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Color tunable upconversion emission in CeO2:Yb,Er three-dimensional ordered macroporous materials
JOURNAL OF RARE EARTHS, Vol. 33, No. 6, Jun. 2015, P. 599
Color tunable upconversion emission in CeO2:Yb,Er three-dimensional ordered macroporous materials CHENG Yanmin (程彦敏), YANG Zhengwen (杨正文)*, LIAO Jiayan (廖佳燕), QIU Jianbei (邱建备), SONG Zhiguo (宋志国), YANG Yong (杨 勇) (College of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China) Received 9 August 2014; revised 6 April 2015
Abstract: The three-dimensional ordered macroporous CeO2:Yb,Er materials were prepared, and the influence of doping concentration of Yb3+ or Er3+ ions on upconversion property was investigated. Green and red upconversion emissions were observed under the excitation of 980 nm at room temperature. It was found that the ratio of red to green upconversion emission intensity increased with increasing of concentration of the Yb3+ or Er3+ ions in the three-dimensional ordered macroporous CeO2:Yb,Er materials. When the concentration of Yb3+ was 10 mol.%, pure red upconversion emission was obtained. The varied mechanism of ratio of red to green upconversion emission intensity was discussed with the concentration of Yb3+ or Er3+ ions. Keywords: three-dimensional ordered macroporous materials; CeO2:Yb,Er; upconversion emission; color tunability; rare earths
In the past few years, rare earth ions doped upconversion (UC) luminescence materials have attracted considerable attention due to their potential applications in many areas such as all-solid compact lasers, displays, optical data storage and fluorescent labels[1–11]. In general, the UC luminescence is produced through multiple processes including ground state absorption and energy transfer, etc. The UC luminescence efficiency depends on the phonon energy of host materials. Fluorides are the most widely used host materials of UC emission due to their low phonon energy. However, poor chemical and thermal stability limited their practical application. Oxide materials are other good choices for UC luminescence because of their good physical and chemical stability[2,6,12,13]. Among the various oxide materials, rare earth oxides such as yttrium oxide, gadolinium oxide and lutetium oxide are good matrixes for UC emission because of their excellent properties including good thermal stability and low phonon energy. In addition, rare earth oxides can be easily doped with other rare earth ions such as Er3+ and Tm3+, which means high concentration doping is possible in these hosts. Very recently, much attention has been focused on the preparation and characterization of three-dimensional ordered macroporous (3DOM) materials due to their potential applications in chemical sensor, solid catalysis, absorbents, lighting and display devices[7,14,15]. For the application of lighting and display devices, control of the
relative intensities of the red, green and blue UC emission is required in the 3DOM materials. Many approaches have already been proposed for this purpose, including using of double near-infrared laser excitation sources and variety of doping concentration of rare earth ions[12,16,17]. Among these methods, changing rare earth ions concentration has been demonstrated to be a convenient and versatile technique for generating color tunable upconversion emission[7,12,18]. In this work, the UC 3DOM materials consisting of CeO2:Yb,Er were prepared and its color tunable action was investigated. Color modification of the UC emission was successfully achieved in the CeO2:Yb,Er 3DOM materials by adjusting the concentration of Yb3+ or Er3+.
