научная статья по теме GREEN AND RED UPCONVERSION EMISSIONS OF ER3+/YB3+-CODOPED SRTIO3 POWDER PREPARED BY POLYMERIC PRECURSOR METHOD Химия

Текст научной статьи на тему «GREEN AND RED UPCONVERSION EMISSIONS OF ER3+/YB3+-CODOPED SRTIO3 POWDER PREPARED BY POLYMERIC PRECURSOR METHOD»

HEOPTAHHóECKHE MATEPHAttbl, 2008, moM 44, № 8, c. 979-982

UDC 546:542

GREEN AND RED UPCONVERSION EMISSIONS OF Er3+/Yb3+-CODOPED SrTiO3 POWDER PREPARED BY POLYMERIC PRECURSOR METHOD

© 2008 F. C. D. Lemos*, J. E. C. da Silva**, D. M. A. Melo*, M. S. C. Camara*, P. S. de Lima*, C. E. J. Carneiro*

* Universidade Federal do Rio Grande do Norte - Departamento de Química, Natal, RN, Brazil **Universidade Federal do Tocantins - Campus Araguaína, R. Humberto de Campus, Araguaína, TO, Brazil

e-mail: lemos@ufrnet.br Received 24.08.2007

Ultrafine Er3+/Yb3+-codoped SrTiO3 (SEYT) powders in cubic form have been successfully prepared by polymeric precursor method. The single phase perovskite for the obtained material was observed at low temperature. An efficient infrared to visible conversion in SEYT perovskite will be reported. Visible emissions at 550 and 663 nm corresponding to the 2S3/2 - 4I15/2 and 4F9/2 - 4I15/2 transitions, respectively, were observed under continuous wave excitation at 980 nm. An enhancement of the visible up-conversion luminescence in Er3+/Yb3+ codoped samples was confirmed due to efficient energy transfer from Yb3+ to Er3+ ions. The quadratic pump power dependence of these emissions indicated the contribution of two photons in the up-conversion process.

INTRODUCTION

Near infrared up-conversion to the visible spectrum has been used for the observation of visible luminescence or laser action in certain rare earth doped materials upon near infrared excitation. This up-conversion is a non-linear process, having two or more low-energy excitation photons converted into one or two high-energy photons [1]. Up-conversion process, such as excited state absorption, energy transfer up-conversion and photon avalanche (PA) absorption produce populations in an excited state whose energy exceeds that of the pump photon. In particular, conversions of infrared radiation (IR), easily obtainable from efficient semiconductor laser diodes, into blue-green wavelength have generated much of the current interest in studying new materials and responsible processes [2].

The trivalent Er was the rare earth ions that was most studied due to the fact that it presents, in the near-infrared spectral region, favorable energy level structure with two transitions (4I15/2 —- 4I11/2 and 4I15/2 —- 4I9/2), being efficiently pumped with high power semiconductor lasers, being able to yield blue, green and red emissions [3]. However, the corresponding transitions have weak ground state absorption, notably at 980 nm. Hence, a sensitizer is indispensable for the achievement of high optical pumping efficiency [4]. Yb3+ seems to be suitable for this role since there is an important spectral overlap between its 2F5/2 —► 2F7/2 emission band and the Er3+ 4I15/2 —- 4I11/2 absorption band, allowing an efficient Yb3+ —- Er3+ transfer of the excitation energy. This fact, combined to a high Yb3+ absorption cross-section at ~980 nm, yields increasing luminescence efficiency in codoped materials [5]. The presence of the sensitizer ions adds the possibility of new ion-ion interactions be-

tween donors and acceptors, resulting in up-conversion processes, which are extremely sensitive to the excitation wavelength. It is also well known that Yb3+/Er3+ codoped oxide systems present great efficiency via energy transfer [6]. It is also important to understand the Er3+ luminescence properties in different host materials and at different concentrations in order to optimize the materials for specific applications.

ABO3 perovskite compounds such as BaTiO3 and SrTiO3 have drawn a good deal of attention due to their properties, in particular SrTiO3, which depict properties such as high dielectric constant, high charge storage capacity, chemical and physical stability, and excellent optical transparency in visible range [7]. In addition, its vibrational frequency is quite low, making it suitable for host matrix as up-conversion phosphors. Recently, we have presented results of efficient IR up-conversion excitation on codoped lead titanate, which indicated that the materials are promising for photonic applications [8]. In this paper we report mainly on the up-conversion luminescence characteristics of Er3+/Yb3+-codoped SrTiO3 at room temperature.

