Yb3+-codoped SrTiO3 ultrafine powders

Yb3+-codoped SrTiO3 ultrafine powders

Journal of Alloys and Compounds 415 (2006) 280–283 Green and red upconversion luminescence in Er3+-doped and Er3+/Yb3+-codoped SrTiO3 ultrafine powde...

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Journal of Alloys and Compounds 415 (2006) 280–283

Green and red upconversion luminescence in Er3+-doped and Er3+/Yb3+-codoped SrTiO3 ultrafine powders Hai Guo a,b , Ning Dong a , Min Yin a , Weiping Zhang a,∗ , Liren Lou a , Shangda Xia a a

Structure Research Laboratory, Department of Physics, University of Science and Technology of China, 230026 Hefei, Anhui, PR China b Department of Physics, Zhejiang Normal University, 321004 Jinhua, Zhejiang, PR China Received 31 August 2004; received in revised form 4 August 2005; accepted 9 August 2005 Available online 19 September 2005

Abstract Er3+ -doped and Er3+ /Yb3+ -codoped strontium titanate (SrTiO3 ) ultrafine powders were prepared in a molten NaCl flux. The intense green and red emissions around 528 nm, 550 nm and 662 nm corresponding to the 2 H11/2 → 4 I15/2 , 4 S3/2 → 4 I15/2 and 4 F9/2 → 4 I15/2 transitions, respectively, of Er3+ ions were observed under excitation at 980 nm. A great enhancement of the visible upconversion emissions in Er3+ /Yb3+ -codoped sample was observed. Excited state absorption and energy transfer process are discussed as the possible mechanisms for these emissions. © 2005 Elsevier B.V. All rights reserved. PACS: 78.55-m Keywords: SrTiO3 ; Upconversion luminescence; Energy transfer

1. Introduction There has been an intense interest in the investigation of ultrafine and nanocrystalline oxide upconversion materials these years for both fundamental research and potential applications in upconversion phosphors, detectors for infrared radiation, fluorescent labels for sensitive detection of biomolecules and two-photon confocal-microscope imaging [1–11]. Trivalent rare earth ions such as Er3+ , Tm3+ , Ho3+ and Yb3+ are doped as absorption and (or) emission centers in these materials. Among these rare earth ions, the Er3+ ion is the most popular as well as one of the most efficient ions for upconversion because the metastable levels 4I 4 3+ can be conveniently populated by 9/2 and I11/2 of Er commercial low-cost high-power 800 nm and 980 nm laser diodes, respectively [4,12]. The sensitization of rare earth ions doped materials with Yb3+ ions is a well-known method for increasing the optical pump efficiency because of the high ∗

Corresponding author. Tel.: +86 551 3606577; fax: +86 551 3601073. E-mail addresses: [email protected] (H. Guo), [email protected] (W. Zhang). 0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.08.008

absorption cross-section of Yb3+ ions around 980 nm and the efficient energy transfer from Yb3+ to other rare earth ions [3,13,14]. The upconversion performance of a material could be enhanced significantly by suitable selection of host matrix. Numerous host matrixes, such as Y2 O3 [1–4], Gd2 O3 [15], ZrO2 [5–7], TiO2 [8] and BaTiO3 [8–10], have been investigated as host matrix for upconversion phosphors due to their low vibrational frequencies. Strontium titanate (SrTiO3 ) is a well-known material because of its good properties, such as its high dielectric constant, high charge storage capacity, good insulating property, its chemical and physical stability and its excellent optical transparency in the visible range [16]. In addition, its vibrational frequency is quite low, which makes it suitable for host matrix as upconversion phosphors. In the present work, rare earth ions doped SrTiO3 ultrafine powders were prepared. Their structural properties were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-TEM) and Fourier transform infrared spectroscopy (FT-IR). Under 980 nm laser excitation, the green and red emissions of Er3+ were recorded.

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Excited state absorption and energy transfer process are discussed as the possible mechanisms for these emissions.

2. Experiment SrTiO3 (STO1), SrTiO3 :Er (1%) (STO2) and SrTiO3 :Er (1%) + Yb (1%) (STO3) powders were prepared in a molten NaCl flux [17]. Appropriate quantities of TiO2 , SrCO3 , Er2 O3 , Yb2 O3 and NaCl were mixed and heated in a crucible at 1000 ◦ C for 3 h, and then cooled quickly to room temperature. The obtained white powders were washed with pure water to get rid of the NaCl flux. The resulting samples were finally dehydrated by heating at 120 ◦ C for 30 h. All the heat treatments were carried out under air atmosphere. X-ray diffraction was carried out on a MAC Science Co. Ltd. (Japan) MXP18AHF X-ray diffraction apparatus with Cu K␣ radiation. Morphology study of the SrTiO3 powders was performed on a JEOL JSM-6700F field emission scanning electron microscopy. The infrared spectra were recorded in the range of 4000–400 cm−1 with a MagnaIR 750 Fourier transform infrared spectrometer. The Stokes emission spectra were measured and analyzed by using a Jobin-Yvon LABRAM-HR laser microRaman spectrometer system equipped with an Ar+ 514.5 nm laser. Intense green and red upconversion emissions, excited by a 980 nm diode laser, were recorded with a Jobin-Yvon HRD1 double monochromater equipped with a Hamamatsu R456 photomultiplier. The output was analyzed by an EG&G 7265 DSP lock-in amplifier and stored into computer memories. All the measurements were carried out at room temperature.