1 Experimental The ordered opal templates were prepared on cleaned quartz substrates by using 350 nm polystyrene (PS) microspheres through a vertical deposition method[19]. The 3DOM CeO2:xmol.%Er3+,ymol.%Yb3+ (x=0.5, y=0, 0.5, 2, 5, 10) was fabricated. Firstly, the 0.1 mol/L CeO2: xmol.%Er3+,ymol.%Yb3+ precursor solution were prepared by using CeO2, Er2O3 and Yb2O3 as raw materials. The Yb(NO3)3 and Er(NO3)3 were prepared by dissolving the Er2O3 and Yb2O3 in the hot concentrated nitric acid, respectively. The Ce(NO3)3 was obtained by dissolving CeO2 into the solution consisted of concentrated nitric
Foundation item: Project supported by the Reserve Talents Project of Yunnan Province (2013HB068) and Applied Basic Research Program of Yunnan Province (2014FB127) * Corresponding author: YANG Zhengwen (E-mail: [email protected]; Tel.: +86-871-65188856) DOI: 10.1016/S1002-0721(14)60459-2
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acid and hydrogen peroxide. The Ce(NO3)3, Yb(NO3)3 and Er(NO3)3 were dissolved in the alcohol, then mixed together. The CeO2:Er3+,Yb3+ transparent solution was obtained. The opal templates were infiltrated with the CeO2:xmol.%Er3+,ymol.%Yb3+ precursor solution. The 3DOM CeO2:Er3+,Yb3+ were finally calcined at 950 ºC for 5 h. Morphology of the CeO2:Er3+,Yb3+ 3DOM materials were observed by a scanning electron microscope (SEM). The elemental analysis of the CeO2:Er3+,Yb3+ 3DOM material was characterized by the energy dispersive X-ray spectrometry (EDS). Transmission electron microscope (TEM) images of the ground 3DOM CeO2:Er3+,Yb3+ powder was taken using a JEOL 2100 transmission electron microscope. The phase structures of the 3DOM CeO2:Er3+,Yb3+ were identified via the X-ray diffraction (XRD) measurement (D8 ADVANCE). The UC luminescence spectra of 3DOM CeO2:Er3+,Yb3+ under a 980 nm infrared laser excitation were recorded by a HITACHIU-F-7000 spectrophotometer.
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The elemental analysis of the CeO2:Er3+,Yb3+ 3DOM material was determined. The EDS spectrum of the CeO2: 0.5 mol.%Er3+,0.5 mol.%Yb3+ 3DOM material is shown in the Fig. 1(c). The Ce, O, Er and Yb elements were observed in this sample. The concentrations of Er and Yb elements are 0.12 mol.% and 0.19 mol.%, respectively. The concentrations of Er and Yb elements in the CeO2:Er3+,Yb3+ 3DOM material are lower than these in the CeO2:Er3+, Yb3+ sol, indicating that only part of Er and Yb elements entered in the CeO2 host. Fig. 2 shows the X-ray diffraction (XRD) patterns of the CeO2:0.5 mol.%Er3+,10 mol.%Yb3+ 3DOM samples and the standard JCPDS Card (No. 01-0800). The broad band range from 17° to 25° in the XRD pattern was considered as the diffraction of quartz substrate. By sintering at 950 ºC, all the diffraction peaks can be indexed to
2 Results and discussion The morphology of opal template and corresponding 3DOM CeO2:Er3+,Yb3+ samples were observed by the SEM. Fig. 1(a) shows the SEM image of opal template made of 350 nm PS microspheres. It can be clearly seen that a long range ordered structure with (111) plane parallel to the surface of quartz substrate is formed for the PS opal template. All the CeO2:Er3+,Yb3+ samples have a long-range ordered hexagonal arrangement regardless of concentration of Er3+ and Yb3+ ions. The concentration of Er3+ and Yb3+ ions has little influence on the ordered structure of CeO2:Er3+,Yb3+ 3DOM samples. Fig. 1(b) shows a long-range ordered hexagonal arrangement of the CeO2:0.5 mol.%Er3+,10 mol.%Yb3+ 3DOM samples. The center-to-center distance between the air macrospores is about 270 nm, which is about 23% smaller than the original size of PS microspheres because of the shrinkage of microspheres’ diameters during calcination.
