Energy transfer processes in Sr3Tb0.90Eu0.10(PO4)3

Optical Materials 33 (2010) 119–122

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Energy transfer processes in Sr3Tb0.90Eu0.10(PO4)3 Marco Bettinelli a,*, Adolfo Speghini a, Fabio Piccinelli a, Jumpei Ueda b, Setsuhisa Tanabe b a b

Laboratorio di Chimica dello Stato Solido, Dipartimento di Biotecnologie, Università di Verona, and INSTM, UdR Verona, Strada Le Grazie 15, I-37314 Verona, Italy Graduate School for Human and Environmental Studies, Kyoto University, Yoshida-nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501, Japan

a r t i c l e

i n f o

Article history: Received 18 May 2010 Received in revised form 12 July 2010 Accepted 13 July 2010 Available online 5 August 2010 Keywords: Energy transfer Luminescence Lanthanide ions

a b s t r a c t The optical spectroscopy and excited state dynamics of the 5D3 and 5D4 levels of Tb3+ and 5D0 level of Eu3+ have been studied in double phosphate materials having the eulytite disordered cubic structure. In the case of Sr3Tb0.90Eu0.10(PO4)3, Tb3+ ? Eu3+ energy transfer is observed upon excitation in the Tb3+ energy levels located in the UV region. This transfer gives rise to strong emission from the 5D0 level of Eu3+, peaking in the red spectral region at 612 nm. The energy transfer efficiency from the 5D4 level of Tb3+ in Sr3Tb0.90Eu0.10(PO4)3 has been evaluated and the luminescence quantum yields of Sr3Tb(PO4)3 and Sr3Tb0.90Eu0.10(PO4)3 have been measured. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Energy transfer processes involving the Tb3+ and Eu3+ ions in codoped materials have attracted attention over the years [1–6]. Despite the relative simplicity of the lowest energy level diagrams of the ions, it has been shown that the energy transfer processes can be quite complicated. In a series of recent papers, Mikhailik et al. [7,8] have proposed to use the Tb3+ ion as an efficient sensitizer of the red luminescence of Eu3+, by exploiting the strong 4f– 5d absorption bands of Tb3+ located in the vacuum ultraviolet (VUV), that can be efficiently excited by noble gas discharge. This would be helpful for the development of efficient phosphors for plasma displays and for lighting. In particular, Mikhailik and Kraus have recently shown that efficient Tb3+ ? Eu3+ energy transfer occurs in the material Ba3Tb0.90Eu0.10(PO4)3 upon VUV excitation [8], but the mechanism and the dynamics of the energy transfer process have not been addressed in detail. For this reason, we have found it interesting to extend their investigation and to provide more insight in the Tb3+ ? Eu3+ energy transfer process in the similar material Sr3Tb0.9Eu0.1(PO4)3.

2. Experimental methods and structural characterization Polycrystalline samples of Sr3Tb(PO4)3, Sr3Tb0.90Eu0.10(PO4)3, Sr3La0.99Tb0.01(PO4)3 and Sr3Y0.99Tb0.01(PO4)3 were obtained by solid state reaction at high temperature (1250 °C, 48 h) starting from SrCO3, NH4H2PO4 (both reagent grade), Tb4O7 (99.999%), Eu2O3 * Corresponding author. E-mail address: [email protected] (M. Bettinelli). 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.07.008

