SONG Ming-Jun ZHANG Yn ZHANG N-N WANG Lin-Tong MENG Qin-Guo

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Up-conversion Properties of Er3+/Yb3+Co-doped Li3Ba2Gd3(MoO4)8Phosphors①

SONG Ming-Juna②ZHANG YanbZHANG Na-NaaWANG Lin-TongaMENG Qin-Guoa

a(261061)b(200235)

Er3+/Yb3+co-doped Li3Ba2Gd3(MoO4)8phosphors were synthesized by conven- tional solid state reaction method, and their structure and spectral properties were investigated. The diffuse reflectance spectra showed that the415/2→411/2transition of Er3+and the27/2→25/2transitionof Yb3+ions were highly overlapped. Under the excitation of 980 nm, three up-conver- sion (UC) luminescence bands around 530, 555 and 660 nm were observed, corresponding to the211/2→415/2,43/2→415/2and49/2→415/2transitions of Er3+ions, respectively. The effects of the concentration and pumping power on theUC intensities of Li3Ba2Gd3(MoO4)8:Er3+/Yb3+phosphors were investigated, and the possible UC mechanism was proposed based on the results.

up-conversion, molybdate compounds, phosphors, energy transfer;

1 INTRODUCTION

As we know, up-conversion (UC) process has provided a possibility to convert infrared pumping source into visible radiation, which has potential applications in the fields of solid-state lasers, mul- ticolor displays, solar cells, temperature sensing,.[1-5]. As a consequence, with the rapid develop- ment of diode lasers in the infrared region over the past decades, considerable attention has been devoted to infrared-to-visible UC luminescence materials. Among the rare earth ions, Er3+may be the most frequently investigated activator for UC process, since its complicated energy level scheme with many meta-stable excited states gives rise to multi-step energy transfer and excited-state absorp- tion process. However, the main disadvantage of Er3+should be its low absorption cross-section in the laser diodes emission range (0.8～1.5 mm), which greatly limits pump efficiency. One possible solution is adding a second ion as sensitizer and Yb3+is often used for this role because of its large absorption cross-section at 980 nm. Furthermore, the energy gap of27/2→25/2levels of Yb3+ion is very close to that of415/2→411/2and411/2→47/2levels of Er3+ion, which allows an efficient energy transfer from Yb3+to Er3+by resonance interaction.

Furthermore, the properties of the host materials, including phonon energy, mechanical robustness and thermal stability, also exert an important effect on the performance of UC. To achieve a high effi- ciency of UC process, one of the most important requirements is that the host materials should have low phonon energy. Therefore, the current choice of potential host materials for UC mainly concentrates on chlorides, fluorides, and sulfur oxides. These materials are characterized by low phonon energy, which give rise to high UC efficiency[6-8], but their applications are restricted by their unstable chemi- cal nature and poor mechanical robustness[9]. Fur- thermore, fluorides and sulfur oxides inevitably cause environmental pollution because of the high content of fluorine and sulfur. In recent years, the molybdate based materials have been intensively studied as promising host for solid state lasers and luminescent phosphors owing to their excellent properties, such as excellent chemical stability, moderate synthesis conditions and pollution-free features. In addition, since they also have relatively low lattice phonon energy, many molybdate com- pounds were also reported as UC host, like CaMoO4:Er3+/Yb3+[1], ALn(MoO4)2:Er3+/Yb3+(A = Li, Na and K; Ln = La, Gd and Y)[9], BaGd2(MoO4)4: Er3+/Yb3+[10],Gd2(MoO)3:Er3+/Yb3+[11], and so forth.

The triple molybdate compounds Li3Ba2Re3(MoO4)8(Re = La～Lu, Y) belong to the monoclinic system with2/group. In the past, the interest in these compounds mainly concentrated on the bulk crystals to take advantages of their laser properties because of their wonderful spectral pro- perties and simple growth techniques. Notably, effi- cient laser performance has already been achieved with Yb3+:Li3Ba2Gd3(MoO4)8[12]and Tm3+:Li3Ba2Lu3(MoO4)8crystals[13]. In recent papers, Li3Ba2Re3(MoO4)8was further investigated as potential hosts for rare earth ions to develop luminescent materials, and Eu3+, Tb3+and Dy3+-doped Li3Ba2Re3(MoO4)8phosphors have been reported for white-light emitting applica- tions[14-16]. However, to the best of our knowledge, these compounds have never been investigated as UC host. So, in the present paper, Er3+/Yb3+co- doped Li3Ba2Gd3(MoO4)8was synthesized by solid state reaction method and their UC properties were recorded under the excitation of 980 nm. Moreover, the effect of Er3+/Yb3+concentration and pump power on the UC emission, as well as the possible UC mechanism wasalso presented.