Fig. 1 SEM images of opal template (a) made of 350 nm microspheres and 3DOM CeO2:0.5 mol.%Er3+,10 mol.% Yb3+ material (b), EDS spectrum of the CeO2:0.5 mol.%Er3+,0.5 mol.%Yb3+ 3DOM material (c)
Fig. 2 XRD patterns of 3DOM CeO2:0.5 mol.%Er3+,10 mol.%Yb3+ material (a), high-resolution TEM images of the CeO2:0.5 mol.%Er3+,2 mol.%Yb3+ (b)
CHENG Yanmin et al., Color tunable upconversion emission in CeO2:Yb,Er three-dimensional ordered macroporous … 601
the pure phase of the CeO2, and the pure cubic phase CeO2 was obtained. Doped Er3+ and Yb3+ ions have no obvious influence on the crystal structure of the CeO2 host. The Er3+ (89 pm) and Yb3+ ions (86.8 pm) are expected to occupy the Ce4+ (87 pm) sites in the CeO2 host. The crystallinity of the the 3DOM CeO2:Er3+,Yb3+ materials was confirmed by the high-resolution TEM image. The TEM image of the ground 3DOM CeO2:0.5 mol.%Er3+,2 mol.%Yb3+ powder is shown in Fig. 2(b). The lattice fringe of the sample is clearly distinguished, indicating that the prepared 3DOM CeO2:Er3+,Yb3+ material are highly crystalline. The distance between the lattice fringes was measured to be about 0.33 nm, which corresponds to the spacing of the (111) lattice planes of the cubic CeO2. Under the excitation of 980 nm, the UC luminescence properties of the CeO2:Er3+,Yb3+ 3DOM materials with different doping concentrations of Yb3+ and Er3+ were investigated in detail. Fig. 3(a) shows the UC luminescence spectra of 3DOM CeO2:0.5 mol.%Er3+,Yb3+ for different Yb3+ concentrations. As seen in Fig. 3(a), the UC emission bands located at 527 (2H11/2→4I15/2), 548 and 562 nm (4S3/2→4I15/2), and 660 and 680 nm (4F9/2→4I15/2) were observed. With increasing Yb3+ concentration, the red UC emission intensity increases, while green UC emission intensity decreases. When the Yb3+ concentration is up to 10 mol.%, near pure red UC emission was obtained. Fig. 3(b) shows the UC luminescence spectra of 3DOM CeO2:Er3+,2 mol.%Yb3+ with different Er3+ concentration dopings. The red and green UC emissions attributed to the transition of Er3+ (2H11/2/4S3/2→ 4 I15/2 and 4F9/2→ 4I15/2) are observed. For the 3DOM CeO2:Er3+,2 mol.% Yb3+ samples with low concentration of Er3+, the green emission is dominant, while the red UC emission becomes dominant in the UC spectra at the high doping concentration of Er3+. The ratio of red to green emission intensity increases with increasing of the Er3+ concentration. Such a result is also observed from other Er3+ doped and Er3+–Yb3+ co-doped hosts, which could
be mainly attributed to the interactions between doping Er3+ and Yb3+ ions. The mechanism of UC emission in theCeO2:Er3+,Yb3+ 3DOM samples was analyzed by investigating the dependence of the UC emission intensity and excitation light power. The UC emission intensity (Iup) depends on the n-th of the excitation power (P) as Iup∝Pn, where n is the number of photons with low energy required to absorb for emitting one high energy photon. The intensity of red and green UC luminescence was measured under different exciting laser power for the 3DOM CeO2:Er3+, Yb3+ sample, as shown in the Fig. 4. The slopes of plots for red and green UC emissions are about 2, which indicates that the two-photon processes were involved in the green and red UC emission mechanisms. The possible UC emission mechanisms are shown in Fig. 5. The Er3+ ion in the ground state is excited to the 4I11/2 state by energy transfer (ET) from Yb3+ ions or ground state absorption (GSA). Er3+ ions in excited state 4I11/2 were transformed to 4F7/2 state by means of the excited state absorption (ESA) or the ET process. Subsequently non- radiative relaxation from the 4F7/2 state populates the next 2 H11/2 and 4S3/2, which results in the 527 and 548 nm green UC luminescence. For red UC emission, the electrons at the 4I11/2 state relaxes non-radiatiely to the next 4 I13/2 level. The 4F9/2 state is populated via transition from the 4I 13/2 state by the ESA and/or ET. The red UC emission is attributed to the transitions from the 4F 9/2 to 4I15/2. In addition, the 4F9/2 state can be populated by cross relaxation process between two nearby Er3+ ions by the route 4F7/2+4I11/2→24F9/2, leading to the red UC emission. The population of 4F9/2 red levels by cross relaxation process occurs indeed, which is confirmed by increasing of ratio of red to green UC emission intensity with increasing of rare earth ions concentration. It is clearly seen from Fig. 3 that the UC emission spectra depends on Yb3+ ions concentration. The ratio of red to green UC emission intensity increases with increasing of Yb3+ concentration, indicating population
