 D7/2 6 D3/2 6 D5/2 6 G7/2 6

6

G11/2 G9/2 6 G5/2 6 G3/2 6

Eexp (cm 21)

Ecal (cm 21)

Oxy¯uoroborate glass:Gd 31

LaCl3:Gd 31 (Ref. [1])

LiYF4:Gd (Ref. [4])

LaF3 (Ref. [4])

0.00 31953.9 32586.2 ± ± 36174.0 ± ± ± ± 39577.0

0.00 32100.35 32702.42 33282.39 35842.92 36185.30 36239.50 36461.31 36579.25 36614.15 39535.83 ± 40580.81 ± ± ±

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 49022 49087 49174 49236 49373 49456 49529 50431 50500 51148 51190 51216 51261 51337 51369

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 49160

40580.81 41839.1 ± 48621.2

± ± ± ±

± ± ± ±

6

G13/2

6

G7/2

±

±

6

F1/2 F3/2

± ±

± ±

6 6

F11/2 p G5/2 6 F9/2 6 F1/2 6 F7/2 6 p G9/2 4 N17/2 4 H(2)7/2 4 D(6)3/2 4 H(2)13/2 4 N19/2 6 H5/2 4 D(6)1/2 4 N23/2 4 H(2)11/2 4 N21/2 6 H15/2 6 H7/2 6

51416.7

54000.0 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

±

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

52277 52351 52554 52706 53631 53717 54002 54168 54330 54475 54546 55559 55661 56189 56300 56808 56880 57091 57162 ± ± ± 58018 58119

49545 49860 50486 51310 51357 51382 51402 51414 51436 51483 52420 52719 53779 54277 54353 54552 54687 54745 55732 55862 56220 56252 56945 57061 57169 57183 57329 57451 57606 58139 58140

A. Kumar et al. / Solid State Communications 117 (2001) 387±392

389

Table 1 (continued) Multiplet

Eexp (cm 21) Oxy¯uoroborate glass:Gd 31

4

Ecal (cm 21) LaCl3:Gd 31 (Ref. [1])

L(2)13/2 L(2)17/2 4 L(2)15/2 6 H11/2 6 H9/2 6 H13/2

± ± ± ± 58953.7 ±

± ± ± ± ± ±

4

± ± ± 61601.2

± ± ± ±

4

L(2)19/2 K(1)11/2 4 F(4)9/2 4 p F(4)7/2 4

of Gd 31 in a glass host. It is expected that the high lying energy levels of the 4f 7 con®guration may decay to the ®nal lower state through a stepwise decay through an intermediate state giving rise to two or more visible photons. This would require a suitable host and the measurements of the lifetime of the different states along with their branching ratios [16]. In this paper we have been able to excite the Gd 31 ion doped in an oxy¯uoroborate glass up to 64,000 cm 21 with two-photon absorption using the 308 nm radiation of a XeCl laser. Fluorescent emissions have been observed both in the UV and visible regions. The lifetime of some of these high lying excited states have been measured. The ¯uorescence spectrum of Gd 31 in oxy¯uoroborate glass, for the transition 6P7/2 ! 8S7/2 also shows phononassisted bands. Concentration quenching of ¯uorescence earlier investigated by several workers [17±22] has also been studied and analysed in terms of the Inokuti and Hirayama [20] model.

2. Experimental Glass with the compositions (70 2 x)H3BO3 1 20 Li2CO3 1 10LiF 1 xGd2O3 with x ˆ 0:5; 1, 3.5 mol% has been prepared by melting the mixed ingredients into a platinum crucible at 10008C. The melt was kept at 10008C for 2 h in order to homogenise it and ®nally poured in a stainless steel cast maintained at 4008C and then allowed to cool slowly to room temperature. The prepared glass is polished before experimentation. The absorption spectrum of the glass was recorded using a Cary 2390 spectrophotometer in the region 300±2000 nm. For ¯uorescence studies, the glass was excited using the 308 nm line of an excimer laser. The IR absorption spectrum of the same glass has also been measured using a Perkin±Elmer model 883 spectrophotometer in the region 600±3000 cm 21. For this the sample was prepared in a pellet form by grinding the glass with KBr in 1:100 ratio.

