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1 Introduction▲
2 Experimental▲
3 Results and discussion▲
4 Conclusion▲
图表▲

Yb³⁺-and Er³⁺-doped Y₂O₃ microcrystals for upconversion photoluminescence and energy transfer with enhancements of near-ultraviolet emission

Jin-Bo Zhao Li-Li Wu

School of Materials Science and Engineering,Qilu University of Technology (Shandong Academy of Sciences)

Key Laboratory for Liquid-Solid Structural Evolution and Process of Materials (Ministry of Education),Shandong University

School of Materials Science and Engineering,Shandong University

作者简介：Li-Li Wu e-mail:wulili@sdu.edu.cn;

收稿日期：26 June 2018

基金：financially supported by The Fundamental Research Funds of Shandong University (No. 2018JC036);

Yb³⁺-and Er³⁺-doped Y₂O₃ microcrystals for upconversion photoluminescence and energy transfer with enhancements of near-ultraviolet emission

Jin-Bo Zhao Li-Li Wu

School of Materials Science and Engineering,Qilu University of Technology (Shandong Academy of Sciences)

Key Laboratory for Liquid-Solid Structural Evolution and Process of Materials (Ministry of Education),Shandong University

School of Materials Science and Engineering,Shandong University

Abstract：

Yb³⁺ and Er³⁺ were doped into Y₂ O₃ to form Y₂ O₃:Yb³⁺,Er³⁺ microcrystals.The effect of doping concentration of Yb³⁺ was studied.The near-ultraviolet(UV) upconversion(UC) emission spectra of Y₂ O₃:Yb3+,Er³⁺ were studied under 1.55-μm excitation.The near-UV UC emissions centered at 391 and 418 nm were enhanced by Yb³⁺ under 1.55-μm laser excitation.Er³⁺ generally acts as activator and is not used as sensitizer in the reference Er³⁺-doped oxides.In the present upconversion process under the 1.55-μm laser excitation,the energy transfer from Er³⁺ to Yb³⁺ and then Yb³⁺ to Er³⁺ was observed.Dopant Er³⁺ acted as not only activator but also sensitizer.The energy transfer between Er³⁺ and Yb³⁺ enhanced the near-UV UC emission of our materials.

Keyword：

Upconversion; Photoluminescence; Y₂O₃: Yb³⁺/Er³⁺; 1.55-μm excitation; Optical materials and properties;

Received： 26 June 2018

1 Introduction

Dopants,introduced impurities in material,can strongly modify electronic,magnetic,optical and other properties of materials [ 1, 2, 3] .Lanthanide-doped upconversion particles are received a great deal of attention due to their captivating photon upconverting ability whereby near-infrared long-wavelength excitation at low power can be converted into shorter wavelength emission [ 4, 5] .This frequency conversion ability allows upconversion (UC) particles to be widely applied in a variety of areas such as solid-state laser,efficient solar cells,display,bioimaging and biomedicine [ 6, 7, 8, 9, 10] .For numerous applications,most of them require the ability to tune the upconversion emission to meet the requirement of a specific application [ 11, 12] .Various attempts have been devoted to tuning the upconversion emission of the photoluminescence materials,and Yb³⁺was used as the sensitizer only under excitation of980-nm wavelength.Er³⁺has unique energy-level structure with abundant metastable excited states [ 13, 14, 15, 16] ,which produce emission peaks at specific wavelengths even in different host materials of varying size.

Yttrium oxide (Y₂O₃) is an attractive non-toxic host rare-earth dopants material,which has intriguing physical and chemical properties,relatively lower phonon cutoff energy and wide range of transparency between 0.23 and0.8μm wavelength [ 17] .Er³⁺has a favorable energy-level structure that matches 1.55μm wavelength,resulting in4U_15/2 to ⁴I_13/2 transition [ 18] .However,most study in Er³⁺-doped materials was focused on 980-nm excitation and near-infrared (NIR)-induced visible UC radiations [ 19, 20, 21] .The ultraviolet (UV) radiation induced by 1.55μm wavelength has been rarely characterized.Particularly,strategies to magnificently increase the UV UC radiations have seldom been reported.The investigation of Er³⁺UC emission under 1.55-μm excitation to ultraviolet regions is of significance in many fields,including the detection of surface plasmon,infrared (IR) quantum counters,increasing the efficiency of energy conversion devices and development of biophotonic materials used in biological labeling.Furthermore,a 1.55-μm photon energy is lower than a 980-nm photon energy [ 17] .In this paper,flowerlike Yb³⁺,Er³⁺doped Y₂O₃ microcrystals were synthesized.The near-UV UC emissions centered at 391 and418 nm were enhanced by Yb³⁺under 1.55μm laser excitation.Er³⁺generally acts as activator and is not used as sensitizer in the reference Er³⁺-doped oxides.The structure,morphology and optical properties of the prepared samples were characterized.Under the 1.55-μm laser excitation,the energy transfer from Er³⁺to Yb³⁺and then Yb³⁺to Er³⁺was observed in the UC process.These results could evoke wide interests in the photovoltaic,biophotonic and photonic applications of these crystals.

