The Y2O2S:Er3+@Y2O2S:Yb3+,Ho3+ core-shell up-conversion (UC) nanoparticles were successfully synthesized by the homogeneous co-precipitation method. The Y2O2S:Er3+@Y2O2S:Yb3+,Ho3+ core-shell nanoparticles exhibit bright green emissions under 980 nm excitation, while the triple-ion doped Y2O2S:Er3+,Yb3+,Ho3+ sample presents mainly red emissions. The intensity ratio of green-to-red emission of the core-shell and conventional triple-ion doped samples are 2.8 and 0.3, respectively. Investigations on the UC mechanisms show that emissions from Er3+ and Ho3+ ions are achieved simultaneously in the core-shell nanoparticles. This is due to the efficient energy transfers of Yb3+→Ho3+ within the shell layer and Yb3+→Er3+ between the shell and the core. While the triple-ion doped Y2O2S: Er3+,Yb3+,Ho3+ sample exhibits mainly the emissions of Er3+ along with weak luminescence of Ho3+ ion. Since the cross relaxation between Er3+ and Ho3+ ions in the Y2O2S:Er3+,Yb3+,Ho3+ nanoparticles can effectively suppress the emissions of Ho3+ ions. Yet, in the core-shell structure, this cross relaxation can be successfully restrained in the core-shell structure where Er3+ is in the core and Ho3+ is in the shell. Therefore, the construction of core-shell structure can improve the luminescence efficiency and provide a route for adjustment of emission color.
Upconversion luminescence (UCL) materials with unique luminescent properties have become the research focus due to their promising applications in anti-counterfeiting, solar cells, three-dimensional display and solid-state lasers [
In this work, we have successfully synthesized the core-shell nanoparticles (NPs) of Y2O2S:Er3+@Y2O2S:Yb3+,Ho3+ by using homogeneous co-precipitation method combined with the solid-gas sulfidation route. The UCL properties of Y2O2S:Er3+@Y2O2S:Yb3+,Ho3+ and triple-ion doped Y2O2S:Er3+,Yb3+,Ho3+ samples were investigated under excitation of 980 nm laser. We found that the core-shell Y2O2S:Er3+@Y2O2S:Yb3+,Ho3+ exhibits bright green emission, while Y2O2S:Er3+,Yb3+,Ho3+ presents red emission due to the new channels of energy transfers in the core-shell structure. Further, the cross relaxation between Er3+ and Ho3+ ions can be successfully restrained in the core-shell structure, which lead to the emissions from both of the Er3+ and Ho3+ ions. Therefore, the core-shell structure provides a new route for adjustment of luminescence color and improvement of luminescence efficiency.
The precipitates of the core material, Y(OH)CO3:12% Er3+, were obtained by mixing the solution of urea (99% purchased from Tianjin Bodi Chemical Co., Ltd) in 0.8 L solution (7.5 mol/L) and rare earth nitrates Re(NO3)3 (99.99%, Guangzhou Rare Earth Industry Group CO., Ltd) in 0.2 L solution. The mole ratio of Y, Er ions is 88 : 12. After the centrifuging and washing with water and isopropanol, the core material of Y2O3:Er3+ powder was achieved by annealing the precipitates at 600°C for 1 h. To coat the shell layer on the core material, we first added the Y2O3:Er3+ into the urea solution (6 mol, 0.8 L) at 60°C, and then mixed the solution (0.2 L) of Re(NO3)3 (Re = Y, Yb, Ho, with mole ratio of 91 : 8 : 1) by bath sonication at 80°C for 30 mins. After the similar processes of cooling down, centrifuging, washing and drying, we obtained the precursor of Y2O3:Er3+@Y(OH)CO3:Yb3+, Ho3+. The precursor of the reference sample, Y(OH)CO3:12% Er3+,8%Yb3+,1%Ho3+, were obtained in the similar processes by mixing the solution of urea with Re(NO3)3 (Re = Y, Er, Yb, Ho with mole ratio of 79 : 12 : 8 : 1). Then, the core-shell NPs of Y2O3: 12%Er3+@Y2O3:8%Yb3+ and Y2O3:Er3+,Yb3+,Ho3+ were finally achieved by annealing the precursor at 600°C. The last step is the sulfidation process. The oxides and sulfur powders were put into a quartz tube which was heated up to 800°C for 30 mins. In this process, Argon was used as protection atmosphere. The above RE-ions doping concentrations were optimized on basis of a series of experimental results as shown in the supporting information.
X-ray diffraction (XRD) patterns were recorded at 40 kV and 40 mA by using a Rigaku D/MAX-Ultima X-ray diffractometer with Cu K
Figure
XRD spectra of (a) Y2O2S: Er3+, (b) Y2O2S: Er3+@Y2O2S: Yb3+, Ho3+ nanocrystals and (c) standard card of hexagonal Y2O2S (JCPDS: No.24-1424).
The size and morphology of the samples are studied by TEM measurements, as shown in Figure
TEM images of (a) the precursor of the core NPs (Y(OH)CO3:12% Er
The core-shell Y2O2S:Er3+@Y2O2S:Yb3+,Ho3+ UC NPs exhibit bright UCL of green color, while the Yb3+, Er3+, Ho3+ triple-doped Y2O2S NPs show red emission color. As shown in Figure
Up-conversion emission spectra of Y2O2S: Yb3+, Er3+, Y2O2S: Yb3+, Ho3+, Y2O2S: Er3+ @ Y2O2S: Yb3+, Ho3+ and Y2O2S: Yb3+, Er3+, Ho3+ samples under 980 nm excitation.
