Yb3+-codoped BaTiO3 nanocrystals by tridoping with Li+ ions

Yb3+-codoped BaTiO3 nanocrystals by tridoping with Li+ ions

Optics Communications 284 (2011) 2046–2049 Contents lists available at ScienceDirect Optics Communications j o u r n a l h o m e p a g e : w w w. e ...

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Optics Communications 284 (2011) 2046–2049

Contents lists available at ScienceDirect

Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m

Upconversion emission enhancement in Er3+/Yb3+-codoped BaTiO3 nanocrystals by tridoping with Li+ ions Xiangqun Chen a, Zhikai Liu a, Qiu Sun b,⁎, Mao Ye b, Fuping Wang b a b

Department of Material Physics and Chemistry, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, PR China

a r t i c l e

i n f o

Article history: Received 13 May 2010 Received in revised form 21 October 2010 Accepted 3 December 2010 Available online 22 December 2010 Keywords: BaTiO3 nanocrystals Intensity Symmetry Lifetime Li+

a b s t r a c t Er3+/Yb3+/Li+-tridoped BaTiO3 nanocrystals were prepared by a sol–gel method to improve the upconversion (UC) luminescence of rare-earth doped BaTiO3 nanoparticles. Effects of Li+ ion on the UC emission properties of the Er3+/Yb3+/Li+-tridoped BaTiO3 nanocrystals were investigated. The results indicated that tridoping with Li+ ion enhanced the visible green and red UC emissions of Er3+/Yb3+codoped BaTiO3 nanocrystals under the excitation of a 976 nm laser diode. X-ray diffraction and decay time of the UC luminescence were studied to explain the reasons of the enhancement of UC emission intensity. X-ray diffraction results gave evidence that tridoping with Li+ ion decreased the local symmetry of crystal field around Er3+, which increased the intra-4f transitions of Er3+ ion. Moreover, lifetimes in the intermediate 4 S3/2 and 4I11/2 (Er) states were enhanced by Li+ ion incorporation in the lattice. Therefore, it can be concluded that Li+ ion in rare-earth doped nanocrystals is effective in enhancing the UC emission intensity. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Barium titanate (BaTiO3) is a potential matrix materials for upconversion (UC) emissions due to its high dielectric constant, high charge storage capacity, good insulating property, its chemical and physical stability, relatively low phonon energy (about 700 cm−1), and so on [1,2]. Up to now, some reports dealing with UC luminescence properties of BaTiO3 (BTO) nanoparticles have been presented [3,4]. For instance, Patra et al. studied fluorescence UC properties of BTO: Er3+ prepared by sol–emulsion–gel method [5]. Zhang et al. discussed the green UC luminescence of the Er-doped BTO films prepared with sol–gel method [6]. Among rare earth ions doped into UC emissions materials, the Er3+ ion is the most popular activator because it shows strong excited state absorption at 980 and 800 nm, which are emission wavelengths of low-cost laser diodes [7–9]. However, the corresponding transitions have weak ground state absorption, notably at 980 nm. Hence, a sensitizer is indispensable to achieve high optical pumping efficiency. In fact, combined with the high Yb3+ absorption cross section at ~980 nm, Er3+ ion yields increased luminescence efficiency in Er3+/Yb3+ codoped materials [10–12]. An efficient energy transfer (ET) often appears due to the large spectral overlap between the 2 F5/2 → 2 F7/2 (Yb3+) NIR emission and the 4I11/2 → 4I15/2 (Er3+) absorption bands.

⁎ Corresponding author. Fax: + 86 451 8641 8409. E-mail address: [email protected] (Q. Sun). 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.12.007