LiYF4:Gd (Ref. [4])

LaF3 (Ref. [4])

58214 58551 58665 ± ± 59613 59730 ± ± 60794 61181

58254 58298 58323 58577 58632 59676 60172 60242 60831 61204

3. Results and discussion 3.1. Two-photon absorption of Gd 31 doped in oxy¯uoroborate glass When the Gd doped glasses were exposed to intense 308 nm (32,458 cm 21) radiation of the excimer laser, a deep orange-red ¯uorescence is observed but no such ¯uorescence is seen in the undoped oxy¯uoroborate glass. Since the ®rst excited state of Gd 31 (i.e. 6P) lies at about 32,000 cm 21 above its ground state there is no chance of orange-red emission (,15,000 cm 21) by relaxation from this 6P state to the ground state. It is therefore clear that this ¯uorescence could either be due to excitation of high lying excited states of Gd 31 (above 60,000 cm 21) relaxing to other excited states or due to some impurities in the Gd doped glass. The UV±Vis absorption measurements reveal no lines due to any other rare earth ion except the 6 P7/2 Ã 8S7/2 transition of Gd 31 at 310 nm. This spectrum has been recorded using an undoped glass of same composition in the reference beam. Other absorption peaks are either submerged into the strong background absorption of the host or they are very weak to be observed. In Fig. 1, the ¯uorescence spectrum of Gd 31 in the wavelength region of 310 nm is shown. This ¯uorescence originates from one-photon absorption of the 308 nm laser radiation. The observation of the ¯uorescent transition 6 P5/2 ! 8S7/2 is explained by phonon coupling to the ground state which compensates the energy mismatch. The relative closeness of the energy of the laser line to that of the 6P level is favourable for near resonant two-photon absorption through 6P as an intermediate level. On focusing the laser beam into the glass two-photon absorption takes place. Stepwise two-photon absorption through the intermediate level 6 P7/2 is also possible as this state has a lifetime of the order of 5.03 ms and the 308 nm radiation of the exciting laser is near resonance. If we compare the relative cross-section of both the excitation processes, the two-step process is seen to be more likely (has a larger cross-section by a factor of

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A. Kumar et al. / Solid State Communications 117 (2001) 387±392

Fig. 2. Fluorescence spectrum of Gd 31 ions doped in oxy¯uoroborate glass in the VUV/UV region on excitation with 308 nm.

10 12). A look at the energy levels for Gd 31 in LaF3 (theoretically calculated) [4] and LiYF4 (experimentally observed) [4] shows that there are a large number of levels viz., 4H(4)13/2, 4G(5)11/2, 4G(6)9/2, 4G(6)7/2 in the energy range 64,000±65,000 cm 21. It is dif®cult to ascertain the particular state in which the two-photon transition (,64,916 cm 21) from the ground state culminates because phonons with energy up to 1000 cm 21 are available through interaction with the host lattice. The energy levels of Gd 31 ion as observed in oxy¯uoroborate glass are compared with other lattices in Table 1. A comparison of the transition energies of 6P5/2,7/2 ! 8S7/2 transitions of Gd 31 in oxy¯uoroborate glass with that in LaCl3 crystal shows that the peaks

Fig. 4. Energy level diagram of Gd 31 ion. Arrows indicate the transitions observed in oxy¯uoroborate glass.