Fig. 3 UC emission spectra of 3DOM CeO2:0.5 mol.%Er3+,xmol.%Yb3+ (x=0, 0.5, 2,5, 10) (a), and 3DOM CeO2:ymol.%Er3+,2 mol.%Yb3+ (y=0.05, 0.5, 1, 2) materials (b)
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Fig. 4 log-log plot of pump power dependence of UC emission of 3DOM CeO2:0.5 mol.%Er3+,0.5 mol.%Yb3+ material
Fig. 5 UC emission mechanism of 3DOM CeO2:Er3+,Yb3+ materials
enhancement of 4F9/2 red levels. There are two possible mechanisms for this population enhancement. One is the cross relaxation process 4F7/2+4I11/2→24F9/2. According to the above UC mechanisms, the cross relaxation process (4F7/2+4I11/2→24F9/2) populated the 4F9/2 red levels. Therefore, the population of the 4F9/2 red levels is related to the 4I11/2 and 4F7/2 level. The larger population of the 4 I11/2 and 4F7/2 level is favorable to the production of 4 F7/2+4I11/2→24F9/2 cross relaxation process. In the CeO2:Er3+,Yb3+ sample, the 4I11/2 and 4F7/2 states were populated by the energy transfer from the Yb3+ to Er3+ ions. The efficiency of energy transfer is governed by the distance between Er3+ and Yb3+ ions, which is inverse proportion to the distance between Er3+ and Yb3+ ions. The distance between the Er3+ and Yb3+ ions decreased with the increasing of the concentration of the Yb3+ ions. Thus the efficiency of energy transfer increases with the increasing of the Er3+ and Yb3+ concentration, resulting in the population enhancement of 4I11/2 and 4F7/2 state of Er3+. In addition, the population increasing of 4I11/2 and 4 F7/2 state of Er3+ enhances 4F7/2+4I11/2→2 4F9/2 cross relaxation process, which causes the population increasing of red emission level. Thus the red to green UC emission intensity increases with increasing of the Yb3+ concentration. On the other hand, there is another important crossrelaxation process in the 3DOM CeO2:Er3+,Yb3+ samples for population enhancement of the 4F9/2 red levels and
JOURNAL OF RARE EARTHS, Vol. 33, No. 6, Jun. 2015
population reduction of the green 2H11/2/4S3/2 levels. It can be described as follows: 2H11/2+4I15/2→4I9/2+4I13/2. In this cross relaxation process, the excited Er3+ ions will relax nonradiatively from 2H11/2 to 4I9/2 state by transferring its energy to a neighboring Er3+ ion in the ground state, exciting the ground state Er3+ to 4I13/2 state. Obviously, the population of the red 4F9/2 level is proportional to population of 4I13/2 sate. With the increasing of Er3+ doping concentration, the cross relaxation process (2H11/2+4I15/2→ 4 I9/2+4I13/2) is more efficient, resulting in the increasing of population of 4I13/2 and 4F9/2 levels. Therefore, the enhancement of red UC emission was observed in the 3DOM CeO2:Er3+,Yb3+ samples with increasing of Er3+ or Yb3+ doping concentration. The red to green UC emission ratio can be modulated by the variety of rare earth ions concentration, resulting in the color tunable UC emission of 3DOM CeO2:Er3+,Yb3+ samples. Fig. 6 shows the CIE chromaticity coordinates of the 3DOM CeO2:Er3+,Yb3+ samples, which were calculated based on the corresponding UC emission spectra shown in Fig. 3. It can be seen that the color of UC emission changed from yellow green to yellow with the variety of Er3+ or Yb3+ concentration. The color tuning of the UC emission was successfully achieved in the 3DOM CeO2:Er3+,Yb3+ materials by controlling the doping concentration of rare earth ions.
Fig. 6 CIE chromaticity coordinates of the 3DOM CeO2: xmol.%Er3+,ymol.%Yb3+ materials (a) x=0.5, y=0; (b) x=0.5, y=0.5, (c) x=0.5, y=2, (d) x=0.5, y=5, (e) x=0.5, y=10, (f) x=0.05, y=2, (g) x=1, y=2, (h) x=2, y=2
3 Conclusions We presented the investigation of color tunability of UC luminescence in the three dimensional ordered macroporous CeO2:Yb3+,Er3+ materials. The ratio of red to green upconversion emission intensity increased with increasing of concentration of the Yb3+ or Er3+ ions in the three-dimensional ordered macroporous CeO2:Yb3+,Er3+ materials, which were attributed to the cross relaxation process between Er3+ ions. When the concentration of Yb3+ was 10 mol.%, pure red UC emission was obtained.