According to the quantum mechanical selection rules, the main intra-4f electronic-dipole transitions of rare-earth ions are forbidden, which can be broken by the local crystal field of the rare-earth ions due to the capability to intermix their f states with higher electronic configurations. Hence, the tailoring rare-earth ions' local environment in the host lattice is a promising route to enhance their optical performance [13–15]. Doping Li+ ion into rare-earth doped nanocrystals can decrease the crystal field symmetry, which can lead to greatly enhance the UC emission intensity of rare-earth ions such as Er3+, and so the UC emission intensity should vary with the Li+ concentration. Realization of efficient NIR to visible UC in BTO nanocrytals will have great impact on deploying their potential advantages. In this paper, effects of Li+ ion on the radiation lifetimes of intermediate 4 S3/2 and 4 F9/2 (Er) states and the increase of UC emission intensity were discussed. 2. Experimental section Er3+ and Yb3+ codoped BTO precursor solutions were synthesized using Ba(CH3COO)2, Er(NO3)3 · 6H2O, Yb(NO3)3 · 6H2O and Ti (OC4H9)4 as the Ba, Er, Yb, and Ti sources, respectively. Required amounts of acetic acid (5 mL) and ethol (20 mL) were used to prepare the sol. The requisite amount (5 mL) of tetrabutyl titanate was added to the mixture of acetic acid and ethanol. Ba(CH3COO)2, Er(NO3)3 · 6H2O, Yb(NO3)3 · 6H2O and LiNO3 were dissolved in 10 mL water followed by adding 10 mL acetic acid and 10 mL ethanol. The titania sol was added dropwise into the solution of Ba(CH3COO)2, under

X. Chen et al. / Optics Communications 284 (2011) 2046–2049

vigorous stirring at ambient temperature. A yellow transparent sol was thus obtained, which formed a gel over a period of an hour at 80 °C, and then the gel was dried at 120–130 °C for several hours. The resulting sol–gel precursor powders were calcined at 800 °C for 2 h to obtain the final nanocrystals. WAXD pattern was obtained for the powder specimen on a D/maxγB X-ray diffractometer using a graphite monochromator CuKα radiation (40 KV, λ = 0.1546 nm). The scanning rate was 0.02°/min over a range of 2θ = 10–80°. Further, the powders were pressed to form smooth and flat disks to be utilized for UC spectral studies. Samples were irradiated with a controllable-power 976 nm diode laser (Hi-Tech Optoelectronics Co. Ltd., Beijing) with the maximum output power of 500 mW. The emitted UC fluorescence was collected by a lens-coupled monochromator of a 3 nm spectral resolution and with an attached photomultiplier tube. Decay profiles of the 548 and 660 nm radiations were measured by square-wave-modulation of the electric current input to the 976 nm diode laser, and by recording the signals via a Tektronix TDS 5052 digital oscilloscope.

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Fig. 2. Measured UC emissions of BTO15 and BTO15 tridoped with Li+ ion concentrations of 1–8 mol% under diode laser excitation of 976 nm.

Fig. 1 presents the UC emission of nanocrystals BTO doped with 1 mol% Er3+ and different amount of Yb3+. As illustrated in Fig. 1, the green and red UC bands centered at 524, 548 and 660 nm arise from the intra-4f electronic transitions 2 H11/2/4 S3/2 → 4I15/2 and 4 F9/2 → 4I15/2 of the Er3+ ions, respectively [8,16,17]. It can be seen that both green and red upconversion emission spectra of Er3+ ions in all Er3+/Yb3+codoped BTO powder show big differences in emission intensity. The higher the concentrations of Yb3+ ion, the stronger the UC emission intensity of red UC bands centered at 660 nm. The UC red emission intensity of BTO: 1Er5Yb (BTO15) is about 4.5 times greater than that of BTO: 1Er. The intensity of green UC bands at 524 nm of BTO14 and BTO15 is about 1.3 times higher than that of BTO: 1Er. However, that of BTO with lower Yb3+ ion concentrations remains nearly constant. In addition, the intensity of green UC bands at 548 nm decreases as the Yb3+ concentration increases, which can be caused by fewer Er3+ ions for transitions 4 S3/2 → 4I15/2 and more Er3+ ions holding at the redemitting level of 4 F9/2 [18]. As a whole, the emission intensity increases with the increasing concentration of Yb3+ and the red emission intensity at 660 nm is stronger than that of the green emission under the excitation at 976 nm. Fig. 2 is the room temperature upconversion spectra of BTO: 1Er5Yb (BTO15) and BTO15 tridoped with different concentration of Li+ samples, under 980 nm laser excitation with the same laser power and keeping the same experimental conditions. The incorporation of