2 Experimental

A series of Y_1.94-xYb_xEr_0.06O₃ (x=0,0.1,0.2,0.3,0.4)samples were synthesized by hydrothermal process.In a typical experiment,Y(NO₃)3·6H₂O and Yb(NO₃)3.5H₂O were mixed in 17 ml deionized (DI) water according to the proper molar ratio and then 0.357 g hexamethylenetetramine was added in the solution.After vigorous stirring for 2 h at room temperature,the final solution was transferred into a sealed 20-ml Teflon-lined stainless autoclave and heated at 180℃for 24 h in an oven.The resulting dispersions were centrifuged and rinsed by DI water and ethanol several times,followed by drying at 60℃in air overnight.After calcined at 700℃for 4 h,the powders were collected for further characterizations.

To demonstrate the morphology of the particles,the sample was characterized by field-emission scanning electron microscope (FESEM Hitachi S-4800,5 kV).The crystalline structure was investigated by X-ray diffractometer (XRD,Rigaku Dmax2200) with Cu Ka radiation(λ=0.15406 nm) at 40 kV and 150 mA.The UC luminescence spectra were taken on a 970CRT fluorescence spectrophotometer (Shanghai Precision Instrument Factory,slit:3 nm),and a 1.55-μm optical fiber laser (DS3-11312,BWT,Beijing Kaipulin Co.,LTD,China) with continuous output maximum power of 2.0 W used as excitation source.All the spectral measurements were taken at room temperature.

3 Results and discussion

XRD patterns of the samples after calcination are presented in Fig.la.All diffraction peaks of the products can be readily indexed to the standard structure of Y₂O₃ (JCPDS File No.71-0099),and none of the diffraction peaks from any other phase are found.From Fig.1b,it can be found that XRD peaks shift toward higher diffraction angles with the concentration of Yb³⁺increasing,which indicates that Yb³⁺has doped into the lattice of Y₂O_3.Additionally,the diffraction peaks from products are sharp and strong,meaning that the samples are well crystallized.

The morphology of the obtained products was examined by FESEM.As a representative,Fig.2a shows FESEM images of Y_1.94Er_0.06O₃ sample.And Fig.2b shows the images of Y_1.54Yb_0.4Er_0.06O₃ sample.It can be seen that both of the samples are flower-like structures,and the sheets with thickness in 500 nm are the base for building the microspheres.The size of a microsphere is about3-4μm.FESEM images indicate that the size and morphology of the samples are independent from Yb³⁺concentration.

The UC emissions of the particles were studied in the emission spectra from 300 to 750 nm.Figure 3 displays UC emission spectra in the range of 300-470 nm for the Y_1.94-xYb_xEr_0.06O₃ (x=0,0.1,0.2,0.3,0.4) samples under excitation at 1.55μm of 1000-mW excitation with an unfocused laser beam.The clear UC emissions centered at410 and 391 nm are observed,which are assigned to the2H_9/2 state to ⁴I_15/2 state and ⁴G_11/2 state to ⁴I_15/2 state of Er³⁺ [ 22] .The effect of Yb³⁺concentration on the upconversion emission spectrum was investigated also,which can be seen in Fig.3.The intensities of both 391-and 410-nm UC emissions gradually increase with the concentration of Yb³⁺increasing.Owing to the quenching effect,UC emission intensity decreases at higher doping level [ 17] .The highest UC emission intensity is achieved with 15 mol%Yb³⁺,corresponding to the sample of Y_1.64Yb_0.3Er_0.06O_3.The inset in Fig.3 presents UC emissions of Y_1.64Yb_0.3Er_0.06O₃ and Y_1.94Er_0.06O₃ in the range of 300-750 nm with pump power of 10 mW.Upconversion emissions centering at 520,550 and 650 nm are observed,corresponding to the transitions from ²H_11/2,⁴S_3/2 and ⁴F_9/2 state to the ground ⁴I_15/2 state of Er³⁺,respectively [ 16] .After doping by Yb³⁺,the UC emission intensity at 520,550 and 650 nm decreases due to the energy transfer from Er³⁺to Yb³⁺ [ 17, 18] .Because of higher intensity of upconversion emissions centering at520,550 and 650 nm,the 391-and 410-nm luminescence emissions appear very weak and cannot be found.

