Yb3+-doped SrAl12O19 nanophosphors

JOURNAL OF RARE EARTHS, Vol. 29, No. 3, Mar. 2011, p. 202

Quenching mechanism of Er3+ emissions in Er3+- and Er3+/Yb3+-doped SrAl12O19 nanophosphors YAN Wuzhao (䮿℺䩞), CHEN Yonghu (䰜∌㰢), YIN Min (ል⇥) (Department of Physics, University of Science and Technology of China, Hefei 230026, China) Received 23 April 2010; revised 26 October 2010

Abstract: Nanoscaled SrAl12O19:Er3+ and SrAl12O19:Yb3+, Er3+ phosphors were synthesized by a combustion method. The emission intensities of every sample were compared by a new method with the emission of codoped Gd3+ ions as a reference. Compared with their bulk material prepared by the solid-state reaction method, a higher Er3+ quenching concentration, as high as 20%, was observed in the nanoscaled phosphors for both visible (VIS) and near infrared (NIR) emissions. The higher quenching concentration in both VIS and NIR regions for nanoscaled samples are related to the structure characteristics of the nano particles. The influence of the introduction of Yb3+ ions on the emission spectra intensity was also investigated and discussed. Keywords: erbium; ytterbium; gadolinium; SrAl12O19; near infrared; nanoscaled phosphor; rare earths

During the past decades, considerable efforts have been made on the development of near infrared (NIR) phosphors because of their applications in security, biosensor, quantum storage and telecommunication areas[1–4]. Er3+ and Yb3+ are the most popular active centers for NIR phosphors due to their suitable energy levels in NIR region. SrAl12O19 (SAO) is considered to be a good host for NIR phosphors because of its good chemical stability and low phonon energy (about 700 cm–1)[5]. In order to get a high photoluminescence, high concentration of luminescent centers are needed. But in the conventional bulk materials, the optimal concentrations of luminescent centers are around 1%[6,7] because of the limitation of the concentration quenching effects. It was reported that the quenching concentration of Eu3+ luminescence is much higher in nanoscaled Y2SiO5:Eu3+ than that in bulk materials[8]. However, in the NIR region, only few reports have been published, wherein the quenching concentrations are around 5% in nanoscaled materials[9,10]. Meanwhile, Yb3+ is often used as the codopant ions to enhance the photoluminescent intensity of Er3+. In this paper, the response of Er3+ ions luminescent intensity with the increasing of Er3+ and Yb3+ concentrations in nanoscaled SAO:Er3+ and SAO:Er3+,Yb3+ phosphors synthesized by a combustion method were investigated. The emission intensity of Gd3+ ions was used as a criterion to normalize that of Er3+ ions in our samples. Er3+ ions were found to have as high as 20% quenching concentrations in nanoscaled SAO:Er3+. Meanwhile, the introduction of Yb3+ was demonstrated to depress the luminescence intensity of Er3+ in SAO:Er3+,Yb3+. The explanation of the high quenching con-

centration was interpreted by the way of distribution of the quenching centers in nanosclaed particles. The energy transfer mechanisms between Er3+ and Yb3+ ions were also presented.

1 Experimental Nanoscaled SAO:Er3+ and SAO:Er3+,Yb3+samples with different concentrations of Er3+ and Yb3+ were synthesized by a combustion method via a highly exothermic redox reaction between metal nitrates and organic fuels. Stoichiometric amounts of host nitrates Sr(NO3)2 and Al(NO3)3·9H2O, dopant nitrates Er(NO3)3·6H2O, Yb(NO3)3·6H2O and Gd(NO3)3·6H2O, and organic fuels urea, glycine and carbohydrazide were calculated using the oxidizing (O) and reduction (F) valences of the components according to the propellant chemistry[11], then mixed and dissolved in a minimum amount of deionized water in a crystallizing dish. The solution was heated at 80 ºC with continuous and mild stirring on a stirring hotplate to evaporate the water slowly. When the water had boiled off, the transparent gel thus produced was quickly transferred into a furnace preheated at 500 ºC, wherein the gel was ignited and started to react with bright incandescent frame to yield white voluminous foamy SAO:Er3+ and SAO:Er3+,Yb3+. The products were ground and used for characterization. To compare the differences of Er3+ quenching concentrations between the nanoscaled phosphors and bulk phosphor, bulk SAO:Er3+ and SAO:Er3+, Yb3+ samples were also synthesized via high temperature solid-state reaction method heated at 1 300 ºC for 4 h.

Foundation item: Project supported by National Natural Science Foundation of China (10774140, 11011120083), Knowledge Innovation Project of the Chinese Academy of Sciences (KJCX2-YW-M11) and Special Foundation for Talents of Anhui Province, China (2007Z021) Corresponding author: YIN Min (E-mail: [email protected]; Tel.: +86-551-3607412) DOI: 10.1016/S1002-0721(10)60431-0

YAN Wuzhao et al., Quenching mechanism of Er3+ emissions in Er3+- and Er3+/Yb3+-doped SrAl12O19 nanophosphors

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The X-ray diffraction (XRD) patterns were obtained by a PANalytical X’PERT PRO diffractometer with Cu KĮ radiation (Ȝ=0.15418 nm). The photoluminescence (PL) measurements were carried out with a Jobin Yvon Fluorolog3-2iHR320 spectrophotometer with a 450 W Xe lamp as the excitation source. The morphology and size of the samples were characterized by a fielding-emission FEI INSPECT F scanning electron microscope (SEM).