Li+ ion drastically intensifies the luminescent intensity compared with that of BTO15. When concentration of Li+ was 7%, the resultant enhancement was about ten times bigger than that of lithium free BTO15. However, UC emission intensity kept unchanged in higher Li+ ion content, for example 8%. The XRD patterns present that the positions and relative intensities of the diffraction peaks are in good agreement with the Joint Committee of Powder Diffraction Standards data (JCPDS No. 310174), showing that the main structure of BTO samples is cubic perovskite. Fig. 3 shows the XRD patterns of peaks (110) of BTO15 powders, and tridoped with 1–8 mol% of Li+ ion. On the basis of the XRD patterns, the average sizes of the BTO nanoparticles can be estimated by the Debye-Scherrer's equation. The strongest peaks (110) were used to calculate for the average crystallite size of the BTO15 samples with different Li+ ion concentrations. The calculated results are about 18.7, 21.4, 19.8, 21.8, 20.9, 23.1, 22.3 and 19.9 nm for the different BTO15 samples with Li+ ion concentrations of 0–8 mol%. The average crystallite sizes of different samples were analogous, which showed that the particle size had not changed by the introduction of Li+ ion. BTO is the typical configuration of ABO3. Li+ ions occupy Ba2+ site (A-site of ABO3) because of the big charge difference between Li+ ion and Ti4+. At the same time, Li+ ion could occupy the interstitial sites due to its small size [19]. In BTO: Er3+/Yb3+ crystals, Yb3+ occupies mainly the B-site of BTO and Er3+ occupies mainly the A-site of BTO [20,21]. It is worth noting that when the Ba2+ ions is substituted by the Li+ ions with smaller radius, the corresponding

Fig. 1. Measured UC emissions of BTO doped with 1 mol% of Er3+ and codoped with Yb3+ ion concentrations of 1–5 mol% under diode laser excitation of 976 nm.

Fig. 3. Measured XRD patterns of peaks (110) of BTO15 powders and further tridoped with Li+ ion concentrations of 1–8 mol%.

3. Results and discussion

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X. Chen et al. / Optics Communications 284 (2011) 2046–2049

lattice constants become a little bigger. The gradual peak shifts illustrate that more Li+ ions can be doped into the host lattice through the substitution of Ba2+ ion below 3 mol%. The constant peak positions for Li+ ions above 4 mol% suggest that no further Li+ ion can enter into the BTO host lattice, and some Li+ ions occupy interstitial sites. It is noted that both the substitution of Ba2+ ions and the occupation of interstitial sites can tailor the local crystal field around Er3+ ions, which is expected to tailor their emission parameters and affect their anti-Stokes luminescence [22]. In upconversion process, I is proportional to the nth power of P, I ∝ Pn, where n is the number of photons involved in the pumping mechanism. In order to understand the effect of Li+ content on the upconversion mechanisms of the observed luminescence bands, the slopes n of BTO15 tridoped different concentration of Li+ samples is studied. Fig. 4 shows a logarithmic plot of the integrated emission intensity of the upconverted fluorescence as a function of pump intensity for BTO15 tridoped Li+ ion concentration of 7 mol% (BTO15: 7 Li+). The slopes n obtained are 1.98 ± 0.02 and 1.77 ± 0.02 for 548 nm and 660 nm emissions, respectively. These results indicate that twophoton processes contribute to the upconversion of green and red emissions. The Li+ ions cannot absorb 980 nm photons and cannot transfer its energy to BTO and Er3+ ions. Energy level diagram of the Er3+ and Yb3+ ions as well as the proposed UC mechanisms for the green and red emissions are shown in Fig. 5. Two energy transfers (ET 1 and 2) from Yb3+ ions can promote the Er3+ ion to the 4 F7/2 state[23,24]. The 4I11/2 level is populated directly by ground state absorption (GSA) and ET1 process: 2 F5/2(Yb3+) + 4I15/2(Er3+) → 2 F7/2 (Yb3+) + 4I11/2(Er3+). The population of Er3+ in 4 F7/2 level is based on excited state absorption (ESA) and the following ET2 process: 2 F5/2(Yb3+) + 4I11/2(Er3+) → 2 F7/2(Yb3+) + 4I7/2(Er3+). The populated Er3 + 4 F7/2 level then relaxes rapidly and nonradiatively to the next lower levels 2 H11/2 and 4 S3/2 green emitting levels. With the increasing concentration of Yb3+, the population in excited levels of 4 S3/2 can be suppressed by energy back-transfer 4 S3/2 (Er3+) + 2 F7/2 (Yb3+) → 4I13/2 (Er3+) + 2 F5/2 (Yb3+), resulting in the decrease of green-light emission (4 S3/2 → 4I15/2). In addition, the populated 4I13/2 level might be excited to the 4 F9/2 red-emitting level in Er3+ ions by cross-relaxation process 4 I11/2 + 4I13/2 → 4I15/2 + 4 F9/2 [15], resulting in the increase of red-light emission (4 F9/2 → 4I15/2). Fig. 6 presents the normalized decay profiles of the 4 S3/2 → 4I15/2 transition at 548 nm and 4 F9/2 → 4I15/2 transition at 660 nm in BTO15 and BTO15: 7 Li+ nanocrystals. All decay profiles can best be fitted by a double exponential function for all the samples. As shown in Fig. 6, the lifetime of the 4 S3/2 state of BTO15: 7 Li+ nanocrystals is longer than that of BTO15 nanocrystals. Lifetimes of the 4 S3/2 and 4 F9/2 states of Er3+ ions for BTO15 tridoped with various Li+ ion concentrations