Figure 4 shows the energy-level diagrams of Er³⁺and Yb³⁺as well as the possible upconversion pathways under laser excitation with 1.55μm.The commonly acceptable mechanisms can be expressed as in Fig.4.It can be briefly described as follows [ 22] :

Fig.1 XRD patterns of a Y_1.94-xYb_xEr_0.06O₃ (x=0,0.1,0.2,0.3,0.4) microspheres and b enlarged spectra in range of 45°-55°

Fig.2 FESEM images of a Y_1.94Er_0.06O₃ microspheres and b Y₁.54Yb_0.4Er_0.06O₃ microspheres

Fig.3 Y_1.94-xYb_xEr_0.06O₃ (x=0,0.1,0.2,0.3,0.4) UC emissions under 1.55-μm diode laser excitation with an unfocused laser beam(pump power of 1000 mW);inset being Y_1.94Er_0.06O₃ and Y_1.64Yb_0.3Er_0.06O₃ UC emission under same laser beam (pump power of 10 mW)

where GSA is ground state absorption and ESA is excited state absorption.The energy transfers (ETs) between Er³⁺and Yb³⁺which leads to the population augment of related Er³⁺levels can well explain the phenomenon that the intensity of UC emissions from Er³⁺centered at 391 and410 nm went up with the concentration of Yb³⁺ [ 22, 23, 24, 25, 26] .It can be seen from Fig.4 that there is an efficient ET process from Er³⁺to Yb³⁺,due to energy matches which are shown as follows:4I_11/2(Er³⁺)+2F_7/2(Yb³⁺)→⁴I_15/2(Er³⁺)+2F₅/2(Yb³⁺) and ⁴S_3/2(Er³⁺)+2F_7/2(Yb³⁺)→4 I_13/2(Er³⁺)+2F_5/2(Yb³⁺) [ 21] .In this process Er³⁺was used as sensitizer to transfer the energy to Yb³⁺.The occurrences of ET process from Yb³⁺to Er³⁺of the4 S_3/2(Er³⁺)+2F_5/2(Yb³⁺)→⁴G_11/2(Er³⁺)+2F_7/2(Yb³⁺)and the ⁴F_9/2(Er³⁺)+2F_5/2(Yb³⁺)→²H_9/2(Er³⁺)+2F_7/2(Yb³⁺) can populate the ⁴G_11/2(Er³⁺) and ²H_9/2(Er³⁺) states [ 23, 24, 25, 26, 27, 28] .In this process,Yb³⁺transferred the energy to Er³⁺and Er³⁺was used as activator.The Er³⁺transitions from ⁴G₁1/2(Er³⁺) and ²H_9/2(Er³⁺) states to the ground state⁴ I_15/2(Er³⁺) produced the 391-and 410-nm radiations [ 23, 29, 30] .In the Er³⁺,Yb³⁺co-doped Y₂O₃ microcrystals,the energy transfer from Yb³⁺to Er³⁺,which is the kind of consecutive process of the energy transfer from Er³⁺to Yb³⁺,can promote the population of Er³⁺near-UV levels and enhance their emission.The UC emission centered at 520,550 and 650 nm has been expressed in our previous work and can be described as Er³⁺transitions from2H_11/2(Er³⁺),⁴S_3/2(Er³⁺) and ⁴F_9/2(Er³⁺) states to the ground state ⁴I_15/2(Er³⁺) [ 17, 25] .The ⁴f_7/2 states of Er³⁺also can be populated from the ⁴I_11/2 of Er³⁺when the excited ²F_5/2 of Yb³⁺to the ground ²F_7/2 state of Yb³⁺.The²H_11/2 and ⁴S_3,2 states can be populated by the non-radiative relaxations [ 17, 18, 19] .

Fig.4 Diagram of energy levels of Er³⁺and proposed upconversion mechanisms

4 Conclusion

Under 1.55-μm wavelength laser excitation,the enhanced near-UV UC emission in 391 and 410 nm was observed in Y₂O₃:Yb³⁺,Er³⁺microcrystals.The near-UV UC emissions were enhanced by Yb³⁺.In the UC process,according to the mechanism analysis,Er³⁺was used as not only activator but also sensitizer.Under the 1.55-μm laser excitation,the energy transfer from Er³⁺to Yb³⁺and then Yb³⁺to Er³⁺was observed.Dopant Er³⁺acted as not only activator but also sensitizer.The energy transfer between Er³⁺and Yb³⁺enhanced the near-UV UC emission of our materials.These results provide a new way to tunable UC radiation in rare-earth ion-doped materials.