(99.99%), La2O3 (99.99%) and Y2O3 (99.99%) following the method described in [9]. All the obtained materials are single phase with a eulytite-type structure, as confirmed by powder X-ray diffraction (XRD) measurements, carried out using a Thermo ARL X’TRA powder diffractometer, operating in the Bragg–Brentano geometry and equipped with a Cu-anode X-ray source (Ka1, k = 0.154056 nm; Ka2, k = 0.154433 nm; Ka1/Ka2 = 2), with a Peltier Si(Li) cooled solid state detector. The XRD patterns were collected with a scan rate of 1.2°/min and an integration time of 1.5 s in the 5–90° 2h range. The phase identification was performed with the PDF-4 + 2007 database supplied by the International Centre for Diffraction Data (ICDD). Polycrystalline samples were ground in a mortar and then put in a low-background sample holder for the data collection. Luminescence emission and excitation spectra were measured at room temperature by using a Shimadzu RF-5000 spectrofluorometer equipped with a Xe lamp, using a spectral bandwidth of 1.5 nm. The room temperature decay curves were measured using a NdYAG laser as the excitation source. The emitted radiation, collected with a fiber, was measured using a half-meter monochromator, equipped with a 150 lines/mm grating and a GaAs detector. The decay curves were recorded with a 500 MHz digital oscilloscope. The crystal structures of the eulytite-type materials Sr3M(PO4)3 (M = La–Lu, Y) are well known to be cubic (space group number 220) and isomorphous with eulytine mineral (Bi4Si3O12) [10]. The Sr2+/M3+ pairs of cations are disordered on a single crystallographic site whilst the oxygen atoms of the phosphate groups are distributed over three partially occupied sites [9]. The cell parameters do not appear to be significantly affected by the nature of the M3+ ion. In fact, Sr3Y(PO4)3 (PDF card 00-044-0320), Sr3Tb(PO4)3 (PDF card 00-033-1353) and Sr3Yb(PO4)3 (PDF card 00-048-0409)

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have almost the same cell volume (1033.29, 1033.06 and 1028.59 Å3, respectively). The Tb3+ and Eu3+ dopant ions substitute in the single disordered cationic sites having C3 point group symmetry.

3.1. Emission and excitation spectra The emission spectrum of Sr3Tb(PO4)3 measured upon excitation at 337 nm (in the 4f8 levels of Tb3+ located above 5D3) is presented in Fig. 1. The bands are broad due to the presence of disorder in the host, as discussed by Blasse [10]. The spectrum clearly shows strong 5D4 emission and no significant 5D3 emission is observed due to efficient cross relaxation processes of the type 5

3þ

3þ

3þ

λem=550 nm 3+

Tb emission

300

350

400

450

500

550

Fig. 2. Room temperature excitation spectra of Sr3Tb(PO4)3 and Sr3Tb0.90Eu0.10(PO4)3 with emission wavelengths of 550 and 596 nm, respectively. The bands marked with a star are Eu3+ excitation bands.

where the phonon energies involved (DEph) are relatively low (less than 500 cm1). Due to fast multiphonon relaxation in the phosphate host, having high energy vibrations in the region of 1000–1100 cm1 [13], energy transfer will lead in all cases only to emission from 5D0. It is interesting to note that for Sr3Tb0.90Eu0.10(PO4)3 the relative intensity of the Eu3+ 5D0 emission bands, with respect to the Tb3+ 5 D4 ones, depends on the excitation pathway. The spectra shown in Fig. 3 are normalized with respect to the peak of the 5D4 ? 7F5 band of Tb3+ around 542 nm. Inspection of the figure shows that upon 5D3 excitation at 377 nm, the Eu3+ emission intensity in the 570–700 nm range is 1.22 times stronger than upon 5D4 excitation at 487 nm. This could be due to partial direct excitation of the Eu3+ ion at 377 nm (26,525 cm1) in the 5G2 and 5G3 levels, whose centres of gravity lie in LaF3 at 26,392 and 26,622 cm1, respectively [14]. 3.2. Excited state dynamics

3þ

Tb ð5 D4 Þ þ Eu3þ ð7 F0 Þ þ DEph ! Tb ð7 F4 Þ þ Eu3þ ð5 D0 Þ 3þ

250

*

Wavelength (nm)

as expected for a fully concentrated Tb3+ material [11] or one in which strong clustering is present [12]. Excitation in the UV in the levels above 5D3 is therefore followed by fast non-radiative processes, resulting in the population of the luminescent 5D4 state. The emission spectrum of Sr3Tb0.90Eu0.10(PO4)3 obtained upon excitation at 337 nm (in the Tb3+ levels) is also shown in Fig. 1. In this case the spectrum is dominated by strong emission bands from the 5D0 level of Eu3+, as only weak 5D4 emission around 545 nm is observed. The data clearly indicate that Tb3+ ? Eu3+ energy transfer occurs. The excitation spectra of Sr3Tb(PO4)3 and Sr3Tb0.90Eu0.10(PO4)3 are shown in Fig. 2. In the case of the former compounds, the excitation profile of the 5D4 emission is composed of transitions to 4f8 levels of Tb3+. Deep UV bands are not detected with the correct intensity due to the use of a Xe lamp, whose output extends from 200/250 to 800 nm. As for Sr3Tb0.90Eu0.10(PO4)3, the excitation spectrum of the Eu3+ 5D0 emission is dominated by Tb3+ bands extending in the UV region. Only a few Eu3+ excitation bands are observed (marked by a star in Fig. 2). These results confirm the Tb3+ ? Eu3+ energy transfer.Various Tb3+ ? Eu3+ phonon assisted energy transfer processes have been proposed in the literature [1–6]; some of the most important are: 3þ