2 EXPERIMENTAL

The Li3Ba2Gd3(MoO4)8:Er3+/Yb3+phosphors were synthesized by means of solid state reaction method with raw materials ofLi2CO3(A.R.), BaCO3(A.R.), Gd2O3(99.999%), Er2O3(99.99%), Yb2O3(99.99%) and MoO3(A.R.). Firstly, a stoichiometric mixture of the raw materials was ball-milled for 10 h using planetary milling with zirconia ball and alcohol media. Then, the dried powders were placed into alumina crucibles and calcined at 900 ℃ for 6 h under air atmosphere. After that, the synthesized samples were grounded into fine powders in an agate mortar for mea- surement.

The structures of Li3Ba2Gd3(MoO4)8:Er3+/Yb3+phosphors were analyzed by X-ray power diffract- tion (D8 Advance diffractometer, Bruker Corpora- tion, Germany) with a Curadiation (= 1.54056 ?, 40 kV, 30 mA). The diffuse reflection spectra were measured using a Shimadzu UV-vis-NIR spectrophotometer (UV-3600). The UC lumines- cence spectra and luminescence decay curves were recorded using an Edinburgh Instruments FLS980 spectrophotometer equipped with an external power-controllable 980 nm semiconductor laser as excitation source. All the measurements were carried out at room temperature.

3 RESULTS AND DISCUSSION

The X-ray diffraction (XRD) patterns of Li3Ba2Gd3(MoO4)8:Er3+/Yb3+phosphors are shown in Fig. 1 and compared with that of pure Li3Ba2Gd3(MoO4)8. As we can see, all the dif- fraction peaks of the synthesized samples are consistent with those of pure Li3Ba2Gd3(MoO4)8on the powder standard card and no additional diffraction peaks were observed, indicating that all the Er3+and Yb3+ions have been completely doped into the host matrix and substitute for the Gd3+ions. However, as shown in Fig. 1(b), the main diffract- tion peaks of the samples gradually shift to higher 2angles with the increasing concentration of Yb3+ions owing to the smaller radius of Yb3+ions compared to that of Gd3+ions.

Fig. 1. XRD patterns of Li3Ba2Gd3(MoO4)8:2%Er3+/xYb3+with different Yb3+concentrations

To investigate the energy level structures of Er3+and Yb3+ions in the Li3Ba2Gd3(MoO4)8host, the absorption spectra of Li3Ba2Gd3(MoO4)8:Er3+/Yb3+phosphors were recorded. Fig. 2 shows the diffuse reflection spectra of Li3Ba2Gd3(MoO4)8:20%Yb3+, Li3Ba2Gd3(MoO4)8:2%Er3+and Li3Ba2Gd3(MoO4)8:2%Er3+/20%Yb3+phosphors. For all the samples, a broad and smooth absorption band around 280 nm can be observed, which belongs to the charge transfer transition of O → Mo. Besides, six eminent absorption bands around 380, 485, 520, 655, 800 and 975 nm can be also observed with the Li3Ba2Gd3(MoO4)8:2%Er3+and Li3Ba2Gd3(MoO4)8:2%Er3+/20%Yb3+phosphors. The former five absorption bands for the two samples are very similar and can be ascribed to the intrinsic transitions of Er3+ions from the ground state of415/2to the excited states of411/2,47/2,211/2,49/2and49/2, respectively. However, it should be noted that the absorption band around 975 nm of Li3Ba2Gd3(MoO4)8:2%Er3+is much weaker than that of Li3Ba2Gd3(MoO4)8:2%Er3+/20%Yb3+. For Li3Ba2Gd3(MoO4)8:2%Er3+, the absorption band peak at 974 nm comes from the415/2→411/2transi- tion of Er3+ions, while the one for Li3Ba2Gd3(MoO4)8:2%Er3+/20%Yb3+at 968 nm should be the superposition of the27/2→25/2transition of Yb3+ions and the415/2→411/2transi- tion of Er3+ions. As for the sample of Li3Ba2Gd3(MoO4)8:20%Yb3+, only an intensive band is found around 975 nm, which arises from the27/2→25/2transition of Yb3+ions. According to Fig. 2, it can be concluded that the energy transfer process between Yb3+and Er3+ions in Li3Ba2Gd3(MoO4)8should be very effective because of the significant overlaps between the415/2→411/2transition of Er3+and the27/2→25/2transitionof Yb3+ions.