We have synthesized Er3+/Yb3+ codoped BTO nanocrystals and Er /Yb3+/Li+ tridoped BTO nanocrystals by sol–gel method. The ratio of the intensity of red luminescence to that of green luminescence has increased with an increase of concentration of Yb3+ in Er3+/Yb3+ codoped BTO nanocrystals. The intensity of green and red bands of BTO15 tridoped with various Li+ ion concentrations has increased with increasing concentration of Li+. This result is interpreted in terms that the observed much longer lifetimes in the Er3+ ions arise from such tailoring induced by Li+ ion. XRD investigations illustrate that the local crystal field of the BTO host lattice can be modified by substituting the Ba2+ site or occupying the interstitial lattice sites with Li+ ions.

Fig. 4. Pump power dependence of the green and red emission of BTO15: 7 Li+ under diode laser excitation of 976 nm.

Fig. 6. Decay profiles of the 4 S3/2 (Er) → 4I15/2 (Er) and 4 F9/2 (Er) → 4I15/2 (Er) transition in BTO15 and BTO15: 7 Li+ under diode laser excitation of 976 nm.

Fig. 5. Energy level diagrams of the Er3+ and Yb3+ ions as well as the proposed UC mechanisms for the green and red emissions.

are listed in Table 1. As shown in Table 1, the lifetime of the 4 S3/2 state increases with Li+ ions of 1–7 mol%, and remains nearly constant at higher concentrations. According to the measured lifetime values of the τ1 and τ2, the green and red UC fluorescence intensity should dramatically increase with doping Li+ ions from 1 mol% to 7 mol%, while decrease slightly at Li+ ions of 8 mol%[25–27]. Such tendency is in good agreement with the experimental observations in Fig. 1.

4. Conclusions

3+

X. Chen et al. / Optics Communications 284 (2011) 2046–2049 Table 1 Lifetimes of the 4 S3/2 and 4 F9/2 states of Er3+ ions for BTO tridoped with various Li+ ion concentrations. Li+ concentration (mol%)

τ1(μs) (4 S3/2)

τ2 (μs) (4 F9/2)

0 1 2 3 4 5 7 8

97.5 99.2 102.7 101.3 103.8 104.1 104.4 99.3

74.4 90.6 93.5 103.8 104.1 109.3 110.6 101.8

Acknowledgements The authors gratefully acknowledge the financial support of Youth Science Foundation of Heilongjiang Province of China (No. QC07C23) and Natural Science Foundation of Heilongjiang Province of China (No. E200905). References [1] S.S.A. Seo, H.N. Lee, T.W. Noh, Thin Solid Films 486 (2005) 94. [2] A. Testino, V. Buscaglia, M.T. Buscaglia, M. Viviani, P. Nanni, Chem. Mater. 17 (2005) 5346. [3] P. Ghosh, S. Sadhu, T. Sen, A. Patra, Bull. Mater. Sci. 31 (2008) 461. [4] Y. liu, W.A. Pisarski, S. Zeng, C. Xu, Q. Yang, Opt. Express 17 (2009) 9089. [5] A. Patra, C.S. Friend, R. Kapoor, P.N. Prasad, Chem. Mater. 15 (2003) 3650.

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