Although the UCL from the Ho3+ and Er3+ ions are realized in both samples of Y2O2S:Yb3+,Er3+,Ho3+ and core-shell Y2O2S:Er3+ @Y2O2S:Yb3+,Ho3+, the green emissions from Ho3+ ions are significantly enhanced in the core-shell sample. While the red UCL from Er3+ ions is dominant in the Y2O2S:Yb3+,Er3+,Ho3+ sample. The intensity ratios (I) of green (G) to red (R) emissions (IG/IR) of Y2O2S:Er3+ @ Y2O2S:Yb3+, Ho3+ and Y2O2S:Yb3+, Er3+, Ho3+ samples are are 2.8 and 0.3, respectively. This is due to the cross relaxation between Er3+ and Ho3+ ions in the Y2O2S:Yb3+, Er3+,Ho3+ sample, which suppresses the emission of Ho3+ and enhances the red emission simultaneously. However, in the core-shell sample of Y2O2S:Er3+@Y2O2S:Yb3+Ho3+, the cross relaxation between Er3+ and Ho3+ ions is effectively suppressed by constructing the core-shell structure where Er3+ is in the core and Ho3+ is in the shell. Therefore, the core-shell sample exhibits dominant green emissions from Ho3+ ions, yet the conventional triple-ion doped sample present red UCL from Er3+ ions.
The possible UCL processes of Y2O2S:Er3+@Y2O2S:Yb3+, Ho3+ sample under 980 nm excitation are shown in Figure
The UCL processes of Y2O2S: Er3+ @ Y2O2S: Yb3+, Ho3+ and Y2O2S: Yb3+, Er3+, Ho3+ samples under 980 nm excitation.
In terms of the UCL processes of Yb3+, Er3+ ions, the Yb3+ is firstly pumped to the excited level of 2F5/2 by absorbing a 980 photon, and then return to the ground state by transferring the energy to an Er3+ ion in the core via ET3 and ET5, as shown in Figure
Notably, due to the small distance between the Er3+ and Ho3+ ions in the Y2O2S: Yb3+, Er3+, Ho3+ sample, the cross relaxations of 5S2, 5F4 (Ho3+) + 4I11/2 (Er3+) → 5I4 (Ho3+) + 4F9/2 (Er3+) (CR3) can easily occur in the conventional triple-ion doped sample. This CR3 process significantly increases the population of Er3+ ion at the 4F9/2 level, which results in the much stronger red emission (due to 4F9/2 → 4I15/2) in the Y2O2S: Yb3+, Er3+, Ho3+ sample than that of the core-shell Y2O2S: Er3+ @ Y2O2S: Yb3+, Ho3+ sample. Meanwhile, this CR3 process also decreases the population of Ho3+ ion at the 5S2, 5F4 states, thereby, suppressing the green emission in the Y2O2S: Yb3+, Er3+, Ho3+ sample. The different UCL and ET processes in the Y2O2S: Yb3+, Er3+, Ho3+ and core-shell Y2O2S: Er3+ @ Y2O2S: Yb3+, Ho3+ samples indicate the formation of the core-shell structure and provide a possible route for adjustment of emission color.
Further, the fluorescence decay curves of Y2O2S: Er3+ @ Y2O2S: Yb3+, Ho3+ (Core-Shell) and Y2O2S: Yb3+, Er3+ samples were measured as shown in Figure
Fluorescence decay curves of red (670 nm) and green (548 nm) emissions of Y2O2S: Er3+ @ Y2O2S: Yb3+, Ho3+ (Core-Shell) and Y2O2S: Yb3+, Er3+ samples under 980 nm excitation.
Calculated luminescence lifetimes of Y2O2S: Er3+ @ Y2O2S: Yb3+, Ho3+ and Y2O2S: Yb3+, Er3+ samples.
Transition | Lifetime ( | |
---|---|---|
Y2O2S: Er3+ @ Y2O2S: Yb3+, Ho3+ | Y2O2S: Yb3+, Er3+ | |
4S3/2 → 4I15/2 (548 nm) | 37.8 | 21.8 |
4F9/2 → 4I15/2 (670 nm) | 139.1 | 46.1 |
The Y2O2S:Er3+@Y2O2S:Yb3+, Ho3+ core-shell NPs are synthesized by the homogeneous co-precipitation method combining with the solid-gas sulfidation route. Investigations on the UCL show that the emissions from Er3+ and Ho3+ ions are achieved simultaneously in the core-shell NPs. This is due to the efficient energy transfers of Yb3+ → Ho3+ within the shell layer and Yb3+ → Er3+ between the shell and the core. However, the core-shell Y2O2S: Er3+ @ Y2O2S: Yb3+, Ho3+ and the triple-ion doped Y2O2S: Yb3+, Er3+, Ho3+ samples present mainly green emission from Ho3+ ions (IG/IR = 2.8) and red luminescence from Er3+ ions (IG/IR = 0.3), respectively. The reason is that the cross relaxation between Er3+ and Ho3+ ions can easily occur due to the small distance between them in the Y2O2S: Yb3+, Er3+, Ho3+ sample. While on the other hand, this cross relaxation can be successfully suppressed by the core-shell structure where Ho3+ is in the shell and Er3+ is in the core. Therefore, the unique core-shell Y2O2S nanostructure could offer new channels for energy transfers and presents novel UC luminescence properties.
The data used to support the findings of this study are included within the article.
There is no conflict of interest regarding the publication of this paper.
The authors thank the National Natural Science Foundation of China (11504039), Fundamental Research Funds for the Central Universities (Grant No. 017192610, 017192617) for their financial support.
The supplementary material provides the experiments on the optimum rare earth doping concentrations and the particle size distribution of the precursors of the core and core-shell nanoparticles.