Fig. 2. Diffraction reflectance spectra of Li3Ba2Gd3(MoO4)8:2%Er3+, Li3Ba2Gd3(MoO4)8:20%Yb3+and Li3Ba2Gd3(MoO4)8:2%Er3+/20%Yb3+

Under the excitation of 980 nm, the UC lumine- scence spectra of Li3Ba2Gd3(MoO4)8:2%Er3+/Yb3+(= 8%, 12%, 16%, 20%, 24%, 28%) phosphors in the wavelength range of 500～700 nm were recorded, as shown in Fig. 3. The UC luminescence spectra of all the recorded samples display similar profiles consisting of three emission bands around 530, 555, and 660 nm. The green emission bands at 530 and 555 nm can be ascribed to the211/2→415/2and43/2→415/2transitions of Er3+ions, respectively. The multi-peaks of the two green bands should be caused by crystal-field stark splitting. The red emission band at 660 nm can be ascribed to the49/2→415/2transition of Er3+ions. In our previous research, a tiny emission band at 490 nm deriving from the47/2→415/2transition of Er3+ions was also observed with Er3+:Li3Ba2Gd3(MoO4)8single crystals under 980 nm excitation[17]. In the present research, however, this emission band is not observed. The main reason may be that the polycrystalline Li3Ba2Gd3(MoO4)8possesses much more defects than single-crystal Li3Ba2Gd3(MoO4)8, which act as nonradiative recombination centers and significantly depopulate of47/2state. The inset of Fig. 3 shows the integrated intensities of green and red emission bands with respect to the Yb3+concentration. As we can see that with increasing the Yb3+concentration, both the green and red emission intensities increase obviously, suggesting an efficient energy transfer of Yb3+→ Er3+. However, when the Yb3+concentration is beyond 20 mol%, both the green and red emission intensities decrease due to the concentration quenching effect. The concentration quenching effect is mainly caused by energy transfer between neighboring Er3+and Yb3+ions. With increasing the Er3+and Yb3+concentrations, the distance between Er3+and Yb3+ions gets shorter, and meanwhile the non-radiative energy transfer becomes more intensive[9]. As a result, the emission intensities of Er3+ions decrease.

In order to understand the UC mechanism of Li3Ba2Gd3(MoO4)8:Er3+/Yb3+, the UC luminescence spectra of Li3Ba2Gd3(MoO4)8:2%Er3+/20%Yb3+phosphors were measured as a function of different pumping powers (Fig. 4). As we can see that there is hardly any difference among the profiles of the UC luminescence spectra, but the emission intensities increase with the increasing pumping powers. The relation between the UC luminescence intensity and the pumping powers can be described by the following equation[18]:

where Iis the UC luminescence intensity,Pis the pumping powers andrepresents the number of photons involved in the UC process. Generally, the above equation is written as:

Fig. 3. UC luminescence spectra of Li3Ba2Gd3(MoO4)8:2%Er3+/xYb3+with different Yb3+concentrations. The inset shows the dependence of green and red UC emission intensity on the Yb3+concentration

Fig. 4. UC luminescence spectra of Li3Ba2Gd3(MoO4)8:2%Er3+/20%Yb3+with different pumping powers. The inset shows the power dependence of UC emission intensity at 530, 555 and 660 nm

The plots of lnIfor the three emission bands versus lnPyield straight lines, as shown in the inset of Fig. 4.By linear fitting, the slope valueswere obtained to be 2.03, 1.85, 1.80 for 530, 555 and 660 nm emission respectively, which are close to 2. So, we can conclude that two photons are involved for the UC process in the system of Li3Ba2Gd3(MoO4)8:Er3+/Yb3+.