*

Sr3Tb(PO4)3

200

3þ

D3 ðTb Þ þ 7 F6 ðTb Þ ! 5 D4 ðTb Þ þ 7 F0 ðTb Þ

3+

Eu emission

Sr3Tb0.90Eu0.10(PO4)3

Intensity (a.u.)

3. Results and discussion

λem=596 nm

*

Decay curves of the 5D3(Tb3+), 5D4(Tb3+) and 5D0(Eu3+) emission were measured at RT upon pulsed laser excitation at 355 nm in the

3þ

Tb ð5 D3 Þ þ Eu3þ ð7 F0 Þ ! Tb ð7 F3 Þ þ Eu3þ ð5 D2 Þ þ DEph

Sr3Tb0.90Eu0.10(PO4)3

5

D0 -> 7

F2

3+

Eu intensity ratio = 1.22

F1

Sr3Tb0.90Eu0.10(PO4)3

Intensity (a.u.)

Intensity (a.u.)

7

7 7

7

F0

F4

F3

7

F5

5

D4 ->

λexc=377 nm

7

F6

7

F4

7

λexc=487 nm

Sr3Tb(PO4)3

F3

450 450

500

550

600

650

700

750

500

550

600

650

700

Wavelength (nm)

Wavelength (nm) Fig. 1. Room temperature emission spectrum of Sr3Tb(PO4)3 and Sr3Tb0.90Eu0.10(PO4)3 upon excitation at 337 nm.

Fig. 3. Room temperature emission spectra of Sr3Tb0.90Eu0.10(PO4)3 upon excitation at 377 (top) and 487 nm (bottom). The spectra are normalized for the intensity of the 5D4 ? 7F5 band at 542 nm.

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M. Bettinelli et al. / Optical Materials 33 (2010) 119–122 Table 1 Decay data for the luminescent levels of Tb3+ and Eu3+ in several eulyite hosts upon excitation at 355 nm. Decay time (ms)

5

1.24 1.22 3.11 2.98 2.68 0.20 2.50

D3 5 D3 5 D4 5 D4 5 D4 5 D4 5 D0

(Tb3+) (Tb3+) (Tb3+) (Tb3+) (Tb3+) (Tb3+) (Eu3+)

rise + decay rise + decay 1/e decay rise + decay

levels lying above 5D3. The obtained decay time values are reported in Table 1, and several representative decay curves are shown in Fig. 4. The decay time of the 5D3 level in the concentrated Tb3+ materials could not be measured due to fast energy transfer processes responsible for the complete quenching, whilst in Sr3La0.99Tb0.01(PO4)3 and Sr3Y0.99Tb0.01(PO4)3 the decay of 5D3 is perfectly exponential with decay times around 1.2 ms. In the diluted eulytites Sr3La0.99Tb0.01(PO4)3 and Sr3Y0.99Tb0.01 (PO4)3 the temporal evolution of the 5D4 emission shows a rise and a decay. The presence of a rise time indicates that this latter level is fed by the upper lying levels, in particular 5D3 that is metastable in these compounds. The decay is exponential with a time constant close to 3 ms. The decay time of 5D4 is only slightly shortened to 2.68 ms when the concentration of Tb3+ increases from 1% to 100%, and as expected no rise time is observed. The decay curve in this case becomes slightly non-exponential and the first e-folding time was evaluated. The small quenching is attributed to energy migration followed by transfer to killer impurities. The presence of 10 mol% Eu3+ in the Strontium Terbium eulytite Sr3Tb0.90Eu0.10(PO4)3 shortens the 5D4 decay time of Tb3+ in a very significant way to 0.20 ms (1/e decay), due to the energy transfer processes discussed above. The decay curve is almost exponential, indicating that the energy transfer to Eu3+ is accompanied by fast energy migration. The efficiency gT of the energy transfer from 5D4 can be calculated using