Fig. 3. Fluorescence spectrum of Gd 31 ions doped in oxy¯uoroborate glass in the visible region on excitation with 308 nm.

in the glass are shifted towards lower energy by 50 and 116 cm 21, respectively. The decrease in energy can be associated to weak covalent coupling of the host with the Gd 31 ion. Fig. 2 shows the ¯uorescence spectrum in the VUV/UV region. This is an example of up-conversion as the emitted radiation has a larger energy quantum than the incident photon energy. The various transitions from the excited multiplets 4F(4)J, 6HJ, 6FJ, 6GJ, 6DJ and 6IJ to the ground

A. Kumar et al. / Solid State Communications 117 (2001) 387±392 Table 2 Assignments of different transitions observed of Gd 31 ion doped in oxy¯uoroborate glass Energy of ¯uorescence peak observed of Gd 31 in oxy¯uoroborate glass (cm 21)

Assignment

61601.2 58953.7 54000.0 51416.7 48621.2 41839.1 40580.5 39577.0 36174.0 32586.5 31953.9 30687.0

4

27981.2 16635.3 14357.4 12060.4 12007.3 11899.0 11711.7 11681.6 11661.6 11556.2 11523.6 11510.7 11491.3 11446.3

F(4)7/2 ! 8S7/2 H9/2 ! 8S7/2 6 F11/2 ! 8S7/2 6 G13/2 ! 8S7/2 6 G7/2 ! 8S7/2 6 D3/2 ! 8S7/2 6 D7/2 ! 8S7/2 4 D9/2 ! 8S7/2 6 I9/2 ! 8S7/2 6 P5/2 ! 8S7/2 6 P7/2 ! 8S7/2 Phonon-assisted band of 6P7/ 8 2 ! S7/2 transition 4 F(4)7/2 ! 6P3/2,5/2,7/2 6 p G7/2 ! 6I7/2; 4H7/2 ! 4D7/2; 6 G7/2 ! 6P7/2 6 F7/2 ! 6D7/2; 4H(2)7/2 ! 6D3/2 4 F(4)7/2 ! 6G7/2; 4L(2)17/ 6 p 2 ! G3/2 6 F1/2 ! 6D1/2 6 F3/2 ! 6D3/2; 4F(4)5/2 ! 6G5/2; 6 G13/2 ! 6D7/2 4 F(4)7/2 6G13/2 ! 6D7/2 6G11/2; 6 G13/2 6G13/2 ! 6D7/2 6D9/2 4 F(4)9/2 6G13/2 ! 6D7/2 6G7/2 6

4

F(4)5/2 6G13/2 ! 6D7/2 6G5/2

4

K(1)13/2 6G13/2 ! 6D7/2 6G13/2

state 8S7/2 have been observed. All the transitions are broad and do not show any structure due to spin orbit multiplets. This is not surprising since for such high energy levels the various spin orbit components are greatly mixed up. The ¯uorescence spectrum observed in the visible region is shown in Fig. 3. The transition 4F(4)J ! 6PJ has a very high intensity as compared to the other 4G7/2 ! 6IJ, 4 HJ ! 4DJ, 4L p(2)17/2 ! 6G3/2, and 4F(4)7/2 ! 6G7/2 transitions. The two broad ¯uorescence pro®les centred at 16,635 and 14,357 cm 21, have almost equal intensity, and assigned as 6GJ ! 6IJ and 4HJ ! 6DJ transitions. These transitions are responsible for the red-orange ¯uorescence observed in Gd 31 doped oxy¯uoroborate glass on 308 nm excitation. None of these transitions shows any structure. A very complex band structure with a few resolved components are seen in the NIR region. The peak centred at 11,899 cm 21 is very broad with a relatively large intensity. This is an admixture of three transitions 6F3/2 ! 6D3/2,