EXPERIMENTAL

The ytterbium-erbium modified strontium titanate ceramics, Sr1 _ + y)YbxEryTiO3, with x and y = 0.025 molar fraction, were prepared by the so-called polymeric precursor method [9]. Strontium nitrate, erbium oxide, ytterbium oxide and titanium IV isopropoxide were used as starting materials. Ethylene glycol and citric acid were used as po-lymerization/complexation agents for the process. Titanium citrate was formed by the dissolution of titanium IV isopropoxide in an aqueous solution of citric acid (60-70°C). After the homogenization of the Ti-citrate solu-

979

6*

700°C

500°C

300 °C

20 30 40 50 60 70 80

26, deg

Fig. 1. XRD Pattern of SEYT powders at different temperatures.

tion, a stoichiometric amount of strontium nitrate was dissolved in water and soon thereafter added to Ti-citrate solution, which was kept under slow agitation until a clear solution was obtained. In the preparation of the ytterbium and erbium solutions, Yb2O3 and Er2O3 were first dissolved in nitric acid and gradually added to the Sr-Ti-cit-rate solution. To achieve total dissolution of the cations, ammonium hydroxide was added dropwise until the pH reached 6-7. After homogenization of the solution containing the Sr, Er/Yb and Ti cations, ethylene glycol was added to promote mixed citrate polymerization by polyes-terification. Upon continuous heating process at 80-90°C, the solution containing the molar ratio among Sr-Er-Yb and Ti cations 0.95 : 0.025 : 0.025 : 1, respectively, became more viscous without any visible phase separation. The citric acid/metal molar ratio was fixed at 1.00 and the

citric acid/ethylene glycol mass ratio was fixed at 60/40. The heat-treatment temperature of the polyester resin thus obtained had to be sufficient to promote polymer pyrolysis without crystallization (300°C for 2h). The resulting porous material was easily deagglomerated in a mortar and annealed at 500 and 700°C for 2h.

The samples were analyzed by powder X-ray diffraction using CuKa radiation in a Shimadzu 600 system with diffraction angle (20) ranging between 5° and 75°. The up-conversion emission spectra were recorded at room temperature with a Jobin-Yvon Ramanor U1000 spectrometer, using an infrared diode laser (GaAs : Si CW, 600 mW) as excitation source. The detector was an RCA C31034 pho-tomultiplier tube.

RESULTS AND DISCUSSION

The X-ray diffraction pattern of the nanocrystals at 300, 500 and 700°C for 2 h is shown in Fig. 1. At 300°C, it was observed that SEYT is amorphous and at 700°C there is a clear indication of crystalline perovskite peaks corresponding to the 100, 110, 111, 200, 210, 211, 220, 221 and 310 orientations of the cubic structure with space

group Pm3 m (JCPDS card no. 86-0178) (Fig. 2). No additional phases were observed, hence the SEYT sample is single phase maintaining the perovskite structure. The crystallite size can be estimated from the Scherrer's equation [10] using the measured full width at half-maximum value peaks. The estimated crystallite size is about 26 nm for the sample obtained at 700°C.

Pure and doped ST synthesis, without secondary or intermediate phases were also reported by several authors [11-14]. In this work, the SEYT was obtained at low temperature without any secondary phase.

Figure 3 shows the visible emission spectra of SEYT perovskite treated at 700°C, under infrared excitation (^980 nm) obtained at room temperature. The spectra

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Exc. 980 nm

2w 4T l-H11/2- a15/2

2S 4t

S3/2- t15/2

4F9/2-4I

- 9/2"

J

15/2

500 550 600 650

Wavelength, nm

700

Fig. 2. The XRD pattern of cubic perovskite.

Fig. 3. Visible emission spectra of SEYT treated at 700°C for 2h.

HEOPrAHHHECKHE MATEPHAÏÏBI tom 44 № 8 2008

GREEN AND RED UPCONVERSION EMISSIONS OF Er3+/Yb3+-CODOPED

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Fig. 4. Diagram of the levels (Yb3+ and Er3+) involved in the optical process.

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show emission bands assigned to the 2H

2S

4I15/2 and 4F9

11/2

4I

15/2'

3/2 -' I15/2 and F9/2 -- 4Ii5/2 transitions for Er3+

around 525, 550 and 663 nm, respectively. It was found that the red emission is dominant and corresponds to the

4F

9/2

4I15/2 transition of Er3+.

The energy level diagram of Yb3+ and Er3+ ions and probable up-conversion mechanism are shown in Fig. 4. The SEYT powder exhibited basically three erbium emissions: one strong emission line at around 663 nm

I15/2 ) and weak emissions 4I15/2) and 525 nm (2H

11/2

around 550 nm 4I15/2). In the

№>/2 (2S3/2

mechanism depicted, an erbium ion is excited to its 4I11/2 level by energy transfer from an excited ytterbium ion (A). A second energy transfer occurs from excited ytterbium and the erbium ion is excited to its 4F7/2 level. The erbium ion can then relax to its 2H11/2, 4S3/2 or 4F9/2 levels by non-radiative mechanisms and emit photons from these levels. The phonon frequency is associated to the host SrTiO3 perovskite at around 550 nm. Therefore, few

3.5 4.0 4.5 5.0 5.5 6.0 In(IR pump power) [a.u.]

6.5 7.0

Fig. 5. Logarithmic plot of the integrated emission intensity of the up-conversion as a function of the pump laser intensity under excitation at 980 nm.

phonons can promote the necessary energy for the nonradioactive process of energy transfer between 4F7/2 erbium excited level and 2H11/2, 4S3/2, 4F9/2 emission levels. The 4F9/2 level is predominantly excited in this material, observed f

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