Non-radiative energy transfer process in the system Sm3+:ZrO2 prepared by sol-gel technique

Non-radiative energy transfer process in the system Sm3:ZrO2 prepared by sot-gel technique E. De la Rosa-Cruz1, L. A. DIaz-Torres, G. A. Kumar, M. Avendaflo Centro de Investigaciones en Optica, A. C. A. P. 1 -948, Leon, Gto. Mexico 37150.

P. Salas, V. M. Castaño Departamento de Fisica Aplicada y TecnologIa Avanzada, IFUNAM, A. P. 1-1010, Querétaro, Qro. Mexico 76000.

J. M. Hemández Departamento de Estado Sólido, IFUNAM, A. P. 18-1027, Mexico, D.F. 07730.

ABSTRACT Photoluminescence (PL) of pure and 0.2 mol% Sm3 doped zirconium oxide prepared by the Sot-Gel process and annealed at 1 000°C to stabilize the monoclinic phase were performed. The experimental spectra suggest the presence of energy transfer processes between the host (Zr02) and the dopant (Sm3+), when the host was excited with a . . . . . . . 3+ centered at 320 nm. The Sm doped monochnic zirconium oxide shows strong emission at the green (569nm) signal and red (607, 613 and 61 8 nm) bands, corresponding to the 4G512 — 6H512 and 4G512 —> 6H712 samarium transition, respectively; whereas the undoped sample only shows a broad band emission centered at 495 nm. The main mechanism that allows the samarium emission under UV-excitation appears to be non-radiative energy transfer from the Zr02 host to the Sm3 ions.

Keyword: sol-gel, photoluminescence, energy transfer, rare earth doped zirconium oxide.

1. INTRODUCCION Zirconium oxide is an excellent material for optical applications due to its hardness, optical transparency and high

refractive index [1]. It is considered as one of the most chemically and photochemically stable material that considering the excellent mechanical, electrical, thermal and optical properties make of it an ideal medium for photonics applications. Zirconium oxide has been widely used as an interferometry filter and for coating high power

laser mirrors [2,3]. Recently, Thermoluminescence (TL) properties of this material was reported showing a dependence of the main TL peak position on the crystalline structure, being centered at 135°C for the monoclinic phase and 440°C for tetragonal phase under UV-irradiation [4,5].

It has been reported that pure Zr02 present low phonon energy, increasing the number and the probability of radiative transitions in rare earth doped samples [6]. This fact has increased the interest to develop rare earth doped zirconium oxide, both waveguides [7] and bulk [8], for photonics applications. The interest on the rare earth ions is to produce emission in the visible range for applications such as active optical windows, new generation television screen and as phosphorus material. Bulk zirconium oxide has been grown by the Skull method and cubic doped . . 3+ have been characterized [8]. Recently, the luminescence of Sm , Tb3+ and Eu3+. in amorphous sol-gel samples zirconium oxide was reported [9, 1 0]. In the present work, experimental results of the photoluminescence of pure Zr02 and Sm3:ZrO2 sol-gel monoclinic polycrystalline powders are presented. Experimental evidence of nonradiative energy transfer from the monoclinic zirconium host to the dopant Sm3 ions was found. I Corresponding author: E. De la Rosa-Cruz, e-mail: elder(foton.cio.mx

Inorganic Optical Materials III, Alexander J. Marker III, Mark J. Davis, Editors, Proceedings of SPIE Vol. 4452 (2001) © 2001 SPIE · 0277-786X/01/$15.00

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25

5000

4500 4000 ('4 ('4

0

3500

:1

-

0

0

3000

2500 2000 10

40 Two theta

Fig. 1 X-ray dffraction patternfor 0.2 mol% samarium oxide doped zirconium oxide annealed at 1000°Cfor 10 h. The spectrum correspond to the monoclinic structure according with the JCPDS 3 7-1484.