4

6

6

391 6

F(4)5/2 ! D5/2 and G13/2 ! D7/2 and the possible overlapping of their inherently broad nature is responsible for this behaviour. Other transitions are also inherently broad but there is less overlapping and hence only weak peaks are seen. The transitions around 11,661 cm 21 show some resolved structure but a correct assignment is quite uncertain since a number of assignments 6G13/2 ! 6D7/2, 4F(4)9/2 ! 6G7/2, 4 F(4)7/2 ! 6G11/2 are possible. All the above-mentioned assignments are made on the basis of very general selection rules DJ # 6; DL # 6: It is to be noted that DS ˆ 0 is not strictly valid [23,24]. However, particularly in Gd 31, the transitions for which the total angular momentum J is conserved …DJ ˆ 0† are seen to be strong. The energy levels of Gd 31 ion and the various transitions observed is shown in Fig. 4 (and in Table 2). From the energy level diagram one can see that there appears a simultaneous ¯uorescence corresponding to transitions 4 F(4)J ! 6GJ, 6GJ ! 6PJ and 6PJ ! 8S7/2 multiplets centred around 12,060, 16,635 and 31,954 cm 21, respectively. This is an interesting example of quantum cutting. The lifetime measurements corresponding to various ¯uorescence peaks in the UV region show a double exponential behaviour. The fast process having a characteristic decay time of a few hundred microseconds is followed by a slow decay with a characteristic time of 2.85 ^ 0.2 ms. The fast relaxation is attributed to the radiative relaxation of 6F, 6 G and 6I multiplets to the ground state and the slow relaxation is attributed to the luminescence of the host. On this basis we have measured the lifetime of 6F, 6G, 6I and 6P multiplets as 0.25 ^ 0.02, 0.28 ^ 0.02, 0.33 ^ 0.020 and 5.03 ^ 0.01 ms, respectively for 1 mol% of Gd 31 doped in oxy¯uoroborate glass. 3.2. Concentration quenching and lifetime measurements Fluorescence spectrum was recorded with different concentrations of Gd doped in the host. It was marked that ¯uorescence intensity increases with the increase of doping concentration up to 3 mol%. The intensity is quenched with a further increase in concentration of the rare earth. Several authors have proposed theoretical models to explain the quenching mechanism [17±24]. The most general model is that of Inokuti and Hirayama [20], which has extensively been used to explain the multipole interactions. According to this model I…t† ˆ I…0† exp‰2t=t0 2 G…1 2 3=S†C=C 0 …t=t0 †3=S Š

…1†

Here I(0) is the intensity of the ¯uorescence immediately after excitation and I…t† is the intensity after time t, C refers to the concentration of the Gd 31 ion, C0 is the critical concentration at which the lifetime of the level is reduced to half of its original value. C0 is given as 3=…4pR30 †; where R0 is the critical separation among the rare earth ions. S is a factor which decides the mechanism of multipole interaction. The value of S value will be 6 for dipole±dipole, 8 for dipole±quadrupole and 10 for quadrupole±quadrupole

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A. Kumar et al. / Solid State Communications 117 (2001) 387±392

interactions. We measured the lifetime of the 6P7/2 level of Gd 31 ion for various concentrations in the oxy¯uoroborate glass. A decrease in lifetime with increase in Gd 31 concentration has been found. Eq. (1) has been ®tted with our observed decay for S ˆ 6; 8 and 10. In the present case a best ®t is obtained for S ˆ 10: This indicates that the quenching mechanism is probably quadrupole±quadrupole type. Reisfeld et al. [25] have studied Gd doped borate glasses and found that the lifetime of 6P7/2 is 4.10 ^ 0.01 ms for 1 mol% of Gd. In our case where the host is oxy¯uoroborate glass, the lifetime is 5.03 ^ 0.01, 3.15 ^ 0.01 and 1.35 ^ 0.01 ms, respectively, for 1, 3 and 5 mol% concentrations of Gd 31. The value of lifetime of the 6P7/2 level in case of oxy¯uoroborate glass (at 1.0 mol% concentration) is about 20% larger than in the case of borate glass. This is probably due to the presence of ¯uoride ions in the host matrix. Dieke and Hall [26] have measured the lifetime of 6 P7/2 state as 7 ms in GdCl3´6H2O. The decreased value in a solid host may be attributed to increased non-radiative relaxations. 3.3. The associated vibrational bands in oxy¯uoroborate glass Two weak broad bands are seen in the lower energy side of the 6P7/2 ! 8S7/2 transition. These bands are interpreted as the phonon-assisted component of the 6PJ ! 8S7/2 transition. The IR absorption in the region 800±1700 cm 21. (shown in Fig. 1a). shows two major peaks at 1400 and 1000 cm 21 attributed to the B±O stretching vibrations of BO3 and BO4 structural units [5]. The ¯uorescence spectrum due to 6P7/2 level appears at 31953.90 cm 21 (see Fig. 1b) and there are two Stokes shifted peaks at 30856.24 and 30632.64 cm 21. The shifts of about 1000 and 1400 cm 21, respectively, are in good agreement with the above-mentioned quanta. The IR peak at 1000 cm 21 is relatively broader than the peak at 1400 cm 21. The same trend is seen in the corresponding ¯uorescence peaks. 4. Conclusion This work clearly shows that glasses doped with Gd have good potential for new devices like quantum cutters, UV sensors, visible to UV converters. However, more theoretical and experimental work is needed to optimise the technology.