3. Results and discussion 3.1. Structural properties XRD patterns of the pure and rare earth ions doped SrTiO3 powders exhibit that the samples crystallized in cubic (Pm3m) SrTiO3 crystalline phase (JCPDS card no. 84-0444). The FE-SEM image of the SrTiO3 powders shows that the particle size distribution was broad and the average diameter was 200 nm. The low concentration dopants of Er3+ and Yb3+ ions do not influence the structure and morphology of SrTiO3 . The upconversion efficiency is governed principally by the nonradiative process of the host materials [7]. Atomic groups with high vibrational frequency, such as OH and CO groups, in materials will increase the multi-phonon nonradiative relaxation rate and hence decrease the upconversion efficiency. FT-IR spectra were used to detect the existence of these groups in SrTiO3 ultrafine powders. Fig. 1 presents the FT-IR transmission spectra of SrTiO3 (a) in paraffin pellets and pure paraffin (b) as a reference. It is clear that there are no absorption bands of OH groups (around 3400 cm−1 ) and CO groups (around 1500 cm−1 ) in FT-IR spectra of SrTiO3 powders, which implies that STO2 and

Fig. 1. FT-IR spectra of: (a) SrTiO3 powders in paraffin pellets and (b) pure paraffin as a reference.

STO3 samples may have higher upconversion luminescence efficiency compared to samples containing CO2 and H2 O in the surface. The bands around 600 cm−1 and 400 cm−1 are assigned to the absorption of cubic SrTiO3 [18], which confirmed the cubic structure obtained by XRD patterns. 3.2. Luminescence properties Fig. 2 shows the Stokes emission spectra of STO2 (a) and STO3 (b) samples under 514.5 nm Ar+ laser excitation at room temperature. Spectra (a) and (b) show almost the same intensity and band shape except the emission band around 978 nm. The spectra exhibit six emission bands corresponding to the radiative decay from some excited states of the Er3+ ions and one excited state of the Yb3+ ions. The bands in the green region 516–537 nm and 537–570 nm are assigned to the transitions 2 H11/2 → 4 I15/2 and 4 S3/2 → 4 I15/2 of Er3+ ions, respectively, while the band in the red region 640–690 nm is associated with the 4 F9/2 → 4 I15/2 transition

Fig. 2. The Stokes emission spectra of: STO2 (a) and STO3 (b) samples under 514.5 nm Ar+ laser excitation at room temperature. Emission from the 2 F5/2 → 2 F7/2 transition denoted with an asterisk (*).

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Fig. 3. Upconversion luminescence spectra of: STO2 (a) and STO3 (b) samples under 980 nm laser excitation at room temperature.

Er3+

2H

→ 4I

of ions. The 11/2 15/2 emissions are observed whenever the 4 S3/2 level is excited because of a fast thermal equilibrium between 2 H11/2 and 4 S3/2 levels [13]. The transitions in the near infrared region can be assigned as follows: 4I 4 4 4 9/2 → I15/2 (783–812 nm), S3/2 → I13/2 (830–870 nm) 4 4 and I11/2 → I15/2 (958–1005 nm) of Er3+ ions. Following irradiation of the codoped STO3 sample with 514.5 nm, only the Er3+ ions should be excited as Yb3+ ion has only one absorption band in the near infrared region (∼980 nm) and therefore no emission from Yb3+ ions should be observed. However, emission attributed to the transition of 2 F5/2 → 2 F7/2 of Yb3+ ions (around 978 nm) in spectra Fig. 2(b), which is denoted with an asterisk (*), is observed. Therefore, it is reasonable to assume that there is an energy transfer from the Er3+ to Yb3+ ions [3]. Fig. 3 is the room temperature upconversion spectra of STO2 (a) and STO3 (b) samples, under 980 nm laser excitation with the same laser power and keeping the same experimental conditions. The bright emission is visible to the naked eyes even for 5 mW of the pump laser power for STO3 sample at room temperature. In both samples, the bands in the region 512–537 nm, 537–570 nm and 640–690 nm are assigned to the transitions 2 H11/2 → 4 I15/2 , 4 S3/2 → 4 I15/2 and 4F 4 3+ ions, respectively. Under 9/2 → I15/2 transitions of Er 514.5 nm laser excitation, single doped and codoped samples show almost the same intensity of green and red emissions as mentioned above. But under 980 nm laser excitation, the upconversion emission intensity of the codoped sample is higher by a factor of ∼4.5 for 4 S3/2 → 4 I15/2 transition and a factor of ∼17.5 for 4 F9/2 → 4 I15/2 transition compared with the single doped one, respectively. Such behavior indicates that there exists an effective Yb3+ to Er3+ energy transfer mechanism, which will be analyzed in the following section. In order to understand the upconversion mechanisms of the observed luminescence bands, the upconverted luminescence intensity I of these transitions was measured as a function of