2. SAMPLE PREPARATION Pure monoclinic Zr02 was obtained by mixing 18.2 ml of zirconium propoxide (ZP) 70% purity (Aldrich Chem. Co.) with 58.7 ml of ethanol 99.5% purity (Baker Co.), 0.5 ml of hydrochloric acid 65% purity (Baker Co.) and 0.5 ml of nitric acid 70% purity (Baker Co.). After strong stirring, 2.9 ml of C02-free distilled water was added dropwise and stirred again. Gelation occurred after 40 minutes at room temperature and the gel was dried at 1 20°C for 24 h, then was annealed at 1000°C for 10 h with a temperature increment rate of 5°C 1mm to obtain the monoclinic structure of the samples. Doped samples were prepared by adding 0.2 mol% of samarium oxide by adding the right quantity of samarium nitrate 99.99% purity into the solution, previously dissolved in the nitric acid.

The samples were characterized by X-ray diffraction using a SIEMENS D-500 equipment with Cu Kcx radiation at 1 .5426 A. The spectra were obtained from 4° to 120° with increments of 0.2° and a swept time of 3 s. Diffraction spectra were treated by DIFFRAC/AC software and the corresponding to samarium doped zirconium oxide is shown in Fig. 1 . The spectrum for undoped sample is similar to this one. The spectrum confirm that the monoclinic structure of the zirconium oxide was obtained, according with the JCPDS 37-1484, and there is no evidence of an amorphous content corresponding to the presence of 5m203, indicating that the samarium ions are well inserted in the crystal lattice.

3. RESULTS AND DISCUSSION The photoluminescence of pure and doped samples was obtained by using a fluorometer Fl 50 de Perkin-Elmer; an appropriate filter was used to reject the undesired lines. The undoped monoclinic Zr02 powder sample was excited with a 5 nm bandwidth pumping signal at 320 nm and the experimental results are shown in Fig. 2. The emission signal was centered at 495 nm and presented a broadband of about 120 nm. These results are in correspondence with data

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0 C a)

0(I) a)

C

E = -J

:\J! %

Excitation spec.

X=614nm

240 280 320 360 400 440 480 520 560 Wavelength (nm)

Fig. 2 Emission (— o— and excitation ( • ) spectra of zirconium oxide. The excitation

spectrum (—) of doped sample centered at 465 nm overlap with the emission of undoped sample opening the possibilities ofenergy transfer process. recently reported for others authors [1 1]. The T.JV uncorrected excitation spectrum presented two peaks centered at 320 and 375 nm when the emitted signal was centered at 490 nm; this confirmed that 320 nm is a good signal to excite the undoped Zr02. Both excitation peaks are associated to the absorption of the zirconium oxide that presents an increment tendency to the deep UV region as was shown in the inset of Fig. 2. Scanning in a broad-spectrum range, there is no evidence of any other excitation or emission peak. Fig. 2 also shows a comparison between the emission spectrum of the undoped sample with the excitation spectrum of the doped sample when the emission was centered at 6 1 4 nm. The clear overlapping between signals suggests the possibility of some kind of energy transfer process from the host to the active ions. Samarium ions present a broadband absorption peak at 470 nm, then the photoluminescence of 0.2 mol% Sm3 doped monoclinic ZrO2:Sm3 was obtained by exciting directly the samarium ions at 465 nm, the experimental results are shown in Fig. 3. The signal emitted showed several peaks at 569 and 579 corresponding to the transitions 4G512 — 6H512; 598, 607, 613 and 61 8 corresponding to the transitions 4G512 —* 6H712; and 657 nm corresponding to the transitions 4G512 — 6H912. The several lines for one transition correspond to the Stark splitting of the of the

corresponding transition 12]. In this figure, it is also shown the excitation spectrum centered at 465 nm and a bandwidth of about 40 nm when the emitted signal was centered at 614 nm.