2 Results and discussion 2.1 XRD patterns and SEM images The obtained XRD patterns of nanoscaled and bulk SAO: Er3+ are shown in Fig. 1, which match perfectly with the standard SrAl12O19 (JCPDS PDF number 80-1195) with hexagonal structure and P63/mmc space group, no extra phase was found. However, when the concentration is up to 20%, a small amount of SrAl2O4 (JCPDS PDF number 10-61) appears in our system. Fig. 2 shows the typical SEM images of SAO:Er3+ 5% and SAO:Er3+ 20% samples synthesized by combustion method, which reveal the surface morphology of the phosphor. The as-synthesized samples have uniform nanorod morphology, whose diameters are about 50–60 nm, the pores formation in the combustion derived samples indicate a large volume of escaped gases.

Fig. 2 SEM images of SAO:Er3+ 5% (a) and SAO:Er3+ 20% (b) phosphors with different concentrations

2.2 Emission and excitation spectra The main emission spectra of nanoscaled SAO:Er3+ under 377 nm excitation are shown in Fig. 3. All the emission peaks are the characteristic transitions among the 4f subshell of Er3+ ions. The 522 and 544 nm emissions come from the (2H11/2, 4S3/2)ĺ4I15/2, 980 and 1530 nm emissions are due to the transitions of (4I11/2, 4I13/2)ĺ4I15/2. In order to get a detailed description of the variations of the Er3+ luminescent intensity with different Er3+ concentrations in the samples, 1% Gd3+ ions are introduced into every sample of the present material systems. Although it was reported that energy transfer could occur between Gd3+ and Er3+ ions in some specific hosts[12,13], when our sample

Fig. 1 XRD patterns of nanoscaled and bulk SAO:Er3+ samples

Fig. 3 Emission spectra of nanoscaled SAO:x% Er3+ (The intensities are scaled by the emission intensity of Gd3+ in every samples); inset: the emission of Gd3+ in SAO host under 272 nm excitation

was irradiated with 272 nm excitation source, the emissions of Er3+ ions did not show up. Gd3+ emissions (inset of Fig. 3) did not appear under the excitation of 377 and 544 nm neither. That is to say, the dopants of Gd3+ and Er3+ ions have no effect on the emission of each other. Thus the emission intensity of Gd3+ ions in respective samples under the excitation of 272 nm can be used as a standard to normalize that of the Er3+ ions under the excitation of 377 nm. By this way, the intensities of Er3+ ion luminescent in the nanoscaled material and in the bulk materials with different concentrations can be compared. The 544 and 1 530 nm emission intensities

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of every nanoscaled and bulk sample with different concentrations are recorded to draw pictures of concentration dependence of Er3+ ions, which are shown in Fig. 4. In the nanoscaled phosphors, the quenching concentrations of 544 and 1 530 nm emissions are as high as 20%, which is 4 times higher than that in the bulk material. To explain the high Er3+ quenching concentration in nanoscaled SAO:Er3+ phosphors, the theory of fluctuation of quenching centers (traps) is adopted[8,14]. The specimens inevitably contain very low concentrations of quenching centers because of the defects produced during the synthesis progress (especially through fast and violent combustion reaction) and trace impurities in raw materials. One or more defects act as traps, when an excited luminescent center (Er3+ ion) is located near to a trap, the energy can be easily transferred to the traps nonradiatively; meanwhile, it can also be transferred to another Er3+ ion if the two Er3+ ions are so close that they can be coupled together by some interaction. When the concentration of the Er3+ ions is low, Er3+ ions can be regarded as “isolated” and only a few Er3+ ions with traps nearby can transfer their energy to the traps, leading to an insignificant concentration quenching. With the concentration increasing, the Er3+ ions become near enough to form a resonant energy transfer net, so energy transfer occurs much more easily among the Er3+ ions because the energy transfer rate is much faster than radiative decay rate resulting from the exchange/superexchange Coulomb interaction and the small average interaction distances under the high Er3+ concentration. Thus, most of the excited energy states go to ground states by means of giving the energy to traps because of shortened average distance between Er3+ ions and traps, leading to the concentration quenching. In nanoscaled materials, because of the limited number of unit cells per particle, there are only an average of a few traps in one particle. Furthermore, the distribution of the traps fluctuates sharply among those particles, and some particles have few traps while others may contain a lot of traps. Owing to the hindering of the particle boundary, the energy of excited Er3+ ions can only be transferred resonantly between the traps and Er3+ ions in one particle. With the concentration of Er3+ ions in-