CHENG Yanmin et al., Color tunable upconversion emission in CeO2:Yb,Er three-dimensional ordered macroporous … 603
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Color tunable upconversion emission in CeO2:Yb,Er three-dimensional ordered macroporous materials CHENG Yanmin (程彦敏), YANG Zhengwen (杨正文)*, LIAO Jiayan (廖佳燕), QIU Jianbei (邱建备), SONG Zhiguo (宋志国), YANG Yong (杨 勇) (College of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China) Received 9 August 2014; revised 6 April 2015
Abstract: The three-dimensional ordered macroporous CeO2:Yb,Er materials were prepared, and the influence of doping concentration of Yb3+ or Er3+ ions on upconversion property was investigated. Green and red upconversion emissions were observed under the excitation of 980 nm at room temperature. It was found that the ratio of red to green upconversion emission intensity increased with increasing of concentration of the Yb3+ or Er3+ ions in the three-dimensional ordered macroporous CeO2:Yb,Er materials. When the concentration of Yb3+ was 10 mol.%, pure red upconversion emission was obtained. The varied mechanism of ratio of red to green upconversion emission intensity was discussed with the concentration of Yb3+ or Er3+ ions. Keywords: three-dimensional ordered macroporous materials; CeO2:Yb,Er; upconversion emission; color tunability; rare earths
In the past few years, rare earth ions doped upconversion (UC) luminescence materials have attracted considerable attention due to their potential applications in many areas such as all-solid compact lasers, displays, optical data storage and fluorescent labels[1–11]. In general, the UC luminescence is produced through multiple processes including ground state absorption and energy transfer, etc. The UC luminescence efficiency depends on the phonon energy of host materials. Fluorides are the most widely used host materials of UC emission due to their low phonon energy. However, poor chemical and thermal stability limited their practical application. Oxide materials are other good choices for UC luminescence because of their good physical and chemical stability[2,6,12,13]. Among the various oxide materials, rare earth oxides such as yttrium oxide, gadolinium oxide and lutetium oxide are good matrixes for UC emission because of their excellent properties including good thermal stability and low phonon energy. In addition, rare earth oxides can be easily doped with other rare earth ions such as Er3+ and Tm3+, which means high concentration doping is possible in these hosts. Very recently, much attention has been focused on the preparation and characterization of three-dimensional ordered macroporous (3DOM) materials due to their potential applications in chemical sensor, solid catalysis, absorbents, lighting and display devices[7,14,15]. For the application of lighting and display devices, control of the
relative intensities of the red, green and blue UC emission is required in the 3DOM materials. Many approaches have already been proposed for this purpose, including using of double near-infrared laser excitation sources and variety of doping concentration of rare earth ions[12,16,17]. Among these methods, changing rare earth ions concentration has been demonstrated to be a convenient and versatile technique for generating color tunable upconversion emission[7,12,18]. In this work, the UC 3DOM materials consisting of CeO2:Yb,Er were prepared and its color tunable action was investigated. Color modification of the UC emission was successfully achieved in the CeO2:Yb,Er 3DOM materials by adjusting the concentration of Yb3+ or Er3+.
1 Experimental The ordered opal templates were prepared on cleaned quartz substrates by using 350 nm polystyrene (PS) microspheres through a vertical deposition method[19]. The 3DOM CeO2:xmol.%Er3+,ymol.%Yb3+ (x=0.5, y=0, 0.5, 2, 5, 10) was fabricated. Firstly, the 0.1 mol/L CeO2: xmol.%Er3+,ymol.%Yb3+ precursor solution were prepared by using CeO2, Er2O3 and Yb2O3 as raw materials. The Yb(NO3)3 and Er(NO3)3 were prepared by dissolving the Er2O3 and Yb2O3 in the hot concentrated nitric acid, respectively. The Ce(NO3)3 was obtained by dissolving CeO2 into the solution consisted of concentrated nitric
Foundation item: Project supported by the Reserve Talents Project of Yunnan Province (2013HB068) and Applied Basic Research Program of Yunnan Province (2014FB127) * Corresponding author: YANG Zhengwen (E-mail: [email protected]; Tel.: +86-871-65188856) DOI: 10.1016/S1002-0721(14)60459-2
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acid and hydrogen peroxide. The Ce(NO3)3, Yb(NO3)3 and Er(NO3)3 were dissolved in the alcohol, then mixed together. The CeO2:Er3+,Yb3+ transparent solution was obtained. The opal templates were infiltrated with the CeO2:xmol.%Er3+,ymol.%Yb3+ precursor solution. The 3DOM CeO2:Er3+,Yb3+ were finally calcined at 950 ºC for 5 h. Morphology of the CeO2:Er3+,Yb3+ 3DOM materials were observed by a scanning electron microscope (SEM). The elemental analysis of the CeO2:Er3+,Yb3+ 3DOM material was characterized by the energy dispersive X-ray spectrometry (EDS). Transmission electron microscope (TEM) images of the ground 3DOM CeO2:Er3+,Yb3+ powder was taken using a JEOL 2100 transmission electron microscope. The phase structures of the 3DOM CeO2:Er3+,Yb3+ were identified via the X-ray diffraction (XRD) measurement (D8 ADVANCE). The UC luminescence spectra of 3DOM CeO2:Er3+,Yb3+ under a 980 nm infrared laser excitation were recorded by a HITACHIU-F-7000 spectrophotometer.