Acknowledgements The authors are thankful to CSIR and DST, Government of India for ®nancial assistance, under various projects, which helped in procurement of the various experimental equipment. References [1] G.H. Dieke, Spectra and Energy Levels of Rare Earth Ions in Crystals, Interscience, New York, 1968. [2] W.T. Carnall, G.L. Goodman, K. Rajnak, R.S. Rana, Argonne National Laboratory Report No. ANL -88-8, 1988, unpublished. [3] G.H. Dieke, H.M. Crosswhite, Appl. Opt. 2 (1963) 675. [4] R.T. Wegh, H. Donker, A. Meijerink, Phys. Rev. B 56 (1997) 13841. [5] J.W.M. Verwey, G.F. Imbusch, G. Blasse, J. Phys. Chem. Solids 50 (1989) 813. [6] K. Binnemans, R. Van Deun, C. Goerller-Walrand, J.L. Adam, J. Non-Cryst. Solids 238 (1998) 11. [7] M.C. Downer, C.D. Cordero-Montalov, H. Crosswhite, Phys. Rev. B 28 (1993) 4931. [8] M. Dagenais, M. Downer, N. Bloembergen, Phys. Rev. Lett. 46 (1981) 561. [9] T. Kundu, A.K. Banerjee, M. Chowdhury, Phys. Rev. B 41 (1990) 10911. [10] L. Kundu, A.K. Banerjee, M. Chowdhury, Chem. Phys. Lett. 181 (1991) 569. [11] J.L. Sommerdijk, A. Bril, A.W. de Jage, J. Lumin. 8 (1974) 341. [12] J.L. Sommerdijk, A. Bril, A.W. de Jage, J. Lumin. 9 (1974) 288. [13] W.W. Piper, J.A. DeLuca, F.S. Ham, J. Lumin. 8 (1974) 344. [14] R.T. Wegh, H. Donker, K.D. Oskram, A. Meijerink, J. Lumin. 82 (1999) 93. [15] R. Pappalardo, J. Lumin. 14 (1976) 159. [16] L.A. Reisberg, M.J. Weber, in: E. Wolf (Ed.), Progress in Optics, Vol. XIV, North-Holland, Amsterdam, 1976, p. 116. [17] R. Reisfeld, Struct. Bonding 13 (1973) 53. [18] D.L. Dexter, J. Chem. Phys. 21 (1953) 836. [19] D.L. Dexter, J.H. Schulman, J. Chem. Phys. 22 (1954) 1963. [20] M. Inokuti, F. Hirayama, J. Chem. Phys. 43 (1965) 1978. [21] M.J. Weber, Phys. Rev. B 4 (1971) 2932. [22] Y.Y. Shin, Y. Do, Y. Kim, Bull. Korean Chem. Soc. 18 (1997) 10. [23] B.R. Judd, Phys. Rev. 127 (1962) 750. [24] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [25] R. Reisfeld, E. Greenberg, R. Velapoldi, J. Chem. Phys. 56 (1972) 1698. [26] G.H. Dieke, L.A. Hall, J. Chem. Phys. 27 (1957) 465.