Fig. 4. Dependence of the upconversion emissions intensity on excitation power in STO3 sample.

pump power P. In upconversion process, I is proportional to the nth power of P, i.e.: I ∝ Pn

(1)

where n is the number of pump photons absorbed per upconverted photon emitted [12,15,19]. A plot of log I versus log P yields a straight line with slope n. The power dependence of I for codoped STO3 sample is shown in Fig. 4. The slopes n obtained were 1.73 ± 0.08, 1.66 ± 0.01 and 1.56 ± 0.07 for 528 nm, 550 nm and 662 nm emissions, respectively. The same behavior is observed in the single doped STO2 sample. These results indicate that two-photon processes contribute to the upconversion of green and red emissions. The excited states for upconversion can be populated by several well-known mechanisms: (1) excited state absorption (ESA), (2) energy transfer (ET) and (3) photon avalanche [4,6]. Photon avalanche was ruled out as a possible mechanism for upconversion because no inflection point was observed in the power study [4]. Fig. 5 gives the energy level

Fig. 5. Energy level diagrams of Er3+ and Yb3+ ions and upconversion mechanisms.

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diagram of Er3+ and Yb3+ ions and the probable upconversion mechanisms [12–14]. For STO2 sample, the Er3+ ions are excited from the ground state to 4 I11/2 level by absorbing one 980 nm laser photon. The ions in the 4 I11/2 level sequentially absorb another 980 nm photon and are raised to 4 F7/2 level. This process is an excited state absorption process, labeled as ESA1 (in Fig. 5). The ions in the 4 F7/2 level undergo multi-phonon relaxation to luminescent levels 2 H11/2 and 4 S3/2 . There is a possible ET route that can also populate those luminescent levels. An excited ion relaxes from 4 I11/2 level to 4 I15/2 level nonradiatively and transfers the excitation energy to a neighboring ion in the same level, promoting the latter to 4F 4 4 4 4 7/2 level: I11/2 + I11/2 → I15/2 + F7/2 (Fig. 5, ET1). The 4 luminescent level F9/2 can be populated via a nonradiative relaxation from the 4 S3/2 excited state. For STO3 sample, the Er3+ ions can be excited to the 4 I11/2 level by ground state absorption and energy transfer from excited Yb3+ ions. This ET process can be described as: 4 I15/2 (Er) + 2 F5/2 (Yb) → 4 I11/2 (Er) + 2 F7/2 (Yb) (Fig. 5, ET2). ET2 process is the dominant one [13], since the Yb3+ ion has a larger absorption cross-section than the Er3+ ions around 980 nm. The ions in the 4 I11/2 level can be excited to the 4 F7/2 level by ESA1 process and energy transfer from excited Yb3+ ions (Fig. 5, ET3: 4 I11/2 (Er) + 2 F5/2 (Yb) → 4 F7/2 (Er) + 2 F7/2 (Yb)). The 4 F9/2 level can also be populated by nonradiative relaxation from the 4 S3/2 level as STO2 sample. Because the intensity of red emissions increases fast than that of green emissions with Yb3+ ions codoping (Fig. 3), there must be another ET process that only populates the 4 F9/2 level (Fig. 5, ET4: 4 I13/2 (Er) + 2 F5/2 (Yb) → 4 F9/2 (Er) + 2 F7/2 (Yb)) [13,14]. The excited 4 I13/2 level may be populated through multi-phonon nonradiative process from 4 I11/2 level and radiative process from upper levels, such as 4 S3/2 level (Fig. 2).

4. Conclusion In conclusion, Er3+ -doped and Er3+ /Yb3+ -codoped cubic SrTiO3 ultrafine powders have been prepared in a molten NaCl flux. Emission from the transition of 2 F5/2 → 2 F7/2 of Yb3+ ions was observed in the Stokes emission spectra under 514.5 nm excitation of STO3 sample and indicated that there is an energy transfer from the Er3+ to Yb3+ ions. The intense green and red emissions around 528 nm, 550 nm and 662 nm corresponding to the 2 H11/2 → 4 I15/2 , 4 S3/2 → 4 I15/2 and 4 F9/2 → 4 I15/2 transitions of Er3+ ions were observed under 980 nm excitation. A great enhancement of the visible upconversion emissions in Er3+ /Yb3+ -codoped sample is

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observed and the energy transfer from Yb3+ to Er3+ is responsible for this enhancement phenomenon. Our results show that rare earth ions doped SrTiO3 ultrafine powders may find potential applications in upconversion phosphors, detectors for infrared radiation.

Acknowledgements This project is supported by the Foundation of Ministry of Education for Training Elitist Project of the Century, and the Doctoral Project Foundation of the National Education Committee of China (No. 20010358016).

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