Taking advantage of the overlapping between the emission and excitation of undoped and doped samples shown in Fig. 2, the samarium-doped sample was excited with a 5 nm bandwidth signal at 320 nm and the emission spectrum was obtained. The experimental results are shown in Fig. 4. The emission spectrum presented a broadband peak centered at 490 nm corresponding to the host emission and several sharp peaks similar to the obtained by exciting directly the active ions, being the main peaks centered at 569, 607, 613 and 61 8 nm. The sharpness of the emission peaks is an evidence of the crystalline structure of the host, showing that the Samarium ions were well incorporated within the crystalline Zr02 lattice. Table 1 resumes all the emission peaks and the corresponding transitions for each one.

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Xexc=465 rim

465 nm

618 rim

613

-.- 0.8 a)

0C 0.6

0 (1) a)

C E -J

569

0.4

0.2 440

480

520

560

600

Wavelength (nm)

Fig. 3 Excitation (—) and Emission ( • ) spectra of ZrO2:Sm3 sample. Emission was obtained exciting directly the active ions with a signal centered at 465 nm, in correspondence with the absorption spectrum. Notice in the inset of Fig. 4 that the three last peaks corresponding to the crystalline field splitting of the transition 6H912 presented a red-shift respect to the corresponding emission in an amorphous host typically centered at 4F312 600 nm [12]. A clear diminishing of the host emission for the doped sample respect to the undoped one was also observed. Probably, the host and samarium ions emission are dependents on the crystallite size as has been reported for the thermoluminescence response {4]; characterization of this phenomena are currently in progress and will be published elsewhere. Table 1 . Corresponding transitions of each peak observed experimentally for samarium doped zirconium oxide sample. The wavelength emission peak and broadband are presented.

Transition

4G512

6H512

4G512—÷6H912

4G512

28

6H912

? (nm)

AX (nm)

569

2.10

579

2.95

598

2.50

607

1.70

613

2.10

618

2.90

657

4.60

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w

0 = ci) 0 (1) ci) = E

-J

Wavelength (nm)

Fig. 4 Comparison between the emission ofpure (— - —) and Sm3 doped (—) zirconium oxide when the sample was excited with a signal centered at 320 nm. The inset shows the four main emission peaks ofthe samarium ions centered at 569, 607, 613 and 618 nm.

Fig. 5 shows the excitation spectrum of the sample when the emission wavelength was centered at 619 nm. Three main peaks were observed in the spectrum, two of them were associated to the samarium absorption and were centered at 405 and 468 nm respectively. The third one was centered at 309 nm and was associated to the zirconium oxide absorption at the deep UV region, in this case the spectra was also uncorrected in the UV. Considering that there is not absorption at 320 nm by the samarium ions, the emission from the samarium transitions appears to be due to excitation of Sm3 ions by energy transfer from the host. That is, once the host Zr02 absorbs the pumping energy either emits a single band centered at 495nm or transfers its excitation energy to the Sm3 ions, which in turn emit at different wavelengths. In addition, since the profile of the host emission does not change considerably from the undoped to the doped sample, one can assume that the involved energy transfer processes are non-radiative in nature. Such an assumption could explain the intensity diminishing of the doped host emission with respect to the undoped host emission, as can be observed in Fig. 4. In conclusion, experimental evidence of non-radiative energy transfer from the host to the active ions in the system ZrO7:Sm3 has been shown. The luminescence of pure monoclinic zirconium oxide present a broad bandwidth emission, opening the possibilities to excite different rare earth ions by energy transfer processes, and thus making this material suitable for photonics applications.

Acknowledgements We acknowledge to Mr. Jesus Nieto for his help in the measurement of the sample emission. This work was partially

supported by CONACyT, Mexico, through grants G34629-E and 3 557-E. G. A. Kumar acknowledges to CONACyT for financial support through grant 990712.

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405 nm

xemi=619nm ':1)

0 0 C,)

309 nm

a)

C

E -J 0.4

300

350

400

450

500

550

Wavelength (nm)

Fig. 5 Excitation spectrum ofthe doped sample when the emission was centered at 619 nm.

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