JOURNAL OF RARE EARTHS, Vol. 33, No. 6, Jun. 2015
The elemental analysis of the CeO2:Er3+,Yb3+ 3DOM material was determined. The EDS spectrum of the CeO2: 0.5 mol.%Er3+,0.5 mol.%Yb3+ 3DOM material is shown in the Fig. 1(c). The Ce, O, Er and Yb elements were observed in this sample. The concentrations of Er and Yb elements are 0.12 mol.% and 0.19 mol.%, respectively. The concentrations of Er and Yb elements in the CeO2:Er3+,Yb3+ 3DOM material are lower than these in the CeO2:Er3+, Yb3+ sol, indicating that only part of Er and Yb elements entered in the CeO2 host. Fig. 2 shows the X-ray diffraction (XRD) patterns of the CeO2:0.5 mol.%Er3+,10 mol.%Yb3+ 3DOM samples and the standard JCPDS Card (No. 01-0800). The broad band range from 17° to 25° in the XRD pattern was considered as the diffraction of quartz substrate. By sintering at 950 ºC, all the diffraction peaks can be indexed to
2 Results and discussion The morphology of opal template and corresponding 3DOM CeO2:Er3+,Yb3+ samples were observed by the SEM. Fig. 1(a) shows the SEM image of opal template made of 350 nm PS microspheres. It can be clearly seen that a long range ordered structure with (111) plane parallel to the surface of quartz substrate is formed for the PS opal template. All the CeO2:Er3+,Yb3+ samples have a long-range ordered hexagonal arrangement regardless of concentration of Er3+ and Yb3+ ions. The concentration of Er3+ and Yb3+ ions has little influence on the ordered structure of CeO2:Er3+,Yb3+ 3DOM samples. Fig. 1(b) shows a long-range ordered hexagonal arrangement of the CeO2:0.5 mol.%Er3+,10 mol.%Yb3+ 3DOM samples. The center-to-center distance between the air macrospores is about 270 nm, which is about 23% smaller than the original size of PS microspheres because of the shrinkage of microspheres’ diameters during calcination.