gT ¼ 1 

sTbEu sTb

where sTbEu and sTb are the decay times of 5D4 in the Eu3+ doped and pure Terbium eulytites, respectively. gT amounts to 0.93, clearly demonstrating that in these conditions the energy transfer process is very efficient.

Intensity (arb. units)

1

Sr3La0.99Tb0.01(PO4)3

0.1

Sr3Tb(PO4)3 0.01

1E-3

Sr3Tb0.90Eu0.10(PO4)3 0

4

8

10

(a)

13

Excited state

Sr3La0.99Tb0.01(PO4)3 Sr3Y0.99Tb0.01(PO4)3 Sr3La0.99Tb0.01(PO4)3 Sr3Y0.99Tb0.01(PO4)3 Sr3Tb(PO4)3 Sr3Tb0.90Eu0.10(PO4)3 Sr3Tb0.90Eu0.10(PO4)3

λex=376 nm Photon distribution (10 cps/nm)

Material

15

(b) 5

0 400

500

600

700

800

Wavelength (nm) Fig. 5. Quantum yield of the luminescence of two eulytite materials in the range 400–800 nm (a, dotted line) Sr3Tb(PO4)3, (b, solid line) Sr3Tb0.90Eu0.10(PO4)3.

The decay curve of 5D0(Eu3+) in Sr3Tb0.90Eu0.10(PO4)3 shows a buildup at short times followed by an exponential decay. The decay constant of 5D0 is 2.50 ms, a value confirmed by measuring the decay curve upon direct excitation in the 5D1 level of Eu3+ at 527 nm. The rise time is about 0.16 ms, a value close to the decay time of 5 D4(Tb3+) in this material, confirming that this latter state is the main feeding level of 5D0(Eu3+) through energy transfer. As shown in Fig. 5, the luminescence quantum yield (QY) in the range 400–800 nm upon excitation in 5D3(Tb3+) at 376 nm is QY = 66.3% for Sr3Tb(PO4)3 and QY = 50.4% for Sr3Tb0.90Eu0.10(PO4)3. The presence of Eu3+ strongly modifies the emission spectrum with only a relatively small decrease in the overall quantum yield. 4. Conclusions In this contribution we have studied the excited state dynamics of the luminescent energy levels of Tb3+ and Eu3+ in several double phosphates having the eulytite structure. In the case of Sr3Tb0.90Eu0.10(PO4)3, efficient Tb3+ ? Eu3+ energy transfer has been clearly evidenced; this transfer appears to originate mainly from the 5D4 level of Tb3+, even though also transfer processes involving 5D3 seem to play a role. The transfer efficiency evaluated from the experimental decay times is 0.93, indicating that the Tb3+ emission is almost completely quenched, with a resulting strong Eu3+ luminescence. The luminescence quantum yield of the Sr3Tb0.90Eu0.10(PO4)3 material appears to be promising for possible technological applications in the field of lighting, as proposed by Mikhailik and Kraus [8]. We point out that the addition of 10 mol% Eu3+ to Sr3Tb(PO4)3 efficiently changes the emission colour of the material from green to red. It is reasonable to predict that intermediate concentrations will give different ratios of the Tb3+ green emission and the Eu3+ red emission. It is therefore conceivable that in this system it is possible to tune the colour of the emitted light by simply changing the concentration of the activator lanthanide ions. Acknowledgements

12

16

20

time (ms) Fig. 4. Room temperature decay curves of the 5D4 emission of Tb3+ in various eulytite hosts upon pulsed excitation at 355 nm.

M.B. wishes to thank the Graduate School of Human and Environmental Studies, Kyoto University, for the award of a Visiting Professorship in the period July–September 2009, during which part of this work has been carried out. Expert technical assistance by Erica Viviani (Univ. Verona) is gratefully acknowledged.

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