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creasing, concentration quenching appears firstly in particles that have a lot of traps, leading to a significant nonradiative cross relaxation and resonant energy transfer mentioned above, while in other particles with few traps and luminescent centers quenching happens at very high concentration. The combined effects of the two kinds of particles lead to higher average quenching concentration of Er3+ ions in nanoscaled SAO:Er3+ phosphors than that in bulk system. In our nanoscaled SAO:Er3+ system, after the Er3+ ions being excited, the 4S3/2ĺ4I15/2 transition (544 nm) is quenched through the assumed cross-relaxation among the Er3+ ions, 4S3/2+4I15/2ĺ4I9/2+4I13/2[15–18], as presented in left frame of Fig. 3. For the Er3+ ions at 4I13/2 level, one ion can get energy from one of its neighboring Er3+ ions and is promoted to 4I9/2 level (4I13/2+4I13/2ĺ4I9/2+4I15/2)[19], as shown in Fig. 5(a), so the 4I13/2ĺ4I15/2 transition (1 530 nm) is quenched, the spectra is also shown in right frame of Fig. 3. The population of 4I9/2 energy level will be enhanced because of the two energy transfer ways mentioned above, however, the population in the 4I9/2 energy level will relax to the 4I11/2 energy level with the assistance of phonons, and then the 4 I11/2ĺ4I15/2 transition (980 nm) is enhanced, as to the 980 nm emission, quenching phenomenon does not show up when the concentration is 25%, as shown in middle frame of Fig. 3. It is in these ways that 544 and 1 530 nm emissions quench. Similar phenomenon was reported in Ref. [19]. 2.3 Yb3+ ions as co-dopants In some previous reports, Yb3+ ions have been introduced as co-dopants to significantly enhance or depress the spectral intensities of the NIR emissions of Er3+ in glass or polycrystalline hosts through energy transfer[18,20]. In our experiments, Yb3+ ions are also introduced into SAO:Er3+ to investigate the changes of the emission intensities of Er3+ ions both in the VIS and the NIR regions, as shown in Fig. 6, we can see that in our nanoscaled samples the introduction of Yb3+ ions suppresses the two emissions (i.e. VIS: 4S3/2ĺ4I15/2 and NIR: 4 I13/2ĺ4I15/2) of Er3+. This result can be explained in terms of the strong energy transfer between Er3+ and Yb3+ ions because of the overlap of the corresponding absorption and

Fig. 4 Quenching concentration comparison of nanoscaled materials with bulk materials for 544 nm emission (a) and 1 530 nm emission (b) (The two intensities are scaled and normalized)

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Fig. 5 Energy level diagram of the transitions after 377 nm excitation (a) Schematic of cross-relaxation mechanisms between two Er3+ ions; (b) Schematic of the non-resonant energy transfer between Er3+ and Yb3+ ions

emission spectra (for Yb3+: 9 613–10 672 cm–1; for Er3+: 9 895–10 344 cm–1)[21–24]. As shown in Fig. 5(b), the Er3+ ions in the 4S3/2 level will relax to the 4I11/2 level through the cross-relaxation energy transfer process (i.e., 4S3/2+2F7/2ĺ 4 I11/2+2F5/2) between Er3+ and Yb3+ ions, which partially quenches the VIS 4S3/2ĺ4I15/2 and NIR 4S3/2ĺ4I13/2 emissions. Furthermore, the NIR emission 4I13/2ĺ4I15/2 will also be quenched, since the 4I13/2 level of Er3+ will get less population in the above assumed nonradiative relaxation. However, the population in the 4I11/2 level will get increased due to the relaxation from 4S3/2 to 4I11/2, the 4I11/2ĺ4I15/2 transition is also quenched by the well-known nonradiative energy transfer from the 4I11/2 level of Er3+ to the 2F5/2 level of Yb3+, so the emission intensity of 4I11/2ĺ4I15/2 does not change obviously with the introduction of Yb3+ ions, but the peak around 980 nm is broadened because of the overlap of 4 I11/2ĺ4I15/2 and 2F5/2ĺ2F7/2 transitions, as indicated in the inset of Fig. 6. From the analyses above, we think that the Yb3+ ions act as quenching centers in our nanoscaled material system.

Fig. 6 Emission spectra of SAO:Er3+ and SAO:Er3+,Yb3+, excited by 377 nm, the intensities are scaled

3 Conclusions A combustion method was used to synthesize SAO:Er3+ and SAO:Er3+,Yb3+ phosphors with different Er3+ ions concentrations. A new method was developed to compare the

emission intensity of every sample. Comparing with the ordinary bulk materials, the nanoscaled SAO:Er3+ specimens had a much higher quenching concentration because of sharp fluctuation of trap numbers in nano-particles and boundary hindering effect of the nano-particles. Meanwhile, the energy quenching mechanism was investigated. We found that the Yb3+ co-dopants decreased the emission intensity by the way of cross relaxation between the 2F5/2 energy level of Yb3+ and the 4I11/2 energy level of Er3+ in SAO:Er3+,Yb3+ system.

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