Fig. 1 SEM images of opal template (a) made of 350 nm microspheres and 3DOM CeO2:0.5 mol.%Er3+,10 mol.% Yb3+ material (b), EDS spectrum of the CeO2:0.5 mol.%Er3+,0.5 mol.%Yb3+ 3DOM material (c)
Fig. 2 XRD patterns of 3DOM CeO2:0.5 mol.%Er3+,10 mol.%Yb3+ material (a), high-resolution TEM images of the CeO2:0.5 mol.%Er3+,2 mol.%Yb3+ (b)
CHENG Yanmin et al., Color tunable upconversion emission in CeO2:Yb,Er three-dimensional ordered macroporous … 601
the pure phase of the CeO2, and the pure cubic phase CeO2 was obtained. Doped Er3+ and Yb3+ ions have no obvious influence on the crystal structure of the CeO2 host. The Er3+ (89 pm) and Yb3+ ions (86.8 pm) are expected to occupy the Ce4+ (87 pm) sites in the CeO2 host. The crystallinity of the the 3DOM CeO2:Er3+,Yb3+ materials was confirmed by the high-resolution TEM image. The TEM image of the ground 3DOM CeO2:0.5 mol.%Er3+,2 mol.%Yb3+ powder is shown in Fig. 2(b). The lattice fringe of the sample is clearly distinguished, indicating that the prepared 3DOM CeO2:Er3+,Yb3+ material are highly crystalline. The distance between the lattice fringes was measured to be about 0.33 nm, which corresponds to the spacing of the (111) lattice planes of the cubic CeO2. Under the excitation of 980 nm, the UC luminescence properties of the CeO2:Er3+,Yb3+ 3DOM materials with different doping concentrations of Yb3+ and Er3+ were investigated in detail. Fig. 3(a) shows the UC luminescence spectra of 3DOM CeO2:0.5 mol.%Er3+,Yb3+ for different Yb3+ concentrations. As seen in Fig. 3(a), the UC emission bands located at 527 (2H11/2→4I15/2), 548 and 562 nm (4S3/2→4I15/2), and 660 and 680 nm (4F9/2→4I15/2) were observed. With increasing Yb3+ concentration, the red UC emission intensity increases, while green UC emission intensity decreases. When the Yb3+ concentration is up to 10 mol.%, near pure red UC emission was obtained. Fig. 3(b) shows the UC luminescence spectra of 3DOM CeO2:Er3+,2 mol.%Yb3+ with different Er3+ concentration dopings. The red and green UC emissions attributed to the transition of Er3+ (2H11/2/4S3/2→ 4 I15/2 and 4F9/2→ 4I15/2) are observed. For the 3DOM CeO2:Er3+,2 mol.% Yb3+ samples with low concentration of Er3+, the green emission is dominant, while the red UC emission becomes dominant in the UC spectra at the high doping concentration of Er3+. The ratio of red to green emission intensity increases with increasing of the Er3+ concentration. Such a result is also observed from other Er3+ doped and Er3+–Yb3+ co-doped hosts, which could
be mainly attributed to the interactions between doping Er3+ and Yb3+ ions. The mechanism of UC emission in theCeO2:Er3+,Yb3+ 3DOM samples was analyzed by investigating the dependence of the UC emission intensity and excitation light power. The UC emission intensity (Iup) depends on the n-th of the excitation power (P) as Iup∝Pn, where n is the number of photons with low energy required to absorb for emitting one high energy photon. The intensity of red and green UC luminescence was measured under different exciting laser power for the 3DOM CeO2:Er3+, Yb3+ sample, as shown in the Fig. 4. The slopes of plots for red and green UC emissions are about 2, which indicates that the two-photon processes were involved in the green and red UC emission mechanisms. The possible UC emission mechanisms are shown in Fig. 5. The Er3+ ion in the ground state is excited to the 4I11/2 state by energy transfer (ET) from Yb3+ ions or ground state absorption (GSA). Er3+ ions in excited state 4I11/2 were transformed to 4F7/2 state by means of the excited state absorption (ESA) or the ET process. Subsequently non- radiative relaxation from the 4F7/2 state populates the next 2 H11/2 and 4S3/2, which results in the 527 and 548 nm green UC luminescence. For red UC emission, the electrons at the 4I11/2 state relaxes non-radiatiely to the next 4 I13/2 level. The 4F9/2 state is populated via transition from the 4I 13/2 state by the ESA and/or ET. The red UC emission is attributed to the transitions from the 4F 9/2 to 4I15/2. In addition, the 4F9/2 state can be populated by cross relaxation process between two nearby Er3+ ions by the route 4F7/2+4I11/2→24F9/2, leading to the red UC emission. The population of 4F9/2 red levels by cross relaxation process occurs indeed, which is confirmed by increasing of ratio of red to green UC emission intensity with increasing of rare earth ions concentration. It is clearly seen from Fig. 3 that the UC emission spectra depends on Yb3+ ions concentration. The ratio of red to green UC emission intensity increases with increasing of Yb3+ concentration, indicating population