The UC mechanism in Er3+/Yb3+co-doped systems has been extensively investigated in many literatures[1-6]. It can be described by the energy level diagram sketched in Fig. 5. Under the excitation of 980 nm, the Yb3+ions can be excited to the excited-state of25/2. Meanwhile, the Er3+can also be exited to its excited-state of411/2through ground state absorption (GSA) process and energy transfer (ET) process:

GSA:415/2(Er3+) + a photon (980 nm) →411/2

ET:415/2(Er3+) +25/2(Yb3+) →

411/2(Er3+) +27/2(Yb3+)

Fig. 5. Energy level scheme of Er3+and Yb3+in Li3Ba2Gd3(MoO4)8and the possible UC mechanisms under 980 nm excitation

Since the lifetime of411/2state of Er3+is very long[19], it is expected that the population of this excited state can be further excited to the upper state of47/2through excited state absorption (ESA) process and energy transfer (ET) process:

ESA:411/2(Er3+) + a photon (980 nm) →47/2

ET:411/2(Er3+) +25/2(Yb3+) →

47/2(Er3+) +27/2(Yb3+)

Some of the populations of the47/2state relax non-radiatively to the211/2and43/2states, giving rise to the green emissions of 530 and 555 nm, respectively. At the same time, some of the popula- tions of the47/2state relax non-radiatively to the49/2state, from which the red emission of 660 nm is observed.

The decay curves of the211/2→415/2(530 nm),43/2→415/2(555nm) and49/2→415/2(660nm) transitions of Er3+ions in Li3Ba2Gd3(MoO4)8:2%Er3+and Li3Ba2Gd3(MoO4)8:2%Er3+/20%Yb3+samples were measured under 980 nm excitation. As shown in Fig. 6, the decay curves of the three transitions in Li3Ba2Gd3(MoO4)8:2%Er3+exhibit a typical single exponential behavior, and the lifetimes were calculated to be 21.6, 20.7 and 22.4s by linear fitting. However, the decay curves of Li3Ba2Gd3(MoO4)8:2%Er3+/20%Yb3+sample are not single exponential and can be fitted with a second-order exponential model:

whereis the UC luminescence intensity,1and2are constants,1and2are the lifetimes for the exponential components. The values of1,2,1and2are presented in Table 1. The mean lifetimes for the three transitions in Li3Ba2Gd3(MoO4)8: 2%Er3+/20%Yb3+sample can be calculated by the following equation[20]:

Fig. 6. Decay curves of2H11/2,4S3/2and4F9/2states for Li3Ba2Gd3(MoO4)8:2%Er3+(black) and Li3Ba2Gd3(MoO4)8:2%Er3+/20%Yb3+(red) samples

The mean lifetimes of Li3Ba2Gd3(MoO4)8:2%Er3+/20%Yb3+are also listed in Table 1. As we can see that the lifetimes in Li3Ba2Gd3(MoO4)8:2%Er3+/20%Yb3+sample are much larger than those in Li3Ba2Gd3(MoO4)8: 2%Er3+sample, which should be caused by energy transfer from Yb3+to Er3+ions.

4 CONCLUSION

A serious of Li3Ba2Gd3(MoO4)8:Er3+/Yb3+phos- phors were synthesized by conventional solid state reaction method and their structures were confirmed by X-ray diffraction method. Under the excitation of 980 nm, the samples exhibited a weak red UC emission at 660 nm and two strong green UC emissions at 530 and 555 nm. With a fixed Er3+concentration of 2 mol%, the effects of Yb3+concentrations on the UC luminescence properties were investigated and the optimum concentration of Yb3+is determined to be 20 mol%. The relations between UC emission intensities and pumping powers were investigated and the result revealed that a two photons process was responsible for all the three observed UC emissions. The lifetimes for both the Li3Ba2Gd3(MoO4)8:2%Er3+and Li3Ba2Gd3(MoO4)8:2%Er3+/20%Yb3+samples were measured and the results showed that the introduce- tion of Yb3+had greatly enhanced the lifetimes of Er3+ions.

Table 1. Lifetimes of the 2H11/2, 4S3/2 and 4F9/2 States of Er3+ Ions for Li3Ba2Gd3(MoO4)8:2%Er3+ and Li3Ba2Gd3(MoO4)8:2%Er3+/20%Yb3+ Samples

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14 June 2017;

3 September 2017

10.14102/j.cnki.0254-5861.2011-1753

① This project was supported by the Natural Science Foundation of Shandong Province ( ZR2014JL029, BS2015CL012, ZR2015BM005)

②Born in 1981, Ph.D, Tel: +86-536-8785283, E-mail: smj521209@126.com