Fig. 3 UC emission spectra of 3DOM CeO2:0.5 mol.%Er3+,xmol.%Yb3+ (x=0, 0.5, 2,5, 10) (a), and 3DOM CeO2:ymol.%Er3+,2 mol.%Yb3+ (y=0.05, 0.5, 1, 2) materials (b)
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Fig. 4 log-log plot of pump power dependence of UC emission of 3DOM CeO2:0.5 mol.%Er3+,0.5 mol.%Yb3+ material
Fig. 5 UC emission mechanism of 3DOM CeO2:Er3+,Yb3+ materials
enhancement of 4F9/2 red levels. There are two possible mechanisms for this population enhancement. One is the cross relaxation process 4F7/2+4I11/2→24F9/2. According to the above UC mechanisms, the cross relaxation process (4F7/2+4I11/2→24F9/2) populated the 4F9/2 red levels. Therefore, the population of the 4F9/2 red levels is related to the 4I11/2 and 4F7/2 level. The larger population of the 4 I11/2 and 4F7/2 level is favorable to the production of 4 F7/2+4I11/2→24F9/2 cross relaxation process. In the CeO2:Er3+,Yb3+ sample, the 4I11/2 and 4F7/2 states were populated by the energy transfer from the Yb3+ to Er3+ ions. The efficiency of energy transfer is governed by the distance between Er3+ and Yb3+ ions, which is inverse proportion to the distance between Er3+ and Yb3+ ions. The distance between the Er3+ and Yb3+ ions decreased with the increasing of the concentration of the Yb3+ ions. Thus the efficiency of energy transfer increases with the increasing of the Er3+ and Yb3+ concentration, resulting in the population enhancement of 4I11/2 and 4F7/2 state of Er3+. In addition, the population increasing of 4I11/2 and 4 F7/2 state of Er3+ enhances 4F7/2+4I11/2→2 4F9/2 cross relaxation process, which causes the population increasing of red emission level. Thus the red to green UC emission intensity increases with increasing of the Yb3+ concentration. On the other hand, there is another important crossrelaxation process in the 3DOM CeO2:Er3+,Yb3+ samples for population enhancement of the 4F9/2 red levels and
JOURNAL OF RARE EARTHS, Vol. 33, No. 6, Jun. 2015
population reduction of the green 2H11/2/4S3/2 levels. It can be described as follows: 2H11/2+4I15/2→4I9/2+4I13/2. In this cross relaxation process, the excited Er3+ ions will relax nonradiatively from 2H11/2 to 4I9/2 state by transferring its energy to a neighboring Er3+ ion in the ground state, exciting the ground state Er3+ to 4I13/2 state. Obviously, the population of the red 4F9/2 level is proportional to population of 4I13/2 sate. With the increasing of Er3+ doping concentration, the cross relaxation process (2H11/2+4I15/2→ 4 I9/2+4I13/2) is more efficient, resulting in the increasing of population of 4I13/2 and 4F9/2 levels. Therefore, the enhancement of red UC emission was observed in the 3DOM CeO2:Er3+,Yb3+ samples with increasing of Er3+ or Yb3+ doping concentration. The red to green UC emission ratio can be modulated by the variety of rare earth ions concentration, resulting in the color tunable UC emission of 3DOM CeO2:Er3+,Yb3+ samples. Fig. 6 shows the CIE chromaticity coordinates of the 3DOM CeO2:Er3+,Yb3+ samples, which were calculated based on the corresponding UC emission spectra shown in Fig. 3. It can be seen that the color of UC emission changed from yellow green to yellow with the variety of Er3+ or Yb3+ concentration. The color tuning of the UC emission was successfully achieved in the 3DOM CeO2:Er3+,Yb3+ materials by controlling the doping concentration of rare earth ions.
Fig. 6 CIE chromaticity coordinates of the 3DOM CeO2: xmol.%Er3+,ymol.%Yb3+ materials (a) x=0.5, y=0; (b) x=0.5, y=0.5, (c) x=0.5, y=2, (d) x=0.5, y=5, (e) x=0.5, y=10, (f) x=0.05, y=2, (g) x=1, y=2, (h) x=2, y=2
3 Conclusions We presented the investigation of color tunability of UC luminescence in the three dimensional ordered macroporous CeO2:Yb3+,Er3+ materials. The ratio of red to green upconversion emission intensity increased with increasing of concentration of the Yb3+ or Er3+ ions in the three-dimensional ordered macroporous CeO2:Yb3+,Er3+ materials, which were attributed to the cross relaxation process between Er3+ ions. When the concentration of Yb3+ was 10 mol.%, pure red UC emission was obtained.
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